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May. 11, 2008 | Useful Definitions for Resin Selection  When selecting a resin or a resin family in the early stages of part design, these basic definitions may be helpful in identifying specific properties important to the end-use application.Glass Transition Temperature: The reversible change in an amorphous polymer or in amorphous regions of a partially crystalline polymer from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. The glass transition generally occurs over a relatively narrow temperature region. Not only do hardness and brittleness undergo rapid changes in this temperature region, but also other properties, such as thermal expansion and specific heat, change rapidly. The glass transition temperature range is generally provided by material suppliers on their data sheets. Heat deflection temperature: The temperature at which a standard test specimen deflects 0.010 inch under a stated load of either 66 or 264 psi. This temperature is one indicator of how a resin might behave at elevated temperatures. Impact resistance: Relative susceptibility of plastics to fracture by shock?e.g., as indicated by the energy expended by a standard pendulum-type impact machine in breaking a standard specimen in one blow. The most common type of test is Izod impact. Izod impact: A test designed to determine the resistance of a plastic material to a shock loading. It involves the notching of a specimen, which is then placed in the jaws of a machine and struck with a weighted pendulum. Tensile strength: The pulling force per area required to yield or break a given specimen. The area used in computing strength is usually the original, rather than the necked-down area. Toughness: The resistance to fracture of a plastic when stressed. More definitions can be found at www.ides.com. |
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