Meaning
Boundary conditions where a fluid moves along a solid surface without adhering to it alter the standard flow profile in pipes and dies. In the processing of certain plastics and elastomers, wall slip occurs when the stress at the wall exceeds a critical value, causing the material to slide rather than stick. This change in behavior reduces the pressure needed to move the fluid through the system.
Velocity Jump
When the material begins to slide, the velocity at the boundary is no longer zero. This wall slip creates a flatter flow profile, which can reduce the amount of shear experienced by the material. While this might seem beneficial, it can also lead to instabilities in the output and fluctuations in the part weight.
Surface Defect
Discontinuous sliding between the melt and the metal tool often results in visual problems like sharkskin or matte finishes. If wall slip is present but not controlled, the finished product may fail to meet the aesthetic requirements of the customer. Engineers often add processing aids to the resin to promote a consistent slip layer.
Friction Reduction
Lowering the resistance at the boundary allows for higher throughput at lower pressures. However, the cost of calling a process ready based on wall slip is the risk that the slip behavior will change with a new batch of material or a slight shift in temperature. Demonstrated rates must be verified over long periods to ensure that the sliding effect remains stable during continuous production and does not lead to intermittent flow surges.