19.0 COMPRESSION/TRANSFER MOULDING DEFECTS
19.1 Introduction:
The defects in both compression moulding and transfer moulding are almost same and given in as common.
| Problem | Moulding condition | Moulding compound | Mould |
| 1. Sticking to surface of mould | -Temperature too low -Cure time too short | Material too fresh (contain moisture) | -Mould stained -New mould not run-in with lubricant -Contaminated with previously run material |
| 2. Sticking due to tightness in cavity | -Temperature too high -Pressure too high | -too stiff in flow | -Insufficient draft |
| 3. Breakage on ejection | -Soft material -Temperature too low -Cure time is too short -Hard material -Mould temperature too high | | -Insufficient draft -Uneven ejection -Undercuts |
| 4. Ejector pins penetrating in the moulding | -Temperature too low -Cure time is too short -Mould temperature too high | -Heat distortion | -Diameter of pins too small -pins not fully returned |
| 5. Thick flash | -Temperature too high -Too much materials -Pressure too high -Press closure too slow | -Too stiff -Too fast curing | -Mould not shutting properly |
| 6. Excessive flash tightness in cavity | -Too much material | -Too easy in flow | -Flash clearance too large |
| 7. Scratches and scars | * | * | -Undercuts in mould -Rough surfaces -Draft angle too shallow |
| 8. Short shot (moulding) | -Temperature too high -Insufficient charge weight -Pressure too low -Closing speed too fast -Fast curing | -Too stiff inflow | -Diameter of pins too small -pins not fully returned |
| 9. Sink mark | -Insufficient charge weight -Pressure too low | -Too easy inflow | -Flash clearance too large |
| 10.Porosity & Voids | -Insufficient charge weight -Air trapment -Closing speed too fast -Incorrect positions of charge | -Too easy flow | -Inadequate venting |
| 11. Surface ripple | -Palletizing & preheating not proper | -Typical long fiber material | -Matt finish sometimes allows to disguise this defect. |
| 12. Cracks | -Very rare | | -Uneven ejection -Undercuts -Rough surface |
| 13. Dull patches | -Insufficient pre heating | | -Stained mould -Worn out chromium plating |
| 14. Blisters | -temperature too low -Too short curing -Poor pre heating -Temperature too high | -Too fresh | -Inadequate venting |
| 15. Knit lines | -Press close too slow –Preheat too high -mould temperature too high | -Too stiff inflow | -Insert or core pin too near to edge -Vent at end of flow |
| 16. Poor gloss | -Temperature too low -Cure time short | | -Mould stained -Unpolished surface |
| 17. White marks | -Temperature too high -Pre heat too high -Burnt material | | |
| 18. Distorted parts | -Uneven mould temp. -Uneven pre heat -insufficient cure time | -flow too easy -shrinkage too high | -Uneven ejection -Variation in wall thickness high -Wrong gate position |
Transfer Moulding defects
All the problems in the compression moulding is also applicable for Transfer moulding
It is based on assumption that the mould in question has adequate runner and gating provision.
Incomplete filling (short shot), knit lines, burning blisters, can all result from excessive frictional heat development, when transferring the moulding compound through runner and gate will occur. A correct mould design is very important.
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