Minimize Sink Marks in Injection Molding

How to Minimize Sink Marks in Injection Molding

Sink marks are caused when a plastic part is not thoroughly packed out. This results from less plastic entering the mold than the volume of the mold was designed to hold. As the plastic cools, it shrinks. When insufficient plastic is injected into the mold, the thicker cross sections will sink as there is not enough plastic there to completely hold the desired structure. The three main reasons for this are poor gating location, poor processing conditions and insufficient gate or runner diameter.

Instructions

1.                            Poor Gating Location

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Thin sections of plastic will freeze off before thick sections. Determine if the part has uniform wall thickness.

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If the wall sections are uniform and the gate is relatively central to the part, move on to the next possibility.

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If the wall sections are not uniform, make sure that the part is gated into the thickest section. This allows the thicker sections to fill with material before the thin sections freeze off and starve them.

2.                            Poor Processing Conditions

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Running a mold too cold or molding a part with too short of a cycle time can cause a part to be insufficiently packed. First, weigh five parts produced at the current molding conditions and determine the average part weight.

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Increase the cycle time by 10% and produce five additional parts.

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Weigh the parts, and determine if there is a noticeable increase in the average part weight.

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If there is an increase in weight, return to step two and repeat the experiment. Continue repeating until there is no significant change in part weight.

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If after step two there is no change in part weight on the first attempt, repeat the series of experiments with a different process variable, such as mold temperature. Increase mold temperature by 10 degrees Fahrenheit and repeat the test.

3.                            Insufficient Gate or Runner Size

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Insufficient gate and runner diameters may allow the runner to freeze before sufficient plastic is injected into the mold. To check for this, first determine the designed volume of the part.

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Determine the specific gravity of the plastic resin that is being used. The material supplier can give that information if it is not readily known.

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Calculate the designed part weight. The part weight in pounds is found by multiplying the part volume in cubic inches x specific gravity x .0361. Weigh a set of five parts and determine the actual average weight of the part.

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If the part weight is below the calculated part weight, increase the gate or runner diameter by one standard size. Mold new parts and weigh.

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Repeat the process as needed. Once the actual part weight reaches the calculated weight, optimal gate and runner size have likely been

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