An Introduction of Injection Plastic Mold Defects

  • Most defects in Injection plastic mold are related to either the flow of the melted material or its non-uniform cooling rate during solidification.

    Here is a list of defects to keep in mind while designing a part for injection molding. In the next section, we'll see how you can avoid each of them by following good design practices.

    Warping
    When certain sections cool (and as a result shrink) faster than others, then the part can permanently bend due to internal stresses.

    Parts with non-constant wall thickness are most prone to warping.

    Sink marks
    When the interior of a part solidifies before its surface, a small recess in an otherwise flat surface may appear, called a sink mark.

    Parts with thick walls or poorly designed ribs are most prone to sinking.

    Drag marks
    As the plastic shrinks, it applies pressure on the mold. During ejection, the walls of the part will slide and scrape against the mold, which can result to drag marks.

    Parts with vertical walls (and no draft angle) are most prone to drag marks.

    Knit lines
    When 2 flows meet, small hair-like discolorations may develop. These knit lines affect the parts aesthetics, but also they generally decrease the strength of the part.

    Parts with abrupt geometry changes or holes are more prone to knit lines.

    Short shots
    Trapped air in the mold can inhibit the flow of the material during injection, resulting in an incomplete part. Good design can improve the flowability of the melted plastic.

    Parts with very thin walls or poorly designed ribs are more prone to short shots.

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