Managing Anisotropic Shrinkage and Restraint Fixturing in Polymer Metrology
Polymer metrology requires force-controlled restraint fixturing and thermal conditioning to isolate mold geometry from anisotropic shrinkage and sag.
Transverse Contraction
Meaning
Material reduction occurs perpendicular to the direction of flow or primary stress as a substance cools or undergoes tensile deformation. In plastic injection molding, transverse contraction describes how the width of a part changes relative to the gate where the material entered. This behavior is a main factor in achieving the correct final dimensions.
Anisotropic Behavior
Difference in shrinkage rates between the flow direction and the cross flow direction is common in fiber reinforced polymers. Because transverse contraction is often greater than the shrinkage along the flow path, parts can become distorted or warped if not properly designed. Engineers must map these differences to predict the final shape accurately.
Mold Compensation
Adjustments to the cavity size are made during the tooling phase to account for the expected material movement. When calculating transverse contraction, designers increase the width of the mold to ensure the finished part meets the specification. This requires a deep understanding of the specific material grade and the cooling rate.
Aspect Ratio
Relationship between the length and width of a part influences how much the material will pull inward. For thin, wide components, transverse contraction can lead to bowing in the center if the cooling is not uniform across the surface. Proper placement of cooling channels in the mold helps to maintain a consistent temperature and minimize internal stresses.
Using simulation software allows designers to visualize these effects before the steel for the mold is cut, saving time and reducing the cost of corrections.