Paper Calendering Techniques Quiz

IWK 203/4: Stock Preparation and Papermaking

• Course code: IWK 203/4
• Course title: Stock Preparation and Papermaking
• Also known as: Penyediaan Stok & Pembuatan Kertas (Malay)
• Instructor: Dr. Kushairi Mohd Salle
• Institution: Universiti Sains Malaysia
• Academic year: 2024/2025

Papermaking Sections

• Introduction to Papermaking (2.1)
• Approach Flow System (2.2)
• Forming Section (2.3)
• Pressing Section (2.4)
• Drying Section (2.5)
• Calendering, Reeling, and Finishing Sections (2.6)

Calendering, Reeling, and Finishing Sections (2.6)

• Calendering:

  • After drying, most paper grades pass through calendering.
  • Calendering uses rolls to press the paper under high pressure.
  • The calender stack consists of horizontal rolls (steel or cast iron) stacked vertically.
  • The objectives of calendering are to reduce paper thickness and create a smooth surface.
  • Calendering is also used to control paper density and smooth watermarks.
  • Rolls may not have equal hardness. Nip pressing time is shorter.
  • Calendering level depends on desired finishing grade.
  • Low-grade paper requires only one or two nips. High-grade paper may pass through several sets (7-9 rolls per set).

• Rolls:

  • Intermediate rolls (upper stack): Heated rolls and variable-crown rolls.
  • Queen rolls: Usually driven rolls.
  • King rolls (bottom): Act as variable rolls.

• Types of Calenders (and types of rolls):

  • On-machine calenders are more economic.
  • High temperature calendering allows for lower pressure. Frequently first two rolls are heated by hot water.
  • Tri-pass drilled rolls are most efficient. Shell rolls are efficient. Bored rolls are least efficient.
  • Machine calenders use a hard-nip system (pair of hard metal rolls) to produce uniform thickness.

• Reeling:

  • Post-drying and calendering, paper is collected on a metal cylinder (reel spool).
  • Reeling gathers paper from the paper machine and forms a loop (full reel).
  • On-machine reeling happens after calendering. Off-machine reeling happens before calendering.
  • Key components: driving drum/reel drum, reel spool, primary arm, secondary arm.

• Reeling process details:

  • Driver drum is the most important component as it controls the speed.
  • Reel spool is initially positioned in the primary arms, then the secondary arms apply pressure for wrapping.
  • Reeling stops upon reaching the desired diameter (full reel).
  • Reeling is stopped, the paper is broken, and the full reel is moved to the end of the rail.

• Winding:

  • Post-reeling, the paper travels through a winding stage.
  • Purpose: Cut and wind the full reel into smaller, usable rolls.

• Size press: Size press distributes sizing agents (like starch) on paper, improving water and air permeability to strengthen the paper.

• Pigment coatings:

  • Improve the sheet's surface for printing by mixing fillers and binders with the paper.

• Modern Paper Machine Finishing:

• Now finishing is done on machine.

• Paper Coating Principles:

  • Blade coating, Size press, Film coating, Curtain coating, Spray coating.

• Blade coating produces smooth but uneven coatings. Curtain coating produces even but rough coatings.

Additional Information

• Typical reeling defect: wrinkles and breaks due to air pockets, especially in dense/low porosity paper.
• Winding process trims both edges of the paper. Broke is recycled. The middle is cut depending on the width.
• Paper rolls are marked with details like grammage, thickness, and width.
• Paper rolls are sent for further treatment to produce additional paper products.