Understanding Polymer Shrinkage Rates: A Practical Guide for Injection Mold Designers

The Mold Designer's Constant Challenge

Every injection molded part shrinks as it cools from melt temperature to ejection temperature. The mold cavity must be larger than the finished part by exactly the shrinkage amount — get it wrong, and every part produced is out of tolerance. For high-performance polymers like PEEK and PPS, shrinkage is both larger and more anisotropic than for commodity materials.

Amorphous vs. Semicrystalline Shrinkage

Amorphous polymers (Ultem PEI, PSU, PES) shrink 0.5-0.7% — relatively isotropic because there is no crystalline phase to create directional differences. Semicrystalline polymers (PEEK, PPS, LCP) shrink 1.0-2.5% depending on crystallinity level — and shrinkage is anisotropic (different in flow vs. transverse directions) because polymer chains align during filling and crystallize preferentially along the flow direction.

Practical Shrinkage Compensation

Do not apply a single 'shrinkage factor' to all mold dimensions. For semicrystalline polymers: apply 1.2x the nominal shrinkage rate for the flow direction and 0.8x for the transverse direction on the first mold trial. Measure actual part dimensions after the mold reaches thermal equilibrium (minimum 50 cycles), then adjust the steel. Modern moldflow simulation can predict shrinkage within 0.1% — well worth the simulation cost for PEEK tooling ($30,000-100,000+).

References & Industry Standards

• ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
• ISO. ISO 1043 — Plastics — Symbols and Abbreviated Terms. iso.org
• National Institute of Standards and Technology (NIST). Polymer Properties Database. nist.gov
• UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
• MatWeb — Material Property Data. matweb.com