Page 186 - Injection Molding Advanced Troubleshooting Guide
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176 19 Buildup
Figure 19.2 Severe vent buildup: notice black buildup that has been scraped off the mold
When buildup is creating negative effects on the parts or process it is important to
verify that the mold has adequate venting and that all of the vents are open. Often
times the buildup will try to escape through the vent and create a large amount of
residue in the vent land, which will further restrict the mold’s ability to vent. If
experiencing venting-related buildup, first verify that the vents are open. To verify
that vents are open the mold surface can have tooling blue applied to the side
opposite where the vents have been cut. If the vents are open at all the blue will not
transfer in the relieved areas of the vent. It can also be critical to know that the
vent is deep enough; the depth can be measured with a depth micrometer or by
closing up on a putty and measuring the putty thickness across the vent location.
If the vents are found to be too shallow they should be opened up by an experi-
enced mold technician.
If the vents are determined to be open the mold should be reviewed to determine if
additional venting can be added. Frequently the old standby of placing a ½″-wide
vent every “X” inches will not be adequate. Rarely will too much venting create an
issue (unless the vents are too deep and flash). Often times a full perimeter vent
will yield excellent results and allow the gas to easily escape the mold. If full pe-
rimeter venting cannot be used an approach of using wider vents will often work
well. A situation that is common is trying to hit an end of fill location with a ½″-
wide vent, so the odds of getting everything perfect are reduced, and if a 2″-wide
vent was used the window to hit the end of fill location is much wider.
Another factor that will have a large effect on buildup is the land length of the vent.
If the land length is too long the vents will not stay as clean and will plug with
buildup much quicker. Land length on vents should be kept below 0.080 in. Often
