Page 276 - Injection Molding Advanced Troubleshooting Guide

268 28 High Fill Pressure

produce a higher shear rate of material during injection, which in turn may lead to
pressures staying low.
The total area of gates is important to consider. A wider gate may help with more
flow volume without increasing the thickness and impacting gate freeze time. Also
review if there are enough gates to adequately fill the part. Every material has a
maximum flow length to wall stock ratio that needs to be considered. If trying to
flow the material across a long part the pressure will climb throughout fill, but
adding a second gate could cut flow length in half. Examine the gate location for
ideal fill: a gate located at the mid-point of the part cuts flow length in half when
compared to a gate located at the end of the part.
Long gate lands are an often-overlooked area. Long gate lands will restrict the flow
of the plastic and lead to increases in fill pressure. Land length should never ex-
ceed 0.030 in on a gate, and dropping it to 0.005 in is often a very effective solution
for a variety of problems including high pressure.
Keep in mind that gate size will also influence the gate seal time and the ability to
pressurize a mold effectively.

28.3.2.2„ Mold: Runner or Sprue Size
It is important to understand that the gate is not the only restriction in the mate-
rial feed system. The runner and sprue can have undersized thickness for a given
part, which in turn can lead to an increase of the required pressure to fill the mold.

Long runners and sprues will also impact the fill pressure of a mold. The further
the plastic flows through a sprue and runner prior to hitting the cavity the higher
the pressure will be. Whenever possible minimize the length of the sprue and run-
ner to minimize the pressure required to fill the mold. In general, longer runners
will require a thicker runner cross section.

28.3.2.3„ Mold: Hot Runner
If there are restrictions within the hot runner system the pressure required to fill
the mold will rise. These restrictions can appear through the hot runner system.
The most common restriction is at a low-vestige-style hot tip. Low-vestige hot tips
can be a much smaller restriction then the downstream runner and gate and lead
to excessive pressures. Hot tips should be evaluated for restriction to the flow and
sized accordingly.

Another factor with hot runner systems can be zones that are not heating properly
leading to cold zones that cause premature freezing of the material. The hot runner
zones should be set so that they neither add nor remove heat from the melted plas-
tic. Hot runner zones should normally be set at the actual melt temperature of the
material.