Meaning
Mathematical adjustment applied to the dimensions of a mould cavity to compensate for the contraction of the cooling polymer. Shrinkage allocation is based on the specific resin shrink rate. The goal is to produce a finished part that meets the design specifications after it reaches room temperature.
Tool Design
Engineers must decide whether to apply a uniform scale factor or to vary the allowance based on wall thickness and flow direction. Shrinkage allocation is critical because semi-crystalline materials like nylon shrink more than amorphous materials like polycarbonate. If the allowance is wrong, the part will be out of tolerance.
Process Variable
Actual amount of contraction is influenced by injection pressure and melt temperature. Shrinkage allocation in the tool is a best-estimate that assumes a stable production process. If the moulder changes the cycle time significantly, the physical dimensions of the part will drift from the intended values.
Steel Safe Strategy
Initial tool builds often use a conservative shrinkage allocation to leave extra metal in critical areas. This approach allows the toolmaker to remove small amounts of steel after the first trial to bring the part into perfect spec. It is much easier to cut more steel away than it is to add it back via welding.
Experienced designers use this buffer to account for the inherent variability in polymer behavior.
