# Application of Additive Manufacturing Technology for Chair Parts Connections
## 1 Introduction
PLA and ABS are the most commonly used materials to manufacture the filaments for AM in furniture manufacturing, but they are confronted with several drawbacks: PLA has a poor toughness, whilst the ABS is damaged by sunlight [15], for which reasons various composite materials are being developed to replace them [16; 17]. In addition to the above remarks, another drawback of FFF technology is that for highly complex parts, printing supports are necessary, which in certain cases are either difficult or even impossible to remove. Instead, SLS is an expensive technology but has the advantage of using a large variety of materials comprised in thermoplastic, ceramic, glass or metal powders, and has no limitations in producing geometrically complex parts for artistic purposes and pieces of furniture without using supports [18]. The powder that is not melted by the laser beam behaves like the supports made with FFF technology, only it is easily removed by brushing or blowing with air. Depending on the needs of the product designer, if you only want a quick check of the shape and the physical-mechanical properties are not very important, then FFF technology is chosen, and if you want a prototype with good properties, high precision, etc., and even if the time to obtain it is longer (compared to FFF technology--but lower than by other classic methods) then SLS technology can be chosen [19]. Using SLS technology, it is possible to realize microstructures at the scale of tens of microns and based on the elastic behavior of the aperiodic microstructures inspired by Voronoi open-cell foams, large objects can be generated with specific elasticity [20; 21; 22], including furniture parts or connectors. L-type corner joints for furniture are tested for bending strain, in the case of chairs [23], or case furniture [24; 25; 26]. The bending moment is calculated, depending on the test type, namely a tensile or compression test. Tests were performed on chair solid wood parts [23; 27], or wood-based panels [24; 25]. Wood dowels [24; 27], dowels and innovative fasteners made from polyamide and obtained by 3D printing using the SLS technology [24; 26] were used for jointing the panels. The strength of the corner joints connected with the wood dowels was proved to be significantly higher than those connected with the 3D-printed dowels [24]. In order to test the strength and stiffness of the L-type corner joints, both for solid wood and wood-based panels, two methods are applied in the literature, namely the diagonal compression and the diagonal tensile test, respectively [11; 23; 24; 25]. A chair was built with 3D-printed connectors designed for chair leg and side rails jointing,made from ABS filament by using the FDM-printing technology [28]. The wooden parts were made of beech wood, and connectors with wall thicknesses of 3 mm and 5 mm, respectively, were bonded with the wooden parts with a polyurethane adhesive. Average failure forces for the connectors (in the range of 790 N-905 N) and for the wood mortise-and-tenon joints (1769 N) were recorded, showing the mechanical performance of the connectors at the level of at most 51% of the mechanical performance of the traditional wooden jointing. The present research investigates the quality and the strength and stiffness of the 3D-printed connectors designed to assemble the leg and two stretchers of a chair. The proposed 3D-printing method in the research is SLS technology and the material used for 3D printing is Polyamide 12 (PA12). The assembly composed of connector and chair components forms an L-type corner joint. Both a diagonal compression test and a diagonal tensile test were performed on the assembled parts and the bending moment was calculated. The results were compared to those of the reference assembly, made of larch solid wood components assembled by a glued mortise and tenon joint.