Stamping Defects: Effective Fixes and Tips

Bending Cracks

Cracks are a serious quality issue in bent parts. Therefore, avoiding cracks during the bending process is a key point to consider.

Main causes of cracks in bent parts:

• Poor mechanical properties of the material (elongation rate).
• Defects on the surface or impurities inside the material.
• The rolling direction of the material is parallel to the bending line.
• Too small a bending radius.
• Shear burrs in the material.
• Sandblasting during material pretreatment, causing surface hardening of the material.

Measures to prevent cracks in bent parts:

To prevent cracking during bending, it is necessary to eliminate the factors causing cracks before bending. The elimination process must start from the design of the bent part.

1) Generally, bending is in a cold state.

When designing the bending radius, consider the elongation rate of the material and the unavoidable situation where the rolling direction of the material is parallel to the bending line. For commonly used Q235 and Q345 plates, the bending radius should not be less than 1.5t.

2) Materials with surface defects or cracks should not be used for bending.

3) When separating materials for bending, consider avoiding the problem of the rolling direction being parallel to the bending line.

4) Remove shear burrs.

5) Annealing the material before bending can improve the cracking situation during bending.

6) If necessary, adjust the order of sandblasting rust removal and bending.

Surface Scratching

Surface scratching is a common quality issue in formed parts. The main causes are excessive friction during material flow in the forming process and poor wear resistance of the mold.

To avoid scratching, measures such as increasing the chamfer of the die, improving the surface roughness of the die, using wear-resistant materials for the die, and applying lubricants on the die can be adopted.

Common Defects in Large Surface Forming

Common defects in large surface forming include cracks and ruptures, wrinkles and folds, unclear edge lines, low stiffness, surface scratches, and surface roughness.

Cracks and Ruptures

Cracks and ruptures during large surface forming occur primarily due to local blanks’ tensile stress exceeding the strength limit. Specific influencing factors include:

1) The material’s stamping performance does not meet process requirements. Deviations in elongation (A), yield strength ratio (R/Rm), grain size, and uniformity can all lead to cracking.

2) Sheet thickness is out of tolerance. When sheet thickness exceeds the upper deviation, material may get stuck in areas with small local gaps, making it difficult for the material to deform and flow into the die, resulting in a rupture.

When the sheet thickness exceeds the lower deviation, the material thins, increasing the compressive stress on the unit area of the cross-section, or due to the thinning material, resistance decreases, causing an excess of sheet material to flow into the die, forming wrinkles, and the material is torn due to obstructed flow.

3) Poor surface quality of the material. For example, scratches on the surface can cause stress concentration during stamping, leading to cracks. Or rust on the surface can significantly increase the frictional resistance during material replenishment, causing deformation and flow difficulties, leading to cracks.

4) The blank holder’s feed resistance is too high. Reasons for excessive feed resistance on the blank holder include large blank dimensions causing increased area resistance; too small die fillet radius causing material flow difficulties; unsmooth blank holder and die fillet radius causing high resistance and difficulty in material flow.

5) Local deep drawing is too much. The local deep drawing deformation of the workpiece exceeds the material deformation limit. The material’s deep drawing deformation in a single forming operation exceeds the ultimate elongation.

6) The die gap is skewed due to manufacturing and assembly errors of the punch and die, causing uneven feed resistance.

7) Not applying lubricant as per the process specifications increases friction resistance, making feeding difficult and causing cracking.

8) Misplacement of the blank during operation can cause one side to be too large and the other side too small.

The larger side has difficulty feeding, causing cracking, while the smaller side has too little resistance, feeds too much, wrinkles easily, and once wrinkled, feeding becomes difficult, leading to rupture.

Wrinkles and Folds

The main causes of wrinkles are due to local blanks becoming unstable under compression and uneven material flow direction, causing local material accumulation and wrinkles. Specific influencing factors include:

1) Poor stamping processability of the part, improper determination of the stamping direction and blank holder face shape, making it difficult to control material flow speed, thus causing wrinkles.

2) The blank holder’s compressive force is insufficient, the feed resistance is too small, and too much feed causes wrinkles.

3) Poor contact of the blank holder face causes uneven compressive force on the blank holder face, leading to local areas lacking sufficient compressive force and producing wrinkles.

4) The punch and die have no gap or an unreasonable gap. Manufacturing and assembly errors of the punch and die lead to local over-pressure of the punch and die, while most are not under pressure, causing a large area of the workpiece to be uneven.

Unclear Edge Lines

From an external observation of the part, the forming edge lines are unclear. The main reason is that the punch and die are not pressed tightly. The causes for not pressing tightly include:

1) The press’s pressure is insufficient. During the forming process, the press’s maximum pressure is less than the forming force of the workpiece. That is, the punch and die are not tightly fitted, and the punch cannot continue downward, leading to unclear edge lines.

2) Manufacturing and assembly errors of the punch and die inserts cause uneven actual gaps of the punch and die. The press’s pressure is all consumed in the local areas where the punch and die gap is smaller. The part of the workpiece in this area is rolled thin, while most of the punch and die are not pressing tightly, causing unclear edge lines.

3) Poor guidance of the press and poor parallelism of the press can cause the punch and die not to press tightly, making the part’s edge lines unclear.

Poor Stiffness

The main reasons for poor stiffness, apart from poor part processability, are primarily due to the blank holder’s feed resistance being too small, and insufficient plastic deformation of the material.

Surface Scratches

The causes of surface scratches include:

1) The die fillet is not smooth enough, scratching the material during the forming process, and possibly causing the material to adhere to the die, forming scratches.

2) Dirt falls into the die, or the lubricating oil is not clean, scratching the surface of the part.

3) When the forming die is composed of several inserts, the inserts are not well joined at the splice, causing scratches.