Dimensional Stability After Molding: Part 1

摘要

Everyone involved in plastics processing knows that molded parts shrink as they cool. The degree to which this occurs is largely dependent upon the composition of the material being processed. Semi-crystalline materials shrink more than amorphous ones, and fillers will reduce the shrinkage rate of any particular polymer to a degree that depends upon the type and amount of filler that is added. Part geometry is also a contributing factor—thin-walled parts shrink less than products with thicker walls simply because they cool to an equilibrium state more quickly. Processing conditions also have an influence on this behavior. Higher packing pressures will reduce the level of shrinkage, and lower mold temperatures also have this effect, although in the case of mold temperature the benefits may be only temporary. One of the most difficult aspects of constructing a mold is determining the compound effects of all these factors on the relationship between the tool dimensions and the size of the related part features.