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Thread: Shrink Calculation on Plastic Part

  1. #1
    Associate Engineer
    Join Date
    Feb 2018

    Shrink Calculation on Plastic Part

    Hi all,

    I have a question regarding shrink rate of a plastic part. I have an over molded part that has a brass insert with ID of .700" & OD of .750". It is over molded with a Silicone thermoset material. The ID is .750" and OD .850". The brass insert is set on a pin and compression molded. There is shrinkage on the part that is collapsing the brass insert to below the .700" ID. The shrink for this part rate for this part is .002-.007 in/in. I am doing the following calculation to get theoretical shrink on this part.

    .750" - {(.850"-.750") * .0055 = .74945", so shrink of .00055

    or would it be the ID of .750 - (.750 * .0055") = .745875, so shrink of .004125"

    We are finding the part to shrink about .002" after it is removed from the locating pins on the mold. I want to verify these experimental values with theoretical values.

    Thanks in advance!
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  2. #2
    Project Engineer
    Join Date
    May 2015
    Is there sufficient pressure in the mold to resize the brass during the injection cycle?

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