Page 57 - Injection Molding Advanced Troubleshooting Guide
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40 4 Gating Details
pressure loss being in the cavity as the study suggested. The process was
pressure limited when the part was 50–60% full and was not able to bury all
the glass on the part at the end of fill. The gates were 0.080 × 0.125 in and
we opened them up to 0.187 × 0.080 in. We saw a very slight improvement
but the part was still pressure limited at 80% full.
Our focus then went to the hot-drop orifices, which were at 0.080 in or 2
mm. We opened them up to 0.14 in (3.5 mm) and our pressure drop subsid-
ed to 16,500 psi. Again, this approach increased the volume of plastic to
the part and ignored what the pressure-drop study was suggesting. This was
eye-opening and cast new light on what education in the plastic industry has
taught for years, and the lack of attention to the influence of gates or hot-
drop orifices. In some cases, it just comes down to reducing the restriction
and increasing the volume of material flow, and not always thinking about
the psi pressure drop.
4.3 Mental Picture Volume versus Pressure
Let’s use a hydraulic cylinder size as an example. A 1.0-in diameter bore in that
cylinder has 0.785 in of surface area. A cylinder with a 2-in diameter bore has
2
3.14 in of surface area in the bore, which is 300% more area. If the hydraulic pres-
2
sure being used was 1,000 psi, the overall force would also be 300% greater, at 785
lb of force with the 1-in bore vs. 3140 lbf with the 2-in bore. But even though the
amount of force is 4 times greater, along with 4 times the volume, the pressure per
square inch is still the same at 1,000 psi. If the cylinder bore were instead the gate
size or area, the increase in potential volume flow and force into the part would
also be 400% and the measurement of injection pressure would show no increase
when in actuality the volume flow and process have drastically changed.
Let’s use some realistic gate sizes and volume changes. Going from a 0.020 × 0.080
in gate to a 0.040 × 0.080 in gate has at minimum a 100% increase in potential
volume flow and force. And going from a 0.050-in diameter to a 0.100-in diameter
gate has an increase of at least 250% in potential volume and force.
Again, this consideration of gate area and volume is important mainly when there
is an issue with a pressure-limited ability to fill or pack the part adequately. Our
current methods using pressure-drop studies focus on one parameter, and assump-
tions of what is relevant can blind us to the reality of what is happening inside the
mold.
