# Application of Additive Manufacturing Technology for Chair Parts Connections
## 3 Results and Discussion  
### Mechanical Properties  
The bending moment capacities under tension (\(Mt\)) and compression (\(Mc\)) of the joints were calculated (in Nm) based on the maximum failure load. The results are presented in the diagram in Figure 7.  
Figure 5: Loading of the L-type corner joint specimens: (**a**) under tensile testing, (**b**) under compression. Figure 6: L-type corner joint test specimens: (**a**) diagonal tensile testing for mortise–tenon jointed parts (**top**) and specimen with a 3D-printed connector (**bottom**); (**b**) diagonal compression testing for mortise–tenon jointed parts (**left**) and specimen with a 3D-printed connector (**right**). The results from Figure 7 show that the mechanical performance of the chair parts assembled with the 3D-printed connector using the sintering technology and PA12 powder is lower than that of the classical mortise-tenon joints made of larch wood. Unsatisfactory results regarding the strength of the L-corner joints for case furniture were obtained by other researchers [24], who replaced the traditional wooden dowels with 3D-printed ones, made of PA12 using the SLS technology. First of all, it has to be noticed that the bending moment under compression (_Mc_) is higher than the bending moment under tensile testing (_M_\(\bar{\text{t}}\)) both for the investigated assembly and the reference sample. Whilst the bending moment at compression for the chair parts assembled with the 3D-printed connector is 24.5% lower than the one of the reference, in the case of the bending moment at tensile testing, that difference is higher, with a percentage of 35.8%. The maximum forces recorded by the L-corner joints were higher in the case of diagonal tensile testing than in the case of diagonal compression testing both for the reference assembly and for the assembly where the 3D-printed connector was used. The average values of the maximum forces recorded when using the 3D-printed connector were of 350 N for the tensile load and 209 N for the compression load, representing 65% and 58%, respectively, of the values recorded by the reference assembly. Comparatively, when testing the chairs [28], the results showed that joints with 3D-printed connectors had a 42-51% lower failure load than that achieved with traditional wooden mortise-tenon joints with similar dimensions. Even so, the chair had enough strength for a person to sit on it [28]. It comes to the conclusion that an excessive strength of the products could bring unnecessary costs, which can be reduced by finding cheaper solutions with a lower but sufficient strength of the product [24]. Based on the conclusions of the researchers mentioned before [24; 28], by using the 3D-printed connectors proposed in the present paper for jointing the chair elements, it is possible to achieve enough strength of the final product and this will be the subject of further research. An explanation of the unsatisfactory results obtained by the structures with 3D-printed connectors could be the fact that the adjacent powder layers are not sintered and bonded together in the same way. A cause can be an insufficient density of the sintered material determined by the scanning rate, laser power or hatching distance [5], or due to the semi-sintered/semi-melted surfaces that can be formed during the SLS processing [6]. In an ideal case, when using optimized laser energy, the laser sinters the new powder layer and bonds it to the adjacent one. In this case, the 3D-printed objects have a high density (728 kg/m\({}^{3}\)) and good mechanical properties in terms of the flexural modulus with a value of 337.1 MPa [9; 29]. The defects in the structure of 3D-printing parts with SLS technology occur because the top layer might be removed during the recoating of new layers, or because  
Figure 7: Tensile and compression bending moments of the joints.