TROUBLESHOOTING

Sink marks are dimple indentations on the surface of a part. Part design, mold function, and processing parameters can all contribute to conditions that cause sink marks (also see Part Dimensions Too Small).

Poor Part Design
Thick wall sections relative to the rest of the part will cause a sink mark on the opposite side of the part. A wall thickness greater than 6.35 mm (0.250") will lead to sink marks. Bosses and ribs are especially susceptible to creating sink marks. Keeping wall thickness as constant as possible and coring thicker areas is recommended.

Mold Function Is Incorrect
First make sure that venting and gate size are not reducing the melt flow in the cavity. A gate that is too small will restrict injection speed and vents that are too small can lead to trapped air, slowing melt flow into the cavity. Reducing the length of sprue bushings and increasing the bushing diameters will also help maintain melt flow pressure and temperature entering the mold cavity. Also, ensure that mold temperatures are not too high. A mold that is too hot, especially with semi-crystalline materials like our TPVs, will cause more shrinkage. Increasing cooling time, especially for thicker parts, will reduce the shrinkage causing the sink mark.

Processing Parameters
Mold under-packing causes poor contact between the shot material and the mold walls, which reduces cooling efficiency. Under-packing of the mold cavity can lead to excessive shrinkage, shrink voids in thicker wall sections, sinks in large surface areas, and poor part appearance (see Start Up Procedures). If these suggestions do not work, it may be necessary to decrease the viscosity of the material entering the mold cavity. A lower viscosity, more homogeneous melt will improve packing out of the cavity. Our TPV materials depend mainly on shear to increase flow. However, increasing melt temperatures will also lower the viscosity of our materials to a lesser extent, and is an option.

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