Deep Drawing
Deep drawing is a process by which flat sheet metal is formed into a cylindrical or box shaped component by means of a punch and draw ring or die. These actions are usually performed in a double action press with a blank holder to prevent wrinkling in the flange as metal is pulled into the die.
Initially the workpiece is stretched over the punch nose as it moves into the die. This stretching continues until the force is sufficient to overcome the force exerted by the hold down ring which is controlled to allow the material to compression and flow into the die. The major mode of deformation changes from pure stretching to drawing at this stage, and is accompanied by an increase in material thickness. This thickening can be as high as 40% for austenitic stainless steel and 10-15% for ferritic stainless steel.
The mechanics of the drawing process are extremely complex and the drawing characteristics of stainless steel differ considerably to those of carbon steels. Some of the factors involved in the process are listed below:
1. Equipment used for deep drawing stainless steel should be up to twice as powerful as that used for carbon steel.
2. Double action hydraulic presses are preferred, single actions can be used if external "cushion" pressure can be applied. Mechanical presses can be used in they can be run at slower speeds than used for carbon steel.
3. Tool materials are similar to those used in press forming, viz. AISI D2, aluminium bronzes and carbide. Due to the large masses of such tools, extensive use is made of cast iron as a backing material, with the more expensive material used as inserts in the appropriate areas (especially in the draw ring). The choice of material for the hold down ring is not usually critical, and cast irons and steel are usually used.
4. Die clearances are similar to carbon steel for ferritic stainless steel (blank thickness plus 10-15%); for austenitic stainless steel clearances are much greater blank thickness plus 35-40%).
5. Draw ring radius should be 4 to 8 times material thickness. If it is too small it can cause fracture of the material and if too large increases tendency to wrinkling.
6. Punch radius should be 4 to 6 times material thickness, again if too small it leads to tearing and if to large it leads to wrinkling.
7. The limiting drawing ratio, i.e. the ratio of blank diameter to punch diameter is between 2.1 and 2.2 for stainless steels compared to 2.15 and 2.5 for carbon steels.
8. Lubricants are selected based on two factors, the most important being to provide a stable film between the workpiece and the tooling to prevent welding galling and seizing as well as reduce friction. Secondly, it must be readily and completely removable after the drawing operation.
9. Lubricants include chlorinated or sulphurised oils and waxes; pigmented pastes; heavy duty emulsions and drawing soaps. PVC and PE coatings are sometimes used, especially with ferritic grades, as they improve formability and protect the surface finish.
10. Stainless steel can be re-drawn after initial drawing. This process is used either to reduce the diameter and increase the height for the full length or for a specified length or to decrease the radii and/or produce flat bottomed components.
11. A spinning operation is sometimes carried out on a drawn component to reduce the "thickening" effects and increase the overall wall height.
12. As a general rule stainless steels cannot be re-drawn to the same extent as carbon steel without the inclusion of interstage annealing.
Columbus produce two materials, viz. 304 DDQ and 430 DDQ specifically for deep drawing. When these materials are ordered, it is advisable to state the exact end use so that the material most suited to the process can be supplies.
Cold Forming - Blanking and Piercing | Press Breaking and Roll Forming | Spinning and Flow Forming | Coining and Embossing | Press Forming | Deep Drawing
Cutting - Sawing | Hand Hacksawing | Shearing | Abrasive Cutting | Thermal Cutting
Hot Rolling - Hot Rolling Process | Hot Rolling Application | Types of Hot Rolling Mill | Hot Rolled Steel Tube | History
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