Tough and defect-free-surfaces PA66/PTFE composite foam with bimodal structure achieved by high-pressure foam injection molding with mold-opening

Polyamide (PA) 66/polytetrafluoroethylene (PTFE) composite foams with bimodal cellular structures were prepared in this work via mold opening foam injection molding (MOFIM) process. PTFE nanofibrils were in-situ fibrillated within the PA66 matrix during the twin-screw extrusion process, and the PTFE fibrous network significantly enhanced the viscoelastic properties and melt strength of PA66, thus improving the foaming characteristics of the composite material. When the content of PTFE was 5 wt%, the PA66/PTFE composite foam showed a distinct bimodal cellular structure owing to the aggregation of saturated nitrogen (N₂) caused by the PTFE network-induced crystallization. The bimodal cells, the PTFE network, and the refined crystalline structure contributed to a significant enhancement in the ductility and mechanical properties of the bimodal PA66/PTFE composite foam, which achieved an improvement in toughness of up to 402.2 %. Moreover, the MOFIM PA66/PTFE composite foam demonstrated superior surface quality due to the two-step foaming in the MOFIM process and the enhanced melt strength by the PTFE network. This work provides new insights into the fabrication of lightweight injection-molded PA composite parts with high performance and surface quality.