Injection moulding of thermoplastics

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Fault

Probable Cause

Suggested Remedy

Silver Streaking

1.1	Plastic temperature too high	1.1	Reduce cylinder temperature.
1.2	Mixture of coarse and fine granules.	1.2	Use uniform granulation.
1.3	Non-uniform plastic temperature. torpedo and then cylinders.	1.3	Reduce temperature of nozzle.
1.4	Exceeding plasticizing capacity	1.4	Pre-heat material.
1.5	Air trapped between the granules at the hopper end.	1.5	Reduce rear cylinder temperature and use uniform granulation.
1.6	Moisture on granules.	1.6	Dry material in an oven.
1.7	Intermittent flow in the cavity.	1.7	Balance gates/relocate gates.
1.8	Injection speed too fast.	1.8	Vent mould.
		1.8.1	Reduce injection pressure.
		1.8.2	Reduce cylinder temperature.
1.9	Mould temperature too low.	1.9	Increase mould temp.
1.10	Injection pressure too high.	1.10	Reduce injection pressure.

Fault

Probable Cause

Suggested Remedy

Black streaks

2.1	Frictional heat caused by movement of cold granules past one another.	2.1	Use external indication.
		2.1.1	Increase rear cylinder temperature.
2.2	Air trapping in the mould.	2.2	Vent mould properly.
		2.2.1	Relocate gates.
		2.2.2	Reduce injection pressure.
2.3	Plunger off-centre causing friction burning of material.	2.3	Relocate plunger.
2.4	Contamination.	2.4	Purge the cylinder.

Fault

Probable Cause

Suggested Remedy

Short shots

3.1	Cold material.	3.1	Increase plastic temperature.
3.2	Cold mould.	3.2	Increase mould temperature.
3.3	Insufficient pressure	3.3	Increase pressure temperature
3.4	Non-uniform mould temperature.	3.4	Rearrange water line.
3.5	Insufficient feed.	3.5	Increase feed.
3.6	Poor venting.	3.6	Increase number and size of vents.
3.7	Insufficient plunger forward time.	3.7	Increase plunger forward time.
3.8	Insufficient injection speed.	3.8	Increase injection speed.
3.9	Improper balance of plastic flow in multiple cavity moulds.	3.9	Correct unbalanced condition.

Fault

Probable Cause

Suggested Remedy

Flash

4.1	Material too hot.	4.1	Reduce material temperature.
		4.1.1	Increase cycle time.
		4.1.2	Decrease mould temp.
4.2	Pressure too high.	4.2	Lower pressure.
4.3	Excessive feed	4.3	Reduce feed.
4.4	Erratic feed	4.4	Check the feeding system.
4.5	Poor parting line or matting surfaces.	4.5	Reface the parting line/matting.
4.6	Erratic cycle time.	4.6	Maintain constant cycles.
4.7	Insufficient clamp.	4.7	Change the mould to machine with greater clamping capacity.

Fault

Probable Cause

Suggested Remedy

Sprue sticking

5.1	Excessive pressure.	5.1	Reduce pressure.
5.2	Hot material.	5.2	Decrease material temp.
5.3	Excessive material.	5.3	Decrease size of the sprue extended nozzle and short sprue bushing.
5.4	Insufficent draft.	5.4	Increase draft angle.
5.5	Improper fit between sprue-bushing and nozzle.	5.5	Hole size in nozzle should be smaller than in the sprue bushing.
5.6	Undercuts or rough surface.	5.6	Eliminate undercuts and polish surface.
5.7	Long plunger swell.	5.7	Reduce plunger forward time.

Fault

Probable Cause

Suggested Remedy

Sink marks

6.1	Not enough plastic in the mould to allow for shrinkage due to-	6.1.1	Redesign and increase injection pressure.
6.1.1	Thick sections, bosses, ribs, etc.	6.1.1	Redesign and increase injection pressure.
6.1.2	Not enough feed.	6.1.2	Increase feed.
6.1.3	Injection pressure too low.	6.1.3	Increase injection pressure.
6.1.4	Plunger forward time too short.	6.1.4	Increase plunger forward time.
6.1.5	Unbalanced gates.	6.1.5	Restrict flow through gates near sprue.
6.1.6	Injection speed too slow.	6.1.6	Raise mould and cylinder temperatures.
6.2	Plastic too hot.	6.2	Reduce cylinder and mould temperatures.
		6.2.1	Decrease cycle time.
6.3	Variation in mould open time.	6.3	Use timers for mould open time.
6.4	Excessive cushion in front of the plunger.	6.4	Reduce feed.

Fault

Probable Cause

Suggested Remedy

Parts sticking to the mould

7.1	Injection pressure or cylinder temperatures too high.	7.1	Reduce.
7.2	Excessive feed.	7.2	Reduce feed.
7.3	Plunger forward time too long.	7.3	Reduce.
7.4	Undercuts in mould.	7.4	Eliminate undercuts, polish and provide sufficient draft.
7.5	Variation of mould open time.	7.5	Maintain constant mould open time.
7.6	Improper finish.	7.6	Polish mould cavity surface to correct finish.
		7.61	Adjust mould temperature to obtain proper shrinkage.

Fault

Probable Cause

Suggested Remedy

Flow lines and folds

8.1	Plastic cold.	8.1	Increase plastic temperature.
8.2	Mould cold.	8.2	Increase mould temperature.
8.3	Gates too small.	8.3	Enlarge gates.
8.4	Non-uniform section thickness.	8.4	Eliminate heavy bosses and ribs.

Fault

Probable Cause

Suggested Remedy

Poor surface finish

9.1	Cold material.	9.1	Increase plastic temperature.
9.2	Cold mould.	9.2	Increase mould temperature.
9.3	Slow injection.	9.3	Increase injection speed.
9.4	Poor flow within mould cavity.	9.4	Redesign parts.
9.5	Poor mould cavity finish.	9.5	Polish mould cavity surface to correct finish.

Fault

Probable Cause

Suggested Remedy

10.

Warpage

10.1	Excessive orientation in the material which manifests itself in differential shrinkages in the direction of flow and vertical to it. Orientation increases as : Material becomes too viscous. The walls become thinner. Runner becomes narrower and longer.	10.1	Avoid orientation.
10.2	Differential thermal shrinkage occurs if : There are different wall-thickness. Mould is unevenly cooled. Temperatures distribution in cylinder is uneven	10.2	Use equal wall-thickness. Use uniform temperature all over moulld surface. Watch cyclinder design.
10.3	As a result of overloading of the moulding due to : Excessive or too long follow-up pressure.	10.3	If possible, use follow-up pressure lower then injection pressure. Shorten follow-up pressure.

Fault

Probable Cause

Suggested Remedy

11.

Fractures, Cracks

11.1	Internal stresses due to : Mould design- Thermal degradation-	11.1	Correct
11.2	Internal stresses due to metal inserts with co-efficient of thermal expansion different from the plastic material.	11.2	Use higher molecular weight materials. Increase wall-thickness Anneal the moulded parts.

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