Page 50 - Injection Molding Advanced Troubleshooting Guide
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4 Gating Details
4.1 Gating
Gating is a subject that has not had extensive research and most do not understand
the impact it has on the process. The gate or hot-drop orifice in most cases is the
most restrictive point in the flow of the plastic and so it should be. Gating can
impact gate seal time, pressures, cavity imbalance, and defects such as high gates,
flaking, jetting, and blush. And when gate modifications are made it is typically an
increase in size or gate volume and not the opposite with a decrease in size or
volume. But to address some of these issues a reduction in gate size or thickness
can be the best approach and varies with the type of plastic you are molding. It is
important to understand exactly what the purpose of the gate is, and there is not
just one standard or one rule when it comes to gate standards with all the variables
involved with part volume, flow lengths, wall stock, and the plastic itself. Always
use the STOP process and make sure to think through what contribution the gate
has on the process and defects.
Just because you decrease the gate size or thickness, it does not always mean that
plastic pressure will increase. And increasing a gate thickness or size does not
always mean your plastic pressure will drop. We mention this because it goes
against common assumptions.
Most people automatically assume you will have an increase in fill pressures if you
go thinner in gate thickness. We have found that it is more about volume, which
can be maintained by increasing the width when reducing the gate thickness.
Some materials prefer the higher shear with thinner gates. On the other hand,
glass-filled materials with higher glass content need runners, hot-drop orifices,
and gates as large as possible to reduce pressures and be able to pack out the glass.
Typically, when a gate is made smaller you can expect to see a change in plastic
pressure because you have reduced the volume of the gate orifice. But a change in
gate size or volume does not always produce a noticeable change in plastic pressure.
