In addition to the deformation zone, the punched hole will exhibit a cylindrical cutting zone and, toward the bottom, a tapered break-away zone. Hole width "w” is measured in the cutting zone. The holes in perforated sheet will generally be slightly tapered or conical in their cross-section.
The burr is formed during punching on the surface at which the ram exits the material. It is impossible to make any generalizations about the extent to which burring will be encountered as this will vary, depending on the material and the punch itself. Perforated plates will require post-punching treatment if they are to be absolutely free of burrs.
Beginning and end of the perforated areas
Standard staggered rows
Staggered rows with ugly start and finish (by two holes)
Staggered rows with ugly start and finish (by three holes)
The unperforated margin is the distance from the edge of the material to the start of the first series of holes. Measurement is to the edge of the hole, not to the center. It is possible to make up perforated sheets with margins which are either not perforated at all or are perforated only in part. There are two designations: the longitudinal margin: e1 + e2 (margin running along the longer edge) or the short edge margin: f1 + f2 (margin running along the shorter edge).
Free open area
The open area – also referred to as the open screening area – is to be calculated according to the various formulas for the different forms of perforation. The open area is defined as a percentage. This is of great significance when calculating the flow rate, the loadbearing capacity and the weight of the perforated plate.