Optical Measurement and Numerical Simulation of Parison Formation in Blow Moulding

Blow moulding is a process whereby a cylindrical parison is extruded first, then pinched between the two halves of a mould and finally blown into the product. Parison size and shape are resulting from complex interactions between mandrel and die geometries, processing parameters and viscoelastic properties of the polymeric material. Moreover, parison size changes with time due to sagging effect. An innovative, contactless and on-line measurement technique of the parison as it is extruded is shown to be an effective tool to measure precisely parison diameter and thickness and to capture dimensional changes during time. This technique employs laser lighting of the parison and hinges on the refractive properties of molten polymer. Images taken with a digital camera are processed to give a precise measurement of diameter and thickness, at different time step during extrusion. Thus, parison swell and sag have been recorded for a commercial HDPE. Influence of processing parameters such as the rotational speed screw speed or die gap width can be brought forward. Moreover, numerical simulations of the parison formation process have been carried out, with an integral viscoelastic constitutive equation.