Injection Molding: The Cause of Warpage
Warp! Good when you want to go past the speed of light; bad when seen in plastic molded parts.
Why do parts warp? Parts warp for 2 reasons: molded-in stress or shrinkage. Most common is molded-in stress due to excessive plastic pressure or differential shrink (direction of flow: shrinks differently than across the flow).
PROBLEM: Product warping due to molded-in stress.
SOLUTION: Corrective annealing of all parts.
RESULTS: No lost product; on-time delivery.
Problem: We recently experienced warping with a long thin-walled part made from glass- filled nylon. The parts didn’t warp right out of the machine; they warped several hours later. We do our inspections in-process, which is generally the best way to catch any production/design challenges –before they have the opportunity to impact the yield of the entire run. In this instance, “immediate inspection” didn’t catch the warp that developed over time and was discovered hours later.
Solution: So what do you do with warped parts? Our first task was to determine if this was a molded-in stress problem or a shrink problem. Going over the history of the part and looking at the run data it became clear this was a molded-in stress problem, which was good. That is to say it was correctable. Molded-in stress can be relieved through annealing. Annealing involves placing the parts in an oven and heating the air to the point where the polymer chains begin to relax, and then leaving the parts at that temperature for a specific amount of time. Optimal temperatures and times for annealing are polymer-specific. Materials manufacturers are the best resource for obtaining these specifics.
Results: All product was corrected, acceptable to client, and delivered on time
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