Page 424 - Injection Molding Advanced Troubleshooting Guide
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42.3 Splay Troubleshooting 423
Case Study: Hot Drop Tips
This was a polycarbonate part with four low-vestige hot drop tips, and the
splay was appearing near the gates. The part needed to be filled fast to fill
and pack out the part, but at these high velocities splay became a problem.
Inspection of the gate and spreader tip showed that the gate orifice was
0.050 in and the spreader tip was 0.025 in, which resulted in a gate area of
0.00147 in . The gate orifices were opened up to a diameter of 0.0625 in,
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which resulted in a gate area of 0.00258 in (a 75% increase in area). By
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opening this restriction at the hot drop tips the splay problem was resolved.
42.3.2.3 Mold: Cold Slug Wells
Cold material, flakes, and drool can contribute to defects appearing to be splay
near the start of fill. Proper cold slug wells must be designed at the transitions
from sprue to runner and gate. Runner pins should not have flash or the flakes
could lead to defects that look like splay. Also verify that undercuts on sprue pull-
ers are not too aggressive or they will leave behind flakes. Many of these issues
with cold material, flakes, or drool can create cosmetic defects that can look like
splay.
42.3.2.4 Mold: Sprue and Nozzle Tip Orifice
The industry has different standards for cold sprue and hot runner nozzle seat ori-
fices. The most common sizes for nozzle tips are 1/8, 3/16, 1/4, 5/16, and 3/8 in,
and standard cold sprue orifices will be 5/32, 7/32, 9/32 in, etc. The standard cold
sprue is designed to accommodate the nozzle orifice with a 1/32 in of clearance,
which helps prevent sprue sticking. So a mold with a 5/32-in sprue orifice will
need a 1/8-in nozzle tip for optimized flow without sprue sticking.
On the other hand, hot runner orifices are designed to a nominal flow channel
without the clearance, so the nozzle tip should match the hot runner orifice one to
one. This allows the hot runner to function as an extension of the machine barrel
and reduces dead spots in the flow path. If a dead spot exists between the tip and
the hot runner orifice, material will tend to degrade, which will lead to splay. Some-
times the root cause of splay is that someone forgot to switch the nozzle tip and the
mismatch is causing these dead spots. Nozzle orifice should be documented for
every mold and the tip should be changed whenever necessary.
Never run a mold with a nozzle tip orifice larger than the sprue opening on the
mold. This will create sticking issues on a cold sprue and create a shear point and
dead spot on a hot runner orifice.
