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The confidence level in successfully forming a sheet metal stamping increases as the simplicity of the part’s topography increases. The goal of forming with stamping technologies is to produce stampings with complex geometric surfaces that are dimensionally accurate and repeatable with a certain strain distribution, yet free from wrinkles and splits. Stampings have one or more forming modes that create the desired geometries. These modes are bending, stretch forming and drawing. Stretching the sheet metal forms depressions or embossments. Drawing compresses material circumferentially to create stampings such as beer cans.As the surfaces of the stamping become more complex, more than one mode of forming will be required. In fact, many stampings have bend, stretch and draw features produced in the form die. The common types of dies that shape material are solid form, stretch form and draw.
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Solid Form7096
The most basic type of die used to shape material is the solid form die. This tool simply displaces material via a solid punch “crashing” the material into a solid die steel on the press downstroke. The result is a stamping with uncontrolled material flow in terms of strain distribution. Since “loose metal” is present on the stamping, caused by uncontrolled material flow, the part tends to be dimensionally and structurally unstable.
Stretch Form
Forming operations that provide for material flow control do so with a blank holder. The blank holder is a pressurized device that is guided and retained within the die set. Stampings formed with a blank holder may be described as having three parts, shown in Fig. 1. They are the product surface(shown in red), blank holder surface (flat area shown in blue) and a wall that bridges the two together. The theoretical corner on the wall at the punch is called the punch break. The punch opening is the theoretical intersection at the bottom of the draw wall with the blank holder. The male punch is housed inside the punch opening, whereas the blank holder is located around the punch outside the punch opening. These tools have a one-piece upper member that contacts both the blank holder and punch surfaces. A blank or strip of material is fed onto the blank holder and into location gauges. On the press downstroke, the upper die member contacts the sheet and forms a lock step or bead around the outside perimeter of the punch opening on the blank holder surface to prevent material flow off the blank holder into the punch. The blank holder then begins to collapse and material stretches and compresses until it takes the shape of the lower punch. The die actions reverse on the press upstroke, and the formed stamping is removed from the die.
Draw
The draw die has earned its name not from the mode of deformation, but from the fact that the material runs in or draws off the blank holder surface and into the punch. Although the draw mode of deformation is present in draw dies, some degree of the stretch forming and bending modes generally also are present. The architecture and operational sequence for draw dies is the same as stretch-form dies with one exception. Material flow off the blank holder in draw dies needs to be restrained more in some areas than others to prevent wrinkling. This is achieved by forming halfmoon-shaped beads instead of lock steps or beads found in stretch-form dies. The first stage of drawing sheet metal, after the blank or strip stock has been loaded into the die, is initial contact of the die steel with the blank and blank holder. The blank, round for cylindrical shells to allow for a circumferential reduction in diameter, is firmly gripped all around its perimeter prior to any material flow. As the press ram continues downward the sheet metal bends over the die radius and around the punch radius. The sheet metal begins to conform to the geometry of the punch.Very little movement or compression at the blank edge has occurred to this point in the drawing operation. Air trapped in the pockets on the die steel is released on the press downstroke through air vents.