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 an unwrinkled sheet metal part.
Deep drawing is a widely used process for producing a range of hardware parts. Depending on the complexity of the part, the process may require one or more drawing operations.
Wrinkling in Deep Drawing
Wrinkling is one of the most common defects in deep drawing. It typically appears in the walls or flange of the part. During the stroke, the blank flange is subjected to radial tensile stress and tangential compressive stress, which can buckle the material into wrinkles. Wrinkling can be prevented with proper design of the drawing system and tooling.
Several factors influence wrinkling in deep-drawn parts:
- Blank holder pressure
- Cavity depth and radius
- Friction between the blank, blank holder, punch, and die cavity
- Clearance 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 alloy of the blank, also affect the drawing process. Any variation in these parameters changes 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 top of the die while the punch forces the material into the die cavity. The sheet metal deforms into the correct shape rather than simply being pulled into the cavity.
If the blank holder does not hold the blank edge securely, the wall of the cup can tear. The blank holder lets the blank slide to a controlled degree, using friction between the holder and the blank. Blank holder force can be applied hydraulically with pressure feedback, using air or nitrogen pads or CNC hydraulic cushions.
The deeper the cavity, the more blank material must be pulled into it, and the greater the risk of wrinkling in the walls and flange. Maximum cavity depth is a balance between wrinkling and fracture initiation — neither of which is acceptable.
The radii of the punch and cavity edges control how blank material flows into the cavity. If the punch and cavity edge radii are too large, the cup wall can wrinkle; if they are too small, the high stress can tear the blank.
Preventing Wrinkling: Blank Holder Control
The simplest way to eliminate wrinkling in deep-drawn parts is to use a blank holder. Most deep drawing processes apply constant blank holder pressure for the full stroke.
Variable blank holder pressure has also proven effective. Pneumatic or hydraulic blank holder pads can vary the pressure linearly with press stroke, which increases the allowable cavity depth.
Numerically controlled (NC) die cushions can deliver variable blank holder pressure during the draw — starting with a higher initial force to establish deformation, then following a tuned pressure profile.
The cushion pads release to pull material into the cavity, then gradually increase pick-up to promote strain hardening in the stretched parts. Used this way, NC die cushions significantly increase the allowable cavity depth while preventing both wrinkling and cracking.
Preventing Wrinkling: Die Design
Punch and die design can be optimized to reduce the probability of wrinkling. Choosing a flange radius just large enough to prevent cracking minimizes the risk of wrinkles. It also helps to reduce part complexity and avoid asymmetries, and a multi-step drawing sequence offers several advantages for preventing wrinkling.
Designing the blank geometry to minimize excess material reduces the chance of wrinkling. Sheet metal blanks have an inherent grain structure, so stress varies with blank layout and grain orientation relative to the die. In asymmetric parts, tuning the blank layout and grain direction to minimize grain stress is worth careful consideration.
Other Factors to Consider
- Excessive compressive stress, thin material, poor lubrication, inadequate blank holding, sharp die geometry, and difficult material properties can all cause wrinkling during the draw. Controlling these factors prevents wrinkles and keeps the process on spec.
- Specific Methods to Prevent Wrinkling
- Surface Prep: Start with clean, smooth blanks. Scale, oil, and surface defects disrupt material flow across the flange and make wrinkling more likely.
- Use Quality Materials: Specify sheet metal with consistent thickness and good drawability. Uniform material is far less prone to wrinkling.
- Control Lubrication: Keep lubrication consistent across the blank and die surfaces. Even friction control lets the flange feed smoothly into the cavity.
- Layered Application: Apply lubricant in thin, even coats rather than one heavy layer. Even coverage prevents localized flow problems at the flange.
- Support Below: Use a properly designed blank holder to support the flange and control how material feeds into the die cavity.
- Apply Gentle Pressure: Set blank holder pressure high enough to hold the flange flat, but not so high that it restricts material flow. The right balance prevents both wrinkling and excessive wall thinning.
- Take Your Time: Run the press at a controlled speed so material flows gradually and evenly into the cavity instead of buckling.
- Edge Preparation: Trim and deburr blanks so the flange feeds evenly into the die. Rough or irregular edges disrupt material flow.
Consider Draw Beads: Use draw beads or pressure pads to control material flow into the cavity and suppress wrinkling in the walls and flange.
- Research Techniques: Review process engineering guidance — material, blank shape, and die geometry all influence wrinkling — and apply proven deep drawing practices from the start.
- Specific Methods to Prevent Tearing
- Material Selection: Choose materials with good ductility and stretchability so the part can deform without tearing. Grades designed for deep drawing are the safest choice.
- Proper Lubrication: Apply the right lubricant between the sheet metal and die surfaces to reduce friction and keep material flowing. Adequate lubrication minimizes the risk of tearing.
- Die Design: Design the die with rounded corners and generous radii so stress distributes evenly and no sharp angles promote tearing. A well-designed die minimizes stress concentrations.
- Blank Holding Force: Apply uniform, sufficient blank holding force throughout the draw. Proper blank holding keeps the material from lifting or tearing at the edges.
- Slow Drawing Speed: Control drawing speed so material flows gradually and evenly. High speeds lead to rapid deformation and tearing.
- Optimal Clearance: Set the proper clearance between punch and die to prevent excessive thinning and material stress.
- Die Radii Ratio: Maintain the proper ratio between punch and die radii. An appropriate ratio reduces localized stress concentrations and minimizes the risk of tearing.
- Blank Shape: Use a blank shape that minimizes stretching and avoids sudden changes in curvature.
- Warm Forming: Consider forming at an elevated temperature, which increases material ductility and reduces the risk of tearing.
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