This property, possessed by all plastics to some degree, dictates that while plastics have solid-like characteristics such as elasticity, strength and form-stability, they also have liquid-like characteristics such as flow depending on time, temperature, rate and amount of loading
The dual nature of polymers, partly viscous fluid and partly elastic solid, is referred to as viscoelasticity In flowing polymers viscoelasticity is responsible for time-dependent properties, such as stress relaxation, normal stresses, very large elongational viscosities, and numerous unusual phenomena such as extrudate swell, entry flow vortices and some flow instabilities