ShrinkPro Shrinkage Calculator

Every injection-molded plastic part shrinks as it cools. The mold cavity has to be cut larger than the desired part dimension to compensate. Get this calculation wrong by 0.5 percent and a 100 mm part comes out 0.5 mm off. On a tight-tolerance assembly, that is the difference between snap-fit success and a $40,000 mold rework. The ShrinkPro shrinkage calculator above gives you the right cavity dimension in seconds for any of 30+ common resins.

The math underneath is straightforward: Mold Dimension = Part Dimension × (1 + Shrinkage Rate). But choosing the correct shrinkage rate is where most engineers and toolmakers go wrong. This guide walks through what shrinkage actually is, why every resin family behaves differently, the variables that make real-world shrinkage drift from the published numbers, and how to use the typical, minimum, and maximum values in the calculator above for production-ready mold sizing.

What is Plastic Shrinkage?

Plastic shrinkage is the dimensional change a thermoplastic undergoes as it transitions from molten state at 200 to 350 degrees Celsius down to room temperature inside the mold. Three things happen during cooling:

• Thermal contraction: the polymer takes up less volume as it cools. This is universal and predictable.
• Crystallization (in semi-crystalline polymers): molecules pack into ordered structures, reducing volume further. PP, PE, PA6, POM, and PBT all crystallize. Amorphous resins like ABS, PC, and PMMA do not.
• Stress release: as the part exits the mold, internal stresses relax and continue contracting for hours or days. Post-mold shrinkage in nylons can add another 0.2 to 0.5 percent beyond initial shrinkage.

The total of these effects, expressed as a percentage of the mold cavity dimension, is the shrinkage rate. ABS shrinks at 0.4 to 0.7 percent (low and predictable). Acetal shrinks at 1.8 to 2.5 percent (high and variable). Nylon 6 shrinks at 0.7 to 1.5 percent depending on moisture content, glass fiber loading, and cooling rate.

Shrinkage Rates by Material Family

Use these as starting points. Every resin grade from every supplier has its own datasheet shrinkage number, and the calculator above carries 30+ specific grades, but here are the rough ranges by family:

Why Real Shrinkage Drifts From the Datasheet

The shrinkage value on a resin datasheet was measured under ASTM D955 test conditions on a 3.2 mm thick rectangular bar with a single gate, optimal cooling, and standard process settings. Your part is almost certainly not that. Five variables shift real-world shrinkage:

• Wall thickness: thicker sections cool slower and shrink more. A 4 mm wall in PP can shrink 0.3 percent more than a 2 mm wall in the same material.
• Flow direction: semi-crystalline resins shrink up to 30 percent more across the flow than along it. Glass-filled grades are even worse, shrinking very little along the flow but normal amounts across.
• Packing pressure and hold time: longer holds with higher packing pressure pack more material in and reduce effective shrinkage. Process drift here is the single biggest cause of "the mold worked yesterday" calls.
• Mold temperature: a hotter mold lets the polymer crystallize more fully, increasing shrinkage. PP run at 60°C cavity temperature shrinks more than the same PP run at 30°C.
• Glass fiber orientation: in glass-filled materials, fibers align with flow. Wherever flow turns sharply, you get a shrinkage anisotropy zone that warps the part.

How to Use the Minimum, Typical, and Maximum Values

The calculator gives you three numbers for a reason. Use them like this:

• Typical: the starting point for cutting steel. This is the dimension to put on the mold drawing.
• Minimum: the dimension if everything aligns to under-shrink (heavy packing, low mold temp, thin walls, low fiber loading variability). Tolerance critical features should be designed so that even at minimum shrinkage they still meet spec.
• Maximum: the dimension if everything aligns to over-shrink (light packing, hot mold, thick sections, high crystallinity). Mating features should be designed so maximum shrinkage still allows fit.

For critical dimensions, run a moldflow analysis before cutting steel. The calculator handles bulk shrinkage; moldflow predicts directional shrinkage and warpage so you can offset the cavity dimensions correctly. On glass-filled materials this is not optional.

Frequently Asked Questions