The process of punching a soft material uses a hallow punch with beveled and sharpened edges that is driven into the material. The backing material is usually a soft metal or wood.
I am working on such a process and have a few questions for the community. I am punching 3 mm discs of thickness 0.2 mm from a elastomer material with tensile modulus of 220 MPa and a max tensile strength of 35 MPa. The disc remains within the hallow punch and is extracted afterwards.
I have measured the force required to punch using a force gauge and found that 140 N was required to punch and retain the disk.
The end process will use a pneumatic cylinder to drive the punch.
1) Is the punch force calculation the following - "Punch Force = perimeter x thickness x shear stress"? Can the shear stress be equated to the materials tensile strength? Using the above, I get a punch force of 66 N. This is well below what is measured using the gauge. Why?
2) What is the effect of a blunt or dull blade on the punch force? Hows does the blades increase in contact area influence the punch force (shear)?
3) In the above example, what is the relationship between the normal force/pressure to the punch force?
4) How would you select a backing material? Softer than the punch's material (HSS Steel), but not too soft as to deform when struck by the punch?