Accuracy evaluation of three different AM technology

CAD/CAM systems have been widely used in dentistry and definitive casts can be fabricated using subtractive or additive technology. Additive manufacturing (AM) is also known as Rapid Prototyping (RP) or 3D printing, and it is a process in which objects are built layer by layer. 3D printing allows the production of complex objects, shortens manufacturing time, and reduces material costs. The common techniques that use light to polymerize resin are stereolithography (SLA), polyjet or material jet (MJ), digital light processing (DLP), and continuous liquid interface production (CLIPTM).

SLA uses an ultraviolet laser to polymerize photosensitive resin liquid into the desired shape. Smooth and fine surface details and high accuracy are the advantages of SLA. MJ is similar to an inkjet printer. It uses materials extruded from nozzles, or a photopolymer jetted over the workspace and then the object is polymerized with a UV light source. FDM comprises an extruder, a nozzle, a print chamber, and a filament feeding system. After accessing the feeding system, the material gets guided to the extruder where it is then melted. Then, the nozzle extrudes melted filament onto the build plate of a print chamber, creating a 3D printed model. DLP uses high power LEDs and photosensitive resin materials to build layers. DLP has moving micro-mirrors to control light reflection. These micro-mirrors correspond to each pixel of the image, and the building time of the object is minimized. CLIPTM uses digital light projection in combination with oxygen permeable optics. This produces high-resolution parts with engineering grade mechanical properties.

Accuracy consists of two parameters: trueness and precision. The trueness is described as the deviation of the printed object from its actual dimensions, and the precision is the deviation between repeated products. High trueness describes the proximity to the original dimensions of the measured object, and high precision defines the ability to manufacture the same product with the dimensions in repetitive products.

In this issue, three scientific articles are reviewed for the accuracy of the different 3D printing technologies in order to consider which ones are suitable for practices and dental laboratories.

Park et al. evaluated the dimensional accuracy and surface characteristics of locations in complete-arch casts made with digital light processing (DLP), fused deposition modeling (FDM), SLA, and photopolymer jetting (Polyjet). A CAD reference cast was created by using a 3D design software program (Rapidform; Inus Technology Inc) and attaching implant scan bodies to a mandibular dental arch. Six cylinders were placed in the left and right canines, second premolars, and second molars. Three additional reference spheres were placed around the left second molar to establish a coordinate system with which to measure deviations. (Fig. 1)

[Fig. 1] The Reference model

Casts were made from 5 different materials by using 4 printers with different printing methods. The groups were classified as per the material used: FDM, DLP1, DLP2, SLA, and Polyjet. The parameters set for the 3D printers in each group are shown in Table. 1. The CAD reference cast data and the scanned data were imported into a 3D analysis program (Geomagic Verify; 3D systems).

[Table. 1] Specification of 3D printers and materials

The FDM casts had a significantly larger systematic deviation (158.1μm) than those of the DLP1 (104.4μm), DLP2 (103.3μm), Polyjet (99.3μm), and SLA casts. (109.9μm) In addition, the FDM casts showed a relatively wide range of deviation and prominent striations. The casts of the DLP 1 and DLP 2 groups had minimal deviation in the anterior regions and contraction in the molar regions. Casts printed with Polyjet and SLA printers generally showed minimal distortion. (Fig. 2)

[Fig. 2] Deviation of 3D printed casts relative to CAD reference cast data

There were significant differences in the median deviation in the X- and Y-axes. In the X-axis, the FDM, DLP 1, and DLP 2 groups showed the most positive values in the left second molar position, and the median value gradually decreased as the measured position moved to the right along the arch. In contrast, for the Polyjet and SLA groups, the left second molar cylinder had the most negative deviation, with the median deviation changing in the positive direction as the measured position moved to the right.

In the Y-axis, cylinders in the FDM group had positive values, with increasing deviations as the measured position moved farther to the right of center. The DLP 1 and DLP 2 groups showed negative values in the anterior region and positive values in the molar region. The Polyjet group showed positive values except at the left second premolar and right second molar position. In the SLA group, the cylinder next to the origin showed the most negative value, as compared with that of the other groups. In the Z-axis, the FDM group had the deviations at both second molar sites, whereas the DLP1 and DLP2 groups showed the deviations at the canine and premolar sites.

When the surface texture of the printed casts was evaluated, interlayer connections were rough and had prominent demarcation lines in the FDM casts. Some areas appeared to have melted, especially the tops of cylinders and sharp points such as the cusp tips. In the DLP 1 and DLP 2 casts, staircase effects between layers were apparent, and in some models, the sharp tips had chipped. The surfaces of the Polyjet casts were the smoothest and had the most gloss but lacked sharpness on the cylinder tops. In the SLA casts, hatched interlayer connections were relatively smooth, and the surface showed some degree of gloss. (Fig. 3a)

The FDM casts showed distinguishable stepped layers of printed filament materials with round margins. The DLP 1 and DLP 2 casts showed marked interlayer configurations and rather jagged edges. The Polyjet casts had no distinguishable interlayer lines but were relatively dull in line angles and points. The SLA casts showed detectable interlayer lines, but the contours were smooth. (Fig. 3b)

[Fig. 3] Photographed models and electron microscope images

Based on the test results, they concluded:

  • The FDM was found to be inferior to the other 3D technologies for complete arch dental cast fabrication
  • Deviations at different cylinder locations showed that FDM and DLP casts tended to contract, whereas casts of the Polyjet and SLA groups expanded buccolingually and anterioposteriorly. Vertically, deviations were generally smaller than those in the other directions, especially for the SLA group.

Rungrojwittayakul et al. studied to evaluate the effect of model base designs on the accuracy of 3D printed models created by two different printer technologies. The horseshoe shaped model with a bar was created and ten solid models and ten hollow models with 2mm thickness using Carbon M2 (CLIPTM, Carbon®) and MoonRay S100 (DLP, SprintRay) were printed. A total of 40 models were included in this study. (Fig. 4) Geomagic Control X was used to measure the trueness and the color maps were created by the software. The data obtained from the tested group were compared within the group using the intraclass correlation coefficient (ICC) value to determine the models’ level of precision.

[Fig. 4] (a) the printed models with CLIPTM technology, (b) the printed models with DLP technology

The Color maps displayed the deviation between the printed models and the reference model. (Fig. 5) The yellow and red areas indicate the expansion of the model and the blue area expresses the shrinkage of the model. The median values of the deviated distance in four tested groups were 0.045 mm for CLIP with solid base, 0.035 mm for CLIP with hollow base, 0.077 mm for DLP with solid base, and 0.077 mm for DLP with hollow base (Table. 2) There was no statistically significant difference between the trueness of the CLIP with solid base group and the CLIP with hollow base group, as well as between the DLP with solid base group and the DLP with hollow base group. However, when comparing the two printers within the same base design and also between the different model base designs, there was a statistically significant difference in trueness. The CLIP technology printer produced higher trueness in model printing than the DLP technology. Previous studies have suggested that the clinically acceptable limit of the model for the fixed prosthesis should be less than 100μm. In this study, the discrepancy of all printed models was found to be below 100μm and was considered clinically acceptable.

[Fig. 5] Color maps indicated the discrepancy between the printed models and the reference model. (a) Hollow model of CLIPTM, (b) Solid model of CLIPTM, (c) Hollow model of DLP, (d) Solid model of DLP

[Table. 2] Test result of each group

With the limitation of this study, they concluded that all of the 3D printed models made using a CLIP printer and a DLP printer had clinically acceptable levels of trueness. However, the models produced using the CLIP technology printer exhibited significantly greater trueness relative to the reference model even though the difference was small. The design of the model base (solid base versus hollow base with a 2.0 mm external shell thickness) did not affect the trueness of the model as much as the technology of the printer.

Emir et al. evaluated the accuracy of the trueness and precision of the models used for the production of fixed restorations that were printed with three different 3D printing technologies. An arch-shaped master model to simulate the mandibular arch was designed with CAD software. Six abutments with a 6° total angle of convergence and 1mm at the circumferential shoulder finish lines were produced to resemble prepared teeth (Canines, 2nd premolars, 2nd molars) with a height of 10.15mm placed on the arch. Cross marks were added at the middle of each abutment’s occlusal surface and were used as reference points to allow measurements of the X, Y, and Z coordinates. Ten models were manufactured by each printer using three different printing technologies: SLA technology (Ultra SP Ortho, envisionTEC), DLP technology (Prefactory Vida, envisionTEC), and Polyjet technology (Objet30 Prime, Stratasys Ltd). (Fig. 6) A total of 30 models were printed. Geomagic Control and SPSS statistical analysis software were used to determine trueness and precision.

[Fig. 6] Printed models using (a) SLA, (b) DLP, and (c) MJ technologies

The comparison of trueness measurements for the three different technologies presented significant differences. The mean trueness was 51.6μm for the SLA models, 46.2μm for the DLP models, and 58.6μm for the MJ models. The DLP models were closer to the reference model. The trueness value differences between DLP and MJ models were found to be statistically significant.

There were significant differences in the mean RMS values of the precision of printed models between all technologies. The mean precision was 37.6μm for the SLA models, 43.6μm for the DLP models, and 30.4μm for the MJ models. The MJ models were more precise than the SLA and DLP models. According to a comparison between the reference marks on the printed models, no statistically significant differences were observed in the X- or Y-direction; only the Z-direction showed significant differences. The SLA models showed the most accurate results in the Z-direction.

According to the color coded maps for the SLA models, a slight contraction was observed on the buccal surfaces of the abutments and the arch. In addition, on the finish lines of the abutments, circumferential shrinkage was detected. However, the lingual surfaces of the posterior abutments and the coronal surface of the arch represented a slight expansion. The DLP models displayed a homogeneous pattern of green surfaces, but a slight expansion was observed on the posterior side and a slight contraction was observed on the lingual surface of the arch. In contrast to the lingual surfaces, the buccal surfaces of posterior abutments displayed expansion. For the MJ models, the buccal surfaces of the arch generally appeared larger than the reference model. A circumferential expansion was observed on the finish lines of abutments and the lingual surfaces of the abutments presented a slight shrinkage. (Fig. 7)

[Fig. 7] The models superimposed over the reference model: (a) SLA, (b) DLP, (c) MJ

In comparison of the STL reference files, the DLP models showed higher trueness than the MJ. However, The MJ models were more precise than SLA and DLP. Additionally, SLA models were more accurate than other printers in the Z-direction. This study demonstrated that the dental models manufactured with tested 3D printers were within clinical tolerance and may be appropriate for the production of fixed restorations.

A basic knowledge of how 3D printing technologies work is essential to understanding possible sources of error. The disadvantage of increasing printing time can be an increased error rate, which also raises the probability of print failures. The layer thickness improves the transitions on the diagonals, but has little effect on vertical and horizontal edges. Therefore, a precise planning phase with regard to the geometry to be printed is required. Overall, the accuracy of a 3D printer is highly dependent on the AM technology, which also determines the printing materials to be used. The physical properties of the different printing materials affect the accuracy.

FDM is the simplest AM technology that performs material extrusion. The printer nozzle loads the thermoplastic filament, melts it with the material’s specific temperature, and extrudes it. The extrusion head is attached to a two-axis system and extrudes the melted material along a previously determined path layer by layer while the platform drops. Advantages are the ease of use, minimal troubleshooting, and low cost for common materials, making it the most affordable option. Limitations are the relatively long printing time, rather low resolution, rough surface with visible layer lines, poor mechanical properties due to layer lines, and warping. Layer lines occur because the melted thermoplastic material is slightly pressed against the previous layer, causing its surface to melt and bond to the new layer. These lines are visible depending on the layer thickness and tend to break up preferentially in case of mechanical stress. As the material hardens, their dimensions change, leading to force on the underlying layers. So, warping is also a common defect in FDM models.

SLA and DLP are slightly more advanced AM technologies that use a vat of a liquid UV-curable photopolymer instead of an extrudable thermoplastic filament. The 3D printer beams a UV beam to cure a photopolymer in a previously determinate point or area, creating a solid layer. Advantages are the average high resolution and highly versatile material selection. Limitations are the high price (especially for printing materials), difficulty in handling liquid resin as it is sensitive to long exposure to UV light, and rather poor mechanical properties due to brittle material and curling. It has isotropic mechanical properties because the resin won’t be fully cured at first and continues curing while post-processing. A technology limitation is the shrinking of the resin during curing, leading it to curling. An important factor of the bottom-up SLA and DLP printers is the polydimethylsiloxane (PDMS) layer at the transparent bottom of the tray which wears out after some time. Additionally, the peeling step, the detachment of the resin from the bottom of the vat, forces an effect on the model.

Material jetting (MJ) is an AM technology in which a print head dispenses droplets of a photosensitive liquid material (acrylic) that cures after ultraviolet (UV) light application, building the model layer by layer. Advantages include high resolution and a unique surface finish. The surface can be either glossy or matte. Limitations include high costs, a long printing time, and poor mechanical properties. This AM technology is relatively expensive, making it impractical for basic applications. Since the layer thickness can be quite thin, the printing time is fairly long.

The clinically acceptable limits for evaluating the accuracy of the working models is related to dental treatments. The accuracy of the working models might affect the misfit of fixed prostheses, which could lead to larger marginal or internal discrepancies before the delivery of the prosthesis. Hazeveld et al. concluded that measurement differences of less than 0.25 mm are clinically acceptable because the tolerances for manual measurements are almost identical to that value. Hirogaki et al. stated that a measurement difference of 0.30 mm could be considered sufficiently accurate for orthodontic diagnostic casts.

According to Brain et al, high resolution is not equivalent to accuracy. Printers that have high resolution can manufacture models with fine details, but printed materials may not be sufficiently accurate. On almost all models, a slight contraction was observed. This contraction on the posterior region might have been due to the higher photopolymer density in the posterior than in the anterior region. Therefore, the reason for the contraction might be polymerization-induced shrinkage.

Despite the existing statistical significance of the differences in accuracy between all the 3D printers studied, when regarded separately, all are minor and acceptable for medical–surgical applications. However, the used printing material determines the field of application. Therefore, when choosing a 3D printer, the focus should no longer be primarily on the technology used, but rather on the desired application depending on the printing materials in relation to the total budget available. Therefore, practitioners and technicians should be aware of the practicability of each technology with its advantages and limitations.

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