Coating Board Paper Making Line: Multi-Ply Forming and Coating Technology
Understanding the Coating Board Market and Production Requirements
Coating board — including folding boxboard (FBB), solid bleached sulfate (SBS), coated recycled board, and white lined chipboard (WLC) — serves the premium packaging segment for consumer goods, pharmaceuticals, cosmetics, and food products. A coating board paper making line represents one of the most capital-intensive investments in the paper industry, requiring sophisticated multi-ply forming technology and precision coating equipment.
The distinguishing feature of coating board production is the multi-layer structure. A typical three-ply board consists of a bleached top liner for printing surface quality, a middle layer of mechanical or recycled pulp for bulk and stiffness, and a back liner that may be bleached or unbleached depending on end-use requirements. Basis weights range from 180 to 600 g/m², with caliper from 0.3 to 1.0 mm.
Multi-Ply Forming Technology
Multi-ply forming is the core technology that differentiates coating board machines from single-ply paper machines. Leizhan Technology engineers forming sections using either multi-fourdrinier configurations or cylinder mould (vat) formers, depending on the product grade and customer requirements.
Multi-Fourdrinier Forming
In this configuration, each ply is formed on a separate fourdrinier wire and then couched together before entering the press section. A typical three-ply machine uses:
- Top wire: Short fourdrinier (8–14 m forming length) for the bleached top liner, basis weight 30–80 g/m². Headbox consistency 0.2–0.5%, producing a sheet with excellent formation for printing surface quality.
- Core wire: Standard fourdrinier (14–22 m forming length) for the middle layer, basis weight 80–300 g/m². This layer provides the bulk and stiffness that define board performance. Headbox consistency 0.5–1.0%. Filler content is minimized (0–5%) to maximize bulk.
- Bottom wire: Short to standard fourdrinier (10–18 m forming length) for the back liner, basis weight 30–80 g/m².
The plies are combined on the core wire at a consistency of 8–12%, using a suction pickup or transfer roll arrangement. Proper ply bonding requires careful control of moisture at the ply interface — too dry and the plies won’t bond; too wet and delamination can occur at the press or during coating.
Cylinder Mould Forming
For recycled board grades where using multiple cylinder moulds in series builds up the sheet thickness, Leizhan supplies cylinder mould formers with 1.0–2.0 m diameter cylinders operating in a vat with stock consistency of 0.2–0.6%. Each cylinder contributes 30–80 g/m² per revolution. A typical machine uses 4–8 cylinders in series, with the top and bottom cylinders producing the liner plies from higher-quality stock, while inner cylinders form the filler plies from lower-cost furnish.
Coating Kitchen and Application Technology
The coating section transforms base board into a premium printing surface. A coating kitchen prepares coating color — a complex formulation of pigments, binders, and additives — which is then applied to the board surface using precision coating heads.
Coating Color Formulation
A typical pre-coating and top-coating formulation includes:
| Component | Pre-coat (parts) | Top-coat (parts) | Function |
|---|---|---|---|
| Coarse GCC (ground calcium carbonate) | 70–90 | 0–20 | Bulk, coverage, cost reduction |
| Fine GCC or PCC | 0–20 | 40–70 | Brightness, smoothness |
| Clay (kaolin) | 10–30 | 20–40 | Gloss, ink holdout, smoothness |
| SBR latex binder | 8–12 | 10–14 | Adhesion, flexibility, pick resistance |
| Co-binder (starch, CMC, PVOH) | 2–4 | 0.5–2 | Water retention, rheology |
| Dispersant (polyacrylate) | 0.1–0.3 | 0.1–0.3 | Pigment dispersion stability |
Coating solids: pre-coat at 60–66%, top-coat at 62–68%. Viscosity: 800–1,500 mPa·s (Brookfield, 100 rpm). Coat weight: pre-coat 8–15 g/m² per side, top-coat 8–12 g/m² per side.
Coating Application Methods
- Blade coating: The dominant technology for board grades. A bent steel blade (0.3–0.5 mm thickness) meters the coating applied in excess by a roll or jet applicator. Produces the smoothest surface. Leizhan blade coaters operate at 300–900 m/min with coat weight uniformity within ±5% across the width.
- Rod/Metering bar coating: A rotating or stationary wire-wound rod meters the coating. Produces a contour coating that follows the base sheet topography — ideal for pre-coating where coverage of surface roughness is more important than absolute smoothness.
- Air knife coating: An air jet meters the coating. Used for specialty grades but less common in high-speed board production due to energy and splash issues.
- Curtain coating: A falling curtain of coating color contacts the web without physical metering element contact. Enables single-pass multi-layer coating application. Growing in adoption for premium board grades.
Drying After Coating
- IR dryers: Electric or gas infrared dryers (2–6 units per coating station) providing rapid, non-contact initial drying. Power: 100–300 kW per meter of width.
- Air flotation dryers: Hot air (150–300°C) impinging on both sides of the web while it floats on air cushions. Typical length: 8–20 meters per coating station.
- Cylinder dryers: Steam-heated cylinders (1.5–1.8 m diameter) in the after-drying section, bringing final moisture to 5.5–7.5%.
Machine Technical Parameters and Production Data
| Parameter | Small Line | Mid-Size Line | Large Line |
|---|---|---|---|
| Production capacity (TPD) | 100–200 | 300–500 | 600–1,200 |
| Trim width (m) | 2.2–3.2 | 3.6–4.8 | 5.0–7.0 |
| Design speed (m/min) | 200–400 | 400–600 | 600–900 |
| Basis weight range (g/m²) | 180–450 | 200–550 | 200–600 |
| Number of plies | 2–3 | 3–4 | 3–5 |
| Coating stations | 1–2 | 2–3 | 3–4 |
| Total installed power (MW) | 4–8 | 10–18 | 20–40 |
Energy and Resource Consumption
Coating board production is energy-intensive due to the multiple forming sections, extensive drying requirements, and coating operations. Benchmark data:
- Total electrical energy: 550–850 kWh/tonne for a complete line from stock preparation through coating and finishing
- Steam consumption: 1.8–2.6 tonnes per tonne of board, with the dryer section accounting for 80–90% of steam use
- Coating color loss: 3–8% of prepared coating color, recoverable through in-line screening and reuse systems
- Water consumption: 10–20 m³/tonne; closed water circuits can reduce fresh water intake to 5–8 m³/tonne
- Broke generation: 5–12% total broke, including edge trim (2–5%), coating breaks (1–3%), and finishing waste (2–4%). All broke is re-pulped and returned to the appropriate ply layer.
Maintenance Schedule for Coating Board Lines
The coating section adds significant maintenance complexity. Blade wear, coating color contamination, and dryer deposition require structured attention:
Daily and Per-Shift Routines
- Coating blade inspection at every reel change — check for wear streaks, edge damage, and coating streaks on the web
- Coating color sieve (screen) inspection — clean or replace plugged screens; typical mesh: 100–150 μm
- IR dryer lamp inspection — replace failed lamps; check for coating mist buildup on reflectors
- Dryer fabric (felt/wire) tracking verification — misalignment causes permanent creasing in coated board
- Calender roll surface check for picking or deposits — clean with solvent wipes if needed
Weekly Maintenance
- Coating head disassembly and thorough cleaning — remove dried coating buildup from all surfaces
- Coating supply system flush — circulate warm water with cleaning agent through all lines, filters, and screens
- Air flotation dryer nozzle inspection — check for blockage, warping, or flame impingement on gas-fired units
- Backing roll surface inspection — measure diameter across width; regrind if variation exceeds 0.05 mm
Monthly Maintenance
- Coating kitchen mixer and disperser inspection — blade wear check, tank cleaning, scale removal
- Coating color viscosity and solids calibration — verify inline measurement accuracy against lab standards
- Dryer can syphon clearance check — accumulated coating dust can affect heat transfer
- Winder slitter blade replacement and anvil inspection
Quarterly Maintenance
- Complete coating blade holder alignment and beam straightness verification
- All coating pump mechanical seal inspection
- Dryer fabric tension system verification — load cell calibration
- Reel drum bearing inspection and lubrication
Annual Shutdown (12–18 Days)
- All coating heads complete rebuild — replace wear strips, backing roll covers, blade holders, and pneumatic/hydraulic components
- Coating kitchen complete clean-out — descaling of all tanks and piping; replace worn mixer blades and pump impellers
- Dryer fabric replacement (fabric life: 180–365 days for coating dryers)
- IR dryer complete cleaning and lamp replacement
- Air flotation dryer internal inspection and nozzle alignment
- Calender roll grinding and bearing replacement
- Multi-ply couching roll regrinding
- All forming wires replacement
- Headbox internal cleaning and slice lip inspection
- Gearbox oil replacement across entire machine
A Leizhan coating board paper making line integrates proven multi-ply forming technology with advanced coating application systems, delivering the print quality, stiffness, and consistency that premium packaging demands. Our engineering team works with customers from concept through commissioning to ensure every component meets production requirements.
Contact us at leizhanzhang@gmail.com | Zhengzhou Leizhan Technology Paper Machinery Co., Ltd.