Sheet Metal Forming Deep drawing

Outline Introduction Mechanics of Drawing Methodology Material and Equipment Defects in Drawing

Deep drawing In Deep Drawing, a flat sheet-metal blank is formed into a cylindrical or box-shaped part by means of a punch that presses the blank into the die cavity. Main products made by this process are: Beverage Cans, Pots and pans Kitchen sinks Automobile panels When the depth of the product is less than its diameter it is referred to as Shallow Drawing. When the Depth is more than that diameter then it is Deep Drawing.

Mechanics of Drawing As the punch proceeds towards its final position, the work piece experiences a complex sequence of stresses and strains as it is formed into its final shape. In step 1, the blank holder force, Fh, is applied and the punch begins to move towards the sheet material. In step 2, the sheet material is subjected to a bending operation. The sheet is bent over the corner of the punch and the corner of the die. In step 3, as the punch continues moving down, a straightening action occurs in the metal that was previously bent over the die radius. Metal from the flange is drawn into the die opening to form the cylinder wall.

Mechanics of Drawing In step 4, As the metal in the flange moves toward the center, it is subjected to the following state of stress: 1- Compression in the circumferential direction (the outer perimeter becomes smaller) 2- Tension in the radial direction 3- A relatively small compression in the thickness direction Since the volume of metal remains constant, and because the circumferential stress is relatively large, the sheet will thicken as it moves in the flange area. (this is why the clearance between the punch and die is higher than the sheet thickness by about 10%) In order for the material to be drawn: 1- Friction between the sheet material and surfaces of the blank holder and die must be overcome. 2- Deformation energy should be provided.

Mechanics of Drawing The downward motion of the punch results in a continuation of the metal flow caused by drawing and compression. Some thinning at the cylinder walls occurs as well. Step 5 shows the completed drawing process.

Deep Drawing Tooling Pneumatic cylinder (3) Guide (3) Blank holder Pins (2) Blank holder Die Punch

Defects in Drawing A number of defects in drawing can occur, which include: (a) Wrinkling in the flange occurs due to compressive buckling in the circumferential direction (blank holding force should be sufficient to prevent buckling from occurring). (b) Wrinkling in the wall takes place when a wrinkled flange is drawn into the cup or if the clearance is very large, resulting in a large suspended (unsupported) region. (c) Tearing occurs because of high tensile stresses that cause thinning and failure of the metal in the cup wall. Tearing can also occur in a drawing process if the die has a sharp corner radius. (d) Earring occurs when the material is anisotropic, i.e. has varying properties in different directions. (e) Surface scratches can be seen on the drawn part if the punch and die are not smooth or if the lubrication of the process is poor.

Defects in Drawing

Key factors in Deep Drawing Properties of the sheet metal The thickness of the sheet The draw ratio The clearance between the die and the punch The corner radii of the die and the punch The blank holder force Friction and lubrication The speed of the punch

Deep drawing of a Beverage Can

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