This research deals with profiling of blank
holder force in deep drawing process. Deep drawing is one of the conventional
sheet metal forming processes. In deep drawing, a blank placed over the die is
pressed by the means of a punch into the die. The blank deforms under the punch
load and assumes the shape of die. This process has numerous applications,
which range from beverage cans to car body panels. A blank holder is used to
restrict the blank from freely flowing into the die. The forced exerted by the
blank holder (blank holder force BHF) plays a particularly important role in
preventing the three major process defects namely tearing, wrinkling and
springback. It has been found that varying BHF during the punch stroke can
improve the part quality. However, researchers have used different BHF
profiles, and claimed success. There is considerable disagreement on the
appropriate shape of BHF profiles.
This research primarily focused on
determining the guiding principles for designing an appropriate BHF profile for
a given drawing process. The conventional deep drawing press available in the
laboratory was only capable of drawing at constant BHF. Therefore, this press
was modified and a controller was fitted to allow for BHF variations during
drawing. The experiments were performed using this modified press. During the
experiments a number of different BHF profile were tested and the effects were
compared.
The results of these experiments lead to a
concept of critical draw depth range. This idea suggests that regardless of the
initial level of BHF in a test. If the BHF is lowered sufficiently in critical
draw depth range, the chances of failure due to tearing will become minimal.
The critical draw depth range can be found from a given condition of maximum
BHF required for wrinkle suppression, and maximum constant BHF value that does
not cause tearing.