Main Cause of Defects
Injection moulding defects may be caused by many different factors. Listed here are common design, maintenance and production issues that can effect the quality of injection moulded products:
• Compounding: melt quality, contaminants and inadequate drying.
• Compatibility of polymer additives such as pigments and stabilizers.
• Suitability of the polymer grade for the application and process conditions.
• Polymer fillers such as glass fibre often influence shrinkage resulting in part distortion.
• Process conditions: incorrect melt temperature, injection speed, switchover onto hold pressure, hold pressure time or pressure setting.
• Maintenance of the mould or moulding machine and particularly inadequate purging, wear of the injection unit barrel, screw and tie bars.
• Hot runner system design, installation in the mould and temperature control.
• Mould design, gate position, gate size, venting, cold slug wells and ejector position.
• Uneven mould cooling design leading to variable temperature profile, long cycle times and distortion.
• Inadequate part design, including large variation in wall sections, sharp corners and difficult profiles that upset the material flow path.
Solution: To solve moulding defects the process engineer must first examine the location of the defict, and the point in time when it becomes evident. Observe the injection moulding process and note down the following points:
• Check, is the defect irregular or does it occur with every shot?
• Check, is the defect always in the same cavity or random?
• Check, is defect always located in the same place on the moulding?
• Check, is defect evident on the sprue or cold runner?
• Check, if a different batch of resin influences the defect?
• Check, if the defect occurs on other moulding machines?
1. Consistent defects.
If the problems always occur in the same location, this usually indicates that there is a problem in the injection nozzle, the hot runner system or shape and design of the cold runner feed and gate.
Also investigate the moulding process conditions such as the injection profile and pressure change-over point.
Product design may also be responsible, e.g. sharp edges or sudden changes in wall thickness etc.
2. Irregular Defects.
Where surface defects occur irregularly in different places, the process engineer should lock at compounding (compound quality, presence of containments). Factors such as low melt temperature, backpressure, screw speed and screw retraction can also play an important part.
3. Defects covering large areas.
This kind of defect usually extends over the entire moulding and is often visible already on the sprue. In this case the process engineer should check for decomposition of the polymer melt.
Melt decomposition may be due to polymer degradation or decomposition of additives, caused by overheating or excessively long residence times. In the case of hygroscopic polymers, hydrolytic degradation can occur when the polymer compound has not been dried sufficiently.