A case history for the process development of an injection molded polyester alloy medical device was presented. It was determined that traditional molecular weight determinations was difficult and not easily correlated with performance. However, a rheological parameter, the steady shear viscosity was found to control the failure rate with good predictive power. In the course of the investigation, it was also found that moisture level much lower than what was called for was required to insure good molecular weight retention and prevent failures. A modified ASTM melt flow procedure was developed that facilitated date comparison between multiple sites along the production path for 'defect free' production.