Calendering Slideshare 3 March

1. Pele Oy Calendering of Paper and Board March 2016 2. Pele Oy Contents  Preface 02  Calendering principle 04  Selected Base Paper…
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  • 1. Pele Oy Calendering of Paper and Board March 2016
  • 2. Pele Oy Contents  Preface 02  Calendering principle 04  Selected Base Paper Challenges 10  Calendering Process 14  Conventional Calendering 26  New Calendering Methods 39  Research Demands 48 2
  • 3. Pele Oy Preface Paper manufacturing is a long sequential process including pulping, papermaking and finishing. Calendering is the last process of finishing having a great effect on overall efficiency and product quality. It is very important to have comprehensive knowledge of all parts of the process to get maximum possible overall efficiency and good product quality. As an example in the following, I will present what is important in calendering and how it has been developed during latest 20 years. Helsinki, 4 March, 2016 Pekka Komulainen 3
  • 4. Pele Oy Calendering Principle
  • 5. Pele Oy Calendering principle  A calender is a series of pressure rollers used to smooth and gloss a sheet of material such as paper, cloth or plastic film.  Important variables are:  Original paper properties  Calender itself with rolling contact against paper surface  Calendering effects on paper properties through:  Replication of roll surface pattern  Paper compression  Particle orientation  Flow of coating Picture: Knowpap 5
  • 6. Pele Oy Calendered grades  It is difficult to make matt but smooth paper, which would be ideal for several purposes. Glossy Paper Gloss 50-80 PPS <1 Silk or semimatt Gloss 20-40 PPS 1-2 Matt Gloss 10-20 PPS >2 Pictures: Jouni Marttila Combinations Smoothness Gloss Uncalendered, matt Low Low Brush polished Low High Special, silk finish High Low Gloss finish High High 6
  • 7. Pele Oy Coated paper surfaces and calendering Hunter Gloss, % PPSRoughness,μm 1 Soft/Soft nip 2-nip Soft Calender Multinip Calender Picture:Jouni Marttila 7
  • 8. Pele Oy Smoothening and glossing mechanisms Pressing of highest tops (smoothness)  Pressure forces are important  Plasticity of total paper is required Roll surface replication (gloss)  Smooth and clean roll surfaces  Only paper surface plasticity needed Coating flow from tops to pits (gloss)  Plasticity of paper surface is required  Tangential forces are important Particle orientation (gloss)  Plasticity of paper surface is required  Pressure and tangential forces important 8
  • 9. Pele Oy Importance of roll surface smoothness  The main glossing and smoothening effect replicates to the side against the hot iron rolls.  Smoothness of the resilient rolls also have effect on the surface of the opposite side of the coated paper quality.  In this example gloss is 8 %- unit lower, when the rough roll is in the bottom position compared to the top position (Nr 2 vs. Nr 7).  If new rolls are rough they must be placed to the top position. Picture: Voith Paper 9
  • 10. Pele Oy Selected Base Paper Challenges
  • 11. Pele Oy Importance of fiber wall thickness  It is important to have several fiber layers in a thin paper to get good formation, smoothness, opacity and gloss. This correlates with thin fiber wall.  To reduce roughening in offset printing it is also important to have thin fiber walls. Area = Perimeter x Wall Thickness, A=P*T Fiber volume = Area x Length, V=A*L=P*T*L Coarseness = fiber weight/Length, C=W/L C = Volume*Density/Length, C=V*ρ/L=P*T*L* ρ/L= P*T*ρ Fiber grammage (g/m2) = Coarseness/fiber width = P*T*ρ/P*2 = 2*T*ρ = 3*T (in µm) Fiber wall density ~ 1500 kg/m3 ~ P/2 Fiber Fiber Wall Thickness Grammage µm g/m2 1 3 2 6 3 9 4 12 5 15 6 18 T Wall density ~ 1500 kg/m3 11
  • 12. Pele Oy Fiber wall thickness of Norway spruce  Average fiber wall thickness of Norway spruce TMP is almost 2 µm but there are some fibers with wall thickness of 3-5 µm. Reme, P. A., Kure, K.-A., Gregersen, O. W., Helle, T., 1999 International Mechanical Pulping Conference Picea abies 12
  • 13. Pele Oy Coating and calendering quality Uneven base + even coating (curtain)  high roughness, gloss mottling and uneven ink absorption after calendering Uneven base + blade coating  good smoothness but uneven ink absorption Even base paper + even coating (curtain or blade)  ideal result, even gloss and no print mottle 13
  • 14. Pele Oy Calendering Process
  • 15. Pele Oy Calender operation Important calender control variables are:  Total nip impulse (linear load, number of nips, speed)  Web temperature and heating (gradient)  Web moisture and moistening (gradient) Main controlled web properties are:  Smoothness  Gloss  Porosity/absorption properties  Caliper  Two-sidedness 15
  • 16. Pele Oy Nip impulse  Paper is viscoelastic. This means that not only the pressure but also the time of pressure has effect on the calendering.  Effect of pressure forces in calendering are related to pressure impulse, which is about the same behavior as in wet pressing. Impulse = Pressure x Time Pressure = linear load / nip length Time = nip length / speed Impulse = linear load / speed time Impulse = area Impulse = Σ pressure x time = speed Σ linear loads Nip pressure 16
  • 17. Pele Oy Calendering effects on paper properties Positive Effects:  Smoothness  (rotogravure)  Gloss  (coated papers)  Absorption and porosity  (SC paper, release paper, cartonboard)  Linting tendency  (offset)  Caliper control (specialty papers)  Two-sidedness control (printing papers) Negative Effects:  Bulk and stiffness   Strength properties   Light scattering   Opacity and brightness   Blackening, mottling   Barring   Wrinkles and calender cuts   Runnability   Costs  Special Effects:  Widening of web <0.5 %  Drying of paper 1-10 %-unit 17
  • 18. Pele Oy Calendering of second side Hot glossing surface Paper before the second side hot roll Elastic roll cover Arch breaks down – smooth surface roughens again Glossing of second side Hot glossing surface 18
  • 19. Pele Oy Effect of moistening on SC paper  Thick-walled mechanical fibers swell in moistening thus reducing paper smoothness and gloss.  If these fibers are compressed in calendering, they easily spring back to the original form. Fibers should be smaller and thin-walled to stay collapsed.  Thick fibers should be compressed at the wet end to get more permanent result. Moistening with water like in offset printing. 19
  • 20. Pele Oy The influence of nip load on pore structure  When linear load is increased compaction of coated paper can be seen in reduced number of large pores. Rescxh et al. January 2010 Tappi Journal 20
  • 21. Pele Oy Soft calendering, gloss and smoothness  It is possible to improve gloss of coated paper by increasing steel roll temperature up to 190 °C.  PPS roughness decreases with temperature only up to 150 °C and is after that constant with increasing temperature. Robert Rounsley, January 1991 Tappi Journal No effect 21
  • 22. Pele Oy Influence of humidity on paper gloss  If paper is not in balance with the ambient air humidity gloss decreases.  To get a good printing result, correct and even moisture content is as important as gloss or smoothness itself.  In the picture gloss decreases 25% when air humidity is increased from dry air to 90% humidity. Gloss improves slightly when paper is dried again. TOSHIHARU ENOMAE AND PIERRE LEPOUTRE: JPPS 23(7):J1-J7(1997) 22
  • 23. Pele Oy Coated paper and COF  Calendering reduces COF only slightly compared to coating formulation.  Adding PCC to clay coated paper formulation increases Coefficient of Friction (picture).  Lubricants reduce friction of coated paper.  It seems that too much friction and shear force can cause dusting and fiber picking to the calender rolls.  Too high COF can also cause vibration problems on customer roll winder. Picture: Toshiharu Enomae, Naoya Yamaguchi and Fumihiko Onabe 23
  • 24. Pele Oy TMP handsheets, PPS and density  Smoothening made only with calender is not retained after offset printing. Forseth, T., Helle, T., Wiik, K., 1996 International Printing and Graphic Arts Conference After remoistening (simulated printing) After calendering Before calendering Density 300  750 450  750 PPS 83 PPS 35 24
  • 25. Pele Oy Temperature gradient calendering Whole paper deformed Only surface deformed 25
  • 26. Pele Oy Coventional Calendering
  • 27. Pele Oy Calendering methods  Machine calendering (hard rolls, MF)  Brush calendering (old method for cartonboard)  Soft calendering i.e. one nip per soft roll • On-machine soft calendering or gloss calendering with hard/soft rolls • On-machine matt calendering with soft/soft rolls  Multi-nip calendering including intermediate soft roll(s) • Off-machine (Supercalender) • On-machine (Janus, OptiLoad etc.)  Wide nip shoe calendering  Hot metal belt calendering (Valmet)  Aqua cooling calendering (Valmet) 27 Early supercalender
  • 28. Pele Oy Typical hard nip calender 28
  • 29. Pele Oy Two-nip soft calender  This kind of soft calender is typical for copy and other uncoated woodfree papers. Sometimes only one nip is needed, if base paper is not symmetrical. 29
  • 30. Pele Oy On-machine calendering Pre-calender before coating Picture: Voith Paper On-line calender after coating Hot iron rolls 30
  • 31. Pele Oy Supercalender  Soft rolls can be paper filled or synthetic covered rolls  To make glossy paper about 10-12 rolls are needed 31
  • 32. Pele Oy Roughness, gloss and calendering  Multi nip calendering is required for WFC, MWC, LWC and SC-A grades. Instead, other grades illustrated in the graph below, can be calendered using soft nip calender. A hard nip calender is not very suitable for offset paper due to mottling tendency. HunterGloss,% News SC-A SC-B Improved News LWC SC-C 10 20 30 40 50 60 70 0 1 2 3 4 5 HunterGloss,% NewsNews SC-ASC-A SC-BSC-B Improved News LWC SC-C FCO WFC MWC Soft Calender Multinip Calender µm PPS 10 Roughness, Multinip or Soft 32
  • 33. Pele Oy Cross Direction Profile Questions
  • 34. Pele Oy Supercalender profiles and temperature  In a cold supercalender edges may have higher pressure than the middle.  When supercalender heats up the hotter middle part presses more than colder edges.  It is important to take nip impressions when calender is warm. ELISABETH H. JONES AND ROBERT H. MOORE, TAPPI Journal, Feb. 1997 34
  • 35. Pele Oy CD-moisture control  Conventional SC paper is overdried to 2-4% and then moistened to 8-10%.  Overdrying evens out the profile, because moist parts have better heat conduction and easier evaporation.  On-line calendering requires good moisture profiles without overdrying.  If profiles are not good, final moisture must be reduced (average printability and runnability are then suffered) Moisture 10% 5% 1. Higher steam cons. 2. Less water sold 3. Lower runnability Front Back 35
  • 36. Pele Oy Moisture level and CD profiles of SC raw paper 36 Max 6.07 Avg 5.00 Min 4.02 Max-Min 2.05 5% Max 7.21 Avg 5.93 Min 4.34 Max-Min 2.87 6% Max 9.37 Avg 7.66 Min 5.60 Max-Min 3.77 8% Max 3.49 Avg 2.96 Min 2.56 Max-Min 0.93 3% Picture: Valmet
  • 37. Pele Oy Example of grinding tolerance effect on paper  Calender roll diameter 500 mm. Best possible grinding accuracy 1 µm = 0.0002 % of roll diameter but 2 % of paper caliper (magnification 10 000).  From a 1000 mm roll diameter 2% is 20 mm (very high difference). Paper maker’s demand is always higher than any maintenance can offer. It is always feasible to grind rolls to the best possible accuracy.  Accuracy cannot be better than measurement. If the measurement accuracy is 0.01 mm the result is ten times worse (quite common). Paper caliper 50 µm +0.5 µm Roll surface +0.5 µm 49 µm = 2% lower caliper Cross machine direction 37
  • 38. Pele Oy Moisture streaks on machine reel  The wide temperature variations seen in the left IR picture, caused by evaporative cooling, correspond to variations in CD moisture.  Picture on the right show severe moisture streakiness. This is so narrow that it is not shown with standard scanning measurements.
  • 39. Pele Oy New Calendering Methods How heavy is your iron? >7 kg < 1 kg
  • 40. Pele Oy Metal belt calender  Dwell time under heated belt and nip is extended. Heated steel belt is only 0.8 mm thick.  Three rolls are oil heated. Surface temperature of rolls and belt is 150 - 200 ºC.  Metal belt precalender improves final smoothness and printability • less re-roughening during coating • uniform coating layer • low final PPS roughness • low mottling values after printing 40
  • 41. Pele Oy 0 5 10 15 20 25 0 20 40 60 80 100 time, ms pressure,MPa Extended calendering zone hard nip soft nip Metal belt calender Metal belt calendering 41
  • 42. Pele Oy Results after final calendering  Metal belt precalender gives lower PPS roughness at same bulk level. 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0.76 0.78 0.80 0.82 0.84 0.86 0.88 Paper Bulk, cm³/g PPSs10roughness,µm no precalendering + blade coating hard nip + blade coating soft nip + blade coating 2 shoe nips + blade coating metal belt calender + blade coating Precalendering + Coating Final calendering 5 nips, 145 °C, 255 kN/m Picture: Valmet 42
  • 43. Pele Oy Precalender concept Bendtsen roughness, ml/min Before After Difference Hard nip, (150 °C, 150 kN/m) 83 120 +36 Soft nip (150 °C, 300 kN/m) 78 111 +33 Metal belt calender (150 °C, 70 kN/m) 90 114 +24 Metal belt calender (150 °C, 100 kN/m) 65 87 +23 Results of rewetting test  Bendtsen roughness of calendered paper before and after surface moisturizing:  Less re-roughening when moisturized for metal belt calendered paper  More stable surface with metal belt calendering 43
  • 44. Pele Oy Metal Belt precalender  Metal belt precalender + multilayer curtain coating gives high gloss and excellent printability • glossing pigment in top layer • more uniform coating color layer • PPS s10 roughness level is comparable to “hard nip precalender + blade coating” (common technology today)  Multinip final calender gives clearly higher gloss and lower PPS than a two-nip soft calender  Metal belt precalender and/or murtilayer curtain coating can not fully compensate for a lighter final calendering 44
  • 45. Pele Oy Valmet metal belt calendering  OptiCalender Metal Belt is easy to use. Linear load is the only control parameter that needs adjusting in the calendering process.  Two-sidedness can be controlled with temperature. Also the operating window is large. The example in the figure below shows the comparison of coated board calendering (the quality level and bulkiness). Picture: Valmet 45
  • 46. Pele Oy Valmet Aqua cooling calender  With conventional roll nip, the best calendering results are achieved when the web is cool and stiff, like in pilot trials.  With aqua cooling technology, pilot-type conditions are brought to production-level, by cooling down the web before the calendering nip.  The first system is delivered to Stora Enso Inkeroinen board machine to produce folding boxboard. With aqua cooling technology it is possible to get almost half of the metal belt calendering benefits with significantly lower investment costs. Picture: Valmet 46
  • 47. Pele Oy Calendering effect of Aqua cooling  The picture shows cooling effect of 200 g/m2 FBB to Bendtsen roughness and bulk.  Speed is 600 mpm, thermo roll surface 200 °C, and nip loads 30 and 150 kN/m.  Compared at the same roughness level (150) after calendering, bulk was 1.72 at an incoming web temperature of 65 °C, while bulk was 1.78 at an incoming web temperature of 36 °C after cooling the web from 65 °C. This means that extra 3.5 % bulk can be obtained. Picture: Valmet 47
  • 48. Pele Oy Research Demands
  • 49. Pele Oy Old truth is still valid  I had a presentation in Turku, Finland 27 October, 2004  The title of the presentation was “Calendering Processes - Future Scenarios and Research Demands”.  The following two pages are a copy of that presentation.  Now, after more than ten years, it is interesting to note, that there has been huge progress in almost all of those areas which I listed.  The only area where I desire more research is calender nip, including forces and their effect on smoothness, gloss and bulk. Not only pressure force but shear forces, friction forces and the length of slip and stick areas in the nip of elastic roll material.  Important would also be the effect of drive torque and Poisson’s ratio to these phenomena. Old studies show that smoothness is different when iron roll drives elastic roll compared to the opposite when elastic roll drives iron roll. 49
  • 50. Pele Oy Research demands More comprehensive research  Trials with variables in papermaking, coating and calendering.  Effect of friction and shear forces as well as Poisson’s ratios of soft rolls  How to simulate on-machine calendering?  More mill research to get correct incoming web properties and CD profiles Two-sidedness control  Total process control in papermaking, coating and calendering  How to get even-sided paper at the same time in gloss, smoothness and oil absorption.  Interactions between (shoe) pressing, drying and calendering  Calendering effect on the soft roll side Improved bulk and stiffness  How to get better gradient effects with not only temperature, but also with moisture and base paper (raw material layering, press section gradient, drying section gradient)  Hot calendering without water evaporation 50
  • 51. Pele Oy Future calendering scenarios Reduced calendering costs  More on-machine calendering  Simple, high-capacity calenders Better two-sidedness control  Simultaneous glossing and smoothening of both sides  Multivariable control for gloss, smoothness and oil absorption Improved bulk and stiffness  Heavy calendering of base paper – light calendering of coated paper  Calendering chemicals and ”chemistry”  Web cooling, variable dwell time moistening  Three-layer base paper (or otherwise different surface than in the middle)  Air conditioning around calender Better CD-profiles and winder rolls  Additional CD profile measurements of temperature, moisture, porosity, blackening, web tension and roll hardness 51
  • 52. Pele Oy Thank You for Your Attention
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