I n Moulding process the rubber is Cured at high temperature and Pressure, the resulting Moulded parts are smaller in size than the actual. The Difference in the thermal expansion of the mold material result in shrink rate Shrink Rate= ( Cavity dimension- Part Length)/ Cavity dimension) (Typical units in/in) This shrinkage value is applied to the desired finished part size to “scale up” the cavity. The resulting cavity dimension is calculated as: Cavity Dimension= finished part length/ (1- Shrink rate) For example: Shrinkage rate testing was performed on a 70 Shore A Nitrile (Buna-N), Ethylene-Propylene (EP), Fluorosilicone (FVMQ), and 75 Shore A Fluorocarbon (FKM). The Fluorosilicone and Fluorocarbon materials undergo a secondary vulcanization process called post-curing. During the post-curing process, the molded parts were baked in a ventilated oven at a specific time and temperature recommended by the material manufacturer, or at a temperature g...
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