Meaning
Variation in flow resistance under the influence of mechanical stress dictates how molten polymer behaves inside a runner or cavity. The term shear induced viscosity refers to the dynamic viscosity of a polymer melt as it is modified by the shear rate experienced during flow. For pseudoplastic polymers, this value decreases as the shear rate rises, allowing the material to flow more easily into thin-walled sections of the mould.
This behavior is fundamental to injection moulding, where high shear forces are used to reduce the required injection pressure.
Flow Dynamic
Velocity gradients across the flow channel create regions of varying shear rate within the melt stream. These gradients mean that the shear induced viscosity is lowest near the mould walls where shear is highest, and highest at the center of the flow. This non-uniformity must be accounted for in hot runner designs to avoid uneven filling.
Process Control
Controlling the speed of the injection ram allows processors to manage the flow resistance of the melt. By adjusting the injection velocity, the technician modifies the shear induced viscosity to achieve the desired fill rate. This adjustment helps in maintaining the process within the capability limits of the moulding machine.
Material Analysis
Determining the relationship between shear rate and viscosity requires specialized testing equipment. Rheological data sheets provide curves that illustrate the shear induced viscosity across a range of temperatures. Designers use these curves to perform mold flow simulations before finalizing the tool configuration.
