Relationships between porosity and mechanical properties of polyamide 12 parts produced using the laser sintering and multi-jet fusion powder bed fusion processes

Laser powder bed fusion (L-PBF, or Laser Sintering) and Multi-Jet Fusion (MJF) are two commercial powder bed fusion (PBF) additive manufacturing (AM) processes for polymers. While the strength of parts produced with polymer PBF processes is typically good, the toughness of AM parts is significantly reduced compared to traditional processes, such as Injection Molding (IM). This study evaluates the impact of residual porosity on the strength and ductility of polyamide-12 (PA-12) parts produced by powder bed fusion. Hot isostatic pressure (HIP) treatment is used to eliminate virtually all porosity from treated L-PBF and MJF parts. The relationship between porosity and mechanical properties (ultimate tensile strength, elongation at break, toughness) is evaluated for groups of parts with and without HIP treatment. For untreated parts, the thickness of the part was found to have a significant impact on mechanical properties, with 5 mm parts having more than 25% higher UTS, 39% higher maximum strains, and 21% higher Young's Moduli than 1 mm parts produced using the same process. In L-PBF parts, the HIP treatment greatly reduced porosity and increased ultimate tensile strengths by an average of 12% compared to parts that were not subjected to HIP, but they also exhibited reduced max strains. While HIP eliminated porosity in MJF parts, MJF parts did not exhibit any statistically significant improvements in mechanical properties from HIP. Though it was not possible to correlate the strength of an untreated part with density, a correlation does exist between density and some mechanical properties (UTS, Young's Modulus) as a result of a density change in L-PBF parts due to a post-processing HIP treatment.