Page 455 - Injection Molding Advanced Troubleshooting Guide

45.3 Voids Troubleshooting 455

ƒ Thick sections
ƒ Gate location

45.3.2.1„ Mold: Cooling
Locations in the mold that are not cooled effectively will run hotter than the rest of
the mold which can lead to increased shrink of the molded part. When this shrink
occurs in a thick wall section the part may either sink or it may have a void.
Verify the part ejection temperature matches the documented process. Examine
the mold after running the process to determine any locations of hot spots. Verify
that the mold is watered correctly and has adequate flow in all circuits.
For further information see Chapter 14 on mold cooling.

45.3.2.2„ Mold: Thick Sections
Although it is more of a part design problem, thick sections will be discussed as
part of the mold design. When parts have thick cross sections, there will be a ten-
dency to form either sinks or voids. This tendency is due to increasing shrink rates
in thick sections. Thick areas cool longer and may not be packed out as well be-
cause surrounding thin wall stock may freeze before the thick area is packed out.
During part design every effort should be made to minimize thick areas on the
part. Coring of thick areas will help to minimize the opportunity for voids to form.

Thick cross sections have a negative impact on cycle time simply because it takes
longer to cool the thick sections of the part. Plastic is an excellent insulator that
will keep heat from conducting through thick sections resulting in longer required
cooling times. To maximize the cooling efficiency of a mold, thick sections must be
cored out to try to maintain nominal wall stock.
In some cases thick wall sections will require the gate to also be increased to en-
sure that the thick section is fully packed out. Increased gate size will lead to a
longer gate seal time which in turn leads to a longer cycle time, which means lower
profit potential.

45.3.2.3„ Mold: Gate Location
If a part is gated into a thin wall and has thick sections away from the gate the odds
of voids will be increased. Nominal wall stock should be maintained whenever pos-
sible. If a gate is located in a thinner wall the thin wall will freeze before the thick
section is packed out, which will result in increased shrink in the thick section.
Whenever a part is designed with nonuniform wall stock the gate should be located
in the thicker section to ensure that the thick area is packed out. Always try to core
out the thick sections first; however, if they cannot be cored try to locate the gate so
that it will have maximum impact on the areas of concern.