Specific Processes for Polyester (PE) Coating Application on ACM for Household Appliance Panels

Polyester (PE) coating for appliance-grade Aluminum Composite Material (ACM) is applied to the aluminum face sheets first (before core bonding), using a continuous roll-coating production line. This industrial-grade process is designed to meet the strict requirements of household appliance manufacturing—ensuring high coating uniformity, strong adhesion to aluminum substrates, and excellent flexibility for subsequent stamping, bending, and forming processes. The stable performance of the coated ACM also guarantees long-term resistance to household environments (humidity, oil fumes, cleaning agents) and maintains consistent appearance over the appliance’s service life. Below is the complete, detailed technical process flow:

Aluminum Substrate Uncoiling & Surface Preparation

The surface preparation stage is critical for ensuring coating adhesion and corrosion resistance—any residual contaminants or oxides will directly affect the final quality of the appliance panel. This stage follows a precise, automated workflow:

Uncoiling of Raw Aluminum Coil

• The raw aluminum coil (alloy grades 3003/3105/5052, thickness 0.2–0.5mm) is loaded onto a hydraulic uncoiler with tension control.

• Uncoiling speed is synchronized with the subsequent production line, typically set at 30–50 m/min (adjustable based on coating thickness and line capacity).

• A tension controller maintains constant pressure (5–8 N/mm²) to prevent aluminum sheet wrinkling or stretching during uncoiling.

Alkaline Degreasing

• Purpose: Remove surface oil (from rolling process), dust, metal filings, and natural oxides to expose a clean aluminum substrate.

• Technical parameters:

• Degreasing agent: Alkaline aqueous solution (sodium hydroxide-based, concentration 3–5%) with surfactant additives.

• Temperature: 50–60 °C (optimized for fast oil emulsification without damaging the aluminum substrate).

• Immersion time: 30–60 seconds (continuous spray or dip-coating process for uniform cleaning).

• Post-degreasing rinse: High-pressure spray rinse with tap water to remove residual degreasing agent.

Multi-Stage Precision Rinsing

• Configuration: 3–4 stages of countercurrent rinsing to eliminate chemical residues step by step.

• Stage 1: Tap water rinse (removes most alkaline residues).

• Stage 2: Low-conductivity water rinse (conductivity ≤50 μS/cm).

• Stage 3: Deionized water rinse (conductivity ≤10 μS/cm) to prevent water spots and ensure coating adhesion.

• Rinsing method: Combined spray and immersion to cover all areas of the aluminum sheet, with water flow rate ≥10 L/m².

Chemical Conversion Treatment (Passivation)

• Type: Two options for appliance-grade applications:

• Chromate passivation (traditional): Forms a chromium-rich conversion layer (thickness 0.5–1 μm) with excellent corrosion resistance (suitable for general appliances).

• Non-chromate passivation (environmentally friendly): Zirconium-titanium-based treatment, compliant with EU RoHS standards (ideal for high-end, export-oriented appliances).

• Process parameters:

• Bath temperature: 30–40 °C.

• Treatment time: 20–30 seconds.

• Film weight: 20–40 mg/m² (ensures optimal bonding between aluminum and primer).

High-Temperature Drying

• Equipment: Continuous hot-air drying oven with temperature zoning control.

• Drying parameters:

• Inlet temperature: 80–90 °C (preheating zone to avoid rapid moisture evaporation).

• Main drying zone: 100–120 °C (core temperature of aluminum sheet ≥90 °C).

• Dwell time: 60–90 seconds (ensures 100% surface dryness, no residual moisture).

• Drying method: Forced convection with air velocity 2–3 m/s, ensuring uniform heat distribution across the sheet width.

Primer Coating

The primer layer acts as a bridge between the aluminum substrate and the PE top coat, enhancing interlayer adhesion, blocking corrosion factors, and improving the flatness of the top coat—critical for the aesthetic and durability of appliance panels.

Precision Roll Coating Application

• Equipment: Reverse roll coater (two-roll or three-roll configuration) with automatic thickness control.

• Working principle: The coating roller transfers primer to the aluminum sheet, while the doctor blade adjusts the coating amount to ensure uniformity.

• Primer characteristics: Polyester-based primer with anti-corrosion pigments (titanium dioxide, zinc phosphate) and adhesion promoters.

• Thickness control:

• Dry film thickness: 5–8 μm (target tolerance ±0.5 μm).

• Online monitoring: Beta-ray thickness gauge installed after the coater, with automatic feedback to adjust roller pressure and speed.

Edge Sealing Treatment (Optional)

• For wide-format aluminum sheets (≥1200mm) used in large appliance panels (e.g., refrigerator side panels), edge sealing is performed after primer coating.

• Purpose: Prevent edge corrosion and ensure consistent coating performance across the entire sheet.

• Method: Automatic edge roller applies a thin layer of primer to the sheet edges, with thickness matching the main surface.

Primer Curing (Intermediate Baking)

Full curing of the primer is essential to achieve its maximum adhesion and corrosion resistance—insufficient curing will lead to coating peeling during subsequent processing.

Convection Oven Curing

• Equipment: Sectional convection oven with independent temperature control for each zone (preheating, curing, cooling transition).

• Curing parameters:

• Peak Metal Temperature (PMT): 160–180 °C (measured by thermal probes attached to the aluminum sheet).

• Dwell time: 25–40 seconds (varies by primer type; thermosetting polyester primer requires full crosslinking).

• Airflow: 3–4 m/s (promotes uniform temperature distribution and removes volatile organic compounds (VOCs) generated during curing).

Curing Quality Control

• Crosslinking degree: ≥90% (tested by solvent rub resistance—no primer removal after 50 rubs with acetone-soaked cotton).

• Color check: No yellowing or discoloration (ΔE ≤0.5 compared to standard sample).

• Surface state: Smooth, no bubbles, pinholes, or orange peel defects.

Polyester (PE) Top Coat Application

The PE top coat defines the appearance and surface performance of the appliance panel, supporting a variety of finishes to meet different design and functional requirements.

Coating Material Preparation

• PE coating type: Thermosetting saturated polyester resin (Tg value 50–70 °C) with high color retention and flexibility.

• Color formulation:

• Pigments: High-purity titanium dioxide (white), iron oxide (colored), or metallic pigments (aluminum powder for metallic finishes).

• Dispersion process: Sand mill grinding to pigment particle size ≤5 μm (ensures color uniformity and gloss consistency).

• Viscosity control: 25–35 s (DIN 4 mm cup at 25 °C) for optimal roll coating performance.

Precision Top Coat Roll Coating

• Equipment: Same reverse roll coater as primer, with dedicated coating rollers (different textures for matte/semi-gloss finishes).

• Thickness specification:

• Standard dry film thickness: 18–25 μm (total coating thickness: primer + top coat = 23–33 μm).

• Customization: 25–30 μm for high-wear surfaces (e.g., washing machine tops) or 15–18 μm for lightweight small appliances.

• Finish options for household appliances:

• Solid color (matte: 10–30 gloss units; semi-gloss: 60–80 gloss units).

• Silky texture: Achieved by textured coating rollers (reduces fingerprint visibility).

• Light embossing: For anti-slip or decorative effects (e.g., oven control panels).

Online Thickness & Gloss Monitoring

• Thickness: Beta-ray gauge (same as primer) with real-time data logging (tolerance ±1 μm).

• Gloss: Inline gloss meter (60° angle) to ensure batch consistency (gloss variation ≤5 units).

High-Temperature Curing for PE Top Coat

This stage completes the crosslinking reaction of the PE top coat, forming a hard, durable film with scratch resistance, chemical resistance, and color stability—key properties for household appliance use.

Sectional Curing Oven Process

• Equipment: Longitudinal sectional curing oven (length 15–20 m) with three zones:

• Preheating zone: 180–200 °C (gradually raises sheet temperature to avoid coating blistering).

• Curing zone: 220–240 °C (PMT, critical for crosslinking).

• Cooling transition zone: 150–180 °C (reduces thermal stress on the coating).

• Curing parameters:

• Total dwell time: 30–60 seconds (adjusted based on coating thickness; thicker coats require longer curing).

• Air circulation: Forced convection with filtered air (prevents dust contamination of the wet coating).

Curing Reaction & Performance Enhancement

• Chemical mechanism: Saturated polyester resin crosslinks with isocyanate curing agents to form a three-dimensional network structure.

• Post-curing performance:

• Hardness: ≥2H (pencil hardness test, GB/T 6739-2006).

• Scratch resistance: No visible scratch after 500g load with steel wool (10 strokes).

• Chemical resistance: No discoloration or peeling after 24-hour exposure to 5% sodium hydroxide solution (kitchen cleaning agent simulation).

Online Cooling & Quality Inspection

Rapid, uniform cooling prevents coating deformation and ensures dimensional stability of the aluminum sheet, while strict quality inspection eliminates defective products before composite bonding.

Forced Air Cooling System

• Configuration: Two-stage cooling:

• Primary cooling: High-velocity cold air (15–20 °C) blown from both sides of the sheet (air velocity 4–5 m/s) to reduce surface temperature to ≤60 °C.

• Secondary cooling: Ambient air cooling (dwell time 30 seconds) to bring the sheet to room temperature (≤35 °C).

• Cooling control: Avoids rapid temperature changes to prevent coating cracking or aluminum sheet warping.

Comprehensive Quality Inspection Items

Coating Thickness Uniformity

• Equipment: Portable magnetic induction thickness gauge (for non-conductive coatings on aluminum).

• Sampling: 5 points per sheet (center + 4 edges) with thickness variation ≤2 μm across the sheet.

Color Difference Detection

• Equipment: Spectrophotometer (D65 light source, 10° observer).

• Standard: ΔE ≤1.0 (batch-to-batch and within-sheet variation) to ensure consistent appliance appearance.

• Rejection criteria: ΔE >1.5 (defective sheets are marked and separated for rework).

Coating Adhesion Test (Cross-Cut Method)

• Test standard: GB/T 9286-1998 (equivalent to ISO 2409).

• Operation:

• Cut 1mm grid (10×10 squares) on the coating with a utility knife.

• Apply adhesive tape (3M 610) to the grid, press firmly, and peel off at 180°.

• Acceptance criteria: ≥4B (no more than 5% of the coating peels off) for general appliances; ≥5B (no peeling) for high-end products.

Surface Defect Inspection

• Method: Combination of automated visual inspection (AVI) and manual sampling.

• Automated inspection: Camera-based system with 0.1mm defect detection accuracy (identifies scratches, pinholes, bubbles, orange peel, and foreign particles).

• Manual inspection: 10% sampling rate to verify AVI results, focusing on edge areas and finish consistency.

Composite Sandwich Forming (ACM Production)

After the aluminum face sheets are coated and cured, they are bonded with the core material to form the final ACM panel—this stage requires precise control of temperature, pressure, and bonding time to ensure structural stability.

Preparation of Core Material & Adhesive

• Core material: PE core (density 0.92–0.94 g/cm³) or fire-retardant PE (FR-PE) core (density 1.1–1.3 g/cm³) for appliance safety.

• Adhesive: Thermosetting polyurethane (PU) adhesive film (thickness 50–80 μm) with high temperature resistance and flexibility.

• Cutting: Core material and adhesive film are cut to match the width of the coated aluminum sheets (tolerance ±2mm).

Continuous Hot-Press Bonding

• Equipment: Continuous double-belt hot press with temperature and pressure zoning control.

• Bonding parameters:

• Temperature: 120–140 °C (core melting and adhesive curing temperature).

• Pressure: 2.5–3.5 MPa (ensures tight bonding between aluminum sheets and core).

• Speed: 5–10 m/min (synchronized with the coating line).

• Bonding process:

• The bottom aluminum sheet → adhesive film → core material → adhesive film → top aluminum sheet are stacked in sequence.

• The stacked structure passes through the hot press, where heat and pressure activate the adhesive and fuse the core with the aluminum sheets.

Post-Bonding Flatness Correction

• Equipment: Roller leveler with 5–7 correction rollers.

• Purpose: Eliminate warpage caused by bonding stress, ensuring ACM panel flatness tolerance ≤0.5 mm/m (meets appliance panel assembly requirements).

• Inspection: Flatness is measured by a laser flatness gauge, with automatic adjustment of roller pressure for deviations.

Protective Film Lamination & Finishing

The final stage involves protecting the coated ACM panels during transportation and subsequent processing, as well as trimming them to the required dimensions for appliance manufacturing.

Protective Film Lamination

• Film specification: PE protective film (thickness 50–80 μm) with medium-low adhesion (180° peel strength 0.3–0.5 N/25mm).

• Lamination process:

• Automatic film laminator applies the film to both sides of the ACM panel, with no bubbles or wrinkles.

• Edge sealing: Film extends 5–10mm beyond the panel edges to prevent moisture and dust ingress.

• Film type options:

• Standard PE film: For general transportation and storage.

• Anti-scratch PE film: With 4H surface hardness (prevents damage during stamping and assembly).

• Anti-fingerprint film: For high-gloss panels (additional protection during appliance assembly).

Slitting & Cutting to Size

• Slitting: For large-width coils (e.g., 1220mm, 2440mm), a rotary slitter cuts the ACM panel into narrower widths (customizable from 300mm to 1200mm) based on appliance requirements.

• Cutting: For sheet products, a hydraulic guillotine cutter or CNC router cuts the panels to length (standard sizes: 1220×2440mm, 1500×3000mm; custom sizes available).

• Tolerance control: Dimension tolerance ±1mm (length and width) for mass production; ±0.5mm for high-precision appliance parts.

Final Packaging & Labeling

• Packaging: Export-standard packaging for appliance-grade ACM:

• Single panel: Wrapped in waterproof kraft paper + PE film.

• Stacked panels: Wooden pallets with moisture-proof layers, corner protectors, and strapping (max stack height 1.2m to avoid deformation).

• Labeling: Each package is labeled with product information (alloy grade, panel thickness, coating type, batch number, production date, and quality inspection certificate number) for traceability.

Pre-Delivery Inspection

• 100% dimensional inspection of finished ACM panels.

• Random sampling (5% of each batch) for re-testing of coating adhesion, color difference, and bonding strength (peeling strength ≥3.0 N/mm, GB/T 1450.2-2005).

• Only panels passing all inspections are released for delivery to appliance manufacturers.

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