A punch is used in the deep drawing process to force a sheet of metal into a die chamber, producing a shaped object. If the depth of a portion is at least half its diameter, the term "deep-drawn" is used to describe it. If not, it is simply referred to as generic stamping.A variety of domestic objects, including soup cans, battery casings, fire extinguishers, and even the kitchen sink, are made using the widely used deep draw stamping technique. Depending on the intricacy of the part, a deep draw procedure may involve one or more drawing operations.
Wrinkling of the sheet metal material, typically in the wall or flange of the part, is one of the main flaws that develops during deep drawing procedures. During the stamping operation, the flange of the blank experiences tangential compressive stress and radial drawing stress, which can occasionally cause wrinkles. If the deep drawing method and stamped part are designed properly, wrinkles can be avoided.
Deeply drawn sections may develop wrinkles due to a number of circumstances, such as:
1.Pressure on blank holders
2.Die cavity height and diameter
3.Friction between the punch and die cavities, the blank, and the blank holder.
4. Clearances between the die cavity, punch, blank, and blank holder
5.Flat dimensions and thickness
6.Final portion sizing
The temperature of the die and the metal alloy of the blank are two additional variables that may impact the drawing. Any one of these variables can change, which affects how likely the deep-drawn section is to wrinkle or crack.
The blank holder, as its name suggests, secures the sheet metal blank's edges to the top of the die while the punch drives the material into the cavity of the die; this prevents the material from simply being dragged into the cavity of the die but instead causes it to deform into the desired shape.
The borders of the blank are not, however, held rigidly in place by the blank holder. If this were the case, the wall of the cup might tear. By creating a force of friction between the blank holder and the blank, the blank holder permits the blank to slide considerably. By using an air or nitrogen cushion, a numerically controlled hydraulic cushion, or a hydraulic cushion with pressure feedback, blank holder force can be provided.
The risk of wrinkles in the part's walls and flange increasing with die cavity depth is due to the need to pull more blank material into the cavity. The maximum die cavity depth strikes a balance between undesirable wrinkle formation and fracture initiation.
The flow of blank material into the die cavity is controlled by the radii of the punch and die cavity edges. If the radii of the punch and die cavity edges are excessively large, cup wall wrinkles may develop. The blank is vulnerable to tearing if the radii are too tiny due to the high pressures.
Using a blank holder is the simplest way to stop wrinkling in deep-drawn portions. The majority of deep drawing procedures include maintaining consistent blank holder pressure throughout the entire drawing process.However, variable blank holder pressure has been used with some degree of effectiveness. The blank holder pressure can be changed linearly over the course of the machine's stroke using a pneumatic or hydraulic blank holder cushion. The permitted die cavity depth is subsequently increased in a little way.
For variable blank holder pressure during the drawing process, a numerically controlled (NC) die cushion can be employed. The initial force is high in an ideal blank holder pressure force profile to produce initial deformations.As material is dragged into the die cavity, the cushion gradually rises again to ensure that the drawn part undergoes strain hardening. While minimizing both wrinkling and cracking, an NC die cushion can significantly increase the allowed die cavity depth.
It is possible to lessen the likelihood of wrinkles by optimizing the design of the punch and die cavity. The likelihood of wrinkles can be reduced by selecting a flange radius that is just big enough to prevent cracking. The part's complexity and any asymmetries should also be kept to a minimum. When it comes to minimizing wrinkling in deep-drawn areas, incorporating a multi-step drawing method has many benefits.
Wrinkling can be prevented by designing the blank geometry to use as little extra material as possible. Due to the inherent grain structure of the sheet metal blank, the stresses might change depending on the die design and grain orientation. One issue to consider is adjusting the grain in an asymmetrical design to reduce the combined grain stresses and the overall stresses of the deep draw operation.
Each component's surface conditions can be customized to enhance overall performance. Lubricants, which can be liquid (wet) or film (dry), minimize friction between the blank and the punch and die cavity. They are typically used on the blank before drawing.
Dry films are becoming more popular today since they eliminate the requirement for part washing following manufacturing. Lubricants can help the metal flow into the die cavity, but you should also think about boosting the blank holding force to make up for the decreased friction.
In the past, operator expertise and trial-and-error improved component and die design. To produce component and die designs and imitate the deep drawing process, computer-aided design and finite element modeling are now used, drastically lowering the costs of tooling and manpower in the design process.