This means that the nylon melt solidifies into varying proportions of crystalline and amorphous regions. The percentage of crystallinity is greatly favored by a slow rate of quench from the melt. This is particularly true for thin parts where the quench temperature greatly affects the rate of cooling. Thick parts cool at a rate that is much more independent of quench temperature. The second basic aspect of processing shrinkage is molded-in stresses.
Many variables affect the degree of stress found in a finished part.
At a given thickness, minimum shrinkage results from high injection pressures, low mold temperatures, and optimum cylinder temperatures and injection speeds. For those grades reinforced with glass fibers, shrinkage values are also reported on 4 inch diameter discs to illustrate the effect of random glass fiber orientation on shrinkage. Note that the thicker bars have the greatest shrinkage for each type of resin. Thicker parts have a slower cooling rate which results in increased crystallization and thus greater shrinkage.