Friction in Forming and Drawing

Friction in metal stamping can have many beneficial as well as detrimental effects on the
tooling and quality of produced parts. It increases the surficial pressure between the tool
and sheet-metal material, which results in deformation of both, with subsequent degradation
of surface quality and wear of tooling. This increases the demand for press force, often
considerably escalating its levels.
Since the area of contact between the part and its tooling constantly changes, the distortion
and degradation of surface affects a widespread portions of both. The roughing
effect on the surface of tooling causes the actual contact areas to diminish in size and
become localized, which subsequently increases the frictional influences in each such segment,
and a faster deterioration of the tooling and parts follows.
The heat along with the damaging effect of surficial pressure, tears out small portions
of sheet-metal material, attaching it permanently to the tooling or elsewhere within the area
of contact. Such small pieces are as if welded; they are difficult to remove and their presence
further affects the quality of parts, their dimensional accuracy, and the condition of
tooling. For example, the force needed to overcome friction during the backward extrusion
of a cup was found to amount to approximately 40 percent of the total force exerted by the
punch.
The problem of friction is quite complex and cannot be readily solved. On the other hand,
some processes, such as metal forming depend on a certain amount of friction, the removal of
which may not be beneficial to the forming process at all. In the absence of this friction, grave
problems with material retention may emerge, which may result in parts that are perhaps
impossible to form at all. Additionally, such a condition may generate a completely different
set of forces acting against the tooling, which may produce such an inner strain within
its material structure that an internal distortion and collapse may become unavoidable.
The only means of controlling friction are lubricants. Lubricating materials are capable of
separating the adjoining surfaces by providing an isolated layer of completely different physical
and mechanical properties between them. With different types of lubricants, different results can be achieved and control of frictional forces may thus be brought to almost perfection.
There are lubricants that are immune to higher temperatures, lubricants that tolerate
extreme pressures, high-viscosity lubricants, low-viscosity lubricants, and other variations.