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.
Dynamic Wall Slip
Meaning
Boundary layer interactions describe the loss of adhesion between a flowing material and the solid surfaces of its container. While most fluid models assume the velocity at the wall is zero, dynamic wall slip occurs when the shear stress exceeds a specific threshold. This behavior is common in highly filled compounds and certain fluoropolymers.
Friction Transition
Sliding begins when the polymer chains can no longer remain entangled with the surface irregularities of the die. In the presence of dynamic wall slip, the flow profile changes from a parabola to a more uniform plug flow. This shift reduces the total pressure drop required to move the material through the system.
Output Variance
Unstable sliding leads to periodic fluctuations in the mass flow rate and the appearance of the product. Because dynamic wall slip is often jerky rather than smooth, it can cause cycles of pressure buildup and release. These cycles result in visible bands or ripples on the surface of the extruded goods.
Processing Boundary
Operators often add process aids to induce a controlled layer of lubrication. Managing dynamic wall slip allows for higher line speeds without the onset of surface defects. Distinguishing between a stable slip and an uncontrolled flow instability is the primary goal of a production audit.
To achieve this, the team monitors the pressure signal for any oscillations that occur at constant screw speeds. If the slip remains erratic, the cost of calling for full production includes a high scrap rate and damaged tooling.