Machinery's Handbook, 31st Edition
Plastics Applications and Properties 559 Density and Specific Gravity: These properties are closely linked. Density is the mass (or weight) per unit volume of a material, used mainly to calculate the mass of material required to make a part whose volume has been calculated from its dimensions. In the plastics industry, commonly used units are lb/in 3 or g/cm 3 . Specific gravity is the ratio of a material’s density to that of water, both measured at the same temperature, at 23 ° C (73.4°F) for plastics. Since the density of water at typical room temperatures is very close to 1.00 gm/cm 3 , specific gravity and density measured in metric units are numerically almost equal. In US customary units, water density equals 0.0361 lb/in 3 . Specific gravities of neat resins range from 0.83 for polymethylpentene to 2.20 for polytetrafluoroethylene (PTFE, Teflon). Because specific gravity is a ratio of densities of like units, it is dimen - sionless. See also Specific Gravity on page 377. Unlike specific gravity, specific volume is not dimensionless, but is defined as the recip rocal of density, with units of in 3 /lb or cm 3 /g. Shrinkage: This is the difference in measurement between a linear dimension of a mold and the corresponding dimension of a plastic article made in the mold, divided by the mold dimension, and expressed in/in, mils/in, or percent. The dominant material property here is the coefficient of linear thermal expansion since the shrinkage is mainly due to cooling from the high temperature of the melt as it solidifies under pressure in the mold to room temperature after its removal from the mold. Expansion coefficients of plastics are much larger, 20 to over 100 times those of metals and ceramics. For type-66 nylon, which freezes at 260 ° C (500°F), shrinkage may be as much as 20 mils/in (0.02 cm/cm), i.e., 2 percent. Shrinkage can also be affected by other molding conditions and by additive content. Amorphous and liquid-crystalline thermoplastics have lower shrinkages than crystalline materials. Glass- or carbon-reinforced and mineral-filled compounds have lower shrinkage than their neat resins. For accurate assessment of shrinkage, test mold- ings at the target part thickness are advised before mold-cavity dimensions are finalized. Water Absorption: This is the increase in weight of a test specimen due to absorption of water, expressed as a percentage of the original dry weight. Standard test specimens (ASTM Method D570) are first dried for 24 hours, then weighed before and after immer - sion in water at 23 ° C (73.4°F) for a specified time. Absorption of water affects not only the dimensions of plastics parts but also mechanical and electrical properties. Parts made from materials with low water absorption tend to have greater dimensional stability. Transparency: This is a measure of the percentage of incident light that is transmitted through a test specimen. It is quantified as luminous transmittance, the ratio of transmit- ted to incident light. Haze measures the percentage of light entering a specimen that is scattered more than 2.5 degrees from the incident beam. Elasticity: Most solid materials are initially elastic, such that when small stresses causing deformation are removed, the material will recover its original shape and dimensions. A more precisely formulated definition of “elastic” is “a material that obeys Hooke’s law of proportionality of strain to stress up to its proportional limit” (see page 565). The mod- ulus of elasticity is Hooke’s law constant E , the quotient of stress divided by strain in the proportional region. In a tensile test, strain is the ratio of observed displacement, relative to the original length, reached under a given stress. Because plastics generally deviate from strict linearity, the moduli reported are usually “one percent moduli,” that is, the quotient of stress and strain at 1 percent strain. For most materials the limit of recoverable strain is a few percent or less. For rubbers and a handful of thermoplastic elastomers, maximum recoverable strain may be as high as 100 percent or more. Stress and strain in relation to plastics part design is discussed on page 563. Plasticity: This refers to the deformation exhibited when materials are stressed be- yond their elastic limits without breaking (i.e., when stress exceeds the yield point ). In this mode, there can be considerable flow within the microstructure of the material, and permanent changes in the dimensions and shape of the deformed article occur. Plastic deformation continues with small increases in stress until finally there is rupture. Plas - ticity is exhibited to some degree by most unreinforced plastics. While parts are normally
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