Surface Quality Starts in Stock Preparation

Coating board buyers judge quality by surface smoothness, brightness, and printability. While the coating kitchen and coater station receive the most attention, the stock preparation system sets the foundation. Fiber length distribution, fines content, ash retention, and contaminant levels entering the approach flow system all determine how uniformly the coating layer bonds to the base sheet.

Inconsistent fiber treatment—pockets of over-refined and under-refined fiber—creates an uneven base sheet surface that no amount of coating can fully compensate for. The result is mottling, poor ink holdout, and in extreme cases, coating layer delamination during printing or converting.

Fiber Refining for Coating Board

Coating board generally requires a combination of softwood for strength and hardwood for smoothness. Refining strategy must balance these opposing requirements:

• Softwood (30–40% of furnish): Refine to 28–35°SR—higher refining develops fiber bonding and internal strength but increases water retention value (WRV), slowing drainage
• Hardwood (40–50% of furnish): Refine lightly to 22–28°SR—excessive refining shortens hardwood fibers and creates excessive fines that hurt drainage
• Broke (10–30%): Re-refine conservatively; broke fibers have already been refined and dried once; target 30–35°SR maximum

Leizhan’s double-disc refiner (Model ZDP series) is well-suited for coating board stock preparation, offering independent control of each disc’s gap with a servo-hydraulic positioning system. This allows precise adjustment on both softwood and hardwood lines from a common control platform. Typical no-load power for a 450 kW refiner installation is below 30 kW—a key efficiency metric that directly affects the mill’s power bill.

Fines Management and Fractionation

The fines fraction (fiber material passing a 200-mesh screen, nominally <76 μm) strongly affects both drainage and sheet surface. Fines content above 25% in the headbox stock slows drainage and creates a dense, low-porosity sheet that resists coating penetration. Below 15%, the sheet surface becomes too open, and coating binder migrates into the sheet rather than staying at the surface.

Leizhan’s approach uses a combination of screening and fractionation to manage fines:

• A fines separation screen with 0.15 mm slot basket diverts the fines-rich stream to the ply that forms the sheet surface
• A disc thickener concentrates the long-fiber fraction for the base ply, where strength properties matter more than surface characteristics

This stratified approach to fines distribution enables the top ply to achieve Parker Print Surf (PPS-10) roughness below 1.5 μm, while the base ply maintains the bulk and stiffness the board requires.

Contaminant Removal for Coating Compatibility

Stickies and hot-melt contaminants from recycled fiber are especially damaging in coating board. A sticky particle as small as 50 μm can break through the coating layer and deposit on a calender roll, picking a hole in the sheet every revolution. The screening and cleaning system must target removal down to this size range:

• Fine slot screening: 0.15 mm slots for 100% throughput, removing shives and fiber bundles
• Reverse cleaners: Operating at 200–250 kPa pressure drop with lightweight reject removal. A light reject rate of 6–10% of feed achieves >85% stickies removal efficiency
• Hot dispersion: A disc disperser (Leizhan Model ZDR) operating at 80–95°C and 30–35% consistency reduces visible stickies below 500 mm²/kg—below this threshold, they no longer cause coating defects

System Monitoring and Quick Checks

• Freeness trending: Measure freeness every 2 hours during production. A drifting freeness value often signals refiner plate wear, not a stock change.
• Slot velocity verification: Measure pressure drop across the fine screen once per shift. A rising delta-P at constant throughput indicates basket blinding—symptom of stickies accumulation or chemical deposit formation.
• Stickies count: Weekly lab analysis of accept stock using a Pulmac MasterScreen or equivalent. Trending upward? Check disperser temperature and disc gap.
• Cleaner reject inspection: Once per shift, take a 1-liter sample from the tertiary cleaner reject line. Visible fibers indicate the secondary or tertiary stage is overloaded—increase reject rate at the primary stage to protect the cascade.

For more information on Leizhan’s stock preparation equipment for coating board production—including disc refiners, pressure screens, and dispersion systems—reach out to our application engineering team.

Contact Leizhan for Your Paper Machinery Needs

Zhengzhou Leizhan Technology Paper Machinery Co., Ltd. supplies complete stock preparation systems and paper machine equipment for tissue, kraft liner, coating board, and corrugated medium production worldwide. For technical consultation or equipment inquiries, reach our team at:

📧 leizhanzhang@gmail.com