Why does my part shrink differently from the datasheet value?

Because datasheet shrinkage comes from standard test plaques, and ISO 294-4 says it is not design data. Real parts vary with wall thickness, flow direction and hold pressure; lower hold pressure packs less material and raises shrinkage.

Is a plus steel change steel-safe?

On a cavity feature, yes: a larger cavity means removing metal. On a core feature such as a hole, a plus change means adding metal, which is difficult or impossible on an existing mold.

How long should parts condition before I measure them?

At least a day at room temperature. DuPont defines mold shrinkage as what occurs within 24 hours, and ASTM D955 measures at 24 and 48 hours.

Should I use flow or cross-flow shrinkage for the steel?

Use this feature's own direction, measured on this part. For most Delrin grades DuPont finds flow shrinkage about 0.1% above cross-flow, and 0.1 to 0.3% higher far from the gate than near it.

Injection Molding calculator

Plastic Shrinkage Calculator

Measure mold shrinkage on one feature after a mold trial and see what steel size brings the part to print nominal. You need the steel dimension, the same feature on a conditioned part, and the print nominal and tolerance.

What this calculator does

Measured mold shrinkage on one feature, whether the part meets print, and the steel size that would.

Formula used

Mold shrinkage (%) = (steel dimension − part dimension) ÷ steel dimension × 100
Tolerance used (%) = |part − print nominal| ÷ print tolerance × 100
Steel for nominal = print nominal ÷ (1 − shrinkage ÷ 100) = print nominal × steel ÷ part
Steel change = steel for nominal − steel dimension; plus makes the steel dimension larger

Inputs explained

Steel Dimension: Cavity or core size at this feature, measured at room temperature.
Molded Part Dimension: Same feature on a part conditioned a day at room temperature.
Print Nominal: Nominal size of this feature on the part drawing.
Print Tolerance (±): Plus-or-minus tolerance on the drawing; use the tighter side if unequal.

How to use the result

Best suited to first-article dimensional report after mold trial, planning a steel-safe cavity correction, qualifying a resin change on an existing mold.
Shrinkage differs with and across flow, and near and far from the gate; correct each feature separately. Parts can keep shrinking after the first day; DuPont finds cold-molded Delrin shrinks most.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

Why does my part shrink differently from the datasheet value? Because datasheet shrinkage comes from standard test plaques, and ISO 294-4 says it is not design data. Real parts vary with wall thickness, flow direction and hold pressure; lower hold pressure packs less material and raises shrinkage.
Is a plus steel change steel-safe? On a cavity feature, yes: a larger cavity means removing metal. On a core feature such as a hole, a plus change means adding metal, which is difficult or impossible on an existing mold.
How long should parts condition before I measure them? At least a day at room temperature. DuPont defines mold shrinkage as what occurs within 24 hours, and ASTM D955 measures at 24 and 48 hours.
Should I use flow or cross-flow shrinkage for the steel? Use this feature's own direction, measured on this part. For most Delrin grades DuPont finds flow shrinkage about 0.1% above cross-flow, and 0.1 to 0.3% higher far from the gate than near it.

Last reviewed 2026-10-01.