Coating board — including folding boxboard (FBB), white lined chipboard (WLC), and solid bleached sulfate board (SBS) — serves the premium packaging market where print quality, stiffness, and surface properties directly influence brand perception at the retail shelf. A coating board paper making line combines multi-ply forming technology with sophisticated coating systems to create products whose surface quality rivals fine printing papers while delivering the stiffness and strength demanded by packaging applications.

The technical complexity of coating board production spans fiber selection for individual plies, multi-headbox forming, coating formulation and application, and finishing operations that together determine whether the board meets exacting customer specifications. This article provides a comprehensive analysis of the equipment and processes that define competitive coating board manufacturing.

Multi-Ply Forming Technology for Coating Board

The multi-ply structure of coating board enables mills to optimize fiber usage by placing the most expensive, highest-quality fiber only in the surface plies where it directly impacts appearance and printability, while using lower-cost fiber in the middle plies that provide bulk and stiffness.

Ply Structure and Fiber Allocation:

• Top Ply (20-40 g/m²): Bleached hardwood kraft (eucalyptus, birch) or DIP with brightness exceeding 82% ISO. Freeness: 400-500 mL CSF for excellent formation. This layer provides the printing surface and must be free of visible dirt, shives, and stickies.
• Middle Ply/Mid-Liner (30-80 g/m²): Mechanical pulp, broke, or lower-grade recycled fiber provides bulk and stiffness at reduced cost. Freeness: 550-700 mL CSF for adequate drainage at high basis weights.
• Back Ply (20-40 g/m²): Bleached or unbleached kraft for SBS/FBB, or selected recycled fiber for WLC. Provides balanced curl control and additional structural properties.

Forming Configurations: Multiple forming configurations produce coating board:

Multi-Fourdrinier: Individual fourdrinier sections form each ply separately before combining on the main wire or at the couch. This configuration offers maximum flexibility for different fiber types and basis weights per ply. Forming roll diameters of 600-900 mm with subsequent foil and vacuum elements progressively dewater each layer before ply bonding.

Top-Wire Former with Secondary Headbox: A primary fourdrinier forms the bottom ply, with secondary headboxes applying subsequent plies. This compact design reduces machine length but limits the basis weight ratio between plies.

Gap Former with Stratified Headbox: A single stratified headbox delivers multiple plies simultaneously into a gap former. This technology provides excellent ply bond and formation at the cost of reduced flexibility in fiber selection between plies.

Ply Bond Strength: Adequate ply bond (measured by Scott Bond or Z-direction tensile) prevents delamination during printing and converting. Values exceeding 250 J/m² (Scott Bond) typically satisfy most packaging applications. Ply bond depends on combining moisture (8-12% at the junction), combining pressure, and the refining level of the fibers at the interface.

Coating Kitchen and Coating Application Systems

The coating operation transforms base board into a high-quality printing surface. The coating kitchen prepares precisely formulated coating colors, while the coating application system meters and applies them to the board surface.

Coating Color Formulation:

• Pigments (80-90 parts by weight): Fine calcium carbonate (0.5-2.0 μm particle size) provides brightness and smoothness. Clay (kaolin, 0.3-5.0 μm) improves gloss and ink holdout. Titanium dioxide (0.2-0.5 μm) boosts opacity when required.
• Binders (10-20 parts): Styrene-butadiene latex provides film strength and flexibility. Starch or polyvinyl alcohol supplements latex while controlling rheology. Binder level significantly affects coating pick strength and print quality.
• Additives (under 5 parts total): Dispersants, lubricants (calcium stearate), rheology modifiers, optical brightening agents, and biocides maintain coating functionality and stability.

Coating solids range from 55-68%, with Brookfield viscosity typically between 800 and 2,500 cP at 100 rpm. Water retention values (AA-GWR) below 100 g/m² prevent binder migration into the base sheet during application.

Blade Coating Systems: The dominant application technology for coating board, blade coaters apply 8-15 g/m² per layer (dry) with excellent surface coverage. Key specifications:

• Blade thickness: 0.3-0.5 mm with bevel angles of 20-40°
• Blade loading pressure: 15-35 kN/m (linear load along the blade)
• Coating speed: 400-1,200 m/min for board applications
• Blade life: 4-12 hours before replacement required

Curtain Coating: Emerging technology for coating board applies a free-falling curtain of coating color onto the moving web. Advantages include complete contour coverage (no blade scratches), lower coat weight capability (5-8 g/m²), and equal coverage across surface roughness variations. Curtain coaters operate with coating solids of 55-65% and curtain heights of 100-300 mm.

Infrared and Air-Foil Drying: After each coating application, infrared dryers (electric or gas-fired) provide rapid non-contact drying. Electric IR units operate at 2.5-3.5 μm wavelength with power densities of 50-150 kW/m. Air-foil dryers follow with high-velocity impingement air at 180-300°C to complete drying before the next coating station or final reel-up.

Energy Consumption and Efficiency in Coating Board Production

Coating board production consumes significant energy across stock preparation, paper machine, and coating operations. Understanding the energy profile enables targeted efficiency improvements.

Electrical Energy Distribution (per ton of coated board):

• Stock preparation (pulpers, refiners, screens, cleaners): 180-300 kWh
• Paper machine drives and vacuum: 280-400 kWh
• Coating kitchen and application: 40-70 kWh
• Coating drying (IR + air dryers): 80-150 kWh
• Calendering, reeling, winding: 30-50 kWh
• Auxiliaries (pumps, compressors, lighting): 80-120 kWh
• Total: 690-1,090 kWh/ton

Thermal Energy Consumption:

• Dryer section steam: 1.6-2.2 tons steam per ton of board
• Coating dryer natural gas (IR + air dryers): 0.8-1.4 GJ per ton
• Heat recovery from dryer hood exhaust recovers 30-50% of exhaust energy for process water heating and combustion air preheating

Optimization Approaches:

• Increasing post-press dryness from 46% to 50% through shoe press installation reduces steam consumption by approximately 9-13%
• Heat recovery from coating dryer exhaust for combustion air preheating saves 10-18% of gas consumption
• Variable frequency drives on vacuum pumps and major agitators reduce electrical load by 15-25% on those services
• Optimizing coating solids from 58% to 63% reduces drying energy by approximately 8% per coat weight maintained

Calendering and Finishing Operations

After coating and final drying, calendering develops the surface properties essential for high-quality printing and converting performance.

Soft-Nip Calendering: Heated steel rolls (120-200°C) against polymer-covered rolls (86-92 Shore D) at linear loads of 80-250 kN/m produce the required smoothness and gloss. The number of nips (typically 1-4) depends on the target surface specifications:

• Standard packaging: 1-2 nips achieving PPS roughness of 1.5-2.5 μm
• Premium graphic packaging: 2-4 nips achieving PPS roughness of 0.8-1.5 μm, gloss of 50-70% (75° TAPPI)

Gloss Calendering: High-temperature steel-on-steel calendering or extended-nip technology for specialty grades requiring gloss exceeding 70%. Roll temperatures reach 180-250°C with linear loads of 200-400 kN/m. The thermal and pressure history of the board surface determines final gloss development.

Moisture Control: Coating board moisture at the reel must be 5.5-7.5% for dimensional stability. Over-dried board (under 4% moisture) becomes brittle and prone to cracking during folding and scoring. Moisturizing systems (steam showers or water sprays) at the calender or reel adjust final moisture content.

Maintenance Requirements for Reliable Operation

Coating board lines integrate mechanical, hydraulic, pneumatic, and thermal systems demanding structured maintenance:

Coating Station Maintenance:

• Blade change every 4-12 hours per coater head (depending on blade quality and operating speed)
• Backing roll grinding/recovering every 3-6 months
• Coating color screening through 100-150 μm filters with continuous recirculation prevents agglomerates
• Daily cleaning of coating pans, pipes, and application heads to prevent dried coating buildup

Dryer Section:

• Monthly IR emitter inspection — replace emitters with broken elements or reduced output
• Quarterly air nozzle inspection for coating dryer — plugged nozzles cause streaky drying and quality defects
• Annual dryer cylinder thickness measurement and internal inspection
• Steam condensate system checks monthly — failed steam traps waste energy and cause moisture profile issues

Calendar Rolls:

• Weekly surface inspection for marks, dents, or coating buildup
• Polymer cover hardness verification monthly — covers that soften below specification reduce calendering effectiveness
• Bearing temperature trending — abnormal temperature rise signals impending failure

Conclusion

Coating board production integrates multiple technologies and scientific disciplines — fiber science, fluid mechanics, colloid chemistry, and heat transfer — into a single manufacturing line. The multi-ply structure enables cost optimization while meeting demanding performance specifications. Coating operations transform functional base board into premium packaging materials. Mills that master the integration of these systems produce board that commands premium pricing in competitive global markets.

Contact us at leizhanzhang@gmail.com to discuss your coating board making line requirements and learn how properly specified stock preparation and paper machine equipment can elevate your product quality.

Zhengzhou Leizhan Technology Paper Machinery Co., Ltd.