Machinery's Handbook, 31st Edition
558 CELLULAR PLASTICS cell foams. In open cell foams, the cells are not completely closed but instead form contin- uous connecting tunnels with other cells in all directions throughout the object. Kitchen sponges are open cell foams. Densities of plastics foams range from 90 percent of the resin’s unfoamed density down to just a few percent. Foaming may be accomplished physically by injection of a gas or volatile solvent during extrusion or molding, or by mixing a chemical blowing agent with the resin. When the mix is processed, the unstable blowing agent decomposes in the melt, generating bubbles. Foams have many uses: as low-cost space filling, cushioning, ther - mal and electrical insulation, and as the interlayer member of sandwich materials. Poly- styrene, polyethylene, and polyurethane (rigid and flexible) are important foam families. Application and Physical Properties.— Selecting a plastic for a specific application re - quires knowledge of many different kinds of properties of candidate materials. Density and mechanical properties will be important considerations in most cases. The plastics must be able to bear tensile, compressive, or bending loads; resist or allow twisting or stretching; survive impacts; withstand thousands or even millions of repeated stress cycles; and resist abrasion. Special service environments—extremely low or high tem - peratures and humidities, outdoor exposure, and the presence of active solvents and chemicals—will modify the selection criteria. Thermal or electrical properties, such as coefficients of thermal expansion, thermal or electrical conductivity, heat capacity (spe - cific heat), dielectric strength, and dielectric-loss factor, may sometimes be important. Over many years, Committee D-20 of ASTM (formerly American Society for Testing Materials) has developed thousands of characterization and test methods for plastics and their products. ASTM Standards, Section 8, “Plastics,” is updated and reissued annually by ASTM. All leading US resin makers and many compounders can furnish the product designer with the results of batteries of tests made on their materials according to num bered ASTM standards. For example, ASTM Test D638, “Tensile Properties,” prescribes the preparation and dimensions of specimens to be tested by pulling from both ends. Several useful mechanical-property values are obtained from this test: tensile modulus, yield strength, tensile strength at break, and elongation (percent stretching) at break. Most plastics products are made by molding from a molten or pre-polymer condition by pro- cesses such as extrusion or injection- or compression-molding. Products so made are apt to have a somewhat different microstructure close to the surface than in their interiors. In theory, the behavior of a simple test bar in bending should be calculable from the straight tensile-test modulus E , as it is with metals. However, because of the “skin effects” found in molded plastics such predictions are not very reliable. For this reason, ASTM Committee D-20 developed Method D790 for flexural properties. Flexural moduli, theoretically the same as E , actually tend to be somewhat higher in molded thermoplastics, as are their flex ural strengths. While tensile and flexural specimens are typically 1 / 8 -inch (3.2 mm) thick, properties do vary with part thickness and processing conditions, so designs should be developed with appropriate safety factors and validation. Even with homogeneous plastics whose properties are expected to be about the same in all principal directions, certain processing conditions, such as stretching while the heat-softened plastic is cooling, can cause alignment orientation of the long polymer chains or crystallites in the stretching or flow direction. This will increase tensile strength in that direction but with some reduction in the perpendicular cross directions. Even more striking anisotropic effects are achieved with long-fiber reinforcements, in which the modulus and strength may be a factor of two or greater in the fiber direction than in the cross-fiber direction. Suppliers of unreinforced sheet, bar, and rod stock for machining can relieve stress by controlled annealing of any residual stresses developed in processing, thereby preventing distortions related to the relief of residual strains during machining. Machining or heat- ing of un-annealed stock can release residual stresses in the material, causing significant dimensional variations and warpage that can be difficult to resolve. Some of the more important application properties are defined here.
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