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476 47 Weld Lines
Venting problems are one of the most common mold-related problems that people
will process around. There have been many cases where “can’t add more venting”
was proven wrong and the defect was resolved after adding more venting. Venting
correctly does not cause a mold to flash! Remember that vents have a depth mea-
sure but also a width measure. Figure 47.4 shows a weld line about to form: this
area must be vented to produce an adequate weld line.
Figure 47.4 Short shot around a boss: weld line will form as the part fills out
When troubleshooting a weld line problem, a good place to start is with an evalua-
tion of the venting. Make sure the mold is clean and all vents are open. If the mold
has alternate venting methods such as sintered metal inserts make sure that they
are still working, and replacing an insert may solve the problem.
See Chapter 7 on venting for more details.
47.3.2.2 Mold: Wall Stock
It is a given that any time the melt front splits around part details such as a core
pin or insert a weld line will form, but what can be overlooked is the influence of
wall stock on the flow front. Thin areas of the mold may cause hesitation for a por-
tion of the melt front resulting in a weld or meld line when the flow fronts rejoin.
