The ejection temperature is the temperature below which the plastic is solid.
When the whole of the part has reached the ejection temperature, the part can be ejected from the mold with no adverse effect on its quality.
Since the cooling time frequently comprises some 80 percent of the cycle time, any reduction in cooling time will significantly reduce the cycle time. Frequently, the cycle time is determined by the time taken to reduce the part temperature to a level at which it can be safely ejected without compromising the quality of the part. The temperature at which a part can be ejected from the mold is affected by a number of factors.
Successful ejection requires the part to be stiff enough to resist any tendency to warp caused by shrinkage variations and residual stresses, and stiff enough to resist the local forces on the part from the ejection system. The geometry of the part, the mold finish, and the degree to which the cavity has been packed during the filling and packing phases can all affect the cycle time.
The overall requirements for cooling the part will always be a compromise between uniform cooling to assure part quality, and fast cooling to minimize production costs.