Meaning
Mechanical force applied to a polymer melt during processing where the rate of deformation varies over time or position. In an injection molding machine, dynamic shear occurs as the screw rotates and advances, forcing the viscous fluid through the narrow channels of the nozzle and gate. This force temporarily reduces the viscosity of the material, a phenomenon known as shear thinning.
Molecular Orientation
Chain entanglement density decreases under high flow rates as the molecules align in the direction of travel. This effect of dynamic shear is critical for filling thin-walled parts that would otherwise freeze before the cavity is full. Over-shearing can lead to molecular chain scission, which permanently reduces the physical properties of the molded part.
Processing Window
Heat generation increases as a result of internal friction during the high speed movement of the melt. Controlling dynamic shear is essential for thermally sensitive resins like unplasticized PVC or acetal. Processors adjust gate size and injection speed to manage these forces and avoid localized burning or jetting defects.
Viscosity Measurement
Capillary rheometers are used to simulate the conditions found in production environments. Data obtained through these tests shows how dynamic shear influences the flow behavior of a specific resin grade. Designers use these results to calculate the pressure drops across complex manifold systems.
