Transient isoviscous hydrodynamic theory is used to investigate the lubrication of plane strain forging processes with flat dies. It is shown that during yielding a film which decreases from a maximum thickness at the center of the contact to zero at the edges is formed. As the deformation continues, the film is transported outwards at half the surface speed of the workpiece. In the case where the dies extend beyond the workpiece, an unlubricated region is formed. The variation in width of this region with reduction predicted by the model is in good agreement with experimental measurements.

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