Measure Twice, Cut Once

April 1, 2015 • Posted in Technical

The old carpenter’s axiom is also true in the world of plastic tooling. A toolmaker always wants to know the shrink rate of a plastic material. With the preponderance of materials available and the numerous individual grades within each polymer family – this can be a daunting task.

Shrinkage data that is calculated by plastic resin producers can be misleading. The geometry of the part used for this data is flat, of a specific size and uniform thickness and a large gating system which does not correlate to most of the complex parts that we mold today. Unless, of course we are molding shrinkage plaques!

Shrinkage is affected by many factors in the molding conversion process of semi-crystalline resins. These include but are not limited to:

• Thicker parts

• Non-uniform wall thickness geometry

• Higher or lower melt temperature

• Hotter tool surface (coolant temperature)/colder tool surface

• Lower or higher packing pressure

• Hotter or colder part ejection

Ideally, a highly confident value for specific shrinkage data can be produced by molding the actual grade of material in a mold of known steel dimensions. One that has geometries similar to the target application utilizing your specific molding conditions and measuring the final part produced – and it pays to measure at least twice…

Many years ago in the gear industry we developed our own in-plant knowledge library of each material’s characteristics at different thicknesses and diameters without the gear tooth geometry to identify the best starting basis of cutting the molds for shrink considerations. The outcome was we were able to be faster to the market than any of our competitors.

For more information on this and other technical questions please contact us at

Brian Santillo is a Sr. Technical Service Engineer, Western Region, for the M. Holland Company.

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