There are times when a part or its sprue (the residual material from getting resin into the mold) stick in a mold and make part extraction a problem for the molder. This sticking can have many causes and comes down to the equipment (mold & press) or the processing parameters. For a molder, fixing a mold is an invoicing activity; process refinement is an internal expense. As a molding customer, it would be beneficial to be knowledgeable about sticking causes to help ferret out what steps should be taken.
Scratches on the core surface are a leading cause of ejection issues. A determination needs to be made as to which half of the mold is sticking to help pin point where the scratch is. Issues with the mold can include its polish. Too little polish can cause a part to grip the mold surface and excessive polish can create a vacuum and suck the part down. The mold ejection system may be worn or misaligned and as such pressing unevenly on the part to eject it. Those are a good number of the issues that would result in a mold repair.
Moving on to equipment and things a customer would not be responsible for would include things like if using a sprue puller or part removal robot. It needs to have been designed for this specific task and of adequate size. There may be problems with the nozzle, like how it mates to the sprue bushing while shooting the sprue/mold core.
Lastly, we consider the process itself. When a part has been running in production and develops sticking issues, while it is tempting to look at the process settings, they should start with the material. Qualified processes rarely go bad without an outside influence like the mold (already looked at), equipment (already looked at) and material variance. Heat degraded material could cause sticking which then leads back to equipment and then process settings. Excessive temperatures, excessive or too low a packing pressure and high backpressures during shot generation can all lead to sticking issues.
Bottom line, ask your molder for a fault assessment and analysis when you are asked to pay for a mold repair. Speaking from a place of knowledge is always preferable to flying blind.
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