Why Coating Quality Starts in the Stock Preparation Room

When a coated board sheet shows mottling, poor print gloss, or coating strike-through, the natural instinct is to adjust the coater or reformulate the coating color. But in many cases, the root cause lies upstream — in stock preparation, where fiber treatment, filler distribution, and sheet formation establish the foundation that coating must build upon.

Coating holdout — the ability of the base sheet to keep coating solids on the surface rather than absorbing them into the sheet — is primarily determined by sheet density, surface pore structure, and internal sizing, all of which are set during stock preparation and forming. This article examines the key stock preparation parameters that influence final coated board quality.

1. Fiber Fractionation for Multi-Ply Board Construction

Coated board grades (folding boxboard, white lined chipboard, solid bleached sulfate board) use multi-ply construction to optimize cost and performance. The top ply — which receives the coating — demands short, well-fibrillated fibers that form a dense, smooth surface. The middle and back plies can use lower-cost fibers with higher freeness.

Fiber fractionation using pressure screens with slotted baskets (0.15–0.35 mm slot width) separates short fibers (top ply) from long fibers (bulk plies). This separation must achieve at least 70% classification efficiency to provide meaningful surface improvement. Poor classification forces the top ply to carry long fibers that create surface roughness and increase coating demand.

2. Refining the Top Ply: Fibrillation Without Shortening

Top-ply refining for coated board must develop bonding area while preserving enough fiber length to maintain ply-bond strength. This is achieved through low-intensity refining with disc or conical refiners operating at specific edge loads of 1.0–1.8 W·s/m.

Target freeness for the top ply is typically 250–350 mL CSF for hardwood and 350–450 mL CSF for a softwood/hardwood blend. Going below 200 mL CSF on the top ply increases water retention and slows drainage, which can reduce machine speed. But going above 400 mL CSF creates a porous sheet with poor coating holdout.

Monitoring recommendation: Measure top-ply freeness at least once per shift. Correlate freeness trends with coater runnability data (blade load, coat weight variability) to establish mill-specific target ranges.

3. Formation Quality and Its Impact on Coating Uniformity

Sheet formation — the uniformity of fiber distribution at the small scale — directly determines how evenly coating is applied. A poorly formed sheet has alternating dense and open areas; coating penetrates the open areas (strike-through) and sits on top of dense areas (creating roughness). The result is visual mottle and variable print gloss.

Formation is controlled by headbox consistency, jet-to-wire ratio, and drainage element configuration. For coated board top plies, headbox consistency of 0.4–0.8% and a jet-to-wire ratio of 0.98–1.02 promote random fiber orientation and uniform formation. Modern headboxes with dilution control profiling (40–60 mm actuator spacing) can reduce basis weight CD variation to below 1.0% coefficient of variation.

4. Internal Sizing and Pore Structure Control

Internal sizing — typically AKD (alkyl ketene dimer) or ASA (alkenyl succinic anhydride) — is essential for controlling coating holdout. Without adequate sizing, the aqueous phase of the coating penetrates the base sheet, carrying binder with it and leaving pigment-starved coating on the surface.

AKD dosage of 1.0–2.5 kg/t (active) with proper retention (cationic starch or polyacrylamide retention aid) typically provides sufficient holdout for single-coated board. Double-coated grades may require slightly higher levels. ASA offers faster curing (on-machine sizing development) at similar dosage levels but requires on-site emulsification equipment.

Troubleshooting tip: If coating holdout suddenly deteriorates, check AKD/ASA dosage, retention aid flow, and system pH before adjusting the coater. A 0.5 pH unit shift can destabilize sizing performance.

5. White Water Management and Fines Balance

Fines — particles passing a 200-mesh screen (76 µm opening) — play a dual role in coated board production. They fill voids between fibers, improving surface smoothness and coating holdout. But excessive fines accumulation in the white water system increases drainage resistance and steam demand.

The optimal fines content in the top ply is 25–35% (by weight). This is managed through a combination of refining control, save-all operation, and white water clarification. Disc save-alls or dissolved air flotation (DAF) systems recover fines and filler for reuse while controlling white water solids below 500 mg/L.

From Stock Prep to Coating Performance

Coating board producers who invest in precise stock preparation — fractionation, controlled refining, formation optimization, and internal sizing — consistently achieve higher coating holdout, better print gloss, and lower coating consumption than mills that treat stock prep as an afterthought.

The equipment selection for stock preparation — pressure screens, refiners, headboxes, and white water recovery systems — must be sized and configured for the specific basis weight range and speed targets of the board machine. Undersized screening or refining capacity creates bottlenecks that force operators to compromise on quality to maintain production.

📧 Contact us for a technical consultation on stock preparation equipment for your coated board machine: leizhanzhang@gmail.com