Page 440 - Injection Molding Advanced Troubleshooting Guide

440 43 Sprue Sticking

Also verify that the correct tip radius is being used. The common tip radii are 1/2
and 3/4 in. Make sure that the correct radius is on the nozzle tip. Most shops stan-
dardize on one radius but takeover molds may have something different. Also there
are occasions where the sprue bushing radius is missing in a mold design review
and the wrong radius shows up on a mold. There are radius gauges available to
check the sprue bushing and the nozzle tip.
Inspect the injection unit for proper nozzle alignment to the mold sprue bushing.
Place a piece of cardboard on the sprue bushing and bring the injection forward.
Back off the injection unit and inspect the cardboard for uniform contact between
the sprue bushing and the nozzle tip. Pressure-sensitive film can also be used in
place of cardboard for a more accurate depiction of the contact force. The injection
unit may need to be aligned if the nozzle is not hitting squarely. There are cases
where the injection unit jumps as it locates into its full forward position, which
indicates an alignment problem.

Figure 43.6 Nylon nozzle tip with undercut from erosion

When molding glass-filled materials the nozzle tip may have erosion problems that
can actually lead to undercuts in the nozzle tip. This undercut area will in turn
lead to sprue sticking. Examination of the sprue will normally highlight if there is
an area that is undercut, and if an undercut is detected the nozzle tip should be
replaced to eliminate the problem. Figure 43.6 shows a nylon tip with an undercut