Complete Process of Silver Coating for Plastic and Metal Parts: UV Spraying Line vs. Vacuum Coating Machine

Silver coating has become a staple finish in industries ranging from automotive interiors and consumer electronics to household appliances, valued for its sleek metallic luster, corrosion resistance, and aesthetic versatility. Two dominant technologies—UV spraying and vacuum coating (Physical Vapor Deposition, PVD)—cater to the distinct requirements of plastic and metal substrates. While UV spraying offers cost-effectiveness and adaptability to complex geometries, vacuum coating delivers superior durability and authentic metallic 质感. This article details the full workflow of both processes, from pre-treatment (polishing and ultrasonic cleaning) to core coating steps, including the composition of silver UV paint and selection of vacuum coating targets, with a focus on substrate-specific adaptations for plastic and metal parts.

Pre-Treatment: Polishing and Ultrasonic Cleaning

Pre-treatment is critical to ensuring coating adhesion, uniformity, and longevity, as surface imperfections, contaminants (oil, dust, oxides), or residual debris can cause peeling, bubbling, or uneven gloss. The process varies slightly between plastic and metal substrates due to differences in hardness, heat resistance, and surface porosity.

Polishing: Substrate-Specific Techniques

Polishing aims to create a smooth, defect-free surface that enhances silver coating’s reflective properties and reduces visible flaws.

For Plastic Parts

Plastic substrates (e.g., ABS, PC, PMMA) are relatively soft (Shore D 60-85) and prone to scratching, requiring gentle polishing methods:

• Abrasive Selection: Fine-grit sandpapers (P1500-P2000) or diamond polishing pastes (1-3 μm) are used to avoid material deformation. For high-gloss requirements, a final buff with a microfiber cloth and polishing compound (containing alumina or silica) achieves a surface roughness of Ra ≤ 0.05 μm.

• Equipment: Manual polishing or low-speed orbital polishers (1000-1500 RPM) prevent overheating, which can warp thermoplastic parts. Textured plastic parts skip aggressive polishing to preserve the original surface pattern, only requiring mild cleaning to remove mold release agents.

For Metal Parts

Metal substrates (e.g., aluminum, steel, zinc alloy) are harder (HV 100-300) and tolerate more rigorous polishing:

• Abrasive Sequence: Rough polishing with aluminum oxide sandpaper (P400-P800) removes machining marks, followed by medium polishing (P1000-P1200) and fine polishing (P1500-P2000) with cerium oxide pastes. For mirror finishes, a cloth wheel polisher (2000-3000 RPM) with chrome oxide paste achieves Ra ≤ 0.02 μm.

• Deburring: Prior to polishing, metal parts undergo deburring to remove sharp edges, which can cause coating buildup or cracking.

Ultrasonic Cleaning: Multi-Stage Decontamination

Ultrasonic cleaning uses high-frequency sound waves (40-80 kHz) to dislodge contaminants from micro-pores and intricate surfaces, outperforming traditional cleaning methods for complex-shaped parts.

Key notes: Ultrasonic frequency is set to 40 kHz for plastics (reduces cavitation damage) and 60-80 kHz for metals (enhances contaminant removal). Parts are placed in mesh baskets with soft liners to avoid scratches during cleaning.

UV Spraying Line: Silver Coating Process

UV spraying applies a silver-colored UV-curable coating via electrostatic spraying, offering fast curing, low VOC emissions, and excellent color consistency. It is ideal for plastic parts and low-wear metal components.

Silver UV Paint Composition

The core material determining the silver finish and performance is the UV-curable paint, formulated as follows:

• Resin System: Acrylate oligomers (urethane acrylate for flexibility, epoxy acrylate for hardness) and monomers (trimethylolpropane triacrylate, TMPTA) form the film matrix. For plastics, flexible oligomers (elongation at break ≥ 50%) prevent cracking; for metals, rigid oligomers (hardness ≥ 2H) enhance scratch resistance.

• Silver Pigment: Aluminum flake powder (aluminum silver paste) is the primary colorant, with particle size 5-20 μm and surface treatment (silane coupling agent) to improve dispersion and weatherability. The pigment loading (10-15% by weight) controls metallic gloss—higher loading creates a brighter, more reflective finish.

• Additives: Photoinitiators (1-hydroxycyclohexyl phenyl ketone, HCPK) absorb UV light (365 nm) to trigger curing; flow agents (polyether-modified polysiloxane) prevent orange peel; anti-yellowing agents (benzotriazole UV absorbers) preserve silver luster.

• Solvents: Low-VOC diluents (ethyl acetate, isobutyl acetate) adjust viscosity (15-20 s, DIN 4 mm cup) for sprayability, with minimal residual solvent ( to avoid bubbling.

Spraying and Curing Process

1. Primer Application (Optional): For plastics with poor adhesion (e.g., PP), a UV-curable primer (acrylic + chlorinated polyolefin) is sprayed first, with a dry film thickness (DFT) of 5-8 μm. It is pre-cured with a UV lamp (80-100 mJ/cm²) for 10-15 seconds.

2. Silver UV Paint Spraying: Electrostatic spray guns (voltage 60-80 kV, spray pressure 0.3-0.5 MPa) apply the silver paint in 1-2 coats, achieving a DFT of 15-25 μm. The spray distance is 20-30 cm, with a conveyor speed of 1-2 m/min to ensure uniform coverage.

3. Flash-Off Stage: Parts are held at room temperature for 5-10 minutes to allow solvent evaporation, reducing pinhole formation during curing.

4. UV Curing: A mercury lamp (power 80-120 W/cm) or LED UV lamp (365 nm) cures the coating at a conveyor speed of 3-5 m/min, with a total energy dose of 300-500 mJ/cm². Curing time is 1-3 seconds, ensuring full crosslinking (gel content ≥ 95%).

5. Topcoat Application (Optional): A clear UV topcoat (DFT 10-15 μm) is applied for high-wear applications, cured with an additional UV pass (200-300 mJ/cm²) to enhance scratch resistance (≥4H pencil hardness) and chemical resistance.

Vacuum Coating Machine: Silver Coating Process

Vacuum coating (magnetron sputtering) deposits a thin silver metallic film in a high-vacuum environment, delivering superior adhesion, wear resistance, and authentic metallic 质感. It is preferred for high-performance metal parts and premium plastic components.

Target Material Selection for Silver Coating

The target determines the silver finish’s purity, durability, and cost. Two primary options are used:

• Aluminum (Al) Target: The most common choice (99.99% purity), offering a bright silver luster similar to polished aluminum. Aluminum is cost-effective, chemically stable, and forms a dense oxide layer (Al₂O₃) that enhances corrosion resistance. Target dimensions vary by chamber size (typically 300×100×5 mm for small-scale equipment), with a sputtering yield of 1.2 atoms/ion.

• Silver (Ag) Target: For a pure, reflective silver finish (e.g., mirror-like surfaces), 99.99% pure silver targets are used. Silver has exceptional reflectivity (≥95% for visible light) but is softer (HV 60-80) and more expensive than aluminum. It is often used for high-end applications (e.g., luxury electronics, automotive trim) with a protective topcoat to prevent tarnishing.

Magnetron Sputtering Process

1. Loading and Fixturing: Pre-treated parts are mounted on rotating fixtures (planetary rotation, 5-10 RPM) to ensure uniform coating. Plastic parts use low-temperature fixtures to avoid thermal damage.

2. Vacuum Evacuation: The chamber is evacuated to a base pressure of 1×10⁻³ Pa using a turbomolecular pump, removing air and moisture that cause film defects (pinholes, oxidation).

3. Ion Bombardment Cleaning: Argon (Ar) gas is introduced (pressure 1×10⁻¹ Pa), and a negative bias (-300 to -500 V) is applied to the parts. Argon ions bombard the surface, removing residual contaminants and activating the substrate (enhances adhesion). This step lasts 5-10 minutes.

4. Sputtering and Deposition: The target is energized with a DC power supply (2-5 kW), creating a plasma that ionizes argon gas. Argon ions bombard the target, ejecting metal atoms (Al or Ag) that deposit onto the rotating parts. Key parameters:

◦ Deposition Pressure: 2×10⁻² Pa (argon flow rate 20-30 sccm).

◦ Deposition Temperature: 80-120°C for plastics, 150-200°C for metals.

◦ Film Thickness: 50-200 nm (controls silver luster—thicker films are more reflective).

1. Post-Deposition Treatment: After deposition, the chamber is purged with nitrogen, and parts are cooled to room temperature (20-30 minutes). A thin protective layer (SiO₂ or Al₂O₃, 20-50 nm) is often sputtered to enhance wear resistance and prevent tarnishing (critical for silver targets).

Post-Coating Inspection and Quality Control

Both processes require rigorous testing to ensure performance compliance:

• Adhesion Test: Cross-cut test (ASTM D3359) with 100/100 rating (no peeling) for metals; tape test (3M 610) for plastics (no coating removal).

• Aesthetic Inspection: Visual assessment under D65 standard lighting checks for unevenness, pinholes, or color deviation (ΔE ≤ 1.0 for silver tone).

• Physical Properties: Pencil hardness (≥2H for UV spraying, ≥3H for vacuum coating), abrasion resistance (≥500 cycles with 500g load, ASTM D4060), and corrosion resistance (48-hour salt spray test, ASTM B117—no rust or discoloration).

• Film Thickness: Digital thickness gauge (precision 1 μm for UV coating, 1 nm for vacuum coating) verifies DFT compliance.

Process Comparison and Application Scenarios

Typical Applications

• UV Spraying: Automotive interior plastics (dashboard trims, door handles), consumer electronics casings (phone back covers), toys, and household appliances.

• Vacuum Coating: Automotive exterior metal parts (wheel rims, grilles), premium electronics (laptop frames), medical devices, and decorative metal fixtures.