Plastics Technology Laboratories  

Mold Shrinkage

ASTM D955, ISO 294, ISO 2577

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Mold shrinkage is the shrinkage of the polymer as it cools after the molding process. It is typically used to properly machine injection molds so that final part dimensions are as desired. Mold shrinkage can be dependant upon the molding parameters used - often the extremes of the processing range for the material plus a midrange value are tested.

Test Procedure:
The length or diameter of the cavity is measured depending on shape of mold. Test specimens are molded under specified conditions. The parts cool and after 48 hours, the molded parts are measured and the shrinkage is calculated. Intertek PTL molds for a 5" x 5" x 0.125" plaque (128 x 128 x 3.2mm), a 4" x 0.125" disk (114 x 3.2mm) and an end gated 5" x 0.5" x 0.125" flame bar (128 x 12.7 x 3.2mm) have been measured and can be used for mold shrinkage evaluations. The molding is done by an approved outside source.

Specimen size:
Enough resin to mold the required samples.

Mold shrinkage is calculated as inches per inch (or percent or mm per mm) for the specified dimensions and conditions.

Equipment Used:
Injection Molds
Digital micrometers
Digital calipers
Weighted digital drop gauge

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**Please note that this test description is intentionally generic in nature and aimed at providing a descriptive summary to enhance test understanding. For more information please contact a Intertek PTL Technical Representative at . Due to copyright restrictions, we are not able to provide copies of standards. Standards can be obtained from appropriate standards authorities.

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