Page 122 - Injection Molding Advanced Troubleshooting Guide
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108 12 Cavity Balance
Cavity balance is also dependent upon the cavities being the same! If the cavities
were cut with wall stock variation or other differences in the steel the mold will be
unbalanced. Also, a family mold is automatically unbalanced. Family molds will
increase all of the challenges experienced in a molding operation and should be
avoided.
12.8 Artificial Balance
When molds are experiencing cavity imbalance the obvious desire is to fix the
problem so that all cavities are subjected to the same process conditions. One
solution that people will sometimes try is a simple fix like adjusting gate sizes or
modifying drop temperatures to try to balance the filling of the mold. Unfortu-
nately, by making these adjustments, although the fill only shot may become “bal-
anced”, things like pressurization of the cavity and gate seal will be different, and
as a result the parts will also be different between the cavities. Note that all gates
should be identical in size, taper, and land length.
Case Study: Imbalance from Gate Taper
An important part of the gate design is the mass and taper leading up to the
orifice on a sub or cashew gate. If the taper is not identical between cavities
the pressure to fill can increase and cause fill imbalances. An example of
this was on a two-cavity mold with sub-gates. This mold had a right-hand
part and a left-hand part that were exactly mirrored so there was no differ-
ence in cavity size but the parts were significantly unbalanced. The sub-gate
orifice was opened up on the short cavity even though the gates were the
same size (note that gates should be identical). After opening the orifice to
the extent it had 50% more volume the parts were still unbalanced; some-
thing was obviously wrong! Close examination showed that the taper of the
sub-gates was slightly different, but not enough to be noticeable without
very close inspection. The taper on the short cavity was increased to match
the other cavity, and what a change! The shorter cavity now filled well ahead
of the other cavity.
With the D-style of sub-gate the taper is taken out of the equation for flow
restrictions because there is not a gradual taper of mass like with a stan-
dard sub-gate.
If you have cavity pressure transducers in multiple cavities it is possible to better
understand the impact of the complete process effects on imbalance and plastic
conditions. The goal is to see consistent cavity fill times, rise in pressure, peak
pressures, and cavity pressure integrals.
