Wall Slip

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.