Meaning
Spatial variations in the velocity gradient across a flow channel determine how much stress different layers of a material experience. In a pipe or a die, the shear rate distribution shows that the fluid moves fastest in the center and slowest at the walls. This variation affects the final properties of the product because the molecular orientation follows the gradient.
Profile Variance
The shape of the flow channel influences how the stress is spread throughout the material as it moves from the center to the walls. A narrow shear rate distribution is often preferred because it results in more uniform heating and molecular alignment across the entire stream. If the gradient is too steep, the outer layers of the material may degrade while the core remains un-sheared and cold, leading to structural failures in the final product.
Processing Window
Understanding these variations helps define the limits of the screw speed and the temperature settings. The shear rate distribution must be kept within a range where the viscosity is stable and the material does not burn. Engineers use this data to determine the maximum throughput that a specific die can handle.
Quality Variance
Differences in the stress history across the thickness of a part can lead to warping or internal stresses. When the shear rate distribution is poorly managed, the finished product may fail in the field due to weak points. Controlling this factor is necessary for achieving a production yield that meets the technical requirements of the application.
