Plastic Troubleshooter
On-Line Solutions To Injection Molding Problems

Improperly Cleaned Hopper

Explanation: When a material change is required, the hopper must be cleaned out. It is not good enough to simply remove the material present. The sides of the hopper must be wiped to remove any material dust or ``fines'' that stick to the sides due to static charges. If not removed, these fines will get picked up by the new material and, because of incompatibility, will appear as contamination in the molded parts.

Solution: Clean the hopper thoroughly between material changeovers. This may require wiping the inside with a cloth slightly dampened with denatured alcohol to remove all traces of fines.


Excessive Lubrication

Explanation: Molds with cams, slides, lifters, and other mechanical actions need periodic lubrication. Sometimes there is a tendency to overdo this and the lubricant may find its way into the cavity of the mold. This is especially true for ejector pin lubricants. The oil-based lubricant is not compatible with the base resin and is evident as contamination on the molded parts. In addition, excessive mold release acts as a contaminant and appears on the molded part as blotches, dark spots, smears, and streaks.

Solution: Optimize the use of lubricants and minimize the use of mold release sprays. Clean up any excess lubricants and use only the amount needed for a specific application. A little lubricant goes a very long way. Investigate the reason for using mold release. Usually it is a temporary approach to a more severe problem, and the problem should be solved to eliminate the need for the release agent.


Contaminated Raw Material

Explanation: The most common cause of contamination in molded parts is molding compound contamination. Such contamination is usually the result of dirty regrind, improperly cleaned hoppers or granulators, open or uncovered material containers, and poor quality virgin material supplied by the manufacturer.

Solution: This type of contamination can be minimized by dealing with high quality, reputable suppliers and by using good housekeeping practices. Properly trained material handlers will also help reduce contamination.


Poor Housekeeping

Explanation: The machine operator may be the source of contamination in a variety of ways. First, if the operator is allowed to have food or drink at the work station, these may accidentally get spilled into containers holding fresh material ready to go into the hopper. Second, the operator may have been instructed to keep the area clean and sweeping dust into the air may result in contaminating raw material or freshly molded parts. Third, a lack of concern or outright sabotage could be the incentive to purposely add contaminants to the molding compound.

Solution: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining contaminant-free areas.



Contamination can be defined as an imperfection in a molded part caused by the presence of a foreign object or material that is not part of the original molding compound.


Some common causes and solutions are listed below

Some common causes and solutions are listed below
Defect - Contamination
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