ACCURACY AND PRECISION OF 3-DIMENSIONAL PRINTED …
Transcript of ACCURACY AND PRECISION OF 3-DIMENSIONAL PRINTED …
ACCURACY AND PRECISION OF 3-DIMENSIONAL PRINTED DENTAL MODELS PRODUCED BY DIFFERENT ADDITIVE MANUFACTURING TECHNOLOGIES.
Greice Oliveira
A thesis submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Master of Science in the Department of Orthodontics in the School
of Dentistry.
Chapel Hill 2019
Approved by:
Tung Nguyen
Tate Jackson
Angela Broome
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© 2019 Greice Oliveira
ALL RIGHTS RESERVED
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ABSTRACT
Greice Oliveira: Accuracy and Precision of 3-Dimensional Printed Dental Models Produced by Different Additive Manufacturing Technologies (Under the direction of Tung Nguyen)
The primary aim of this investigation was to evaluate the accuracy of dental models manufactured
by different additive manufacturing processes: Stereolithography Apparatus (SLA), Liquid Crystal
Display with Unidirectional Peel (LCD-SLA/UDP), and Digital Light Processing (DLP). The secondary
objective was to test the accuracy of the models printed at different Z-plane resolutions. To accomplish
these goals, twelve 3-D digital surface models were selected, exported as surface model stereolithography
files, and printed using the additive manufacturing processes mentioned. Models were printed at 50 and
100µm resolution. The printed models were rescanned to create digital STL models and superimposed to
the original scan. No statistically significant difference between layer thicknesses was found for the SLA,
and for the DLP. The LCD-SLA/ UDP technology showed statistical significant differences between
different layer thicknesses. The SLA technology showed statistically significant difference from DLP and
LCD-SLA/ UDP. No significant difference was observed between DLP and LCD-SLA/ UDP.
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ACKNOWLEDGEMENTS
Thank you to my committee members, Dr. Nguyen, Dr. Jackson, and Dr. Broome, for your
guidance throughout this project. Thank you to my mentor, Dr. Nguyen, whose expertise was invaluable
in the formulating of the research topic and methodology. You provided me with the tools that I needed to
successfully complete my dissertation. Thank you to Ceib Phillips for her expertise in statistics. Thank
you to Nicki Parkstone and Cary Shepherd for their help and dedication with the laboratory work. Thank
you to the Southern Association of Orthodontists for the awarded research grant. A very special thank you
to my parents, Dejair and Cleide, for their constant support, and to my loving husband, Gustavo, and my
two daughters, Antonia and Cecilia for providing unending inspiration.
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TABLE OF CONTENTS
LIST OF TABLES …………………………………………………………………………….…………vii
LIST OF FIGURES …………………………………………………………….……………………….viii
LIST OF ABBREVIATIONS …………………………………………………………………………….ix
LIST OF SYMBOLS ………………………………………………………………………….…………..x
REVIEW OF THE LITERATURE …………………………………………………………….……….…1
Plaster Versus Digital Models ……………………………………………………………….….…1
3D Printing Technology ……………………………………………………………………..…….4
3D Manufacturing Technologies classification ……………………………………………..…….4
Additive Manufacturing Technologies …………………………………………………….….…..5
3D Printing Resolution ……………………………………………………………….………..…..6
Influence of Layer Thickness on 3D Printing …………………………………………..…………7
Accuracy of 3D Printed Dental Models ……………………………………………….….……….9
Clinical Applications of 3D Printed Dental Models ……………………………………………..10
References ………………………………………………………………………………………..12
ACCURACY AND PRECISION OF 3-DIMENSIONAL PRINTED DENTAL MODELS PRODUCED BY DIFFERENT ADDITIVE MANUFACTURING TECHNOLOGIES………………...15 Introduction ……………………………………………………………………………..………..15
Methods …………………………………………………………………………………..………17
Results ……………………………………………………………………………………...…….18
Discussion …………………………………………………………………………………..……19
Conclusion ………………………………………………………………………………….……22
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References……………………………………………………………………………………...…23
APPENDIX 1: TABLES AND FIGURES ………………………….……………………………………25
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LIST OF TABLES
Table 1: Difference in overall accuracy between different layer thicknesses for each 3D printer………..25
Table 2: Difference in overall accuracy between 3D printers by different layer thicknesses …………….26
Table 3. Photopolymer resins based on the manufacturers recommendations……………………………26
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LIST OF FIGURES
Figure 1: Study methodology flowchart ……………………………………………………………….....27
Figure 2: Chart comparing the 3D printing technologies and printers investigated in this study.……...…28
Figure 3: Close up view of models printed at 50 micron and 100 micron resolution ………………….....29
Figure 4: 3M™ High-Resolution Scanning Spray applied on printed models. Close up view of the spray coating…………………...…………………………………………………………………….……29
Figure 5: Superimposition of 3D printed models to their original counterpart …………………………..30
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LIST OF ABBREVIATIONS
3D Three Dimensional
AM Additive Manufacturing
RP Rapid Prototyping
CAD Computer-Aided Design
SLA Stereolithography Apparatus
DLP Digital Lighting Processing
FDM Fused Deposition Modeling
SLS Selective Laser Sintering
LCD-SLA/UDP Liquid Crystal Display with Unidirectional Peel
UV Ultraviolet
STL Stereolithography
ANOVA Analysis of Variance
SD Standard Deviation
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LIST OF SYMBOLS
© Copyright symbol
® Registered Trademark
™ Trademark symbol
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A REVIEW OF THE LITERATURE
Plaster versus digital models
Successful orthodontic treatment is dependent on comprehensive diagnosis and treatment
planning, and an accurate model analysis is critical for correct diagnosis. Three-dimensional (3D)
imaging has seen significant advances recently, allowing clinicians to develop the virtual orthodontic
patient. Advancements in 3D facial imaging, cone-beam computed tomography (CBCT), scanning
techniques, and dental model technology facilitate accurate reconstructions of teeth, soft tissue, and
bone.1
Plaster models are certainly the historic gold standard in dental diagnosis and treatment
procedures. Is has been shown that they present precise and reliable information about the patient’s dental
arches, teeth position and its dimensions. However, its use comes with a few important disadvantages.
Plaster models require physical storage space, they are prone to breakage and degradation over the years,
they lack in terms of efficiency when recovery is needed, and they represents a hurdle when sharing data
with other professionals is needed for multidisciplinary treatment.2-5
The benefits of using digital models in orthodontic practices are numerous. Primarily, digital
models are less labor intensive. Orthodontists can immediately send a digital scan of the teeth and
surrounding oral structures to an outside laboratory or share it with another dentist in cases which
multidisciplinary care is involved. Similarity, a digital diagnostic or treatment simulation can be
completed or models can be superimposed to evaluate treatment progress without risk of degradation or
loss of the model. There is no demand for physical storage as the patient data can be digitally archived
and only printed when needed.5-8
Digital study models were introduced commercially in 1999 by OrthoCad™ (Cadent, Carlstadt,
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NJ, USA), and in 2001 by E-model™ (GeoDigm, Chanhassen, MN, USA). New technologies were
introduced to the market later. 6 Currently, different technologies can be used to generate digital study
models, which may render discrepancies in accuracy between plaster and digital models.9
In 2003, Santoro et al. conducted a study to compare OrthoCad™ digital models to plaster
models. Two alginate impressions were taken from seventy-six subjects. One impression was
immediately poured to fabricate a standard plaster model, while the second impression was sent overnight
to OrthoCad™ to be digitized. Measurements such as tooth size, overjet, and overbite were taken from
the physical and digital models to the nearest 0.1mm. Physical models were measured with a Boley
gauge, and OrthoCad™ models were measured with OrthoCad™ software. The results regarding tooth
size and overbite showed significantly smaller measurements for digital models when compared to their
plaster counterpart. There was no statistically significant difference for overjet measurements. The
authors concluded that despite the differences reported, digital models produced by OrthoCad™ would be
considered an acceptable option for the purpose of routine diagnosis.4
In a similar study by Stevens et al. conducted in 2006, conventional plaster models were
compared to their digital models produced by E-Model™ software. Twenty-four subjects with eight
malocclusion types were enrolled in the study. Mean differences were recorded for Peer Assessment
Rating (PAR) and Bolton analysis. Measurements were recorded to the nearest 0.01mm. Conventional
models were measured with a digital caliper, while digital models were measured with the use of E-
Model™ software. Mean differences ranged from 0.04 - 0.83 for PAR index and from 0.04 - 0.38mm for
Bolton analysis. Although statistically significant differences were found between the 2 methods, the
authors concluded that no measurement associated with PAR index or Bolton analysis appeared to be
clinically significant, and that digital models are not a compromised choice for treatment planning and
clinical diagnosis.10
In 2011, Fleming et al. conducted a systematic review that included literature to assess validity of
the use of digital models. Various databases including MEDLINE, LILACS, BBO, ClinicalTrials.gov, the
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National Research Register and Pro-Quest Dissertation Abstracts and Thesis database were used to search
studies that would compare linear and angular measurements from conventional plaster and digital
models. Forty abstracts were initially selected. However, only 17 of them met the inclusion criteria. The
mean difference between measurements on digital versus plaster models was low. In all studies, the
differences were considered to be clinically insignificant. The authors concluded that digital models offer
a high degree of validity when compared to direct measurement on plaster models, and can be considered
clinically acceptable. However, the overall quality of the selected studies was variable, with generally
inadequate descriptions of the sample populations and rare reports of confidence intervals and standard
errors between different techniques.6
Another review by Luu et al., conducted in 2012, evaluated intrarater reliabilities in terms of
mean differences, intraclass correlation coefficient (ICC), and Pearson correlation coefficients (PCCs) of
measurements of digital models compared with plaster models. Agreement of measurements was
excellent. The authors were in agreement with Fleming et al., concluding that digital models are clinically
acceptable compared with plaster models in respect to intrarater reliability and validity of the selected
linear measurements.11
A more recent systematic review published in 2016 by Rossini et al. was conducted to assess
accuracy, validity, and reliability of measurements of digital versus plaster models. The databases used to
search for peer-reviewed articles included PubMed, PubMed Central, National Library of Medicine
Medline, Embase, Cochrane Central Register of Controlled Clinical trials, Web of Knowledge, Scopus,
Google Scholar, and LILACs. Thirty-five articles were selected. This review is an update from previous
reviews, with more articles with a lower risk of bias, and the addition of articles containing qualitative
ordinal measures (PAR, ABO Objective Grading System, and ICON). For most studies, no significant
differences in measurements were found with the exception of the Objective Grading System (ABO).12
With regard to this grade system, studies reported that digital models should be substituted by plaster
models for calculation of this index due to incorrect articulation and difficulties in landmark identification
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on digital models, and bias in the software used to calculate the ABO Objective Grading System.13-16
More recently, an improvement in this grading system was observed. Although the virtual setup model
resulted in greater deductions in overjet, occlusal contact, and total score compared with the plaster setup
model, the total reductions were not greater than 25, which represent the threshold for passing. This
finding provides evidence that supports clinical acceptance.13 In view of the many advantages such as
cost, time, and storage space required, digital models could be considered the new gold standard in
current practice.
3D printing technology
The evolution of 3D imaging and modelling in dentistry is progressing towards a more efficient
and cost-effective workflow using state-of-the-art technology. The practicability of this technique is
expanding in several dental fields such as prosthodontics, oromaxillofacial surgery and prosthesis, and
production of surgical guides or physical models in dental implant treatment. Three-dimensional printed
models can also be used to fabricate orthodontic appliances. Over the last years, the traditional
orthodontic workflow is shifting to a digital format. The 3D printing technology is becoming more
affordable, faster, and it is claimed to be able of producing dental models with a high resolution and
accuracy.
3D manufacturing technology classification
The 3D manufacturing process can be classified as subtractive or additive manufacturing.
Subtractive manufacturing is based on milling methodology while additive manufacturing is based on
layering methodology, usually joining materials layer upon layer. In subtractive manufacturing, a
prefabricated block of material is milled with the use of burs, diamonds, or diamond disks to manufacture
the desired shaped part.17 Although subtractive manufacturing can create complete shapes effectively, it is
not an economic alternative since material is wasted during the process. Around ninety percent of the
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prefabricated block is removed to fabricate a dental restoration, for instance.18 Other limitations can be
attributed to this method. Milling tools can only withstand short running cycles due to exposure to heavy
abrasion and wear. Also, the precision depends on the size of the smallest milling tool for each
material.19,20
The additive manufacturing methodology has the potential to overcome these limitations. In additive
manufacturing, the final physical part is fabricated by adding layer upon layer of material to form a 3D
object. The 3D computer-aided design (CAD) model is sliced into many layers and the geometric data
provided by the equipment software is used by the 3D printer to build each layer sequentially until the
part is completed. The use of this technique allows the fabrication of complex parts that include undercuts
and cavities, which is seen in dental anatomy. 21,22 Additionally, additive manufacturing is more cost-
effective since less material is used, and more efficient since more than 1 print can be done
simultaneously. Nowadays, there are a vast number of 3D printers with the ability to print various 3D
objects using different technologies. 23
Additive manufacturing technologies
A stereolithography apparatus (SLA) was one of the first additive manufacturing technologies to
be theorized by in the early 1980s, when Japanese researcher Dr. Hideo Kodama invented the modern
layered approach to stereolithography by using ultraviolet light to cure photosensitive polymers. 24,25 This
technique was then patented by Chuck Hull in 1984. 26 The process was defined as a "system for
generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed". The
SLA is a printing technique where an ultraviolet laser cures resin-based material in a desired shape. This
technique builds parts one layer at a time, in a vat of photopolymer resin. Each layer is traced-out and
light-cured by the laser on the surface of the liquid resin, at which point a build platform moves according
to the layer thickness desired, and another layer of resin is wiped over the surface. The process is then
repeated until complete. Supports are needed to attach overhanging parts to the build platform, prevent
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deflection due to gravity, and retain newly created sections during the peel process. The support is created
by the 3D printer software during the preparation of the 3D models and should be removed manually once
the printing process is concluded. Once printing is completed and the parts have reached their final shape,
the photopolymerization reaction is not yet driven to completion. At this point, mechanical and thermal
properties are not fully set. Approximately 80% of the total polymerization takes place while the part is
built in the 3D printer. Post curing is necessary to complete the remaining 20% by means of a
conventional UV curing unit. This enables parts to reach the highest possible strength and become more
stable. 27,28
Digital Light Processing (DLP) is a similar process to SLA. The major difference is the light
source. DLP uses an arc lamp with a liquid crystal display projector, which is applied to the entire surface
of the vat of photopolymer resin in a single pass, making the printing time significantly faster when
compared to SLA technology. The similarities also include the use of support to attach the printed parts
and the post-curing process to complete the solidification of the printed part.
Liquid Crystal Display-Stereolithography with Unidirectional Peel (LCD-SLA/UDP) technology
carries the same fundamentals from SLA. The LCD technology works similarly to DLP. However,
instead of using a light projector which beams light outward, a LCD screen beams light in straight lines.
Due to the nature of light reaching outward from a DLP projector, the light essentially creates a curved
lens or magnifying glass effect. As a result, the wider the cross section of the model, the lower the
resolution will be. In contrast, an LCD screen resolution is uniform across the entire area so it does not
present the same issue. What this allows is to print multiple models simultaneously, spread out around the
entire build platform, while maintaining the quality. The DLP technology would not be able to manage
this since objects towards the edge of the build platform would start to degrade in quality. As far as UDP
mode is concerned, it is an experimental technology that aims to produce large models extremely quickly.
Uni-directional peeling technology was created in an attempt to reduce the up-and-down peel to one-
directional peel action. However, due to its geometric limitations, it has seen limited marketability and
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usability in the dental field. 29
3D printing resolution
The 3D printer axis systems provide the object depth and design and are represented by the X, Y,
and Z planes. The XY plane is also known as the horizontal plane, while the Z plane is known as the
vertical plane. Normally, the X and Y-axes correspond to lateral movement while the Z axis corresponds
to vertical motion during the printing process. In other words, X and Y-axes refer to those that are parallel
to the 3D printer build platform. On the other hand, the Z-axis represents the direction in which the
printed part is built, which is perpendicular to the build platform. The Z resolution was the first major
numerical differentiation between early 3D printers. 30
The resolution that is measured microscopically in the XY dimension is described as the
minimum feature size. This is represented by the diameter of the laser beam and radical polymerization
kinetics in SLA technology and by the size of the pixel for technologies such as DLP and LCD-SLA, in
which a screen projector is used to solidify the liquid polymer. The resolution recommended for
orthodontic purposes should be equivalent to a minimum of 100 µm. Most resolution reports are related to
the Z dimension, represented by the layer thickness.
Influence of Layer Thickness on 3D printing
Generally, the layer thickness is based on the morphologic features and intended use of the object
to be printed. This feature is controlled by how much the 3D printer build platform elevates after a layer is
cured. Usually, if high accuracy is needed, the layer thickness is minimized. On the other hand, if cost and
time are more relevant, thicker layers are used. Improved resolution can be manifested by a smoother
surface finish and greater detail. Higher resolution in the z-direction corresponds to a reduced layer height
on a printed part. It is often assumed that an improved resolution should also equate to improved
accuracy. Although smaller layer height should result in a printed piece with more detail and improved
surface finish, it cannot be assumed that the printed piece has an improved accuracy compared to models
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presenting increased layer height or lower resolution. Also, high resolution comes with a tradeoff.
Thinner layers will require more repetitions, which in turn means more opportunities for artifacts and
errors. Layer thickness selection must provide a satisfactory balance between speed and resolution. 31
It was not until 2017 that Favero et al. published a study that investigated the effect of layer
thickness on the accuracy of 3D printed dental models. Thirty-six typodont models were scanned and then
fabricated by a SLA printer. Three layer thicknesses were evaluated: 25, 50, and 100 µm (N=12).
Forty-eight additional models were printed using 4 commercially available 3D printers: Juell 3D (12
models printed with 100 µm layer), Objet Eden260V Dental Advantage (12 models printed with 28 µm
layer), large-frame Vector 3SP (12 models printed with 100 µm layer height), and Perfactory Desktop
Vida (EnvisionTEC) (12 models printed with 100 µm layer height). All printed models were digitally
scanned and superimposed using a best-fit algorithm to assess accuracy. As the layer thickness decreased,
the deviation values increased, with the 25 µm layer models showing the greatest deviations, and the 100
µm layer models showing the least deviation. The authors attributed that to the increased potential for
errors and artifacts as the quantity of layers are added. An inconsistent X-Y resolution may have also
produced surface irregularities. Similarly, as the number of layers increased, it also increased the amount
of potential variation cause by the x-y resolution. Although statistically significant differences were found
in the average overall deviations, all values were considered clinically acceptable. Based on this study, it
can be stated that increased resolutions do not necessarily mean increased accuracy. Factors such as cost,
efficiency, surface quality, and accuracy should be considered when selecting layer thickness.30
More recently, a study was conducted by Loflin et al. to compare the accuracy of 3D printed
models and traditional plaster models using three layer heights: 25, 50, and 100 µm. Twelve sets of stone
models previously submitted as ABO cases were digitally scanned and then printed on a SLA Form2
printer using three layer heights. All stone models and 3D printed models were scored by 4 faculties and 7
residents from the Department of Orthodontics at the University of Texas Health Science Center at
Houston School of Dentistry. The ABO Cast-Radiograph Evaluation (CRE) grading system was used to
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score each model. Based on that, 8 criterias were evaluated: alignment/rotations, marginal ridges,
buccolingual inclination, overjet, occlusal contacts, occlusal relationships, interproximal contacts, and
root angulation.31
Models with a 100 µm layers showed the strongest correlation of total score to stone models,
while 25 µm models showed the weakest correlation of total score. The 100 µm layer models were highly
correlated to stone models in regards to buccolingual inclination, overjet, occlusal relationship, and
interproximal contacts, and presented a weak correlation in regards to alignment/rotations and marginal
ridges. All models showed very strong correlation with stone models for occlusal contacts.31
Accuracy of 3D printed dental models
In order to incorporate the use of 3D printed models in orthodontics, accuracy must be warranted.
Accuracy can be determined by the superimposition of the printed model to the original CAD model and
it is reported as a percentage of its original volume. Precision is the closeness of the superimposition of
repeatedly printed models. In other words, it can be referred to the reproducibility of the printer to
fabricate multiple models with the exact same dimensions. Precision is especially important when
fabricating clear aligners.
The accuracy of 3D printed models can range from 20 to 100 µm. This can be influenced by the
3d printer technology and its resolution. Dietrich et al reported a superior precision, but less accuracy for
SLA technology when compared with Polyjet. Although Kim et al showed better trueness values for DLP
and Polyjet, SLA had better accuracy when the occlusion was evaluated. However, it is important to
recognize the discrepancy observed in the results from those studies ranged from 20 to 50 µm, which may
not be significant for many clinical applications. Also, since then the laser resolution of the current SLA
3D printers have improved.33,34
Other factors can also influence the accuracy of a 3D printed part. The material selection can play
a role on that. The use a flexible resin, like those used for indirect bonding trays have less dimensional
stability than the standard rigid resins. If the resin does not flow properly, the cured layer will be uneven.
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The color can also affect it since the more transparent the resin is, the easier for the laser to pass through
and cure undesired areas, introducing artifacts.
Accuracy can also be altered by the print orientation. Horizontal prints are known to be more
accurate than vertical ones due to shrinkage of unsupported parts. To avoid distortion, current 3D
software will add the support to the printed part.
It is critical that the build platform rises perfectly parallel. Otherwise it will create distortion and
may skew the print. Different technologies are used to allow this parallel raise of the platform. Despite of
the mechanism used, 3D printers should be calibrated in a regular basis to provide reliable prints. The
intervals between calibrations should not exceed 3 to 4 months.
Clinical Applications of 3D printed dental models
The use of 3D printed models in orthodontics is not limited for diagnosis and treatment planning
but also include the fabrication of indirect bonding trays, metal appliance fabrication, retainers and clear
aligners. Two different methods can be used to fabricate indirect bonding trays. One method consists of
printing the dental model with the brackets. The indirect tray is then fabricated with the material of
choice, and the brackets are inserted into the tray. The materials used can be silicone-based, thermoplastic
or dual materials. The second option is to directly print the indirect bonding tray. The disadvantage of this
method is that the absence of a custom base may allow for excess of composite around the brackets. For
both methods, the patient’s dentition is digitized and the software provides the clinical outcome of the
orthodontic treatment and based on that position the brackets in a “straight-wire” setup, but also allowing
the clinician to change the position of the bracket as desired. The bracket position relative to each tooth is
then transferred to the initial alignment. Unfortunately, there are only a few companies that fabricates
these trays with transparent material to allow proper light curing while also providing adequate material
flexibility to prevent debonding of brackets during tray removal. Additionally, literature is still lacking
data on accuracy and precision of 3D printed indirect bonding trays.
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Metal appliances can be produced by Selective Laser Sintering (SLS) technology. This technology
can use several materials such as Stainless Steel, Cobalt Chromium, T64 Titanium, porcelain, plastics,
and glass, providing high strength and stiffness to the printed part. Orthodontic metal appliances such as
skeletal anchorage attachments, lingual holding arches, Rapid Palatal Expanders can be fabricated using
SLS. However, a multi-piece part will often involve a 2-step process in which bands, support arms, and
lingual pads are 3D printed and then welded to the commercially available expansion screw to complete
the RPE appliance.35
As a result of the more reasonable prices of current 3D printers and the availability of commercial
software that will align the teeth and produce sequential models following the teeth movement, it is
possible to fabricate “in-office” clear aligners. The scan of the patient’s dentition is imported into a
software such as OrthoAnalyzer (3Shape, Copenhagen, Denmark), Orchestrate 3D (Orchestrate
Orthodontic Technologies, Rialto, CA), or OnyxCeph (Image Instruments, Chemnitz, Germany). After
models are cleaned, oriented and based, the teeth are segmented so it can be positioned to achieve ideal
alignment. The number of trays fabricated is calculated by the software based on tooth movement and its
preset algorithm. Another option is to use online services such as AccuSmile, ArchForm or ARCAD. The
setup is completed by them and the STL files with the tooth movement is sent to the clinician for a cost.
Also, companies are developing resins that will permit to directly print clear aligners, making the process
more efficient and cost-effective.36
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digital study models. Biomed Eng Online 2013;12:1-11.
24. Hideo Kodama. A Scheme for Three-Dimensional Display by Automatic Fabrication of Three Dimensional Model. IEICE Transactions on Electronics (Japanese Edition) 1981; J64-C:237–41.
25. Hideo Kodama. Automatic method for fabricating a three-dimensional plastic model with photo-hardening polymer. Review of Scientific Instruments 1981; 52: 1770–73. 26. Hull CV. Apparatus for production of three-dimensional objects by stereolithography. U.S. Patent No. 4,575,330. March 11, 1986. 27. Dandekeri S, Sowmya MK, Bhandary S. Stereolithographic surgical template: A review. J Clin Diagnostic Res 2013; 7:2093-5. 28. www.formlabs.com 29. www.uniz.com 30. Favero CS, English JD, Cozad BE, Wirthlin JO, Short MM, Kasper FK. Effect of print layer height and printer type on the accuracy of 3-dimensional printed orthodontic models. Am J Orthod
14
Dentofac Orthop 2017;152:557-565. 31. www.all3dp.com 32. Loflin, W. (2018) The Effect of Print Layer Thickness on the American Board of orthodontics
(ABO) Cast-Radiograph Evaluation (CRE) Grading of 3D-printed Models (masters dissertation) 33. Dietrich CA, Ender A, Baumgartner S, Mehl A. A validation study of reconstructed rapid prototyping models produced by two technologies. Angle Orthod 2017;87:782-787. 34. Kim SY, Shin YS, Jung HD, Hwang CJ, Baik HS, Cha JY. Precision and trueness of dental models manufactured with different 3-dimensional printing techniques. Am J Orthod Dentofac Orthop. 2018;153:144-153. 35. Graf S, Cornelis MA, Hauber Gameiro G, Cattaneo PM. Computer-aided design and manufacture of hyrax devices: Can we really go digital? Am J Orthod Dentofac Orthop. 2017;152:870-874. 36. EnvisionTec Press release, Feb 2018. https://envisiontec.com/orthodontic-materials-launched-at-lmt-lab-day-chicago
15
ACCURACY AND PRECISION OF 3-DIMENSIONAL PRINTED DENTAL MODELS PRODUCED BY DIFFERENT ADDITIVE MANUFACTURING TECHNOLOGIES.
Introduction
Three-dimensional (3D) printing technology has undergone significant advances in recent years,
rapidly changing dentistry. This technology is becoming more affordable, efficient, and capable to
provide high printing accuracy. In orthodontics, 3D printing is used to produce study models for diagnosis
as well as working models for appliance fabrication.1,2 Additive Manufacturing (AM), also referred to as
Rapid Prototyping (RP) is described as a variety of techniques used to rapidly fabricate a scale model of a
physical part or assembly using three-dimensional computer-aided design (CAD) data. . The printer reads
the CAD data and deposits consecutive layers of liquid, powder, or plastic filaments to build a model
from a series of cross sections that are joined together to establish the final shape.3
As defined by its name, three-dimensional printing involves 3 dimensions where “X” and “Y”
represent the horizontal plane. The “Z” dimension represents the vertical axis and it is related to the layer
height in which a part will be printed. As with any 3D printed structures, the final accuracy of dental
models produced by AM can be influenced by the 3D printing system of choice. Currently, many
different AM technologies are available. It is important for clinicians to be aware of each system’s
properties and characteristics so to appropriately utilize it for the different orthodontic needs.4
Some of the most common 3D printing techniques used in dentistry are Stereolithography (SLA),
Digital Lighting Processing (DLP), Fused Deposition Modeling (FDM), and Selective Laser Sintering
(SLS), and Liquid Crystal Display with Unidirectional Peel (LCD-SLA/UDP). In our study, we
investigated SLA, DLP, and LCD-SLA/UDP. The LCD-SLA/UDP is a more recent technology. The
principles of SLA, DLP, and LCD-SLA/UDP printing are essentially the same. These technologies use a
16
similar mode of operation in which a light source is used to promote photopolymerization of resin
monomers. The method used to apply this principle is what sets them apart. The SLA printing technique
uses a laser beam as the light source, the DLP technique uses a projector, and the LCD-SLA/UDP uses an
array of ultraviolet (UV) LCD.5,6
In the SLA technique, the laser must pass through every point of the cross-sectional area to be
printed. The DLP projector and the LCD screen permits that an entire layer of polymer be
photopolymerized at once, allowing to a faster printing cycle when compared to SLA. The light directed
from a DLP projector expands as it reaches the cross-section area to be printed. Due to the nature of the
projected cone of light, the periphery of a printed part may have a higher chance of presenting distorted
pixels. Unlike previous technologies, the LCD technology uses panels that have the ability to project the
light in a parallel mode, instead of expanding the light. It also selectively filters UV light, which allows
the entire layer to be printed faster than DLP.6
Currently, the two most popular printers used amongst orthodontists are the Form2 (Formlabs,
Somerville, MA, USA) and the MoonRay (SprintRay, Los Angeles, CA, USA). They employ SLA and
DLP technologies, respectively. The Slash Plus UDP uses LCD printing method, which is a more recent
technology and, for that reason, not as popular as Form2 and Moonray.5,6
As the adoption of 3D printing technology by orthodontics practices sees a significant increase, it
is important to investigate the accuracy of different systems, notably of 3D printed dental models since
they are used for a variety of clinical purposes. To this day, limited data exists on the accuracy of 3D
printed dental models and possible differences between various additive processes using different layer
thicknesses.
The aims of this study were to evaluate the accuracy of dental models manufactured by different
3D print technologies (SLA, DLP, and LCD-SLA/UDP), and to test the accuracy of the dental models
printed at Z-plane resolutions of 50 and 100 microns (µm). The null hypotheses were that there would be
no significant differences in accuracy between the printed models manufactured by different 3D printing
17
technologies, and that there would be no significant differences between printed models printed at
different Z-plane resolutions.
Methods
Ethical approval for this study was obtained from the Institutional Review Board (IRB) at the
University of North Carolina (Institutional Review Board number 18-3245; reference number 236961). A
flowchart illustrating the study methodology is shown in Figure 1. Twelve consecutive 3D digital models
(6 maxillary and 6 mandibular) from the UNC Orthodontic database were randomly selected. According
to power analysis done on similar precursor studies, a total of 12 dental models were required to
demonstrate significant results in regards to dimensional accuracy of different additive manufacturing
technologies.7,8 Models were selected based on the following inclusion criteria: 1) Complete intra-arch
permanent dentition from first molar to first molar; 2) Morphology of all involved teeth within normal
limits.
Digital scans were obtained via TRIOSⓇ scanner (3Shape, Copenhagen, Denmark) and exported as
surface model stereolithography (STL) files. Digital models were coded to remove any identifiable
information, and stored in an encrypted drive. The STL files were converted into physical models using 3
different AM techniques: 1) SLA (Form2), 2) DLP (MoonRay S100), and 3) LCD-SLA/ UDP (Slash Plus
UDP) (Figure 2). For every technology, dental models were printed at resolutions of 50 and 100 microns
in the Z plane (Figure 3). Photopolymer resins were selected based on the manufacturers
recommendations. All 3D printed models were printed as solid models with a horseshoe-shaped base and
in the horizontal position. The occlusal plane was parallel to the building platform, and the tooth cusps
were oriented away from it. At the completion of each printing cycle, each model was carefully removed
from the building platform and immersed in 2 separate tanks with 90% isopropyl alcohol, for 20 minutes,
to remove any uncured resin. Then, the model was removed from the second tank and allowed to air dry
at room temperature. Each model was post-cured for 15 minutes. All 72 replicas were stored in a dark
18
container to prevent exposure to sunlight.
The printed models were rescanned with the TRIOSⓇ scanner to create new digital STL models.
The scanner was calibrated before every use and all models were placed in the same orientation during
the scanning process. Each model was positioned on the back of its base and was oriented with the dental
arch pointing toward the scanner. A scan spray (3M™ High-Resolution Scanning Spray) (Figure 4) was
applied in each printed model prior to scanning. A minimal surface coating was sprayed to ensure optimal
scanning properties of the models. These new digital models produced STL files that were superimposed
to the original ones, which were obtained by the first scanning process. Prior to the superimposition, all
models, including the original ones, were cropped digitally to eliminate non-anatomic parts.
A Procrustes analysis, which overlaps 3D images, was done using an open source software (Slicer
3D CFM 4.0). 9 Reports were generated for each superimposition that listed any deviations between the
mesh surfaces of the original files and their corresponding 3D printed models. Figure 4 shows examples
of superimpositions of the digitized SLA, DLP, and LCD-SLA/UDP replicas with the corresponding STL
original files. Positive discrepancies are represented by a range from yellow to red, and show areas of the
print that are larger when compared to the original dataset. Negative discrepancies are represented by a
range from light to dark blue, and show areas of the print that are smaller when compared to the original
dataset. The green areas indicate few or no deviations between the superimposed models equivalent to
less than 0.1 millimeter when compared to the original scan. Five randomly selected models were re-
measured after 2 weeks to ensure the measurements could be accurately replicated. The same investigator
recorded all measurements on two separate occasions. Any differences between the two sets of
measurements greater than or equal to 0.5mm were identified and repeated until acceptable reliability was
achieved. Absolute and root mean square errors were calculated along the entire surface of the models to
determine accuracy. The T-test and ANOVA statistical analysis were performed.
19
Results
The results of the difference in the overall accuracy between 50 and 100 µm layer thicknesses for
each 3D printer are presented in Table 1. No statistical significant difference between 50 and 100 µm
layer thicknesses was found for the SLA technology (Form2) (p 0.0248), and for the DLP technology
(MoonRay S100) (p 0.426). The LCD-SLA/ UDP technology (Slash Plus UDP) showed statistical
significant differences between 50 and 100 µm layer thicknesses (p 0.019), presenting a lower mean error
value for the 50 µm layer (0.147 mm) when compared to the 100 µm layer (0.164 mm). Among the 3D
printers investigated, the lowest mean error value was found for models produced by the SLA technology
(Form2), with a 50 µm layer thickness (0.172 mm). The highest mean error value was observed on
models produced by the DLP technology (MoonRay S100), also with a 50 µm layer thickness (0.172
mm). The SLA technology (Form2) showed the lowest value for mean maximum error, followed by
MoonRay, and Slash Plus UDP.
Table 2 shows the results of overall accuracy between 3D printers by different layer thicknesses.
The SLA technology (Form2) showed statistically significant difference from DLP (MoonRay S100) and
LCD-SLA/ UDP (Slash Plus UDP) for both 50 µm (p 0.0001) and 100 µm (p 0.039) layer thicknesses. No
significant difference was observed between DLP (MoonRay) and LCD-SLA/ UDP (Slash Plus UDP).
Discussion
The objectives of the present study were to investigate the accuracy of dental models manufactured
by different 3D printing technologies, and to evaluate the accuracy of the 3D-printed dental models at
different layer thicknesses..
The accuracy of a printed structure is determined by factors such as the XY resolution, the Z
resolution or layer thickness, and the printing technology. A higher Z resolution means a reduced layer
thickness on a 3D printed model. In theory, a reduced layer thickness would be physically expressed by a
20
smoother surface finish and better surface detail of the model. For this reason, it could be assumed that a
50 µm layer thickness 3D print would exhibit a better resolution than a 100 µm layer thickness print.
Despite the fact that a reduced layer thickness is expected to provide more detail in a 3D printed model, it
does not necessarily mean that it will provide superior accuracy when compared to a model produced with
a larger layer thickness, as it is often assumed. Our data showed no statistical significance in accuracy for
different layer thicknesses for both SLA (Form2) and DLP (MoonRay S100) technologies. Although the
difference between 50 and 100 µm layer models produced by the LCD-SLA/ UDP (Slash Plus UDP)
technology was statistically significant. This difference corresponds to 0.017mm and is not considered
clinically relevant.
The superior overall accuracy found for the SLA (Form2) technology, when compared to DLP
(MoonRay S100) and LCD-SLA/UDP (Slash Plus UDP), may be attributed to the fact that this
technology uses a laser beam that cures every point of the entire printing layer, turning the process time-
consuming but also highly accurate. The DLP and LCD-SLA/UDP technologies are able to cure an entire
layer at once. The cone of light projected by the DLP can also interfere with the XY resolution since it
may create distorted pixels, which can affect the periphery of a printed part. The resulting irregularities
found on the boundaries of each layer will affect the accuracy of the 3D printed model. The XY
resolution, also known as “minimal feature size”, corresponds to 140 µm, 100 µm, and 75 µm for SLA,
DLP, and LCD-SLA UDP, respectively. This feature is set by the manufacturer, so standardization for
comparison purposes is not possible.10-12
An important characteristic of our methodology is that the original digital models were obtained
directly from the oral environment, and then compared to the scanned 3D printed models. This allowed us
to assess accuracy of 3D printed models while eliminating variables that could have been introduced
during acquisition by means of alginate impressions. On the other hand, the 3D printed models had to be
scanned to be digitally superimposed with the original models. However, the TRIOSⓇ scanner provides
high accuracy, and the errors introduced by the digitalization process should be considerably less
21
significant than those introduced by the printing techniques.13
Previous studies on accuracy of 3D printed models compared plaster to 3D printed models through
linear measurements using a digital caliper.14-17 To evaluate the discrepancies between the original scans
and the 3D printed models, we opted to use digital superimposition. Mathematical superimposition has
the ability to overcome human errors related to landmark identification.
A digital model produced by additive manufacturing can be influenced by factors that would
include a possible distortion produced with data conversion and manipulation during formatting to STL
file, and also model shrinkage during fabrication and post-curing process. If these errors were part of the
present study, all 3D printed models would have been influenced. Previous studies have determined a
range of error that would be considered clinically acceptable when evaluating accuracy of 3D printed
models. The data from these studies showed 0.20 to 0.50 mm to be an acceptable range for clinical
accuracy.14,18-22 Taking this range in consideration, the 3D printing technologies included in our study
produced 3D printed models that are suitable for diagnostic and treatment planning.
A previous study conducted by Favero et al. showed an increase in deviation values directly
proportional to an increase in Z resolution values. The 100 µm layer thickness group showed increased
accuracy when compared to the 50 µm and 25 µm layer thickness groups in terms of deviation from the
original digital model. The 25 µm layer models presented the greatest deviations, while the 100 µm layer
models showed the smallest deviation. The authors attributed that to the increased potential for errors and
artifacts as the amount of layers required to print a given part increases. Despite of the differences, all
values were considered clinically acceptable. The selection of layer thickness must take in consideration
not only accuracy, but also factors such as cost, surface quality, and efficiency. In our study, No
statistically significant differences were observed between the 50 µm and 100 µm layer thickness, except
for the LCD-SLA/UDP technology. The methodology used in both studies were similar, however our
study used a different scanner and scan protocol since we sprayed the models with a scan powder, and
also used a different software to superimpose the digital models onto their original counterparts.23
22
The intraoral scanner (TRIOSⓇ) used to scan the printed 3D models was the same used to obtain
the original intraoral scans. The TRIOSⓇ scanner is a powder-free system based on ultrafast optical
scanning technology and has documented high accuracy.24, 25 Although the original intraoral scan used
does not require a scan spray, difficulties were found during the scanning process of the 3D printed
models. The scan was not able to capture continuous images of the models, likely due to the highly
different optical properties of oral tissues and printing polymers. The scanners produced were grossly
inaccurate and irregular. In an attempt to eliminate the issue and obtain quality scans of the models, we
opted for applying a thin layer of the 3M™ High-Resolution Scanning Spray over all printed models.
This scanning spray has the ability to adjust the optical reflective properties of intraoral surfaces to allow
for proper capture of 3D digital images. In our study, we found this to be true for models surfaces as well.
Although the 3D printed models had to be coated by scanning spray powder, the coating thickness was
kept to a minimal. A previous study comparing the surface morphologies and coating thickness of 3
different spray scan systems was done using confocal laser scanning microscopy. The mean coating
thickness ranged from of 18.9 µm to 25.3 µm. The study concluded that all coating thickness values were
clinically acceptable.26
The color map illustrated in Figure 4, shows examples of superimposition of a 3D printed model to
their original model. Less pronounced errors were observed on the SLA (Form2) technology. The SLA
model was slightly wider by approximately 0.1mm labiolingually, which translates into an appliance that
is slightly bigger, but still fits properly. The DLP (Moonray S100) and LCD-SLA/UDP (Slash Plus UDP)
technologies showed shrinkage of the model. Additionally, while the bulk of the error for the SLA is
located on the labial surface, the DLP and LCD-SLA/UDP presented errors along all the cusp tips, which
is not desirable in a tight-fitting appliance. This is also reported by the maximum error values that were
greater for the LCD-SLA/UDP models. The post-curing process of the 3D printed models can lead to
additional shrinkage of the resin. This impact is more significant to the Z plane.27
23
Conclusion
Models produced with the Form2 were more accurate than the models produced with the MoonRay
and Slash Plus UDP. Changing the print resolution from 50 to 100 µm showed to affect the overall
accuracy only for models produced with the Slash Plus UDP.
24
REFERENCES
1. Christensen LR. Digital workflows in contemporary orthodontics. APOS Trends Orthod 2017; 7: 12-8. 2. Graf S, Cornelis M, Gameiro G, and Cattaneo P. Computer-aided design and manufacture of hyrax devices: Can we really go digital? Am J Orthod Dentofacial Orthop 2017; 152: 870-4. 3. Beguma Z, Chhedat P. Rapid prototyping—when virtual meets reality. Int J Comput Dent 2014; 17: 297-306. 4. Wu W, Ye W, Wu Z, Geng P, Wang Y, Zhao J. Influence of Layer Thickness, Raster Angle, Deformation Temperature and Recovery Temperature on the Shape-Memory Effect of 3D-Printed Polylactic Acid Samples. Materials 2017; 10: 970. 5. Nguyen T, Jackson T. Understanding the Fundamentals of 3D printing. In: Kim-Berman H, Franchi L, Ruellas A, eds. Effective, Efficient and Personalized Orthodontics: Patient-centered Approaches and Innovations. Craniofacial Growth Series, Center for Human Growth and Development, The University of Michigan, Ann Arbor, MI 2018; 55:281-293. 6. LCD vs DLP - Resin 3D Printing Technologies Compared. https://all3dp.com/2/lcd-vs-dlp-3d-printing-technologies-compared/ 7. Hazeveld A, Huddleston Slater, and Ren Y. Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques. Am J Orthod Dentofacial Orthop no. 2014; 104: 108-15. 8. Rebong RE, Stewart KT, Utreja A, Ghoneima AA. Accuracy of three-dimensional dental resin models created by fused deposition modeling, stereolithography, and Polyjet prototype technologies: A comparative study. Angle Orthod. 2018; 3: 363-69. 9. 3D Slicer. 2017 [cited 2017 March 7, 2017]; Available from: slicer.org. 10. Form2 Tech Specs. https://formlabs.com/3d-printers/form-2/tech-specs/ 11. Technical Specifications. https://sprintray.com/moonray-desktop-3d-printer/technical-specifications/ 12. Slash plus UDP. https://www.uniz.com/us_en/page/3d-printers-slashplus-udp 13. Nedelcu RG, Persson AS. Scanning accuracy and precision in 4 intraoral scanners: an in vitro comparison based on 3-dimensional analysis. J Prosthet Dent 2014; 6:1461-71. 14. Hazeveld A, Huddleston Slater, and Ren Y. Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques. Am J Orthod Dentofacial Orthop 2014; 145:108-15. 15. Rebong RE, Stewart KT, Utreja A, Ghoneima AA. Accuracy of three-dimensional dental resin models created by fused deposition modeling, stereolithography, and Polyjet prototype technologies: A comparative study. Angle Orthod. 2018; 3: 363-69. 16. Kasparova M et al. Possibility of reconstruction of dental plaster cast from 3D digital study models. Biomed. Eng. Online 2013; 1:1.
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17. Murugesan K, Anandapandian PA, Sharma SK, and Kumar MV. Comparative evaluation of dimension and surface detail accuracy of models produced by three different rapid prototype techniques. J. Indian Prosthodont 2012; 1:16-20. 18. Hassan WN, Yusoff Y, Mardi NA. Comparison of reconstructed rapid prototyping models produced by 3-dimensional printing and conventional stone models with different degrees ofcrowding. Am J Orthod Dentofacial Orthop 2017; 151:209-18. 19. Schirmer UR, Wiltshire WA. Manual and computer-aided space analysis: a comparative study. Am J Orthod Dentofacial Orthop 1997; 112:676-80. 20. Hirogaki Y, Sohmura T, Satoh H, Takahashi J, Takada K. Complete 3-D reconstruction of dental cast shape using perceptual grouping. IEEE Trans Med Imaging 2001; 20:1093-101. 21. Halazonetis DJ. Acquisition of3-dimensional shapes from images. Am J Orthod Dentofacial Orthop 2001; 119:556-60. 22. Bell A, Ayoub AF, Siebert P. Assessment of the accuracy of a three- dimensional imaging system for archiving dental study models. J Orthod 2003; 30:219-23. 23. Favero CS, English JD, Cozad BE, Wirthlin JO, Short MM, Kasper FK. Effect of print layer height and printer type on the accuracy of 3-dimensional printed orthodontic models. American Journal of Orthodontics and Dentofacial Orthopedics 2017; 4:557-565. 24. Hack GD, Patzelt SBM. Evaluation of the Accuracy of Six Intraoral Scanning Devices: An in-vitro Investigation. ADA Professional Product Review 2015;4:1-5. 25. Ender A, Zimmermann M, Attin T, Mehl A. In vivo precision of conventional and digital methods for obtaining quadrant dental impressions. Clin Oral Investig 2015; 7:1495-504. 26. Lehmann KM, Azar MS, Wentaschek S, Scheller H. The Effect of Optical Conditioning of Preparations with Scan. Acta Stomatol Corat 2011; 2:86-92. 27. Keating AP, Knox J, Bibb R, Zhurov AI. A comparison of plaster, digital and reconstructed study model accuracy. J. Orthod. 2008; 3:191-201.
26
APPENDIX 1: TABLES AND FIGURES
Table 1. Difference in overall accuracy between different layer thicknesses (Z resolution) for each 3D
printer. Level of significance set to ≤ 0.05.
Printer Z Resolution
(µm)
Mean
Maximum
Error (mm)
Mean Error
(mm)
SD P-Value
50 0.488 0.093 0.041
Form2 100 0.593 0.117 0.044
0.248
50 0.741 0.172 0.036
MoonRay 100 0.726 0.159 0.030
0.426
50 0.907 0.147 0.017
Slash Plus
UDP
100 1.001 0.164 0.019
0.037
27
Table 2. Difference in overall accuracy between 3D printers by different layer thicknesses (Z resolution).
Level of significance set to ≤ 0.05.
Resolution Form2 vs. MoonRay Form2 vs. Slash Plus
UDP
MoonRay vs. Slash Plus UDP
50 µm 0.0001 0.002 0.066
100 µm 0.039 0.007 0.679
Table 3. Photopolymer resins based on the manufacturers recommendations.
Printer Form2 MoonRay Slash Plus / UDP
Resin Formlabs Gray Resin (RS-F2-GPGR-04)
SprintRay Model Gray (SGR – 12419)
zOrtho Gray (ZSLM07GR001)
28
Figure 1: Study methodology flowchart.
Original scans (N=12)
Conversion to STL format
50 µm layer 100 µm layer 50 µm layer 100 µm layer 50 µm layer 100 µm layer
Models superimposition
29
Figure 2: Chart comparing the 3D printing technologies and printers investigated in this study.
Printer
Form2
MoonRay
Slash Plus UDP
Technology SLA DLP LCD-SLA/UDP
Layer thickness 25, 50, and 100 µm 20, 50, and 100 µm 10, 25, 50, 100, 150, 200, and 300 µm
Build Volume 5.7” x 5.7” x 6.9” 5” x 3.2” x 8”
7.5" × 4.7" × 7.9"
Footprint 13.5” x 13” x 20.5” 15” x 15” x 20” 14" × 16" × 21"
MSRP $ 3,499 $ 4,430 $ 3,349
Image sources: formlabs.com, sprintray.com, uniz.com
30
Figure 3: Close up view of models printed at 50 micron and 100 micron resolution.
Form2 MoonRay Slash Plus UDP
50 microns
100 microns
Figure 4: 3M™ High-Resolution Scanning Spray (A) applied on printed models (B). Close up view of
the spray coating.
31
Figure 5: Superimposition of 3D printed models to their original counterpart.
Form2 MoonRay Slash Plus UDP