Polyether–ether–ketone (PEEK) as an engineering plastic with excellent mechanical properties requires strict processing conditions. Introducing microcells in PEEK parts is beneficial for reducing the material cost and improving their tenacity. Herein, foamed PEEK parts are fabricated using microcellular injection molding with supercritical nitrogen as the blowing agent. Using optimum processing conditions obtained from orthogonal array experiments based on finite‐element simulation, the foamed PEEK parts achieve a 10% average weight reduction and display a typical sandwich structure with an average cell size of 30 μm in the core region. The cell size is larger, whereas the cell density and skin layer thickness are smaller in the region near the gate than the region far from the gate. The foamed PEEK parts show a slight drop in the tensile modulus and tensile strength but a significant improvement in the elongation at break when compared with the solid parts. Moreover, the foamed PEEK parts with weld lines exhibit less reduction in tensile properties than the solid counterparts, attributed to their tighter interfacial adhesion at the weld line. This study successfully produces foamed PEEK parts with high weight reduction and ductility and provides insights into the weld line mitigation through microcellular injection molding.

Polyformaldehyde (POM) and Polyamide 66 (PA66) are engineering plastics with excellent mechanical properties and thermal stability. Producing microcellular injection molded POM and PA66 parts with high dimensional accuracy would be beneficial to reduce material cost and product quality. In this research, foamed POM and PA66 gear parts were fabricated by using microcellular injection molding with supercritical nitrogen as the blowing agent. Compared to conventional injection molded parts (parts that foaming is not involved), the foamed POM and PA66 gear parts achieved 5% and 10% average weight reduction, respectively. The foamed parts displayed a lower shrinkage ratio when compared to the solid counterparts, which was attributed to the cell expansion that offset part of the inward shrinkage stress. Moreover, POM gear parts with a higher crystallinity degree presented more serious shrinkage ratio compared to the PA66 gear parts, which contributed to the denser polymer molecular chains arrangement. The shrinkage ratio in both directions of PA66 foamed gear parts depended on the injection volume, and the lowest shrinkage ratio of 0.043‰ was obtained at the injection volume of 74 mm, when the polymer reached the maximum foaming ratio. The findings from this study could provide practical guidance for preparing microcellular injection molded products with high dimensional accuracy.