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Paper roll quality and roll hardness 1 jan 2018

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Focus on paper roll quality, roll defects and roll hardness testing

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Paper roll quality and roll hardness 1 jan 2018

  1. 1. Pele Oy Paper Roll Quality and Roll Hardness January 2018
  2. 2. Pele Oy Paper compressibility and roll hardness  Newsprint is compressible and bulky paper compared to LWC and coated woodfree.  During winding newsprint loses the original wound-in tension very fast and the rolls are easily too soft. High density papers and especially plastic foils are not compressed as much and rolls are harder. 0 5 10 15 20 25 0 50 100 150 200 Compression,% Radial Pressure, kPa Newsprint LWC Coated woodfree, gloss 2
  3. 3. Pele Oy Roll stresses  Every wound paper layer on the roll surface increases radial pressure and compress the inner portion of the roll. Compression reduces the original roll periphery and thus circumferential tension as long as there will be balance between the outer tension band and the spring-back of the inner roll body.  When the balance is reached, there is a negative circumferential tension inside the roll. This tries to buckle the layers, but paper stiffness and radial pressure prevent the buckling in a good roll.  The positive tension band of a bulky paper is only some centimeters from the roll surface.  On glossy SC and LWC rolls this positive tension band is much thicker and the highest pressure is reached deeper in the roll. Compressed diameter after winding additional layers Outside diameter Original diameter 3
  4. 4. Pele Oy Internal roll stresses in a good roll  In a good roll the radial pressure between paper layers is highest against the core and then constant or slightly decreasing to the roll periphery.  If radial pressure P can be measured, circumferential tension T can be calculated: • T = - P - dP/dr x r • where r = roll radius/(core outside radius) • P = internal roll pressure, kPa • T = internal web tension, kPa  In a newsprint roll P can be 300 kPa (3 bar) and if the curve is flat in the middle (dP/dr = 0) tension T is also - 300 kPa (= MD compression).  If paper caliper is 0.07 mm we get a negative tension of - 21 N/m. 4
  5. 5. Pele Oy Roll stresses and paper properties  The positive tension band on top of the roll is only some centimeters thick. Inside this band paper will be plastically strained and under the band plastically compressed during storage. This paper deformation reduces all roll stresses.  In the following unwind process paper on the roll surface has lower elongation and dynamic stretch. This might have influence on paper runnability and breaks during flying splice can happen.  Paper inside the roll is more compressed and reaches better smoothness, lower bulk and caliper, lower porosity and better elongation and dynamic stretch properties.  Main part of the paper inside a roll is not similar as a sample from the roll surface. Can be runnability problem 5
  6. 6. Pele Oy Example of paperboard properties in a roll  Physical properties of paper are changed in a roll due to the stresses during winding and on roll storage. Paperboard (here FBB) changes more than printing paper.  The picture below is an example of paper gloss, which is about 10 % higher (about 6 %- unit) on the core compared to the roll surface.  This is from a two-drum winder. Belt supported winder gives more even result. 6 Distance from roll core0 750 mm Gloss,% JUHO WALDÉN, 2014
  7. 7. Pele Oy Winding and ”hardness memory”  Radial pressure in a machine reel is normally decreasing towards the periphery so that the surface is softer than the start of the winding. One reason is that the nip widens when the reel diameter grows. In addition, all paper reels have low radial pressure on the surface due to less and less compressing layers.  Paper layers on the machine reel surface are less compressed than inside the reel (they are thicker on the surface).  Winding is a very dynamic process. Web travel from unwind to rewind can be some 10 m and winder speed 40 m/s. The delay time from unwind to rewind is then 0.25 s. In this short time paper is without compression and it tries to expand to the original thickness. However, the time is so short that the thickness inside the machine reel is almost ”copied” to the winder roll and the compression due to winder parameters still reduce the thickness.  The result is that the first set wound from the machine reel surface tends to be soft at start, and hardness is increasing towards the roll surface. The last set tends to be more constant in hardness and especially soft starts or roll deformations are not as big problems. Diameter Hardness after winder 3rd set 1st set Diameter Radial pressure of parent reel 7
  8. 8. Pele Oy Roll density measurement  Online roll density measurements can be made in winding or unwinding supposing that the original paper thickness is constant. This is possible by calculating paper caliper inside the roll from the angular and surface speeds of the roll.  The picture below shows typical newsprint roll density distributions and how the first set on the winder is softer than the other sets. 8 660 665 670 675 680 685 690 695 100 300 500 700 900 Roll diameter, mm Rolldensity,kg/m3 First set Second and last sets First online density measurements Pekka Komulainen, 1977 Veitsiluoto, Finland
  9. 9. Pele Oy Paper roll is a spiral  Paper roll is not a cylinder formed of paper rings but a continuous spiral of paper with continuous MD tension (plus or minus).  CD differences of web tension, strain and original web length are important due to the spiral form.  Differences in web length and strain also have effect on roll hardness in a spiral.  Often an accuracy of 0.1% is enough in papermaking. If we have this difference in original paper length, it means that after winding only 1000 m we would have the longer part of the web one meter ahead of the shorter part. We need web tension to even out these differences.  When crepe wrinkles inside the roll are formed, it is not only one layer slipping but sometimes two or more layers. This causes a longer total movement and more severe damages between the slipping layers.  Due to the slipping, telescoping can be an additional problem. 9
  10. 10. Pele Oy Example of the spiral web effect  If you measure drum speed difference in a two-drum winder you will find, that the front drum has slower speed in spite of higher torque.  The explanation is that there is all the time one web layer less on the front drum compared to the rear drum.  Theoretical curve can be calculated. The speed difference is higher in the beginning and depends on the paper caliper.  Some winders have speed difference control instead of torque control. However, if there are several paper grades it will be very difficult to find correct speed difference curve for each paper grade. 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 100 400 700 1000 Roll diameter, mm Speeddifference,% 10
  11. 11. Pele Oy Winding parameters for two-drum winder 1 Web tension, T 2a Rider roll load, N 2b Roll weight, N 3 Torque (Winding force), T Roll diameter Roll tightness 11 = TNT Pictures: Valmet
  12. 12. Pele Oy Roll hardness in radial direction  Ideally the hardness should be slightly decreasing from the core to the periphery. However, this is only possible when the winder has center drive and support.  More common than center drive winder is a two-drum winder, where roll’s own weight increases nip force and normally some other optimization must be made. Picture: R. Duane Smith 12
  13. 13. Pele Oy Typical two-drum winder hardness distribution  It is quite easy to get a tight and hard start to the roll by using high torque.  When the roll diameter increases the roll nip force decreases due to the geometry. In addition, effect of torque decreases very fast in the beginning.  The only way to keep roll hardness constant would be fast increased rider roll load. For most papers the rider roll load of large roll diameters should be so high that there would be wrinkles and bursts in the roll.  When the roll grows, its own weight causes very high nip load. To compensate this hardening effect, web tension should be reduced. However, too small web tension together with high nip load can cause wrinkles and bursts. Roll diameter Roll Hardness OK Too hard Too soft 13
  14. 14. Pele Oy Winding and CD profile properties  It would be easier to make directly sheeted paper without winding like in a pulp dryer.  Total thickness of a paper pile under pressure is related to original caliper and compression properties of the paper.  The diameter of a paper roll is related not only to the caliper and compressibility but also to the original web length and MD strain properties of the paper web.  To make good rolls we should have online measurements of all the four variables. Sometimes we measure and control only paper caliper profile without any information of the three other profiles.  If there are CD profile related problems after winding, papermakers usually argue that grammage, moisture and caliper profiles are straight.  One should first think if these measurements are correct and relevant and then look, how much worse the other profiles become by controlling the basic profiles straight. 14 Pile of paper Pile thickness depends on - paper caliper - paper compressibility Paper roll Roll diameter depends on - caliper & compressibility - web length & strain
  15. 15. Pele Oy Paper elongation and roll hardness  The picture below shows how sheet length or elasticity effect on roll hardness and bagginess.  If web caliper is constant, longer part of the web forms softer roll or even bagginess and wrinkles. 15
  16. 16. Pele Oy Web caliper, length and reel hardness  To make even reel hardness in cross-machine direction it is important to have correct web length related to the roll diameter in every CD position.  For heavy calendered, high density papers constant caliper is more important (paper is not any more compressible to even out reel diameter differences). For bulky webs, length and caliper differences are both important. Length Caliper Thin Average Thick Short Good Hard Rock- hard Average Soft Good Hard Long Very soft Soft Good When = π = 3,14 it means that roll hardness is even L D L D 16
  17. 17. Pele Oy Roll Quality and Hardness Testing
  18. 18. Pele Oy Paper roll quality testing  Cross Direction Testing  Backtender’s Billy club (wooden stick)  Beloit Rho-meter  Schmidt hammer  Parotester  Tapio RQP  ACA Systems RoQ  On-line backtender’s friend  Roll diameter profile measurement  Valmet iRoll  Radial Testing  Cameron gap test  Smith needle  Improved needles or plastic strips  J-line measurement  Core torque wrench test  Wit-Wot Roll Analyzer  On-line density measurement 18
  19. 19. Pele Oy Principle of general hardness testers  General hardness testers are of two types: static or dynamic.  Hardness with static testers can be defined as the resistance of a material to permanent, plastic deformation. The traditional hardness tests used for metals and concrete are based on well-defined physical indentation tests.  The original dynamic scleroscope test consists of dropping a hammer, which falls inside a glass tube under the force of its own weight from a fixed height onto the test specimen. The height of the rebound travel of the hammer is measured. In original use for hard materials the scleroscope test does not mark the material under test.  The scleroscope measures hardness in terms of the elasticity of the material and the hardness number depends on the height to which the hammer rebounds, the harder the material, the higher the rebound. Advantages of this method are portability and non-marking of the test surface. 19 Old scleroscope
  20. 20. Pele Oy Dynamic Leeb hardness  According to the dynamic Leeb principle (invented by Leeb and Brandestini, 1975), hardness value is derived from the energy loss of a defined impact body after impacting on a sample, similar to the Shore scleroscope.  The Leeb quotient (vr /vi) is taken as a measure of the energy loss by plastic deformation: the impact body rebounds faster from harder test samples than it does from softer ones, resulting in a greater value of 1000 × vr /vi.  While in the traditional static tests the force is applied uniformly with increasing magnitude, dynamic testing methods apply an instantaneous load. A test takes a mere 2 seconds and, using the standard probe D, leaves an indentation of ~0.5 mm in diameter on steel or steel casting with a Leeb hardness of 600 HLD.  Proseq Equotip and PaperSchmidt are examples of Leeb principle. 20 Picture: Proseq
  21. 21. Pele Oy Needs of paper roll hardness testing  The main general need of hardness testing is the level of the hardness. For paper roll the main need is to measure the cross-direction stable variation of the hardness, not the universal level of the hardness. Hardness curves as a function of CD position are important, not only one variation number.  Because paper roll hardness tester should be nondestructive, portable and fast, static testers commonly used for metals and concrete are not suitable for paper roll.  Paper roll is a layered structure of paper sheets and air between. With porous papers there is air also inside the sheet. Without air paper roll could be totally elastic and very hard. The more there is air, the softer the roll is.  One can understand that on the top of the roll there are much thicker air layers than inside and close to the core. Actually hardness test measures more air content in the roll than material itself. Air is the viscous element causing deformation. Energy is used to this plastic deformation or air removal (inside fibers, between fibers, from surface roughness and finally between paper layers). 21
  22. 22. Pele Oy Hardness test and roll structure  Paper roll structure is normally understood to be pressure distribution between paper layers. In a good roll pressure increases from roll surface to the core. This increase is very fast on top of the roll but much slower inside.  It is very interesting question how deep hardness testers measure. This should be studied.  If the hardness measurement depth is less than 20 mm, the result can vary very much depending on the ratio of paper elastic moduli Et/Er.  Positive tension band on roll surface is thicker with dense papers (LWC) and narrower with bulky papers (Newsprint). 22
  23. 23. Pele Oy Conventional paper roll hardness testers  The basic and conventional methods are Beloit Rho meter, Schmidt hammer and Paro tester. All these measure with a quite strong impact to the roll.  There are no studies how deep hardness testers measure. Schmidt hammer measures concrete to about 10 cm depth. Paper roll is softer and contains layers and air. Hence the measuring depth must be much lower. 23 Picture: DIPESH DILIP MISTRY
  24. 24. Pele Oy PaperSchmidt technical information  Proceq has published more information than the other suppliers.  For example Tapio tells only: “Hitting velocity and the shape of the hammer's tip can be selected to suit each customers preferences. The goal is to mimimize the possible damage inflicted to the tested material (paper, plastic rolls or aluminium foil rolls) and to provide accurate measurements.” 24 PaperSchmidt roll hardness tessting
  25. 25. Pele Oy Fishbone diagram of hardness testing 25
  26. 26. Pele Oy Tapio RQP and ACA Systems RoQ  The newest type of testers such as Tapio RQP and ACA Systems RoQ measure 30-50 times per second. The measuring time of one point for the conventional testers is several seconds. It is supposed that the impact of the newest testers must be much lower than with the older testers. With low impact the measurement depth is also low. This is not studied, but the measurement depth should be calculated in number of paper layers, not in centimeters.  The lower the measurement depth is, the more the test measures paper properties such as porosity, roughness, stiffness, friction and elastic moduli of paper. 26
  27. 27. Pele Oy TAPIO RQP v2.0 and hammer penetration  Tapio Technologies states that the new RQP v2.0 tester is based on the position measurement instead of the acceleration measurement used earlier.  The penetration of the hammer is also measured providing new interesting information about the measured surface.  What could this interesting information be? It is well known that paper moisture and temperature have effect on paper plasticity and static hardness testers measure plasticity. Very often when there is moisture variation in the sheet, moist areas are cool before calendaring. After calendering paper is cooled down and dry areas cool faster than moist areas i.e. moist areas are hot and very plastic.  If penetration is something else e.g. more correlated to paper moisture than to hardness number, it would help in understanding CMD profiles better. When test methods develop, more studies are needed! 27
  28. 28. Pele Oy Plastic and elastic deformation  Elastic and plastic deformation of multiple sheets. The maximum force used in these experiments is 100 N.  These blue curves are the experimental force-deformation curves, the red points show the positions of the biggest deformations, the green points show the plastic (residual) deformations.  The sheet numbers are 16, 32, 48 and 72.  It can be seen that higher pressure force gives higher plastic deformation.  Modern hardness testers use very small impact and the deformation is mostly elastic. 28 Dissertation Jian Chen
  29. 29. Pele Oy Calendering variables  Paper plasticity has effect on calendering and paper caliper. In addition, plasticity has effect on roll hardness directly. It is important to remember that moisture and temperature change, when the roll travels from paper machine to winder and finally to the customer. 29 Source: Fapet
  30. 30. Pele Oy Need of CD test sampling frequency  The main need of roll hardness test is to get so accurate CD profile that decisions of rejected paper can be made. Online scanning of paper never can separate MD and CD variations.  The best real CD measurement to compare with hardness test is thermographic IR camera. It will mainly react to CD moisture streaks and they can be very narrow as in the picture on the right.  It depends on the process what is the correlation between the measured profiles. Very often the root cause is dry basis weight linked to moisture of IR camera → caliper → paper length → roll hardness.  Higher density papers react mainly through caliper and low density papers through paper length/tension differences.  Anyway, hardness test must be fast to get at least some points per second. 30
  31. 31. Pele Oy Moisture streaks on machine reel  The wide temperature variations seen on the left picture, caused by evaporative cooling, correspond to variations in moisture.  On the right a reel having severe moisture streakiness is presented. This variation probably is so narrow that it is not shown in online scanning measurement. Photo: Albany International 31
  32. 32. Pele Oy Tapio RQP measurement speed  New testers can measure with a continuous high speed. An example below shows that 1 m/s (lowest CD profile) is too high speed and 0.1 m/s is too low and accurate (red).  Suitable speed can be about 0.5 m/s (blue). If reel width is 5 m, profiling time is 10 s. 32 Picture: Sami Uhlbäck, 2008
  33. 33. Pele Oy Overall hardness and CD profile  Newest roll hardness testers are good and fast in CD profile measurement but probably not as good in overall roll hardness as the conventional methods. For overall hardness calculated roll density is still a good method. Operators often compare web length to roll diameter and then notice if something happens.  In addition, it must be remembered that all instruments require maintenance. The following picture is from Proceq and tells that all testers have some wear, which has influence on the hardness measurement level (not on CD profile form). 33
  34. 34. Pele Oy Tester comparisons  It is important to compare available testers to get the best for a specific purpose.  Important point is to study measurement variation. This picture is from Proceq showing measurement of one roll and corresponding variation.  One important point is usability – how fast the test is and how much the procedure requires learning. Additionally spare parts and maintenance are always important questions.  Today the prices of testers are so low compared to the benefits that return on investment is always guaranteed. 34
  35. 35. Pele Oy Human errors in hardness testing  The following publication is one of the best concerning winding and CD profiles: WINDING EXPERIMENTS ON NONUNIFORM THICKNESS WEBS, By JARED WILLIAM GALE  sequence=1  Jared Gale noticed in his study: “There were some slight inconsistencies within the hardness test. This can be attributed to human error due to the difficulty in insuring that the trigger was pulled at same rate and pressure for each test. It was also difficult maintaining the Rho meter in a tangent position to the roll.”  There are basic differences between conventional point-by-point hardness testers and the newest higher speed continuous testers. For comparison it is very important that several persons make tests of the same roll and the results are compared.  Hand-held moving testers can have different speed, angles to CMD/roll tangent and pressure against roll depending on the individual person. In addition, top layer can be more or less loose. 35
  36. 36. Pele Oy Theory of Mill Assist new RHO meter  There is also a new version of conventional RHO-meter available.  There is very little published information about the theory. ACA and Tapio tell impact frequency, but very little about theory and what important roll properties the tester really measure.  Something can be found about BEL-2000 GEN II, which is copied to the right. 36
  37. 37. Pele Oy Modern hardness testing and real life Modern hardness testing  Very dynamic and fast  Low adjustable impact  Narrow local pressure  Deformation mainly elastic Knowledge of hardness testing  No good theories or studies  Not clear what is measured  In laboratory studies paper pile is under pressure, but not under MD tension like in reality Real roll life  Long time from paper machine to the customer  Reel-wide pressure and tension  Several impacts during handling  Deformation viscoelastic and plastic  Creep and relaxation have effect  Varying humidity and temperature environment 37
  38. 38. Pele Oy Bagginess probability  Amy Thuer has published the picture below in a study of plastic film rolls. This might also be quite valid for paper rolls. However, it must be remembered that paper is different compared to plastic foil. High density papers (D>1000 kg/m3) behave more like plastic, but other papers have voids inside the material and then the differences in caliper are not as dominant. Paper can compress and even out the hardest lanes of the roll. 38 Picture: Amy Thuer, WHEN ROLL HARDNESS CAN PREDICT BAGGINESS AND WHEN IT CAN’T
  39. 39. Pele Oy Roll density vs. roll hardness  Roll density and hardness were measured from three different kraftliner grades.  Correlation is very good. Density was calculated from roll weight and compared to average measured hardness. 39 RollDensity Roll Hardness Picture: Linus Söremark
  40. 40. Pele Oy Thickness scanner vs. iRoll CD profile (Valmet)  Hardness profile is a basic roll quality measurement. It cannot be replaced by online caliper measurement. Valmet iRoll is a good alternative, but very expensive. The following is what Valmet comments on caliper measurement:  There are four main issues with thickness scanner performance. These are: 1. Resolution is too low, in practice 1 micrometer or worse. The needed resolution should be at least ten times better. Quality Control System (QCS) suppliers sometimes state that they have better resolution but that is not true in reality. 2. Requires constant care. There is a constant need for cleaning, calibration and other maintenance and tuning. Without this the whole measurement concept fails to work. In practice, it's not possible to provide this constant service in a mill environment. 3. Caliper is not the right thing to measure. To assure good reel buildup and runnability, the primary parameter is the hardness/diameter profile. Thickness is only a secondary parameter that effects hardness and diameter. 4. Web breaks are caused by contacting caliper measurement scanners. 40
  41. 41. Pele Oy Continuous hardness measurement on winder  Due to the shrinkage on paper drying the edges are normally soft and loose under the rider roll on winder. On a two-drum winder this is a demanding situation.  It is dangerous to press edges more than the center under rider roll or otherwise wrinkles will arise (loose web and high nip load at the same time). To avoid this, roll hardness profile is normally slightly lower at edges or rider rolls are installed slightly higher at edges. 41 Picture: Valmet
  42. 42. Pele Oy Selected Roll Hardness Profiles Example TAPIO RQP Plastic Foil Roll
  43. 43. Pele Oy Machine reel hardness profiles  The shape of the CD hardness profile in a machine reel is quite constant.  This picture shows relative hardness profiles of four sets measured on the unwind roll surface from the winder. Picture: Sami Hyötynen Metso Paper Oy 43
  44. 44. Pele Oy Parent reel and roll profile forms  The shape of the curves from parent reel compared to customer rolls is very similar. The average hardness of customer rolls is higher.  Soft spots in parent reel are about 20 units softer than in customer rolls. D.M.S. WANIGARATNE et al. APPITA 2010 44
  45. 45. Pele Oy Parent reel and roll hardness profiles  Customer rolls are harder than parent reel.  First set made from the parent reel surface is softer than the second set from the inner part of the parent reel. First set from parent reel surface Second set from parent reel surface Customer roll hardness profiles Parent reel hardness profile Pictures from: 58/TMP.objres.156.pdf?sequence=1 45
  46. 46. Pele Oy Caliper and reel hardness  Reel hardness of 205 g/m2 packaging board correlates very well with caliper profile. 46 D.M.S. WANIGARATNE et al.. APPITA 2010-317
  47. 47. Pele Oy Effect of supercalender on hardness profile  After coating the hardness profile of LWC paper is quite even (blue).  After supercalendering there is more short and long range variation (red with hard center). Picture: Ilari Ikonen, 2010 47
  48. 48. Pele Oy CD caliper profile and reel hardness  Caliper as such is not very important. However, papermakers try to control the process so that the shape of the CD caliper profile is straight (why?).  This does not guarantee a straight hardness profile and it is common, that edge rolls have too soft ends after winder.  The second roll from the right shows a too large local hardness difference leading to problems such as bagginess, bursts or corrugations. Four rolls in a set, edge rolls are soft at edges. 48
  49. 49. Pele Oy Roll hardness and paper caliper profiles  PAROtester2 CD hardness of uncoated magazine paper. The profiles are from the top: grammage, caliper and roll hardness.  It can be seen that caliper follows grammage and hardness caliper.  Final problem is the marked red range of corrugation, where all profiles reach the minimum.  Hardness is an accurate measurement compared to the other measurements. The biggest relative difference is in hardness profile (over 10%). Picture: Juha Turkki 49
  50. 50. Pele Oy Example of hardness and corrugation  The picture below shows clearly that quite a narrow resolution in CD direction is needed to predict corrugations in uncoated SC paper.  The corrugated roll was the first from the left (blue). Roll width was only 60 cm and there were 3 clear peaks in hardness (20 cm interval). TAPIO RQP measurement. 50 Hardness Roll width, cmPicture: Juha Turkki
  51. 51. Pele Oy Coated paper hardness profiles  The picture below shows machine reel hardness profiles of woodfree coated paper. Profiles have been measured from winder unwind before winding each set of rolls. There were totally five sets.  It is typical that the first set is always softer than the other four sets. Picture: Teemu Pasi 51
  52. 52. Pele Oy Example of several hardness profiles  Example of 17 successive SC paper roll hardness profiles from the same CD position. The stability of the curve form is quite good. 52 Picture: Sami Uhlbäck 2008
  53. 53. Pele Oy SC paper runnability in gravure printing  The upper curve caused web breaks in printing (too much roll hardness variation).  The lower curve run without problems. Picture: Sami Uhlbäck, 2008 53
  54. 54. Pele Oy Local faults in hardness profile  Slack areas increase web breaks and paper waste.  In this picture a 20 mm wide slack area is found 100 mm from the roll edge.  This requires high resolution fast measurement to be found. Slack area 54 Picture: Sami Uhlbäck, 2008
  55. 55. Pele Oy Caliper and hardness control  Caliper control with calender is directly seen on hardness profile. 55
  56. 56. Pele Oy CD shrinkage profile in paper drying  Web edges normally have different hardness in the winder rolls. This is due to the higher shrinkage at the edges.  This shrinkage profile is from a modern newsprint machine with single felted dryers. One meter from the edges has 3-8% shrinkage while it is in the middle less than 2%. 56 Picture: Steve I’Anson
  57. 57. Pele Oy CD hardness profile and bagginess  This picture from ACA Systems shows very accurate correlation of bag formation and hard spots on the roll. This is typical for high density papers and plastic foils.  For bulky and porous papers this correlation is not that simple. 57
  58. 58. Pele Oy Aluminium foil and bagginess  The picture on the right shows web tension CD profiles at different radial positions in the reel, for the case where aluminium foil was wound throughout the reel.  Close to the core higher web grammage and thickness give more tension (is stiff).  When more layers are wound there will be more plastic MD strain in the middle and this compensates the original tightness.  Finally on the surface, the plastic strain has higher effect on the web tightness and the web will be slack in the middle.  The correlation between web thickness, roll hardness and web bagginess depends on the radial position in the final roll and how many windings there are after first winding. Loose surface will be close to the core in next winding. 58 09/06/doctoral_thesis_c_land_16754.pdf
  59. 59. Pele Oy Typical Roll Defects Air to roll Uneven hardness profile More info from Roisum:
  60. 60. Pele Oy Main problems of machine reels  The material is from 2004. The mills were printing paper mills. The most common problems seem to be wrinkles, TNT control and CD profiles/bursts.  The main problems are connected together and could be reduced with new type of reels. 60 Collected from several newsprint and magazine paper mills
  61. 61. Pele Oy Pressure from spool to paper The problem: Wrinkles and bursts on reel bottom. Coated board: glossy spots close to the spool. Bottom broke up to 2% = 2000 m of 100 km. Problem areas  There is more problems at edges when the spool is thin, reel is wide, diameter is large, basis weight is low and paper density is high.  Thin groundwood papers are more sensitive. CD profiles are important.  Reel hardness, hardness distribution, air between layers and machine speed have influence.  Center drive, larger diameter reel spools and new type of reel changes can solve the problem. 61 Picture: Valmet
  62. 62. Pele Oy Main problems on printing paper winders  The most common problems seem to be dishing or uneven roll edge, CD profiles, crepe wrinkles and loose cores or bad start. 0 2 4 6 8 10 12 14 Dishing/uneven edge Profile Wrinkles Loose cores/bad start Trim Bursts Splicing SMB's Spreading Roll bouncing Slitting Vibration 62 Collected from several newsprint and magazine paper mills
  63. 63. Pele Oy Product quality control by hardness profiler  The end product of a paper mill is the customer roll. Even if paper is sheeted, it must first form a good roll.  Papermakers normally measure grammage, moisture and caliper online. However, the correlation of roll quality with online measurements is often very poor and the evaluation of roll quality must be made by hand and visually.  Roll hardness measurement reflects roll quality much better than any other profiler from paper itself.  If the hardness measurement is easy, fast and accurate, it could even replace in many cases online CD profile measurements.  Mill information system can draw complete CD profiles from individual roll measurements and send the information to the laboratory and operators.  Normally the need to adjust CD profiles is only when some changes are made and that would suit very well for roll hardness measurements to control paper quality. Severe roll hardness and diameter variation of wrapped LWC roll 63 Picture: Pekka Komulainen
  64. 64. Pele Oy Corrugations or rope marks  Roll hardness/roll diameter is different on each side of rope mark.  Rope mark propensity can be found with hardness measurement before it can be seen.  Root causes for corrugations are: • Differences in paper cross-direction profiles of caliper, compressibility, tension and elasticity • Differences of nip load in cross-direction or web travel from unwind to rewind • Too hard winding • Most problematic with high density papers (glossy papers) 64 Look deeper Roisum:
  65. 65. Pele Oy Bagginess  Bagginess can be hidden inside the roll and will show up when paper is unwound in converting. Roll hardness and web tension profiles can easily show this kind of bagginess propensity before it can be seen.  Typical for bagginess is:  Out-of-plane buckling (at low tension)  Bagginess is caused by CD profile variation (grammage, moisture, caliper etc.) together with hard winding and low elasticity of paper  Typical for thin, hard calendered papers with coating or high filler content  Can be seen only inside the roll or on top of the roll 65 Look closer Roisum:
  66. 66. Pele Oy 66 Cross machine direction burst  This burst is identified by a break in the sheet across the roll face. This burst is generally found near the core or in the outer few centimeters of the roll that is wound too tight (hard).  The burst can be across the full face of the roll or a partial burst just enough to break down the web under stress.  If rider roll is pressing too much, this can be in the middle of the roll radius.  Reasons are: • Too hard roll • Tight web together with high caliper • High MD tension and nip pressure • Low MD strength and stretch (low TEA) • Very dry paper
  67. 67. Pele Oy Crepe wrinkles and their reasons  Combination of low web tension and high nip load  Hard area on top of soft area or increasing hardness to the roll periphery  Low web caliper/grammage and low MD stiffness  Low coefficient of friction, slippery inside the roll (DIP is today well washed and COF is quite high, the first DIP plants produced very slippery paper)  Hardness variation in CD profile, loose and thick web at edges  Uneven pressure from rider roll MD 67 Cross sections of crepe wrinkles (David Mcdonald, 2014) Crepe wrinkles start from the roll edge
  68. 68. Pele Oy J-line and crepe wrinkles  Internal slipping of paper layers under the roll surface (decreasing tension) is measured with J-line measurement.  Shooting or marking a roll radius and then winding layers on top of that will move the marked layers towards the winding direction (= loosening the original web tension).  If the original tension is low and slippage high there will be a negative tension in the machine direction resulting in buckle of the layers (= crepe wrinkle).  Very bad combination is a high nip load, a low web tension and a low COF. This is most common at the machine reel edge, where caliper can be high and web length long (=loose edges). In addition edges can be dry and thus COF is low.  It is best to have curved CD profile = slightly lower caliper at edges to avoid crepe wrinkles and other problems. Read more: 68
  69. 69. Pele Oy Sensitivity to crepe wrinkles  There will be slippage and possibility to get crepe wrinkles, when - F > µ • p • A + S - F = buckling force from nip action = f(N and nip width) - A = area of possible sliding - S = paper stiffness force  To avoid crepe wrinkles paper static COF must be high as well as roll hardness, but nip load should be low.  Low paper stiffness and caliper increase possibilities to get crepe wrinkles.  With a soft drum cover the buckling force and J-lines are smaller than with a hard winder drum. Possibility to get crepe wrinkles is very small. Picture: Valmet 69
  70. 70. Pele Oy J-line measurement  When measuring J-line all possible safety measures should be considered. A safe method is to stop the winder, draw a radial line to the end, and then start winding again.  After winding the J-line can be measured, photographed and analyzed.  This method, however, is not correct, because stopping and starting have effect on winding forces and slippage of the layers.  The old, but not so safe method is to shoot a chalked special string against the rotating roll end and then look the line after winding (right picture). 70 Picture: J.K. Good
  71. 71. Pele Oy Process effects on crepe wrinkle formation  Crepe wrinkle is a common defect at web edges. All parameters from raw materials to the winder have effect on crepe wrinkle formation – especially at edges.  In unwinding it is easy to feel the wrinkles by hand. If there are wrinkles already in the machine reel you can feel them on unwind reel at winder.  Main winder wrinkles must be checked at salvage winder.  To improve the situation all the papermaking process should be improved – not only winder parameters. At the edges the situation can be described as follows: Higher CD shrinkage CD control of B.W Lower B.W. to dryers Faster drying at edges Large roll diameter Permanent edge strain Lower MD E modulus Strain on reel surface Loose edges Larger reel diameter Web run, spreading Higher rider roll load High nip load Dry edges to dryers Higher caliper Higher porosity Higher dryer temperature Sheet flutter High paper density Low web tension Crepe wrinkles Low paper static COF Low paper MD stiffness 71
  72. 72. Pele Oy Effect of splicing on roll structure  Rolls having winder splices are never as good as normal rolls.  If the splice is made because of web break in the machine reel, the winder must be decelerated before the splice, which increases web tension.  During the splicing the web can be loose, which decreases roll hardness.  After the splice winder is accelerated, which reduces roll hardness.  On top of this loose part nip effect is higher and roll hardness is increased again.  On a rewinder (salvage winder) dynamic forces are smaller and spliced rolls can be better. Roll hardness Roll diameter Splice 72 Picture: Pekka Komulainen
  73. 73. Pele Oy Transport and roll storage  Sometimes rolls are stored horizontally on each other.  If rolls are too soft they will form out-of-round.  On a two-drum winder soft rolls always have harder surface, and they easily get buckled and show starring on the end.  Internally soft rolls are formed in the positions where rider roll is not contacting the rolls. 73 Picture: Pekka Komulainen
  74. 74. Pele Oy Roll ridges and hardness  Rolls with hardness variation can look quite nice on the winder, but after storage and transport there can be severe bagginess on the hard ridge areas.  This kind of bagginess is higher on the roll surface, where the nip load and diameter differences have been highest.  Storage time, humidity and temperature of the environment/paper have effect on the final hardness profiles and bagginess formation. 74 Pictures: David Roisum Look closer:
  75. 75. Pele Oy Starred roll and hardness  Stars or paper buckling in the machine direction is caused by negative MD tension inside the roll.  The basic reason is increasing roll hardness towards the roll surface.  Starring could occur later when the roll gets external forces and impacts in handling, transport and storage.  It is very important that rolls are not stored like in the picture. 75
  76. 76. Pele Oy Dished roll or telescoping Reasons can be:  Bad spreading (D-bar), misalignment of incoming webs  Misalignment of winder rolls  Unsymmetrical diameter and hardness of paper roll (CD caliper or tension differences)  Soft cores in CD direction  Core chuck pressure is too low  Air entrainment with high speed  Low paper-to-paper COF (Coefficient Of Friction) 76 Picture: David Roisum et al.
  77. 77. Pele Oy Why rewinder (salvage winder) makes hard rolls?  If only one roll is pressed with rider rolls there is good contact. On the main winder edge rolls may be smaller in diameter and not pressed at all at the end of winding.  Bulky paper like newsprint is compressed plastically and deformed after main winder. Thinner and denser paper makes harder rolls. Especially roll surface is made on rewinder from the hardest part of the unwinding roll. Hardness is easily increased towards the roll surface at the rewinder.  Sometimes web tension control has same total force for wide and narrow rolls. Then the narrowest rolls can have very high tension (= force/width).  Drum radius of a two-drum rewinder is normally smaller than that on the main winder (e.g. 550 mm vs. 850 mm). Narrow nip – harder roll from own weight.  Winder speed of a rewinder is about half or the main winder speed (e.g. 1200 vs. 2400 m/min). Higher speed – more air into the roll, higher centrifugal force and faster nip impact all reduce roll hardness on the main winder. 77 Picture: Voith/Jagenberg
  78. 78. Pele Oy Optimum roll hardness Average hardness Hardnessvariation Toosoftroll Toohardroll Too much hardness variation Good roll quality Soft roll starring 78  If the roll is too soft, especially in the center, there will be handling damages such as starring and out-of-roundness. Additionally, there can be crepe wrinkles or telescoping.  If the roll is too hard there can be bagginess and bursts.  Too soft or too hard rolls tolerate less roll hardness variation than rolls with optimum hardness. There is always some CD profile variation in the hardness and it is important to find the optimum average hardness level to avoid roll broke.
  79. 79. Pele Oy Roll hardness and roll deformation  It is clear that harder rolls keep roundness better in handling than soft rolls. Permanent deformation grows very fast when the roll is softer.  However, it is evident that the roll should be hard inside but softer on the top. Unfortunately two-drum winder tends to make the opposite. 79 Picture: D.MCDONALD, J. HAMEL AND A.MÉNARD, 2005
  80. 80. Pele Oy Conclusion The following four things are most important to be remembered about roll quality:  The end product of papermaking process is a high quality paper roll and its quality must be controlled.  The highest cost broke of the papermaking process is at the end.  All faults should be corrected where they first appear (in the process phase and position). It is never possible to totally compensate earlier fault with later correction.  Winding cannot make paper better, but in many cases product quality can be much lower after winding. 80 Picture: Valmet