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| Industry Terms
Mechanical Tensile Strength A measurement of the force necessary to pull a material apart, and how much the material stretches before breaking. Tensile Modulus A measurement of the stiffness of a material. The higher the modulus, the stiffer the material; the lower the number, the more flexible the material. Falling Dart Test A test for toughness – the practical impact resistance – of plastics materials. The test gives breakage resistance data comparable to what the material might be subjected to in its actual end-use application. A specified weight is dropped onto an "impact tup" resting on the surface of the sample; the tup transfers the force of the impact to the sheet. The results are expressed as foot/pounds of energy, based on the length of drop and the amount of weight required to cause breakage. The higher the number, the tougher the material. Imapact Strength The correlation between impact test results and abuse resistance of a finished part is very difficult. The two most common methods of measuring toughness of plastic materials are izod impact and falling dart impact. Izod values are a measure of the notch sensitivity of various materials and should not exclusively be used to predict the toughness of a material. Some very tough sheet materials are notch sensitive. (ASTM D-256) Falling Dart impact is usually a better measure of toughness for sheet products than is izod. There are also variables in the falling dart test such as diameter of dart, etc. In general, Impact tests are a guide only to the toughness of the plastic sheet material. Rockwell Hardness Rockwell Hardness of Plastic sheet is obtained by placing a steel ball against a plastic sheet under pressure. But since elastic recovery is involved as well as hardness it is not valid to compare hardness of various kinds of plastic sheet entirely on the basis of this test. Rockwell Hardness is not an index of wear qualities or abrasion resistance. For instance, styrene sheet has high Rockwell Hardness, but poor scratch resistance. (ASTM D-785) Shore Hardness (Indentation Hardness) The hardness of a material as determined by the load necessary to produce penetration of the indenter to a predetermined depth. In this case, a Shore Durometer is used. (ASTM 2240) The Scale Shore "A" indicates the softer durometers, while the Shore D represents the more rigid durometers. Numerical hardness values represent either depth of penetration, or convenient arbitrary units derived from depth of penetration.  |
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