Multi-Layer Forming Technology for Coating Board Production

Multi-Layer Forming Technology for Coating Board Production

Coating board — used extensively in packaging for pharmaceuticals, cosmetics, food, and consumer goods — requires a multi-layer sheet structure that combines bulk, stiffness, surface smoothness, and printability. The base sheet typically consists of 3-5 layers: a top liner for coating reception, middle layers for bulk and stiffness, and a back liner for strength. Each layer demands specific fiber treatment, chemical addition, and forming conditions to achieve the final board properties specified by end-users.

Layer-Specific Stock Preparation Requirements

Each layer in a coating board sheet has distinct fiber requirements that dictate separate stock preparation lines:

• Top liner (bleached chemical pulp, 30-50 gsm): CSF 400-450 mL, requires disc refining at low intensity (SEL 0.5-1.0 J/m) to preserve fiber length while developing bonding
• Middle layer (mechanical pulp or DIP, 100-200 gsm): CSF 300-350 mL, higher refining intensity (SEL 1.5-2.0 J/m) acceptable since bulk is the priority
• Back liner (unbleached kraft or mixed waste, 30-50 gsm): CSF 350-400 mL, moderate refining to provide bending stiffness

Using a single stock preparation line for all layers compromises quality — the top liner becomes over-refined (losing opacity) while middle layers remain under-refined (reducing bulk). Separate lines with dedicated refiners for each layer represent a capital investment that typically pays back within 18-24 months through reduced fiber cost (using lower-cost furnish for middle layers) and improved quality consistency.

Coating Base Preparation: The Role of Screening and Cleaning

The top liner surface must be free of contaminants larger than 0.05 mm — any speck, shive, or stickies particle will telegraph through the 8-12 gsm coating layer and appear as a visible defect. A dedicated fine screening and cleaning system for the top liner line is essential:

• Pressure screen: 0.08-0.12 mm slotted basket, accept consistency 0.8-1.2%
• Forward cleaners: 75 mm cones in 3-stage cascade, reject rate below 5% at tertiary stage
• Deaeration tank: removes entrained air that causes pinholes in coating layer

The combined screening and cleaning system achieves debris removal efficiency above 99% for particles in the 0.05-0.15 mm range, reducing coating defects (fisheyes, streaks) by an estimated 60-75% compared to single-stage screening alone. For a 250 TPD coating board machine, this translates to a 2-3% improvement in first-quality production output.

Energy and Water Management in Multi-Ply Forming

Multi-layer forming sections consume significant process water — each layer requires independent dilution and recirculation. Optimized water management reduces both fresh water consumption and effluent treatment costs:

• Disc filter or dissolved air flotation (DAF) save-all: recovers 95-98% of fiber from white water, returning clarified water for shower and dilution use
• White water recirculation: up to 80% of process water can be reused, reducing fresh water intake to 8-12 m3 per ton of board
• Gravity disc thickener: thickens middle layer stock from 0.8% to 4-5% consistency, reducing steam demand in the dryer section by 8-12%

Overall specific energy consumption for a well-designed multi-layer coating board machine producing 200-400 gsm board ranges from 450-550 kWh per ton, with the stock preparation section accounting for approximately 80-100 kWh/t. Energy benchmarking against industry standards identifies optimization opportunities that typically yield 5-10% annual savings.

Preventive Maintenance for Coating Board Production Equipment

• Pressure screen rotor seals: inspect every 400 hours, replace at signs of leakage to prevent stock dilution consistency variations
• Disc thickener filter bags: monitor filtrate clarity weekly, replace bags at 1,500-2,000 hours or when solids carryover exceeds 50 mg/L
• Refiner plate alignment: perform tram test monthly, acceptable parallelism tolerance under 0.05 mm across plate diameter
• Save-all disc sectors: inspect every 6 months for warping or media detachment that reduces fiber recovery efficiency

Multi-layer forming technology continues to evolve toward higher speeds, lighter basis weights, and greater recycled content. Leizhan Technology offers complete stock preparation systems with integrated process control for coating board grades.

Contact: leizhanequipment@gmail.com | www.pulprefining.com