Optical measurement and modelling of parison sag and swell 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 result 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 sag. An innovative, contactless and online measurement technique of the parison is shown to be an effective tool to measure precisely parison diameter and thickness and to capture dimensional changes with 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 screw speed or die gap width can be brought forward. From the measurements, thickness swell is found to possess a different behaviour from diameter swell. Moreover, sag has been measured and can be modelled from a Newtonian perspective using one dimensional convected coordinates. Parison sag is shown to be governed by two parameters: an extrusion velocity and a a single coefficient of sagging susceptibility which value has been deduced from experiments.