The temperature duiring ejection of a molded part directly determines the molding cycle. A higher temperature duiring ejection means the part spends less time in the mold — cycle time shortens and productivity increases. However, this comes with trade‑offs in part dimensions, residual stress, and demolding behavior.
Understanding these relationships is essential for setting the right temperature duiring ejection.
Plastic continues to shrink after ejection as it cools outside the mold. The higher the temperature duiring ejection, the more shrinkage occurs after ejection — and therefore, the smaller the final part will be. If the part stays in the mold longer and cools more completely, its final dimensions will be closer to the cavity and core sizes.
A part that cools completely inside the mold tends to develop internal stresses, which can lead to cracking, warping, or fracture. A hotter ejection helps the part relieve some of these stresses during demolding — the material is still compliant enough to relax. However, as mentioned above, this also means the part will shrink more after ejection.
Parts with internal undercuts or threads are easier to eject when warm. The material expands more readily, allowing it to clear the core without breaking. If the part is too cool, it becomes rigid and brittle, making demolding difficult or impossible.
The allowable expansion depends on material type, undercut depth, and part size. For example, a small cap with the same undercut depth as a larger cap is much harder to eject because it has less material available to stretch.
Higher temperature duiring ejections may reduce the force required to push the part out of the mold — but in some cases, especially with deep undercuts, the mold may still require additional ejection force. This could mean specifying a machine with higher ejection capacity, or adding a hydraulic ejection system to the mold.
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