The journey from raw pulp to a finished coated board sheet involves multiple interdependent process stages. Every decision made in the wet end, press section, and drying section directly affects how the coating will perform — and ultimately what grade of finished product the mill can sell. Understanding these linkages is what separates mills that consistently produce premium coated board from those that struggle with quality claims.

Wet End Chemistry for Coatable Base Sheet

The foundation of a good coated board is a well-formed base sheet with controlled absorption characteristics. Internal sizing levels must be carefully managed: too much sizing (Cobb values below 20 g/m²) prevents adequate coating adhesion and causes picking during printing; too little sizing (Cobb values above 35 g/m²) causes excessive coating penetration and binder depletion at the surface.

For multi-ply board, the top ply — which will receive the coating — should use refined chemical pulp with freeness of 35-45°SR, while the middle and back plies can use higher-freeness mechanical or recycled fibre (25-35°SR) to optimize bulk and bending stiffness. This layered structure is the key to achieving the stiffness-to-grammage ratio that packaging converters demand.

Formation and Surface Smoothness

Base sheet formation directly correlates with coating quality. A well-formed sheet (formation index above 80 on the Ambertec scale) provides a uniform substrate for coating. Poor formation creates localized density variations that cause uneven coating absorption and visible mottling in the finished product.

The wet pressing section must deliver 42-48% dryness before the pre-dryer section. Higher dryness reduces drying energy and also compacts the surface fibres into a smoother substrate. Sheet surface roughness (PPS-10) should target below 6.0 μm before the first coating station — achievable through a combination of good formation, optimized press loading (80-120 kN/m), and surface sizing where required.

Drying Section Configuration

Pre-dryers before the coating station serve two purposes: removing water and developing internal sizing through heat-activated reactions (for AKD or ASA sizing systems). The pre-dryer temperature profile should ramp from 60-80°C on the first cylinders to 110-130°C on the final cylinders before the size press or first coater, with a web exit temperature of 55-70°C.

After each coating application, IR dryers and air flotation dryers working in sequence provide non-contact drying that preserves the wet coating layer’s surface integrity. Post-coating after-dryers complete moisture removal to the final 5-7% target.

Process Integration and Quality Loops

• Basis Weight Control: Cross-machine basis weight variation should be maintained within ±1.5% of target. Modern QCS systems with dilution profiling headboxes can achieve ±0.8%.
• Moisture Profile: Pre-coater moisture should be 5-6% with cross-machine variation under ±0.5%. Uneven moisture causes uneven coating pickup.
• Caliper Control: Calendered base sheet caliper variation exceeding ±2% will telegraph through to the coated product. Hot soft-nip calenders with zone-controlled rolls provide the best profile control.
• Coating Weight Automation: Online coat weight sensors with feedback control to blade loading or metering element position maintain the target within ±0.5 g/m² in modern installations.

Coating board making is not just about the coating station — it is about building quality into the base sheet at every stage of the process. Mills that treat base sheet preparation and coating as an integrated system consistently outperform those that manage them as separate operations.

For technical consulting on optimizing your coating board making line from stock preparation through to finished product, contact our team.

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