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This standard covers the measurement of specimen shrinkage for injection and compression molding. Data for mold shrinkage should be used for material comparison. Actual mold shrinkage values are highly dependant on part geometry, mold configuration, and processing conditions.

Scope of test
1.1 This test method is intended to measure shrinkage from mold cavity to molded dimensions of thermoplastics when molded by compression or injection processes with specified process conditions.

1.2 This test method covers initial shrinkage measurements. The method also accommodates shrinkage at 24 and 48 h, which may be critical for some materials.

1.3 This method will give comparable data based on standard specimens and can not predict absolute values in actual molded parts with varying flow paths, wall thicknesses, pressure gradients and process conditions. Differences in mold shrinkage may also be observed between the three specimen geometries described in this test method.

1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are given for information only.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

NOTE 1--This test method is technically identical to ISO 294-4 where Type D2 specimens are used except that pressure transducers are an option in this test method and required in ISO 294-4.

A test often reported in product literature is Shrinkage From the Mold (ASTM D955). This relatively simple test is performed on specimens molded in a master unit die or MUD mold. The injection molded specimen, either the bars or discs, are measured. The difference between the molded part and the mold cavity, parallel to flow, is reported as the shrinkage from the mold. The specimens are required to be conditioned by cooling at room temperature for two hours prior to measurement. The test measures batch to batch uniformity in shrinkage as molded.(http://online.sfsu.edu/)

Mold shrinkage for many materials differs for flow and transverse (or across flow) directions. Flow direction is taken as the direction the molten material is traveling when it exits the gate and enters the mold.

Measurement Orientation

Three specimen types may be used to determine mold shrinkage, Type A, Type B, and Type D2, see more at http://www.ides.com/property_descriptions/ASTMD955.asp

old shrinkage in the flow direction is calculated by:
SFlow = 100 * (LM - LS) / LM
where LM is the length of the test section of the mold cavity and LS is the corresponding length of the test specimen after it has cooled.

Mold shrinkage in the transverse direction is calculated by:
STransverse = 100 * (WM - WS) / WM M is the width of the test section of the mold cavity and WS is the corresponding width of the test specimen after it has cooled.
Similar Standards for thermoplastic measurement are ISO 294-4

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