Sheet metal drawing is a more complex operation than cutting or bending operation, and more things can go wrong during sheet metal drawing. A number of defects can occur in a sheet metal drawn product. Following is a list of common defects during sheet metal operation, with sketches are shown in Figure 1.
Figure 1: Sheet Metal defects in drawn parts: (a) Wrinkling in the flange, (b) Wrinkling in the wall, (c) Tearing, (d) Earing, and (e) Surface scratches.
Common Sheet Metal defects in drawn parts:
Let us discuss these defects below in brief.
(a) Wrinkling in the flange:
Wrinkling in a drawn part consists of a series of ridges that form radially in the undrawn flange of the workpart due to compressive buckling.
(b) Wrinkling in the wall:
If and when the wrinkled flange is drawn into the cup, these ridges appear in the vertical wall.
(c) Tearing:
Tearing is an open crack in the vertical wall, usually near the base of the drawn cup, due to high tensile stresses that cause thinning and failure of the metal at this location. This type of failure can also occur as the metal is pulled over a sharp die corner.
(d) Earing:
Earing is the formation of irregularities in the upper edge of a deep drawn cup, caused by anisotropy in the sheet metal drawing. If the material is perfectly isotropic, earing doesn’t form.
(e) Surface scratches:
Surface scratches can occur on the drawn part if the punch and die are not smooth or if lubrication is not sufficient enough.
