Engineering tools

Mould shrinkage calculator — from part dimension to tool dimension

Plastics shrink as they cool, so mould cavities are cut larger than the finished part: tool dimension = part dimension ÷ (1 − shrinkage). Amorphous materials (ABS, PC) shrink little and predictably — around half a percent. Semi-crystalline materials (PP, HDPE, nylons, POM) shrink two to five times more, direction-dependently, which is why they dominate warpage conversations and why glass fibre — which anchors the structure — cuts their shrinkage sharply.

Why one number is never enough

Published shrinkage is measured on standard plaques; your part is not a plaque. Thick sections shrink more than thin; flow direction shrinks differently from cross-flow in glass-filled grades; generous packing reduces shrinkage near the gate and starves the far end. This is why critical-dimension tooling follows a discipline: mould-flow prediction, steel-safe cutting, first-article measurement, then final steel correction — a sequence, not a lookup.

Plan the whole part, not one dimension

Pair this with the materials guide when the material itself is still open, and the cycle-time and cost tools when it is. Machine fit: shot size and clamping force.