Injection Molding Shrinkage Guide — What Buyers Must Know

Shrinkage is the dimensional reduction a plastic part undergoes as it cools from melt temperature to room temperature. Every material shrinks differently, and if the mold is not designed with the correct shrinkage factor, the finished part will be the wrong size — sometimes by several millimeters on a 100mm dimension.

This guide covers shrinkage rates for the most common injection molding materials, how mold makers compensate for shrinkage, and what importers need to verify before production begins.

Why Shrinkage Matters in Mold Design

When a mold is cut, the cavity dimensions must be larger than the final part dimensions by the shrinkage percentage of the material. If a part calls for ABS with 0.5% shrinkage, a 100mm cavity dimension must be cut to 100.5mm. If the mold is designed for ABS but later run with PP (1.5-2.5% shrinkage), the parts will be undersized by roughly 1-2mm per 100mm.

Shrinkage Rates by Material Family

Amorphous Materials (Low Shrinkage)

Semi-Crystalline Materials (Higher Shrinkage)

High-Performance / Specialty Materials

Post-Molding Shrinkage (Secondary Shrinkage)

Many materials continue to shrink after ejection — especially semi-crystalline types. PA6 can shrink an additional 0.2-0.5% over 24-48 hours as it absorbs moisture and relaxes. POM continues crystallizing for days after molding, causing dimension drift. When specifying tolerances for precision parts, request that measurements be taken 48 hours after molding, not immediately at the press.

Factors That Affect Actual Shrinkage

Wall Thickness

Thicker walls cool slower and allow more time for crystallization, resulting in higher shrinkage. A 3mm wall in PP may shrink 2.0% while a 1mm wall shrinks only 1.2%. When a part has mixed wall thicknesses, the mold maker must either use an average shrinkage factor or make the mold adjustable — rare in standard tooling.

Gate Location and Packing

Areas near the gate receive more pack pressure and shrink less. Areas farthest from the gate receive less compensation and may exhibit greater shrinkage. This is why long parts with a single gate can be larger at the gate end and smaller at the far end. Multi-gate designs distribute shrinkage more evenly but introduce weld lines.

Mold Temperature

Higher mold temperatures increase crystallization in semi-crystalline materials, leading to higher shrinkage. A POM part molded at 40°C mold temp may shrink 1.8%, while the same part at 90°C shrinks 2.3%. The molder must document and control mold temperature within ±5°C for consistent shrinkage.

Glass Fiber Orientation

Glass-filled materials shrink less in the flow direction (fibers align with flow) and more in the cross-flow direction. For PA6-GF30, flow-direction shrinkage might be 0.3% while cross-flow is 0.7%. This anisotropic behavior must be accounted for in the mold design, especially for parts with critical dimensions in both axes.

How Mold Makers Compensate for Shrinkage

• Steel-safe vs. Steel-wrong: Most experienced mold makers cut cavities on the "steel-safe" side — the cavity is cut slightly undersize, then steel is added (EDM or welding) if the part comes out too small. Beginners often cut too large, requiring costly steel removal.
• Shrinkage factor entry: The mold designer enters the material-specific shrinkage factor into the CAD/CAM software. The software then scales the cavity geometry by that factor. If the wrong factor is entered, every cavity is wrong.
• Multi-cavity adjustment: For complex parts, some mold makers cut one cavity, test it, measure actual shrinkage, then adjust the remaining cavities. This iterative approach is more accurate but takes 2-3 extra weeks.
• Adjustable cores: For critical dimensions, some molds include adjustable core pins that can be moved in/out to compensate for shrinkage variation during production.

What to Verify with Your Mold Maker

1. Written shrinkage assumption: Ask for the exact shrinkage percentage used in the mold design, and whether it's based on the material supplier's data sheet or their own experience.
2. Post-molding measurement timing: Request "48-hour dimensions" on the first article inspection report, not just as-molded readings.
3. Anisotropic acknowledgment: For glass-filled materials, confirm the mold design accounts for differential shrinkage between flow and cross-flow directions.
4. Shrinkage correction plan: Ask what their process is if first-shot parts are outside tolerance — do they adjust the steel or change process parameters first?

Related Guides

• Injection Molding Tolerances Guide
• Plastic Material Selection Guide
• Mold Design Overview
• Injection Molding Process Guide