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Additives Additives are materials added to plastics with the intent on improving specific properties of the plastic. Examples include flame retardants, thermal stabilizers, and UV stabilizers.
BMC Bulk Molding Compound
Coefficient of Linear Thermal Expansion (CTE) As with other materials, the CTE is defined as the change in length per unit length per unit rise in temperature. Polymer material CTE's are in general much higher than those of metals. Therefore, when metal inserts are placed in a molded part, it is important to verify that with temperature excursions the maximum tensile strength of the plastic is not exceeded. Hooke's Law can be used here: s = E e = E (aplastic - ainsert) DT. CTE can be affected by fillers, and directional fillers can create anisotropy (directionality) in the CTE.
Creep Creep results from the fact that the long polymer chains tend to slide over each other so that there is a time-dependency to the stress-strain diagram. A load placed on a polymer material will result in an initial deformation, but with the load remaining over time, permanent deformation will occur. Creep data is difficult to find, but is often expressed in terms of a creep modulus.
Fillers Fillers are materials added to plastics whose intent is to modify the physical nature of the plastic without significantly modifying its mechanical properties. Filler examples include mica (phogopite and muscovite mica), glass spheres, silica, and calcium carbonate.
Glass Transition Temperature (TG) The TG can loosely be defined as a temperature point where a polymer experiences a significant change in properties. Typically, a large change in Young's Modulus is experienced. The TG is where a polymer structure turns "rubbery" upon heating and "glassy" upon cooling. Amorphous polymers are structural below TG . Amorphous materials go through one stage of the change from a glassy to a rubbery consistency with a simultaneous loss in stiffness (modulus of elasticity or Young's Modulus). This stage of going from stiff to flowing is over a wide temperature range. Crystalline, materials, on the other hand, go through a stage of becoming leathery before becoming rubbery. There is a loss of stiffness (modulus of elasticity or Young's Modulus) in both of these stages. However, crystalline materials have a sharp, defined melting point.
Heat Deflection Temperature The overall definition of heat deflection temperature is a temperature at which a polymer sample deflects a certain amount under heat and load. There is not good consistency between test methods, making data sheet reading difficult. The test can be conducted on three different thicknesses, two different loading conditions, and yet the specific test is rarely called out. What follows is one test definition: The temperature where the material shows a deformation of .01 inches under a constant load (66 or 264 psi). The heat deflection temperature is a function of the temperature, strain rate, and stress.
Mold Shrinkage Mold shrinkage refers to the amount of contraction that a part experiences after cooling to ambient after removal from a mold. Since material shrinkage begins the instant that resin is injected into the tool, good mold design involves judgment as to the best gate position, size, cycle time, etc. so that delaminations and voids can be prevented.
Reinforcement The addition of strong reinforcement fibers significantly increases the mechanical properties of thermoplastic and thermoset materials. Typical reinforcements include glass fibers, carbon fibers, and aramid fibers. Fiber morphologies include continuous strand, woven, roving, and/or chopped fibers; the nature of the fiber used depends on the application of the plastic. For example, chopped fiber is typically used in resins destined for injection or compression molding.
SMC Sheet Molding Compound
Thermal Conductivity Thermal conductivity represents the rate at which heat is transferred by conduction through a given unit area of a given material when the temperature difference or gradient is normal to the cross sectional area. The coefficient of thermal conductivity can be defined as the quantity of heat that travels through a unit volume of a polymer in a given time when the temperature gradient is one degree. Plastics typically have lower thermal conductivity coefficients compared to those of metals.
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