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458 46 Warp
46.3 Warp Troubleshooting
Warp can be a challenging defect to resolve. By the very nature of heating and cool-
ing a thermoplastic there will be shrinkage and stresses that can lead to warp. The
most common reason for warp is differential shrink throughout a molded part.
This differential shrink can come from a number of factors including:
Differential cavity pressure
Differential cooling
Differential orientation especially with semi-crystalline materials
Each of the above factors is covered specifically below in each appropriate section.
When it comes to warp, part design drives many of the problems encountered in
molding. The key design rules for plastic are absolutes when it comes to avoiding
warp. The four design rules that are most important are:
1. Maintain nominal wall stock
Plastics will shrink differently depending on the nominal wall stock. Wall stock
impacts how far a part will fill and pack from a gate, cooling rate, and orienta-
tion. If a part has varying wall stock the plastic will have different shrinkage
rates, which will result in warp. Trying to process around a fundamental design
flaw will result in manufacturing process windows that are unacceptable for
producing quality parts.
2. Avoid gating thin to thick
If nominal wall stock varies (see #1) do not locate a gate in the thinner wall
stock regions. Locating a gate in such a region will result differential shrink be-
cause the ability to pack out the thick section will be driven by the thin wall.
Once the thin wall freezes the thick areas can no longer be packed, which re-
sults in an under-pack situation in the thick regions resulting in increased
shrink. Another result of gating thin to thick is the thin areas may be over-
packed by trying to pack out sink in the thick areas, which again leads to differ-
ential shrink and warp.
3. Rib to wall ratio
To avoid thick hot areas around ribs design standards for a rib to wall ratio
should be followed. Typical standards would be a 40–70% thickness for a rib
depending on the material. This will help minimize sink but also helps avoid a
hot thick junction at the base of the rib that can lead to warp due to excess
shrink. Thin ribs will freeze faster than the nominal wall and may lead to warp
opposite of the rib location.
4. Utilize rounded corners
Try to avoid sharp corners whenever possible. A rounded corner with an appro-
priate radius will allow even filling and cooling. Also sharper corners will create
