In sheet metal drawing, several types of defects can occur due to improper material flow, tooling design, or process parameters. Here are the most common defects found in drawn parts
1. Wrinkling
Description: Formation of waves or folds in the flange or wall area.
Causes:
Insufficient blank holder pressure.
Excessive metal flow from the flange.
Too much material between die and blank holder.
Prevention:
Increase blank holder pressure.
Use draw beads or restraining features.
Optimize lubrication and blank size.
2. Tearing (or Fracture)
Description: Cracking or complete tearing of the metal, usually near the die radius or punch corner.
Causes:
Excessive tensile stress or too small a die radius.
Poor lubrication.
Blank too small or insufficient material flow.
Prevention:
Use larger die corner radii.
Apply proper lubrication.
Adjust blank size or reduce punch travel.
3. Earing
Description: Uneven or wavy edges on the rim of a deep-drawn cup (looks like “ears”).
Causes:
Material anisotropy (directional properties from rolling).
Prevention:
Use materials with low planar anisotropy.
Adjust blank shape to compensate.
Re-draw or trim after drawing.
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4. Thinning (or Necking)
Description: Excessive reduction in wall thickness, especially in the wall or corner regions.
Causes:
High drawing ratio or poor material flow.
Sharp punch or die radii.
Non-uniform friction or lubrication.
Prevention:
Optimize draw ratio.
Increase corner radii.
Use consistent lubrication.
5. Galling or Scoring
Description: Scratches or rough patches on the surface due to metal pickup on tooling.
Causes:
Inadequate lubrication.
Tool material too soft or improperly finished.
Prevention:
Use better lubrication or coated tools (TiN, CrN, etc.).
Polish tool surfaces.
6. Springback
Description: Elastic recovery after forming, causing deviation from intended shape.
Causes:
High yield strength or insufficient forming pressure.
Prevention:
Use overbending or restriking operations.
Choose materials with lower yield strength.
Optimize die design.
7. Surface Distortion or Orange Peel
Description: Rough, uneven surface texture after drawing.
Causes:
Coarse grain structure in material.
Excessive deformation.
Prevention:
Use fine-grain sheet metal.
Apply proper annealing.
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