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One or two plastic parts are sticking in injection mold

This is true I have gone through this problem not in one company but in many, from the period when I started carrier from trainee engineer. Here are some techniques I used to solve this kind of issue through process. (By considering that injection mold design and mold maintenance is ok in all aspects)
  • Decrease the shot wt. Decrease the hold pressure at starting.
  • If cold water circulation required in injection mold, keep it off initially.
  • Increase the cooling time 4-5 secs for startup.
  • Ensure the purging of barrel for degrading material.
  • Dipper cavity mold, optimize injection mold initial opening speed some vacuum pull back component to cavity.
  • After repair of injection mold, ejector plate become much free in movement, (as all the contamination has cleaned, and if ejector rod is not coupling type). When injection mold clamping drops to zero, ejector plate move forward push the component to cavity.
It's like the age old problem of injection mold makers greasing ejector pins, the plastic part design seems to be a closed cup so inevitably the ejector pins are acting as gas vents, if your mold makers are greasing the pins they are blocking this venting system and after a short time the grease clears. I have been fighting the grease or no grease battle for many years with countless injection mold makers. EJECTOR PINS SHOULD NOT BE GREASED. As for the mold surface I find that the best finish for elastomers is a bicarb blast not sand or glass.

Since this is occurring when the injection mold is returned from the tool shop after service, I would suspect you have a surface tension issue going on. Check and see if they are using diamond to clean the surfaces. They may be getting the surface too smooth causing surface tension issues that will lead to sticking. As the injection mold and the surface finish dules, your surface tension changes and you have less sticking issues.

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