3 Fundamentals of press design

Deep drawing and stretch drawing
– plastic deformation at the peaks of the roughness of contacting surfaces,
– surface expansion brought about by the forming process, which
removes the adhesion presenting boundary layers and brings bright
metal surfaces in contact with one another,
– similar material structures of the frictional partners.
The greater is the difference between the metals and alloys of the frictional
partners, the lower is the tendency to develop cold welding. This
tendency is at its lowest between metallic and non-metallic materials of
corresponding hardness.
Abrasion
Abrasive wear includes all parting or separation processes that take place
at the interface of frictional partners, where material is released in submicroscopic
particles through a material removal process. The difference
in the hardness between the two surfaces of frictional partners is an
important factor for assessing abrasive friction. The wear resistance of a
die increases with the hardness of its surface. Due to the relative movement
of the two surfaces of the frictional partners, a part of the energy
used is converted into heat. With most materials, the hardness drops as
the temperature increases and the resistance to wear is reduced. Abrasive
wear represents a generally inevitable long-term wear effect on the die.
Surface fatigue
Fatigue wear is considered to be the separation of micro- and macroscopic
material particles. This separation is caused through fatigue
cracking, progressive cracking and residual fracture due to mechanical,
thermal or chemical stress-related conditions on the loaded surfaces. As
a rule, surface fatigue plays a minor role in sheet forming dies. However,
fatigue is common in cutting tools, as an increasing normal tension
stress is applied onto the front surface in addition to a variable frictional
shear stress that exists over the whole external surface area.
Lubrication
Lubricants are subject to different temperatures and compressive stresses
during their use. Important variables influencing the correct choice
of suitable lubricants are viscosity, density and compressibility, where-
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