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Recent Developments in Fiber-Reinforced Additive Manufacturing of Injection MoldingInserts

Hofstätter, Thomas; Pedersen, David B.; Tosello, Guido; Hansen, Hans N.

Published in:Proceedings - 2018 ASPE and euspen Summer Topical Meeting: Advancing Precision in Additive Manufacturing

Publication date:2018

Document VersionPublisher's PDF, also known as Version of record

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Citation (APA):Hofstätter, T., Pedersen, D. B., Tosello, G., & Hansen, H. N. (2018). Recent Developments in Fiber-ReinforcedAdditive Manufacturing of Injection Molding Inserts. In Proceedings - 2018 ASPE and euspen Summer TopicalMeeting: Advancing Precision in Additive Manufacturing (pp. 110-115). American Society for PrecisionEngineering.

RECENT DEVELOPMENTS IN FIBER-REINFORCED ADDITIVEMANUFACTURING OF INJECTION MOLDING INSERTS

Thomas Hofstatter1, David B. Pedersen1,Guido Tosello1, Hans N. Hansen1

1Department of Mechanical Engineering, Technical University of DenmarkProduktionstorvet 427A2800 Kongens Lyngby

Denmark

Keywords: Additive Manufacturing, Vat Pho-topolymerization, Fiber-Reinforcement, InjectionMolding, Simulation

INTRODUCTIONDigitization and industry 4.0 (I4.0) processeshave increased the need and possibilities ofsmart, flexible, and cost efficient production. Fo-cusing this development to the injection molding(IM) industry, IM inserts manufactured from addi-tive manufacturing (AM) vat photopolymerization(VP) have been developed and increased theirmarket share over the past years. [1]–[3] The in-serts comprise the possibility of digital part de-velopment at low entry costs and therefore are atechnology used for pilot production.

The authors conclude that challenges regardingthe lifetime of polymer inserts in connection withthe involvement of thermal stresses, crack propa-gation, and molding cycle time can be tackled byfiber-reinforced AM. Lifetime has been increasedby a factor of 20 reaching an average lifetime of4.500 shots and an increase the attractivenessof the inserts for prototyping and pilot productionhas been achieved. So far, short, virgin, andunseized carbon fibers (CFs) with diameters of7.2µm and average lengths of 100µm as well asglass fibers (GFs) have been utilized in a numberof investigations.

Considering the increased lifetime of the dis-cussed inserts, further factors such as the mold-ing cycle time increase in their significance for fur-ther developments. The tested molding configu-rations included a 1.5 s mold closing and injection,7 s packing and cooling, 1.5 s mold opening andejection, followed by 10 s natural cooling. Bothpacking and cooling, as well as natural coolingare characterized by longer times as required forcompetitive materials such as brass, aluminum,

Step 1 Small AM Insertwithout Reinforcement

Step 2 Small AM Insertwith Composite Reinforcementa) classical, b) para-conformal cooling

Step 3 Multiscale AM Insertwith Composite Reinforcementa) low, b) high throughput cooling

FIGURE 1. AM IM insert development steps con-sidering fiber-reinforcement and cooling as cru-cial factor for an enlargement of the applicationspace.

steel, and silicium carbide (SiC) and are tackledby the presented research.

SYNTHESIS OF CURRENT CAPABILITIESThe authors have performed both experimentaland numerical investigations to face the chal-lenges described below facing moreover the chal-lenge of effusivity of polymer insert material in anIM process.

Heat Transfer and Cooling MechanismsNumerical simulations as well as investigationsusing a thermographic camera were conducted inorder to understand the modified cooling mecha-nisms of polymer inserts produced by VP. An un-derstanding of both convectional as well as liquidcooling was achieved for multiple insert develop-ment steps shown in Figure 1. [4], [5]

The inserts were modeled with dimensions of (20x 20 x 2.7)mm3 (small inserts) and (60 x 80 x10)mm3 (multiscale inserts) including effusivity inrelation to the thermal properties in comparison tothe injected acrylonitrile butadiene styrene (ABS)material. The interface temperature was calcu-

FIGURE 2. Temperature of conventional (left)and para-conformal (right) cooling channels at athroughput of 5 L/min. Doe to the improved flowa-bility of the fluid in the channels with rounded cor-ners, a higher throughput of up to 10 L/min is pos-sible.

lated as

Ti =b1T1 + b2T2b1 + b2

, (1)

bi =√kiρicpi . (2)

Whereas:

Ti = temperature at interfaceki = thermal conductivityρi = densitycpi

= heat capacity

This resulted in an interface temperature withABS of 32.2 ◦C (steel), 121.0 ◦C (photopolymer),30.0 ◦C (brass), and 27.4 ◦C (SiC) whereas a 4-fold difference between the photopolymer and thecomparison materials can be noted and signifi-cantly influences the insert temperature. [4]

Investigations confirmed the crucial importanceof advanced cooling mechanisms such as para-conformal cooling where the standardized moldgeometry is modified by techniques availablefor conformal cooling providing a uniform sur-face temperature and therefore reducing thermalstresses. The amount of heat removed fromthe system by the cooling liquid has been in-creased from 2.9 kW to 5.7 kW after the changefrom conventional cooling to para-conformal cool-ing shown in Figure 2. This factor reaches signif-icance as insert and part volumes increase whileat the same time utilizing the freedom of designthat comes with AM techniques.

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Standard CoolingEnhanced Cooling

Enhanced Cooling - HT

MultiscaleMultiscale - HT

FIGURE 3. Surface temperature of small AM in-serts with standard cooling, enhanced cooling,with and without high throughput (HT, 10 L/min),and multiscale inserts.

By the use of enhanced cooling, the overall tem-perature of the insert was reduced (as shown inFigure 3), which allows a reduction of the natu-ral cooling time by 6 s while starting the moldingcycle at the same insert temperature. The useof larger insert geometries corresponds to longercooling times, which can be reduced by an ad-vanced cooling of the standardized steel mold inthe background.

Fiber Orientation and ControlConventional VP processes include a flat buildplate in a bottom-up or top-down machinewhereas the part is manufactured in layers atusual heights of lower 2-digit µm forcing the fibersinto a layered orientation by removing one degreeof freedom (DOF).

Simulations have been conducted predicting thefinal fiber orientation as a result of the fluid flow ofthe resin due to moving boundaries representingthe build plate. Several potential solutions havebeen found to control the fiber orientation and in-crease the orientation angles out of the build plateorientation. [6]

Streamline analysis was performed according to[7]–[9] predicting the fiber orientation in bottom-up and top-down technologies including a mod-ified build plate design. The usually horizon-tal alignment of fibers in bottom-up printing pro-cesses can be changed in top-down processesand isotropic fiber orientation can be achievedby a modified build plate with cylindrical, verticalholes shown in Figure 4.

Practical experiments at an open top-down ma-chine have shown furthermore an improvement inwarping of the free surface due to surface tension

FIGURE 4. Streamlines as fiber orientation pre-diction (top) and vertical velocity factors normal-ized by the build plate velocity (bottom) for a mod-ified geometry with vertical cylindrical holes in thebuild plate.

of the thermoset photopolymer resin when usingmodified build plates. Due to the suction effectof the moving build plate, a velocity is induced inthe fluid, which again results in a warped surface.The waiting time for relaxation of the surface ten-sion effect has been reduced by 50% when us-ing the modified build plate design. The resultsalso allow the conclusion that fiber orientation ina thermoset photopolymer can be controlled bythe orientation of the part in the resin vat.

FAILURE MECHANISMSCracks represent the biggest threat for insert life-time and their understanding is crucial for a fur-ther enhancement of lifetime and product quality.

Fiber-Matrix Interface and Internal DefectsInvestigations on thermal curing mechanisms ofmulti-component photopolymer resins have beenconducted facing the low level of chemical bond-ing of CFs (and less GFs) to the curing polymer. Itcould be concluded that a combination of thermaland optical energy introduction during the printingprocess allows an improved fiber-matrix interfacequantified by an increased strength and stiffnessof the final part.

Visualized in Figure 5, it was possible to in-crease the specific load at break by a factor

0

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/mm

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Young

s M

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ulu

s in

N/m

m2

Curing Temperature in degC

Tensile Tests at 5 %wt Fiber Content (DIN EN ISO 527-2BB)

Specific Load at BreakYoungs Modulus

FIGURE 5. Tensile properties of a thermoset pho-topolymer with 5%wt glass fiber content at differ-ent curing temperatures utilizing both thermal andoptical curing of the photopolymer.

of up to 3 at certain cases utilizing both opti-cal as well as thermal curing of the thermosetphotopolymer filled with 5%wt glass fiber. 50%trimethylolpropane triacrylate containing 600 ppmmonomethyl ether hydroquinone as inhibitor incombination with 50% 2-Hydroxyethyl methacry-late containing 250 ppm monomethyl ether hy-droquinone as inhibitor and 0.2% Irgacure819 (chemical name: Bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide) was utilized as thermosetphotopolymer. Youngs modulus was improved athigher temperatures whereas a significant differ-ence was measured at 80 ◦C following investiga-tions presented in [10] concluding a gelation ofthe thermoset photopolymer at higher tempera-tures.

Other weakening factors such as air intrusionshave been investigated using X-ray computed to-mography (CT) whereas a significant potentialfor improvement has been identified, which canbe realized by reducing the air intrusion dur-ing both the mixture and the manufacturing pro-cess. Potential solutions under investigation area modified stirring mechanism as well as a vac-uum mixture, which is already in use e.g. forcold-molding of probes for scanning electron mi-croscopy (SEM). [11]

Crack Propagation and Fiber AlignmentIt could be shown by the use of SEM, CT, and op-tical microscopy analysis that two types of crackscan be identified: (1) smaller surface cracks prop-agating over the heated surface in a random ori-entation perpendicular to the surface; and (2)deep cracks penetrating the insert up to 1.8mm,oriented in parallel to the printed layers (if exist-ing, and therefore also in parallel to the fiber ori-

entation) and finally leading to a total failure ofthe part. Examples are given in Figure 6 show-ing both randomly oriented surface cracks as wellas penetrating cracks oriented along the printedlayer and therefore the fibers. [11], [12]

X-ray tomography was conducted using a ”ZEISSXRadia 410 Versa” device at an operation volt-age of 40 kV and a power of 10W using low en-ergy 1 (LE1) filter and a macro objective (LFOV)or a 4X objective. A resolution of 2µm was ob-tained as highest resolution. Prior investigationson the fiber orientation during a layer-wise man-ufacturing process in a bottom-up machine wereconfirmed using an elliptic approximation of theresolved fibers following the method presented in[13]. [11] A tensor estimation for one of threesample regions is shown in Figure 7 for fiber ori-entation outside of the printing plane. Orientationalong the printed plane is randomly distributed.

Surface cracks have a limited influence on thesurface quality of the final product if the injectionpressure is reduced and the representation of thesurface is limited. Otherwise, the importance ofsurface quality needs to be considered in the con-text of an internal prototyping process. Surfaceroughness was concluded to be stable at an av-erage of Ra = 0.45 µm.

It could be shown that fibers have a significantinfluence on the crack propagation which is re-duced to 1.25% as compared to a non-reinforcedpolymer. The concluded crack orientation anddimensions lead to the understanding that anisotropic fiber orientation should be achieved forfurther development of AM IM inserts since thematerial deals both with periodical mechanicaland thermal stresses during the process.

Life Cycle and Environmental FactorsA full life cycle assessment (LCA) has been con-ducted investigating the influence of CFs on theenvironmental impact of IM inserts in comparisonto conventional materials such as brass, steel,and aluminum under the consideration of contri-butions by [14]–[17]. The improved lifetime playsan important role in the reduction of the investi-gated factors of global warming potential (GWP)and human toxicity (HT). Both AM and computernumerical control (CNC) machining were investi-gated in a comparison assessment which did notinclude the injection molding process itself sincethe process is identical for both process chains.The boundaries are shown in Figure 8 including

0.65 mm0.64 mm

0.12 mm

1.2 mm

0.27 mm

0.23 mm

2.85 mm

0.43 mm

1.2 mm

0.27 mm

0.23 mm

2.85 mm

0.43 mm

FIGURE 6. Deep cracks aligned along the printedlayers and propagated due to the corner effect ofthermal and mechanical stresses (top) and ran-domly oriented surface cracks (middle) with ahigher density on the right side of the image dueto the orientation of the material injection from theright. Individually identified fibers in an overview(bottom).

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-3 -2 -1 0 1 2 3

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bab

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Θ

Structure Tensor Estimate Position 1

FIGURE 7. Structure tensor estimation outsidethe printing layers. Due to geometrical restric-tions in a bottom-up printing process, only a slightangle between the printed plane and the fiber ispossible.

FIGURE 8. System boundaries of the conductedLCA with exemption of the IM process itself.

the influencing factors such as materials used inthe production process and waste generation.

The LCA showed a huge potential of productionoptimization in such a way that the amount ofwasted resin is reduced therefore especially re-ducing the HT level. Moreover, there are currentlyno sustainable end of life (EoL) scenarios avail-able. [18]

CONCLUSIONWhile industrial applications slowly start their ap-pearance on the market for non-fiber-reinforcedIM inserts, fiber-reinforcement has shown to bringfurther improvement to the material. With anoverall understanding of the crack propagationmechanisms in correlation to the fiber orientationas well as a predictive description of flow mech-anisms in the resin vat, a determined fiber orien-tation can be achieved helping to further improvethe mechanical and thermal properties of the in-serts. These properties play an important role in

parameters for further industrial production whendirectly influencing the molding cycle time. Sincethe natural cooling time has been reduced by over50% and LCA showed a significant reaction onthe lifetime of the inserts, they can be considereda sustainable method for future production in asmart, digital fabrication environment.

ACKNOWLEDGMENTSThe Technical University of Denmark, Depart-ment of Mechanical Engineering is gratefully ac-knowledged for funding the PhD project ”Addi-tive Manufacturing of Fiber-Reinforced Polymers”.The research on X-ray CT presented in this pa-per has been conducted in cooperation with the3D Imaging Center at DTU in the frameworkof the activities of the Manufacturing Academyof Denmark (MADE, http://en.made.dk/), WorkPackage 3 ”3D Print and New Production Pro-cesses”, funded by Innovation Fund Denmark(https://innovationsfonden.dk/en).

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