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Holes (part 2)
Plastics One Injection Molding
As easy as it is to make holes in molded parts it does not come without some concerns for
the strength of the part. For every cored or molded hole there will be a weld line. The
weld lines are caused by the flow of the melted plastic around the core pins. These weld
lines are not as strong as the surrounding plastic material, and also may detract from the
overall appearance of the molded part. The part designer should consider these points
when designing holes in a molded part.
The coring of holes is easy when the axis is parallel to the parting line. But when
holes and other features run perpendicular to the parting line then retractable cores
(or cams) are required. Split pins and cores (called passing steel shutoffs) can be used
to create some of the features. The designer needs to be aware of the problems of
side action cores and the added expenses associated with these types of molds. With a
little understanding of how the mold opens and where the parting line will exist, these
costly features can be modified. Rule of thumb: whenever possible all design features
should be incorporated in the same direction of the mold opening so that cam action
can be avoided.
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