← Defect Guide
Dimensional Instability
Dimensions drift between lots, cavities, or over time under the same conditions. Distinct from warpage (shape change).
◆ Common Causes
- •Cooling/hold variation in crystalline resins
- •Moisture expansion in PA (1% moisture ≈ 0.2–0.3% growth)
- •Inconsistent measurement timing
- •Cavity-to-cavity fill/cooling variation
- •Post-mold shrinkage (continued crystallization)
◆ Solutions
- ✓Increase cooling; stabilize hold pressure
- ✓Standardize measurement timing (after 24 h at 65% RH)
- ✓Anneal PA to pre-empt post-shrinkage
- ✓Check cavity balance and cooling circuits
- ✓Monitor shot weight within ±0.5%
◆ Resin-Specific Notes
PA
★ Grows over days as it absorbs moisture — a measurement-timing/material issue, not a process knob. Hold off on hasty condition changes.
POM
High crystallinity and post-shrinkage. Cooling and annealing.
PBT
Fast crystallization. Manage gate freeze and hold.
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This content is a field-reference summary. Final judgment requires on-site engineer verification.