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Melt viscosity, the resistance of a polymer melt to flow, varies with processing conditions, particularly flow rate (i.e., shear rate), temperature, and pressure, and with the polymeric material, i.e., the structure, molecular weight, and additives. Viscosity increases for more rigid repeat units, with molecular weight, and when filler and fibers are incorporated into the polymer.
As shown in Figure 14, low shear rates do not affect melt viscosity. When the shear rate reaches a critical value, however, the polymer chains begin to align in the direction of flow and the melt viscosity begins to decrease with increasing shear rate. The linear decrease (on a log-log curve) in viscosity with increasing shear rate is known as the power law region and can be modeled using:
 (3)
where k is the consistency index and n is the power law index. Not all polymers have the same sensitivity to shear rate, and so the power law index varies from about 0.15 for thermoset rubbers to 0.30 for polystyrene and polyvinyl chloride to 1.0 for Newtonian fluids like water.
Melt viscosity decreases with increasing temperature. For narrow ranges of melt temperature, this decrease can modeled using an Arrhenius equation:
 (4)
where hR is a reference viscosity, Ea is the activation energy for flow, R is the universal gas constant, and T is the absolute temperature. Activation energies typically vary from about 7 kJ/mol for thermoplastic vulcanizates to 100 kJ/mol for materials like PMMA and polycarbonate. The WLF equation is employed when the temperature dependence of melt viscosity is modeled over larger temperature ranges. Higher pressures produce in an increase melt viscosity that can be expressed as:
 (5)
where a is an empirical constant.
Figure 14. Effect of shear rate on melt viscosity[44].
3.6 Degradation
Thermal stability plastic injection mold, the ability of a polymer to withstand processing and service (use) temperatures, is a function of the polymer structure and molecular weight. Aromatic rings in the polymer backbone tend to enhance stability whereas some functional groups like halides (e.g.., chloride in PVC) often leave the chain easily. High molecular weight causes more entanglement, which limits the mobility needed for degradation, and reduces the number of unstable end groups.
During processing, thermal, mechanical, chemical, and rfillers, plasticizers and softeners, lubricants and flow promoters, anti-aging additives, flame-retardants, colorants, blowing agents, cross-linking agents, and UV stabilizers. Fillers, such as glass fibers, carbon black, and talc,injection molding, enhance the mechanical properties. Plasticizers aid in the flowability of the polymer while the lubricants reduce the friction of the moldings. Flame retardants prevent or limit the burning of polymers. Blowing agents are used to produce gas-filled cells in the polymer matrix which enhance the properties and lower the weight of the polymer.
3.5. Melt Viscosity
Melt viscosity, the resistance of a polymer melt to flow, varies with processing conditions, particularly flow rate (i.e., shear rate), temperature, and pressure, and with the polymeric material, i.e., the structure, molecular weight, and additives. Viscosity increases for more rigid repeat units, with molecular weight, and when filler and fibers are incorporated into the polymer.
As shown in Figure 14, low shear rates do not affect melt viscosity. When the shear rate reaches a critical value, however, the polymer chains begin to align in the direction of flow and the melt viscosity begins to decrease with increasing shear rate. The linear decrease (on a log-log curve) in viscosity with increasing shear rate is known as the power law region and can be modeled using:
(3)
where k is the consistency index and n is the power law index. Not all polymers have the same sensitivity to shear rate, and so the power law index varies from about 0.15 for thermoset rubbers to 0.30 for polystyrene and polyvinyl chloride to 1.0 for Newtonian fluids like water.
Melt viscosity decreases with increasing temperature. For narrow ranges of melt temperature, this decrease can modeled using an Arrhenius equation:
(4)
where hR is a reference viscosity, Ea is the activation energy for flow, R is the universal gas constant, and T is the absolute temperature. Activation energies typically vary from about 7 kJ/mol for thermoplastic vulcanizates to 100 kJ/mol for materials like PMMA and polycarbonate. The WLF equation is employed when the temperature dependence of melt viscosity is modeled over larger temperature ranges. Higher pressures produce in an increase melt viscosity that can be expressed as:
(5)
where a is an empirical constant.
Figure 14. Effect of shear rate on melt viscosity[44].
3.6 Degradation
Thermal stability, the ability of a polymer to withstand processing and service (use) temperatures, is a function of the polymer structure and molecular weight. Aromatic rings in the polymer backbone tend to enhance stability whereas some functional groups like halides (e.g.., chloride in PVC) often leave the chain easily. High molecular weight causes more entanglement, which limits the mobility needed for degradation, and reduces the number of unstable end groups.
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