Theory of Cooling in Injection Molding

The plastic starts to cool down as soon as it hits the walls of the mold. Once the holding time is over, the cooling time starts. The mold remains closed till the end of the cooling time. The mold then opens and the part is ejected. Before the mold opens, the part must reach the ejection temperature of the plastic. If the part is ejected before it reaches ejection temperature, the part is too soft and will get deformed during ejection. Excessive cooling time is only a waste of machine time and therefore profits. Cooling time should also be set such that the part dimensions remain consistent and the process is capable.

Determination of the right cooling time can get complicated. With parts with thick sections, it is difficult to measure the internal temperature in the center of the thickest section. In some parts of the mold, it is difficult to get enough cooling and therefore cooling times have to be increased to increase the heat transfer. In some cases, the mold temperature can stabilize after as long as a couple hours. Shrinkage can also be influenced by changes in cooling times.

In the picture below, you will notice that some dimensions may be more sensitive than others. Dimension A is not influenced by the cooing time range experimented with. However, Dimension B changes with the cooling time. The target value for dimension B is 0.135”. So we can either set the cooling time at around 17 seconds or make steel changes to run it faster and achieve the same dimensions. Identifying the lower and upper limits on the graph will also present a graphical representation of where the cooling time can be set.