Page 269 - Injection Molding Advanced Troubleshooting Guide

27.3 Excessive Cycle Time Troubleshooting 261

27.3.2.5„ Mold: Poor Ejection
Ejection issues can lead to longer cycle times. The two most common problems
encountered with ejection are:
1. Lack of ejection surface area leading to pin push.
To process around this issue the cooling time is often increased. The mold
should be designed with adequate ejection to ensure that the part will be pushed
off efficiently during ejection. Bear in mind that materials with higher shrink
rates will often require more ejection area for effective part removal. Also be
sure to examine the part design for areas with poor draft, deep draws for ejec-
tion, and shrink on details, because all will demand more ejection area to push
the part from the mold.
2. Extra ejection strokes.
Sometimes a part will not eject well due to a variety of issues and extra ejection
strokes may be added to ensure proper ejection. Each extra time the ejector
strokes is increasing the cycle time. If the part is sticking the mold should be
examined for improvements. In some cases the solution can be as simple as
allowing increased injection stroke. Also keep in mind that every extra ejector
stroke is increasing the wear on the mold and machine. Look for the reason for
the extra strokes and eliminate the issue.

27.3.2.6„ Mold: Thick Sections
Molds with thick wall sections will require additional cooling time. For most cases
this must be addressed in part design (STOP: Think). Maintaining a consistent
nominal wall stock is a fundamental design rule that cannot be overemphasized. A
mold can be built with thick sections even where the part design does not call for
increased thickness. A prime example of this problem is when a core pin is shorter
than required, which leaves a thick section under the pin. If a part has sink oppo-
site a core pin location check the depth of the core pin to ensure that it is long
enough to avoid the risk of sink over the pin.
Also if a part wall thickness must be locally increased try to blend the wall stock
rather than having an abrupt transition. This blend may allow less risk of sink and
read-through (variation in gloss levels due to differential shrink) at the wall stock
increase.

27.3.2.7„ Mold: Thermal Isolation of Hot Runner

A hot runner system adds a large heat load to a mold that needs to be isolated from
the cavity. If the heat in the hot runner is not well managed extra heat may transfer
into the cavity leading to extended cycle times. The manifold should be thermally
isolated and controlled to minimize the heat rise in the cavity. Watch for issues