Meaning
Shear thinning behavior in molten polymers causes a decrease in viscosity as the rate of shear deformation increases. This characteristic, known as non-newtonian pseudoplastic flow, enables the plastic melt to flow easily through thin gates and complex cavity geometries during injection. This behavior distinguishes polymers from simple fluids like water or oil.
Shear Behaviour
During the injection phase, high pressure forces the polymer through narrow channels at high speed, generating high shear rates. This action straightens the tangled polymer chains, which is the physical mechanism behind non-newtonian pseudoplastic flow in polymer melts. As the chains align, the internal friction drops and the material flows with much less resistance.
Processing Effect
Melt temperature and injection speed must be balanced to take advantage of this behavior without degrading the material. Since non-newtonian pseudoplastic flow reduces the required injection pressure at higher speeds, moulders can fill thin walled sections quickly. However, excessively high shear rates can cause localized shear heating, which can burn the polymer or cause material degradation.
Tooling Consideration
Runner and gate dimensions must be calculated to achieve the optimal shear rate for the chosen resin during filling. If the gates are too large, the shear rate will be too low, and the melt will remain highly viscous, leading to fill problems. Conversely, too small a gate will cause excessive shear and pressure drop.
Therefore, understanding this flow behavior is essential for designing efficient feed systems that minimize material stress and energy consumption.
