Both the shrink mark and shrinkage porosity are defects of injection products due to the molding shrinkage at thick walls where no sufficient follow-up compensation is made. During the injection process, the surface layer of products often condenses first, followed by the center layer, which will cause sinking on the surface of parts with thick walls, thereby producing obvious inner concave is called shrink mark and the large concave is called depression; conversely, when parts with thick walls have condensed and when the surface intensity and rigidity are large enough to resist the shrinkage stress resulted from the subsequent shrink of the center part, a large shrinkage porosity or a series of minute shrinkage holes will form inside thick walls. Usually when such shrinkage porosities or holes appear, depression will more or less occur at the surface of products.
Although shrink mark has no impact on the structural intensity of products, it will cause rather obvious visual defect on the surface whereof; depression influences both surface quality and structural property of products; shrinkage porosity can bring bad effect on the structural property of products. The three defects usually occur at parts with relatively thick walls or at hot spot of products such as the back of lug boss, stiffening rib as well as straight gate of spure.
During actual production, optimization of structure and material of product, the injection technics as well as mold structure are usually adopted to avoid shrinkage porosity and meanwhile to control the surface shrink mark of products at the minimum degree that can be accepted by users.
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