Page 51 - Injection Molding Advanced Troubleshooting Guide

34 4 Gating Details

It is important not to focus on the gate as the only restriction to plastic flow. The
gate land can be a contributor to pressure loss and is often overlooked. There is no
advantage to excessive gate land and it can contribute to other issues also. Many
people also overlook the role of hot-drop tip orifices. We have often seen people
open up gates or runners when the hot-drop tip was the restriction—as would occur,
for example, when a hot runner feeds into a cold runner. Remember it is about the
area or volume of the hot-drop orifice or gate.
Plastics are understood to flow in what is referred to as fountain flow. Let’s assume
you have a round runner and experience textbook fountain flow. Then what hap-
pens when you take this volume of flow and ram it through a small round orifice
into the large space of the cavity? But what if you could change the gate geometry
to improve the transition from the runner to the part? Let’s use a pressure washer
as an example to help paint a mental picture. If you were to put a tip with small
round orifice on your pressure washer, what type of stream would you expect to
see? Now if you put a tip with a rectangular orifice that was thin and wide, what
would the flow look like? The difference is pretty drastic between a small, straight,
jetting stream and a fanned-out stream. With the thin-and-wide concept we have
been able to reduce cycle times with a quicker gate seal, maintain or reduce fill
pressures, eliminate high gates or vestiges on cashew gates, help minimize gate
blush, eliminate jetting, eliminate pulls, and eliminate flaking.

Case Study

One example is a PC/ABS part where we were addressing high gate vestige.
It had a cashew gate with a 0.040-in diameter orifice. We changed the
orifice from 0.040 in round to a rectangular shape of 0.020 × 0.080 in,
which increased the gate volume. In this case we were not only able to help
reduce the high-gate defect but were able to drop the fill pressures from
16,000 to 11,000 psi. This created a larger process window on a part that
had struggled with flash and shorts.
In another example we were able to eliminate two defects—jetting and
pulls—on a larger glass-filled PP part that was causing a lot of scrap. The
750-ton tool had no process window allowance to address the defects. The
part had two cashew gates with 0.110-in diameter round orifices. In this
case we could go thinner but not wider because the geometry of the taper
would not allow it. We started by welding up the orifice and going from the
0.110 in round to a 0.050 × 0.110 in rectangle. We were concerned about
increasing fill pressures because we were reducing the orifice volume.
When we ran the tool after the change there was no increase in fill pres-
sures. Jetting was improved but we still had some issues with the pulls. The
pulls were the result of the part shrinking away from the gate area before
ejection. So, we had another idea: If we made the gate thinner than 0.050
in, would it break while the part was shrinking away, reducing the pull