Coating Board Making: Layer Formation and Machine Configuration Guide
Why Multi-Layer Formation Matters in Coating Board
Modern coating board machines produce 2, 3, or 4 layers in a single forming process. Each layer serves a distinct function — top liner for printability and brightness, middle layers for bulk and stiffness, back liner for strength. Achieving uniform formation across all layers simultaneously is one of the most demanding tasks in papermaking. Poor layer-to-layer bonding, uneven basis weight distribution, or inter-layer contamination can render a full production run unusable.
1. Headbox Technology for Multi-Ply Board
The headbox is the critical component that determines layer quality. For coating board making, two configurations dominate:
- Multi-channel headbox with single slice: 2–4 separate stock channels converge inside the headbox, exiting through a single adjustable slice (8–18 mm opening). This provides the best layer-to-layer adhesion because plies contact each other before the jet hits the forming fabric. The downside: fiber mixing at layer boundaries can contaminate bright top plies with darker middle-layer stock — controlled by maintaining higher jet velocity in the top layer channel (3–5% faster than middle layer).
- Separate headboxes with on-wire ply bonding: Individual mini-fourdriniers or hybrid formers apply each layer sequentially. Allows complete independence of layer stock preparation and jet-to-wire ratios. The risk is poor ply bonding if the first layer’s surface becomes too dry before the next ply is applied — maintain first-ply moisture above 88% at the ply-bonding point.
2. Forming Section Design
Forming section configuration depends heavily on production speed and grammage range:
- Fourdrinier with top former: The workhorse for coating board at 200–600 m/min and 200–600 gsm. A fourdrinier table with 8–12 drainage elements (foils + vacuum boxes) forms the base; a top former (hybrid or mini-fourdrinier) applies the top liner. The base sheet must reach 6–8% dryness before the top former applies its layer.
- Gap former configuration: At higher speeds (500–1,000 m/min), gap formers with two fabrics provide superior dewatering symmetry. Both sides of the sheet drain simultaneously, reducing two-sidedness — important because coating is typically applied to the top side only, and any asymmetry in base sheet surface properties becomes visible after calendering.
- Cylinder mold machines: For thick boards (>600 gsm) at lower speeds, multiple cylinder molds apply successive layers. Each mold operates at 6–12 m/min peripheral speed. While older technology, they remain cost-effective for high-bulk, lower-speed production.
3. Critical Process Parameters
| Parameter | Recommended Range | Impact of Deviation |
|---|---|---|
| Jet-to-wire ratio (each layer) | 0.97–1.03 | ±0.05 causes fiber orientation problems, affecting stiffness profile |
| Headbox consistency (total) | 0.5–1.2% | Lower consistency improves formation but increases white water load |
| Dewatering rate (foil angle progression) | 0.5° → 1.0° → 2.0° → 3.0° | Too aggressive early dewatering seals the sheet, causing poor drainage |
| Vacuum box level (max) | 15–25 kPa | Excessive vacuum marks the sheet and increases drive load |
| Sheet dryness at ply bonding | 88–92% moisture (8–12% dryness) | Below 88%: good bonding but risks layer mixing; Above 92%: risk of delamination |
4. Press Section for Multi-Ply Board
The press section must dewater the thick multi-ply sheet without causing delamination or crushing. Key design considerations:
- Gradual pressure application: A triple-nip press with increasing loads (60 → 90 → 120 kN/m) prevents sheet crushing. The first nip must remove water gently — high initial pressures trap water in the sheet center.
- Double-felting for middle-density board: Boards below 450 gsm can use single-felted nips. Heavier boards (>450 gsm) require double-felted first and second nips to provide water escape paths from both sides.
- Shoe press for final nip: An extended-nip shoe press (200–250 mm nip length) at 1,000–1,200 kN/m line load in the third position maximizes dryness while preserving bulk — critical for coating board where caliper is a customer specification.
5. Post-Forming: Sizing and Pre-Coating
Before the main coating stations, the base sheet typically receives:
- Surface sizing: A size press or film press applies starch solution (4–8% solids) to seal the surface and reduce coating penetration. Pickup: 1.5–2.5 g/m² per side.
- Pre-coating: A single blade coater applies a thin pre-coat (8–15 g/m²) that fills surface voids and fibers, creating a smoother substrate for top coats. Pre-coat solids: 58–65%.
6. Drive and Tension Control
Multi-ply board at heavy grammages exerts significantly higher web tension than lighter grades. Drive sections between each forming unit, press, and dryer group must maintain precise speed matching (±0.1% draw) to prevent sheet breaks and ply delamination. AC vector drives with load-sharing algorithms handle the wide torque range needed across different production speeds.
Get Support for Your Coating Board Production Line
Whether you are commissioning a new coating board machine or retrofitting an existing line, equipment configuration — headbox selection, forming section design, and press configuration — significantly impacts your product quality and operating costs. For technical guidance, reach out to our team.
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