A study of parison development in extrusion blow molding

The dynamics of parison development in the extrusion blow molding process have been studied by employing a pinch-off mold technique in conjunction with experimental photography. The experiments yielded quantitative information of the parison length, diameter, and weight distribution as a function of extrusion time and axial distance. At a given extrusion time, parison shape is controlled by the two competing influences of swell and drawdown. The initial phase of parison extrusion is dominated by swell effects, which are then gradually overshadowed by the growing influence of drawdown arising from the weight of the parison. In addition, the results indicate the strongly time-dependent nature of both the diameter and weight swell functions. The relationship between these swell functions suggests that the deformation is slightly anisotropic for the polyethylene resin used in this study. Moreover, a modelling approach, based on a combination of the competing effects due to swell and drawdown, has been proposed in order to develop an approximate quantitative analysis capable of predicting parison dimensions.