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Processes introduction:
Deep Drawing
Deep drawing is a
compression-tension metal forming process in which a sheet metal blank is
radially drawn into a forming die by the mechanical action of a punch[1]. It
is thus a shape transformation process with material retention. The flange
region (sheet metal in the die shoulder area) experiences a radial drawing
stress and a tangential compressive stress due to the material retention
property. These compressive stresses (hoop stresses) result in flange
wrinkles (wrinkles of the first order). Wrinkles can be prevented by using a
blank holder, the function of which is to facilitate controlled material
flow into the die radius.
The total drawing load consists of the ideal forming load and an additional
component to compensate for friction in the contacting areas of the flange
region and bending forces at the die radius. The forming load is transferred
from the punch radius through the drawn part wall into the deformation
region (sheet metal flange). Due to tensile forces acting in the part wall,
wall thinning is prominent and results in an uneven part wall thickness. It
can be observed that the part wall thickness is lowest at the point where
the part wall loses contact with the punch, i.e. at the punch radius. The
thinnest part thickness determines the maximum stress that can be
transferred to the deformation zone. Due to material volume constancy, the
flange thickens and results in blank holder contact at the outer boundary
rather than on the entire surface. The maximum stress that can be safely
transferred from the punch to the blank sets a limit on the maximum blank
size (initial blank diameter in the case of rotationally symmetrical
blanks). An indicator of material formability is the limiting drawing ratio
(LDR), defined as the ratio of the maximum blank diameter that can be safely
drawn into a cup without flange to the punch diameter. Determination of the
LDR for complex components is difficult and hence the part is inspected for
critical areas for which an approximation is possible.
Commercial applications of this metal shaping process often involve complex
geometries with straight sides and radii. In such a case, the term stamping
is used in order to distinguish between the deep drawing (radial
tension-tangential compression) and stretch-and-bend (along the straight
sides) components.
Deep drawing has been classified into conventional and unconventional deep
drawing. The main aim of any unconventional deep drawing process is to
extend the formability limits of the process. Some of the unconventional
processes include hydromechanical deep drawing, Hydroform process, Aquadraw
process, Guerin process, Marform process and the hydraulic deep drawing
process to name a few.
The Marform process, for example, operates using the principle of rubber pad
forming techniques. Deep-recessed parts with either vertical or slopped
walls can be formed. In this type of forming, the die rig employs a rubber
pad as one tool half and a solid tool half, similar to the die in a
conventional die set, to form a component into its final shape. Dies are
made of cast light alloys and the rubber pad is 1.5-2 times thicker than the
component to be formed. For Marforming, single-action presses are equipped
with die cushions and blank holders. The blank is held against the rubber
pad by a blank holder, through which a punch is acting as in conventional
deep drawing. It is a double-acting apparatus: at first the ram slides down,
then the blank holder moves: this feature allows it to perform deep drawings
(30-40% transverse dimension) with no wrinkles.
Industrial uses of deep drawing processes include automotive body and
structural parts, aircraft components, utensils and white goods. Complex
parts are normally formed using progressive dies in a single forming press
or by using a press line.
Stamping parts