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98 11 Hot Runners
Selection of proper orifice size must be analyzed carefully to ensure a balance be-
tween all factors. Computer-aided flow analysis can help with predictions of how
well a given gate orifice will be at filling and packing a mold. Consult with material
suppliers and hot runner manufacturers for recommendations on orifice sizes.
Case Study
This concerns a part having a scrap issue with splay where the orifice size
ended up being the solution. This was a part with four low vestige tips that
had a spreader tip in the center of the gate orifice. Many will just consider
the orifice diameter and not include the area of the spreader tip that
reduces the volume. In this case the orifice diameter was 0.050″ and the
spreader tip in the center of the gate was 0.025″ diameter. The area of the
spreader tip reduced the area/volume by 25%. The fill speeds with this gate
needed to be on the high end to make a good part but we struggled with a
lot of splay. We opened the orifice diameter to 0.060″, which was an in-
crease of 55% in area/volume when considering the area that the spreader
tip took away. This allowed us to adjust the fill speed and eliminate our
splay/shear issue.
11.8 Leakage
Hot runner leakage can be a major problem that will shut down a mold. A big
development toward reducing hot runner leakage occurred when the hot drops
started being threaded to the manifold versus relying on the stack height, seal
rings, and bolt patterns in the mold plates to hold the hot runner together. Some
toolmakers complained from a maintenance perspective that this was more diffi-
cult to work on when the drops had to be removed for service. In some cases this
was true with threads getting galled up and creating another mess. But improve-
ments with thread designs and coatings have reduced this concern among tool-
makers. So in the big picture of hot runner issues this has been a big improvement
for maintaining molds.
Another observation over the years, but not a common one recently, is with sup-
port in the hot half. If the areas cleared out for the hot runner are excessive, cavity
pressure can lead to deflection within the mold plates leading to leakage.
