Meaning
Physical phenomenon in polymer processing where the viscosity of a melt decreases toward the center of a flow channel due to the alignment of molecular chains.
Flow Behavior
Molten plastic moving through a runner or cavity experiences different shear forces depending on the distance from the frozen layer at the wall. While the highest shear occurs near the static boundary, core shear thinning defines the behavior of the hot, fast-moving material in the middle of the stream. Long polymer molecules disentangle and orient themselves parallel to the flow direction.
This reduction in internal resistance allows the material to travel further into thin-wall sections than a Newtonian fluid could achieve.
Moulding Impact
Tool designers account for this behavior when calculating the pressure drop across a part. Because core shear thinning reduces the force needed to move the melt, it allows for lower injection pressures. If a resin exhibits insufficient thinning, the center of the part may cool too quickly and cause sink marks.
Uniform filling depends on the predictability of this viscosity drop.
Material Specification
Resin manufacturers adjust molecular weight distribution to control how the melt responds to velocity. A material with high core shear thinning is often preferred for high-speed packaging applications. Testing verifies that the batch-to-batch consistency remains within the limits required for stable cycle times.
