Lubrication limits in backward can extrusion at low reductions.

The tribological conditions in backward can extrusion at the contact between the can wall and the punch land are very severe due to heavy surface expansion. In the present paper these problems are analysed especially at low reduction (r < 50%) where they are particularly pronounced. The influence of degree of reduction, punch nose geometry, friction and strain hardening of the specimen material on the material flow is studied by experimental analyses in model material as well as steel. On the basis of these flow analyses the transition between sound flow and stiction of workpiece material to the punch has been estimated as function of the above mentioned parameters. The local surface expansion inside the can and the resulting lubricant film distribution has been measured. The results point out that the punch design normally recommended is suboptimal at large reductions and inappropriate at small reductions. A proposal for appropriate punch geometry depending on the degree of reduction is given.