Machinery's Handbook, 31st Edition
578 ENVIRONMENTAL CONSIDERATIONS presence of high or low temperatures, chemical substances, energy sources, and radia- tion. Design also must consider that plastics components may be stressed by processing, assembly, finishing, and cleaning operations that affect their end-use properties. The stress level in the plastics product greatly affects performance. Generally, in - creased stress levels resulting from injection molding, forming, assembly work, and end-use forces reduce resistance to environmental factors. Although many plastics are hygroscopic and absorption of water results in dimensional and property changes, plastics are widely accepted because of their relative compatibility with the environment com - pared to metals. Some chemicals attack the polymer chain directly by reaction, result- ing in a progressive lowering of the molecular weight of the polymer and changes in the short-term mechanical properties; others dissolve the material, although high-molecu - lar-weight plastics dissolve very slowly. Swelling, changes in weight and dimensions, and loss of properties are evidence of solvation. Plasticization may result if the chemical is miscible with the polymer, resulting in loss of strength, stiffness, and creep resistance, and increased impact resistance. The mate- rial may swell and warp due to relaxation of molded-in stresses. Environmental stress cracking may cause catastrophic failure when plastics are stressed, even when the product appears to be unaffected by exposure to a chemical. Chemical compatibility data are obtained from standard test bars exposed to or placed in the chemical of study and tested as previously described for such properties as tensile strength, flexural modulus, dimensional change, weight, and discoloration. Chemical re - sistances from some commonly used thermoplastics materials are shown in Table 4, but these are only general guidelines and cannot substitute for values derived from tests on the end product. More extensive tests expose samples to a chemical in the presence of fixed stress or fixed strain distribution along its length, followed by examination for the stress or strain location at which damage begins. While the preceding tests may provide useful data about chemical compatibility, they do not generate reliable information on performance properties for design purposes. The only tests that provide such information are the creep modulus and rupture tests, conducted at ap - propriate temperatures in the environment that will be encountered by the product, preferably on prototype parts. Many plastics are slowly degraded to varying degrees by ultraviolet light (UV), which causes fading, chalking, and embrittlement. Acrylics and a few others are not damaged by UV. However, hundreds of additives are available that, when compounded into neat resins, will give compounds that are resistant to heat, burning, UV, staining, etc. Mechanical Properties.— Almost all end-use applications involve some degree of load ing, so mechanical properties are of prime importance in designing with plastics. Ma- terial selection is usually based on manufacturers’ marketing data sheets listing tensile strength, modulus of elasticity ( E ), elongation, impact strength, stress and strain behavior, and shear strength. Suppliers’ data often are generated under standard test conditions, so they may not be directly transferable to the components produced. Determination of the governing mechanical properties and their relation to end-use re - quirements is of vital importance in design. In practical applications materials are seldom subjected to steady deformation without the influence of other factors such as environment and temperature. A thorough understanding of mechanical properties and tests used to de- termine such properties and of the effects of adverse or beneficial conditions on mechanical properties over long time periods is extremely important. Some resin suppliers and contract manufacturers offer design and technical advice to customers who need design assistance. Design Analysis Structural Analysis.— Plastics parts may be subjected to stresses caused by assembly, handling, temperature variations, and other environmental effects. Simple analysis using information in Moment of Inertia on page 239 and Beam Calculations on page 256 can be used to make sure that newly designed parts can withstand these stresses. These methods may also be used for product improvement, cost reduction, and failure analysis of existing parts.
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