Causes of Surface Galling and Scratches on Deep Drawn Parts

Causes of Surface Galling and Scratches on Deep Drawn Parts

Surface galling and scratches are common quality defects in deep drawing and metal forming processes. These defects not only affect the appearance of the product but may also reduce corrosion resistance, coating quality, sealing performance, and assembly precision. In industries such as automotive, appliances, electronics, and stainless steel products, surface quality requirements are especially strict, making defect prevention extremely important.

1. Friction Between Material and Tooling

1.1 Excessive Friction During Drawing

During deep drawing, the sheet continuously slides against the punch, die, and blank holder surfaces. Excessive friction can cause the material surface to be torn or scratched.

Main reasons include:

• High contact pressure

• Long sliding distance

• Inadequate lubrication film

1.2 Adhesive Wear (Galling)

Soft metals or stainless steel tend to adhere to the die surface under pressure and heat.

This leads to:

• Material transfer onto tooling

• Repeated scratching of subsequent parts

• Severe surface scoring marks

Galling is particularly common in stainless steel and aluminum alloys.

2. Poor Tooling Surface Condition

2.1 Rough Die Surface

If the punch or die surface roughness is too high, the metal sheet will experience abrasive scratches during movement.

Common causes:

• Insufficient polishing

• Machining marks remaining on tooling

• Surface damage after long-term use

2.2 Tool Wear or Damage

As tooling wears, sharp edges or damaged areas can form.

Consequences include:

• Continuous linear scratches

• Local indentation marks

• Surface tearing

Regular die maintenance is critical for preventing these issues.

3. Improper Lubrication

3.1 Insufficient Lubrication

Without enough lubricant, direct metal-to-metal contact increases friction and surface damage.

Results include:

• Surface scoring

• Heat generation

• Material sticking

3.2 Uneven Lubricant Distribution

Inconsistent lubrication causes certain areas to experience excessive friction while others flow normally.

This creates:

• Local scratches

• Uneven surface finish

• Unstable material flow

3.3 Incorrect Lubricant Selection

Lubricants that cannot withstand high pressure or temperature may fail during forming.

Especially for stainless steel deep drawing, ordinary lubricants may not provide sufficient anti-galling performance.

4. Material Surface Problems

4.1 Defects on Raw Material Surface

Pre-existing defects on the metal sheet may worsen during forming.

Examples include:

• Surface oxide scale

• Dirt or contamination

• Coil scratches

• Rolling marks

4.2 Hard Particles or Foreign Matter

Dust, metal chips, or abrasive particles trapped between the sheet and tooling can scratch the surface during movement.

5. Improper Process Parameters

5.1 Excessive Drawing Speed

Very high forming speeds increase frictional heat and material adhesion.

This may result in:

• Increased galling tendency

• Surface burn marks

• Deeper scratches

5.2 Excessive Blank Holder Force

Overly high blank holder pressure increases contact stress and friction, damaging the surface.

5.3 Improper Die Clearance

Incorrect clearance can cause excessive squeezing or unstable material movement, leading to surface defects.

6. Equipment and Operational Factors

6.1 Tool Misalignment

Misalignment between punch and die creates uneven pressure distribution and localized scratching.

6.2 Poor Equipment Stability

Machine vibration or inconsistent movement may cause abnormal contact between tooling and material.

6.3 Improper Handling During Production

Scratches may also occur during transportation, stacking, or manual handling after stamping.

7. Common Locations of Surface Scratches

8. Improvement and Prevention Measures

8.1 Improve Tooling Surface Quality

• Polish die surfaces to lower roughness

• Apply hard coatings such as TiN or DLC

• Repair worn tooling promptly

8.2 Optimize Lubrication

• Use high-performance drawing lubricants

• Ensure even lubricant application

• Select lubricants suitable for stainless steel or aluminum

8.3 Optimize Process Parameters

• Reduce excessive drawing speed

• Adjust blank holder force properly

• Optimize die clearance and drawing radius

8.4 Improve Material and Cleanliness Control

• Use high-quality sheet materials

• Remove dust and contaminants before forming

• Maintain clean production environments

8.5 Strengthen Equipment Maintenance

• Ensure precise alignment of tooling

• Reduce machine vibration

• Regularly inspect guiding systems

Conclusion

Surface galling and scratches on deep drawn parts are mainly caused by excessive friction, poor tooling conditions, inadequate lubrication, and unstable process parameters. Effective prevention requires comprehensive control of tooling quality, lubrication systems, material cleanliness, and forming conditions. By combining proper maintenance with advanced surface engineering technologies, manufacturers can significantly improve surface quality and reduce defect rates.

References

1. Altan, T., & Tekkaya, A. E. Sheet Metal Forming: Fundamentals. ASM International.

2. Kalpakjian, S., & Schmid, S. R. Manufacturing Engineering and Technology. Pearson Education.

3. Lange, K. Handbook of Metal Forming. McGraw-Hill.

4. ASM International. ASM Handbook, Volume 18: Friction, Lubrication, and Wear Technology.

5. Hosford, W. F., & Caddell, R. M. Metal Forming: Mechanics and Metallurgy. Cambridge University Press.