A route to improving elongation of high-temperature laser sintered PEKK

Laser sintering (LS) is one of the most popular additive manufacturing (AM) techniques as it produces parts of complex geometry with high dimensional accuracy and good mechanical strength. However, the nature of the LS process often leads to brittle behavior characterized by a low elongation at break if compared to conventional polymer processing techniques, e.g., injection molding (IM). For poly(ether ketone ketone) (PEKK), such elongation is currently below 3%. This study determines and then optimizes the relationship between cooling time and crystallization of PEKK during LS and the resulting elongation at break. The elongation at break of PEKK was successfully improved by using shorter cooling times. The combination of the slow crystallization kinetics of PEKK and a short cooling time of 1 h increased elongation at break to 14%; this is a striking result never achieved for PAEKs in LS before. A calibration curve was developed that can be used to correlate PEKK structure and mechanical properties to cooling conditions according to the application. This methodology can also be applied to select and optimize the mechanical properties of other LS polymers sharing similar kinetics of crystallization and processing temperatures. This work suggests a great potential for a wide range of "post-processing" heat treatments to be used in AM to tailor the ultimate mechanical properties.