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Sink marks (injection moulding defect)
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A sink mark is a localized shrinkage of the material during Injection Moulding process. Sink marks appear as depressions on the surface of an injection molded part which is not aesthetically acceptable in Plastics Manufacturing. Effect * Visual defect * Sink marks reflect light in different directions to the rest of the part Causes * Thick sections in product design Areas of the part that are significantly thicker are prone to sink mark. * Improper design of the ribs, bosses, and gussets. These features are given for strengthening of the product. Thickness of the ribs, bosses, and gussets in a product design should be 50 to 60 percent of the attached (base) wall thickness. * Improper gate location Gate should be placed near to the thicker sections |
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