Behaviour of molten polyolefins extruded from an annular die

In the extrusion blow molding process, the behaviour of the parison prior to inflation is of central importance. Properties of the finished article, such as wall thickness and thickness uniformity, are determined by the simultaneous phenomena of swell and sag.In the present study, an apparatus was built to study the behaviour of the extrudate from an annular die under isothermal conditions. First, sag was eliminated by extruding into an oil bath, and both thickness and diameter swell were determined as functions of time. Then, extruding into heated air, sag behaviour was observed.These procedures allowed the study of the individual contributions of swell and sag to the overall parison behaviour.Three high density polyethylene resins and a polypropylene resin were studied. The materials were thoroughly characterized to obtain the traditional rheological properties. In addition, an extensional rheometer was modified to measure the viscosity under uniaxial extension.A model to predict the total length of the parison at any given time is proposed. This model is expressed in terms of rheological properties of the materials and process variables only