During the stretch stamping process, the punch presses the sheet metal blank into the die cavity to form a contoured part. A part is called stretch if its depth is at least half its diameter. Otherwise, it is simply called universal stamping.
Close-up of unwrinkled sheet metal
Draw stamping is a widely used process that produces a range of hardware parts, and the deep drawing process may have one or more drawing operations, depending on the complexity of the part.
Creping and stretching stamping process
One of the main defects that occurs in deep drawing operations is wrinkling of sheet metal material, usually in the walls or flanges of the part. During the stamping process, the blank flange is subjected to radial pulling stress and tangential compressive stress, and sometimes wrinkles are generated. Wrinkling can be prevented if the stretching system and stampings are designed properly.
There are several factors that cause wrinkles in deep-drawn parts, including:
- Blank support pressure
- Cavity depth and radius
- Friction between blank, blank holder, punch and cavity
- Gap between blank, blank holder, punch and cavity
- Blank shape and thickness
- Final Part Geometry
- Punch speed
Other factors, such as die temperature and the metal alloy of the blank, can also affect the drawing process. Variations in any of these factors can affect the likelihood of wrinkling or cracking in deep drawn parts.
As the name suggests, the blank holder holds the edge of the sheet metal blank in place on the top of the die, while the punch forces the sheet metal into the die cavity – the sheet metal deforms into the correct shape, rather than simply being pulled into the die cavity.
However, the blank holder will not hold the edge of the blank securely in place, and if this is the case, the wall of the cup may tear. The blank holder allows the blank to slide to some extent by providing friction between the blank holder and the blank itself. Blank support force can be applied hydraulically through pressure feedback by using air or nitrogen pads or CNC hydraulic buffers.
The greater the cavity depth, the more blank material must be pulled into the cavity, and the greater the risk of wrinkling in the part walls and flanges. The maximum cavity depth is a balance between wrinkling and fracture initiation, neither of which is desirable.
The radii of the punch and cavity edges control the flow of blank material into the cavity. Cup wall wrinkling may occur if the radius of the punch and cavity edge is too large. If the radius is too small, the blank is prone to tearing due to high stress.
The method to prevent the wrinkling of deep-drawn parts: use the blank support
The easiest way to eliminate wrinkling in deep-drawn parts is to use a blank holder. In most deep drawing processes, a constant blank holder pressure is applied throughout the drawing process.
However, there has been some success with the use of variable blank holder pressure, pneumatic or hydraulic blank holder pads that can vary the blank holder pressure linearly depending on the stroke of the machine. This provides some increase in the allowable cavity depth.
Numerically controlled (NC) die pads can be used to provide variable blank holder pressure during the drawing operation, with a larger initial force in a suitable blank holder pressure force profile to provide initial deformation.
Cushion pads come off to pull material into the cavity, then slowly increase pick-up to ensure strain hardening in stretched parts, NC die pads can significantly increase the allowable cavity depth while preventing wrinkling and cracking.
Methods to prevent wrinkling of deep-drawn parts: cavity design
The punch and cavity design can be optimized to reduce the probability of wrinkling. Choosing a flange radius just large enough to prevent cracking minimizes the possibility of wrinkles. Also, it can be helpful to consider minimizing part complexity and any asymmetries. Using a multi-step drawing process offers several advantages in preventing wrinkling in deep-drawn parts.
Designing the blank geometry to minimize excess material can reduce the possibility of wrinkling. Sheet metal blanks have an inherent grain structure, so stress can vary depending on the design of the mold and the orientation of the grains. The general stress of compounding and deep drawing processes to tune the grain in an asymmetric design to minimize grain stress is a consideration.
Other factors to consider
The surface conditions of each component can be customized to improve overall performance. Lubricants reduce friction between the blank and the punch and die cavity and can be liquid (wet) or thin film (dry). Usually, they are applied to the stock before drawing.
Today, dry films are gaining acceptance because they reduce the need for part cleaning after manufacture, and while lubricants can facilitate the flow of metal into the cavity, consider increasing the blank holding force to account for the reduced friction.
Previously, trial and error and operator experience optimized part and mold designs. Today, computer-aided design and finite element modeling are used to create part and die designs and to simulate deep-drawing processes, significantly reducing tooling and labor costs in the design process.