Molding 101
Polymers
Polymer Viscosity
Properties of Plastics
Injection Molding
Screw and Check Ring
Commonly Used Calculations
Polymer Viscosity

a. What is viscoelasticty?

When liquids are subjected to a force, the deformation is permanent. This property is called viscosity. Viscosity also is an indicator of the resistance to flow. Liquids are said to be viscous. When solids, example: metals, are subjected to a certain limited force, the deformation is not permanent. When the force is released, the solid recovers to its original shape. This property is called elasticity. The word ‘limited’ is used because excessive force causes permanent deformation, another topic of discussion. Plastics exhibit both a viscous component and an elastic component. They are said to be viscoelastic and the behavior is called visoelasticity. This can be visualized with the spring and dash pot model.

 

b. What are Newtonian and Non-Newtonian fluids

 Newtonian Fluid - A fluid whose viscosity is not influenced by the shear rate that is being applied to it. The viscosity remains constant as the shear changes. Shear rate in injection molding is synonymous to injection speed. Non-Newtonian Fluid - A fluid whose viscosity changes with the change in the shear rate that is being applied to it. The viscosity does not remain constant as the shear changes.

c. How is the melt viscosity in polymers different in polymers?

All plastics are non-Newtonian. This means that their viscosity does not remain constant over a given range of shear rates. In the strict sense, the rheological behavior of a plastic is a combination of non – Newtonian and Newtonian behavior. At lower shear rates, the plastic is non-Newtonian but as the shear rate increases, the plastic tends to exhibit a Newtonian behavior. This happens because with increasing shear rate, the polymer molecules start to untangle from each other and start to align themselves in the direction of flow. Refer to the picture below.

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