Dimensional variation and shrinkage differences
Part dimensions fall outside the tolerance or vary from cycle to cycle.
Symptoms
- Dimensions differ in every shift
- Shrinkage in a new mold differs from the expectation
- Together with warpage
Tip: Measure the part also 24 hours after production; semi-crystalline materials keep shrinking over time.
Order of solutions
- Fix the process parameters (cushion, V/P changeover, mold temperature)
- Check the condition of the check ring and the screw
- Standardize the moisture of the raw material and the regrind ratio
- Monitor the balance of the mold temperature controller
- Revise the shrinkage allowance in the mold according to the measurement results
Likely causes
Process setting
- Holding pressure/time varies
- Mold temperature fluctuates
- Metering/cushion inconsistent
- Cooling time insufficient (dimensions not stable)
Raw material
- Batch differences, moisture, regrind ratio
- Crystallization differences in semi-crystalline material
Machine
- Check ring leakage
- Hydraulic/servo repeatability
Mold
- Shrinkage allowance wrong in the mold design
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Other defects
Short shot (incomplete fill)
The cavity does not fill completely; the farthest or thinnest area of the part stays incomplete.
Flash
Plastic escapes through the parting line or around the ejector pins.
Sink mark
A dent on the surface opposite a thick section; the surface is drawn inwards.
Void (internal vacuum void)
Internal cavity in a thick section of the part; the outer surface may look perfect.