Previous studies have shown that defects like pin-holes and air entrainment are common during polymer electrolyte membrane (PEM) manufacturing. For mass production of defect-free membranes, an optimized manufacturing process is required. The objective of this work is to understand the nature of defect generation during the manufacturing process of high-temperature PEM membranes and the factors influencing them. An extrusion process is simulated using a multiphase, laminar model using computational fluid dynamics (CFD) techniques. The effects of processing conditions on quality of the cast and pressure drop are analyzed. The results showed that for a constant inlet to substrate velocity ratio, both the substrate velocity and temperature will have an insignificant effect on the entrained air height. However, their effect on pressure drop is found to be substantial. Further, it is shown that the inlet velocity and slot gap width have significant effect on the entrained air height and pressure drop. POLYM. ENG. SCI., 51:1701-1709,2011. (C) 2010 Society of Plastics Engineers