On-line optical measurement of parisons in blow moulding

Blow moulding is a process whereby a cylindrical parison is extruded first, then pinched by the two halves of a closing mould and finally blown into the product. The actual size and shape of the parison is the result of complex interactions between the die and mandrel geometries, the viscoelastic properties of the polymer and the processing parameters. In practice, obtaining a satisfactory parison is a time-consuming trial and error process. A novel technique using laser lighting of the parison at different angles, during extrusion, gives access to the size and thickness along the entire length of the parison and to the shape of the cross-section at a given location. Results are given for a polypropylene an a polyethylene materials, together with extrusion simulations of a parison with an elliptic cross-section. Increasing the flow rate is increasing the swelling of the parison and its thickness. This effect is more marked with Polypropylene than with Polyethylene.