How to Avoid Plastic Part Shrinkage?
Shrinkage is an inherent part of injection molding – as molten plastic cools and solidifies, it contracts. While some shrinkage is unavoidable, excessive or uneven shrinkage leads to sink marks, warpage, and out‑of‑tolerance dimensions. The good news: with careful planning and process control, you can minimize these defects and produce consistent, dimensionally stable parts.
What Causes Shrinkage?
Material nature – Crystalline polymers (e.g., PP, PA, POM) shrink more than amorphous ones (e.g., PS, PC, ABS) because their molecular chains pack tightly during cooling.
Wall thickness – Thicker sections cool more slowly, causing higher volumetric shrinkage and sink marks.
Mold temperature – Higher mold temperatures slow cooling, increasing total shrinkage.
Melt temperature – Higher melt temperatures mean more thermal expansion and, consequently, more shrinkage upon cooling.
Packing & holding pressure – Insufficient packing fails to compensate for material contraction, leading to voids and sink marks.
Gate location – Poorly placed gates can cause anisotropic shrinkage (different shrink rates in flow vs. cross‑flow directions), resulting in warpage.
Practical Strategies to Control Shrinkage
Choose low‑shrinkage grades (e.g., filled materials – glass fiber, talc, or mineral fillers significantly reduce shrinkage).
For crystalline materials, consider nucleated grades that promote more uniform crystallization and reduce total shrink.
Uniform wall thickness – Avoid abrupt transitions; design ribs and bosses with care to prevent thick sections.
Balanced cooling – Place cooling channels close to the cavity and ensure uniform temperature distribution across the part.
Gate sizing and location – Use larger gates and locate them at thickest areas to maximize packing effectiveness. Multiple gates can help balance flow and reduce anisotropic shrinkage.
Proper venting – Prevent trapped air that can impede packing.
Increase packing pressure – Higher pressure forces more material into the cavity to compensate for volumetric shrinkage.
Extend packing/holding time – Continue packing until the gate freezes off; too short a hold time will starve the cavity.
Optimize melt and mold temperatures – Lower temperatures (within resin limits) reduce total shrink, but may affect flow. Find a balance.
Adjust injection speed – Faster fill can generate shear heat, which may increase shrinkage; slower fill often leads to more uniform packing.
Annealing – For certain materials, heat‑treating the part just below its glass transition temperature can relieve internal stresses and stabilize dimensions.
Common Shrinkage Defects – Causes & Solutions
| Defect | Likely Cause | Corrective Action |
|---|---|---|
| Sink marks (surface depressions over thick areas) | Insufficient packing pressure or time; too‑thick section. | Increase packing pressure/hold time; reduce wall thickness; move gate to thick area. |
| Warpage / distortion | Non‑uniform shrinkage (differential cooling or anisotropic orientation). | Balance cooling; use uniform wall thickness; adjust gate location to minimize flow‑direction orientation. |
| Voids / bubbles | Material shrinks internally without external compensation. | Increase packing pressure; enlarge gate; reduce melt temperature. |
| Overall dimensional shrink above spec | Too‑high melt or mold temperature; low packing pressure. | Lower temperatures; raise packing pressure; extend holding time. |
| Inconsistent shrink from batch to batch | Material viscosity variation or process drift. | Tighten process parameters; use spectrophotometer for material verification; standardize drying. |
A Quick Reference Checklist
The Bottom Line
You can’t eliminate shrinkage entirely, but you can control it. The most effective approach combines thoughtful material selection, balanced mold design, and fine‑tuned processing. When all three work together, you’ll achieve parts that meet their dimensional targets with minimal cosmetic defects – saving time, material, and rework costs.
