The Machine Configuration Challenge

Building a coating board machine is fundamentally different from building a packaging paper machine. The multi-ply structure — typically 3 to 5 plies — requires separate stock preparation lines, individual forming sections, and careful ply bonding before the sheet ever reaches the coating stations. Machine configuration decisions made at the design stage determine productivity, quality, and operating cost for decades.

This guide walks through the key configuration decisions for coating board making, from forming section layout through drying and coating.

1. Multi-Ply Forming Section Layouts

Coating board machines use one of three forming configurations:

Cylinder mold (vat) formers — the traditional choice, with each ply formed on a rotating cylinder mold in a vat of stock. Advantages include natural ply bonding and low energy consumption. Limitations include speed ceilings (typically below 400 m/min) and basis weight profile control challenges.

Fourdrinier + secondary formers — a top Fourdrinier wire forms the base ply, with secondary forming units (mini-Fourdriniers or hybrid formers) depositing additional plies on top. This configuration supports higher speeds (up to 800 m/min) and better formation control.

Gap former + multilayer headbox — for premium coated board, a gap former with a multilayer headbox can produce a 3-ply sheet in a single forming zone. This compact design reduces machine length but places higher demands on headbox design and stock preparation uniformity.

The choice depends on target production volume, basis weight range, and fiber furnish flexibility requirements.

2. Press Section Design for Ply Bond and Bulk Preservation

The press section in coating board making must achieve two sometimes-contradictory goals: maximum ply-bond strength (through intimate fiber contact at ply interfaces) and bulk preservation (to meet caliper specifications with minimal fiber).

Shoe presses and extended-nip presses are increasingly common because they apply higher peak pressure (800–1,200 kN/m) with longer residence time, improving ply bonding without crushing the sheet. Typical press dryness targets after the press section are 48–52% solids — going higher than 54% can reduce bulk by 3–5%, while lower than 46% increases dryer steam demand substantially.

Double-felted first press nips and single-felted subsequent nips optimize water removal while maintaining surface smoothness on the top (coated) side.

3. Dryer Section Configuration and Energy Strategy

Board machines have long dryer sections because the high basis weight (typically 200–500 g/m² for coated board) and multi-ply structure resist heat transfer. Multi-cylinder dryers arranged in groups or tiers are standard.

Key dryer section parameters:

• Specific drying rate: 15–25 kg H₂O/m²·h for multi-cylinder sections
• Dryer cylinder surface temperature: 110–160°C, increasing from wet end to dry end
• Pocket ventilation: Supply air at 90–110°C with humidity ratio below 0.15 kg/kg
• Steam consumption: 1.3–1.6 kg steam per kg water evaporated (with efficient condensate removal)

Heat recovery from dryer hood exhaust (typically 75–85°C, 0.12–0.18 kg/kg humidity) can preheat combustion air, white water, or building ventilation, recovering 15–25% of the energy that would otherwise be discharged.

4. Coating Station Configuration and Placement

Coating board machines typically include 1–3 coating stations depending on quality requirements. Common configurations:

• Single coating (pre-coat only): One blade or rod coater applying 8–15 g/m² per side. Used for basic coated board where functional performance (not print gloss) is the priority.
• Double coating (pre-coat + top coat): Pre-coat of 8–12 g/m² fills surface voids; top coat of 8–10 g/m² delivers print gloss and smoothness. This is the standard for folding boxboard and carrier board.
• Triple coating: Pre-coat + mid-coat + top coat for premium grades requiring very high print fidelity. Total coat weight can reach 30–35 g/m² per side.

Coating stations are positioned after sufficient drying — typically at 90–95% dry solids before the first coater. Applying coating to a sheet that is too wet causes binder migration and coating defects. Infrared or air-float dryers between coating stations provide non-contact drying that preserves surface quality.

5. Calendering and Reel Building

Soft-nip or multi-nip calenders with heated rolls (120–180°C) provide the final surface finish before reeling. Calender nip load (typically 80–200 kN/m) and roll temperature are the primary levers for gloss and smoothness. Higher temperature at lower pressure generally produces better gloss with less bulk loss than high pressure at lower temperature.

The reel must handle large-diameter parent rolls (up to 3.5 m diameter on modern machines) with consistent tension control to avoid starring (internal bursts caused by winding tension exceeding sheet strength).

Designing for the Full Production Window

Coating board machine configuration is not a one-size-fits-all decision. The forming section, press, dryers, coaters, and calender must be matched to the intended product mix — basis weight range, number of plies, coating layers, and speed targets.

Equipment that provides flexibility — such as adjustable forming zone lengths, interchangeable coating application heads, and variable-speed dryer groups — allows the mill to shift product mix without expensive rebuilds. When evaluating suppliers, look beyond individual component specifications to the integrated performance of the entire line.

📧 Discuss your coating board machine project with our engineering team: leizhanzhang@gmail.com