Page 235 - Injection Molding Advanced Troubleshooting Guide

226 24 Cracking

Residence time must be considered for hot runner manifolds as well as the barrel.
The hot runner manifold is a continuation of the melt delivery stream and can
impact the residence time of the material. Most manifolds are designed to clear out
every shot but there can be cases of greater than one shot capacity in the manifold
which adds to the overall residence time of the material at temperature.
Another factor impacting residence time is any hang up area in the system. If for
example there is a step between the nozzle and the adapter there will be a dead
spot that traps material. This trapped material can break free over time leading to
degraded material being injected into the mold. This issue often will result in splay
or black specks and streaking; however, it could also contribute to cracking in the
molded part. To avoid this situation, it is important to match components on a
molding machine and inspect for damage. Any known dead spots in material flow
should be eliminated.

24.3.1.4„ Molding Process: Ejection
Cracking will frequently occur during ejection. This is usually related to how hard
the part is sticking in the core half of the mold. To identify if ejection is creating
the crack on the part, check the part for cracks prior to ejecting (do this by stop-
ping the molding cycle before ejecting). If cracks are not present then eject the
part from the mold again looking for cracks. If the part appears to be cracking
during ejection there is probably a root cause such as excessive shrink on the core
or undercuts on the core. To try to eliminate cracking on ejection it is sometimes
possible to adjust the ejection velocity to improve the problem. Please note that if
the ejection velocity is slowed there will be a negative impact on cycle time.
Rather than modifying the ejection velocity a better solution is often to resolve the
true cause. As an example, sometimes long cycle times will cause excessive shrink
onto the core making ejection difficult, so speeding up the cycle can at times cure
the problem. It is also important to review the mold for undercuts, EDM/machining
marks, damage, and surface finish. Also verify that adequate ejection is provided
during tooling design. Tooling concerns will be addressed in the tooling section
below.

24.3.1.5„ Molding Process: Part Removal
If cracking is not occurring during ejection it may be occurring after ejection. An
example of this could be if the part is extracted by a robot and the robot may be
damaging the part by hitting excessively hard or by making a move that damages
the part. Another possible point of cracking a part is as the part hits the chute or
conveyor if the part is not robotically picked. If the parts are dropping on a chute
and cracking it is possible to cushion the chute with cardboard to prevent the dam-
age. Some parts by design should be robotically handled to avoid damage from im-