Moulded Part Shrinkage and the Cavity Dimension to Cut

The relation divides rather than multiplies -- cavity = part / (1 - shrinkage) -- and the difference from the approximate part x (1 + shrinkage) is small at low shrinkage and real at high, which matters because the semi-crystalline materials that shrink most are where the error is largest. Material class is the first-order decision. Amorphous materials such as ABS, polycarbonate and polystyrene shrink roughly 0.004 to 0.008 in/in; semi-crystalline materials such as polypropylene, polyethylene, acetal and nylon shrink roughly 0.010 to 0.025, several times more. A mould cut for one and run in the other misses by enough to scrap a fitted part, and the tool is the thing that has to be corrected. Two things make a single shrinkage figure insufficient. The first is ANISOTROPY: flow-direction and cross-flow shrinkage differ, sharply in filled and semi-crystalline materials, so a square feature cut uniformly comes out rectangular and a glass-filled part warps out of a mould cut on one number. The second is that the published figure is a RANGE, not a value, and mould temperature, hold pressure and hold time all move the actual shrinkage within it -- so the range is the real uncertainty and the process is part of the dimension. Some materials also continue shrinking for hours or days after ejection, which is why a part measured at the press can be out of tolerance the next morning. The steel-safe rule follows from an asymmetry rather than from caution. Cutting the cavity to the LOW end of the shrinkage range makes it small, which makes the part large, and a large part is corrected by removing metal from the cavity -- a normal, cheap operation. Cutting to the high end makes the part small, and correcting that means welding the cavity and re-cutting it, which is expensive and leaves a repair in the tool. The two errors are not equally recoverable, so the choice between them is not a coin toss. This computes a dimension from an entered shrinkage. It does not predict shrinkage from the material and the process, model warpage or differential shrinkage across a part, account for gate location and its effect on flow direction, handle post-mould shrinkage timing, or size a tool. Mould flow analysis, the resin supplier's shrinkage data for the specific grade, and the mould designer govern.

cavity = part dimension / (1 - shrinkage), and part = cavity x (1 - shrinkage) inverts it; the flow and cross-flow directions are computed separately.

Moulding shrinkage allowance as tool design states it, with the shrinkage rate ENTERED in in/in from the resin supplier's data for the specific grade -- amorphous materials run roughly 0.004 to 0.008 and semi-crystalline roughly 0.010 to 0.025.

One division and one multiplication.

Estimate. AHJ and licensed professional govern.

Field names used by the API: part_dimension_in, shrinkage_flow_in_in, shrinkage_cross_in_in, shrinkage_low_in_in, shrinkage_high_in_in, existing_cavity_in, cavity_flow_in, cavity_cross_in, anisotropy_in, cavity_low_in, cavity_high_in, part_from_cavity_in

• Shrinkage is entered per grade published as a range, not a valuethe resin supplier's data sheet
• Process moves it within the range mould temperature, hold pressure and hold timethe moulder's process record
• Post-mould shrinkage some materials keep shrinking for hours or daysthe material data sheet