# Production and Processing of a Spherical Polybutylene Terephthalate Powder for Laser Sintering
## 2 Materials and Methods  
Materials: PEG Polyglykol 35000 S (Clariant, Muttenz, Switzerland) flakes were ordered and used without further purification or drying. PBT TORAYCON 1200M (TORAY, Tokyo, Japan) was purchased in granulate form. The material did not contain any stabilizers or other additives so as to prevent these from negatively influencing the laser sintering process. The material was dried before use. A second reference PBT CCP PBT 1100-5600 (Chang Chung Plastics, Taipei, Taiwan) granulate was acquired as well. This material contained stabilizers and process aids. The material was dried before use. Fumed silica flow aid Aerosil R812 (Evonik, Essen, Germany) used without further alteration. Extrusion: the materials were mixed in a laboratory scale Extrusiograph (Brabender Technologie, Duisburg, Germany) single-screw extruder with a barrel temperature profile of 230-240-250-250 \({}^{\circ}\)C going from inlet to nozzle. The screw had a 19 mm base diameter and L/D ratio of 25.7. A wide-slit nozzle was used to produce the bulk of the material, which was collected and passively cooled down against ambient air on a steel sheet. Powder extraction: the slabs obtained from extrusion were broken into small pieces and divided into batches of 2.5 kg each. To each batch, 10 L water was added and stirred in a container inside a concrete mixer for two hours. The mixture was then allowed to settle for 12 hours, after which the watery phase was removed. Another 10 L fresh water was added to the remaining slurry, and the process was repeated. The complete washing cycle was carried out three times for each batch. Removal of the PEG phase was confirmed by DSC measurement. Powder classification and drying: the fine fraction of the powder, consisting of particles smaller than 10 \(\upmu\)m, was removed using a specially designed settling tank. Depending on the flow rate of a dilute dispersion of powder in water through the settling tank, particles of certain size settle to its bottom. Small enough particles with reduced settling velocity do not reach the bottom of the tank before the water reaches the outlet, and are washed out in this way. In a subsequent step, the slurry at the bottom of the tank was collected, and the majority of water was removed by vacuum filtration over a Buchner funnel. The final traces of water were removed by drying the material for 24 hours at  
Figure 2: Schematic overview of the powder production process. PBT: polybutylene terephthalate. 80 \({}^{\circ}\)C in a vacuum drying oven. The powder was then sieved over a sieve with a 150 \(\upmu\)m mesh width. The powder was dry blended with 0.05 wt. % funnel silica as flowing aid. Scanning electron microscopy: scanning electron microscopy images were obtained on a JSM 7100F scanning electron microscope (JEOL, Tokyo, Japan). Samples were deposited on a piece of carbon tape on an aluminium SEM stub and sputter coated with a 10 nm layer of Pt/Pd (80/20). Optical microscopy: to assess the morphology and microstructure of built parts, a sample was fractured and embedded in resin. The sample was sanded, polished, and observed using a DM6 optical microscope (Leica Microsystems, Wetzlar, Germany) with incident light and differential interference contrast. Particle size distribution: the particle size distribution was measured by dynamic light scattering on an LS230 (Beckman Coulter, Brea, CA, United States) instrument. Approximately 0.1 g of powder was added to approximately 20 ml demineralized water and thoroughly stirred. Before measurement, the dispersion was treated with ultrasound to break apart any agglomerates. The particle size was calculated based on the Fraunhofer model, for the range between 0.4-2000 \(\upmu\)m.  
Particle size and shape distribution: the size and shape of particles was recorded optically with a DM6 optical microscope (Leica Microsystems, Wetzlar, Germany) at 100x magnification in transmission.