Page 394 - Injection Molding Advanced Troubleshooting Guide

392 40 Short Shot

Barrel diameters can be measured with a cylinder bore gauge to evaluate barrel
wear. Screw diameters can be measured with an outside micrometer and a parallel
bar that spans two flights of the screw.

40.3.3.4„ Machine: Machine Performance
The machine must be capable of achieving the established set point of all setup
parameters. Verification of machine performance is an important part of injection
molding troubleshooting.
See Chapter 8 on machine performance for more details.

40.3.3.5„ Machine: Nozzle Leak
If plastic leaks at the machine nozzle the ability of the process to provide con-
sistent cavity pressure will be compromised. The main areas to verify for leaks are
as follows:
1. Nozzle tip to sprue bushing. If the either the nozzle tip or the radius on the
sprue bushing is damaged the plastic can leak through the gap. Another con-
cern is the use of a nozzle tip with the wrong radius, which will create a gap at
the contact between the nozzle tip and sprue bushing radius. See Figure 40.10
for an example of a nozzle to sprue plastic leak.
2. Before bringing the injection unit forward ensure that all leaking plastic has
been removed from the nozzle tip and the sprue, and if using a hot runner make
sure that the sprue bushing seat is clean. Repeated crushing of plastic at this
interface will cause damage over time resulting in a leak.
3. If any of the end components of the machine are not tightened correctly a leak
may occur. Possible leak areas could be between the nozzle tip and the nozzle,
between the nozzle and the adapter, between the adapter and the end cap, and
between the end cap and the barrel. When installing any of these components
make sure that the seat surfaces are clean (including no melted plastic). Also
make sure that all components are adequately torqued.