# Application of Additive Manufacturing Technology for Chair Parts Connections
## 3 Results and Discussion
### Mechanical Properties
of warping which stops the deposition and sintering of new powder layer, or due to the balling effect which hinders the uniform deposition of the new powder on the previous one, favoring the occurrence of cracks and porosity [9]. According to the results obtained by other researchers [13; 24], the strength and stiffness of the corner joints connected with auxetic dowels made from Polyamide 12 powder by using the 3D-printing SLS technology were significantly lower than those connected with the reference traditional wooden dowels used for case furniture joints. As in the case of case-type furniture [13; 24], the scope of the present study is to provide a one-time ready-to-assemble (RTA) construction of the chairs. As remarked by the authors of the previous mentioned research works, the aim of the engineering design of the products is a compromise between technical and economic considerations, in terms of strength-costs appreciation. Over the traditional glued wooden joints used in the furniture industry, the 3D-printed fasteners or connectors are easy to assemble and have the advantages of shortening the production time by reducing the mounting operations and implicitly the final cost of the product. Figure 8 presents the typical defects occurring after applying tensile (a) and compression (b) loads on the mortise-tenon jointed chair elements. Both compression and tensile testing affect the joint and destroy the adherence between the mortise and tenon and in other cases lead to the occurrence of deep cracks in the leg element. The failure of these assembled parts happens for breaking forces in the range 333 N-573 N for the tensile test and 329 N-415 N for the compression test. Under these loads, the assembled parts were destroyed so that the jointing became nonfunctional. Contrarily, the use of 3D-printed connector in the assembled parts did not affect the integrity of the wood stretchers and leg after mechanical testing, all stresses being supported by the connector in this case, and the joint failing in the 3D-printed materials. The same remark was made by other researchers [28] for the connectors used between the leg and side rails of a chair made of beech wood, for which an ABS filament was used and the FDM-printing technology was employed. In this case, the FEM analyses have shown that the maximal values of stresses occurred at the upper edges of the connectors, in the area of the connection between the side rail and the leg, showing that the end part of the rail presses from inside to the wall of the connector, as noticed in the present research. Figure 8: Typical defects occurred in the reference sample after mechanical testing: (**a**) at tensile; (**b**) at compression. Figure 9 shows the typical defects occurring on the connector after conducting the mechanical tests under tensile (a) and compression (b) loads. For both types of loads, cracks occurred on the corner of the connector part where the stretchers were mounted, on the longitudinal direction of the stretcher. Only one connector was totally broken perpendicular to the stretcher direction, because of the force exerted by the end of the stretcher to the connector wall (Figure 9b right). In this case, the stress was applied along with the layers and the fracture occurred at the weak area, probably between two adjacent layers sintered by partially melting. This phenomenon was explained by other researchers [9], and it is related to the sizes of the powder particles, reaching a maximum of 100 \(\upmu\)m, as presented in Figure 3. The small powder particles are melted during the sintering process and work as a functional binder, whilst the large particles remain solid. Other researchers noticed that the size distribution of the particles has an influence on the porosity formation of the sintered layers [30]. Thus, the smallest particles are molten even with a low energy density, and the molten material is welding the layers.