Why Base Sheet Quality Defines Coating Board

In coating board production — whether for folding box board, white-lined chipboard, or solid bleached board — the base sheet determines 70–80% of final board quality. No coating formulation can fully compensate for a rough, poorly formed base sheet. Mills that invest in optimized stock preparation consistently achieve better coating holdout, lower coating consumption, and higher calendering efficiency.

1. Fiber Fractionation: The Core Strategy

Coating board typically uses a multi-layer structure, with different fiber fractions in each layer:

• Top liner: Bleached kraft pulp (softwood + hardwood blend) for brightness and smoothness. Typical freeness: 350–450 mL CSF after refining.
• Middle/filler layers: Mechanical pulp, broke, or recycled fiber. Freeness: 200–350 mL CSF.
• Back liner: Unbleached or lightly bleached kraft for strength. Freeness: 400–500 mL CSF.

Fractionation with pressure screens using 0.15–0.25 mm slots separates long and short fiber fractions. The long-fiber fraction (retained on screen) provides strength in middle layers; the short-fiber fraction (passed through) goes to top layers for improved smoothness and coating holdout.

A properly designed fractionation system achieves 20–35% long-fiber rejection with 3–5% consistency in the reject stream, ready for refining and recombination.

2. Refining for Coating Board Grades

Refining objectives differ by layer:

• Top liner refining: Gentle refining at 1.0–1.5 W·s/m SEL to fibrillate fiber surfaces without cutting — this improves bonding and reduces surface roughness. Net energy: 40–80 kWh/t for hardwood bleached kraft.
• Middle layer refining: More aggressive refining at 1.5–2.5 W·s/m SEL to develop bulk and stiffness while maintaining drainage. Target freeness drop of 100–150 mL CSF.
• Back liner refining: Moderate refining at 1.2–1.8 W·s/m with softwood to achieve strength targets.

Disc refiners with fine-bar plates (1.5–2.0 mm bar width, 2.5–3.5 mm groove width) produce the surface fibrillation needed for coating holdout. Avoid over-refining — fines content above 20–25% causes coating penetration problems and reduces sheet gloss.

3. Screening & Cleaning for Surface Quality

Any contaminant in the base sheet becomes amplified after coating. A speck or shive that measures 0.3 mm becomes a visible 0.5+ mm defect after a 15–20 g/m² coating layer is applied.

• Pressure screens: 0.20–0.30 mm slots in primary position for top layer stock. Reject rates of 12–18% to secondary slotted screens (0.15–0.20 mm).
• Cleaners: Forward cleaners for heavy contaminants, reverse cleaners or lightweight cleaners for stickies and low-density debris — critical when using recycled fiber in middle layers.
• Machine screen (approach): The final barrier — 0.15–0.20 mm slots or 1.0–1.2 mm holes with minimum pulsation. A machine screen with controlled-velocity rotor and carefully matched accept/reject piping diameter prevents pressure surges that cause formation problems at the headbox.

4. Broke Handling: Consistency Is Key

Coated broke (trim, reject rolls, start-up waste) returns coating pigments to the stock system. Key considerations:

• Dedicated broke pulper: Coated broke requires 15–20 minutes pulping time at 4–6% consistency for complete fiberization. Pigment particles are inert but can accumulate if the water system isn’t balanced.
• Broke screening: 1.6–2.0 mm holes remove any coating lumps or undispersed fiber bundles before the broke joins the stock system.
• Consistency control: Broke chest consistency must be tightly controlled (±0.2% of setpoint) because broke represents 10–35% of total furnish. Variations here directly affect basis weight control at the headbox.

5. Water Quality & White Water Management

Coating board mills operate with partially closed water systems. Dissolved solids, particularly calcium from coating pigments, can accumulate and cause:

• Deposits on screens and headbox surfaces
• Reduced retention aid efficiency
• Microbiological growth in the stock system

Maintain conductivity below 3,000–4,000 µS/cm in the short circulation. A controlled fresh water make-up rate of 2–5 m³/t combined with save-all filtration (disc filter or DAF unit) achieves this balance.

Key Quality Parameters: Base Sheet vs. Coated Board

Talk to a Coating Board Equipment Specialist

From fractionation screens to refiner plate selection, the right equipment configuration makes a measurable difference in your coating board quality and coating efficiency. For technical guidance specific to your mill’s production goals, contact us.

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