Factors Affecting Productivity and Quality in the Extrusion of Polyvinylchloride Powder

Abstract

Microscope examination of material from a single screw extruder screw channel following removal from the extruder barrel showed that the fusion mechanism and fusion rate depended on the lubricants used. One of two different fusion mechanisms occurred depending on the formulation, The "rear of the channel fusion model" produced rapid fusion and promoted mixing, while the 'forward fusion model" resulted in incomplete fusion and the presence of unfused PVC particles in the extrudate. Rate of fusion depended on the level of the various additives. Thus increasing the level of incompatible lubricant decreased the foster rate, while incorporation of process aids, impact modifiers and filler increased the fusion rate. Output rate was dependent on the rate of fusion with the result that increasing the fusion rate decreased the output rate. This means that factors causing fusion rate variations will also result in output rate fluctuations. For maximum thermal stability an optimum fusion rate exists as earlier fusion results in overheating while late or incomplete fusion results. in late or inadequate incorporation of stabiliser. The existance of two possible mechanisms explains why, following (for economic reasons) the superseding of granule extrusion by direct powder extrusion, British extrusion practice favoured the imported twin screw extruder, whereas United States practice was to continue with the cheaper rugged single screw machine.