Influence of printing procedure and printing axis of dental alloys on dimensional accuracy, surface roughness, and porosity

INTRODUCTION

Computer-aided design and computer-aided manufacturing are increasingly used in dentistry, especially in orthodontics.^[1] 3D printing is currently mainly used to produce models with resin materials but is also available for removable dental prostheses,^[2-4] oral splints,^[5] drilling templates,^[6] or orthodontic appliances.^[7-11] To process dental alloys for the framework of removable dental prostheses or orthodontic appliances, different printing processes are currently available. The first printing technique used for the processing of metal alloys was selective laser sintering (SLS). Metal particles are partially melted in layers of 20–100 µm and are subsequently reheated and completely joined, resulting in volumetric differences.^[12] SLS has, therefore, been replaced by selective laser melting (SLM), where particles are already fused within the first step. Another approach is electron beam melting (EBM), where metal powder is melted using an electron beam instead of a laser source. In contrast to those technologies joining pure metal alloy powders, another approach deposits a binder over metal powder layers. This so-called binder jetting (BJ) technique requires additional curing steps involving depowdering, sintering imperfections, and annealing of the object.^[13] The printing technique may result in varying mechanical properties due to insufficiently fused particles and incorporated porosities.^[12-15] Furthermore, the production time of printed specimens is highly dependent on the chosen printing method and the required post-processing procedures. Even when selecting the same printing procedure but a different printer model, the outcome may vary. Factors influencing the outcome are the size of the building platform (chamber), the temperature within the chamber, laser energy (melting temperature), printing speed, or layer thickness.^[14-20]

Dental materials that are placed within the oral cavity over a longer period should have a smooth surface with a surface roughness value Ra <0.2 µm to prevent biofilm accumulation.^[21]

From 3D printing with resin materials, it is known that the positioning of the specimens on the building platform influences its mechanical properties^[22] and printing accuracy.^[17] For metal processing, the influence of the positioning of a specimen on the building platform on dimensional accuracy, surface roughness, and porosities is still unclear. The purpose of this study was therefore to compare specimens produced using either BJ, EBM, or SLM in three printing axes (X-, Y-, and Z-axis) to the original stereolithography (STL) file. Null hypotheses were that (1) the printing method does not affect dimensional accuracy and surface roughness and (2) the printing axis does not affect dimensional accuracy and surface roughness.

MATERIAL AND METHODS

Specimen production

Specimens with a dimension of 50 mm × 12 mm were produced using BJ, EBM, and SLM techniques and were printed either along the X (Group III), Y (Group II), or Z-axis (Group I) [Table 1 and Figure 1] (n = 8 per group). Specimen dimension was chosen according to the ISO standard 6892-1 for tensile strength test specimens. Unfortunately, it was not possible to process the same alloy with varying printing procedures.

Table 1: Specimens produced with either BJ, EBM, or SLM in three different printing axis along each axis (X, Y, and Z).

Code	Processing	Printing axis	Material	Company
BJ I	Binder	Z-axis	316L steel	Digital Metal
BJ II BJ III	jetting	Y-axis X-axis	(Fe, Cr, Ni, Mo, C)	AB, Höganas (Sweden)
EBM I	Electron	Z-axis	Arcam	GE Additive,
EBM II	beam	Y-axis	CoCrMo	Concept Laser,
EBM III	melting	X-axis	Powder	Lichtenfels (Germany)
SLM I	Selective	Z-axis	CoCr	CAD/Tools,
SLM II	laser	Y-axis	Remanium	Augsburg
SLM III	melting	X-axis	Star	(Germany)

BJ: Binder jetting, EBM: Electron beam melting, SLM: Selective laser melting

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Figure 1:

(a) Specimen printing direction in X-, Y-, and Z-axis). (b) Specimens of SLM, electron beam melting, and binder jetting.

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BJ specimens were produced out of 316L steel with a DM P2500 (Digitalmetal, Höganas, Sweden). Specimens were built in layers of 42 µm with a binding material without supporting structures at room temperature. Afterward, the residual powder was removed and specimens were sintered. Printing time for a thickness of 2 mm was 50 min plus additional curing-, debinding, and sintering accumulating to 24 h.

EBM specimens were produced with cobalt chrome molybdenum (CoCrMo) powder on an Arcam EBM spectra L (GE Additive, Lichtenfels, Germany). The building chamber was heated just below the melting point of the powder under a vacuum. The printing time of a 2 mm structure was 7 min, but the complete process with heating up and cooling down took 4 h. No supportive structures were needed, as the whole building chamber is heated up just below the melting point of the chosen material.

SLM specimens were produced of CoCr at the laboratory of concept laser (Lichtenfels, Germany, run by Caddent, Germany). This device can print 50–100 layers of 0.03 mm within 14 h. A layer of metal powder is, therefore, distributed evenly on a building platform. Layers are fused with a laser beam. The complete build-up platform lowers for the next layer to be distributed on the complete platform. As every melted part starts to cool when the laser moves on, shrinking occurs and stress is induced. To release tension out of the final product, further heat-treatment of 1 h at 1140°C was required. To reduce shrinkage, a supporting structure was attached to the edges of the triangular structure of the specimens. After specimen production, supporting structures were removed but no additional polishing steps were performed.

RESULTS

Surface roughness

The surface roughness measurements with statistics are displayed in [Table 2]. Two-way ANOVA revealed a significant effect of printing method and printing axis for all roughness parameters (all P < 0.001, Ssk axis P = 0.025). Mean Sa values for the different printing procedures ranged for BJ I 5.9 to BJ II 12.88 µm, for EBM II 47.48 to EBM I 52.2 µm, and for SLM II 21.9 to SLM I 71.77 µm. For Sz, the mean values were for BJ I 60.67 to BJ II 121.39 µm, for EBM II 449 to EBM I 535.46 µm, and for SLM 131.55 III to SLM I 579.45 µm. The Lowest Sa and Sz values were revealed for specimens printed with the BJ I method. Post hoc tests showed no significant effect of the printing axis for BJ and EBM (P > 0.05). While for SLM roughness values, Sa and Sz were significantly higher for Group I compared with III and II (P < 0.001).

Table 2: Surface roughness parameters mean and standard deviation of BJ, EBM, and SLM (n=3 per group).

Group	Sa (µm)	Sz (µm)	Sq (µm)	Ssk
BJ I	5.90±1.06A	60.67±4.47A	7.33±1.25A	-0.196±0.05A
BJ II	12.88±2.51A	121.39±10.73A,B	16.14±2.73A,B	0.33±0.09B
BJ III	8.65±0.83A	93.13±7.59A,B	10.84±1.04A	0.027±0.16A
EBM I	52.2±2.81B	535.46±66.99C,D	72.37±5.13C	1.36±0.15C
EBM II	47.48±8.24B	449.18±69.21D	60.18±9.38D	0.52±0.36B
EBM III	51.44±8.38B	455.44±51.29D	63.61±9.68C,D	0.27±0.18B
SLM I	71.77±0.93C	579.45±38.84C	91.91±1.32E	0.15±0.08A,B
SLM II	21.9±0.1D	157.04±30.61B	26.16±1B,F	-0.320±0.5A
SLM III	25.88±5.56D	131.55±22.28A,B	29.93±6.46F	-0.08±0.04A,B
ANOVA
Printing method	P<0.001	P<0.001	P<0.001	P<0.001
Printing axis	P<0.001	P<0.001	P<0.001	P=0.025

P-values obtained with two-way ANOVA for each effect are displayed. Differences within subgroups tested with Fisher LSD test are indicated with varying superscript letters (P<0.05). BJ: Binder jetting, EBM: Electron beam melting, SLM: Selective laser melting

For BJ specimens, a homogeneous surface with few and very regular depths and heights of the troughs and peaks is observable in [Figure 2]. For EBM all groups and SLM I, an irregular height distribution is visible, and the surface is dotted with peaks and troughs at irregular intervals. SLM II and SLM III appear smoother than SLM I, confirming roughness measurements.

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Figure 2:

Surface topography of binder jetting (BJ), selective laser melting, and BJ (×10 objective, image size 4600 µm × 3400 µm, scale in µm).

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Dimensional accuracy

[Figure 3] shows the dimensional deviation of the specimens compared with the original STL-file using different colors. Green indicates a precise print. Negative deviations are indicated in blue. Additive deviations are given in red. Most accurate specimen dimensions were achieved with BJ-process followed by EBM and then SLM.

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Figure 3:

Dimensional accuracy visualization of the specimens. Green indicates a high conformity with the original file used for printing.

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Deviations were quantified in [Figure 4]. BJ showed the smallest deviation with no significant differences between the printing axis (P > 0.05). Significantly larger deviations were recorded for EBM I and II that did not vary significantly from each other (P = 0.433), compared with EBM III (both P < 0.001). SLM was the least accurate printing method displaying low dimensional accuracy with significantly higher discrepancies for SLM I than for II and III (all P < 0.001).

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Figure 4:

Dimensional deviation in mm of the specimens from the original stl using best fit comparison (boxplot indicate standard deviation and min/max, mean = 0) (n = 2 per group).

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Porosity

Scanning electron microscopic images are displayed in [Figure 5]. BJ samples were the most porous. Large holes and a diffuse non-homogeneous surface can be observed. The EBM process is less porous. Various precipitates are visible. For SLM specimens, the various porous cluster can be seen in the image, surrounded by a homogeneous non-porous surface.

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Figure 5:

Scanning electron microscopy images (magnification bar: 10 µm) of polished and etched specimens printed using binder jetting, electron beam melting, and selective laser melting technique in three different printing axes.

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DISCUSSION

This study aimed to evaluate the null hypotheses that (1) the printing method does not affect dimensional accuracy and surface roughness and (2) the printing axis does not affect dimensional accuracy and surface roughness to estimate the potential of additive manufacturing in orthodontic dentistry. Both printing method and printing axis affected surface roughness parameters, dimensional accuracy as well as porosities, and tensile strength values. Therefore, the null hypotheses (1) and (2) were both rejected.

The surface roughness measurements revealed that with BJ processing, the most homogeneous surface was achieved. EBM and SLM processes created a rather rough and wavy surface that requires further post-processing to obtain a smooth surface finish and accurate dimensions. Surfaces of materials that are placed in the oral cavity require a surface with a roughness value of Ra < 0.2 µm to prevent biofilm accumulation.^[21] As Sa values of all tested specimens superseded this threshold value, further polishing is recommended after all printing procedures. Most manufacturers of orthodontic appliances add extra volume that is later polished off during a post-printing procedure, resulting in the final dimension of the planned product. The different polishing processes are either by hand, automated with polishing machines, or electrochemical. Manufacturers are encouraged to improve the surface quality of EBM and SLM to limit extensive post-processing.

The printing axis did affect the surface roughness of the specimens as well. Overall, the lowest surface roughness was achieved using printing axis II > III > I. Hence, the largest surface area of a specimen should not be placed parallel to the printing axis when a smooth surface finish is to be achieved.

When analyzing the dimensional accuracy, it was observed that all specimens within all processes and axis displayed a certain deviation. Using printing method BJ, the lowest discrepancies were found. With EBM processing deviation was larger and worst with SLM. Printing SLM in axis III provided accurate specimens while for I and II, the deviation was not acceptable. Another study that examined trifurcated 3D printed 316L steel joints using SLM processing showed that maximal discrepancies were < 5%.^[23]

As specially the printing axes I and II showed the highest deviations, it might be necessary to avoid printing large solid volumes. During the cooling process, the solid specimens may deform due to their weight. Rather small volumes or cooling channels within the printed body might be a useful tool to be further investigated.

Although BJ displayed the best surface roughness properties and dimensional accuracy, unfortunately, high porosity was observed in SEM images. Manufacturers are encouraged to improve these porosities as BJ processing is the least expensive (BJ CoCr). Furthermore, tensile strength values of BJ were lower than for EBM and probably SLM which superseded the limit of the testing machine. However, the tensile strength values may have been influenced by the different materials used for the printing methods, which is a limitation of this study. It has been found that CoCr alloy specimens processed by SLM technique mechanical properties (ultimate tensile strength 1158 ± 10 MPa and Vickers hardness 399 ± 24 HV) were improved compared with specimens prepared by casting and milling techniques. Hence, SLM printing provided a microstructure with relatively homogeneously distributed fine grains and more second-phase particles, which can significantly increase strength.^[24] Another study compared EBM and SLM using a CoCr alloy for metallic orthopedic implant applications. Tensile strength values of 562–884 MPa for SLM and 960 MPa for EBM were recorded.^[25]

All three printing methods have their advantages. SLM is currently the most common method applied in orthodontics, despite its accuracy and surface deficiencies observed in this study. However, the design of orthodontic appliances is much more filigree than that of the tested specimens in this study. Orthodontic appliances are printed using supportive structures that allow faster cooling without deformation [Figure 6]. The production of larger test specimens for tensile strength testing used in this study may have caused greater deformations. Hence, further studies may be conducted to measure printing accuracy with more clinically relevant specimen dimensions. In the long-term, the easiest procedure regarding printing speed, material properties, and pre-and post-processing will find its way to in-house 3D metal printing in dental and orthodontic clinics.

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Figure 6:

An example of printed orthodontic appliances for clinical application with support structures using selective laser melting technique.

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CONCLUSION

Within the limitations of this study, it can be concluded that with the BJ printing method of a dental alloy, the highest dimensional accuracy with smoothest surfaces, irrespective of the printing axis was achieved. EBM and SLS processes created rather rough and wavy surfaces that required further post-processing to obtain a smooth surface finish and accurate dimensions. Those two printing techniques were highly affected by varying the printing direction.