Page 463 - Injection Molding Advanced Troubleshooting Guide

46.3 Warp Troubleshooting 463

When a part has areas that have differences in cavity pressure the shrink will vary
in these areas. Variation in shrink across a part will result in warp because the
areas of higher shrink pull the part resulting in warpage.
A challenge in molding is minimizing the pressure drop across the cavity and try-
ing to generate as uniform of a cavity pressure as possible. By minimizing differ-
ences in cavity pressure, the impact of differential pressure will be minimized.
One key to minimizing pressure drop is filling the mold fast because this helps
maintain a more consistent viscosity and provide a more uniform cavity pressure.
Distance from the gate has a major impact on cavity pressure: the farther away
from the gate the lower the pressure. Evaluation of the gate location is critical be-
fore a mold is built; if the end of the cavity is too far from the gate it will be impos-
sible to provide a uniform cavity pressure. In many cases adding additional gates
to provide better pressure distribution can be the key to success. This is another
problem that CAE flow analysis can provide a great deal of help with.
Wall thickness will also play an impact on cavity pressure distribution especially
when combined with distance from gates. Thinner wall areas will freeze faster and
may restrict the ability to provide adequate and equal cavity pressures. This is es-
pecially important if the part has variation in wall stock. Wall thickness differ-
ences are further magnified when molding a semi-crystalline material because the
impact on crystallinity will show as increased risk of warp.
Another factor to keep in mind is that areas that are not pressurized well against
the mold cavity will tend to shrink away from the mold quicker than areas with
higher cavity pressure. Once the plastic has shrunk away from the walls of the
mold the effective cooling rate will decrease due to the lack of contact with the
mold (remember the mold is serving as a heat exchanger). Improved pressure dis-
tribution and higher overall cavity pressure can lead to more efficient cooling, thus
reducing warp.
Verify that the second-stage pressure is set correctly. Confirm that the transfer
from velocity control to pressure control is responding well. The machine should
transfer quickly into the set second-stage pressure and do so consistently. If the
machine does not respond well the material may start to freeze prior to effective
pressurization of the cavity.

46.3.1.4„ Molding Process: Gate Seal
There are times when running with gate seal will lead to warp problems. When
running with gate seal the plastic that is packed into the cavity will be held in the
cavity. Gate seal can result in locking in excess pressure at the gate area of the
part. This is more common on center direct-gated parts where high local pressure
radiates from the gate.