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.

Date of Award	1972
Original language	English