Process-Controlled Crystallinity in PA12 Produced by Multi Jet Fusion and Hot Pressing: Tailoring Cooling Profiles to Enhance Fatigue Resistance

This work investigates the role of manufacturing-induced crystallinity on the fatigue behavior of Polyamide 12 (PA12), with particular emphasis on the role of temperature in fatigue performance. Specimens produced by Multi Jet Fusion (MJF) are compared with hot-pressed (HP) counterparts manufactured from the same powder batch under controlled cooling conditions. An extended cooling (EC) protocol is introduced to tailor crystallinity by prolonging the residence time within the crystallization window. Comprehensive thermal, microstructural, and mechanical characterization is combined with fatigue testing and in-situ infrared thermography. Results show that higher crystallinity significantly reduces viscoelastic dissipation and self-heating, leading to improved fatigue resistance. Despite lower defect content, standard HP specimens exhibit inferior fatigue performance compared to MJF due to reduced crystallinity. The EC protocol effectively bridges this gap, enhancing fatigue life and matching the MJF fatigue performance. Based on these findings, optimized cooling parameters are proposed and experimentally validated, demonstrating that crystallinity control is an effective process-level strategy to improve the fatigue performance of PA12.