Theory of Gate Seal
The holding pressure must pack
into the cavity, the plastic equivalent to this volumetric shrinkage that
occurs during the cooling down of the plastic as it hits the cold walls of the
mold. The plastic enters the cavity through the gate. As long as the gate is
not frozen, the plastic can enter the cavity or leave the cavity. The holding
pressure must therefore be applied till the gate is frozen. This is where the
time factor gets into the picture. If enough time is not given, two of the
following can happen. First, enough plastic will not get into the cavity and
second, the plastic inside the cavity which is under high pressure will come
out of the cavity. It is therefore imperative that the time for which the
holding pressure is applied is enough to freeze the gate of the part.
A very simple study is performed
to determine this time. Samples molded with different holding times are weighed
and the time after which the weight remains constant is set as the holding
time. As the holding time is increased more and more plastic enters the cavity
increasing the weight. But as soon as the gate is frozen, the plastic cannot
get into the cavity and the part weight remains constant. This is called the
gate freeze time or the gate seal time. See the picture on the next page.
You will notice that the part
weight remains constant after 9 seconds. The holding time is set to one second
higher than the gate seal time to ensure that the gate is frozen during every
shot. In the case of the graph below the time was set to 10 seconds. This will
ensure consistency and any small variations will be compensated for.