When plastic material is injected into a mold cavity, it is cooled and solidifies to form a part that is made to final part dimensions. However, if the cooling of this part in the mold is not uniform, differences in part shrinkage can occur, causing the part to distort. This condition is referred to in the injection molding industry as warpage. Warpage is also the result of excessive stresses that are built into a part during the molding process. Here are some causes and solutions below for warpage of plastic injected parts.
An injection speed that is too fast will increase the melt temperature, forcing the melt to overpack the cavity. This overpacking creates molded-in stresses in the part, which warp the part. Solutions: Decreasing the melt temperature if the injection speed is too fast.
The injection speed is too slow, the viscosity of the material increases the amount of packing pressure needed to fill the cavity. This higher packing pressure adds molded-in stresses in the part, causing warpage. Solutions: Increasing the injection speed to decrease the holding pressure needed to fill the cavity.
Long injection forward times contribute to part warpage by allowing more time for more material to overpack the cavity. Solutions: Limiting the injection forward time.
If not enough gates are used to fill a part, higher packing pressure occur to fill out the plastic injected part, producing molded-in stresses. Solutions: Increasing the number of gates reduces the packing pressure by distributing the pressure over a number of gates.
Running the mold haves at different temperatures can cause shrinkage in one direction to help overall warpage. Plastic injected parts have a tendency to warp toward hotter mold halves since this causing higher shrinkage. Solutions: Location and placement of cooling lines should be reviewed to ensure that even efficient cooling is taking place to the part.
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