What Is Stock Preparation System?
The stock preparation system is a critical part of a multi-layer paperboard machine. Its main function is to prepare different pulp furnishes separately for each layer of the board (top layer, middle layer, and back layer) to achieve desired strength, surface quality, and cost optimization.
Objective of Stock Preparation
The stock preparation system ensures:
- Proper fiber separation and cleaning
- Controlled refining for each layer
- Accurate consistency and flow
- Stable quality before the headbox
“Each layer has independent stock preparation to meet its specific performance requirements.”
Typical Layers in Multi-Layer Board
- Top Layer (Top Ply)
- High-quality fibers (Virgin pulp / Bleached pulp)
- Focus: Printability, smoothness, brightness
- Middle Layer (Core Ply)
- Recycled fibers / BCTMP
- Focus: Bulk, stiffness, thickness
- Back Layer (Bottom Ply)
- Recycled or semi-chemical pulp
- Focus: Strength and runnability
Main Equipment Used in Stock Preparation
1. Pulping System
Purpose: Fiber disintegration
Equipment:
- Hydra-pulper (Drum or Vat type),
- Bale pulper (for virgin pulp),
- Broke pulper (for wet & dry broke)
“Each raw material source has a dedicated pulper.”
2. Screening System
Purpose: Removal of contaminants
Equipment:
- Coarse screens (hole screens),
- Fine screens (slot screens),
- Multi-stage screening with reject handling
“Protects downstream equipment and improves sheet quality.”
3. Cleaning System
Purpose: Removal of heavy & light impurities
Equipment:
- High-density cleaners (sand, metals),
- Low-density cleaners (plastics, stickies)
“Ensures trouble-free machine operation.”
4. Thickening & Storage
Purpose: Consistency control and buffering
Equipment:
- Disc filters / Drum thickeners
- Storage chests (Machine chest, Blend chest)
“Provides process stability during grade changes.”
5. Refining System
Purpose: Fiber development
Equipment:
- Double Disc Refiners (DDR)
- Conical refiners (specific layers)
Refining is layer-specific:
- Top layer → moderate refining (surface strength)
- Middle layer → light refining (bulk retention)
- Back layer → higher refining (bond strength)
6. Blending & Additive Dosing
Purpose: Furnish optimization
Additives Used:
- Starch
- Retention aids
- Dyes / Optical Brightening Agents (OBA)
- Wet strength / Dry strength chemicals
“Blending is done in blend chests for uniform stock.”
Simplified Process Flow of Stock Preparation:
Raw Material → Pulper → Coarse Screening → Fine Screening → Cleaning → Thickening → Storage Chest → Refining → Blend Chest → Machine Chest
“Each layer follows a parallel but independent flow line.”
Importance of Stock Preparation in Board Quality
- Controls GSM, stiffness, and strength
- Improves formation and runnability
- Reduces breaks and defects
- Optimizes fiber and chemical cost
Conclusion
The stock preparation system is the foundation of a multi-layer board machine. By preparing separate, optimized furnishes for each layer, it enables manufacturers to produce high-performance paperboard with consistent quality and cost efficiency.