Page 462 - Injection Molding Advanced Troubleshooting Guide
P. 462
462 46 Warp
The fact that plastic warps toward heat can be used as an advantage in some parts.
By running an intentional mold temperature from cover to ejector it is possible to
drive warp in the direction opposite of what it normally would go. Use caution
when setting intentional water temperature differential because the steel will ex-
pand differently depending on the temperature. Depending on the shut off condi-
tions in the mold too much offset can lead to galling and mold damage. Check with
a tooling engineer or mold maker to determine how much temperature differential
is permissible for the mold in question.
Case Study: Cooling Warp
In this case a talc-filled polypropylene was being molded in a part that had a
rectangular section that had a tendency to warp inwards. If the part warped
inwards it created assembly problems with the mating components. A water
circuit was run through the mold surrounding the rectangular section. This
outer water line was run at 120 °F and the rest of the mold was run at
80 °F. When running with this water offset the rectangular section would
actually want to warp outwards rather than inwards, which resulted in a part
that had no assembly issues.
46.3.1.2 Molding Process: Melt Temperature
High melt temperatures can lead to a hotter part at ejection. In injection molding
the management of the heat energy put into the plastic and its removal is critical.
If the plastic is hotter and all other process conditions remain the same, the part
will be hotter when ejected, which in turn may result in increased warp.
Changes in melt temperature will also impact the pressure across the cavity. If the
melt temperature is cooler than prior molding runs the pressure drop will increase
across the cavity. This increased pressure drop can lead to higher cavity pressure
differentials that result in differential shrink resulting in warp. The worst-case
scenario would be a melt temperature that is so low that there is a short shot (0 psi
cavity pressure) at end of fill.
Verify that the melt temperature matches the established process. Uncontrolled
high melt temperatures could lead to increased cooling time to compensate. Docu-
ment melt temperature and verify when troubleshooting.
46.3.1.3 Molding Process: Pressure Differential
Another process factor that impacts warp is differential cavity pressure. When
plastic is heated the distance between the molecules increases but during cooling
the plastic contracts, which results in shrinkage. The amount of shrink of plastic
will change based on how much pressure is used to “pack” the molecules together.
