Page 55 - Injection Molding Advanced Troubleshooting Guide

38 4 Gating Details

reducing the gate thickness to 0.006 in. You probably would have bet every-
thing you owned that the pressures would have increased. A thickness
reduction of 0.006 in may not seem like much, but it was a 50% reduction in
gate volume.


Gating and its geometry can have a big impact on shear rates, gate seal, pressures,
and quality issues. Most molders and mold makers have their own opinion on gate
sizing and many do not keep an open mind on the impact gate size can have on the
process and part quality.
To establish gate standards, you have to consider material variation, part wall
thickness, and flow lengths. Typically, we would tend to err on the small side, go-
ing thin and wide when possible; but with some materials, such as glass-filled
nylon, we tend to go much larger, especially with larger parts where there is more
volume of plastic to move.

Most of the efforts in the industry to control the injection process involve measur-
ing and controlling injection pressure. This is, of course, a very critical parameter
to control and to confirm you have a quality part. But consider the measurement of
actual injection volume through the gate, and how it relates to injection pressure,
which possibly would be a great tool for process analysis. The gate is typically not
a big focus unless there are pressure-loss issues. Pressure-drop studies are the
common means to observe the impact of the gate on the process but the industry
standard process is flawed and does not take into consideration the impact the gate
has.

Case Study: Pressure Drop
A part and process was pressure limited at 24,000 psi, which was the maxi-
mum machine pressure available. This was a polypropylene part that had
two gates and excessive flow length. The pressure-drop study showed that
through the gates we had 9,000 psi pressure drop, and the rest was gener-
ated through the cavity. This specific grade of PP had a high viscosity, which
contributed to the process being pressure limited. (Running a standard PP
required a plastic pressure of only 9,000 psi.)
Some people would have suggested using a higher-pressure machine.
I thought I could bring the pressure down slightly with a larger gate size but
did not think I would be able to eliminate the pressure-limited issue. The
gates were originally 0.020 × 0.080 in and I opened them up to 0.030 ×
0.080 in. We then did a pressure-drop study through the new gates, and
there was no change: it was still at 9,000 psi, which I found a little sur-
prising. I then opened the gates to 0.040 × 0.080 in and the pressure-drop
study still showed 9,000 psi pressure loss through the gates.