Defects in metal stamping occur when the physical forces applied to sheet metal exceed the material’s formability limits or when variations occur in tooling, machinery, or process parameters. Because stamping relies on severe localized plastic deformation (shearing, bending, stretching, or drawing), even minor mismatches can result in dimensional errors or structural failures.
The root causes of stamping defects are categorized into the following core factors:
1. Improper Punch-and-Die Clearance (模具間隙不當)
Clearance—the space between the punch and die cutting edges—directly governs edge quality and part accuracy:
- Clearance Too Large: Causes excessive plastic bending, heavy edge rounding, thick drag burrs, and dimensional shrinkage.
- Clearance Too Small: Forces secondary shearing, creating double-burnished bands, edge cracking, tight burrs, and rapid tool wear or punch chipping.
- Uneven Clearance: Results in single-sided burrs, non-uniform wall thickness, and off-center punch displacement.
2. Material Elasticity and Springback (彈性回復與反彈)
Metal inherently retains elastic strain during plastic deformation. When the stamping load is released at Bottom Dead Center (BDC), elastic recovery causes springback (angular shifts, bow, or twist). Fluctuations in sheet thickness, yield strength, or rolling grain direction alter the amount of springback between production batches.
3. Incorrect Blankholding Pressure and Die Radii (壓邊力與模具圓角不當)
In forming and deep drawing operations, balancing material flow is critical:
- Wrinkling (起皺): Occurs when blankholding pressure is too low or die corner radii are too large, allowing unconfined sheet metal to buckle under compressive hoop stress.
- Cracking and Fracturing (破裂/斷裂): Occurs when blankholding pressure is too high, drawing speed is excessive, or die entry radii are too sharp, restricting material flow and exceeding the material’s ultimate tensile strength.
4. Tool Wear and Alignment Deviations (模具磨損與安裝對中不良)
- Cutting Edge Dullness: As punch and die edges wear down from continuous friction, shearing action deteriorates, leading to severe burrs and rough, fractured edges.
- Misalignment: Non-concentric alignment, worn guide posts, or unparallel upper and lower shoe plates cause uneven clearance, pitch location errors, or die clashing (galling/kicking).
5. Press Machine Deflection and BDC Instability (沖床剛性與下死點變動)
Under heavy loads, press frame stretching or slide tilting causes Bottom Dead Center (BDC) height variation. This dynamically alters punch penetration depths and local clearances during operation, resulting in inconsistent part dimensions and surface distortion.
6. Inadequate Lubrication and Surface Galling (潤滑與摩擦問題)
High contact pressures generate extreme friction and localized heat. Without proper high-pressure lubricant, metal-to-metal contact leads to galling / scoring (かじり/擦傷), thermal metal adhesion, and severe surface scratches on both the part and the die
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