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Finishing
Kristian Goldszer
Papermaking Course Montevideo
Part II Paper Processes
March 19 21, 2009
Sources: Metso Paper, KnowPap, Cepatec AB, Skogsindustrierna
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Finishing
Calendering
Coating
Reeling
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Objectives of Calendering
Pre Calendering
Controls caliper profile
Controls bulk
Creates a good smooth surface for coating
Controls porosity dewatering of coating
Finish Calendering
Creates gloss
Creates smoothness
Improves the printing properties
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Effects on Paper Properties
Desirable Changes
Smoothness +++ Gloss +++ Oil absorption - - Air porosity - - - Two-sidedness - -
Undesirable Changes Density +++ Thickness - - -
Stiffness - - - Compressibility - - - Opacity - - Brightness - Tear strength - -
+ = increases- = decreases
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Calendering Process Variables
The main parameters that contributes to the calendering result are:
Pressure Loading Roll hardness and dimension
Temperature Web Roll
Web Moisture
Water Steam
Time Machine speed
Nip length Number of nips
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Specific Pressure ( max ) =
[kN/m]
[mm]
Nip width
Specific pressure [MPa]
Specific Pressure
Linear load (kN/m)
Nip width (mm)
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Specific Pressure and Nip Width
100 kN/m 100 kN/m100 kN/m
Nip width 1-2 mm
Max nip pressure 80 Mpa
Nip width 10-20 mm
Max nip pressure 40 MPa
Nip width 30-250 mm
Max nip pressure 10 MPa
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Effect of Linear Load
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Effect of Temperature
The higher the temperature, the easier it is to plasticize the paper
Lignin and hemi cellulose soften
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Effect of Moisture
The softening temperatures of lignin and hemi cellulose drop as the
moisture content increases
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Effect of Machine Speed
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Effect of Number of Nips
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Calender Types
Hard Calender
Soft Calender
Long Nip Calender
Multi Nip Calender
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Hard Calender
Metal rolls
Narrow nip Short dwell time in nip
High maximum pressure
Used for: pre and final calendering of board
final calendering of news
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Soft Calender
At least one of the rolls forming the nip is a soft cover roll
Can replace a hard calender in all applications More even density profile
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Comparison of Hard and Soft Calender
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Long Nip Calender
Especially for board
Excellent smoothness with reduced loss of bulk
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230 1.3 6
15
5
Soft Calender
Long-Nip Calender
Nip dwell times based on speed 700 m/min
70 mm shoe length
Nip
Pressure(MPa
)
Nip Dwell Time (ms)
Nip Load Soft Calender versus Long Nip Calender
Shoe calender has much higher potential in treating the sheet surface
than a soft roll calender due to lower pressure and longer dwell time
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More Effective Heat Transfer - Melts the Surface
Soft calendered 250 C Shoe calendered 250 C
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Calender Type Bulk versus Printability
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New Technology Metal Belt Calender
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Multi Nip Calender
Mainly for printing papers
Densifies the web, in particularly the surface
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Coating
Paper and Board are coated in order to improve printability
Coating impacts Paper and Board surface in the following way:
Improved smoothness => more uniform print result
Increased porosity in the surface layer => uniform colourabsorption
Increased opacity => less risk for print through
Improved brightness
Increased gloss
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Coated Products
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Main Coating Processes
Two principles:
Application of coating colour in excess, 200 g/m2, metering ofexcess coating colour to final amount, 5 30 g/m2
Pre-metered amount corresponding to desired coat weight isapplied to the substrate to be coated
Drying:
IR dryer
Air dryer
Cylinder dryer
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Coating Colour
Pigments, 80 95 %
Binders, 5 20 % Additives, 1 2 %
Solids content = 40 60 %
Viscosity 1200 mPas
Base Layer:
Coarse pigments
Coverage important Improve smoothness
Even out brightness
Affect colour absorption
Top Layer:
Fine pigments
Increase gloss Increase brightness
Improve smoothness
Desired colour absorption
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Demands on Pigments
Compatible with other substances in coating colour
Easy to disperse in water and good runnability
Low average particle size
Low binder demand
Not wearing
Low price
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Important Pigments
Clay
70 75 % < 2 m High viscosity High Aspect Ratio, 25 - 40 Gloss. smoothness Good coverability => used for low
coat weights such as LWC, ULWC
Calcium Carbonate
90 % < 2 m Irregular spherical particles
Low Aspect Ratio, 3 - 7 Rougher and duller surface
Brightness, opacity Higher pigment brightness than clay,
but not the same coverage
High Aspect Ratio => Gloss
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Important Pigments
Calcined Clay
75 95 % < 2 m Lower density
Higher light scattering
Brightness , Opacity
Titanium Dioxide
Used together with other pigment, 5 15 %
Very high refractive index and small particles, 0.3 0.4 m
Brightness , Opacity
Plastic pigments
Gloss, Opacity
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Clay Calcium Carbonate
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Function of Binder in Coating Colour
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Important Binders
Latex
Styrene Butadiene Styrene Acrylate
PvAc
Starch Low Binder Cost High Binding Power
Good Water Retention
Reduces Solids Content of Total Mix
Increased Risk for Uneven Binder Migration
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Additives
Dispersants
NaPa Increases viscosity
Wet Strength Agent Harden coating, cross link binders Ammonium Zirconium Carbonate Glyoxale
Melamine Formaldehyde Resins
Lubricant Ammonium Stearate
Optical Brightener
Biocide Prevent bacterial growth Glutharaldehyde
Defoamer
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Coating Processes
Excess Processes
Roll application Jet application
Short dwell time application (SDTA)
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Roll Application
Application pressure = 0.5 4 Bar
Dwell time = 15 75 ms Applicator roll:
Rubber covered
Diameter 25 % of backing roll
Speed, 25 % of machine speed 0.5 2 mm gap between applicator roll
and substrate to be coated
Dewatering controlled by pressure in
high speeds and by dwell time in lowspeeds
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Jet Application
Application pressure = 1 Bar
Dwell time = 15 75 ms Dewatering controlled by dwell time
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Short Dwell Time Application
Application pressure = 1 Bar
Dwell time = 1.25 6.25 ms Dewatering controlled by
dwell time
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Blade Coating
Most frequently used coating method
Machine speeds up to 2000 m/min High solids content, 62 67 %
Blades are 0.4 0.5 mm thick and 75 mm wide
Gives a uniform surface
Stiff or bent blade
The coating evens out irregularities
in the base paper filling in method
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Blade Coating
The coat weight is controlled by
applying a blade pressure Important with constant blade
angle for uniformity
For stiff blades the coat weightdecreases with increasedpressure and vice versa
Stiff blades for high speedmachines with low coat weight
Bent blades for low speedmachines with high coat weight
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Roll Blade
Alternative to blade for pre-coating of board
Coat weight controlled with a rubber hose behind the rod blade thatcontrols the pressure
Coat weight not as sensitive to pressure changes as blade coating
Higher roughness compared to blade coating
Solids content around 3 5 % lower than for blade coating
The coating follows the contour ofthe base paper contour method
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Air Knife
Excess coating colour is metered with an air jet
Excess coating colour is collected in a vacuum box Earlier common for top coating of board
Low solids content, 40 45 %
Low speeds, 400 500 m/min
Speed limitation is main reason for replacing air knifes with blade
The coating follows the contour ofthe base paper contour method
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Drying of Coating Colour IR Drying
electrically or gas heated normally 1st stage
evaporation rate 100 140 kg/h,m2
Hot Air Drying steam or gas heated evaporation rate 35 160 kg/h,m2
Cylinder Drying coating surface must be non sticking final drying evaporation rate 10 15 kg/h,m2
Water 30 - 35 % of total coating composition
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Coated White Top Liner
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Liquid Packaging Board
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Light Weight Coated
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Reeling
Production of large diameter parentrolls for further processing in the mill:
Calendering
Coating
Winding to customer roll
Sheeting to customer sheets
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Roll Requirements
Correct roll dimensions
Diameter (also cross CD) Weight Web length
Round rolls Core in centre
Avoid vibrations Straight roll edges
In further processing Storing
Good roll structure Hardness and roll tension
Clean and dust free
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Reeling Processes
The reeling process includes the following processes:
1. Web control before reeling
2. Reeling3. Reel change
4. Reel handling
We control web inspection web tension machine speed vibrations
Reeling roll structure
Reel change deceleration stopping
moving
Reel handling coating calendering
storage converting
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Roll Structure
Radius
Hardness
The first layers are wound as hard as required to prevent slippage
between the layers The hardness decreases continuously from the intermediate zone to
the surface
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Effect of Line Pressure on Roll Structure
Constant Pressure
bad roll structure
Controlled Pressure good roll structure
Line pressure between roll and reel drum
Reel
Reel
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Effect on Paper Properties of Reeling Line pressure, web tension and internal tensions effects paper
properties
The effect is high close to the core and low at the surface
DecreasesPorosity
IncreasesGloss
DecreasesRoughness
DecreasesBulk
DecreasesStiffnessDecreasesThickness
ResultProperty
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Reel Types Pope
Simple model suitable for:
Small rolls
Qualities that are insensitive toline pressure
Compressible paper
Peripheral drive
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Turn Up Pope
High initial nip load due to reel spool weight
Hard to control nip load in transfer from primary- to secondary arms
Sudden increase in nip load at transfer
Nip load
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Center drive torque
Advantages with center drive torque: Bigger rolls
Better roll structure Bigger tolerance to variations in line pressure
Reel Types Center Drive Torque
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Turn Up Methods
Common methods:
Tape Gooseneck
Water Jet
Choice depends on: Grade
Operating speed
Web width
Reel type
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Water Jet Turn-Up
Reel drum
WJ beam
Water pressure max 2000 barHigh speed operation,tail cutting and turn-upwith air blow
Nip
Pick-up air blow
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Tape Turn-Up 1. Control panel2. Tape feeding and cutting
unit
3. Tape transfer rail
4. End of the tape
5. Break6. Empty reel spool in the
primary arms
7. The tape is fed into thenip and the tape sticks
onto the reel spool8. The tape cuts and guides
the tail around the reelspool
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