Scope:

ASTM D955 is a standard method to measure mold shrinkage that is the contraction in the molded article compared to the mold cavity used for compression or injection molding processes in specified conditions. It is generally used to adjust machine injection molds so that ultimate part dimensions are as expected. Mold shrinkage can be reliant upon the molding parameters that are used frequently. In this test, the extremes of the processing range and a midrange value of material are tested. Other international standard methods related to plastic shrinkage are ISO 294-4 (for thermoplastics) that measures the dimensions of plastic and polymeric articles by using a 3-D measuring device and ISO 2577 (for thermosets). 

Test Procedure:

For the Mold Shrinkage ASTM D955 test, the required length or diameter of the cavity is measured based upon the shape of the mold. The test samples are molded under stated parameters. The parts are cooled for a given period of time. The molded parts are measured and the shrinkage is determined after 48 hours. The molds are a (5 × 5 × 0.125 inches) for plaque (128 × 128 × 3.2 mm), a (4 × 0.125 inches) for a disk (114 × 3.2 mm). An end gated (5 × 0.5 × 0.125 inches), flame bar (128 × 12.7 × 3.2 mm) is measured and used for mold shrinkage calculations.

Specimen size:

The required samples are molded using sufficient resin.

Data:

Mold Shrinkage ASTM D955, ISO 294-4, and ISO 2577 are calculated as inches per inch (or percent or mm per mm) for the stated dimensions and conditions.

Conclusion:

ASTM D955 is a standard test method of measuring shrinkage from mold dimensions of thermoplastics.