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How to Test Plastic Properties

Specific Gravity

Specific Gravity is a strong element in the price factor and thus has great importance. Beyond the price/volume relationship, however, specific gravity is used in production control, both in raw material production and molding and extrusion. Polyethylenes, for example, may have density variation, depending upon the degree of "packing" during molding, or the rate of quench during extrusion. Although specific gravity and density are frequently used interchangeably, there is a very slight difference in their meaning. Specific gravity is the ratio of the weight of a given volume of material at 73.4 °F (23 °C) to that of an equal volume of water at the same temperature. It is properly expressed as Specific Gravity, 23/23 °C. Density is the weight per unit volume of material at 23 °C and is expressed as D23C, g/cm3. The discrepancy enters from the fact that water at 23 °C has a density slightly less than one. ASTM D-792.