# Production and Processing of a Spherical Polybutylene Terephthalate Powder for Laser Sintering
## 4 Conclusions  
The melt emulsification method is exceptionally suited to make powders with spherical geometry in the size range that is required for laser sintering. Compared to conventional laser sintering powders, the powder that was fabricated in this study showed outstanding flowability and packing density properties. The measured properties could be confirmed through the application of the powder in a commercial laser sintering system.  
As a powder production technology for polyesters in general and PBT in particular, melt emulsification however presents critical limitations. It could be demonstrated that the polymer degraded considerably over the course of the powder production process. This initially translated to a narrower process window. For parts that could be built, inferior mechanical properties were found for both laser sintered and compression molded parts made from powder, compared to injection-molded parts made from the starting granulate.  
The initial melt emulsification process step in particular leads to degradation, first of all because a PBT grade without heat and hydrolysis stabilization was used in this study. A similar PBT grade with additives showed comparable degradation effects, which may be traced back to the water content in the matrix phase and washing out of stabilizers. The selection of a different water-soluble matrix polymer such as polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP) could help reducing hydrolysis during extrusion. Enhanced drying, for example in a circulating hot air dryer would also lead to an improvement. Finally, the addition of more or, other additives can reduce thermal degradation and hydrolysis during all process steps.  
This study could validate the viability of the melt emulsification process as a means to produce laser sintering powders with outstanding particle shape, size and flowability. The process is scalable and can be extended to other material systems to allow the production of a wide spectrum of laser sintering powders. The economically viable production of powders on an industrial scale can be realized by combination of individual process steps. Extrusion, washing, and sedimentation can be performed in a single continuous step for example. By such optimizations, the method need not be much more expensive than other powder production processes, such as cryogenic milling. Possible additional costs for producing the powder with melt emulsification can be offset by the benefit of obtaining powders with spherical particles.  
[PERSON] conducted the trials, and wrote the paper. [PERSON] initiated the project, assisted with planning trials, and interpretation of results. [PERSON] is supervisor of the group, initiated the idea of producing new powders for laser sintering, and gave valuable input on all steps of the study progress.  
This project was financially supported by the Swiss Commission for Technology and Innovation (CTI) under project grant nr. 19194.1.  
[PERSON] of Geberit AG is gratefully acknowledged for his assistance with blend preparation, tensile testing, and rheological measurements. The authors also wish to extend their gratitude to [PERSON] of Geberit AG for his invaluable feedback and contributions to this work. [PERSON] of the Scientific Center for Optical and Electron Microscopy (ScopeM) at ETH Zurich is acknowledged for his assistance with acquiring SEM images.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.