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.
Flow Direction Shrinkage
Meaning
Dimensional changes occur in a molded plastic part along the axis of material injection as it cools and solidifies. Engineers must account for flow direction shrinkage during the design of a mold to ensure final parts meet specifications. The magnitude of this change depends on the material type and the processing conditions.
Molecular Orientation
Alignment of polymer chains along the path of the melt increases the contraction in that specific direction. When flow direction shrinkage is high, it is often because the molecules have been stretched and then relax during the cooling phase. This orientation creates a difference in properties between the length and width of the part.
Part Geometry
Variation in wall thickness can lead to uneven cooling rates and inconsistent dimensional results. Because flow direction shrinkage varies across different sections, complex parts may experience internal stress that leads to warping. Uniform thickness helps to minimize these localized differences.
Thermal Effect
Temperature control of the mold surface determines how quickly the plastic transitions from a liquid to a solid. Lower temperatures often increase flow direction shrinkage because the material freezes before the molecules can reach a more relaxed state. Holding pressure applied by the injection machine can counteract some of this contraction by forcing more material into the cavity as it cools.
Engineers use sensors to monitor the cooling rate and adjust the cycle time to maintain dimensional stability across production batches.