Fine Screen System in Stock Preparation: Working Principle, Benefits & Applications

“Final Fiber Protection Before the Head box”

What is a Fine Screen System?

A Fine Screen System is the last and most critical screening stage in stock preparation.
It removes small contaminants before stock is delivered to the approach flow

Such as: 1.Stickies, 2. Plastics, 3. Wax particles, 4. Shives, 5. Sand & fine debris

 In board machines (FBB, Duplex, Kraft liner, Multi-layer board), fine screening ensures sheet cleanliness, and runnability.

Location in Stock Preparation Line

The Position of Fine screening system is after HD cleaner/Coarse Screen and Before LD Cleaners.

Pulper → Coarse Screening → HD Cleaners → Fine Screening System → LD Cleaners→ Deflaker/Refiner → Machine Chest → Headbox

For multi-layer board machines, separate fine screens are also used in approach flow system for:

• Top layer
• Middle layer
• Back layer

Working Principle of Fine Screen:

Basic Principle

Fine screening works on pressure screening with narrow slots or small holes.

• Stock enters the screen basket
• Acceptable fibers pass through slots/holes
• Contaminants are rejected and discharged

Key Operating Parameters:

Parameter	Typical Range
Consistency	0.6 – 1.2%
Slot Width	0.15 – 0.30 mm
Hole Diameter	1.0 – 1.6 mm
Pressure Drop	0.3 – 0.8 bar

Main Components of Fine Screen:   

1. Screen Housing
2. Screen Basket
   1. Slotted (most common for board machines)

   1. Holed (earlier stage)

3. Rotor / Foil
   

   1. Maintains slot cleanliness

4. Accept Outlet
5. Reject Outlet
6. Dilution & Reject Control System

Types of Fine Screens Used in Board Machines:

a) Single-Stage Fine Screen:

• Used in clean fiber lines
• Lower capital cost

b) Multi-Stage Cascade Fine Screening:

• Accept of first screen goes forward
• Reject is re-screened in secondary screens
• Higher fiber recovery
• Common in recycled board mills

c) Approach Flow Fine Screen:

• Installed just before headbox
• Ensures final cleanliness

Importance of Fine Screening in Board Grades:

Board Grade	Fine Screen Benefit
Duplex Board	Clean white top layer
FBB	Stickies-free printing surface
Kraft Liner	Better strength & runnability
Multilayer Board	Layer separation & cleanliness

Merits and Demerits of Fine Screen System:

Merits	Demerits
Removes micro contaminants	Higher power consumption
Improves sheet cleanliness	Basket wear & replacement cost
Reduces breaks & holes	Reject handling required
Protects head-box & forming fabrics	Sensitive to consistency fluctuations
Improves print-ability	Improper operation can cause fiber loss
Higher machine speed possible

Common Operating Problems & Remedies:

Problem	Cause	Remedy
High reject	Worn basket	Replace basket
Slot plugging	High consistency	Reduce consistency
Poor screening	Wrong slot size	Optimize basket
Fiber loss	Improper cascade	Improve reject screening

Leading Fine Screen Suppliers in India:

Indian & Global Suppliers Active in India

• Andritz India
• Valmet India
• Voith Paper India
• Kadant India
• Parason Machinery (India)
• Aikawa Fiber Technologies (AFT)

Slot Selection for Board Machine Approach flow:

Layer	Slot Size
Top Layer	0.15 – 0.20 mm
Middle Layer	0.20 – 0.25 mm
Back Layer	0.25 – 0.30 mm

Conclusion

The Fine Screen System is the backbone of cleanliness in a board machine.
For recycled fiber–based board mills in India, proper design, correct slot selection, and multi-stage screening are essential to achieve high quality board with stable machine runnability.