Surface friction effects related to pressforming of continuous fibre thermoplastic composites

In the pressforming of thermoplastic composite sheet, the heated laminate is rapidly formed into a mould. The moulding force is transmitted using either matched metal dies or by a rubber pad/metal mould combination. Friction must occur between the metal or rubber mould surface and the heated composite as the laminate moves across the tool surface until it is fully formed. This paper describes work carried out to characterize and measure these frictional forces. Composites such as unidirectional carbon fibre-reinforced poly(ether ether ketone) and glass fibre fabric-reinforced PA-12 have been tested, with rubber and tool steel as the mould materials. Two methods of testing were used, one comprising two fixed heated platens, between which the surfaces to be tested were placed while the composite was sheared from between the surfaces. Pulling-out force was achieved using a variable velocity shearing rig and by using dead-weight loading. A heated friction sled was also built which allowed various samples of metal and rubber material to be dragged across a heated composite sheet. The effects of varying surface temperature, normal pressure, surface fibre orientation and mould release agent were investigated. An adhesive bond was found to occur if the surfaces were left in contact during heating. By varying the shearing velocity, the friction between the composite and tool surface was found to be hydrodynamic in nature, i.e. velocity-dependent, at forming temperature.