Meaning
Dimensional and density gradients occurring across different locations of a single molded component reflect local cooling and packing variations within the tool. A within cavity variance measures property discrepancies across distinct sections of a single mold impression during part solidification. The calculation evaluates local thickness, volumetric shrinkage and internal stress differences within one physical component.
It highlights localized thermal imbalance or uneven packing pressure distribution inside complex mold geometries. Quality engineers track this internal variation to prevent part warpage and localized structural failure.
Thermal Gradient
Non-uniform heat extraction across thick and thin wall sections creates localized cooling rate differentials within the mold steel. A within cavity variance develops when cooling channels extract heat faster from outer edges than from heavy internal bosses. Slow cooling in thick sections extends polymer crystallization time, leading to localized volumetric shrinkage and internal voids.
Fast cooling near thin walls freezes resin quickly, locking in residual stress profiles that alter local physical dimensions. Mold cooling channel placement must balance thermal extraction to reduce internal property variation across the component geometry.
Gate Proximity
Pressure decay along the flow path causes density differences between material near the gate and material at the flow end. Observing a within cavity variance reveals higher packing density near the injection gate where holding pressure remains effective throughout solidification. Distant regions experience pressure drops, resulting in lower local density and increased localized shrinkage.
Optimizing gate location and holding pressure duration reduces the density gradient across extended flow lengths.
Structural Consequence
Internal material variations distort final component geometry and induce premature mechanical failure under load. Excessive within cavity variance triggers post-molding warpage as differential residual stresses relax over time. High stress concentration points around un-packed features reduce impact strength and fatigue life during operational assembly service.
