Coating board — including folding boxboard (FBB), white line chipboard, and solid bleached board — relies on a multi-ply construction where each layer contributes specific functional properties. A well-designed coating board paper making line requires independent stock preparation systems for each ply, sophisticated multi-layer forming technology, and careful process integration to achieve the surface quality, stiffness, and printability demanded by packaging converters. This guide examines the technical requirements for each ply’s stock preparation and the overall line configuration.

1. Multi-Ply Architecture: Individual Stock Preparation Lines

A typical coating board machine producing 250-500 TPD produces board with three to five plies. Each ply requires dedicated stock preparation equipment optimized for its specific furnish and quality requirements.

Top Liner (Bleached Chemical Pulp): The top ply (30-60 g/m²) provides surface smoothness and printability. Stock preparation includes:

• Low-consistency pulper: 15-25 m³ capacity, 110-160 kW motor at 200-250 rpm
• Fine screening: Slotted baskets at 0.15-0.25 mm slot width, 37-55 kW motor
• Refining: Disc refiner at 132-200 kW with SEL of 0.6-1.0 J/m targeting 350-400 mL CSF
• Filler addition system: GCC or PCC slurry at 8-15% of fiber weight for opacity and smoothness
• Consistency control: Stock delivered to the headbox approach at 0.3-0.6%

Under Liner / Middle Ply (Mechanical or Recycled Fiber): This ply (40-80 g/m²) provides bulk and stiffness. For recycled fiber-based mills:

• Drum pulper or batch pulper processing mixed waste paper at 12-16% consistency
• Coarse screening: Holes at 2.0-2.8 mm, followed by fine slotted screens at 0.20-0.30 mm
• Forward cleaning: 75-100 mm cone diameter cleaners for lightweight contaminant removal
• Refining: Disc refiner at 200-315 kW with SEL of 1.0-1.8 J/m targeting 380-480 mL CSF
• Optional fractionation: Separating long and short fiber for independent refining optimization

Back Liner (Unbleached or Recycled Fiber): The back ply (50-80 g/m²) requires adequate strength but does not need high brightness. Stock preparation is similar to the middle ply but typically uses lower-cost furnish and minimal refining (40-60 kWh/ton).

Filler Ply (Low-Cost Fiber): For grades with a separate filler ply (board thickness >0.5 mm), this layer uses the lowest-cost furnish — typically mixed waste with minimal treatment. Coarse screening at 2.5-3.5 mm holes and basic cleaning suffices, with energy consumption below 30 kWh/ton.

2. Screening and Cleaning Configurations by Ply

Each ply demands a different screening and cleaning intensity based on furnish quality and surface requirements.

Top Liner Fine Screening: The top ply screen system must achieve exceptional cleanliness:

• Basket type: Wedge-wire slotted, 0.12-0.18 mm slot width
• Inlet consistency: 0.6-1.2%
• Rotor speed: 800-1,000 rpm (foil rotor for gentle screening)
• Motor power: 37-55 kW per screen
• Two-stage cascade: Primary accept goes forward; primary reject feeds secondary screen
• Cleaner protection: Forward cleaners upstream of the screen protect baskets from abrasive particles

Middle/Back Ply Screening: Less demanding specifications allow wider slots:

• Basket slot width: 0.20-0.35 mm
• Operating consistency: 1.0-1.8%
• Rotor speed: 700-900 rpm
• Motor power: 55-90 kW
• Three-stage cascade for maximum fiber recovery

Cleaning Cascade Design: A three-stage forward cleaner system recovers fiber while concentrating rejects:

• Stage 1 (primary): Accept goes forward; reject at 8-12% feeds stage 2
• Stage 2 (secondary): Accept returns to stage 1 inlet; reject at 15-20% feeds stage 3
• Stage 3 (tertiary): Accept returns to stage 2 inlet; reject (2-3% of original flow) is sewered
• Overall fiber recovery: 97-99%

3. Energy Consumption Across Multi-Ply Systems

Operating multiple independent stock preparation lines multiplies energy consumption. Understanding per-ply energy patterns enables targeted reduction strategies.

Total Stock Preparation Energy (kWh/ton of finished board, 4-ply configuration):

• Top liner line: 25-40 kWh/ton of board (high refining energy, lower tonnage)
• Under liner line: 18-30 kWh/ton of board
• Back liner line: 15-25 kWh/ton of board
• Filler ply line: 8-15 kWh/ton of board
• Shared systems (water, vacuum, compressed air): 10-18 kWh/ton
• Total stock preparation: 76-128 kWh/ton

Optimization Strategies:

• Installing VFDs on all pumps above 22 kW in stock preparation — 15-20% energy reduction
• Optimizing refiner plate selection per ply — using the correct bar pattern for each furnish reduces energy by 8-12%
• Implementing automatic refiner control with online freeness measurement — 5-10% energy saving
• Recovering heat from refiner cooling water for process heating — reduces steam consumption by 3-5%
• Operating fillers and additives at optimal concentration to minimize pumping energy

A 350 TPD coating board line implementing a comprehensive energy program can reduce stock preparation energy from 100 to 85 kWh/ton, saving approximately 1,785,000 kWh annually — equivalent to $178,500 at $0.10/kWh.

4. Basis Weight and Moisture Control for Coating Base Sheet

The uncoated base sheet quality directly determines final coated board performance. Consistent basis weight and moisture profiles are essential for uniform coating pickup and calender finishing.

Headbox and Forming Section: Multi-ply headboxes deploy successive jet layers onto a fourdrinier or hybrid former. Key control parameters:

• Jet-to-wire speed ratio: 0.95-1.05 (adjusted per ply for orientation control)
• Headbox slice opening: 8-15 mm depending on basis weight and speed (200-600 m/min typical for board)
• Consistency at headbox: 0.3-0.8% per ply
• CD basis weight variation target: <1.5% 2-sigma after forming

Press Section Dewatering: Shoe press technology with extended nip (200-300 mm) achieves 48-52% dryness after pressing. This reduces dryer section steam consumption by 15-20% compared to roll presses at 42-46% outgoing dryness. Linear loads of 800-1,200 kN/m are typical, with double-felted configurations for maximum water removal.

Pre-Coating and Surface Sizing: Before the coating section, a pre-coating or surface sizing station applies starch or pigment at 2-5 g/m² to seal the surface and reduce coating holdout variation. Surface-sized base sheet shows 20-30% lower coating penetration, resulting in better coverage at reduced coat weight.

5. Maintenance for Multi-Ply Stock Preparation Systems

Maintaining four or more independent stock preparation lines multiplies the maintenance workload. A systematic approach organized by equipment type rather than by ply line is most efficient.

Daily Checks (All Plies):

• Record pressure screen DP for each ply — investigate trends exceeding 0.15 bar drift
• Monitor refiner motor loads and compare against production rate — 10% deviation triggers investigation
• Check cleaner bank pressures and visual reject appearance
• Verify basis weight and moisture profile stability from QCS trends

Weekly Maintenance:

• Clean fine screen baskets (top liner: weekly; other plies: bi-weekly)
• Inspect cleaner cone tips for wear
• Check refiner plate condition — amperage drop exceeding 15% at constant gap indicates plate change needed
• Verify chemical dosing accuracy through drawdown checks

Monthly Service:

• Replace cleaner cones showing tip diameter wear >2 mm
• Calibrate all consistency transmitters on all ply lines
• Inspect fan pump impellers for cavitation damage
• Lubrication audit on all grease points

Quarterly Overhaul:

• Replace fine screen baskets: top liner at 1,500-2,000 hours; other plies at 2,000-3,000 hours
• Replace refiner plates: top liner at 1,200-1,800 hours; middle/back at 800-1,200 hours
• Pump wear ring inspection and replacement as needed
• Pipe thickness survey at all elbows and tees

Annual Shutdown (2-3 weeks planned):

• Complete overhaul of all stock preparation pumps
• Replace all pressure screen rotors, bearings, and seals
• Refiner rebuild including bearing replacement and shaft alignment
• Tank and chest inspection/repair with full cleaning
• Replace all mechanical seals in stock preparation area
• Control system calibration and loop tuning across all ply lines
• Full vibration analysis and motor testing on all drives above 15 kW

A coating board paper making line with well-engineered stock preparation and disciplined multi-ply maintenance achieves consistent product quality with ply bond strength exceeding 150 J/m², surface smoothness (PPS) below 2.5 μm at 1 MPa, and machine availability above 90%. The complexity of multi-ply operation is offset by the flexibility to optimize each ply’s furnish and processing independently.

Contact us at leizhanzhang@gmail.com | Zhengzhou Leizhan Technology Paper Machinery Co., Ltd.