Common Defects in Sheet Metal Forming and Their Prevention Methods

Sheet metal forming is one of the most critical processing steps in modern manufacturing, widely used in electronic enclosures, medical equipment, automation equipment, home appliances, communications, and automotive industries. Despite continuous improvements in CNC machining, various defects still occur during bending, stamping, and stretching processes, affecting appearance, assembly accuracy, and structural reliability.

We have summarized the most common defects in sheet metal forming and effective control methods to help engineers and manufacturing personnel improve yield and quality stability.

Burrs

Defect Manifestation: Sharp protrusions appear on the edges after cutting or punching, affecting safety and dimensional accuracy.

Causes:

• Tool dulling, excessive die clearance.

• Punch edge wear

• Excessively high material hardness

• Inappropriate cutting parameters (laser/CNC punching)

Preventive Measures:

• Regular maintenance and grinding of punches and tools

• Adjust die clearance (generally 5-10% of the plate thickness)

• Appropriately reduce speed for high-hardness materials

• Optimize focal point position and air pressure during laser cutting

• Add a deburring process if necessary (roller deburring, vibratory deburring, etc.)

Bending Springback

Defect Manifestation: Bending angle is larger than the design value, making assembly difficult.

Causes:

• High material elastic modulus

• Excessive bending radius

• Insufficient bending force

• Mismatch between upper and lower die structures

Preventive Measures:

• Determine the compensation angle through trial molding (e.g., pre-press to 88°, naturally spring back to 90°)

• Use a smaller bending radius (ideally ≥ sheet thickness)

• Increase blank holder force and bending depth

• Select materials with lower springback characteristics (e.g., soft aluminum)

• Include angle compensation in the design

Scratches and Indentations

Defect Manifestation: Scratches and indentations appear on the board surface, affecting the appearance quality.

Causes:

• Rough mold surface or foreign objects

• Lack of mold protection

• Lack of protection during board transportation and handling

• Excessive friction with the mold during bending

Preventive Measures:

• Regularly polish the mold and keep it clean

• Apply protective adhesive to the mold surface or use nylon molds

• Add soft padding film between stacked boards

• Optimize the bending sequence to reduce the contact area between the board and the equipment

• Control the bending speed and avoid strong scratches

Punching Deformation

Defect Manifestation: Enlarged hole diameter, wrinkled hole edges, or irregular shape appear after punching.

Causes:

• Incorrect die clearance

• Uneven sheet thickness or high material strength

• Unstable stamping speed or pressure

• Too close hole spacing leading to material deformation

Preventive Measures:

• Select a reasonable die clearance according to material properties (generally 6-8% of sheet thickness)

• Shorten the distance between punching and bending, and optimize layout

• Use a high-precision stamping press to avoid pressure fluctuations

• For high-strength steel, reduce stamping speed and increase punch strength

Crack

Defect Manifestation: Cracks and whitening appear in the drawn parts, flanging, or bending areas.

Causes:

• Excessive drawing depth

• Insufficient material ductility

• Insufficient radius (R) angle design

• Insufficient die clamping force

• Inappropriate forming method

Preventive Measures:

• Increase the radius (R) angle and reduce the drawing ratio in the design.

• Perform multiple drawing operations (segmented forming) if necessary.

• Use materials with good ductility (such as deep-drawing steel DC04).

• Add lubricant to reduce friction.

• Increase the clamping force to prevent excessive material flow.

Bend Crack

Defect Manifestation: Tear and white edge appear at the outer corner of the bend.

Causes:

• Insufficient bending radius

• Material fiber direction is consistent with the bending direction

• Bending speed is too fast

• Sheet material is too hard or not annealed

Preventive Measures:

• Select the minimum bending radius recommended by the material (e.g., ≥ 1.5t for 5052-H32)

• Make the bending direction as perpendicular as possible to the rolling direction of the material

• Anneal hardened materials before bending

• Reduce bending speed and apply force in stages

Weld Distortion

Defect Manifestation: Warping and twisting of the plate surface after welding, affecting dimensional stability.

Causes:

• Uneven heating of thin plates generates thermal stress.

• Improper welding sequence.

• Excessive heat input.

Preventive Measures:

• Use TIG/MIG low-current welding to reduce heat input.

• Use spot welding instead of long welds.

• Use fixtures for fixation and symmetrical welding.

• Use skip welding to distribute heat.

• Immediately perform cold pressure straightening after welding.

Dimension Inaccuracy

Defect Manifestation: Height and side length after bending do not conform to the drawing dimensions.

Causes:

• Insufficient K-value/Bend allowance calculation

• Bending coefficient not adjusted according to material properties

• Inaccurate workpiece positioning

Preventive Measures:

• Set accurate K-values according to different materials and plate thicknesses

• Use standard bending compensation formulas for flattening

• Strictly adhere to tooling positioning to avoid misalignment

• Regularly calibrate the accuracy of dies and bending machines

Warping / Oil Canning

Defect Manifestation: Bulges, ripples, or dents appear after bending or stamping large areas of sheet metal.

Causes:

• Residual stress in the sheet metal

• Inappropriate processing sequence

• Uneven localized stress

Preventive Measures:

• Use a leveling machine to eliminate sheet metal stress

• Add stiffening ribs to improve rigidity

• Optimize bending and stamping sequence

• Avoid large-area single operations (e.g., large-area stamping in one go)

In summary, sheet metal forming defects are rarely caused by a single factor, but rather by a combination of factors including material properties, mold condition, process parameters, and equipment precision.

To effectively reduce defects and improve finished product quality, the following must be done:

• Optimize the process from the design stage

• Strictly control the precision of molds and equipment

• Standardize processing parameters and bending compensation

• Establish a comprehensive inspection and feedback mechanism