For a processor, finished parts sticking in the mold can be an absolute headache. The parts may be picked by a robot, ejected off the mold to fall onto a conveyor system, or hand-picked by an operator after each cycle. Parts sticking in the mold can cause massive downtime not to mention mold safety alarms if the part remains in the mold for the next cycle. There are ways to fix this issue and we will cover them here under three categories; Machine, Mold, and Material.
1.) Machine: Within the process parameters a part sticking in the mold usually means that the part is over-packed. If this is the case the processor can try decreasing injection or hold pressures, reduce shot volumes, or decrease screw forward time.
2.) Mold: One cause for parts sticking relating to the mold could be scratched or marred surfaces in the cavity. For this you would want to remove the blemishes and polish the mold surface. Insufficient draft angles could also be an issue. You want a minimum draft angle of 0.5 degree per side. The greater the draft angle the easier the part will be to eject. The issue could also stem from a poor ejection mechanism. In order to solve this, options include installing new ejector pins or increasing the number of ejectors per cavity.
3. )Material: As far as material is concerned, there could be inadequate lubrication in the resin causing the parts to stick. In this case, you could incorporate using a mold release agent or mix a small percentage of lubricant to the resin itself before processing.
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Jeremy Cooley is an Asaclean Technical Service Representative & Purging Expert living in South Carolina. He's worked in injection molding & purging for over 15 years, and frequently presents at national plastics trade shows and conferences.