Yayue Pan

Chi Zhou

Yong Chen1

e-mail yongchenuscedu

Daniel J Epstein Department of Industrial

and Systems Engineering

University of Southern California

Los Angeles CA 90089

A Fast Mask ProjectionStereolithography Processfor Fabricating Digital Modelsin MinutesThe purpose of this paper is to present a direct digital manufacturing (DDM) processthat is an order of magnitude faster than other DDM processes that are currently avail-able The developed process is based on a mask-image-projection-based stereolithogra-phy (MIP-SL) process in which a digital micromirror device (DMD) controls projectionlight to selectively cure liquid photopolymer resin In order to achieve high-speed fabri-cation we investigate the bottom-up projection system in the MIP-SL process A two-waylinear motion approach has been developed for the quick spreading of liquid resin intouniform thin layers The system design and related settings for achieving a fabricationspeed of a few seconds per layer are presented Additionally the hardware softwareand material setups for fabricating three-dimensional (3D) digital models are presentedExperimental studies using the developed testbed have been performed to verify the effec-tiveness and efficiency of the presented fast MIP-SL process The test results illustratethat the newly developed process can build a moderately sized part within minutesinstead of hours that are typically required [DOI 10111514007465]

Keywords additive manufacturing high-speed fabrication mask image projectionstereolithography fast recoating

1 Introduction

Layer-based additive manufacturing (AM) processes such asstereolithography apparatus (SLA) can fabricate parts directlyfrom computer-aided design (CAD) models without part-specifictooling or fixtures As a direct manufacturing approach AM proc-esses can cost-effectively fabricate truly complex three-dimensional (3D) shapes that were previously impossible Unliketraditional prototyping approaches that take days AM-based rapidprototyping can build physical objects in hours Due to such timeand cost benefits AM processes have been widely adopted in theproduct development process for building prototypes of a design

Although the speed of AM systems has significantly increasedover the years the building process of a moderate sized 3D modelis typically measured in hours In a recent NSF workshop ondeveloping the roadmap for AM [1] the development of AMmachines with higher throughput was identified to be critical forfuture rapid manufacturing requirements Future high-speed AMsystems require new approaches evolving from point-processingor line-processing methods such as a laser or an extruding nozzleto area-processing or volume-processing methods

In this research we investigate the building speed of an area-processing approach that is based on a DMD A DMD is a micro-electromechanical system device that enables one to simultaneouslycontrol 1 million small mirrors that turn on or off a pixel at over5 KHz Using this technology a light projection device can projecta dynamically defined mask image onto a resin surface to selec-tively cure liquid resin into layers of the object Consequently therelated AM process MIP-SL can be much faster than the laser-based SLA process by simultaneously forming the shape of a wholelayer An illustration of the MIP-SL process is shown in Fig 1 In

the MIP-SL process a 3D CAD model of an object is first sliced bya set of horizontal planes Each thin slice is converted into a two-dimensional (2D) mask image The mask image is then sent to theDMD and projected onto a photocurable resin surface to form theshape of the related layer By repeating the process 3D objects canbe fabricated on a layer-by-layer basis

11 Building Speed Limitation of the MIP-SL Process Inthe MIP-SL process the building time of each layer consists ofspreading liquid resin into a uniform thin layer and curing theformed liquid layer into a solid layer Compared to a laser beam thatis used in the SLA process the DMD used in the MIP-SLA processcan dramatically decrease the curing time of a layer Hence the bot-tleneck for achieving a fast building speed is the spreading of liquidresin into uniform thin layers which is the focus of the paper

Research systems (eg Refs [2ndash8]) and commercial systems(eg Refs [910]) have been developed before based on the maskimage projection approach Most of the developed systems arebased on the top-down projection as shown in Fig 1 Suppose dLT

is the layer thickness After a previous layer has been cured theplatform in such a system usually moves down a certain distanced and then up by d-dLT in order to spread liquid resin into a uni-form thin layer In addition to the Z movement a recoating pro-cess is usually required to sweep through the platform such thatthe top surface can be flattened For resin with low viscosity adeep-dip recoating approach has also been developed to replacethe surface sweeping approach After the up and down movementsin the Z axis a sufficient waiting time is required for the liquidresin to settle down into a flat surface However such recoatingmethods typically take over a minute which limits the buildingspeed of the MIP-SL process Consequently the building time ofsuch MIP-SL systems is still measured in hours

12 Contributions To address the building speed limitationof the MIP-SL process we present a novel approach for quickly

1Corresponding authorContributed by the Manufacturing Engineering Division of ASME for publication

in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING Manuscript receivedDecember 9 2011 final manuscript received August 10 2012 published online Sep-tember 10 2012 Assoc Editor Bin Wei

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-1Copyright VC 2012 by ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

spreading liquid resin into uniform thin layers Our approach isbased on a two-way movement design in a bottom-up projectionsystem We addressed the related challenge of large attachingforces in the bottom-up projection system By optimizing the pro-cess settings we illustrate that the preparation of a uniform thinlayer can be done within seconds Consequently the developedfast MIP-SL process can build moderate sized parts in minutesinstead of hours

2 Bottom-Up Projection Based MIP-SL System

In addition to the top-down projection approach as shown inFig 1 another projection approach used in the MIP-SL process isthe bottom-up projection as shown in Fig 2 That is the maskimage is projected onto the bottom of a transparent tank After alayer is cured at the bottom of the built part the platform is movedup and then down to form a small gap with the bottom surface ofthe resin tank A uniform thin layer can be achieved after theformed gap is filled with liquid resin

21 Advantages of the Bottom-Up Projection System Abottom-up projection based system has several advantages over atop-down projection based system (1) The container depth isindependent of the part height Thus a shallow vat can be used toreduce the required volume of the liquid resin During the buildingprocess liquid resin can be added by a pump when needed (2)Recoating is achieved by constraining liquid resin between thepreviously cured layers and the resin tank Hence no additional

sweeping is needed for flattening the resin surface (3) Muchsmaller layer thickness can be achieved since the gap size is onlydetermined by the Z stage resolution regardless of the fluid proper-ties of liquid resin (4) The curing of liquid resin is sealed fromthe oxygen-rich environment By eliminating the oxygen inhibi-tion effect the liquid photopolymer resin can be cured faster

22 Challenges of the Bottom-Up Projection System Despitethe advantages the bottom-up projection based system has notbeen widely used in the SLA and MIP-SL processes A main rea-son is that the separation of the cured part from the tank surfaceis difficult That is in the bottom-up projection based MIP-SLprocess a cured layer is sandwiched between the previous layerand the resin vat The solidified material may adhere strongly tothe corresponding rigid or semirigid transparent solidificationsubstrate causing the object to break or deform when the buildplatform moves up from the vat during the building process

One approach to prevent the detachment of a cured layer fromthe built part is to increase its exposure such that the cured layercan strongly bond to the previous layers However such overcuringwill also lead to poor surface quality and inaccurate dimensionsAnother approach to address the problem is to apply a certain typeof coating on the resin vat to reduce the attachment force of a curedlayer Suitable coatings including Teflon and silicone films canhelp the separation of the part from the vat [1112] A coated Teflonglass has also been used in the machines of Denken [13] and Envi-sionTEC [9] However even with the intermediate material theseparation force can still be large Huang and Jiang [12] investi-gated the attachment force for the coating of an elastic siliconefilm Based on a developed on-line force monitoring system testresults indicate that the pulling force increases linearly with the sizeof the working area Experiments indicate that for a square of60 60 mm the pulling force to separate the part from the film isgreater than 60 N Such a large attachment force between the curedlayer and the vat is a key challenge that needs be addressed in thebottom-up projection based MIP-SL process

To reduce the large attachment force another approach devel-oped by EnvisionTEC in its Perfactory Systems [9] is to incorpo-rate additional mechanisms that add tilting motions in the partseparation That is during the separation one side of the platformcan be moved up slowly before the other side In this way insteadof the pulling-up the part can be peeled off from the vat surfaceHence the detaching force would be significantly reduced How-ever such additional tilting motion will also reduce the buildingspeed of the MIP-SL process

We address the large separation force and the related speedproblem by developing a two-way movement method For thepurpose of easy sliding between the built part and the resin tankwe used another type of coating material polydimethylsiloxane(PDMS Sylgard 184 Dow Corning) The coating selection isbased on the ability of PDMS in inhibiting free-radical polymer-ization near its surface as shown by Dendukuri et al [14] In theirresearch it was identified that a very thin oxygen-aided inhibitionlayer (25 lm) is formed that can prevent the cured layer fromattaching to the PDMS film Thus cured layers can easily slide onthe PDMS film Based on the approach a fast MIP-SL process hasbeen developed that can build a CAD model in minutes

The remainder of the paper is organized as follows Part separa-tion study based on the PDMS film is presented in Sec 3 A two-way movement approach to reduce the part separation force ispresented in Sec 4 The process settings and the related buildingtime analysis are presented in Sec 5 The experimental setup forperforming physical experiments is discussed in Sec 6 Theexperimental results of multiple test cases are presented in Sec 7Finally conclusions with future work are given in Sec 8

3 Part Separation Study Based on PDMS

In order to separate the cured part from the PDMS film a sim-ple and intuitive approach is to directly move the platform up aFig 2 An illustration of the bottom-up projection system

Fig 1 An illustration of the MIP-SL process

051011-2 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

certain distance d and then down by d-dLT where dLT is the layerthickness Since one layer thickness is usually very small(50ndash200 lm) the distance d is usually much larger in order for theresin to fully fill the gap (eg 5 mm) We studied the part separa-tion force of a cured layer from a coated PDMS glass based onsuch movements The PDMS film thickness is set at 1 mm A setof physical experiments has been designed and performed tounderstand the separation force based on such an approach

Figure 3 shows the setup for measuring the pulling-up forceTwo FlexiForce sensors (Tekscan South Boston MA) with arange of 0ndash25 lbs are sandwiched between the fixture and thevat The two sensors are connected to a microcontroller whichcan sample and record the sensorsrsquo readouts at over 3 KHzSince the vat is free at the bottom and the side and only fixed atthe top the pulling force by the part will be transferred to thesensors when the platform rises In the experiments we first usea given mask image to build a certain number of layers (eg 25layers) The layer thickness is set at 02 mm We then begin torecord the separation force in the building process of the nextfew layers For each layer after the designed mask image hasbeen exposed for a certain time the platform is raised up slowlyat 06 mms for 5 mm and the related readouts of the sensors arethen recorded

In our study we considered three factors that may affect theseparation force including (1) exposure time (2) image area and(3) image shape To understand the effects of these factorsdesigned experiments were conducted Seven projection patternswere used for testing the effect of image shape They are shown inFig 3 which include circle band hexagon t-shape square star-shape triangle and u-shape For comparison all the projectionpatterns have the same area in the tests The separation forces of acured layer were measured based on each of the seven projectionpatterns Figure 4 shows the measured separation forces of a sen-sor for different test cases The horizontal axis indicates the dis-tance in the Z direction (in the unit of 10 lm) and the vertical axisindicates the measured pulling force (in ounces)

It can be observed from the experimental results that

(1) As the Z stage moves up the separation force increasesgradually After the cured layer is detached from the PDMSfilm the separation force will drop rapidly from the peakvalue to 0

(2) Due to the flexibility of the PDMS film the pulling-upforce is rather small within a moving distance that is lessthan 200 lm

(3) The peak force gets larger when the same mask image isexposed longer

(4) The peak force gets larger when a larger image area isprojected

(5) The image shape has more complex effects on the peakforce In addition their effects may interact with the expo-sure time and the projection area

(6) With the coated PDMS film on the vat the separation forceis still considerably large (100 oz or 278 N for an imagearea of 625 mm2 with 1 s exposure)

4 Two-Way Movement Design for the Fast

MIP-SL Process

The experiment results indicate that the suction force betweenthe cured layer and the PDMS film is large during the pulling-upprocess Such a large force on the cured layer may cause thebuilding process to fail if the bonding force between the currentlayer and previous layers is smaller than the suction force In addi-tion after building multiple layers such forces on the PDMS filmmay lead to cracks in the film due to material fatigue caused bythe cyclic loading

Based on the PDMS film a two-channel design [15] was pre-sented for the multimaterial MIP-SL process However such anapproach mainly designed for switching tanks is not suitable forthe fast building process In the two-channel design the building

Fig 3 Experimental setup for studying part separation forcesin the MIP-SL process

Fig 4 Pulling-up forces of a cured layer from a PDMS film indifferent settings (a) T 5 1 s area 5 625 mm2 (b) T 5 05 sarea 5 625 mm2 and (c) T 5 1 s area 5 156 mm2

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-3

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

of each layer requires a full cyclic motion including both movingthe platform up and down in the Z axis and moving the tank backand forth in the X axis Such motions will slow down the buildingprocess To facilitate a high-speed MIP-SL process based on thebottom-up projection we develop a novel two-way movementdesign that requires much less motions than the two-channeldesign The developed approach is motivated by the followingobservations

(1) As shown in Fig 4 the pulling-up force is negligible forthe Z movement of a small distance (eg 50 or 100 lm)based on the 1 mm thick PDMS film

(2) As demonstrated in Ref [14] the oxygen-aided inhibitionaround the PDMS surface leaves a nonpolymerized lubri-cating layer near the PDMS film Therefore the cured layercan easily slide on the PDMS surface

The two-way movement design as discussed in Sec 41 caneffectively address the large separation force that is problematicwhile achieving a fast building speed at the same time

41 Two-Way Movement Design An illustration of the fastMIP-SL process based on the two-way movement design is shownin Fig 5 In our method a transparent PDMS film is first appliedon the bottom surface of a transparent glass vat

(1) After a mask image is exposed to cure a layer the platformis moved up in the Z axis for one layer thickness (eg50 lm) Accordingly the regions of the PDMS film relatedto the shape of the cured layer will be pulled up by the suc-tion force However the force is small due to the superelasticity of the PDMS film Note that there is no liquidresin between the cured layer and the PDMS film at thismoment

(2) The tank is moved along the X axis for a certain distanceDx A good property of the PDMS film is that a very thinoxygen-aided inhibition layer (25 lm) is formed near thePDMS film that can provide a nonpolymerized lubricatinglayer for easy sliding [14] If the moving distance is suffi-ciently large (eg larger than the extent size of the curedlayer in the X axis) the elastic deformation of the pulled-upPDMS film will be released by such a sliding movementHence at the end of the X movement liquid resin will befilled in the small gap between the cured layer and thePDMS film

(3) The mask image of a new layer can now be projected at thebottom surface to cure the next layer These three steps canthen be repeated by moving the tank in an opposite direc-tion Note that to achieve the motion in the X direction weonly move the tank and the related frame There is no rela-tive motion between the platform and the projection deviceHence the XY accuracy of the MIP-SL system will not beaffected by the X translations

42 Separation Force Study Based on the Two-WayMovement Design To verify the proposed two-way movementdesign a set of experiments was conducted based on the setup asshown in Fig 3 The same set of mask patterns was used in build-ing test layers The same exposure time and layer thickness wereused (1 s and 02 mm respectively) The building process asshown in Fig 5 was first used in building a set of layers In thetests the tank was translated in the X axis by 20 mm The movingspeed is set at 25 mms After the layers have been built thepulling-up forces in the Z axis during building the next layer wererecorded However instead of curing a new layer as shown in step3 the part is moved up slowly at 06 mms for 25 mm The meas-ured forces of a sensor in the Z axis during the aforementionedthree steps are shown in Fig 6 In each figure the curves recordthe test results based on a sampling resolution of 80 ms

The figures show that the force in the Z direction is rather smallwhen the platform is moved up by 02 mm During the remaining

two steps (ie sliding on the PDMS film and the platform pulling-up) the peak separation forces are also relatively small (around2ndash6 oz or 056ndash167 N) Such measured forces are only 3ndash4 ofthe related ones as shown in Fig 4 Hence the two-way movementdesign can effectively reduce the large separation force in thebottom-up projection system

43 Shearing Force Study in the X Axis In the two-waymovement design cured layers can easily slide on the PDMS sur-face The FlexiForce sensors were used in a modified setup tomeasure the shearing force in the X direction However no mean-ingful readouts were recorded from the sensors To quantitativelyestimate the value of the shearing force in the X axis a set ofsquare rods with different sizes was built using the two-waymovement design The built rods shown in Fig 7 are 10 mm tallThe minimum cross section size is 04 04 mm Note that wealso successfully built rods with even smaller sizes However therods were so fragile that they lost the mechanical strength to

Fig 5 The MIP-SL process based on the two-way movementdesign with PDMS

051011-4 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

sustain themselves when the part was taken out of the resin vatand washed in isopropyl alcohol

Nevertheless for a rod with a size of 04 04 mm the maxi-mum tangential force that can be added on it can be analyticallyestimated As shown in Fig 7 the testing rods in the experimentcan be modeled as a cantilever beam Suppose the length of thebeam is L the size of the beam section is b b the force in tangentdirection is F The maximum bending stress at the end can be cal-culated as r frac14 Mc=I where I is the section modulus I frac14 b4=12and c frac14 b=2 Substituting these values for their variables the

resultant equation is r frac14 6FL=b3 Suppose the allowable blendingstress is [r] and the minimal beam section size is [b] we will havethe following equation F frac12rfrac12b3=6L The material used in ourtests has the following parameters [r]frac14 65 MPa [b]frac14 04 mmand Lfrac14 10 mm According to the equation the upper bound of thetangential force is only 007 N or 025 oz Compared with the sepa-ration force in the Z direction the shearing force in the X directionis rather small

5 A Fast MIP-SL Process and Its Building Speed

Analysis

The two-way movement design enables the quick spreading ofliquid resin into a uniform thin layer In addition the DMD-baseddigital mask projection enables the fast curing of the spread liquidresin into a desired solid layer Consequently for a given 3DCAD model a fast MIP-SL process can fabricate a physical objectwithin a short building time The curing characteristics and thetwo-way movement settings of the developed MIP-SL process arepresented as follows A detailed analysis of its building time isalso discussed

51 Curing Characteristics There are two types of photo-polymer systems acrylate chemistry and cationic photopolymeri-zation in the SLA process [16] Acrylate chemistry polymerizesvia a free-radical mechanism while cationic photopolymerizationundergoes ring-opening reactions in the presence of cationic pho-toinitiators The monomer propagation for cationic reactionsrequires relatively higher activation energy Consequently thephotospeed of acrylate-based photopolymers is higher due to thelower activation energy for free-radical reactions Considering thephotospeed difference we selected the photopolymer resins basedon acrylate chemistry for the developed fast MIP-SL process Asshown in Sec 6 our projection system can cure a layer within ashort exposure time (lt500 ms) Such a fast curing time contrib-utes to the desired fast MIP-SL process for building 3D objects inminutes

After an image is exposed for a certain time (Tprojection) a wait-ing time Twait_projection is required before the layer can be movedup for one layer thickness (ie step 1 in Fig 5) Such a waitingtime is critical in order for the acrylate resin to complete the solid-ification process and gain sufficient strength for the Z movementOtherwise the building process may fail The waiting time is de-pendent on the resinrsquos curing property Due to the fast photospeedof the acrylate resins the waiting time in our system is short(300 ms in our tests)

Fig 7 Shearing force verification test

Fig 6 Pulling-up forces of a cured layer based on the two-waymovement design in different settings (a) T 5 1 s area 5 625mm2 and (b) T 5 1 s area 5 156 mm2

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-5

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

52 Two-Way Movement Settings In the two-way move-ment design the cured part is first moved up for one layer in the Zaxis and the tank is then translated in the X axis for a certain dis-tance The two linear movements have different accuracy andspeed requirements

(1) The Z movement needs to be accurate since it will determinethe layer thickness of the next layer The Z stage also needsto have a resolution that is much smaller than a layer thick-ness Accordingly to ensure the desired accuracy and resolu-tion we set small acceleration and velocity values in the Zmovement The slow movement of the cured part also ena-bles the PDMS film to fully elastically deform for a smallattaching force However the movement time in the Z axis(TZ) is still reasonably short (04 s in our tests) since only asmall moving distance is required (eg 50 or 100 lm)

(2) The tank needs to be moved in the X axis for a certain dis-tance to release the elastic deformation of the PDMS filmThe X moving distance is related to the shape and size of thecured layer and less than the extent size of the cured layer inthe X axis Since the relative position of the platform and theprojection system will not change during the X movementthe accuracy and resolution requirements on the X movementare not as high as those on the Z movement Hence a muchlarger acceleration and velocity can be applied in the Xmovement to reduce the movement time in the X axis (TX)

In our testbed we used a Z linear stage with a thread of05 mmround and a X linear stage with a thread of 254 mmround The moving time for different displacement distances inour prototyping system was calibrated for both linear stages Theresults are plotted as diamond-shape and square-shape lines inFig 8 for the Z and X stages respectively As shown in the figurethe movement in the Z axis is much slower than that in the X axisThe movement time required to complete a moving distance inthe X axis can be identified based on the calibration data

After the X movement another waiting time Twait_X is requiredin order for the flowing liquid resin to settle Otherwise the build-ing process may fail The waiting time caused by the X motions isrelated to the movement distance and the moving speed Due tothe small gap between the cured part and the PDMS film therequired waiting time is typically short (100 ms) After the wait-ing time of Twait_X the liquid resin forms a uniform thin layerwhich is ready for the next layer to be built The process can thenbe repeated after the related mask image is exposed

53 The Building Time of a Layer As shown in Fig 9 thebuilding time of each layer is thus the sum of all the aforemen-tioned steps

TLayer frac14 TProjection thorn Twait_Projection thorn TZ thorn TX thorn TWait_X

The first two items TProjection and Twait_Projection are related to thecuring characteristics of the photopolymer resins used in theMIP-SL process The photopolymer resins based on acrylate chem-istry can be quickly cured A stronger light source used in the pro-jection system can further reduce the projection time TProjection

The other three items TZ TX and TWait_X are related to thetwo-way movement design TZ is related to the layer thickness and

Fig 8 The movement time in the X and Z axes in our prototyping system

Fig 9 The building time of a layer in the two-way movementbased MIP-SL process

Fig 10 The prototype hardware system for the fast MIP-SLprocess

051011-6 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

the moving velocity in the Z axis TX is related to the size of thecured layer and the moving velocity in the X axis A linear stagewith a higher speed can be used to further reduce the movementtime TX TWait_X is determined by the gap distance between thePDMS and the cured layer the moving velocity of the tank theshape of the cured layer and the flow properties of the liquidresin For a typical layer thickness that is usually small TWait_X isreasonably short (100 ms in our tests)

Note that the projection time TProjection for the first few layers ismuch longer (eg 3ndash4 s) to ensure the initial layers can bestrongly bonded to the build platform For all the other layers thetotal building time of a layer is usually short (a few seconds in ourtests) Hence a fast fabrication speed can be achieved in thedeveloped process (eg building 3 mm height per minute)

6 Experimental Setup

61 Hardware System A prototype system has been built toverify the developed process The hardware setup of the fastMIP-SL system is shown in Fig 10 In the designed system an off-the-shelf projector (CASIO XJ-S36) was used The optical lenses of

the projector were modified to reduce the projection distance Vari-ous projection settings including focus key stone rectificationbrightness and contrast were adjusted to achieve a sharp projectionimage on the designed projection plane The DMD resolution in oursystem is 1024 768 and the envelope size is set at 48 36 mm Aprecise linear stage from Aerotech Inc (Pittsburgh PA) is used asthe elevator for driving the platform in the Z axis A fast linear stagefrom Servo Systems Co (Montville NJ) is used to drive the tanksback and force in the X axis A high performance 4-axis motion con-trol board with 28 bidirectional IO pins from Dynomotion Inc (Cal-abasas CA) is used for driving the linear stages A flat and clearglass Petri dish is used as resin tank A PDMS film (Sylgard 184Dow Corning) is coated on the glass dish

62 Software System A mask planning testbed has beendeveloped using the Cthornthorn language with Microsoft VISUAL Cthornthorncomplier The testbed integrates the geometry slicing and themotion controlling It also synchronizes the image projection withthe X and Z movements The graphical user interface of the devel-oped software system is shown in Fig 11 The flowchart of thefast MIP-SL process is also shown in Fig 11

63 Materials PerfatoryTM SI500 (yellow) and Acryl R5(red) from EnvisionTEC Inc (Ferndale MI) were used in testingthe developed fast MIP-SL process Both resins belong to acry-late For curing depths of 005 mm and 01 mm the exposure timesfor SI500 based on our projection system are set at 03 s and045 s respectively The exposure times for Acryl R5 are set at04 s and 055 s for curing depths of 005 mm and 01 mmrespectively

7 Experimental Results and Discussion

Tests have been performed to verify the building speed of thedeveloped prototyping system The results of the designed testshave demonstrated that the presented MIP-SL process can build3D models in minutes instead of hours

71 Test Procedures and Results A set of CAD modelswith different complexity was used in our tests The screenshotsof six input CAD models are shown in Figs 12(a)ndash17(a) Therelated STL files have triangle numbers ranging from several hun-dreds to 12 million (refer to Table 1)

Two different layer thicknesses commonly used in the MIP-SLprocess were tested A 50 lm layer thickness was used in the fab-rication of a gear model The mask image projection time was035 s for each layer except the base The projection waiting timewas set at 01 s For all the other models a 100 lm layer thicknesswas used in their building processes Due to the larger layer

Fig 11 Flow chart of the fast MIP-SL system and related soft-ware system

Table 1 Building time statistics

Model Gear Head Statue Teeth Shell Brush

Figure number Fig 12 Fig 13 Fig 14 Fig 15 Fig 16 Fig 17Triangle number 660 24190 5204 133806 32762 1259246SizeX (mm) 254 25 177 24 24 76Thickness (mm) 005 01 01 01 01 01Tprojection (s) 035 (04)a 045 045 (055)a 045 (055)a 045 045Twait_projection (s) 01 03 03 03 03 03TZ (s) 032 042 042 042 042 042MoveX (mm) 76 25 127 25 25 635 (13)b

TX (s) 058 11 067 046 046 056 (039)b

TX_Wait (s) 005 01 01 005 005 005TLayer (s) 14 (145)a 237 194 (204)a 168 (178)a 168 178 (161)b

HeightZ (mm) 493 285 305 73 223 83Layer number 98 285 305 73 223 83Ttotal_building (min) 231 (239)a 1126 986 (104)a 204 (217)a 624 225

aThe projection and waiting time for R5 resin others are for SI500bDifferent X moving distances were used in building the bottom (1st layerndash19th layer) and the top (20th layerndash83rd layer) portions of the brush

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-7

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

thickness a longer image exposure and projection waiting timeswere used (045 and 03 s respectively in the tests) Accordinglythe Z movement will also take a longer time for a larger layerthickness In the tests the movement time in the Z axis (TZ) is032 and 042 s for the layer thickness of 50 lm and 100 lmrespectively

The required moving distance in the X axis is related to the sizeand shape of the cured layer For a layer with a big cross-sectionalarea (eg the models of a head and a statue) the X translation dis-tance is set to a value that is close to the X extent size Due to thelarge movement the X waiting time was also set longer In com-parison for a layer with a small cross-sectional area (eg themodels of a hearing aid shell and the top portion of a brush) the Xtranslation distance can be much smaller than the extent size ofthe layer in the X axis However due to the fast moving speed inthe X axis the differences on TX are usually small (less than 1 s asshown in Table 1)

Two types of resins SI500 and Acryl R5 were tested Theircuring characteristics are slightly different For the same layerthickness the curing of Acryl R5 takes 01 s longer than that ofSI500 The viscosities of the two resins are also slightly differentHowever the same settings can be used in the two-way movementdesign based on the two resins

Figures 12ndash17 show the built objects based on the developedfast MIP-SL process The quality of the built objects was exam-ined to be satisfactory Both surface finish and dimension were an-alyzed to be acceptable In our prototyping system the nominalsize of a pixel is 47 lm The fine image resolution enables themesoscale features (ie in the range of 01ndash1 mm) to be well cap-tured in the built physical objects eg the lip of the human headthe cloth folds in the Beethoven statue and the dentures in theteeth model

72 Building Time Analysis All the models shown in Figs12ndash17 were built within 12 min using our prototyping system Themodels with less than 100 layers (eg the gear the teeth and thebrush) only require 2ndash3 min to be built A statistic of the buildingtime is given in Table 1

A much larger exposure time (eg 4ndash5 s) was required for thefirst few layers in order to build a base Consequently the built

objects and the build platform can be well bonded For all otherlayers as shown in Fig 18 the building time of a layer (TLayer) inour MIP-SL process is only 14ndash25 s The variation on TLayer ismainly due to different layer thicknesses and the X moving distan-ces For an average of 2 s per layer and a layer thickness of01 mm the building speed of the developed MIP-SL process is3 mm per minute or 180 mm per hour To the best of our knowl-edge such a MIP-SL process is one of the fastest layer-based

Fig 12 A test of a gear (a) CAD model and (b) built objects intwo liquid resins

Fig 13 A test of a head (a) CAD model and (b)ndash(c) two viewsof the built object

Fig 14 A test of a statue (a) CAD model and (b)ndash(e) two viewsof the built objects in two liquid resins

Fig 15 A test of teeth (a) CAD model and (b)ndash(c) built objectsin two liquid resins

Fig 16 A test of a hearing aide shell (a) CAD model and(b)ndash(c) two views of the built object

Fig 17 A test of a brush (a) CAD model and (b) built object

051011-8 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

additive manufacturing processes that have been developed Avideo of building the gear model as shown in Fig 12(a) can befound in Ref [17]

8 Conclusions

A novel mask-image-projection-based stereolithography pro-cess has been presented for fabricating 3D objects with fast build-ing speed The proposed approach is based on projecting maskimages bottom-up on a PDMS coated glass substrate A new two-way movement design has been presented for quickly spreadingliquid resin into uniform thin layers Such a design can signifi-cantly reduce the separation force between cured layers and theresin tank Experimental results verified that the separation forceas well as the sliding force is relatively small during the two-waymovement process The motions related to the two-way move-ment design can also be performed quickly The MIP-SL processdeveloped based on such a recoating approach can achieve highfabrication speed for input CAD models The experimental resultsdemonstrate that the newly developed MIP-SL process can suc-cessfully fabricate 3D objects with satisfactory quality in a shorttime (usually in minutes)

Some future work to further improve the building speed of thedeveloped MIP-SL process includes (1) investigating the settingsof Tx and Twait_X based on given layers for the best performance(2) testing faster moving speeds in the X and Z axes and (3) test-ing the two-way movement design in prototyping systems withlarger XY extent sizes

References[1] Bourell D Leu M and Rosen D eds 2009 NSF WorkshopmdashRoadmap for

Additive Manufacturing Identifying the Future of Freeform ProcessingWashington DC March Available at httpwohlersassociatescomroadmap2009html

[2] Chatwin C Farsari M Huang S Heywood M Birch P Young R andRichardson J 1998 ldquoUV Microstereolithography System That Uses SpatialLight Modulator Technologyrdquo Appl Opt 37 pp 7514ndash7522

[3] Bertsch A Bernhard P Vogt C and Renaud P 2000 ldquoRapid Prototypingof Small Size Objectsrdquo Rapid Prototyping J 6(4) pp 259ndash266

[4] Stampfl J Fouad H Seidler S Liska R Schwager F Woesz A andFratzl P 2004 ldquoFabrication and Moulding of Cellular Materials by RapidPrototypingrdquo Int J Mater Product Technol 21(4) pp 285ndash296

[5] Sun C Fang N Wu D and Zhang X 2005 ldquoProjection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Maskrdquo Sens ActuatorsA 121 pp 113ndash120

[6] Lu Y Mapili G Suhali G Chen S C and Roy K 2006 ldquoA DigitalMicro-Mirror Device-Based System for the Microfabrication of Complex Spa-tially Patterned Tissue Engineering Scaffoldsrdquo J Biomed Mater Res A77A(2) pp 396ndash405

[7] Limaye A S and Rosen D W 2007 ldquoProcess Planning Method for MaskProjection Micro-Stereolithographyrdquo Rapid Prototyping J 13(2) pp 76ndash84

[8] Choi J Wicker R B Cho S Ha C and Lee S 2009 ldquoCure Depth Controlfor Complex 3D Microstructure Fabrication in Dynamic Mask ProjectionMicrostereolithographyrdquo Rapid Prototyping J 15(1) pp 59ndash70

[9] EnvisionTEC retrieved Sept 1 2011 httpwwwenvisiontecde[10] V-Flash desktop modeler retrieved Sept 1 2011 httpwwwmodelin3dcom[11] Chen Y Zhou C and Lao J 2011 ldquoA Layerless Additive Manufacturing

Process Based on CNC Accumulationrdquo Rapid Prototyping J 17(3)pp 218ndash227

[12] Huang Y M and Jiang C P 2005 ldquoOn-Line Force Monitoring of PlatformAscending Rapid Prototyping Systemrdquo J Mater Process Technol 159pp 257ndash264

[13] Denken Corp 1997 SLP-4000 Solid Laser Diode Plotter Product BrochureDenken Corporation Japan

[14] Dendukuri D Pregibon D C Collins J Hatton T A and Doyle P S2006 ldquoContinuous-Flow Lithography for High-Throughput Microparticle Syn-thesisrdquo Nature Mater 5 pp 365ndash369

[15] Zhou C Chen Y Yang Z G and Khoshnevis B 2011 ldquoDevelopmentof Multi-Material Mask-Image-Projection-Based Stereolithography for theFabrication of Digital Materialsrdquo Annual Solid Freeform Fabrication Sympo-sium Austin TX

[16] Jacobs P F 1995 Stereolithography and Other RPampM Technologies FromRapid Prototyping to Rapid Tooling Society of Manufacturing EngineeringDearborn MI

[17] Pan Y Y Zhou C and Chen Y retrieved Nov 23 2011 httpwwwyoutubecomwatchvfrac140TZWCwnvyew

Fig 18 Layer building time of the test cases

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-9

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

spreading liquid resin into uniform thin layers Our approach isbased on a two-way movement design in a bottom-up projectionsystem We addressed the related challenge of large attachingforces in the bottom-up projection system By optimizing the pro-cess settings we illustrate that the preparation of a uniform thinlayer can be done within seconds Consequently the developedfast MIP-SL process can build moderate sized parts in minutesinstead of hours

2 Bottom-Up Projection Based MIP-SL System

In addition to the top-down projection approach as shown inFig 1 another projection approach used in the MIP-SL process isthe bottom-up projection as shown in Fig 2 That is the maskimage is projected onto the bottom of a transparent tank After alayer is cured at the bottom of the built part the platform is movedup and then down to form a small gap with the bottom surface ofthe resin tank A uniform thin layer can be achieved after theformed gap is filled with liquid resin

21 Advantages of the Bottom-Up Projection System Abottom-up projection based system has several advantages over atop-down projection based system (1) The container depth isindependent of the part height Thus a shallow vat can be used toreduce the required volume of the liquid resin During the buildingprocess liquid resin can be added by a pump when needed (2)Recoating is achieved by constraining liquid resin between thepreviously cured layers and the resin tank Hence no additional

sweeping is needed for flattening the resin surface (3) Muchsmaller layer thickness can be achieved since the gap size is onlydetermined by the Z stage resolution regardless of the fluid proper-ties of liquid resin (4) The curing of liquid resin is sealed fromthe oxygen-rich environment By eliminating the oxygen inhibi-tion effect the liquid photopolymer resin can be cured faster

22 Challenges of the Bottom-Up Projection System Despitethe advantages the bottom-up projection based system has notbeen widely used in the SLA and MIP-SL processes A main rea-son is that the separation of the cured part from the tank surfaceis difficult That is in the bottom-up projection based MIP-SLprocess a cured layer is sandwiched between the previous layerand the resin vat The solidified material may adhere strongly tothe corresponding rigid or semirigid transparent solidificationsubstrate causing the object to break or deform when the buildplatform moves up from the vat during the building process

One approach to prevent the detachment of a cured layer fromthe built part is to increase its exposure such that the cured layercan strongly bond to the previous layers However such overcuringwill also lead to poor surface quality and inaccurate dimensionsAnother approach to address the problem is to apply a certain typeof coating on the resin vat to reduce the attachment force of a curedlayer Suitable coatings including Teflon and silicone films canhelp the separation of the part from the vat [1112] A coated Teflonglass has also been used in the machines of Denken [13] and Envi-sionTEC [9] However even with the intermediate material theseparation force can still be large Huang and Jiang [12] investi-gated the attachment force for the coating of an elastic siliconefilm Based on a developed on-line force monitoring system testresults indicate that the pulling force increases linearly with the sizeof the working area Experiments indicate that for a square of60 60 mm the pulling force to separate the part from the film isgreater than 60 N Such a large attachment force between the curedlayer and the vat is a key challenge that needs be addressed in thebottom-up projection based MIP-SL process

To reduce the large attachment force another approach devel-oped by EnvisionTEC in its Perfactory Systems [9] is to incorpo-rate additional mechanisms that add tilting motions in the partseparation That is during the separation one side of the platformcan be moved up slowly before the other side In this way insteadof the pulling-up the part can be peeled off from the vat surfaceHence the detaching force would be significantly reduced How-ever such additional tilting motion will also reduce the buildingspeed of the MIP-SL process

We address the large separation force and the related speedproblem by developing a two-way movement method For thepurpose of easy sliding between the built part and the resin tankwe used another type of coating material polydimethylsiloxane(PDMS Sylgard 184 Dow Corning) The coating selection isbased on the ability of PDMS in inhibiting free-radical polymer-ization near its surface as shown by Dendukuri et al [14] In theirresearch it was identified that a very thin oxygen-aided inhibitionlayer (25 lm) is formed that can prevent the cured layer fromattaching to the PDMS film Thus cured layers can easily slide onthe PDMS film Based on the approach a fast MIP-SL process hasbeen developed that can build a CAD model in minutes

The remainder of the paper is organized as follows Part separa-tion study based on the PDMS film is presented in Sec 3 A two-way movement approach to reduce the part separation force ispresented in Sec 4 The process settings and the related buildingtime analysis are presented in Sec 5 The experimental setup forperforming physical experiments is discussed in Sec 6 Theexperimental results of multiple test cases are presented in Sec 7Finally conclusions with future work are given in Sec 8

3 Part Separation Study Based on PDMS

In order to separate the cured part from the PDMS film a sim-ple and intuitive approach is to directly move the platform up aFig 2 An illustration of the bottom-up projection system

Fig 1 An illustration of the MIP-SL process

051011-2 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

certain distance d and then down by d-dLT where dLT is the layerthickness Since one layer thickness is usually very small(50ndash200 lm) the distance d is usually much larger in order for theresin to fully fill the gap (eg 5 mm) We studied the part separa-tion force of a cured layer from a coated PDMS glass based onsuch movements The PDMS film thickness is set at 1 mm A setof physical experiments has been designed and performed tounderstand the separation force based on such an approach

Figure 3 shows the setup for measuring the pulling-up forceTwo FlexiForce sensors (Tekscan South Boston MA) with arange of 0ndash25 lbs are sandwiched between the fixture and thevat The two sensors are connected to a microcontroller whichcan sample and record the sensorsrsquo readouts at over 3 KHzSince the vat is free at the bottom and the side and only fixed atthe top the pulling force by the part will be transferred to thesensors when the platform rises In the experiments we first usea given mask image to build a certain number of layers (eg 25layers) The layer thickness is set at 02 mm We then begin torecord the separation force in the building process of the nextfew layers For each layer after the designed mask image hasbeen exposed for a certain time the platform is raised up slowlyat 06 mms for 5 mm and the related readouts of the sensors arethen recorded

In our study we considered three factors that may affect theseparation force including (1) exposure time (2) image area and(3) image shape To understand the effects of these factorsdesigned experiments were conducted Seven projection patternswere used for testing the effect of image shape They are shown inFig 3 which include circle band hexagon t-shape square star-shape triangle and u-shape For comparison all the projectionpatterns have the same area in the tests The separation forces of acured layer were measured based on each of the seven projectionpatterns Figure 4 shows the measured separation forces of a sen-sor for different test cases The horizontal axis indicates the dis-tance in the Z direction (in the unit of 10 lm) and the vertical axisindicates the measured pulling force (in ounces)

It can be observed from the experimental results that

(1) As the Z stage moves up the separation force increasesgradually After the cured layer is detached from the PDMSfilm the separation force will drop rapidly from the peakvalue to 0

(2) Due to the flexibility of the PDMS film the pulling-upforce is rather small within a moving distance that is lessthan 200 lm

(3) The peak force gets larger when the same mask image isexposed longer

(4) The peak force gets larger when a larger image area isprojected

(5) The image shape has more complex effects on the peakforce In addition their effects may interact with the expo-sure time and the projection area

(6) With the coated PDMS film on the vat the separation forceis still considerably large (100 oz or 278 N for an imagearea of 625 mm2 with 1 s exposure)

4 Two-Way Movement Design for the Fast

MIP-SL Process

The experiment results indicate that the suction force betweenthe cured layer and the PDMS film is large during the pulling-upprocess Such a large force on the cured layer may cause thebuilding process to fail if the bonding force between the currentlayer and previous layers is smaller than the suction force In addi-tion after building multiple layers such forces on the PDMS filmmay lead to cracks in the film due to material fatigue caused bythe cyclic loading

Based on the PDMS film a two-channel design [15] was pre-sented for the multimaterial MIP-SL process However such anapproach mainly designed for switching tanks is not suitable forthe fast building process In the two-channel design the building

Fig 3 Experimental setup for studying part separation forcesin the MIP-SL process

Fig 4 Pulling-up forces of a cured layer from a PDMS film indifferent settings (a) T 5 1 s area 5 625 mm2 (b) T 5 05 sarea 5 625 mm2 and (c) T 5 1 s area 5 156 mm2

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-3

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

of each layer requires a full cyclic motion including both movingthe platform up and down in the Z axis and moving the tank backand forth in the X axis Such motions will slow down the buildingprocess To facilitate a high-speed MIP-SL process based on thebottom-up projection we develop a novel two-way movementdesign that requires much less motions than the two-channeldesign The developed approach is motivated by the followingobservations

(1) As shown in Fig 4 the pulling-up force is negligible forthe Z movement of a small distance (eg 50 or 100 lm)based on the 1 mm thick PDMS film

(2) As demonstrated in Ref [14] the oxygen-aided inhibitionaround the PDMS surface leaves a nonpolymerized lubri-cating layer near the PDMS film Therefore the cured layercan easily slide on the PDMS surface

The two-way movement design as discussed in Sec 41 caneffectively address the large separation force that is problematicwhile achieving a fast building speed at the same time

41 Two-Way Movement Design An illustration of the fastMIP-SL process based on the two-way movement design is shownin Fig 5 In our method a transparent PDMS film is first appliedon the bottom surface of a transparent glass vat

(1) After a mask image is exposed to cure a layer the platformis moved up in the Z axis for one layer thickness (eg50 lm) Accordingly the regions of the PDMS film relatedto the shape of the cured layer will be pulled up by the suc-tion force However the force is small due to the superelasticity of the PDMS film Note that there is no liquidresin between the cured layer and the PDMS film at thismoment

(2) The tank is moved along the X axis for a certain distanceDx A good property of the PDMS film is that a very thinoxygen-aided inhibition layer (25 lm) is formed near thePDMS film that can provide a nonpolymerized lubricatinglayer for easy sliding [14] If the moving distance is suffi-ciently large (eg larger than the extent size of the curedlayer in the X axis) the elastic deformation of the pulled-upPDMS film will be released by such a sliding movementHence at the end of the X movement liquid resin will befilled in the small gap between the cured layer and thePDMS film

(3) The mask image of a new layer can now be projected at thebottom surface to cure the next layer These three steps canthen be repeated by moving the tank in an opposite direc-tion Note that to achieve the motion in the X direction weonly move the tank and the related frame There is no rela-tive motion between the platform and the projection deviceHence the XY accuracy of the MIP-SL system will not beaffected by the X translations

42 Separation Force Study Based on the Two-WayMovement Design To verify the proposed two-way movementdesign a set of experiments was conducted based on the setup asshown in Fig 3 The same set of mask patterns was used in build-ing test layers The same exposure time and layer thickness wereused (1 s and 02 mm respectively) The building process asshown in Fig 5 was first used in building a set of layers In thetests the tank was translated in the X axis by 20 mm The movingspeed is set at 25 mms After the layers have been built thepulling-up forces in the Z axis during building the next layer wererecorded However instead of curing a new layer as shown in step3 the part is moved up slowly at 06 mms for 25 mm The meas-ured forces of a sensor in the Z axis during the aforementionedthree steps are shown in Fig 6 In each figure the curves recordthe test results based on a sampling resolution of 80 ms

The figures show that the force in the Z direction is rather smallwhen the platform is moved up by 02 mm During the remaining

two steps (ie sliding on the PDMS film and the platform pulling-up) the peak separation forces are also relatively small (around2ndash6 oz or 056ndash167 N) Such measured forces are only 3ndash4 ofthe related ones as shown in Fig 4 Hence the two-way movementdesign can effectively reduce the large separation force in thebottom-up projection system

43 Shearing Force Study in the X Axis In the two-waymovement design cured layers can easily slide on the PDMS sur-face The FlexiForce sensors were used in a modified setup tomeasure the shearing force in the X direction However no mean-ingful readouts were recorded from the sensors To quantitativelyestimate the value of the shearing force in the X axis a set ofsquare rods with different sizes was built using the two-waymovement design The built rods shown in Fig 7 are 10 mm tallThe minimum cross section size is 04 04 mm Note that wealso successfully built rods with even smaller sizes However therods were so fragile that they lost the mechanical strength to

Fig 5 The MIP-SL process based on the two-way movementdesign with PDMS

051011-4 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

sustain themselves when the part was taken out of the resin vatand washed in isopropyl alcohol

Nevertheless for a rod with a size of 04 04 mm the maxi-mum tangential force that can be added on it can be analyticallyestimated As shown in Fig 7 the testing rods in the experimentcan be modeled as a cantilever beam Suppose the length of thebeam is L the size of the beam section is b b the force in tangentdirection is F The maximum bending stress at the end can be cal-culated as r frac14 Mc=I where I is the section modulus I frac14 b4=12and c frac14 b=2 Substituting these values for their variables the

resultant equation is r frac14 6FL=b3 Suppose the allowable blendingstress is [r] and the minimal beam section size is [b] we will havethe following equation F frac12rfrac12b3=6L The material used in ourtests has the following parameters [r]frac14 65 MPa [b]frac14 04 mmand Lfrac14 10 mm According to the equation the upper bound of thetangential force is only 007 N or 025 oz Compared with the sepa-ration force in the Z direction the shearing force in the X directionis rather small

5 A Fast MIP-SL Process and Its Building Speed

Analysis

The two-way movement design enables the quick spreading ofliquid resin into a uniform thin layer In addition the DMD-baseddigital mask projection enables the fast curing of the spread liquidresin into a desired solid layer Consequently for a given 3DCAD model a fast MIP-SL process can fabricate a physical objectwithin a short building time The curing characteristics and thetwo-way movement settings of the developed MIP-SL process arepresented as follows A detailed analysis of its building time isalso discussed

51 Curing Characteristics There are two types of photo-polymer systems acrylate chemistry and cationic photopolymeri-zation in the SLA process [16] Acrylate chemistry polymerizesvia a free-radical mechanism while cationic photopolymerizationundergoes ring-opening reactions in the presence of cationic pho-toinitiators The monomer propagation for cationic reactionsrequires relatively higher activation energy Consequently thephotospeed of acrylate-based photopolymers is higher due to thelower activation energy for free-radical reactions Considering thephotospeed difference we selected the photopolymer resins basedon acrylate chemistry for the developed fast MIP-SL process Asshown in Sec 6 our projection system can cure a layer within ashort exposure time (lt500 ms) Such a fast curing time contrib-utes to the desired fast MIP-SL process for building 3D objects inminutes

After an image is exposed for a certain time (Tprojection) a wait-ing time Twait_projection is required before the layer can be movedup for one layer thickness (ie step 1 in Fig 5) Such a waitingtime is critical in order for the acrylate resin to complete the solid-ification process and gain sufficient strength for the Z movementOtherwise the building process may fail The waiting time is de-pendent on the resinrsquos curing property Due to the fast photospeedof the acrylate resins the waiting time in our system is short(300 ms in our tests)

Fig 7 Shearing force verification test

Fig 6 Pulling-up forces of a cured layer based on the two-waymovement design in different settings (a) T 5 1 s area 5 625mm2 and (b) T 5 1 s area 5 156 mm2

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-5

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

52 Two-Way Movement Settings In the two-way move-ment design the cured part is first moved up for one layer in the Zaxis and the tank is then translated in the X axis for a certain dis-tance The two linear movements have different accuracy andspeed requirements

(1) The Z movement needs to be accurate since it will determinethe layer thickness of the next layer The Z stage also needsto have a resolution that is much smaller than a layer thick-ness Accordingly to ensure the desired accuracy and resolu-tion we set small acceleration and velocity values in the Zmovement The slow movement of the cured part also ena-bles the PDMS film to fully elastically deform for a smallattaching force However the movement time in the Z axis(TZ) is still reasonably short (04 s in our tests) since only asmall moving distance is required (eg 50 or 100 lm)

(2) The tank needs to be moved in the X axis for a certain dis-tance to release the elastic deformation of the PDMS filmThe X moving distance is related to the shape and size of thecured layer and less than the extent size of the cured layer inthe X axis Since the relative position of the platform and theprojection system will not change during the X movementthe accuracy and resolution requirements on the X movementare not as high as those on the Z movement Hence a muchlarger acceleration and velocity can be applied in the Xmovement to reduce the movement time in the X axis (TX)

In our testbed we used a Z linear stage with a thread of05 mmround and a X linear stage with a thread of 254 mmround The moving time for different displacement distances inour prototyping system was calibrated for both linear stages Theresults are plotted as diamond-shape and square-shape lines inFig 8 for the Z and X stages respectively As shown in the figurethe movement in the Z axis is much slower than that in the X axisThe movement time required to complete a moving distance inthe X axis can be identified based on the calibration data

After the X movement another waiting time Twait_X is requiredin order for the flowing liquid resin to settle Otherwise the build-ing process may fail The waiting time caused by the X motions isrelated to the movement distance and the moving speed Due tothe small gap between the cured part and the PDMS film therequired waiting time is typically short (100 ms) After the wait-ing time of Twait_X the liquid resin forms a uniform thin layerwhich is ready for the next layer to be built The process can thenbe repeated after the related mask image is exposed

53 The Building Time of a Layer As shown in Fig 9 thebuilding time of each layer is thus the sum of all the aforemen-tioned steps

TLayer frac14 TProjection thorn Twait_Projection thorn TZ thorn TX thorn TWait_X

The first two items TProjection and Twait_Projection are related to thecuring characteristics of the photopolymer resins used in theMIP-SL process The photopolymer resins based on acrylate chem-istry can be quickly cured A stronger light source used in the pro-jection system can further reduce the projection time TProjection

The other three items TZ TX and TWait_X are related to thetwo-way movement design TZ is related to the layer thickness and

Fig 8 The movement time in the X and Z axes in our prototyping system

Fig 9 The building time of a layer in the two-way movementbased MIP-SL process

Fig 10 The prototype hardware system for the fast MIP-SLprocess

051011-6 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

the moving velocity in the Z axis TX is related to the size of thecured layer and the moving velocity in the X axis A linear stagewith a higher speed can be used to further reduce the movementtime TX TWait_X is determined by the gap distance between thePDMS and the cured layer the moving velocity of the tank theshape of the cured layer and the flow properties of the liquidresin For a typical layer thickness that is usually small TWait_X isreasonably short (100 ms in our tests)

Note that the projection time TProjection for the first few layers ismuch longer (eg 3ndash4 s) to ensure the initial layers can bestrongly bonded to the build platform For all the other layers thetotal building time of a layer is usually short (a few seconds in ourtests) Hence a fast fabrication speed can be achieved in thedeveloped process (eg building 3 mm height per minute)

6 Experimental Setup

61 Hardware System A prototype system has been built toverify the developed process The hardware setup of the fastMIP-SL system is shown in Fig 10 In the designed system an off-the-shelf projector (CASIO XJ-S36) was used The optical lenses of

the projector were modified to reduce the projection distance Vari-ous projection settings including focus key stone rectificationbrightness and contrast were adjusted to achieve a sharp projectionimage on the designed projection plane The DMD resolution in oursystem is 1024 768 and the envelope size is set at 48 36 mm Aprecise linear stage from Aerotech Inc (Pittsburgh PA) is used asthe elevator for driving the platform in the Z axis A fast linear stagefrom Servo Systems Co (Montville NJ) is used to drive the tanksback and force in the X axis A high performance 4-axis motion con-trol board with 28 bidirectional IO pins from Dynomotion Inc (Cal-abasas CA) is used for driving the linear stages A flat and clearglass Petri dish is used as resin tank A PDMS film (Sylgard 184Dow Corning) is coated on the glass dish

62 Software System A mask planning testbed has beendeveloped using the Cthornthorn language with Microsoft VISUAL Cthornthorncomplier The testbed integrates the geometry slicing and themotion controlling It also synchronizes the image projection withthe X and Z movements The graphical user interface of the devel-oped software system is shown in Fig 11 The flowchart of thefast MIP-SL process is also shown in Fig 11

63 Materials PerfatoryTM SI500 (yellow) and Acryl R5(red) from EnvisionTEC Inc (Ferndale MI) were used in testingthe developed fast MIP-SL process Both resins belong to acry-late For curing depths of 005 mm and 01 mm the exposure timesfor SI500 based on our projection system are set at 03 s and045 s respectively The exposure times for Acryl R5 are set at04 s and 055 s for curing depths of 005 mm and 01 mmrespectively

7 Experimental Results and Discussion

Tests have been performed to verify the building speed of thedeveloped prototyping system The results of the designed testshave demonstrated that the presented MIP-SL process can build3D models in minutes instead of hours

71 Test Procedures and Results A set of CAD modelswith different complexity was used in our tests The screenshotsof six input CAD models are shown in Figs 12(a)ndash17(a) Therelated STL files have triangle numbers ranging from several hun-dreds to 12 million (refer to Table 1)

Two different layer thicknesses commonly used in the MIP-SLprocess were tested A 50 lm layer thickness was used in the fab-rication of a gear model The mask image projection time was035 s for each layer except the base The projection waiting timewas set at 01 s For all the other models a 100 lm layer thicknesswas used in their building processes Due to the larger layer

Fig 11 Flow chart of the fast MIP-SL system and related soft-ware system

Table 1 Building time statistics

Model Gear Head Statue Teeth Shell Brush

Figure number Fig 12 Fig 13 Fig 14 Fig 15 Fig 16 Fig 17Triangle number 660 24190 5204 133806 32762 1259246SizeX (mm) 254 25 177 24 24 76Thickness (mm) 005 01 01 01 01 01Tprojection (s) 035 (04)a 045 045 (055)a 045 (055)a 045 045Twait_projection (s) 01 03 03 03 03 03TZ (s) 032 042 042 042 042 042MoveX (mm) 76 25 127 25 25 635 (13)b

TX (s) 058 11 067 046 046 056 (039)b

TX_Wait (s) 005 01 01 005 005 005TLayer (s) 14 (145)a 237 194 (204)a 168 (178)a 168 178 (161)b

HeightZ (mm) 493 285 305 73 223 83Layer number 98 285 305 73 223 83Ttotal_building (min) 231 (239)a 1126 986 (104)a 204 (217)a 624 225

aThe projection and waiting time for R5 resin others are for SI500bDifferent X moving distances were used in building the bottom (1st layerndash19th layer) and the top (20th layerndash83rd layer) portions of the brush

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-7

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

thickness a longer image exposure and projection waiting timeswere used (045 and 03 s respectively in the tests) Accordinglythe Z movement will also take a longer time for a larger layerthickness In the tests the movement time in the Z axis (TZ) is032 and 042 s for the layer thickness of 50 lm and 100 lmrespectively

The required moving distance in the X axis is related to the sizeand shape of the cured layer For a layer with a big cross-sectionalarea (eg the models of a head and a statue) the X translation dis-tance is set to a value that is close to the X extent size Due to thelarge movement the X waiting time was also set longer In com-parison for a layer with a small cross-sectional area (eg themodels of a hearing aid shell and the top portion of a brush) the Xtranslation distance can be much smaller than the extent size ofthe layer in the X axis However due to the fast moving speed inthe X axis the differences on TX are usually small (less than 1 s asshown in Table 1)

Two types of resins SI500 and Acryl R5 were tested Theircuring characteristics are slightly different For the same layerthickness the curing of Acryl R5 takes 01 s longer than that ofSI500 The viscosities of the two resins are also slightly differentHowever the same settings can be used in the two-way movementdesign based on the two resins

Figures 12ndash17 show the built objects based on the developedfast MIP-SL process The quality of the built objects was exam-ined to be satisfactory Both surface finish and dimension were an-alyzed to be acceptable In our prototyping system the nominalsize of a pixel is 47 lm The fine image resolution enables themesoscale features (ie in the range of 01ndash1 mm) to be well cap-tured in the built physical objects eg the lip of the human headthe cloth folds in the Beethoven statue and the dentures in theteeth model

72 Building Time Analysis All the models shown in Figs12ndash17 were built within 12 min using our prototyping system Themodels with less than 100 layers (eg the gear the teeth and thebrush) only require 2ndash3 min to be built A statistic of the buildingtime is given in Table 1

A much larger exposure time (eg 4ndash5 s) was required for thefirst few layers in order to build a base Consequently the built

objects and the build platform can be well bonded For all otherlayers as shown in Fig 18 the building time of a layer (TLayer) inour MIP-SL process is only 14ndash25 s The variation on TLayer ismainly due to different layer thicknesses and the X moving distan-ces For an average of 2 s per layer and a layer thickness of01 mm the building speed of the developed MIP-SL process is3 mm per minute or 180 mm per hour To the best of our knowl-edge such a MIP-SL process is one of the fastest layer-based

Fig 12 A test of a gear (a) CAD model and (b) built objects intwo liquid resins

Fig 13 A test of a head (a) CAD model and (b)ndash(c) two viewsof the built object

Fig 14 A test of a statue (a) CAD model and (b)ndash(e) two viewsof the built objects in two liquid resins

Fig 15 A test of teeth (a) CAD model and (b)ndash(c) built objectsin two liquid resins

Fig 16 A test of a hearing aide shell (a) CAD model and(b)ndash(c) two views of the built object

Fig 17 A test of a brush (a) CAD model and (b) built object

051011-8 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

additive manufacturing processes that have been developed Avideo of building the gear model as shown in Fig 12(a) can befound in Ref [17]

8 Conclusions

A novel mask-image-projection-based stereolithography pro-cess has been presented for fabricating 3D objects with fast build-ing speed The proposed approach is based on projecting maskimages bottom-up on a PDMS coated glass substrate A new two-way movement design has been presented for quickly spreadingliquid resin into uniform thin layers Such a design can signifi-cantly reduce the separation force between cured layers and theresin tank Experimental results verified that the separation forceas well as the sliding force is relatively small during the two-waymovement process The motions related to the two-way move-ment design can also be performed quickly The MIP-SL processdeveloped based on such a recoating approach can achieve highfabrication speed for input CAD models The experimental resultsdemonstrate that the newly developed MIP-SL process can suc-cessfully fabricate 3D objects with satisfactory quality in a shorttime (usually in minutes)

Some future work to further improve the building speed of thedeveloped MIP-SL process includes (1) investigating the settingsof Tx and Twait_X based on given layers for the best performance(2) testing faster moving speeds in the X and Z axes and (3) test-ing the two-way movement design in prototyping systems withlarger XY extent sizes

References[1] Bourell D Leu M and Rosen D eds 2009 NSF WorkshopmdashRoadmap for

Additive Manufacturing Identifying the Future of Freeform ProcessingWashington DC March Available at httpwohlersassociatescomroadmap2009html

[2] Chatwin C Farsari M Huang S Heywood M Birch P Young R andRichardson J 1998 ldquoUV Microstereolithography System That Uses SpatialLight Modulator Technologyrdquo Appl Opt 37 pp 7514ndash7522

[3] Bertsch A Bernhard P Vogt C and Renaud P 2000 ldquoRapid Prototypingof Small Size Objectsrdquo Rapid Prototyping J 6(4) pp 259ndash266

[4] Stampfl J Fouad H Seidler S Liska R Schwager F Woesz A andFratzl P 2004 ldquoFabrication and Moulding of Cellular Materials by RapidPrototypingrdquo Int J Mater Product Technol 21(4) pp 285ndash296

[5] Sun C Fang N Wu D and Zhang X 2005 ldquoProjection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Maskrdquo Sens ActuatorsA 121 pp 113ndash120

[6] Lu Y Mapili G Suhali G Chen S C and Roy K 2006 ldquoA DigitalMicro-Mirror Device-Based System for the Microfabrication of Complex Spa-tially Patterned Tissue Engineering Scaffoldsrdquo J Biomed Mater Res A77A(2) pp 396ndash405

[7] Limaye A S and Rosen D W 2007 ldquoProcess Planning Method for MaskProjection Micro-Stereolithographyrdquo Rapid Prototyping J 13(2) pp 76ndash84

[8] Choi J Wicker R B Cho S Ha C and Lee S 2009 ldquoCure Depth Controlfor Complex 3D Microstructure Fabrication in Dynamic Mask ProjectionMicrostereolithographyrdquo Rapid Prototyping J 15(1) pp 59ndash70

[9] EnvisionTEC retrieved Sept 1 2011 httpwwwenvisiontecde[10] V-Flash desktop modeler retrieved Sept 1 2011 httpwwwmodelin3dcom[11] Chen Y Zhou C and Lao J 2011 ldquoA Layerless Additive Manufacturing

Process Based on CNC Accumulationrdquo Rapid Prototyping J 17(3)pp 218ndash227

[12] Huang Y M and Jiang C P 2005 ldquoOn-Line Force Monitoring of PlatformAscending Rapid Prototyping Systemrdquo J Mater Process Technol 159pp 257ndash264

[13] Denken Corp 1997 SLP-4000 Solid Laser Diode Plotter Product BrochureDenken Corporation Japan

[14] Dendukuri D Pregibon D C Collins J Hatton T A and Doyle P S2006 ldquoContinuous-Flow Lithography for High-Throughput Microparticle Syn-thesisrdquo Nature Mater 5 pp 365ndash369

[15] Zhou C Chen Y Yang Z G and Khoshnevis B 2011 ldquoDevelopmentof Multi-Material Mask-Image-Projection-Based Stereolithography for theFabrication of Digital Materialsrdquo Annual Solid Freeform Fabrication Sympo-sium Austin TX

[16] Jacobs P F 1995 Stereolithography and Other RPampM Technologies FromRapid Prototyping to Rapid Tooling Society of Manufacturing EngineeringDearborn MI

[17] Pan Y Y Zhou C and Chen Y retrieved Nov 23 2011 httpwwwyoutubecomwatchvfrac140TZWCwnvyew

Fig 18 Layer building time of the test cases

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-9

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

certain distance d and then down by d-dLT where dLT is the layerthickness Since one layer thickness is usually very small(50ndash200 lm) the distance d is usually much larger in order for theresin to fully fill the gap (eg 5 mm) We studied the part separa-tion force of a cured layer from a coated PDMS glass based onsuch movements The PDMS film thickness is set at 1 mm A setof physical experiments has been designed and performed tounderstand the separation force based on such an approach

Figure 3 shows the setup for measuring the pulling-up forceTwo FlexiForce sensors (Tekscan South Boston MA) with arange of 0ndash25 lbs are sandwiched between the fixture and thevat The two sensors are connected to a microcontroller whichcan sample and record the sensorsrsquo readouts at over 3 KHzSince the vat is free at the bottom and the side and only fixed atthe top the pulling force by the part will be transferred to thesensors when the platform rises In the experiments we first usea given mask image to build a certain number of layers (eg 25layers) The layer thickness is set at 02 mm We then begin torecord the separation force in the building process of the nextfew layers For each layer after the designed mask image hasbeen exposed for a certain time the platform is raised up slowlyat 06 mms for 5 mm and the related readouts of the sensors arethen recorded

In our study we considered three factors that may affect theseparation force including (1) exposure time (2) image area and(3) image shape To understand the effects of these factorsdesigned experiments were conducted Seven projection patternswere used for testing the effect of image shape They are shown inFig 3 which include circle band hexagon t-shape square star-shape triangle and u-shape For comparison all the projectionpatterns have the same area in the tests The separation forces of acured layer were measured based on each of the seven projectionpatterns Figure 4 shows the measured separation forces of a sen-sor for different test cases The horizontal axis indicates the dis-tance in the Z direction (in the unit of 10 lm) and the vertical axisindicates the measured pulling force (in ounces)

It can be observed from the experimental results that

(1) As the Z stage moves up the separation force increasesgradually After the cured layer is detached from the PDMSfilm the separation force will drop rapidly from the peakvalue to 0

(2) Due to the flexibility of the PDMS film the pulling-upforce is rather small within a moving distance that is lessthan 200 lm

(3) The peak force gets larger when the same mask image isexposed longer

(4) The peak force gets larger when a larger image area isprojected

(5) The image shape has more complex effects on the peakforce In addition their effects may interact with the expo-sure time and the projection area

(6) With the coated PDMS film on the vat the separation forceis still considerably large (100 oz or 278 N for an imagearea of 625 mm2 with 1 s exposure)

4 Two-Way Movement Design for the Fast

MIP-SL Process

The experiment results indicate that the suction force betweenthe cured layer and the PDMS film is large during the pulling-upprocess Such a large force on the cured layer may cause thebuilding process to fail if the bonding force between the currentlayer and previous layers is smaller than the suction force In addi-tion after building multiple layers such forces on the PDMS filmmay lead to cracks in the film due to material fatigue caused bythe cyclic loading

Based on the PDMS film a two-channel design [15] was pre-sented for the multimaterial MIP-SL process However such anapproach mainly designed for switching tanks is not suitable forthe fast building process In the two-channel design the building

Fig 3 Experimental setup for studying part separation forcesin the MIP-SL process

Fig 4 Pulling-up forces of a cured layer from a PDMS film indifferent settings (a) T 5 1 s area 5 625 mm2 (b) T 5 05 sarea 5 625 mm2 and (c) T 5 1 s area 5 156 mm2

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-3

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

of each layer requires a full cyclic motion including both movingthe platform up and down in the Z axis and moving the tank backand forth in the X axis Such motions will slow down the buildingprocess To facilitate a high-speed MIP-SL process based on thebottom-up projection we develop a novel two-way movementdesign that requires much less motions than the two-channeldesign The developed approach is motivated by the followingobservations

(1) As shown in Fig 4 the pulling-up force is negligible forthe Z movement of a small distance (eg 50 or 100 lm)based on the 1 mm thick PDMS film

(2) As demonstrated in Ref [14] the oxygen-aided inhibitionaround the PDMS surface leaves a nonpolymerized lubri-cating layer near the PDMS film Therefore the cured layercan easily slide on the PDMS surface

The two-way movement design as discussed in Sec 41 caneffectively address the large separation force that is problematicwhile achieving a fast building speed at the same time

41 Two-Way Movement Design An illustration of the fastMIP-SL process based on the two-way movement design is shownin Fig 5 In our method a transparent PDMS film is first appliedon the bottom surface of a transparent glass vat

(1) After a mask image is exposed to cure a layer the platformis moved up in the Z axis for one layer thickness (eg50 lm) Accordingly the regions of the PDMS film relatedto the shape of the cured layer will be pulled up by the suc-tion force However the force is small due to the superelasticity of the PDMS film Note that there is no liquidresin between the cured layer and the PDMS film at thismoment

(2) The tank is moved along the X axis for a certain distanceDx A good property of the PDMS film is that a very thinoxygen-aided inhibition layer (25 lm) is formed near thePDMS film that can provide a nonpolymerized lubricatinglayer for easy sliding [14] If the moving distance is suffi-ciently large (eg larger than the extent size of the curedlayer in the X axis) the elastic deformation of the pulled-upPDMS film will be released by such a sliding movementHence at the end of the X movement liquid resin will befilled in the small gap between the cured layer and thePDMS film

(3) The mask image of a new layer can now be projected at thebottom surface to cure the next layer These three steps canthen be repeated by moving the tank in an opposite direc-tion Note that to achieve the motion in the X direction weonly move the tank and the related frame There is no rela-tive motion between the platform and the projection deviceHence the XY accuracy of the MIP-SL system will not beaffected by the X translations

42 Separation Force Study Based on the Two-WayMovement Design To verify the proposed two-way movementdesign a set of experiments was conducted based on the setup asshown in Fig 3 The same set of mask patterns was used in build-ing test layers The same exposure time and layer thickness wereused (1 s and 02 mm respectively) The building process asshown in Fig 5 was first used in building a set of layers In thetests the tank was translated in the X axis by 20 mm The movingspeed is set at 25 mms After the layers have been built thepulling-up forces in the Z axis during building the next layer wererecorded However instead of curing a new layer as shown in step3 the part is moved up slowly at 06 mms for 25 mm The meas-ured forces of a sensor in the Z axis during the aforementionedthree steps are shown in Fig 6 In each figure the curves recordthe test results based on a sampling resolution of 80 ms

The figures show that the force in the Z direction is rather smallwhen the platform is moved up by 02 mm During the remaining

two steps (ie sliding on the PDMS film and the platform pulling-up) the peak separation forces are also relatively small (around2ndash6 oz or 056ndash167 N) Such measured forces are only 3ndash4 ofthe related ones as shown in Fig 4 Hence the two-way movementdesign can effectively reduce the large separation force in thebottom-up projection system

43 Shearing Force Study in the X Axis In the two-waymovement design cured layers can easily slide on the PDMS sur-face The FlexiForce sensors were used in a modified setup tomeasure the shearing force in the X direction However no mean-ingful readouts were recorded from the sensors To quantitativelyestimate the value of the shearing force in the X axis a set ofsquare rods with different sizes was built using the two-waymovement design The built rods shown in Fig 7 are 10 mm tallThe minimum cross section size is 04 04 mm Note that wealso successfully built rods with even smaller sizes However therods were so fragile that they lost the mechanical strength to

Fig 5 The MIP-SL process based on the two-way movementdesign with PDMS

051011-4 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

sustain themselves when the part was taken out of the resin vatand washed in isopropyl alcohol

Nevertheless for a rod with a size of 04 04 mm the maxi-mum tangential force that can be added on it can be analyticallyestimated As shown in Fig 7 the testing rods in the experimentcan be modeled as a cantilever beam Suppose the length of thebeam is L the size of the beam section is b b the force in tangentdirection is F The maximum bending stress at the end can be cal-culated as r frac14 Mc=I where I is the section modulus I frac14 b4=12and c frac14 b=2 Substituting these values for their variables the

resultant equation is r frac14 6FL=b3 Suppose the allowable blendingstress is [r] and the minimal beam section size is [b] we will havethe following equation F frac12rfrac12b3=6L The material used in ourtests has the following parameters [r]frac14 65 MPa [b]frac14 04 mmand Lfrac14 10 mm According to the equation the upper bound of thetangential force is only 007 N or 025 oz Compared with the sepa-ration force in the Z direction the shearing force in the X directionis rather small

5 A Fast MIP-SL Process and Its Building Speed

Analysis

The two-way movement design enables the quick spreading ofliquid resin into a uniform thin layer In addition the DMD-baseddigital mask projection enables the fast curing of the spread liquidresin into a desired solid layer Consequently for a given 3DCAD model a fast MIP-SL process can fabricate a physical objectwithin a short building time The curing characteristics and thetwo-way movement settings of the developed MIP-SL process arepresented as follows A detailed analysis of its building time isalso discussed

51 Curing Characteristics There are two types of photo-polymer systems acrylate chemistry and cationic photopolymeri-zation in the SLA process [16] Acrylate chemistry polymerizesvia a free-radical mechanism while cationic photopolymerizationundergoes ring-opening reactions in the presence of cationic pho-toinitiators The monomer propagation for cationic reactionsrequires relatively higher activation energy Consequently thephotospeed of acrylate-based photopolymers is higher due to thelower activation energy for free-radical reactions Considering thephotospeed difference we selected the photopolymer resins basedon acrylate chemistry for the developed fast MIP-SL process Asshown in Sec 6 our projection system can cure a layer within ashort exposure time (lt500 ms) Such a fast curing time contrib-utes to the desired fast MIP-SL process for building 3D objects inminutes

After an image is exposed for a certain time (Tprojection) a wait-ing time Twait_projection is required before the layer can be movedup for one layer thickness (ie step 1 in Fig 5) Such a waitingtime is critical in order for the acrylate resin to complete the solid-ification process and gain sufficient strength for the Z movementOtherwise the building process may fail The waiting time is de-pendent on the resinrsquos curing property Due to the fast photospeedof the acrylate resins the waiting time in our system is short(300 ms in our tests)

Fig 7 Shearing force verification test

Fig 6 Pulling-up forces of a cured layer based on the two-waymovement design in different settings (a) T 5 1 s area 5 625mm2 and (b) T 5 1 s area 5 156 mm2

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-5

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

52 Two-Way Movement Settings In the two-way move-ment design the cured part is first moved up for one layer in the Zaxis and the tank is then translated in the X axis for a certain dis-tance The two linear movements have different accuracy andspeed requirements

(1) The Z movement needs to be accurate since it will determinethe layer thickness of the next layer The Z stage also needsto have a resolution that is much smaller than a layer thick-ness Accordingly to ensure the desired accuracy and resolu-tion we set small acceleration and velocity values in the Zmovement The slow movement of the cured part also ena-bles the PDMS film to fully elastically deform for a smallattaching force However the movement time in the Z axis(TZ) is still reasonably short (04 s in our tests) since only asmall moving distance is required (eg 50 or 100 lm)

(2) The tank needs to be moved in the X axis for a certain dis-tance to release the elastic deformation of the PDMS filmThe X moving distance is related to the shape and size of thecured layer and less than the extent size of the cured layer inthe X axis Since the relative position of the platform and theprojection system will not change during the X movementthe accuracy and resolution requirements on the X movementare not as high as those on the Z movement Hence a muchlarger acceleration and velocity can be applied in the Xmovement to reduce the movement time in the X axis (TX)

In our testbed we used a Z linear stage with a thread of05 mmround and a X linear stage with a thread of 254 mmround The moving time for different displacement distances inour prototyping system was calibrated for both linear stages Theresults are plotted as diamond-shape and square-shape lines inFig 8 for the Z and X stages respectively As shown in the figurethe movement in the Z axis is much slower than that in the X axisThe movement time required to complete a moving distance inthe X axis can be identified based on the calibration data

After the X movement another waiting time Twait_X is requiredin order for the flowing liquid resin to settle Otherwise the build-ing process may fail The waiting time caused by the X motions isrelated to the movement distance and the moving speed Due tothe small gap between the cured part and the PDMS film therequired waiting time is typically short (100 ms) After the wait-ing time of Twait_X the liquid resin forms a uniform thin layerwhich is ready for the next layer to be built The process can thenbe repeated after the related mask image is exposed

53 The Building Time of a Layer As shown in Fig 9 thebuilding time of each layer is thus the sum of all the aforemen-tioned steps

TLayer frac14 TProjection thorn Twait_Projection thorn TZ thorn TX thorn TWait_X

The first two items TProjection and Twait_Projection are related to thecuring characteristics of the photopolymer resins used in theMIP-SL process The photopolymer resins based on acrylate chem-istry can be quickly cured A stronger light source used in the pro-jection system can further reduce the projection time TProjection

The other three items TZ TX and TWait_X are related to thetwo-way movement design TZ is related to the layer thickness and

Fig 8 The movement time in the X and Z axes in our prototyping system

Fig 9 The building time of a layer in the two-way movementbased MIP-SL process

Fig 10 The prototype hardware system for the fast MIP-SLprocess

051011-6 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

the moving velocity in the Z axis TX is related to the size of thecured layer and the moving velocity in the X axis A linear stagewith a higher speed can be used to further reduce the movementtime TX TWait_X is determined by the gap distance between thePDMS and the cured layer the moving velocity of the tank theshape of the cured layer and the flow properties of the liquidresin For a typical layer thickness that is usually small TWait_X isreasonably short (100 ms in our tests)

Note that the projection time TProjection for the first few layers ismuch longer (eg 3ndash4 s) to ensure the initial layers can bestrongly bonded to the build platform For all the other layers thetotal building time of a layer is usually short (a few seconds in ourtests) Hence a fast fabrication speed can be achieved in thedeveloped process (eg building 3 mm height per minute)

6 Experimental Setup

61 Hardware System A prototype system has been built toverify the developed process The hardware setup of the fastMIP-SL system is shown in Fig 10 In the designed system an off-the-shelf projector (CASIO XJ-S36) was used The optical lenses of

the projector were modified to reduce the projection distance Vari-ous projection settings including focus key stone rectificationbrightness and contrast were adjusted to achieve a sharp projectionimage on the designed projection plane The DMD resolution in oursystem is 1024 768 and the envelope size is set at 48 36 mm Aprecise linear stage from Aerotech Inc (Pittsburgh PA) is used asthe elevator for driving the platform in the Z axis A fast linear stagefrom Servo Systems Co (Montville NJ) is used to drive the tanksback and force in the X axis A high performance 4-axis motion con-trol board with 28 bidirectional IO pins from Dynomotion Inc (Cal-abasas CA) is used for driving the linear stages A flat and clearglass Petri dish is used as resin tank A PDMS film (Sylgard 184Dow Corning) is coated on the glass dish

62 Software System A mask planning testbed has beendeveloped using the Cthornthorn language with Microsoft VISUAL Cthornthorncomplier The testbed integrates the geometry slicing and themotion controlling It also synchronizes the image projection withthe X and Z movements The graphical user interface of the devel-oped software system is shown in Fig 11 The flowchart of thefast MIP-SL process is also shown in Fig 11

63 Materials PerfatoryTM SI500 (yellow) and Acryl R5(red) from EnvisionTEC Inc (Ferndale MI) were used in testingthe developed fast MIP-SL process Both resins belong to acry-late For curing depths of 005 mm and 01 mm the exposure timesfor SI500 based on our projection system are set at 03 s and045 s respectively The exposure times for Acryl R5 are set at04 s and 055 s for curing depths of 005 mm and 01 mmrespectively

7 Experimental Results and Discussion

Tests have been performed to verify the building speed of thedeveloped prototyping system The results of the designed testshave demonstrated that the presented MIP-SL process can build3D models in minutes instead of hours

71 Test Procedures and Results A set of CAD modelswith different complexity was used in our tests The screenshotsof six input CAD models are shown in Figs 12(a)ndash17(a) Therelated STL files have triangle numbers ranging from several hun-dreds to 12 million (refer to Table 1)

Two different layer thicknesses commonly used in the MIP-SLprocess were tested A 50 lm layer thickness was used in the fab-rication of a gear model The mask image projection time was035 s for each layer except the base The projection waiting timewas set at 01 s For all the other models a 100 lm layer thicknesswas used in their building processes Due to the larger layer

Fig 11 Flow chart of the fast MIP-SL system and related soft-ware system

Table 1 Building time statistics

Model Gear Head Statue Teeth Shell Brush

Figure number Fig 12 Fig 13 Fig 14 Fig 15 Fig 16 Fig 17Triangle number 660 24190 5204 133806 32762 1259246SizeX (mm) 254 25 177 24 24 76Thickness (mm) 005 01 01 01 01 01Tprojection (s) 035 (04)a 045 045 (055)a 045 (055)a 045 045Twait_projection (s) 01 03 03 03 03 03TZ (s) 032 042 042 042 042 042MoveX (mm) 76 25 127 25 25 635 (13)b

TX (s) 058 11 067 046 046 056 (039)b

TX_Wait (s) 005 01 01 005 005 005TLayer (s) 14 (145)a 237 194 (204)a 168 (178)a 168 178 (161)b

HeightZ (mm) 493 285 305 73 223 83Layer number 98 285 305 73 223 83Ttotal_building (min) 231 (239)a 1126 986 (104)a 204 (217)a 624 225

aThe projection and waiting time for R5 resin others are for SI500bDifferent X moving distances were used in building the bottom (1st layerndash19th layer) and the top (20th layerndash83rd layer) portions of the brush

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-7

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

thickness a longer image exposure and projection waiting timeswere used (045 and 03 s respectively in the tests) Accordinglythe Z movement will also take a longer time for a larger layerthickness In the tests the movement time in the Z axis (TZ) is032 and 042 s for the layer thickness of 50 lm and 100 lmrespectively

The required moving distance in the X axis is related to the sizeand shape of the cured layer For a layer with a big cross-sectionalarea (eg the models of a head and a statue) the X translation dis-tance is set to a value that is close to the X extent size Due to thelarge movement the X waiting time was also set longer In com-parison for a layer with a small cross-sectional area (eg themodels of a hearing aid shell and the top portion of a brush) the Xtranslation distance can be much smaller than the extent size ofthe layer in the X axis However due to the fast moving speed inthe X axis the differences on TX are usually small (less than 1 s asshown in Table 1)

Two types of resins SI500 and Acryl R5 were tested Theircuring characteristics are slightly different For the same layerthickness the curing of Acryl R5 takes 01 s longer than that ofSI500 The viscosities of the two resins are also slightly differentHowever the same settings can be used in the two-way movementdesign based on the two resins

Figures 12ndash17 show the built objects based on the developedfast MIP-SL process The quality of the built objects was exam-ined to be satisfactory Both surface finish and dimension were an-alyzed to be acceptable In our prototyping system the nominalsize of a pixel is 47 lm The fine image resolution enables themesoscale features (ie in the range of 01ndash1 mm) to be well cap-tured in the built physical objects eg the lip of the human headthe cloth folds in the Beethoven statue and the dentures in theteeth model

72 Building Time Analysis All the models shown in Figs12ndash17 were built within 12 min using our prototyping system Themodels with less than 100 layers (eg the gear the teeth and thebrush) only require 2ndash3 min to be built A statistic of the buildingtime is given in Table 1

A much larger exposure time (eg 4ndash5 s) was required for thefirst few layers in order to build a base Consequently the built

objects and the build platform can be well bonded For all otherlayers as shown in Fig 18 the building time of a layer (TLayer) inour MIP-SL process is only 14ndash25 s The variation on TLayer ismainly due to different layer thicknesses and the X moving distan-ces For an average of 2 s per layer and a layer thickness of01 mm the building speed of the developed MIP-SL process is3 mm per minute or 180 mm per hour To the best of our knowl-edge such a MIP-SL process is one of the fastest layer-based

Fig 12 A test of a gear (a) CAD model and (b) built objects intwo liquid resins

Fig 13 A test of a head (a) CAD model and (b)ndash(c) two viewsof the built object

Fig 14 A test of a statue (a) CAD model and (b)ndash(e) two viewsof the built objects in two liquid resins

Fig 15 A test of teeth (a) CAD model and (b)ndash(c) built objectsin two liquid resins

Fig 16 A test of a hearing aide shell (a) CAD model and(b)ndash(c) two views of the built object

Fig 17 A test of a brush (a) CAD model and (b) built object

051011-8 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

additive manufacturing processes that have been developed Avideo of building the gear model as shown in Fig 12(a) can befound in Ref [17]

8 Conclusions

A novel mask-image-projection-based stereolithography pro-cess has been presented for fabricating 3D objects with fast build-ing speed The proposed approach is based on projecting maskimages bottom-up on a PDMS coated glass substrate A new two-way movement design has been presented for quickly spreadingliquid resin into uniform thin layers Such a design can signifi-cantly reduce the separation force between cured layers and theresin tank Experimental results verified that the separation forceas well as the sliding force is relatively small during the two-waymovement process The motions related to the two-way move-ment design can also be performed quickly The MIP-SL processdeveloped based on such a recoating approach can achieve highfabrication speed for input CAD models The experimental resultsdemonstrate that the newly developed MIP-SL process can suc-cessfully fabricate 3D objects with satisfactory quality in a shorttime (usually in minutes)

Some future work to further improve the building speed of thedeveloped MIP-SL process includes (1) investigating the settingsof Tx and Twait_X based on given layers for the best performance(2) testing faster moving speeds in the X and Z axes and (3) test-ing the two-way movement design in prototyping systems withlarger XY extent sizes

References[1] Bourell D Leu M and Rosen D eds 2009 NSF WorkshopmdashRoadmap for

Additive Manufacturing Identifying the Future of Freeform ProcessingWashington DC March Available at httpwohlersassociatescomroadmap2009html

[2] Chatwin C Farsari M Huang S Heywood M Birch P Young R andRichardson J 1998 ldquoUV Microstereolithography System That Uses SpatialLight Modulator Technologyrdquo Appl Opt 37 pp 7514ndash7522

[3] Bertsch A Bernhard P Vogt C and Renaud P 2000 ldquoRapid Prototypingof Small Size Objectsrdquo Rapid Prototyping J 6(4) pp 259ndash266

[4] Stampfl J Fouad H Seidler S Liska R Schwager F Woesz A andFratzl P 2004 ldquoFabrication and Moulding of Cellular Materials by RapidPrototypingrdquo Int J Mater Product Technol 21(4) pp 285ndash296

[5] Sun C Fang N Wu D and Zhang X 2005 ldquoProjection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Maskrdquo Sens ActuatorsA 121 pp 113ndash120

[6] Lu Y Mapili G Suhali G Chen S C and Roy K 2006 ldquoA DigitalMicro-Mirror Device-Based System for the Microfabrication of Complex Spa-tially Patterned Tissue Engineering Scaffoldsrdquo J Biomed Mater Res A77A(2) pp 396ndash405

[7] Limaye A S and Rosen D W 2007 ldquoProcess Planning Method for MaskProjection Micro-Stereolithographyrdquo Rapid Prototyping J 13(2) pp 76ndash84

[8] Choi J Wicker R B Cho S Ha C and Lee S 2009 ldquoCure Depth Controlfor Complex 3D Microstructure Fabrication in Dynamic Mask ProjectionMicrostereolithographyrdquo Rapid Prototyping J 15(1) pp 59ndash70

[9] EnvisionTEC retrieved Sept 1 2011 httpwwwenvisiontecde[10] V-Flash desktop modeler retrieved Sept 1 2011 httpwwwmodelin3dcom[11] Chen Y Zhou C and Lao J 2011 ldquoA Layerless Additive Manufacturing

Process Based on CNC Accumulationrdquo Rapid Prototyping J 17(3)pp 218ndash227

[12] Huang Y M and Jiang C P 2005 ldquoOn-Line Force Monitoring of PlatformAscending Rapid Prototyping Systemrdquo J Mater Process Technol 159pp 257ndash264

[13] Denken Corp 1997 SLP-4000 Solid Laser Diode Plotter Product BrochureDenken Corporation Japan

[14] Dendukuri D Pregibon D C Collins J Hatton T A and Doyle P S2006 ldquoContinuous-Flow Lithography for High-Throughput Microparticle Syn-thesisrdquo Nature Mater 5 pp 365ndash369

[15] Zhou C Chen Y Yang Z G and Khoshnevis B 2011 ldquoDevelopmentof Multi-Material Mask-Image-Projection-Based Stereolithography for theFabrication of Digital Materialsrdquo Annual Solid Freeform Fabrication Sympo-sium Austin TX

[16] Jacobs P F 1995 Stereolithography and Other RPampM Technologies FromRapid Prototyping to Rapid Tooling Society of Manufacturing EngineeringDearborn MI

[17] Pan Y Y Zhou C and Chen Y retrieved Nov 23 2011 httpwwwyoutubecomwatchvfrac140TZWCwnvyew

Fig 18 Layer building time of the test cases

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-9

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

of each layer requires a full cyclic motion including both movingthe platform up and down in the Z axis and moving the tank backand forth in the X axis Such motions will slow down the buildingprocess To facilitate a high-speed MIP-SL process based on thebottom-up projection we develop a novel two-way movementdesign that requires much less motions than the two-channeldesign The developed approach is motivated by the followingobservations

(1) As shown in Fig 4 the pulling-up force is negligible forthe Z movement of a small distance (eg 50 or 100 lm)based on the 1 mm thick PDMS film

(2) As demonstrated in Ref [14] the oxygen-aided inhibitionaround the PDMS surface leaves a nonpolymerized lubri-cating layer near the PDMS film Therefore the cured layercan easily slide on the PDMS surface

The two-way movement design as discussed in Sec 41 caneffectively address the large separation force that is problematicwhile achieving a fast building speed at the same time

41 Two-Way Movement Design An illustration of the fastMIP-SL process based on the two-way movement design is shownin Fig 5 In our method a transparent PDMS film is first appliedon the bottom surface of a transparent glass vat

(1) After a mask image is exposed to cure a layer the platformis moved up in the Z axis for one layer thickness (eg50 lm) Accordingly the regions of the PDMS film relatedto the shape of the cured layer will be pulled up by the suc-tion force However the force is small due to the superelasticity of the PDMS film Note that there is no liquidresin between the cured layer and the PDMS film at thismoment

(2) The tank is moved along the X axis for a certain distanceDx A good property of the PDMS film is that a very thinoxygen-aided inhibition layer (25 lm) is formed near thePDMS film that can provide a nonpolymerized lubricatinglayer for easy sliding [14] If the moving distance is suffi-ciently large (eg larger than the extent size of the curedlayer in the X axis) the elastic deformation of the pulled-upPDMS film will be released by such a sliding movementHence at the end of the X movement liquid resin will befilled in the small gap between the cured layer and thePDMS film

(3) The mask image of a new layer can now be projected at thebottom surface to cure the next layer These three steps canthen be repeated by moving the tank in an opposite direc-tion Note that to achieve the motion in the X direction weonly move the tank and the related frame There is no rela-tive motion between the platform and the projection deviceHence the XY accuracy of the MIP-SL system will not beaffected by the X translations

42 Separation Force Study Based on the Two-WayMovement Design To verify the proposed two-way movementdesign a set of experiments was conducted based on the setup asshown in Fig 3 The same set of mask patterns was used in build-ing test layers The same exposure time and layer thickness wereused (1 s and 02 mm respectively) The building process asshown in Fig 5 was first used in building a set of layers In thetests the tank was translated in the X axis by 20 mm The movingspeed is set at 25 mms After the layers have been built thepulling-up forces in the Z axis during building the next layer wererecorded However instead of curing a new layer as shown in step3 the part is moved up slowly at 06 mms for 25 mm The meas-ured forces of a sensor in the Z axis during the aforementionedthree steps are shown in Fig 6 In each figure the curves recordthe test results based on a sampling resolution of 80 ms

The figures show that the force in the Z direction is rather smallwhen the platform is moved up by 02 mm During the remaining

two steps (ie sliding on the PDMS film and the platform pulling-up) the peak separation forces are also relatively small (around2ndash6 oz or 056ndash167 N) Such measured forces are only 3ndash4 ofthe related ones as shown in Fig 4 Hence the two-way movementdesign can effectively reduce the large separation force in thebottom-up projection system

43 Shearing Force Study in the X Axis In the two-waymovement design cured layers can easily slide on the PDMS sur-face The FlexiForce sensors were used in a modified setup tomeasure the shearing force in the X direction However no mean-ingful readouts were recorded from the sensors To quantitativelyestimate the value of the shearing force in the X axis a set ofsquare rods with different sizes was built using the two-waymovement design The built rods shown in Fig 7 are 10 mm tallThe minimum cross section size is 04 04 mm Note that wealso successfully built rods with even smaller sizes However therods were so fragile that they lost the mechanical strength to

Fig 5 The MIP-SL process based on the two-way movementdesign with PDMS

051011-4 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

sustain themselves when the part was taken out of the resin vatand washed in isopropyl alcohol

Nevertheless for a rod with a size of 04 04 mm the maxi-mum tangential force that can be added on it can be analyticallyestimated As shown in Fig 7 the testing rods in the experimentcan be modeled as a cantilever beam Suppose the length of thebeam is L the size of the beam section is b b the force in tangentdirection is F The maximum bending stress at the end can be cal-culated as r frac14 Mc=I where I is the section modulus I frac14 b4=12and c frac14 b=2 Substituting these values for their variables the

resultant equation is r frac14 6FL=b3 Suppose the allowable blendingstress is [r] and the minimal beam section size is [b] we will havethe following equation F frac12rfrac12b3=6L The material used in ourtests has the following parameters [r]frac14 65 MPa [b]frac14 04 mmand Lfrac14 10 mm According to the equation the upper bound of thetangential force is only 007 N or 025 oz Compared with the sepa-ration force in the Z direction the shearing force in the X directionis rather small

5 A Fast MIP-SL Process and Its Building Speed

Analysis

The two-way movement design enables the quick spreading ofliquid resin into a uniform thin layer In addition the DMD-baseddigital mask projection enables the fast curing of the spread liquidresin into a desired solid layer Consequently for a given 3DCAD model a fast MIP-SL process can fabricate a physical objectwithin a short building time The curing characteristics and thetwo-way movement settings of the developed MIP-SL process arepresented as follows A detailed analysis of its building time isalso discussed

51 Curing Characteristics There are two types of photo-polymer systems acrylate chemistry and cationic photopolymeri-zation in the SLA process [16] Acrylate chemistry polymerizesvia a free-radical mechanism while cationic photopolymerizationundergoes ring-opening reactions in the presence of cationic pho-toinitiators The monomer propagation for cationic reactionsrequires relatively higher activation energy Consequently thephotospeed of acrylate-based photopolymers is higher due to thelower activation energy for free-radical reactions Considering thephotospeed difference we selected the photopolymer resins basedon acrylate chemistry for the developed fast MIP-SL process Asshown in Sec 6 our projection system can cure a layer within ashort exposure time (lt500 ms) Such a fast curing time contrib-utes to the desired fast MIP-SL process for building 3D objects inminutes

After an image is exposed for a certain time (Tprojection) a wait-ing time Twait_projection is required before the layer can be movedup for one layer thickness (ie step 1 in Fig 5) Such a waitingtime is critical in order for the acrylate resin to complete the solid-ification process and gain sufficient strength for the Z movementOtherwise the building process may fail The waiting time is de-pendent on the resinrsquos curing property Due to the fast photospeedof the acrylate resins the waiting time in our system is short(300 ms in our tests)

Fig 7 Shearing force verification test

Fig 6 Pulling-up forces of a cured layer based on the two-waymovement design in different settings (a) T 5 1 s area 5 625mm2 and (b) T 5 1 s area 5 156 mm2

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-5

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

52 Two-Way Movement Settings In the two-way move-ment design the cured part is first moved up for one layer in the Zaxis and the tank is then translated in the X axis for a certain dis-tance The two linear movements have different accuracy andspeed requirements

(1) The Z movement needs to be accurate since it will determinethe layer thickness of the next layer The Z stage also needsto have a resolution that is much smaller than a layer thick-ness Accordingly to ensure the desired accuracy and resolu-tion we set small acceleration and velocity values in the Zmovement The slow movement of the cured part also ena-bles the PDMS film to fully elastically deform for a smallattaching force However the movement time in the Z axis(TZ) is still reasonably short (04 s in our tests) since only asmall moving distance is required (eg 50 or 100 lm)

(2) The tank needs to be moved in the X axis for a certain dis-tance to release the elastic deformation of the PDMS filmThe X moving distance is related to the shape and size of thecured layer and less than the extent size of the cured layer inthe X axis Since the relative position of the platform and theprojection system will not change during the X movementthe accuracy and resolution requirements on the X movementare not as high as those on the Z movement Hence a muchlarger acceleration and velocity can be applied in the Xmovement to reduce the movement time in the X axis (TX)

In our testbed we used a Z linear stage with a thread of05 mmround and a X linear stage with a thread of 254 mmround The moving time for different displacement distances inour prototyping system was calibrated for both linear stages Theresults are plotted as diamond-shape and square-shape lines inFig 8 for the Z and X stages respectively As shown in the figurethe movement in the Z axis is much slower than that in the X axisThe movement time required to complete a moving distance inthe X axis can be identified based on the calibration data

After the X movement another waiting time Twait_X is requiredin order for the flowing liquid resin to settle Otherwise the build-ing process may fail The waiting time caused by the X motions isrelated to the movement distance and the moving speed Due tothe small gap between the cured part and the PDMS film therequired waiting time is typically short (100 ms) After the wait-ing time of Twait_X the liquid resin forms a uniform thin layerwhich is ready for the next layer to be built The process can thenbe repeated after the related mask image is exposed

53 The Building Time of a Layer As shown in Fig 9 thebuilding time of each layer is thus the sum of all the aforemen-tioned steps

TLayer frac14 TProjection thorn Twait_Projection thorn TZ thorn TX thorn TWait_X

The first two items TProjection and Twait_Projection are related to thecuring characteristics of the photopolymer resins used in theMIP-SL process The photopolymer resins based on acrylate chem-istry can be quickly cured A stronger light source used in the pro-jection system can further reduce the projection time TProjection

The other three items TZ TX and TWait_X are related to thetwo-way movement design TZ is related to the layer thickness and

Fig 8 The movement time in the X and Z axes in our prototyping system

Fig 9 The building time of a layer in the two-way movementbased MIP-SL process

Fig 10 The prototype hardware system for the fast MIP-SLprocess

051011-6 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

the moving velocity in the Z axis TX is related to the size of thecured layer and the moving velocity in the X axis A linear stagewith a higher speed can be used to further reduce the movementtime TX TWait_X is determined by the gap distance between thePDMS and the cured layer the moving velocity of the tank theshape of the cured layer and the flow properties of the liquidresin For a typical layer thickness that is usually small TWait_X isreasonably short (100 ms in our tests)

Note that the projection time TProjection for the first few layers ismuch longer (eg 3ndash4 s) to ensure the initial layers can bestrongly bonded to the build platform For all the other layers thetotal building time of a layer is usually short (a few seconds in ourtests) Hence a fast fabrication speed can be achieved in thedeveloped process (eg building 3 mm height per minute)

6 Experimental Setup

61 Hardware System A prototype system has been built toverify the developed process The hardware setup of the fastMIP-SL system is shown in Fig 10 In the designed system an off-the-shelf projector (CASIO XJ-S36) was used The optical lenses of

the projector were modified to reduce the projection distance Vari-ous projection settings including focus key stone rectificationbrightness and contrast were adjusted to achieve a sharp projectionimage on the designed projection plane The DMD resolution in oursystem is 1024 768 and the envelope size is set at 48 36 mm Aprecise linear stage from Aerotech Inc (Pittsburgh PA) is used asthe elevator for driving the platform in the Z axis A fast linear stagefrom Servo Systems Co (Montville NJ) is used to drive the tanksback and force in the X axis A high performance 4-axis motion con-trol board with 28 bidirectional IO pins from Dynomotion Inc (Cal-abasas CA) is used for driving the linear stages A flat and clearglass Petri dish is used as resin tank A PDMS film (Sylgard 184Dow Corning) is coated on the glass dish

62 Software System A mask planning testbed has beendeveloped using the Cthornthorn language with Microsoft VISUAL Cthornthorncomplier The testbed integrates the geometry slicing and themotion controlling It also synchronizes the image projection withthe X and Z movements The graphical user interface of the devel-oped software system is shown in Fig 11 The flowchart of thefast MIP-SL process is also shown in Fig 11

63 Materials PerfatoryTM SI500 (yellow) and Acryl R5(red) from EnvisionTEC Inc (Ferndale MI) were used in testingthe developed fast MIP-SL process Both resins belong to acry-late For curing depths of 005 mm and 01 mm the exposure timesfor SI500 based on our projection system are set at 03 s and045 s respectively The exposure times for Acryl R5 are set at04 s and 055 s for curing depths of 005 mm and 01 mmrespectively

7 Experimental Results and Discussion

Tests have been performed to verify the building speed of thedeveloped prototyping system The results of the designed testshave demonstrated that the presented MIP-SL process can build3D models in minutes instead of hours

71 Test Procedures and Results A set of CAD modelswith different complexity was used in our tests The screenshotsof six input CAD models are shown in Figs 12(a)ndash17(a) Therelated STL files have triangle numbers ranging from several hun-dreds to 12 million (refer to Table 1)

Two different layer thicknesses commonly used in the MIP-SLprocess were tested A 50 lm layer thickness was used in the fab-rication of a gear model The mask image projection time was035 s for each layer except the base The projection waiting timewas set at 01 s For all the other models a 100 lm layer thicknesswas used in their building processes Due to the larger layer

Fig 11 Flow chart of the fast MIP-SL system and related soft-ware system

Table 1 Building time statistics

Model Gear Head Statue Teeth Shell Brush

Figure number Fig 12 Fig 13 Fig 14 Fig 15 Fig 16 Fig 17Triangle number 660 24190 5204 133806 32762 1259246SizeX (mm) 254 25 177 24 24 76Thickness (mm) 005 01 01 01 01 01Tprojection (s) 035 (04)a 045 045 (055)a 045 (055)a 045 045Twait_projection (s) 01 03 03 03 03 03TZ (s) 032 042 042 042 042 042MoveX (mm) 76 25 127 25 25 635 (13)b

TX (s) 058 11 067 046 046 056 (039)b

TX_Wait (s) 005 01 01 005 005 005TLayer (s) 14 (145)a 237 194 (204)a 168 (178)a 168 178 (161)b

HeightZ (mm) 493 285 305 73 223 83Layer number 98 285 305 73 223 83Ttotal_building (min) 231 (239)a 1126 986 (104)a 204 (217)a 624 225

aThe projection and waiting time for R5 resin others are for SI500bDifferent X moving distances were used in building the bottom (1st layerndash19th layer) and the top (20th layerndash83rd layer) portions of the brush

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-7

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

thickness a longer image exposure and projection waiting timeswere used (045 and 03 s respectively in the tests) Accordinglythe Z movement will also take a longer time for a larger layerthickness In the tests the movement time in the Z axis (TZ) is032 and 042 s for the layer thickness of 50 lm and 100 lmrespectively

The required moving distance in the X axis is related to the sizeand shape of the cured layer For a layer with a big cross-sectionalarea (eg the models of a head and a statue) the X translation dis-tance is set to a value that is close to the X extent size Due to thelarge movement the X waiting time was also set longer In com-parison for a layer with a small cross-sectional area (eg themodels of a hearing aid shell and the top portion of a brush) the Xtranslation distance can be much smaller than the extent size ofthe layer in the X axis However due to the fast moving speed inthe X axis the differences on TX are usually small (less than 1 s asshown in Table 1)

Two types of resins SI500 and Acryl R5 were tested Theircuring characteristics are slightly different For the same layerthickness the curing of Acryl R5 takes 01 s longer than that ofSI500 The viscosities of the two resins are also slightly differentHowever the same settings can be used in the two-way movementdesign based on the two resins

Figures 12ndash17 show the built objects based on the developedfast MIP-SL process The quality of the built objects was exam-ined to be satisfactory Both surface finish and dimension were an-alyzed to be acceptable In our prototyping system the nominalsize of a pixel is 47 lm The fine image resolution enables themesoscale features (ie in the range of 01ndash1 mm) to be well cap-tured in the built physical objects eg the lip of the human headthe cloth folds in the Beethoven statue and the dentures in theteeth model

72 Building Time Analysis All the models shown in Figs12ndash17 were built within 12 min using our prototyping system Themodels with less than 100 layers (eg the gear the teeth and thebrush) only require 2ndash3 min to be built A statistic of the buildingtime is given in Table 1

A much larger exposure time (eg 4ndash5 s) was required for thefirst few layers in order to build a base Consequently the built

objects and the build platform can be well bonded For all otherlayers as shown in Fig 18 the building time of a layer (TLayer) inour MIP-SL process is only 14ndash25 s The variation on TLayer ismainly due to different layer thicknesses and the X moving distan-ces For an average of 2 s per layer and a layer thickness of01 mm the building speed of the developed MIP-SL process is3 mm per minute or 180 mm per hour To the best of our knowl-edge such a MIP-SL process is one of the fastest layer-based

Fig 12 A test of a gear (a) CAD model and (b) built objects intwo liquid resins

Fig 13 A test of a head (a) CAD model and (b)ndash(c) two viewsof the built object

Fig 14 A test of a statue (a) CAD model and (b)ndash(e) two viewsof the built objects in two liquid resins

Fig 15 A test of teeth (a) CAD model and (b)ndash(c) built objectsin two liquid resins

Fig 16 A test of a hearing aide shell (a) CAD model and(b)ndash(c) two views of the built object

Fig 17 A test of a brush (a) CAD model and (b) built object

051011-8 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

additive manufacturing processes that have been developed Avideo of building the gear model as shown in Fig 12(a) can befound in Ref [17]

8 Conclusions

A novel mask-image-projection-based stereolithography pro-cess has been presented for fabricating 3D objects with fast build-ing speed The proposed approach is based on projecting maskimages bottom-up on a PDMS coated glass substrate A new two-way movement design has been presented for quickly spreadingliquid resin into uniform thin layers Such a design can signifi-cantly reduce the separation force between cured layers and theresin tank Experimental results verified that the separation forceas well as the sliding force is relatively small during the two-waymovement process The motions related to the two-way move-ment design can also be performed quickly The MIP-SL processdeveloped based on such a recoating approach can achieve highfabrication speed for input CAD models The experimental resultsdemonstrate that the newly developed MIP-SL process can suc-cessfully fabricate 3D objects with satisfactory quality in a shorttime (usually in minutes)

Some future work to further improve the building speed of thedeveloped MIP-SL process includes (1) investigating the settingsof Tx and Twait_X based on given layers for the best performance(2) testing faster moving speeds in the X and Z axes and (3) test-ing the two-way movement design in prototyping systems withlarger XY extent sizes

References[1] Bourell D Leu M and Rosen D eds 2009 NSF WorkshopmdashRoadmap for

Additive Manufacturing Identifying the Future of Freeform ProcessingWashington DC March Available at httpwohlersassociatescomroadmap2009html

[2] Chatwin C Farsari M Huang S Heywood M Birch P Young R andRichardson J 1998 ldquoUV Microstereolithography System That Uses SpatialLight Modulator Technologyrdquo Appl Opt 37 pp 7514ndash7522

[3] Bertsch A Bernhard P Vogt C and Renaud P 2000 ldquoRapid Prototypingof Small Size Objectsrdquo Rapid Prototyping J 6(4) pp 259ndash266

[4] Stampfl J Fouad H Seidler S Liska R Schwager F Woesz A andFratzl P 2004 ldquoFabrication and Moulding of Cellular Materials by RapidPrototypingrdquo Int J Mater Product Technol 21(4) pp 285ndash296

[5] Sun C Fang N Wu D and Zhang X 2005 ldquoProjection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Maskrdquo Sens ActuatorsA 121 pp 113ndash120

[6] Lu Y Mapili G Suhali G Chen S C and Roy K 2006 ldquoA DigitalMicro-Mirror Device-Based System for the Microfabrication of Complex Spa-tially Patterned Tissue Engineering Scaffoldsrdquo J Biomed Mater Res A77A(2) pp 396ndash405

[7] Limaye A S and Rosen D W 2007 ldquoProcess Planning Method for MaskProjection Micro-Stereolithographyrdquo Rapid Prototyping J 13(2) pp 76ndash84

[8] Choi J Wicker R B Cho S Ha C and Lee S 2009 ldquoCure Depth Controlfor Complex 3D Microstructure Fabrication in Dynamic Mask ProjectionMicrostereolithographyrdquo Rapid Prototyping J 15(1) pp 59ndash70

[9] EnvisionTEC retrieved Sept 1 2011 httpwwwenvisiontecde[10] V-Flash desktop modeler retrieved Sept 1 2011 httpwwwmodelin3dcom[11] Chen Y Zhou C and Lao J 2011 ldquoA Layerless Additive Manufacturing

Process Based on CNC Accumulationrdquo Rapid Prototyping J 17(3)pp 218ndash227

[12] Huang Y M and Jiang C P 2005 ldquoOn-Line Force Monitoring of PlatformAscending Rapid Prototyping Systemrdquo J Mater Process Technol 159pp 257ndash264

[13] Denken Corp 1997 SLP-4000 Solid Laser Diode Plotter Product BrochureDenken Corporation Japan

[14] Dendukuri D Pregibon D C Collins J Hatton T A and Doyle P S2006 ldquoContinuous-Flow Lithography for High-Throughput Microparticle Syn-thesisrdquo Nature Mater 5 pp 365ndash369

[15] Zhou C Chen Y Yang Z G and Khoshnevis B 2011 ldquoDevelopmentof Multi-Material Mask-Image-Projection-Based Stereolithography for theFabrication of Digital Materialsrdquo Annual Solid Freeform Fabrication Sympo-sium Austin TX

[16] Jacobs P F 1995 Stereolithography and Other RPampM Technologies FromRapid Prototyping to Rapid Tooling Society of Manufacturing EngineeringDearborn MI

[17] Pan Y Y Zhou C and Chen Y retrieved Nov 23 2011 httpwwwyoutubecomwatchvfrac140TZWCwnvyew

Fig 18 Layer building time of the test cases

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-9

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

sustain themselves when the part was taken out of the resin vatand washed in isopropyl alcohol

Nevertheless for a rod with a size of 04 04 mm the maxi-mum tangential force that can be added on it can be analyticallyestimated As shown in Fig 7 the testing rods in the experimentcan be modeled as a cantilever beam Suppose the length of thebeam is L the size of the beam section is b b the force in tangentdirection is F The maximum bending stress at the end can be cal-culated as r frac14 Mc=I where I is the section modulus I frac14 b4=12and c frac14 b=2 Substituting these values for their variables the

resultant equation is r frac14 6FL=b3 Suppose the allowable blendingstress is [r] and the minimal beam section size is [b] we will havethe following equation F frac12rfrac12b3=6L The material used in ourtests has the following parameters [r]frac14 65 MPa [b]frac14 04 mmand Lfrac14 10 mm According to the equation the upper bound of thetangential force is only 007 N or 025 oz Compared with the sepa-ration force in the Z direction the shearing force in the X directionis rather small

5 A Fast MIP-SL Process and Its Building Speed

Analysis

The two-way movement design enables the quick spreading ofliquid resin into a uniform thin layer In addition the DMD-baseddigital mask projection enables the fast curing of the spread liquidresin into a desired solid layer Consequently for a given 3DCAD model a fast MIP-SL process can fabricate a physical objectwithin a short building time The curing characteristics and thetwo-way movement settings of the developed MIP-SL process arepresented as follows A detailed analysis of its building time isalso discussed

51 Curing Characteristics There are two types of photo-polymer systems acrylate chemistry and cationic photopolymeri-zation in the SLA process [16] Acrylate chemistry polymerizesvia a free-radical mechanism while cationic photopolymerizationundergoes ring-opening reactions in the presence of cationic pho-toinitiators The monomer propagation for cationic reactionsrequires relatively higher activation energy Consequently thephotospeed of acrylate-based photopolymers is higher due to thelower activation energy for free-radical reactions Considering thephotospeed difference we selected the photopolymer resins basedon acrylate chemistry for the developed fast MIP-SL process Asshown in Sec 6 our projection system can cure a layer within ashort exposure time (lt500 ms) Such a fast curing time contrib-utes to the desired fast MIP-SL process for building 3D objects inminutes

After an image is exposed for a certain time (Tprojection) a wait-ing time Twait_projection is required before the layer can be movedup for one layer thickness (ie step 1 in Fig 5) Such a waitingtime is critical in order for the acrylate resin to complete the solid-ification process and gain sufficient strength for the Z movementOtherwise the building process may fail The waiting time is de-pendent on the resinrsquos curing property Due to the fast photospeedof the acrylate resins the waiting time in our system is short(300 ms in our tests)

Fig 7 Shearing force verification test

Fig 6 Pulling-up forces of a cured layer based on the two-waymovement design in different settings (a) T 5 1 s area 5 625mm2 and (b) T 5 1 s area 5 156 mm2

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-5

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

52 Two-Way Movement Settings In the two-way move-ment design the cured part is first moved up for one layer in the Zaxis and the tank is then translated in the X axis for a certain dis-tance The two linear movements have different accuracy andspeed requirements

(1) The Z movement needs to be accurate since it will determinethe layer thickness of the next layer The Z stage also needsto have a resolution that is much smaller than a layer thick-ness Accordingly to ensure the desired accuracy and resolu-tion we set small acceleration and velocity values in the Zmovement The slow movement of the cured part also ena-bles the PDMS film to fully elastically deform for a smallattaching force However the movement time in the Z axis(TZ) is still reasonably short (04 s in our tests) since only asmall moving distance is required (eg 50 or 100 lm)

(2) The tank needs to be moved in the X axis for a certain dis-tance to release the elastic deformation of the PDMS filmThe X moving distance is related to the shape and size of thecured layer and less than the extent size of the cured layer inthe X axis Since the relative position of the platform and theprojection system will not change during the X movementthe accuracy and resolution requirements on the X movementare not as high as those on the Z movement Hence a muchlarger acceleration and velocity can be applied in the Xmovement to reduce the movement time in the X axis (TX)

In our testbed we used a Z linear stage with a thread of05 mmround and a X linear stage with a thread of 254 mmround The moving time for different displacement distances inour prototyping system was calibrated for both linear stages Theresults are plotted as diamond-shape and square-shape lines inFig 8 for the Z and X stages respectively As shown in the figurethe movement in the Z axis is much slower than that in the X axisThe movement time required to complete a moving distance inthe X axis can be identified based on the calibration data

After the X movement another waiting time Twait_X is requiredin order for the flowing liquid resin to settle Otherwise the build-ing process may fail The waiting time caused by the X motions isrelated to the movement distance and the moving speed Due tothe small gap between the cured part and the PDMS film therequired waiting time is typically short (100 ms) After the wait-ing time of Twait_X the liquid resin forms a uniform thin layerwhich is ready for the next layer to be built The process can thenbe repeated after the related mask image is exposed

53 The Building Time of a Layer As shown in Fig 9 thebuilding time of each layer is thus the sum of all the aforemen-tioned steps

TLayer frac14 TProjection thorn Twait_Projection thorn TZ thorn TX thorn TWait_X

The first two items TProjection and Twait_Projection are related to thecuring characteristics of the photopolymer resins used in theMIP-SL process The photopolymer resins based on acrylate chem-istry can be quickly cured A stronger light source used in the pro-jection system can further reduce the projection time TProjection

The other three items TZ TX and TWait_X are related to thetwo-way movement design TZ is related to the layer thickness and

Fig 8 The movement time in the X and Z axes in our prototyping system

Fig 9 The building time of a layer in the two-way movementbased MIP-SL process

Fig 10 The prototype hardware system for the fast MIP-SLprocess

051011-6 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

the moving velocity in the Z axis TX is related to the size of thecured layer and the moving velocity in the X axis A linear stagewith a higher speed can be used to further reduce the movementtime TX TWait_X is determined by the gap distance between thePDMS and the cured layer the moving velocity of the tank theshape of the cured layer and the flow properties of the liquidresin For a typical layer thickness that is usually small TWait_X isreasonably short (100 ms in our tests)

Note that the projection time TProjection for the first few layers ismuch longer (eg 3ndash4 s) to ensure the initial layers can bestrongly bonded to the build platform For all the other layers thetotal building time of a layer is usually short (a few seconds in ourtests) Hence a fast fabrication speed can be achieved in thedeveloped process (eg building 3 mm height per minute)

6 Experimental Setup

61 Hardware System A prototype system has been built toverify the developed process The hardware setup of the fastMIP-SL system is shown in Fig 10 In the designed system an off-the-shelf projector (CASIO XJ-S36) was used The optical lenses of

the projector were modified to reduce the projection distance Vari-ous projection settings including focus key stone rectificationbrightness and contrast were adjusted to achieve a sharp projectionimage on the designed projection plane The DMD resolution in oursystem is 1024 768 and the envelope size is set at 48 36 mm Aprecise linear stage from Aerotech Inc (Pittsburgh PA) is used asthe elevator for driving the platform in the Z axis A fast linear stagefrom Servo Systems Co (Montville NJ) is used to drive the tanksback and force in the X axis A high performance 4-axis motion con-trol board with 28 bidirectional IO pins from Dynomotion Inc (Cal-abasas CA) is used for driving the linear stages A flat and clearglass Petri dish is used as resin tank A PDMS film (Sylgard 184Dow Corning) is coated on the glass dish

62 Software System A mask planning testbed has beendeveloped using the Cthornthorn language with Microsoft VISUAL Cthornthorncomplier The testbed integrates the geometry slicing and themotion controlling It also synchronizes the image projection withthe X and Z movements The graphical user interface of the devel-oped software system is shown in Fig 11 The flowchart of thefast MIP-SL process is also shown in Fig 11

63 Materials PerfatoryTM SI500 (yellow) and Acryl R5(red) from EnvisionTEC Inc (Ferndale MI) were used in testingthe developed fast MIP-SL process Both resins belong to acry-late For curing depths of 005 mm and 01 mm the exposure timesfor SI500 based on our projection system are set at 03 s and045 s respectively The exposure times for Acryl R5 are set at04 s and 055 s for curing depths of 005 mm and 01 mmrespectively

7 Experimental Results and Discussion

Tests have been performed to verify the building speed of thedeveloped prototyping system The results of the designed testshave demonstrated that the presented MIP-SL process can build3D models in minutes instead of hours

71 Test Procedures and Results A set of CAD modelswith different complexity was used in our tests The screenshotsof six input CAD models are shown in Figs 12(a)ndash17(a) Therelated STL files have triangle numbers ranging from several hun-dreds to 12 million (refer to Table 1)

Two different layer thicknesses commonly used in the MIP-SLprocess were tested A 50 lm layer thickness was used in the fab-rication of a gear model The mask image projection time was035 s for each layer except the base The projection waiting timewas set at 01 s For all the other models a 100 lm layer thicknesswas used in their building processes Due to the larger layer

Fig 11 Flow chart of the fast MIP-SL system and related soft-ware system

Table 1 Building time statistics

Model Gear Head Statue Teeth Shell Brush

Figure number Fig 12 Fig 13 Fig 14 Fig 15 Fig 16 Fig 17Triangle number 660 24190 5204 133806 32762 1259246SizeX (mm) 254 25 177 24 24 76Thickness (mm) 005 01 01 01 01 01Tprojection (s) 035 (04)a 045 045 (055)a 045 (055)a 045 045Twait_projection (s) 01 03 03 03 03 03TZ (s) 032 042 042 042 042 042MoveX (mm) 76 25 127 25 25 635 (13)b

TX (s) 058 11 067 046 046 056 (039)b

TX_Wait (s) 005 01 01 005 005 005TLayer (s) 14 (145)a 237 194 (204)a 168 (178)a 168 178 (161)b

HeightZ (mm) 493 285 305 73 223 83Layer number 98 285 305 73 223 83Ttotal_building (min) 231 (239)a 1126 986 (104)a 204 (217)a 624 225

aThe projection and waiting time for R5 resin others are for SI500bDifferent X moving distances were used in building the bottom (1st layerndash19th layer) and the top (20th layerndash83rd layer) portions of the brush

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-7

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

thickness a longer image exposure and projection waiting timeswere used (045 and 03 s respectively in the tests) Accordinglythe Z movement will also take a longer time for a larger layerthickness In the tests the movement time in the Z axis (TZ) is032 and 042 s for the layer thickness of 50 lm and 100 lmrespectively

The required moving distance in the X axis is related to the sizeand shape of the cured layer For a layer with a big cross-sectionalarea (eg the models of a head and a statue) the X translation dis-tance is set to a value that is close to the X extent size Due to thelarge movement the X waiting time was also set longer In com-parison for a layer with a small cross-sectional area (eg themodels of a hearing aid shell and the top portion of a brush) the Xtranslation distance can be much smaller than the extent size ofthe layer in the X axis However due to the fast moving speed inthe X axis the differences on TX are usually small (less than 1 s asshown in Table 1)

Two types of resins SI500 and Acryl R5 were tested Theircuring characteristics are slightly different For the same layerthickness the curing of Acryl R5 takes 01 s longer than that ofSI500 The viscosities of the two resins are also slightly differentHowever the same settings can be used in the two-way movementdesign based on the two resins

Figures 12ndash17 show the built objects based on the developedfast MIP-SL process The quality of the built objects was exam-ined to be satisfactory Both surface finish and dimension were an-alyzed to be acceptable In our prototyping system the nominalsize of a pixel is 47 lm The fine image resolution enables themesoscale features (ie in the range of 01ndash1 mm) to be well cap-tured in the built physical objects eg the lip of the human headthe cloth folds in the Beethoven statue and the dentures in theteeth model

72 Building Time Analysis All the models shown in Figs12ndash17 were built within 12 min using our prototyping system Themodels with less than 100 layers (eg the gear the teeth and thebrush) only require 2ndash3 min to be built A statistic of the buildingtime is given in Table 1

A much larger exposure time (eg 4ndash5 s) was required for thefirst few layers in order to build a base Consequently the built

objects and the build platform can be well bonded For all otherlayers as shown in Fig 18 the building time of a layer (TLayer) inour MIP-SL process is only 14ndash25 s The variation on TLayer ismainly due to different layer thicknesses and the X moving distan-ces For an average of 2 s per layer and a layer thickness of01 mm the building speed of the developed MIP-SL process is3 mm per minute or 180 mm per hour To the best of our knowl-edge such a MIP-SL process is one of the fastest layer-based

Fig 12 A test of a gear (a) CAD model and (b) built objects intwo liquid resins

Fig 13 A test of a head (a) CAD model and (b)ndash(c) two viewsof the built object

Fig 14 A test of a statue (a) CAD model and (b)ndash(e) two viewsof the built objects in two liquid resins

Fig 15 A test of teeth (a) CAD model and (b)ndash(c) built objectsin two liquid resins

Fig 16 A test of a hearing aide shell (a) CAD model and(b)ndash(c) two views of the built object

Fig 17 A test of a brush (a) CAD model and (b) built object

051011-8 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

additive manufacturing processes that have been developed Avideo of building the gear model as shown in Fig 12(a) can befound in Ref [17]

8 Conclusions

A novel mask-image-projection-based stereolithography pro-cess has been presented for fabricating 3D objects with fast build-ing speed The proposed approach is based on projecting maskimages bottom-up on a PDMS coated glass substrate A new two-way movement design has been presented for quickly spreadingliquid resin into uniform thin layers Such a design can signifi-cantly reduce the separation force between cured layers and theresin tank Experimental results verified that the separation forceas well as the sliding force is relatively small during the two-waymovement process The motions related to the two-way move-ment design can also be performed quickly The MIP-SL processdeveloped based on such a recoating approach can achieve highfabrication speed for input CAD models The experimental resultsdemonstrate that the newly developed MIP-SL process can suc-cessfully fabricate 3D objects with satisfactory quality in a shorttime (usually in minutes)

Some future work to further improve the building speed of thedeveloped MIP-SL process includes (1) investigating the settingsof Tx and Twait_X based on given layers for the best performance(2) testing faster moving speeds in the X and Z axes and (3) test-ing the two-way movement design in prototyping systems withlarger XY extent sizes

References[1] Bourell D Leu M and Rosen D eds 2009 NSF WorkshopmdashRoadmap for

Additive Manufacturing Identifying the Future of Freeform ProcessingWashington DC March Available at httpwohlersassociatescomroadmap2009html

[2] Chatwin C Farsari M Huang S Heywood M Birch P Young R andRichardson J 1998 ldquoUV Microstereolithography System That Uses SpatialLight Modulator Technologyrdquo Appl Opt 37 pp 7514ndash7522

[3] Bertsch A Bernhard P Vogt C and Renaud P 2000 ldquoRapid Prototypingof Small Size Objectsrdquo Rapid Prototyping J 6(4) pp 259ndash266

[4] Stampfl J Fouad H Seidler S Liska R Schwager F Woesz A andFratzl P 2004 ldquoFabrication and Moulding of Cellular Materials by RapidPrototypingrdquo Int J Mater Product Technol 21(4) pp 285ndash296

[5] Sun C Fang N Wu D and Zhang X 2005 ldquoProjection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Maskrdquo Sens ActuatorsA 121 pp 113ndash120

[6] Lu Y Mapili G Suhali G Chen S C and Roy K 2006 ldquoA DigitalMicro-Mirror Device-Based System for the Microfabrication of Complex Spa-tially Patterned Tissue Engineering Scaffoldsrdquo J Biomed Mater Res A77A(2) pp 396ndash405

[7] Limaye A S and Rosen D W 2007 ldquoProcess Planning Method for MaskProjection Micro-Stereolithographyrdquo Rapid Prototyping J 13(2) pp 76ndash84

[8] Choi J Wicker R B Cho S Ha C and Lee S 2009 ldquoCure Depth Controlfor Complex 3D Microstructure Fabrication in Dynamic Mask ProjectionMicrostereolithographyrdquo Rapid Prototyping J 15(1) pp 59ndash70

[9] EnvisionTEC retrieved Sept 1 2011 httpwwwenvisiontecde[10] V-Flash desktop modeler retrieved Sept 1 2011 httpwwwmodelin3dcom[11] Chen Y Zhou C and Lao J 2011 ldquoA Layerless Additive Manufacturing

Process Based on CNC Accumulationrdquo Rapid Prototyping J 17(3)pp 218ndash227

[12] Huang Y M and Jiang C P 2005 ldquoOn-Line Force Monitoring of PlatformAscending Rapid Prototyping Systemrdquo J Mater Process Technol 159pp 257ndash264

[13] Denken Corp 1997 SLP-4000 Solid Laser Diode Plotter Product BrochureDenken Corporation Japan

[14] Dendukuri D Pregibon D C Collins J Hatton T A and Doyle P S2006 ldquoContinuous-Flow Lithography for High-Throughput Microparticle Syn-thesisrdquo Nature Mater 5 pp 365ndash369

[15] Zhou C Chen Y Yang Z G and Khoshnevis B 2011 ldquoDevelopmentof Multi-Material Mask-Image-Projection-Based Stereolithography for theFabrication of Digital Materialsrdquo Annual Solid Freeform Fabrication Sympo-sium Austin TX

[16] Jacobs P F 1995 Stereolithography and Other RPampM Technologies FromRapid Prototyping to Rapid Tooling Society of Manufacturing EngineeringDearborn MI

[17] Pan Y Y Zhou C and Chen Y retrieved Nov 23 2011 httpwwwyoutubecomwatchvfrac140TZWCwnvyew

Fig 18 Layer building time of the test cases

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-9

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

52 Two-Way Movement Settings In the two-way move-ment design the cured part is first moved up for one layer in the Zaxis and the tank is then translated in the X axis for a certain dis-tance The two linear movements have different accuracy andspeed requirements

(1) The Z movement needs to be accurate since it will determinethe layer thickness of the next layer The Z stage also needsto have a resolution that is much smaller than a layer thick-ness Accordingly to ensure the desired accuracy and resolu-tion we set small acceleration and velocity values in the Zmovement The slow movement of the cured part also ena-bles the PDMS film to fully elastically deform for a smallattaching force However the movement time in the Z axis(TZ) is still reasonably short (04 s in our tests) since only asmall moving distance is required (eg 50 or 100 lm)

(2) The tank needs to be moved in the X axis for a certain dis-tance to release the elastic deformation of the PDMS filmThe X moving distance is related to the shape and size of thecured layer and less than the extent size of the cured layer inthe X axis Since the relative position of the platform and theprojection system will not change during the X movementthe accuracy and resolution requirements on the X movementare not as high as those on the Z movement Hence a muchlarger acceleration and velocity can be applied in the Xmovement to reduce the movement time in the X axis (TX)

In our testbed we used a Z linear stage with a thread of05 mmround and a X linear stage with a thread of 254 mmround The moving time for different displacement distances inour prototyping system was calibrated for both linear stages Theresults are plotted as diamond-shape and square-shape lines inFig 8 for the Z and X stages respectively As shown in the figurethe movement in the Z axis is much slower than that in the X axisThe movement time required to complete a moving distance inthe X axis can be identified based on the calibration data

After the X movement another waiting time Twait_X is requiredin order for the flowing liquid resin to settle Otherwise the build-ing process may fail The waiting time caused by the X motions isrelated to the movement distance and the moving speed Due tothe small gap between the cured part and the PDMS film therequired waiting time is typically short (100 ms) After the wait-ing time of Twait_X the liquid resin forms a uniform thin layerwhich is ready for the next layer to be built The process can thenbe repeated after the related mask image is exposed

53 The Building Time of a Layer As shown in Fig 9 thebuilding time of each layer is thus the sum of all the aforemen-tioned steps

TLayer frac14 TProjection thorn Twait_Projection thorn TZ thorn TX thorn TWait_X

The first two items TProjection and Twait_Projection are related to thecuring characteristics of the photopolymer resins used in theMIP-SL process The photopolymer resins based on acrylate chem-istry can be quickly cured A stronger light source used in the pro-jection system can further reduce the projection time TProjection

The other three items TZ TX and TWait_X are related to thetwo-way movement design TZ is related to the layer thickness and

Fig 8 The movement time in the X and Z axes in our prototyping system

Fig 9 The building time of a layer in the two-way movementbased MIP-SL process

Fig 10 The prototype hardware system for the fast MIP-SLprocess

051011-6 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

the moving velocity in the Z axis TX is related to the size of thecured layer and the moving velocity in the X axis A linear stagewith a higher speed can be used to further reduce the movementtime TX TWait_X is determined by the gap distance between thePDMS and the cured layer the moving velocity of the tank theshape of the cured layer and the flow properties of the liquidresin For a typical layer thickness that is usually small TWait_X isreasonably short (100 ms in our tests)

Note that the projection time TProjection for the first few layers ismuch longer (eg 3ndash4 s) to ensure the initial layers can bestrongly bonded to the build platform For all the other layers thetotal building time of a layer is usually short (a few seconds in ourtests) Hence a fast fabrication speed can be achieved in thedeveloped process (eg building 3 mm height per minute)

6 Experimental Setup

61 Hardware System A prototype system has been built toverify the developed process The hardware setup of the fastMIP-SL system is shown in Fig 10 In the designed system an off-the-shelf projector (CASIO XJ-S36) was used The optical lenses of

the projector were modified to reduce the projection distance Vari-ous projection settings including focus key stone rectificationbrightness and contrast were adjusted to achieve a sharp projectionimage on the designed projection plane The DMD resolution in oursystem is 1024 768 and the envelope size is set at 48 36 mm Aprecise linear stage from Aerotech Inc (Pittsburgh PA) is used asthe elevator for driving the platform in the Z axis A fast linear stagefrom Servo Systems Co (Montville NJ) is used to drive the tanksback and force in the X axis A high performance 4-axis motion con-trol board with 28 bidirectional IO pins from Dynomotion Inc (Cal-abasas CA) is used for driving the linear stages A flat and clearglass Petri dish is used as resin tank A PDMS film (Sylgard 184Dow Corning) is coated on the glass dish

62 Software System A mask planning testbed has beendeveloped using the Cthornthorn language with Microsoft VISUAL Cthornthorncomplier The testbed integrates the geometry slicing and themotion controlling It also synchronizes the image projection withthe X and Z movements The graphical user interface of the devel-oped software system is shown in Fig 11 The flowchart of thefast MIP-SL process is also shown in Fig 11

63 Materials PerfatoryTM SI500 (yellow) and Acryl R5(red) from EnvisionTEC Inc (Ferndale MI) were used in testingthe developed fast MIP-SL process Both resins belong to acry-late For curing depths of 005 mm and 01 mm the exposure timesfor SI500 based on our projection system are set at 03 s and045 s respectively The exposure times for Acryl R5 are set at04 s and 055 s for curing depths of 005 mm and 01 mmrespectively

7 Experimental Results and Discussion

Tests have been performed to verify the building speed of thedeveloped prototyping system The results of the designed testshave demonstrated that the presented MIP-SL process can build3D models in minutes instead of hours

71 Test Procedures and Results A set of CAD modelswith different complexity was used in our tests The screenshotsof six input CAD models are shown in Figs 12(a)ndash17(a) Therelated STL files have triangle numbers ranging from several hun-dreds to 12 million (refer to Table 1)

Two different layer thicknesses commonly used in the MIP-SLprocess were tested A 50 lm layer thickness was used in the fab-rication of a gear model The mask image projection time was035 s for each layer except the base The projection waiting timewas set at 01 s For all the other models a 100 lm layer thicknesswas used in their building processes Due to the larger layer

Fig 11 Flow chart of the fast MIP-SL system and related soft-ware system

Table 1 Building time statistics

Model Gear Head Statue Teeth Shell Brush

Figure number Fig 12 Fig 13 Fig 14 Fig 15 Fig 16 Fig 17Triangle number 660 24190 5204 133806 32762 1259246SizeX (mm) 254 25 177 24 24 76Thickness (mm) 005 01 01 01 01 01Tprojection (s) 035 (04)a 045 045 (055)a 045 (055)a 045 045Twait_projection (s) 01 03 03 03 03 03TZ (s) 032 042 042 042 042 042MoveX (mm) 76 25 127 25 25 635 (13)b

TX (s) 058 11 067 046 046 056 (039)b

TX_Wait (s) 005 01 01 005 005 005TLayer (s) 14 (145)a 237 194 (204)a 168 (178)a 168 178 (161)b

HeightZ (mm) 493 285 305 73 223 83Layer number 98 285 305 73 223 83Ttotal_building (min) 231 (239)a 1126 986 (104)a 204 (217)a 624 225

aThe projection and waiting time for R5 resin others are for SI500bDifferent X moving distances were used in building the bottom (1st layerndash19th layer) and the top (20th layerndash83rd layer) portions of the brush

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-7

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

thickness a longer image exposure and projection waiting timeswere used (045 and 03 s respectively in the tests) Accordinglythe Z movement will also take a longer time for a larger layerthickness In the tests the movement time in the Z axis (TZ) is032 and 042 s for the layer thickness of 50 lm and 100 lmrespectively

The required moving distance in the X axis is related to the sizeand shape of the cured layer For a layer with a big cross-sectionalarea (eg the models of a head and a statue) the X translation dis-tance is set to a value that is close to the X extent size Due to thelarge movement the X waiting time was also set longer In com-parison for a layer with a small cross-sectional area (eg themodels of a hearing aid shell and the top portion of a brush) the Xtranslation distance can be much smaller than the extent size ofthe layer in the X axis However due to the fast moving speed inthe X axis the differences on TX are usually small (less than 1 s asshown in Table 1)

Two types of resins SI500 and Acryl R5 were tested Theircuring characteristics are slightly different For the same layerthickness the curing of Acryl R5 takes 01 s longer than that ofSI500 The viscosities of the two resins are also slightly differentHowever the same settings can be used in the two-way movementdesign based on the two resins

Figures 12ndash17 show the built objects based on the developedfast MIP-SL process The quality of the built objects was exam-ined to be satisfactory Both surface finish and dimension were an-alyzed to be acceptable In our prototyping system the nominalsize of a pixel is 47 lm The fine image resolution enables themesoscale features (ie in the range of 01ndash1 mm) to be well cap-tured in the built physical objects eg the lip of the human headthe cloth folds in the Beethoven statue and the dentures in theteeth model

72 Building Time Analysis All the models shown in Figs12ndash17 were built within 12 min using our prototyping system Themodels with less than 100 layers (eg the gear the teeth and thebrush) only require 2ndash3 min to be built A statistic of the buildingtime is given in Table 1

A much larger exposure time (eg 4ndash5 s) was required for thefirst few layers in order to build a base Consequently the built

objects and the build platform can be well bonded For all otherlayers as shown in Fig 18 the building time of a layer (TLayer) inour MIP-SL process is only 14ndash25 s The variation on TLayer ismainly due to different layer thicknesses and the X moving distan-ces For an average of 2 s per layer and a layer thickness of01 mm the building speed of the developed MIP-SL process is3 mm per minute or 180 mm per hour To the best of our knowl-edge such a MIP-SL process is one of the fastest layer-based

Fig 12 A test of a gear (a) CAD model and (b) built objects intwo liquid resins

Fig 13 A test of a head (a) CAD model and (b)ndash(c) two viewsof the built object

Fig 14 A test of a statue (a) CAD model and (b)ndash(e) two viewsof the built objects in two liquid resins

Fig 15 A test of teeth (a) CAD model and (b)ndash(c) built objectsin two liquid resins

Fig 16 A test of a hearing aide shell (a) CAD model and(b)ndash(c) two views of the built object

Fig 17 A test of a brush (a) CAD model and (b) built object

051011-8 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

additive manufacturing processes that have been developed Avideo of building the gear model as shown in Fig 12(a) can befound in Ref [17]

8 Conclusions

A novel mask-image-projection-based stereolithography pro-cess has been presented for fabricating 3D objects with fast build-ing speed The proposed approach is based on projecting maskimages bottom-up on a PDMS coated glass substrate A new two-way movement design has been presented for quickly spreadingliquid resin into uniform thin layers Such a design can signifi-cantly reduce the separation force between cured layers and theresin tank Experimental results verified that the separation forceas well as the sliding force is relatively small during the two-waymovement process The motions related to the two-way move-ment design can also be performed quickly The MIP-SL processdeveloped based on such a recoating approach can achieve highfabrication speed for input CAD models The experimental resultsdemonstrate that the newly developed MIP-SL process can suc-cessfully fabricate 3D objects with satisfactory quality in a shorttime (usually in minutes)

Some future work to further improve the building speed of thedeveloped MIP-SL process includes (1) investigating the settingsof Tx and Twait_X based on given layers for the best performance(2) testing faster moving speeds in the X and Z axes and (3) test-ing the two-way movement design in prototyping systems withlarger XY extent sizes

References[1] Bourell D Leu M and Rosen D eds 2009 NSF WorkshopmdashRoadmap for

Additive Manufacturing Identifying the Future of Freeform ProcessingWashington DC March Available at httpwohlersassociatescomroadmap2009html

[2] Chatwin C Farsari M Huang S Heywood M Birch P Young R andRichardson J 1998 ldquoUV Microstereolithography System That Uses SpatialLight Modulator Technologyrdquo Appl Opt 37 pp 7514ndash7522

[3] Bertsch A Bernhard P Vogt C and Renaud P 2000 ldquoRapid Prototypingof Small Size Objectsrdquo Rapid Prototyping J 6(4) pp 259ndash266

[4] Stampfl J Fouad H Seidler S Liska R Schwager F Woesz A andFratzl P 2004 ldquoFabrication and Moulding of Cellular Materials by RapidPrototypingrdquo Int J Mater Product Technol 21(4) pp 285ndash296

[5] Sun C Fang N Wu D and Zhang X 2005 ldquoProjection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Maskrdquo Sens ActuatorsA 121 pp 113ndash120

[6] Lu Y Mapili G Suhali G Chen S C and Roy K 2006 ldquoA DigitalMicro-Mirror Device-Based System for the Microfabrication of Complex Spa-tially Patterned Tissue Engineering Scaffoldsrdquo J Biomed Mater Res A77A(2) pp 396ndash405

[7] Limaye A S and Rosen D W 2007 ldquoProcess Planning Method for MaskProjection Micro-Stereolithographyrdquo Rapid Prototyping J 13(2) pp 76ndash84

[8] Choi J Wicker R B Cho S Ha C and Lee S 2009 ldquoCure Depth Controlfor Complex 3D Microstructure Fabrication in Dynamic Mask ProjectionMicrostereolithographyrdquo Rapid Prototyping J 15(1) pp 59ndash70

[9] EnvisionTEC retrieved Sept 1 2011 httpwwwenvisiontecde[10] V-Flash desktop modeler retrieved Sept 1 2011 httpwwwmodelin3dcom[11] Chen Y Zhou C and Lao J 2011 ldquoA Layerless Additive Manufacturing

Process Based on CNC Accumulationrdquo Rapid Prototyping J 17(3)pp 218ndash227

[12] Huang Y M and Jiang C P 2005 ldquoOn-Line Force Monitoring of PlatformAscending Rapid Prototyping Systemrdquo J Mater Process Technol 159pp 257ndash264

[13] Denken Corp 1997 SLP-4000 Solid Laser Diode Plotter Product BrochureDenken Corporation Japan

[14] Dendukuri D Pregibon D C Collins J Hatton T A and Doyle P S2006 ldquoContinuous-Flow Lithography for High-Throughput Microparticle Syn-thesisrdquo Nature Mater 5 pp 365ndash369

[15] Zhou C Chen Y Yang Z G and Khoshnevis B 2011 ldquoDevelopmentof Multi-Material Mask-Image-Projection-Based Stereolithography for theFabrication of Digital Materialsrdquo Annual Solid Freeform Fabrication Sympo-sium Austin TX

[16] Jacobs P F 1995 Stereolithography and Other RPampM Technologies FromRapid Prototyping to Rapid Tooling Society of Manufacturing EngineeringDearborn MI

[17] Pan Y Y Zhou C and Chen Y retrieved Nov 23 2011 httpwwwyoutubecomwatchvfrac140TZWCwnvyew

Fig 18 Layer building time of the test cases

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-9

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

the moving velocity in the Z axis TX is related to the size of thecured layer and the moving velocity in the X axis A linear stagewith a higher speed can be used to further reduce the movementtime TX TWait_X is determined by the gap distance between thePDMS and the cured layer the moving velocity of the tank theshape of the cured layer and the flow properties of the liquidresin For a typical layer thickness that is usually small TWait_X isreasonably short (100 ms in our tests)

Note that the projection time TProjection for the first few layers ismuch longer (eg 3ndash4 s) to ensure the initial layers can bestrongly bonded to the build platform For all the other layers thetotal building time of a layer is usually short (a few seconds in ourtests) Hence a fast fabrication speed can be achieved in thedeveloped process (eg building 3 mm height per minute)

6 Experimental Setup

61 Hardware System A prototype system has been built toverify the developed process The hardware setup of the fastMIP-SL system is shown in Fig 10 In the designed system an off-the-shelf projector (CASIO XJ-S36) was used The optical lenses of

the projector were modified to reduce the projection distance Vari-ous projection settings including focus key stone rectificationbrightness and contrast were adjusted to achieve a sharp projectionimage on the designed projection plane The DMD resolution in oursystem is 1024 768 and the envelope size is set at 48 36 mm Aprecise linear stage from Aerotech Inc (Pittsburgh PA) is used asthe elevator for driving the platform in the Z axis A fast linear stagefrom Servo Systems Co (Montville NJ) is used to drive the tanksback and force in the X axis A high performance 4-axis motion con-trol board with 28 bidirectional IO pins from Dynomotion Inc (Cal-abasas CA) is used for driving the linear stages A flat and clearglass Petri dish is used as resin tank A PDMS film (Sylgard 184Dow Corning) is coated on the glass dish

62 Software System A mask planning testbed has beendeveloped using the Cthornthorn language with Microsoft VISUAL Cthornthorncomplier The testbed integrates the geometry slicing and themotion controlling It also synchronizes the image projection withthe X and Z movements The graphical user interface of the devel-oped software system is shown in Fig 11 The flowchart of thefast MIP-SL process is also shown in Fig 11

63 Materials PerfatoryTM SI500 (yellow) and Acryl R5(red) from EnvisionTEC Inc (Ferndale MI) were used in testingthe developed fast MIP-SL process Both resins belong to acry-late For curing depths of 005 mm and 01 mm the exposure timesfor SI500 based on our projection system are set at 03 s and045 s respectively The exposure times for Acryl R5 are set at04 s and 055 s for curing depths of 005 mm and 01 mmrespectively

7 Experimental Results and Discussion

Tests have been performed to verify the building speed of thedeveloped prototyping system The results of the designed testshave demonstrated that the presented MIP-SL process can build3D models in minutes instead of hours

71 Test Procedures and Results A set of CAD modelswith different complexity was used in our tests The screenshotsof six input CAD models are shown in Figs 12(a)ndash17(a) Therelated STL files have triangle numbers ranging from several hun-dreds to 12 million (refer to Table 1)

Two different layer thicknesses commonly used in the MIP-SLprocess were tested A 50 lm layer thickness was used in the fab-rication of a gear model The mask image projection time was035 s for each layer except the base The projection waiting timewas set at 01 s For all the other models a 100 lm layer thicknesswas used in their building processes Due to the larger layer

Fig 11 Flow chart of the fast MIP-SL system and related soft-ware system

Table 1 Building time statistics

Model Gear Head Statue Teeth Shell Brush

Figure number Fig 12 Fig 13 Fig 14 Fig 15 Fig 16 Fig 17Triangle number 660 24190 5204 133806 32762 1259246SizeX (mm) 254 25 177 24 24 76Thickness (mm) 005 01 01 01 01 01Tprojection (s) 035 (04)a 045 045 (055)a 045 (055)a 045 045Twait_projection (s) 01 03 03 03 03 03TZ (s) 032 042 042 042 042 042MoveX (mm) 76 25 127 25 25 635 (13)b

TX (s) 058 11 067 046 046 056 (039)b

TX_Wait (s) 005 01 01 005 005 005TLayer (s) 14 (145)a 237 194 (204)a 168 (178)a 168 178 (161)b

HeightZ (mm) 493 285 305 73 223 83Layer number 98 285 305 73 223 83Ttotal_building (min) 231 (239)a 1126 986 (104)a 204 (217)a 624 225

aThe projection and waiting time for R5 resin others are for SI500bDifferent X moving distances were used in building the bottom (1st layerndash19th layer) and the top (20th layerndash83rd layer) portions of the brush

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-7

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

thickness a longer image exposure and projection waiting timeswere used (045 and 03 s respectively in the tests) Accordinglythe Z movement will also take a longer time for a larger layerthickness In the tests the movement time in the Z axis (TZ) is032 and 042 s for the layer thickness of 50 lm and 100 lmrespectively

The required moving distance in the X axis is related to the sizeand shape of the cured layer For a layer with a big cross-sectionalarea (eg the models of a head and a statue) the X translation dis-tance is set to a value that is close to the X extent size Due to thelarge movement the X waiting time was also set longer In com-parison for a layer with a small cross-sectional area (eg themodels of a hearing aid shell and the top portion of a brush) the Xtranslation distance can be much smaller than the extent size ofthe layer in the X axis However due to the fast moving speed inthe X axis the differences on TX are usually small (less than 1 s asshown in Table 1)

Two types of resins SI500 and Acryl R5 were tested Theircuring characteristics are slightly different For the same layerthickness the curing of Acryl R5 takes 01 s longer than that ofSI500 The viscosities of the two resins are also slightly differentHowever the same settings can be used in the two-way movementdesign based on the two resins

Figures 12ndash17 show the built objects based on the developedfast MIP-SL process The quality of the built objects was exam-ined to be satisfactory Both surface finish and dimension were an-alyzed to be acceptable In our prototyping system the nominalsize of a pixel is 47 lm The fine image resolution enables themesoscale features (ie in the range of 01ndash1 mm) to be well cap-tured in the built physical objects eg the lip of the human headthe cloth folds in the Beethoven statue and the dentures in theteeth model

72 Building Time Analysis All the models shown in Figs12ndash17 were built within 12 min using our prototyping system Themodels with less than 100 layers (eg the gear the teeth and thebrush) only require 2ndash3 min to be built A statistic of the buildingtime is given in Table 1

A much larger exposure time (eg 4ndash5 s) was required for thefirst few layers in order to build a base Consequently the built

objects and the build platform can be well bonded For all otherlayers as shown in Fig 18 the building time of a layer (TLayer) inour MIP-SL process is only 14ndash25 s The variation on TLayer ismainly due to different layer thicknesses and the X moving distan-ces For an average of 2 s per layer and a layer thickness of01 mm the building speed of the developed MIP-SL process is3 mm per minute or 180 mm per hour To the best of our knowl-edge such a MIP-SL process is one of the fastest layer-based

Fig 12 A test of a gear (a) CAD model and (b) built objects intwo liquid resins

Fig 13 A test of a head (a) CAD model and (b)ndash(c) two viewsof the built object

Fig 14 A test of a statue (a) CAD model and (b)ndash(e) two viewsof the built objects in two liquid resins

Fig 15 A test of teeth (a) CAD model and (b)ndash(c) built objectsin two liquid resins

Fig 16 A test of a hearing aide shell (a) CAD model and(b)ndash(c) two views of the built object

Fig 17 A test of a brush (a) CAD model and (b) built object

051011-8 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

additive manufacturing processes that have been developed Avideo of building the gear model as shown in Fig 12(a) can befound in Ref [17]

8 Conclusions

A novel mask-image-projection-based stereolithography pro-cess has been presented for fabricating 3D objects with fast build-ing speed The proposed approach is based on projecting maskimages bottom-up on a PDMS coated glass substrate A new two-way movement design has been presented for quickly spreadingliquid resin into uniform thin layers Such a design can signifi-cantly reduce the separation force between cured layers and theresin tank Experimental results verified that the separation forceas well as the sliding force is relatively small during the two-waymovement process The motions related to the two-way move-ment design can also be performed quickly The MIP-SL processdeveloped based on such a recoating approach can achieve highfabrication speed for input CAD models The experimental resultsdemonstrate that the newly developed MIP-SL process can suc-cessfully fabricate 3D objects with satisfactory quality in a shorttime (usually in minutes)

Some future work to further improve the building speed of thedeveloped MIP-SL process includes (1) investigating the settingsof Tx and Twait_X based on given layers for the best performance(2) testing faster moving speeds in the X and Z axes and (3) test-ing the two-way movement design in prototyping systems withlarger XY extent sizes

References[1] Bourell D Leu M and Rosen D eds 2009 NSF WorkshopmdashRoadmap for

Additive Manufacturing Identifying the Future of Freeform ProcessingWashington DC March Available at httpwohlersassociatescomroadmap2009html

[2] Chatwin C Farsari M Huang S Heywood M Birch P Young R andRichardson J 1998 ldquoUV Microstereolithography System That Uses SpatialLight Modulator Technologyrdquo Appl Opt 37 pp 7514ndash7522

[3] Bertsch A Bernhard P Vogt C and Renaud P 2000 ldquoRapid Prototypingof Small Size Objectsrdquo Rapid Prototyping J 6(4) pp 259ndash266

[4] Stampfl J Fouad H Seidler S Liska R Schwager F Woesz A andFratzl P 2004 ldquoFabrication and Moulding of Cellular Materials by RapidPrototypingrdquo Int J Mater Product Technol 21(4) pp 285ndash296

[5] Sun C Fang N Wu D and Zhang X 2005 ldquoProjection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Maskrdquo Sens ActuatorsA 121 pp 113ndash120

[6] Lu Y Mapili G Suhali G Chen S C and Roy K 2006 ldquoA DigitalMicro-Mirror Device-Based System for the Microfabrication of Complex Spa-tially Patterned Tissue Engineering Scaffoldsrdquo J Biomed Mater Res A77A(2) pp 396ndash405

[7] Limaye A S and Rosen D W 2007 ldquoProcess Planning Method for MaskProjection Micro-Stereolithographyrdquo Rapid Prototyping J 13(2) pp 76ndash84

[8] Choi J Wicker R B Cho S Ha C and Lee S 2009 ldquoCure Depth Controlfor Complex 3D Microstructure Fabrication in Dynamic Mask ProjectionMicrostereolithographyrdquo Rapid Prototyping J 15(1) pp 59ndash70

[9] EnvisionTEC retrieved Sept 1 2011 httpwwwenvisiontecde[10] V-Flash desktop modeler retrieved Sept 1 2011 httpwwwmodelin3dcom[11] Chen Y Zhou C and Lao J 2011 ldquoA Layerless Additive Manufacturing

Process Based on CNC Accumulationrdquo Rapid Prototyping J 17(3)pp 218ndash227

[12] Huang Y M and Jiang C P 2005 ldquoOn-Line Force Monitoring of PlatformAscending Rapid Prototyping Systemrdquo J Mater Process Technol 159pp 257ndash264

[13] Denken Corp 1997 SLP-4000 Solid Laser Diode Plotter Product BrochureDenken Corporation Japan

[14] Dendukuri D Pregibon D C Collins J Hatton T A and Doyle P S2006 ldquoContinuous-Flow Lithography for High-Throughput Microparticle Syn-thesisrdquo Nature Mater 5 pp 365ndash369

[15] Zhou C Chen Y Yang Z G and Khoshnevis B 2011 ldquoDevelopmentof Multi-Material Mask-Image-Projection-Based Stereolithography for theFabrication of Digital Materialsrdquo Annual Solid Freeform Fabrication Sympo-sium Austin TX

[16] Jacobs P F 1995 Stereolithography and Other RPampM Technologies FromRapid Prototyping to Rapid Tooling Society of Manufacturing EngineeringDearborn MI

[17] Pan Y Y Zhou C and Chen Y retrieved Nov 23 2011 httpwwwyoutubecomwatchvfrac140TZWCwnvyew

Fig 18 Layer building time of the test cases

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-9

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

thickness a longer image exposure and projection waiting timeswere used (045 and 03 s respectively in the tests) Accordinglythe Z movement will also take a longer time for a larger layerthickness In the tests the movement time in the Z axis (TZ) is032 and 042 s for the layer thickness of 50 lm and 100 lmrespectively

The required moving distance in the X axis is related to the sizeand shape of the cured layer For a layer with a big cross-sectionalarea (eg the models of a head and a statue) the X translation dis-tance is set to a value that is close to the X extent size Due to thelarge movement the X waiting time was also set longer In com-parison for a layer with a small cross-sectional area (eg themodels of a hearing aid shell and the top portion of a brush) the Xtranslation distance can be much smaller than the extent size ofthe layer in the X axis However due to the fast moving speed inthe X axis the differences on TX are usually small (less than 1 s asshown in Table 1)

Two types of resins SI500 and Acryl R5 were tested Theircuring characteristics are slightly different For the same layerthickness the curing of Acryl R5 takes 01 s longer than that ofSI500 The viscosities of the two resins are also slightly differentHowever the same settings can be used in the two-way movementdesign based on the two resins

Figures 12ndash17 show the built objects based on the developedfast MIP-SL process The quality of the built objects was exam-ined to be satisfactory Both surface finish and dimension were an-alyzed to be acceptable In our prototyping system the nominalsize of a pixel is 47 lm The fine image resolution enables themesoscale features (ie in the range of 01ndash1 mm) to be well cap-tured in the built physical objects eg the lip of the human headthe cloth folds in the Beethoven statue and the dentures in theteeth model

72 Building Time Analysis All the models shown in Figs12ndash17 were built within 12 min using our prototyping system Themodels with less than 100 layers (eg the gear the teeth and thebrush) only require 2ndash3 min to be built A statistic of the buildingtime is given in Table 1

A much larger exposure time (eg 4ndash5 s) was required for thefirst few layers in order to build a base Consequently the built

objects and the build platform can be well bonded For all otherlayers as shown in Fig 18 the building time of a layer (TLayer) inour MIP-SL process is only 14ndash25 s The variation on TLayer ismainly due to different layer thicknesses and the X moving distan-ces For an average of 2 s per layer and a layer thickness of01 mm the building speed of the developed MIP-SL process is3 mm per minute or 180 mm per hour To the best of our knowl-edge such a MIP-SL process is one of the fastest layer-based

Fig 12 A test of a gear (a) CAD model and (b) built objects intwo liquid resins

Fig 13 A test of a head (a) CAD model and (b)ndash(c) two viewsof the built object

Fig 14 A test of a statue (a) CAD model and (b)ndash(e) two viewsof the built objects in two liquid resins

Fig 15 A test of teeth (a) CAD model and (b)ndash(c) built objectsin two liquid resins

Fig 16 A test of a hearing aide shell (a) CAD model and(b)ndash(c) two views of the built object

Fig 17 A test of a brush (a) CAD model and (b) built object

051011-8 Vol 134 OCTOBER 2012 Transactions of the ASME

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

additive manufacturing processes that have been developed Avideo of building the gear model as shown in Fig 12(a) can befound in Ref [17]

8 Conclusions

A novel mask-image-projection-based stereolithography pro-cess has been presented for fabricating 3D objects with fast build-ing speed The proposed approach is based on projecting maskimages bottom-up on a PDMS coated glass substrate A new two-way movement design has been presented for quickly spreadingliquid resin into uniform thin layers Such a design can signifi-cantly reduce the separation force between cured layers and theresin tank Experimental results verified that the separation forceas well as the sliding force is relatively small during the two-waymovement process The motions related to the two-way move-ment design can also be performed quickly The MIP-SL processdeveloped based on such a recoating approach can achieve highfabrication speed for input CAD models The experimental resultsdemonstrate that the newly developed MIP-SL process can suc-cessfully fabricate 3D objects with satisfactory quality in a shorttime (usually in minutes)

Some future work to further improve the building speed of thedeveloped MIP-SL process includes (1) investigating the settingsof Tx and Twait_X based on given layers for the best performance(2) testing faster moving speeds in the X and Z axes and (3) test-ing the two-way movement design in prototyping systems withlarger XY extent sizes

References[1] Bourell D Leu M and Rosen D eds 2009 NSF WorkshopmdashRoadmap for

Additive Manufacturing Identifying the Future of Freeform ProcessingWashington DC March Available at httpwohlersassociatescomroadmap2009html

[2] Chatwin C Farsari M Huang S Heywood M Birch P Young R andRichardson J 1998 ldquoUV Microstereolithography System That Uses SpatialLight Modulator Technologyrdquo Appl Opt 37 pp 7514ndash7522

[3] Bertsch A Bernhard P Vogt C and Renaud P 2000 ldquoRapid Prototypingof Small Size Objectsrdquo Rapid Prototyping J 6(4) pp 259ndash266

[4] Stampfl J Fouad H Seidler S Liska R Schwager F Woesz A andFratzl P 2004 ldquoFabrication and Moulding of Cellular Materials by RapidPrototypingrdquo Int J Mater Product Technol 21(4) pp 285ndash296

[5] Sun C Fang N Wu D and Zhang X 2005 ldquoProjection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Maskrdquo Sens ActuatorsA 121 pp 113ndash120

[6] Lu Y Mapili G Suhali G Chen S C and Roy K 2006 ldquoA DigitalMicro-Mirror Device-Based System for the Microfabrication of Complex Spa-tially Patterned Tissue Engineering Scaffoldsrdquo J Biomed Mater Res A77A(2) pp 396ndash405

[7] Limaye A S and Rosen D W 2007 ldquoProcess Planning Method for MaskProjection Micro-Stereolithographyrdquo Rapid Prototyping J 13(2) pp 76ndash84

[8] Choi J Wicker R B Cho S Ha C and Lee S 2009 ldquoCure Depth Controlfor Complex 3D Microstructure Fabrication in Dynamic Mask ProjectionMicrostereolithographyrdquo Rapid Prototyping J 15(1) pp 59ndash70

[9] EnvisionTEC retrieved Sept 1 2011 httpwwwenvisiontecde[10] V-Flash desktop modeler retrieved Sept 1 2011 httpwwwmodelin3dcom[11] Chen Y Zhou C and Lao J 2011 ldquoA Layerless Additive Manufacturing

Process Based on CNC Accumulationrdquo Rapid Prototyping J 17(3)pp 218ndash227

[12] Huang Y M and Jiang C P 2005 ldquoOn-Line Force Monitoring of PlatformAscending Rapid Prototyping Systemrdquo J Mater Process Technol 159pp 257ndash264

[13] Denken Corp 1997 SLP-4000 Solid Laser Diode Plotter Product BrochureDenken Corporation Japan

[14] Dendukuri D Pregibon D C Collins J Hatton T A and Doyle P S2006 ldquoContinuous-Flow Lithography for High-Throughput Microparticle Syn-thesisrdquo Nature Mater 5 pp 365ndash369

[15] Zhou C Chen Y Yang Z G and Khoshnevis B 2011 ldquoDevelopmentof Multi-Material Mask-Image-Projection-Based Stereolithography for theFabrication of Digital Materialsrdquo Annual Solid Freeform Fabrication Sympo-sium Austin TX

[16] Jacobs P F 1995 Stereolithography and Other RPampM Technologies FromRapid Prototyping to Rapid Tooling Society of Manufacturing EngineeringDearborn MI

[17] Pan Y Y Zhou C and Chen Y retrieved Nov 23 2011 httpwwwyoutubecomwatchvfrac140TZWCwnvyew

Fig 18 Layer building time of the test cases

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-9

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms

additive manufacturing processes that have been developed Avideo of building the gear model as shown in Fig 12(a) can befound in Ref [17]

8 Conclusions

A novel mask-image-projection-based stereolithography pro-cess has been presented for fabricating 3D objects with fast build-ing speed The proposed approach is based on projecting maskimages bottom-up on a PDMS coated glass substrate A new two-way movement design has been presented for quickly spreadingliquid resin into uniform thin layers Such a design can signifi-cantly reduce the separation force between cured layers and theresin tank Experimental results verified that the separation forceas well as the sliding force is relatively small during the two-waymovement process The motions related to the two-way move-ment design can also be performed quickly The MIP-SL processdeveloped based on such a recoating approach can achieve highfabrication speed for input CAD models The experimental resultsdemonstrate that the newly developed MIP-SL process can suc-cessfully fabricate 3D objects with satisfactory quality in a shorttime (usually in minutes)

Some future work to further improve the building speed of thedeveloped MIP-SL process includes (1) investigating the settingsof Tx and Twait_X based on given layers for the best performance(2) testing faster moving speeds in the X and Z axes and (3) test-ing the two-way movement design in prototyping systems withlarger XY extent sizes

References[1] Bourell D Leu M and Rosen D eds 2009 NSF WorkshopmdashRoadmap for

Additive Manufacturing Identifying the Future of Freeform ProcessingWashington DC March Available at httpwohlersassociatescomroadmap2009html

[2] Chatwin C Farsari M Huang S Heywood M Birch P Young R andRichardson J 1998 ldquoUV Microstereolithography System That Uses SpatialLight Modulator Technologyrdquo Appl Opt 37 pp 7514ndash7522

[3] Bertsch A Bernhard P Vogt C and Renaud P 2000 ldquoRapid Prototypingof Small Size Objectsrdquo Rapid Prototyping J 6(4) pp 259ndash266

[4] Stampfl J Fouad H Seidler S Liska R Schwager F Woesz A andFratzl P 2004 ldquoFabrication and Moulding of Cellular Materials by RapidPrototypingrdquo Int J Mater Product Technol 21(4) pp 285ndash296

[5] Sun C Fang N Wu D and Zhang X 2005 ldquoProjection Micro-Stereolithography Using Digital Micro-Mirror Dynamic Maskrdquo Sens ActuatorsA 121 pp 113ndash120

[6] Lu Y Mapili G Suhali G Chen S C and Roy K 2006 ldquoA DigitalMicro-Mirror Device-Based System for the Microfabrication of Complex Spa-tially Patterned Tissue Engineering Scaffoldsrdquo J Biomed Mater Res A77A(2) pp 396ndash405

[7] Limaye A S and Rosen D W 2007 ldquoProcess Planning Method for MaskProjection Micro-Stereolithographyrdquo Rapid Prototyping J 13(2) pp 76ndash84

[8] Choi J Wicker R B Cho S Ha C and Lee S 2009 ldquoCure Depth Controlfor Complex 3D Microstructure Fabrication in Dynamic Mask ProjectionMicrostereolithographyrdquo Rapid Prototyping J 15(1) pp 59ndash70

[9] EnvisionTEC retrieved Sept 1 2011 httpwwwenvisiontecde[10] V-Flash desktop modeler retrieved Sept 1 2011 httpwwwmodelin3dcom[11] Chen Y Zhou C and Lao J 2011 ldquoA Layerless Additive Manufacturing

Process Based on CNC Accumulationrdquo Rapid Prototyping J 17(3)pp 218ndash227

[12] Huang Y M and Jiang C P 2005 ldquoOn-Line Force Monitoring of PlatformAscending Rapid Prototyping Systemrdquo J Mater Process Technol 159pp 257ndash264

[13] Denken Corp 1997 SLP-4000 Solid Laser Diode Plotter Product BrochureDenken Corporation Japan

[14] Dendukuri D Pregibon D C Collins J Hatton T A and Doyle P S2006 ldquoContinuous-Flow Lithography for High-Throughput Microparticle Syn-thesisrdquo Nature Mater 5 pp 365ndash369

[15] Zhou C Chen Y Yang Z G and Khoshnevis B 2011 ldquoDevelopmentof Multi-Material Mask-Image-Projection-Based Stereolithography for theFabrication of Digital Materialsrdquo Annual Solid Freeform Fabrication Sympo-sium Austin TX

[16] Jacobs P F 1995 Stereolithography and Other RPampM Technologies FromRapid Prototyping to Rapid Tooling Society of Manufacturing EngineeringDearborn MI

[17] Pan Y Y Zhou C and Chen Y retrieved Nov 23 2011 httpwwwyoutubecomwatchvfrac140TZWCwnvyew

Fig 18 Layer building time of the test cases

Journal of Manufacturing Science and Engineering OCTOBER 2012 Vol 134 051011-9

Downloaded From httpasmedigitalcollectionasmeorg on 06192013 Terms of Use httpasmeorgterms