# Application of Additive Manufacturing Technology for Chair Parts Connections
## 1 Introduction  
Nowadays, additive manufacturing (AM) is gaining more and more attention in the industrial field for rapid prototyping, rapid manufacturing, mass customization and mass production [1]. Based on CAD modelling, this technology is supported by a diversity of professional software able to create bold shapes, with no limits of creativity and has the potential to transform new ideas and methods outside of 'conventional' industry norms [2]. One of the methods applied in additive manufacturing is selective laser sintering (SLS). Powder raw material, such as nylon, bio-plastics, ceramics, wax, bronze, stainless steel, cobalt chrome and titanium, is sintered or fused by the application of a laser beam for each layer specified by the 3D model [1, 3, 4]. The minimum layer thickness for this technology is 0.1 mm and the advantage of applying this method is the fast build speed, but the quality of the surface is appreciated as an average one [4]. According to the findings in the literature, the process input parameters, such as scanning rate, laser power, hatching distance or scanning pattern, can cause defects in the structure of the printed part or an insufficient density of the sintered material [5]. During the SLS processing, sintered, semi-sintered/semi-melted or completely melted surfaces can be formed [6]. The most important factor affecting the quality of the SLS printed surfaces is the scanning pattern, varying from the raster in X or Y direction, alternating in the X and Y direction, spiral, fractal, or scanning of successive sectors [5]. Applying a raster in alternating in the X and Y direction resulted in a higher fractional density comparing to the use of the raster strategy in the X or Y direction [7]. On the other hand, the particle fusion can be improved by high energydensity, with a benefit on the mechanical properties of the final part. The energy radiated into the powder bed produces the melting and the layers adhesion, so the higher the energy, the better the connection between the two successive sintered layers, and the sintered part can be generated without layer delamination [8]. Despite its advantages, the ability of SLS to process polymers is affected by some defects, such as the porous structure and balling effect [6; 9]. The causes of these defects were found to be the high laser power and the fast scan speed and interruptions near fused powders. The balling defect affects the uniformity of the powder layer deposited on the previously sintered one and often leads to cracks, porosity and a rough surface [9]. Another particularity of this technology is that the final product needs a long cooling time and also, the non-fused powder remaining on the part exterior surface has to be removed. Accompanying the Industry 4.0 digital revolution, additive manufacturing (AM) is considered a key technology [10]. At first being applied for fast prototyping in medicine, aerospace and automotive industry, it became rapidly widespread in other sectors, including the furniture industry. There are three types of applications where 3D-printing technology is used in furniture manufacturing, as follows: the entire piece of furniture is manufactured in 3D printing, some parts of the furniture are 3D printed, or only connection parts or fasteners are produced using this technology. The variety of materials available today for AM and the corresponding 3D-printing technologies and printers are the source of inspiration for freedom design with complex geometries. In the furniture industry, the most used materials are thermoplastic materials in the form of filaments, used in the case of applying fused filament fabrication (FFF), or fused deposition modeling (FDM) [10; 11; 12; 13; 14], and thermoplastic powders, used when applying selective laser sintering (SLS) technology [10].