Assessment of non-uniform shrinkage in the laser sintering of polymer materials

Laser sintering technologies are well established in the rapid prototyping and rapid manufacturing industries, producing a wide range of products for an ever-increasing number of industry sectors. In theory, these processes facilitate direct digital fabrication from 3D CAD models, creating items that have very sophisticated geometries with high levels of quality and accuracy. However, the laser sintering fabrication process is susceptible to part shrinkage which in turn may lead to dimensional inaccuracies (and therefore quality degradation) of the finished item. This paper reviews the issue of shrinkage for laser sintering in the z-axis, and using an EOS P700 machine operating with polyamide-12 material, demonstrates a technique for the assessment of this problem. The experimental results quantify the magnitude of z-axis shrinkage, finding it to be both non-linear and linked to thermal inconsistencies in the build chamber. The study shows that z-axis non-linear shrinkage is likely in all builds; however, its linearity may be improved through placement of products in the centre of the build chamber. The data presented highlight that current operational practices of factorial adjustment for each axis of the build chamber are inadequate and demonstrates the implications of these inaccuracies across different size build components.