plastic molding term

plastic molding term

Heat-distortion temperature (°F) at fiber stress of 264 psi (ASTM D 648) is a statement of the temperature required to permit an arbitrary standard amount of deflection by a standard load, in a test in which the specimen is subjected to a gradually increasing temperature. It is thus an indication of the ability of a material to retain its stiffness, or resistance to deformation, against the weakening influence of high temperature. A similar test,under the lower load of 66 psi (ASTM D 648), in which, of course, a higher temperature is recorded, has similar significance for material subjected to lighter load. Values determined by this second test are shown, for some materials, in the 6th column of the table. A large difference between heat-distortion temperatures in these two tests gives warning of the need to pay particular attention to designing so as to minimize the stress in service of articles subjected to heat.

Impact strength, Izod {ft-lb per iti. of notch’) (ASTM D 256) is the energy required to break a notched test bar by means of a sharp blow, and thus is indication of the shock-resistance of a material. A brittle or notch-sensitive material has low impact strength.

Tensile strength {psi) (ASTM D 638), i.e., the force required to break by simple tension, is seldom of direct significance in the choice of a plastic for a specific application, because usually the design is governed by the need to avoid distortion, which occurs under much lower stresses, but it may serve as a preliminary indication of the general mechanical strength of a plastic, and as a guide in applications which are primarily structural.

Recommended service temperature (°F) for molded articles subject to no stress cannot be stated in terms of any single definitive test. The values given in this column are based merely on general experience, and have no consistent relationship with respective heat-distortion temperatures under high and low loads. All three figures should be taken into account, and also, of course, the fact that continuous contact with water or chemicals may lower very much the temperature which a molded article can tolerate without distortion.

Heat-distortion temperature (°F) at fiber stress of 66 ps’t (ASTM D 648) has been mentioned above, in connection with the companion test at 264 psi.

(Ma) Flexural strength (psi(ASTM D 790) is a direct measure of the load required to cause failure by bending. Since usually a molded article must be designed to hold its shape under flexure, rather than merely not to break, a test for flexural stiffness, or for modulus of elasticity, is more generally significant than this one, but flexural strength is included here because it forms part of certain specifications for plastics.

(Mb) Compressive strength (psi) (ASTM D 695), the measure of compressive load required to cause failure, is seldom of direct significance for thermoplastic materials, since an article must usually be designed to resist deformation, which of course occurs under loads much less than that which will cause failure — and particularly so if the load is a permanent one, and large enough to cause cold flow, even though no immediate permanent distortion. Compressive strength, then, can serve usually only as a base to which to apply a large factor of safety.

(Me) Modulus of elasticity in tension (psi) (ASTM D 638) is a measure of the force required to produce a given change in dimension. Thus a high modulus is associated with rigidity and ability to hold shape under momentary loading. Under prolonged loading, however, deformation may develop in a plastic through cold flow; and under repeated loading through fatigue. The moduli of the plastics are low in comparison with those of structural metals.

(Md) Rockwell hardness (ASTM D 785) expresses resistance to penetration by brief contact with a heavily loaded small spherical bail, and thus measures one type of hardness. It is not to be interpreted as a measure of saatch-resistance.

(Me) Specific gravity (ASTM D 792) is usually important only as affecting the number of articles obtainable per unit weight of material.

(Ea, Eb, Ec) Dissipation factor at 60, 10s and 106 cycles per second (ASTM D 150) is a measure of the percentage of AC electrical energy lost as heat within a material used as a dielectric. A low dissipation factor is desirable in electrical insulation because it minimizes not only the waste of energy but also the immediate and cumulative effects of heat upon the material. On the other hand, a material of very low dissipation factor cannot be preheated or heat-sealed by high-frequency methods of heating.

(Ed, Ee, Ef) Dielectric constant (specific inductive capacity) at 60, 10z and 106 cycles per second (ASTM D 150) of a material is the ratio of the capacitance of a capacitor in which the material is the dielectric to the capacitance of the same capacitor with vacuum as the dielectric; thus, dielectric constant is directly a measure of the value of the material as dielectric in a capacitor.

(Eg, Eh) Dielectric strength short-time and step-by-step {volts per mil) (ASTM D 149) are measures of the voltage required to cause puncture, and thus are indicative of the thickness of insulation required to withstand a given voltage. (Actually, the thickness is usually more likely to be governed by mechanical considerations than by electrical.)

(Ei) Volume resistivity (megohm-cm) (ASTM D 257) is a measure of internal electrical resistance, i.e., resistance to steady passage of current through the material (not resistance to puncture), and thus a measure of suitability to serve as an insulator.

(Ej) Insulation resistance (megohms) (ASTM D 257) is a more realistic measure of insulating value, since it is calculated from the current passing both through the material and along its surface, and is determined on specimens conditioned at 50 per cent relative humidity.

(Ek) Arc-resistance (seconds) (ASTM D 495) is a measure of suitability for use under arcing conditions.

(Oa) Index of refraction (ASTM D 542) influences the ”brilliance” of a transparent plastic, determines the limit of curvature in a light-piping system, and is a necessary factor in the calculation of the shape of a lens or reflector.

(Ob) Light-/ransviissio;; (per cent、(ASTM D 791) is important in applications in optics and illumination.

(Oc) Haze (per cent) (ASTM D 1003) is a factor in the appearance of transparent plastics, and important in applications in optics and illumination.

(Ta) Coefficient of linear thermal expansion per °C (ASTM D 696) represents the magnitude of change in dimensions with change of temperature, which may be important in a device graduated for accurate measurement, or in an assembly of plastic with metal or other material, where differential expansion must be considered. Also it is a factor in the shrinkage of molded articles from the dimensions of the mold, and hence in the design of molds to yield articles of desired dimensions.

(Tb) Rate of burning {in. per minute) (ASTM D 635)i.e., the rate of propagation of flame in a test piece, is less important than the separation, by this test, of materials which when ignited will continue to burn and those which will not support combustion — i.e., are self-extinguishing.

(Ca) Water-absorption (per cent) (ASTM D 570) is usually important only because of the effects of absorbed water on other properties, e.g., impairment of electrical properties; swelling and warpage; impairment of rigidity; enhancement of toughness and impact strength.

(Aa) Weight loss (per cent) is a measure of the tendency of a plastic to lose weight upon aging, through evaporation of plasticizers or softeners. It is determined by weighing a specimen, heating it at 180 2°F for 72 hr, reweighing, and calculating the percentage loss in weight.