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The concept of injection molding

The concept of injection molding is to fill plastic materials to the cavity with the least pressure. Because of the compressible nature of a polymer melt there will be a pressure difference between the last point to fill and the gate with a matching difference in density and therefore shrinkage. If you can fill the cavity with a low pressure then the pressure difference from lptf and gate is smallest. To fill the cavity with the least pressure means that the apparent viscosity of the melt must be as low as possible and this is a function of temperature and shear rate (which for any single injection mold can be related to speed of filling). Ultimate filling speed is frequently compromised by poor venting resulting in gas burns or 'Dieseling'. Often the filling speed is reduced (wrongly) in preference to pulling the injection mold and improving the venting - but it is easier to do!

Adiabatic heating of the polymer melt means that it is possible heat the polymer above the manufacturers recommended maximum without degradation because is only at such a high temperature for a short time. You can get a similar beneficial effect by making cold runners small in cross-section so you raise the temperature of the melt by a further 20 - 30 Celsius. In this way you produce plastic parts with lower stresses meaning that you can de-mold at higher temperatures in shorter cycles without compromise to dimensional stability - and improve the bottom line all at the same time.

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