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46.3 Warp Troubleshooting 467
Cavity cooling rate
Gate seal
If the above conditions are not kept consistent between cavities warp is an expected
potential outcome. It is absolutely critical for a multicavity mold to be as balanced
as possible. If the mold is not balanced and some cavities warp and others do not
the mold will have to be modified to ensure balance.
For more details on cavity balance see Chapter 12.
46.3.2.4 Mold: Gating
Warp can be impacted by the gates on a molded part. Gates need to be evaluated in
regards to the overall part to try to provide adequate fill and uniform pressure dis-
tribution within the cavity.
If a part does not have enough gates the flow length becomes excessive for the ma-
terial and can lead to a stalled flow front and lack of fill. Just as important a consid-
eration is the distance from the gate that the pressure is applied during pack.
Pressure will vary across the cavity as the distance from the gate increases and
this variation in pressure can lead to warp. Adding a second gate to a part may
allow for a much better distribution of pressure and reduce the warpage of a part.
Another factor relating to flow length and packing length is where the gate is lo-
cated on a part. During mold design it is important to consider the flow length and
how the pressure will be distributed through the cavity. This is another area that
doing a CAE evaluation of the part can yield very valuable insight when looking at
“Sink Index” and “Frozen Layer” plots.
It is important to keep in mind that if a gate is too thick the part may become over-
packed near the gate. This uneven packing can cause warp on the part, especially
if the rest of the part is freezing while the gate area is still being packed. There
have been many cases, especially with semi-crystalline materials, where going to a
thinner gate reduced warp on a part.
46.3.3 Warp Troubleshooting Machine Issues
Machine factors influencing warp include:
Machine performance
Robot handling
46.3.3.1 Machine: Machine Performance
As with many other defects, knowing that the machine is actually achieving its set
points is important to avoiding warp.
See Chapter 8 on machine performance.
