Page 56 - Injection Molding Advanced Troubleshooting Guide

4.2 Gate Size, Shape, and Taper 39

Looking back, I see it was not logical to expect this to change much, be-
cause we were just shooting a minimal amount of material through the
gates, and no matter how big they were we would not see the impact with-
out putting more volume through the gate. We then shot the whole part, and
to our amazement the overall pressure drop was now down to 17,000 psi,
despite the pressure-drop study still showing the same 9000-psi pressure
loss through the gate. Enlarging the gates increased the volume of material
that could move through the gate in the same amount of time, allowing the
cavity to fill more easily.
Most would assume that the first enlargement of the gates from 0.020 ×
0.080 in to 0.030 × 0.080 in was a 50% increase in gate area, and would
allow a comparable increase in flow volume per unit time. But you could
also argue this was an even greater increase in effective flow orifice. I have
no idea of how thick the skinning of plastic around the gate would be, but
let’s suggest 0.005 in per side. This may be a stretch, but I just want to
paint a picture. If we had skinning of 0.005 in per side at the original 0.020-
in gate thickness, the remaining flow-channel thickness would be 0.010 in.
With the new 0.030-in gate size, the flow channel would be 0.020 in, a 100%
increase in effective area and potential flow volume. With this theory, going
to 0.040 × 0.080 in gate size would leave a flow-channel thickness of 0.030
in, an increase of 200% over the original size.

You really need to keep an open mind about when you need larger gates and when
you need smaller gates. As mentioned earlier, typically reducing gate sizes can
eliminate other defects and molded-in stress and in some cases reduce gate seal
time.
Many people tend to focus on runner size as much as gate size. Although we agree
that the runner should always be a consideration, from our experience 99% of the
time the problem is not the runner but the gate or hot-drop orifice. It depends on
the length of the runner; if the runner is short, we really do not care if it is round
or square, because it has minimal impact on the pressure loss unless it is ex-
tremely undersized. Runners in most cases are much larger than they need to be,
contributing to excessive waste and in some cases additional cycle time with in-
creasing gate seal times.

Case Study: Hot Drop Restriction

On a part running glass-filled nylon the process was pressure limited at
24,000 psi. Based on the pressure-drop study showing relatively little pres-
sure loss through the runners and gates, it was assumed not much could be
done to remedy the situation. This tool had four cavities and a hot runner
with two hot drops, each feeding a cold runner and one gate to each cavity.
The parts in this case had a long flow length, which also pointed to the ▸