Molding PVC With a Cushion…

The basics of scientific molding is the same whether you are molding CPVC or PP as it is with large parts vs. small parts. Essentially, the difference between is in the range and scale of parameters and not the scientific approach.

Question: Isn’t it dangerous to mold PVC with a cushion since the extra material in front of the screw will degrade?
My Response: This has a negligible affect on the residence time since the cushion is a very small fraction of entire volume of material within the barrel. With PVC , it is vitally important to mold with a cushion since the performance and repeat ability of the smear tip is greatly influenced by the material viscosity. Frequent variation in part weight and quality will occur if a cushion is not used because viscosity of PVC is greatly influenced by small material temperature fluctuations.
Additional Information: Since the screw typically contains approximately 2 times the maximum shot capacity of the molding machine, a 5-10% cushion will not affect the integrity of the PVC within the barrel.

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