The dynamics of parison free inflation in extrusion blow molding

Parison free inflation behavior, associated with the extrusion blow molding process, is considered both experimentally and theoretically. Experimental observations indicate1 that the parison assumes a rather complex shape under conditions of unrestricted inflation. In particular, the time‐dependent shape is markedly ellipsoidal rather than cylindrical in nature. This nonuniform behavior, however, becomes more prominent in relation to the entire length as the parison‐length‐to‐diameter ratio is decreased. Based on the experimental observations, a simplified analytical treatment of the free inflation of a viscoelastic parison is presented. The theoretical results illuminate the influence of material properties and process conditions on the inflation process. Expectedly, inflation is enhanced by an increase in the pressure driving force as well as by a decrease in viscosity. However, melt elasticity is also found to exert a significant influence on the inflation behavior. Moreover, the theoretical analysis suggests that the initial parison dimensions play a central role in controlling the inflation process.