Precautions for Sheet Metal Surface Treatment Processing

Surface treatment of sheet metal parts directly affects appearance, corrosion resistance, wear resistance, and assembly quality after cutting, bending, and welding. Common processes include powder coating, electroplating, anodizing, brushing, painting, and silk-screen printing. The following are the key precautions in sheet metal surface treatment processing.

1. Pretreatment Precautions (Most Critical)

Proper surface cleaning determines coating adhesion and final product quality.

1.1 Oil and Contaminant Removal

It is essential to completely remove:

• Stamping oil

• Fingerprints and sweat

• Welding residue

• Anti-rust oil

Common methods:

• Alkaline cleaning

• Ultrasonic cleaning

• Degreasing agents

1.2 Deburring

Laser cutting and stamping edges often produce burrs.

If not removed, they may cause:

• Uneven coating

• Burnt edges during electroplating

• Safety hazards to operators

Common methods:

• Vibratory finishing

• Brushing machines

• Manual grinding

1.3 Rust and Oxide Scale Removal

Especially important for carbon steel parts.

Acid pickling time must be controlled to avoid:

• Over-corrosion

• Dimensional changes

• Hydrogen embrittlement

2. Precautions for Different Surface Treatment Processes

2.1 Powder Coating (Electrostatic Powder Spraying)

One of the most widely used methods.

Key Points

(1) Proper Grounding

Poor grounding may result in:

• Low powder transfer efficiency

• Missed coating areas

• Orange peel defects

(2) Coating Thickness Control

Typical thickness:

• 60–100 μm

If too thick:

• Sagging

• Orange peel texture

If too thin:

• Poor coverage

• Weak corrosion resistance

(3) High-Temperature Deformation

Curing temperature is usually:

• 180–200°C

Thin sheet metal parts may:

• Warp

• Deform

Therefore:

• Large thin panels should use fixtures during curing

(4) Faraday Cage Effect in Corners

Complex bent structures may suffer from:

• Poor powder penetration

• Insufficient coating in inner corners

Solutions:

• Adjust spray gun angle

• Apply secondary touch-up spraying

2.2 Spray Painting

Precautions

A clean environment is extremely important.

Dust contamination may cause:

• Rough or grainy surface finish

Common Problems

• Sagging

• Cratering

• Color difference

• Blistering

Key Control Factors

• Paint viscosity

• Spray gun pressure

• Flash-off time

• Baking temperature

2.3 Electroplating (Zinc, Nickel, Chrome Plating)

Precautions

(1) Edge Burning at Sharp Corners

Current concentration at sharp edges may cause:

• Excessive coating thickness

• Blackening or burning

Solutions:

• Add chamfers

• Use auxiliary anodes

(2) Chemical Residue in Blind Holes

Blind holes may trap plating solution, leading to:

• Residual corrosion

• Rust after assembly

Design recommendation:

• Avoid enclosed structures whenever possible

(3) Hydrogen Embrittlement Risk

Especially critical for high-strength steel.

After plating, it is recommended to:

• Perform dehydrogenation treatment

• Bake parts to remove hydrogen

2.4 Anodizing (For Aluminum Parts)

Precautions

(1) Material Grade Affects Color

Different aluminum alloys may show noticeable color variation.

Especially:

• 5052

• 6061

• Die-cast aluminum

Mixing different material batches is not recommended.

(2) Sensitive to Scratches

Avoid impact or scratches before anodizing because:

• Surface defects become more visible after anodizing

(3) Reserve Conductive Areas

Anodized layers are electrically insulating.

If conductivity is required:

• Reserve grounding/contact points

• Use masking for specific areas

2.5 Brushing / Hairline Finish

Precautions

• Brushing direction must be consistent

• Batch consistency is important

Otherwise:

• Appearance differences become obvious

Additional Considerations

After brushing:

• Surface oxidizes easily

• Fingerprints are more visible

Usually combined with:

• Clear coating

• Anodizing

• Anti-fingerprint treatment

3. Precautions During Structural Design Stage (Often Overlooked)

3.1 Avoid Treatment Dead Zones

Examples:

• Deep bending grooves

• Fully enclosed welded cavities

These may lead to:

• Incomplete coating

• Uneven plating

3.2 Reserve Fixture Hanging Points

Powder coating and electroplating require hanging fixtures.

Design should consider:

• Hanging locations

• Masked areas

• Appearance impact

3.3 Allowance for Coating Thickness

Surface treatment increases part dimensions.

Examples:

• Powder coating: increases single-side thickness by 0.06–0.1 mm

• Electroplating: increases by several to dozens of microns

Precision assemblies must:

• Reserve appropriate tolerances

4. Quality Control Focus

Common Inspection Items

Appearance Inspection

• Color difference

• Particles

• Scratches

• Missed coating

• Sagging

Performance Testing

• Adhesion test (cross-hatch test)

• Salt spray test

• Coating thickness test

• Hardness test

5. Workshop Management Precautions

Environmental Control

Especially for painting:

• Stable temperature and humidity

• Dust prevention

Workpiece Protection

During transportation and handling:

• Prevent scratches

• Prevent dents and pressure marks

Batch Consistency

For the same production batch:

• Use the same curing cycle

• Use the same powder or paint batch

To minimize:

• Color variation

6. Summary of Common Problems