Reference Target Placement for Flexible Plastic Parts
Position reference targets on flexible plastic parts at high-stiffness rib intersections using explicit 3-2-1 restraint forces to eliminate gravity sag and inspection disputes.
Shrinkage Differential
Meaning
Discrepancies in the rate of volume reduction occur in different directions or regions of a plastic part as it cools from melt to solid. A shrinkage differential is caused by polymer molecule alignment during injection, leading to different contraction rates along the flow direction versus across it. This variation is a primary source of warp in molded components.
Molecular Orientation
Molten polymer flowing through narrow gates undergoes high shear, which stretches and aligns the polymer chains. This alignment causes a shrinkage differential where the material shrinks more along the flow direction than perpendicular to it. Minimizing flow rates and using wider gates reduces this orientation.
Tooling Adaptation
Mold designers must account for non-uniform contraction when dimensioning the mold cavities. Semicrystalline polymers exhibit a greater shrinkage differential than amorphous resins, requiring more complex cavity adjustments during tool design. Calculating these differences accurately before cutting steel prevents the fabricated part from falling out of tolerance, which avoids expensive tool rework.
Quality Defect
When contraction rates vary across the part geometry, internal stresses build up until the part twists. This shrinkage differential leads to functional failures like poorly fitting housings and unstable assemblies. Proper gate positioning minimizes these issues.