Page 439 - Injection Molding Advanced Troubleshooting Guide

43.3 Sprue Sticking Troubleshooting 439

43.3.2.5„ Mold: Sharp Corners
All edges on the sprue and runner transitions should be broken to reduce stress
points for breaking and reducing shear. This also includes sprue-style tips on hot
runners. In cases where this does not help, thin gusset ribs can be added. These
will cool quickly and add structure/support to the sprue runner transition. In
cases where gusset ribs are needed, the root cause is usually excessive sized
runner and sprue diameters. Adding larger radius to corners in these cases will
add more mass and can make the problem worse.

43.3.2.6„ Mold: Oversized Sprue or Runner

Sprue bushings and runners that are excessive in size create unwanted mass that
will require excessive cycle time to cure to prevent the sprue from breaking off
from the runner. This is very common in the industry and can contribute to un-
wanted waste with runner and sprue scrap.
Oversized sprues and runners will often lead to increased cycle time to allow for
cooling. If the sprue and runner is overly soft at the desired cycle time it probably
is oversized or does not have adequate cooling in the runner sprue area. There are
many cases where the replacement of a sprue bushing with a smaller diameter
sprue bushing would lead to dramatic drops in cycle time. The cycle time should be
dictated by the part and not the sprue or runner cooling.

43.3.3„ Sprue Sticking Troubleshooting Machine Issues

Potential machine-related causes of sprue sticking include:
ƒ Nozzle problems
ƒ Machine performance
ƒ Injection unit forward pressure

43.3.3.1„ Machine: Nozzle Problems
A damaged nozzle or nozzle tip can cause the sprue to stick. Some common prob-
lems are damage to the face of the nozzle tip which allows plastic to leak at the
interface between the nozzle tip and the sprue bushing. Damaged nozzle tips can
be resurfaced or replaced.
Another item to watch for is matching the nozzle tip orifice to the sprue bushing.
The nozzle tip should be 1/32 in under the size of the sprue orifice to allow maxi-
mum flow through the tip and avoid the situation where the nozzle tip is oversized.
An oversized nozzle tip orifice will result in sprues sticking. There are nozzle ori-
fice gauges for checking tips and sprue bushings.