The quality of coated board begins with the base sheet. No coating formulation — no matter how advanced — can compensate for a base paper with poor formation, high surface roughness, or excessive two-sidedness. Stock preparation and wet-end screening are the critical upstream processes that determine whether your coating section delivers a premium surface.

What Coating Demands from the Base Sheet

Coating board grades — folding boxboard, white-lined chipboard, solid bleached sulfate board — require base sheets with:

• Smoothness (Bendtsen): <300 mL/min before coating, ideally <150 mL/min for single-coated grades
• Formation uniformity: Low basis weight variation (<2.5% coefficient of variation) to prevent coating skip and mottling
• Internal bond strength: >250 J/m² Scott bond to withstand blade coating forces and printing stresses
• Low porosity: <1,200 mL/min Gurley to control coating holdout and minimize binder migration
• Dimensional stability: Minimal CD shrinkage during drying to prevent curl and register issues in converting

Screening: The Gatekeeper of Coating Base Quality

Contaminants in the base sheet — even those invisible to the naked eye — become magnified after coating. A single shive or stickie particle 200 μm in diameter can cause a coating blade streak that runs the entire reel length. For coating board grades, the screening approach must be more aggressive than for uncoated grades:

• Primary pressure screen slots: 0.15–0.25 mm for top liner stock; 0.20–0.30 mm for filler/middle plies
• Screening consistency: 0.8–1.5% in approach flow system (after machine chest dilution)
• Reject rate: 10–15% for top liner screening to ensure effective contaminant removal without excessive recirculation
• Basket design: Wedge-wire or C-bar profile baskets provide the best combination of slot accuracy and hydraulic capacity

Refining Strategy for Coating Base: Different Goals for Different Plies

Multi-ply coating board requires differentiated refining strategies:

• Top liner (bleached kraft / high-white furnish): Moderate refining at SEL 0.8–1.5 J/m to develop surface smoothness and internal bond. Target freeness reduction: 40–80 mL CSF. Over-refining creates excessive fines that reduce bulk and increase drying load
• Under liner / middle plies (mechanical pulp / broke): Light refining or no refining — these plies provide bulk and stiffness. Excessive refining reduces caliper and increases steam consumption with minimal quality benefit
• Back liner: Similar to top liner if the board is to be printed on both sides. For one-side coated board, minimal refining is acceptable

Approach Flow Stability: The Hidden Quality Factor

Flow pulsations in the approach system directly cause basis weight variations. For coating board, where MD basis weight variation as low as 1.5% can cause visible coating non-uniformity:

• Fan pump pulsation: Should not exceed ±1.5% of discharge pressure at vane-passing frequency
• Machine screen pulsation: Air-loaded or hydraulic pulsation dampening systems should be maintained to keep pressure variation <0.5%
• Headbox consistency: 0.3–0.8% depending on grade — lower consistency promotes better formation but increases dewatering load

Fractionation for Multi-Ply Board

For top liner quality, fiber fractionation is a powerful tool. Separating the furnish into long-fiber (>1.2 mm) and short-fiber (<0.8 mm) fractions allows:

• The long fiber fraction to be refined specifically for the top liner ply, achieving maximum surface strength
• The short fiber fraction to be directed to the under liner, improving formation and smoothness without fiber cutting
• Overall energy savings of 10–18% compared to refining the full furnish stream

Key Equipment Components for Coating Board Stock Prep

An optimized stock preparation line for coating board includes:

• Pulping system with extraction holes 8–10 mm (virgin) or 6–8 mm (recycled)
• High-density cleaner (3.5–4.5% consistency) for heavy contaminant removal before screening
• 2- or 3-stage pressure screening cascade with slot baskets 0.15–0.25 mm for top liner
• Disc or conical refiners with fillings selected for the specific furnish type
• Disc thickener or screw press for final consistency control at 3.5–4.5% before the machine chest

The difference between a coating board that runs smoothly through the coater and one that causes constant blade changes and quality claims often traces directly to stock preparation decisions made months or years earlier.

📧 Contact us at leizhanzhang@gmail.com for coating board stock preparation equipment consultation.