# Production and Processing of a Spherical Polybutylene Terephthalate Powder for Laser Sintering
## 1 Introduction  
Industry increasingly adopts additive manufacturing (AM) as a way to produce prototypes and small series of end-use parts in a fast, reliable, and cost-effective manner. Multiple studies [1; 2; 3] indicate that amongst the AM technologies that use polymers as a feedstock, laser sintering offers high levels of design freedom and productivity. The limitations of the laser sintering process are the high equipment investment costs, and the limited number of usable materials. Laser sintering is a powder-based process wherein parts are created layer-by-layer. A roller or blade first deposits a thin powder layer of typically 100-150 \(\upmu\)m thickness. The cross-sections of parts present in this layer are molten by scanning the surface with a CO\({}_{2}\) laser. As a result, the powder particles fuse together with the underlying layer and with each other. Once scanning has finished, the build platform lowers by a single layer thickness, and the process is repeated. In this way, three-dimensional objects are created. A powder material must fulfill a number of requirements in order to be suitable for laser sintering, as summarized by [PERSON] [4]. Materials must be carefully designed, both on an intrinsic polymer level, and on an extrinsic powder level. The most important requirements are summarized in Figure 1.  
The intrinsic polymer properties in the bottom section of the diagram in Figure 1 can be established by choice of the material grade, or better, by specific design of the polymer chains and architecture via chemistry. The extrinsic properties in the top part of the diagram are those associated with the shape, size, and mechanics of the powder particles the material consists of. Tuning and optimization of these properties is done by selection of an appropriate powder production technology and subsequent classification methods. Because of these many requirements, there are only a handful materials on the market that can be used with the laser sintering process. Market studies [6] show that the largest portion of parts built with laser sintering is based on polyamide (PA), but that there is a strong desire for parts made from different materials. One of these materials is polybutylene terephthalate (PBT). PBT has a good chemical resistance and is characterized by a high insulation resistance and dielectric strength. This makes it the material of choice for housings for electrical appliances and various under-the-hood car parts. The ability to produce such parts with laser sintering, thereby making use of the full design freedom the process offers, opens up many possibilities. Examples are weight reduction, downsizing, and consolidation of many parts into a single laser sintered one. Various authors described the production and use of PBT powders for laser sintering. Investigating five different PBT grades, [PERSON] et al. [7] reported the production of powders by cryogenic grinding, and their subsequent processing on an SLS machine. The parts exhibited a relatively high Young's modulus. However, tensile strength and elongation at break compared to injection-molded parts were limited, signaling a need for further improvement. [PERSON] et al. [8; 9; 10; 11] produced powders by wet grinding PBT granules, then running the milled powder through a so-called downer reactor to obtain spherical particles. Parts consisting only of a single layer could be produced with laser sintering, but no mechanical properties were tested. The performance of dry blends of PA12 mixed with up to 30% PBT was investigated by [PERSON] et al. [12]. The authors reported a minimal increase of flexural modulus upon blending PA12 with 10% PBT. The  
Figure 1: Critical properties for a polymer laser sintering material. Adapted with permission from [PERSON] et al. [5]. blends were otherwise featured by phase separation and inferior mechanical properties compared to the PA12 base component.