In the deep drawing process, a punch pushes a sheet metal blank into a die cavity, resulting in a contoured part. A part is said to be deep-drawn if the depth of the part is at least half of its diameter. Otherwise, it is simply called general stamping.
Deep draw stamping is a widely used process that produces a range of household items, such as soup cans, battery casings, fire extinguishers, and even the kitchen sink. A deep draw process may have one or more drawing operations, depending on the complexity of the part.
Wrinkling and Deep Drawing Operations
One of the primary defects that occurs in deep drawing operations is the wrinkling of sheet metal material, generally in the wall or flange of the part. The flange of the blank undergoes radial drawing stress and tangential compressive stress during the stamping process, which sometimes results in wrinkles. Wrinkling is preventable if the deep drawing system and stamped part are designed properly.
Causes Of Wrinkling In Deep Drawn Parts
Several factors can cause wrinkles in deep drawn parts, including:
• Blank holder pressure
• Die cavity depth and radius
• Friction between the blank, blank holder, punch and die cavity
• Clearances between the blank, blank holder, punch and die 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. A variation in any of these factors influences the potential for wrinkling or cracking in the deep-drawn part.
The blank holder, as the name implies, holds the edges of the sheet metal blank in place against the top of the die while the punch forces the sheet metal into the die cavity—the sheet metal deforms into the proper shape, instead of simply being pulled into the die cavity.
The blank holder, however, does not hold the edges of the blank rigidly in place. If this were the case, tearing could occur in the cup wall. The blank holder allows the blank to slide somewhat by providing frictional force between the blank holder and the blank itself. Blank holder force can be applied hydraulically with pressure feedback, by using an air or nitrogen cushion, or a numerically controlled hydraulic cushion.
The greater the die cavity depth, the more blank material has to be pulled down into the die cavity and the greater the risk of wrinkling in the walls and flange of the part. The maximum die cavity depth is a balance between the onset of wrinkling and the onset of fracture, neither of which is desirable.
The radii degrees of the punch and die cavity edges control the flow of blank material into the die cavity. Wrinkling in the cup wall can occur if the radii of the punch and die cavity edges are too large. If the radii are too small, the blank is prone to tearing because of the high stresses.
Methods for Preventing Wrinkling in Deep Drawn Parts: Using a Blank Holder
The simplest method for eliminating wrinkling in deep-drawn parts is using a blank holder. In most deep drawing processes, a constant blank holder pressure is applied throughout the entire drawing action.
Variable blank holder pressure, however, has been employed with some success. A pneumatic or hydraulic blank holder cushion can vary the blank holder pressure linearly over the stroke of the machine. This provides some increase in the allowable die cavity depth.
A numerically controlled (NC) die cushion can be used to provide a variable blank holder pressure over the course of drawing action. In an optimal blank holder pressure force profile, the initial force is large so as to provide initial deformations.
The cushion drops off to pull material into the die cavity, and then slowly increases back up to ensure strain hardening in the drawn part. An NC die cushion can dramatically increase the allowable die cavity depth while preventing both wrinkling and cracking.
Methods for Preventing Wrinkling in Deep Drawn Parts: Die Cavity Design
The design of the punch and die cavity can be optimized to reduce the probability of wrinkling. Choosing a flange radius that is just large enough to prevent cracking can minimize the potential for wrinkles. Additionally, considering minimizing the part complexity and any asymmetry can also help. Incorporating a multi-step drawing process offers a variety of advantages in preventing wrinkling in deep-drawn parts.
Designing the blank geometry to minimize excess material can reduce the potential for wrinkling. The sheet metal blank has an inherent grain structure, so the stresses can vary depending on the design of the die and the orientation of the grain. Adjusting the grain in an asymmetrical design to minimize the compound of grain stresses and the general stresses of the deep draw process is something to take into consideration.
Other Factors To Consider
Surface conditions of each component can be tailored to improve overall performance. Lubricants reduce the friction between the blank and the punch and die cavity and can be liquid (wet) or films (dry). Generally, they are applied to the blank before drawing.
Today, dry films are gaining acceptance because they reduce the need for part washing after fabrication. While lubricants can facilitate the metal flow into the die cavity, consider increasing the blank holding force to account for the reduced friction.
In the past, trial and error and operator experience optimized part and die design. Today, computer aided design and finite element modeling are used to create part and die designs and to simulate the deep drawing process, significantly reducing the costs of tooling and labor in the design process.
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