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.