Shrinkage behaviour of semi-crystalline polymers in laser sintering: PEKK and PA12

Abstract

Shrinkage is extensively mentioned in the literature as one of the main causes for dimensional instability or poor performance in Laser Sintering (LS). This study proposes and examines a methodology to describe shrinkage in cooling from a material perspective. Thermal behaviour and crystallisation effects were measured to determine the influence of powder structure and density on the overall LS shrinkage. PEKK and PA12 powders were used to assess such behaviour. The shrinkage parameter associated with powder bulk properties has the greatest impact in PEKK, contributing to 57% of the total shrinkage observed in cooling for this material as supported by the low values of bulk density, irregular morphology and internal porosity observed for these particles. For PA12, crystallisation is responsible for 60% of the overall shrinkage observed. In LS, PEKK shows an overall shrinkage approximately 30% lower than PA12, which makes it a promising material for maintaining final part dimensions. Keywords: Laser sintering, Shrinkage, Powder bulk density, Crystallisation, PEKK, PA12