Tooling · Note 01
Why the part comes out smaller than the steel
Every moulded part shrinks as it cools. The mould is cut oversize to compensate, which is why the resin has to be chosen before the tool is cut, not after.
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- Polydyne LLC
A moulding tool is never cut to the size of the finished part. It is cut larger, by an amount that depends on the resin, the wall thickness, the gate, and the direction the material flowed. When the part cools from melt to room temperature it contracts onto those dimensions. Get the compensation right and the first article measures on print. Get it wrong and you are cutting steel again.
Shrinkage is a property of the material, not of the shop
Mould shrinkage is quoted by the resin supplier, usually in inches per inch or as a percentage, and it varies widely between families. A semi-crystalline resin such as acetal or unfilled nylon shrinks several times as much as an amorphous resin such as ABS or polycarbonate, because its molecules pack into an ordered structure as they solidify.
| Resin | Structure | Typical shrinkage |
|---|---|---|
| ABS | Amorphous | 0.004 – 0.007 in/in |
| Polycarbonate | Amorphous | 0.005 – 0.007 in/in |
| Nylon 6/6, unfilled | Semi-crystalline | 0.007 – 0.018 in/in |
| Nylon 6/6, 30% glass | Semi-crystalline | 0.002 – 0.006 in/in |
| Acetal (POM) | Semi-crystalline | 0.018 – 0.025 in/in |
| Polypropylene | Semi-crystalline | 0.010 – 0.025 in/in |
| PBT, unfilled | Semi-crystalline | 0.009 – 0.022 in/in |
Typical mould shrinkage ranges by resin family. Always work from the supplier data sheet for the specific grade.
Glass fibre changes the answer in two directions at once
Adding glass fibre cuts shrinkage sharply, but it also makes shrinkage directional. Fibres line up with the flow, so the part contracts less along the flow path than across it. On a long, thin part that difference shows up as warp rather than as an out-of-tolerance dimension, and no amount of holding pressure will take it out. It has to be designed for, in the gate position and the wall layout.
Process moves shrinkage, within a band
Holding pressure, hold time, melt temperature, and mould temperature all shift the final dimension a little. Higher hold pressure packs more material into the cavity and reduces shrinkage; a hotter mould lets a semi-crystalline resin crystallise further and increases it. That gives a process window to tune inside, but it is a band of a few thousandths, not a way to recover a tool that was cut to the wrong shrink rate.