TROUBLESHOOTING

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Stresses and material orientation are leading causes of distorted parts. The finished part will appear to be warped in some fashion. Non-uniform shrinkage caused by stresses in the part traditionally causes warping of a part. The source of the stresses in a part is very complex, and often a combination of many forces.

Molded Part Stresses
Material will generally shrink toward the hotter areas of the melt. If there is a large temperature differential existing in the melt, stresses will be created in the part. To address this issue, increase the mold temperature to increase the cooling time. Prevent over-packing at the gate by reducing hold pressure or hold time. Also, increase the melt temperature and delay ejection to ensure the part has sufficiently cooled and solidified prior to ejection.

Difference In Packing Density
If there are non-uniform pressures in the mold during the injection and pack/hold phases, the part will not have uniform densities. Increase the injection fill speed to reduce material viscosity as it enters the cavity. Increase screw RPM to ensure a more homogeneous melt and reduce viscosity of flow. Also, increase the hold pressure. If there is a sharp variation in cross-sectional thickness, this could lead to pressure variations in the part, which will also affect the packing density.

Ejection Is Operating Improperly
If ejector pins are actuated too early, are sized too small or are located improperly, they may be deforming the part. First, ensure that the ejector pins are moving in a uniform manner across the part. Ejector pins should be located in thicker, stronger areas of the part. If the bearing area of the ejector pins is too small, they can damage the part. Increase the diameter and number of ejector pins to spread the ejection pressures more uniformly over the surface of the part. With delicate parts it may be necessary to use air instead of pins to eject the part.

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