Regarding this reserach, only few quality requirements for dental frameworks are defined, as they are an intermediate step in the production process of dental restorations. Nevertheless, specifications for materials and for the final restorations exist and many requirements can be transferred to the intermediate product.
In general, the quality demands for dental frameworks can be subdivided into chemical, physical, mechanical and geometrical properties. Chemical properties, e.g. biocompatibility, corrosion resistance and resistance to tarnishing, are subject to different dental material standards. Physical properties are also subject to standards relating to density, solidus and liquidus temperature and thermal expansion. These standards also include minimum requirements for different mechanical properties [14, 15, 16].
An overview of the requirements according to German standards is given in table 1.
Table 1: Standardized quality requirements for dental frameworks, according to [14, 15, 16].
Category Characteristic Standard
Chemical Biocompatibility DIN EN ISO 10993
Corrosion resistance DIN EN ISO
22674:2006 ISO 10271:2001 Resistance to tarnishing DIN EN ISO
22674:2006
In addition to these standardized requirements, different quality needs can be ascertained according to the application of the final part, as well as from the production process. These include geometrical properties, e.g.
dimensional accuracy or surface roughness, as well as a number of qualitative characteristics, for example visual appearance or processability.
In contrast to the well-defined standardized characteristics, no generally accepted specified values and tolerances exist for these properties. Their evaluation is based on the process participants’ individual expertise. While this approach delivers satisfactory results for the final product, it makes the application of quality management techniques, such as determination of process or machine capability, extremely difficult.
Few attempts to define dimensional tolerances can be found in literature. For example, Uckelmann determined maximum values for the deviation in shape of 50 µm for crowns and 100 µm for larger bridges [5]. This specification is based on studies regarding the maximum acceptable gap width between the edge of the dental restoration and the remaining teeth. It needs to be taken into consideration that these values are only valid for the crown margin of the final restoration and thus are not necessarily transferable to all the dimensions of the metal framework.
Another important factor is the surface roughness. While a certain roughness is helpful for the ceramic veneering to form a strong bond between the metal framework and the ceramic material, the interface to the remaining teeth needs to be very smooth. This leads to varying requirements for different areas of the framework, which are not universally defined.
4.2 Specific Requirements for Additive Manufacturing
These product requirements can be transferred into requirements for the Additive Manufacturing process. An approach for this is made based on the authors´ experience. Not all of the defined requirements can be influenced by the parameter settings of the SLM process. For example the chemical properties mostly depend on the composition of the alloy used. The microstructure that is formed during solidification is the only factor influencing the process for these properties. It may differ from the one formed in a casting process due to different thermal conditions.
Nevertheless, it can be assumed that a material will usually fulfill the chemical requirements once it is qualified. The same applies to the physical properties, e.g. solidus and liquidus temperature or thermal expansion. They are subject to the raw material production.
In contrast to this, the density of the final part is strongly influenced by the SLM process. Only complete powder melting, without vaporization of single alloy components, will lead to a fully dense part. This density also has a strong influence on the mechanical properties. They are further influenced by the microstructure and the surface quality of the part, the latter can be particularly influenced in the SLM process. Besides its impact on the mechanical properties, the surface roughness additionally affects the adhesion of the ceramic veneering.
The dimensional accuracy is also influenced by the parameters of the SLM process. It is, for example, strongly dependent on the layer thickness, the powder grain size and the laser beam focus diameter. Accuracy can be improved by reducing all three parameters, however, this results in a decrease of building speed and will thus lead to higher production costs.
Based on analysis of the required properties, density, 0.2% proof stress, elongation at break, dimensional accuracy and surface roughness can be identified as the crucial quality needs that can be influenced by the SLM process. The parameters and their required values are listed in table 2.
Table 2: Required parameters for dental frameworks.
Parameter Required value
Specific density d ± 5% from material specification [15]
0.2% proof stress Rp0.2 ≥ 360 MPa 1) [15] according to the intended application of the material. Dental frameworks can belong to type 3 or 4. In each case the higher value is considered here.
4.3 Comparison to the State of the Technology
Comparisons to references can be considered to get an initial impression of the ability of the SLM process to reliably fulfill these requirements. Though the specified dimensional accuracy of 0.02 - 0.05 mm (cf. chapter 2.3) in principle meets the required value of 50 µm, the reference frame is not completely clear. The specified value does not necessarily refer to the deviation in shape, so that the two values are not exactly comparable.
A clear definition of a required surface roughness is not given. Nevertheless, the achievable values presumably do not fulfill the requirements for the smooth interface between the restoration and teeth. Here manual or mechanical post processing work is inevitable.
As little research work deals with the repeatability of the SLM process and, furthermore, results for one material are not transferable to other materials, pre-tests were carried out for dental materials. In this case tensile specimens were produced by SLM as well as by precision casting. Two different dental laboratories performed the precision casting and the SLM samples were produced on a Realizer SLM 50 machine with a predefined data set for the CoCr material used. Tensile tests with six samples each were performed according to DIN EN ISO 22674 [15]. Figure 3 shows the tensile bar that was used according to this standard.
Figure 3: Tensile bar, according to [15].
Additional diameter measurements were carried out for these samples with a micrometer gauge. [17] Though it does not directly evaluate the shape deviation, this measurement of the dimensional accuracy can give a rough idea of the viable accuracy. For all sets of samples the mean values were calculated as well as the standard deviation. The results compared to the required values (cf. table 2) are shown in Figure 4.
It can be observed that in this test the SLM parts fulfill the requirements for the mechanical properties and the density. Compared to the precision cast parts, the mechanical properties of the SLM parts are higher and tend to show a smaller standard deviation. But a definite statement is not possible due to the small number of samples.
The SLM parts as well as the precision cast parts show small variations in density compared to the specified limits. As the density strongly influences the mechanical properties, it can be assumed that the given material specification tolerance of ± 5 % is far too high for the entire process. For dimensional accuracy evaluation, only a dimensional diameter tolerance was calculated. This is not comparable to the shape deviation.
As all the SLM samples for the pre-test were built within one build job, process variations are not considered. For a comprehensive understanding of the determining factors, further tests with parameter setting variations are necessary, consideration also needs to be given to repeatability.
Figure 4: Comparison between precision cast and SLM samples, according to [17].