Fluoropolymer Processing Aid Dosing Economics under High Shear Profile Tooling
Optimize fluoropolymer dosing between 400 and 800 ppm to balance wall slip against startup induction scrap and avoid masterbatch working capital drag.
Wall Shear Rate
Meaning
Fluid mechanics parameters describe how fast the layers of a molten polymer move past each other at the point of contact with a solid surface. Because the material at the wall is ideally stationary while the center moves quickly, the wall shear rate represents the maximum intensity of deformation in the flow. It is measured in reciprocal seconds.
Velocity Gradient
The steepness of the change in speed from the center of the channel to the edge determines how much the polymer chains are stretched. A high wall shear rate indicates that the material is being subjected to intense forces that could lead to chain scission or degradation. This value is calculated based on the volumetric flow rate and the dimensions of the passage.
Stress Calculation
Multiplying the speed of deformation by the viscosity of the melt gives the total force acting on the wall. When the wall shear rate exceeds the ability of the polymer to stay together, surface defects such as melt fracture will occur. Engineers use this figure to predict the onset of processing instabilities during a production run.
Design Constraint
Sizing the openings in a die requires a balance between high output and the limits of the material. If the wall shear rate is too high for a given resin, the only solutions are to widen the gap or slow down the line. Comparing the lab results to the demonstrated rate in the plant ensures that the tooling is capable of meeting the production forecast.
This calculation prevents the error of designing a die that can only run at half its intended speed.