Meaning
A distinct family of thermoplastic polyolefin resins exhibits non-Newtonian pseudoplastic flow behavior, wherein melt viscosity decreases non-linearly as applied shear rate increases during melt processing. Utilizing shear thinning polyolefins allows molders to fill intricate, thin-walled part geometries rapidly under high injection speeds without exceeding hydraulic pressure limits of processing equipment. Polymer chain branching, molecular weight distribution, and polymer architecture govern the magnitude of viscosity reduction under shear.
Resins regain high melt viscosity once shear forces cease inside the mold cavity, preventing sag and drool during gate solidification.
Rheological Behavior
Entangled polymer chains align parallel to the flow direction under high shear rates within runner channels and narrow gate orifices. Molecular alignment reduces intermolecular friction, causing dramatic drops in apparent viscosity during high-speed injection. Evaluating shear thinning polyolefins using capillary rheometry generates viscosity curves essential for mold filling simulation software.
Wide molecular weight distribution grades display earlier onset of shear thinning compared to narrow molecular weight metallocene polyolefins.
Processing Advantage
High flowability under shear allows processors to reduce barrel temperatures, saving heating energy and shortening overall cooling cycle times. Rapid cavity filling minimizes thermal skin formation during injection, yielding uniform wall density and reducing required injection pressures. In shear thinning polyolefins, lower processing temperatures reduce risk of thermal degradation and color shifting in heat-sensitive compounds.
High melt strength at low shear prevents parison sag during blow molding operations.
Tooling Consideration
Gating design must generate sufficient shear to capitalize on the viscosity reduction characteristics of the resin without causing shear degradation. Small pin gates increase shear rates substantially, lowering injection resistance as polymer enters the cavity. Processing shear thinning polyolefins requires careful vent positioning, because high-speed melt fronts displace cavity air rapidly and risk burning part edges if venting is restricted.
