Integrating Free Volume Diffusion Models into Mold Thermal Calculations
Coupling free volume mass diffusion to transient mold heat transfer equations prevents premature part ejection and cycle time miscalculations.
Polymer Shrinkage
Meaning
Dimensional change metrics measuring the contraction of plastic materials during the cooling and solidification phase of the molding process establish the final physical boundaries of a manufactured part. Analyzing polymer shrinkage allows tool designers to oversize mold cavities to compensate for this reduction in volume. The metric is influenced by the polymer’s cooling rate or processing pressure.
Measurement Method
Engineers use standardized test methods, such as those defined by ASTM D955, to determine the percentage of contraction that occurs after molding. The calculation of polymer shrinkage requires measuring the dimensions of the mold cavity at room temperature and comparing them to the dimensions of the molded part after it has cooled. This measurement ensures that production parts meet the specified tolerances.
Factor Influence
Amorphous and semi-crystalline plastics exhibit very different contraction behaviors due to their distinct molecular arrangements. While amorphous polymers experience low, uniform polymer shrinkage, semi-crystalline materials show much higher and anisotropic contraction, which can lead to warpage if not managed. Processing parameters, such as melt temperature or injection speed, also affect the final degree of contraction.
Industrial Consequence
Failing to account for contraction during the tool design phase can result in out-of-tolerance parts that must be scrapped, leading to expensive mold modifications and production delays. Precision applications require accurate prediction of polymer shrinkage to ensure that mating components fit together correctly during assembly. This predictive capability reduces development costs, minimizes trial-and-error iterations and ensures that the final production yield meets the performance requirements of the end-use application.