Page 477 - Injection Molding Advanced Troubleshooting Guide
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47.3 Weld Lines Troubleshooting 477
Maintaining uniform nominal wall stock is rule one when designing for plastics
and this is one of the reasons why.
If a weld or meld line is occurring in an area that does not make sense check short
shots to look for flow front hesitation. Computer flow analysis will help identify
these issues on part designs. Sometimes the wall stock may have been uninten-
tionally thickened in an area which in turn leads to a meld or weld forming.
47.3.2.3 Mold: Gate Location
The location of the gate as well as the number of gates can impact weld lines. If a
part is gated with multiple gates the part will have weld lines between each of the
flow fronts (unless sequentially valve gating). It is important to keep the weld lines
in mind when designing gates, and a flow analysis software can help with predict-
ing all potential weld line locations.
Sometimes during mold design, it is possible to position a gate so that any weld
lines forming from mold cores will be in a noncritical location. Look for opportuni-
ties during design to try to influence the impact of weld line location. It is critical
to understand the customer expectations for weld line appearance, location, and
strength.
Sequential valve gates can be used with multiple gates on a part to provide a part
without visible weld lines from gate flow fronts. Sequential valve gating is when an
initial start of fill valve is open and downstream gates are not opened until the flow
front reaches them. This avoids the weld line that is formed from the flow fronts
from multiple start of fill gates.
47.3.3 Weld Lines Troubleshooting Machine Issues
Potential machine impacts on weld lines can come from:
Clamp tonnage
Velocity to pressure (V to P) transition
Machine performance
47.3.3.1 Machine: Clamp Tonnage
As mentioned in Section 47.3.2.1 venting of a mold is critical to formation of qual-
ity weld lines. If the mold is clamped with excess tonnage there is a good chance
that the vents will hob shut. Too much tonnage on a mold literally moves the steel
resulting in a lack of venting.
Inspect the clamp tonnage setting to determine if the clamp force is set correctly.
Just because a machine has 500 tons of clamp force does not mean that it is appro-
