How do you calculate plastic shrinkage?

Subtract the molded part dimension from the cavity dimension, divide by the cavity dimension, and multiply by 100. A 100.00 mm cavity producing a 98.20 mm part gives a 1.80 mm difference, or about 1.8% shrinkage.

What is a typical shrinkage rate for plastic?

It depends on the resin. Amorphous materials like ABS and polycarbonate run about 0.4-0.8%, while semi-crystalline resins like nylon, POM, and polypropylene shrink more, often 1.0-2.5%. Always check the specific grade's TDS.

Why measure shrinkage after 24 hours of conditioning?

Parts continue to shrink for hours to days after ejection, especially semi-crystalline resins. Measuring after conditioning gives a stable dimension that reflects the part as it will be used, not the size at ejection.

What is the difference between in-flow and cross-flow shrinkage?

In-flow shrinkage runs along the direction the plastic flowed and is often higher in semi-crystalline materials due to molecular orientation. Cross-flow runs perpendicular. Glass-filled resins reverse this, shrinking more cross-flow.

How do I use shrinkage to size a mold cavity?

Divide the target part dimension by (1 minus shrinkage as a decimal). For a 98.20 mm target part at 1.8% shrink, cut the steel to about 100.00 mm so the part lands on size.

What causes higher-than-expected shrinkage?

Low pack and hold pressure, short hold time, high melt temperature, thick walls, or a hot mold all increase shrink. If measured shrink exceeds the TDS, check pack pressure and cooling first.

Injection Molding calculator

Plastic Shrinkage Calculator

Mold shrinkage is the percentage a molded part contracts relative to the steel cavity that formed it, measured after the part has cooled and conditioned. Toolmakers and process engineers use it to size cavities so the finished plastic part lands on print, since the steel must be cut larger than the target dimension by the expected shrink. It matters because shrinkage varies by resin, wall thickness, and flow direction, and a wrong allowance means a tool that produces out-of-tolerance parts and an expensive steel rework. Comparing measured shrinkage to the resin technical data sheet tells you whether your process is in the expected window.

What this calculator does

Calculate mold shrinkage as a percentage from the mold cavity dimension and the measured molded part dimension.
Use this to verify actual shrinkage against resin data sheet values, adjust mold cavity dimensions for new resins, or diagnose dimensional issues from process changes.
It computes the percentage difference between the steel cavity dimension and the molded part dimension after conditioning.

Formula used

Shrinkage = ((Cavity dimension - Part dimension) / Cavity dimension) x 100
Compare to resin TDS values. In-flow shrinkage often differs from cross-flow.

Inputs explained

Mold cavity dimension (steel size):
Molded part dimension (after 24hr conditioning):

How to use the result

Use it during tool design to set cavity sizes, and after first trials to compare actual shrink against the resin TDS.
Shrinkage differs in-flow versus cross-flow and varies with wall thickness and packing, so a single measured value may not represent the whole part.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 319.371 (BLS, May 2026), up 19.5% 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

How do you calculate plastic shrinkage? Subtract the molded part dimension from the cavity dimension, divide by the cavity dimension, and multiply by 100. A 100.00 mm cavity producing a 98.20 mm part gives a 1.80 mm difference, or about 1.8% shrinkage.
What is a typical shrinkage rate for plastic? It depends on the resin. Amorphous materials like ABS and polycarbonate run about 0.4-0.8%, while semi-crystalline resins like nylon, POM, and polypropylene shrink more, often 1.0-2.5%. Always check the specific grade's TDS.
Why measure shrinkage after 24 hours of conditioning? Parts continue to shrink for hours to days after ejection, especially semi-crystalline resins. Measuring after conditioning gives a stable dimension that reflects the part as it will be used, not the size at ejection.
What is the difference between in-flow and cross-flow shrinkage? In-flow shrinkage runs along the direction the plastic flowed and is often higher in semi-crystalline materials due to molecular orientation. Cross-flow runs perpendicular. Glass-filled resins reverse this, shrinking more cross-flow.
How do I use shrinkage to size a mold cavity? Divide the target part dimension by (1 minus shrinkage as a decimal). For a 98.20 mm target part at 1.8% shrink, cut the steel to about 100.00 mm so the part lands on size.

Last reviewed 2026-05-12.