Winder Two Drum

2.4 Winding For the final winding of customer rolls, there are basically two types of winders: two-drum winders and multistation winders. In a two-drum winder, the roll weight lies on two winding drums and the growing weight of the roll increases nip load against the drums. Depending on the roll diameter and paper density, the weight of a one-meter-wide printing paper roll is  between 200 and 2000 kg. Because the weight is divided between two supporting drums, nip load against these drums is roughly between 1 and 10 kN/m. The maximum acceptable nip loads for thin papers are between 3 and 5 kN/m. Modified two-drum winders and multistation winders can minimize problems of crepe wrinkles, bursts, bags, corrugations, out-ofroundness, etc. Modified two-drum winders are used to reduce the maximum nip load. Basically, there are four methods. In the first method, nip pressure can be decreased by installing a controlled method of relieving air pressure between the drums and the roll. This method is well suited for  porous papers, where air can be ventilated out of the roll through the incoming paper web if  paper feed is between the drums. A second alternative is to widen the drum nip using a supporting belt in place of the front drum. When the winder is inclined, more and more of the roll weight is supported by the belt. This results in a lower maximum nip pressure. The third type of modified two-drum winder widens the nip by using one or two soft material covered drums. With a compressive material, the nip is wider and maximum pressure lower. A fourth type of modified two-drum winder utilizes the variable geometry provided by a  pivoted front drum in combination with compliant nips at the winder drums and rider roll to widen the gap between the drums as the roll builds up. Multistation winders are used for the most demanding paper grades, where two-drum winding is insufficient. The following grades and paper properties typically require multistation winding: - Thin papers with low stiffness, low COF (coefficient of friction), and low tensile strength - Coated grades with high density and high roll diameter - Glossy papers with low friction, high density, and high roll diameter - Coated rotogravure papers with high smoothness and gloss, low COF, low binder content, low stiffness, and low tensile strength.

As a summary, the following table (Table 4) describes the suitability of winder types for paper grades: Table 4. Suitability of winder types for paper grades. B = Belt support, A = air relief, S = soft roll, 1 = most suitable, 2 = also possible. 2.4.1 Problems and challenges by paper grade 2.4.1.1 Newsprint The basis weight of newsprint has decreased from 48.8-52.0 g/m2 to 40-45 g/m2. This is a big difference in stiffness and strength. At the same time, roll diameters have grown and deinked  pulp is today the main raw material. Increased density and filler content together with changes in COF make the paper so prone to roll defects and process disturbances that multistation or modified two-drum winders are recommended. Conventional two-drum winders have  problems with wrinkles, bursts, out-of-roundness, and roll structure. One reason for this is the low COF due to flotation deinking and its chemicals. In some cases, when ONP and washing deinking is used, the COF can be too high, resulting in roll bouncing and eccentricity.  Newsprint is normally offset printed, where dustfree paper is required. This places high demands on the slitting system and dust removal. Multistation winders and even high-speed two-drum winders can make soft rolls from newsprint. Because web tension must be kept low to achieve good runnability and high nip loads produce nip-related defects, the only way is to use tangential force to make a hard roll. However, the effect of center torque on the roll  periphery rapidly decreases as roll diameter increases. 2.4.1.2 Glossy grades The density of uncoated and coated glossy papers is normally more than 1000 kg/m3. This is the limit where crepe wrinkles, bursts, bagginess, ridges, corrugations ¾ which are typical defects of hard rolls and high nip loads ¾ become common. Wrinkles and bursts are also  problems with multistation winders at the core and in the roll edge area, because the main part of the supporting force comes through the core and roll bottom. Glossy grades will more often be calendered on-machine with high-temperature soft-calenders or supercalenders. If the parent reel is not cooled after high-temperature calendering, there will  be drying problems before the winder and at the winder. Surface layers of the parent reel will dry, especially during winter in northern countries due to dry ambient air. Starting a new  parent reel, long set change and splicing are problematic because drying will shrink the web, relax frozen tensions, and cause length differences between the center and the edges. Change of the CD slitting position and loose web edges are common problems encounterd without web

cooling or air conditioning. This is also a problem with other grades, but it is most severe with the highest web temperatures. 2.4.1.3 Uncoated woodfree The main problem with uncoated woodfree grades is the winder vibration related to a high COF and roll eccentricity. An increased use of carbonates instead of clay pigments (even in surface sizing) and also ASA-type hydrophobic sizes instead of AKD have increased the COF, resulting in vibration. Also, the two-drum winder speed must be kept at a high level because  paper machine speeds are increasing. Good results have been achieved by optimizing the use of different chemicals to obtain the optimum COF level.

The running speed of woodfree machines has increased considerably, but is still less than the speed of wood-containing paper machines. The capacity of a two-drum winder for a new paper machine is completely utilized and a second winder is required. However, many high-speed machines have only one winder in spite of the speed increase of the paper machine. If vibration is a problem, reduced speed or special programming of speed controls is needed, which will reduce winder capacity. Also, narrow and small-diameter rolls are slit on the same winder, which has a negative influence on capacity. High maximum running speed and automation level are critical success factors for woodfree winders. 2.4.1.4 Coated woodfree Problems with coated woodfree are similar to those of coated mechanicals. The main differences are: - Higher basis weight, better strength, and stiffness (bursts and crepe wrinkles are not common). - Matte, dull, and silk grades are also produced, resulting in problems with gloss marking, high COF, and vibration. - A major part of production is sheeted, which requires soft, large-diameter rolls. - Air permeability is low, air is pumped into the roll, and web stability is not good. This can reduce the maximum possible running speed. - Mineral content of the paper can be up to 50%, which increases slitter blade wear. Because the basis weight range of coated woodfree grades is large, the web tension range is also large and the highest web tensions required are close to board web tensions.

2.4.1.5 Containerboard Containerboards, i.e., corrugating medium and linerboards, are normally wound with conventional two-drum winders. Boards are bulky, web length in a roll is short, and board machine production is high. Normally, there is only one winder per board machine. If the  board machine is modern, a winder of the highest capacity is needed. However, the web is strong and high running speeds and fast acceleration rates can be used. Usually, problems encountered are other than roll quality, i.e., noise, dust, and winder capacity related to automation level and availability. The edge trim transport system is also critical. A good system is equipped with two separate high-capacity pulpers directly under the winder for narrow and wide edge trims. 2.4.1.6 Cartonboard and graphical board Cartonboards are normally coated glossy or matte products. Bulk and stiffness are important and should be retained after winding and storage. Matte and dull grades are demanding with respect to marking and bulk decrease. Deinked pulp (DIP) is also used in the middle layer. DIP and coating are demanding due to slitter wear and dust. Roll bottom curl is also problematic, and large-diameter cores must be used. 2.4.2 Future trends 2.4.2.1 Raw materials - More recycled fibers made with better deinking processes will be used. This will increase roll density and filler content and have an effect on the COF level (normally more slippery webs than with virgin fibers). - More coated grades with a higher coating amount, but also lower basis weights will be common. Fiber content of the papers will decrease while pigments and chemicals increase. Paper density, slitter wear, roll weight, and variation in the COF level are consequences. - Chemical pulps of several types of short fibers will increasingly be used (nonwood fibers, Eucalyptus, Acacia, etc.). - Because of closed water systems, fine material of fibers and pigments will be retained in the  paper together with sizes and functional chemicals like retention aids, foam control agents etc. These have an effect on the COF level and thus on winder performance. - Carbonates and special pigments will be increasingly used. These normally have a higher COF than conventional pigments (kaolin, talc).

2.4.2.2 Papermaking and winding The average paper machine width has increased annually about 100 mm. In 1952, the maximum trim was about 5200 mm, in 1972 about 7200 mm, and in 1992 about 9200 mm. It is estimated that in 2000 this rule will become invalid, and width development is estimated to decrease. To get the same production increase as earlier, the speed increase should be greater than has historically been the case. Today, the annual speed increase is about 50 m/min or 3%. Design speeds of printing paper machines will be 2000 m/min in the year 2000. Coating and calendering are developing more and more toward on-machine processes. Consequently, only one coater and one calender are needed for a paper machine. However, two winders are needed. This increases investment cost. Competition with electronic media requires reduction of total winding costs, which means lower operating costs, less breaks, less broke, and fewer culled rolls. Winders must be highly automated, working with a minimum crew (one operator per winder). Universal winders are needed, where high automation level, good capacity, and the best roll quality are combined in the same winder. This means that the capacity is better than with conventional two-drum winders and roll quality better than with a conventional multistation winder, regardless of the grade. 2.4.2.3 Roll size Rotogravure roll widths have increased continuously. The widest printing machines are 3600 mm. Maximum practical printing paper roll diameters are 1300 mm for offset and rotogravure. Maximum SC and LWC roll densities can be 1300 kg/m3. The weight of this kind of rotogravure roll is over six metric tons. If this roll would be made at a diameter of 1500 mm, the maximum roll weight would be over nine tons! Offset printing machines are still mainly around one meter wide. However, more and more machines will be 1440 mm. Printing machine speeds have increased and will increase at about the same rate as paper machine speeds, but the speed level is about half that of the fastest paper machines. Rotogravure and offset presses can run 900 m/min. To reduce the number of flying splices, roll diameters must grow accordingly. In some cases, the winder must be suitable for both rotogravure and offset rolls. This can require different core diameters. If these rolls must be trimmed in the same set, a multistation winder can be used. However, development seems to focus production on one grade, i.e.,

rotogravure or offset. Less variation in roll width, weight, and core diameter is always a better situation for a modern winder when capacity and roll quality are concerned. 2.4.3 Two-drum winders On two-drum winders, a set of rolls is wound side-by-side on two winding drums. A conventional two-drum winder winds rolls on two drums of equal diameter and symmetrical geometry (Fig. 42). The weight of the set is supported equally by the drums and the nipload at the end of the set is defined by the weight of the rolls. Designs with different drum diameters and geometries, which divide the roll weight nonequally, have been developed but the basic  principle remains the same. To overcome the problems that arise from excessive nipload, new models of two-drum winders have been developed. These include winders with air relief, belt support, variable geometry, and soft nip covers. The advantages of a two-drum winder are simple operation and maintenance as well as high production capacity. Figure 42. Two-drum winder. 2.4.3.1 Winder functions A basic two-drum winder consists of equipment to fulfill the main functions of a winder: unwind, slitting, and windup. Equipment to handle parent reel change and set change is also needed. An unwind stand with a brake generator or mechanical brake maintains the web tension needed for web handling through the winder. The unwind can be maneuvered sideways and oscillated to get the web into the right position and to spread local profile variations over a wider area. By moving only one end of the parent reel in the machine direction, a skewed web tension profile can be corrected. Steering the web through the winder requires a set of guide/lead rolls. Full width lead rolls usually need to be driven to maintain equal speed with the paper web. Sectional guide rolls are undriven. The web is spread and flattened with bowed rolls for slitting under tension. After slitting, the sheets must be separated so that paper rolls on the winding drums do not run together and roll dishing is avoided (see "Web spreading"). A special type of guide roll with segments that presses more on high tension areas of the web than on slack areas equalizes cross tension variation but increases length variation of the web. Slitting of the web takes place with a pair of shear-cut rotating blades. The tangential shearslitting method is widely used in the paper industry. In this slitting method, the web path is tangent or nearly tangent to the bottom band (Fig. 43). The number of slitter pairs needed is

defined by the customer, with the maximum number being limited by the minimum roll width to be slit. Positioning of the slitters can be fully automated. Some special grades need very narrow roll widths, which can be achieved by a special arrangement of narrow top slitters on slitting rings assembled on a slitting drum. Other techniques for slitting are crush slitting, waterjet, and laser, but these are not widespread in the  paper industry. In the tangential shear slitting method, the cut point should be as close as  possible to the point where the web first contacts the bottom band (Fig. 43). The cut point is the point where the top blade contacts the bottom band. To ensure the highest cut quality, the cut point must be located inside the wrap or preferably at the beginning of it; the wrap is the total area where the web contacts the bottom band. The highest quality cut is made when the  blades are new and the cut point is at its furthest point forward. As the blades wear, the cutting nip opens up and the cut point moves back. This open nip reduces the quality of the cut  because paper tends to tear before the slit. The further back the cut point moves, the worse the cut becomes. The bottom band should penetrate the sheet 0.5-2.5 mm, depending on paper grade, but typically 1.5 mm. This small amount of penetration helps stabilize the web at the cut  point. On highly coated grades, this penetration must be minimized in order to prevent marking of the web in the vicinity of the cut edge. Figure 43. Tangential shear slitting method. The optimum position of the cut point is ensured by the slitter blade geometry and the proper cant angle (shear angle or toe-in) between top and bottom blades. The depth or overlap of the top slitter blade to the bottom slitter band should be between 0.5 and 2.5 mm depending on the grade of paper to be wound. For grades where the slitting result is not critical, the overlap is greater. There should be a light top slitter side load against the bottom slitter, typically 20-45  N depending on paper grade and axial runout of the blades. In principle, the side load should  be as light as possible because this will increase blade life. If there is axial runout in the  blades, the side load must be increased to get the best slitting result. The speed at which the bottom slitter band is driven is very important. The bottom band is responsible for driving the top slitter blade. Because the top blade is overlapped with the  bottom band, it will rotate slightly slower than the bottom band. For this reason, the bottom  band is usually driven a bit faster than the web. This ensures that the top slitter blade is also rotating slightly faster than the web, which reduces the chance of the web bunching up and causing a break at the cut point. Typically the overspeed of the bottom band is 3%-5% faster than the web speed. In practice, however, on thick board grades, the top slitter blade follows the speed of the web. In this case, it is recommended to decrease the overspeed of the bottom

 band in order to minimize rubbing between the top slitter blade and bottom band, and increase the blade life. The web is separated after slitting by spreader bars, D-bars, bowed rolls, or sectional spreader rolls. A dual spreader arrangement allows a folding type of web separation. The dual spreader (Fig. 44) assembly maintains an equal path length from unwind to windup and thus does not affect the web tension profile. Using a dual spreader, the spreading effect is increased with the wrap angle. In a dual spreader, the first element folds the web outward and the second element folds the web straight again in the machine-direction with a gap between each slit sheet. Simple web spreading and separating devices are the single spreader roll, fixed spreader bar, or bowed tube which are mainly used in applications with a narrow winder or few rolls in a set. (See "Separating the cut webs in the two-drum winder"). Figure 44. Web separation with a dual spreader. The windup section consists of winding drums and a rider roll that applies the necessary load at the beginning of a set when the weight of the paper rolls does not provide enough nipload. During running, the rolls at each end are held in place with core chucks. The first/rear winding drum is speed controlled, and the second/front drum is usually torque controlled to give a tightening effect during winding. Winding drums can be friction coated with tungsten carbide or other coatings to increase traction and wear resistance. Drum grooving is designed to prevent air entrapment in rolls and air bags in front of winding nips. Excess air in the set easily results in air bursts when running non-porous thin papers. The conventional rider roll is of straight stiff construction that loads more in the high caliper/diameter areas than the low caliper areas, somewhat equalizing roll diameter variation. The articulating rider roll consists of segments which load each position equally, thus allowing different roll and core diameters in a set. Set change equipment consists of a cutting device, set ejector and lowering cradle. New cores are inserted manually or with core-loading equipment. Further automation of the set change sequence includes automatic core gluing and tail fastening with tape or glue. Manual parent reel change takes place such that the empty reel spool is ejected on rails and a new reel is inserted with a crane. Automated reel change equipment consists of transfer rails and waiting stations for full parent reels and storage rails for empty reels. An automated reel change might include an automatic back splicing device, perhaps with the capability of a commercial printing quality butt-joint splice.

2.4.3.2 Automated functions In order to keep up with a continuously running paper machine, the winder needs to run at a high speed and acceleration rate because, as a batch process, it must stop for each set. However, the productivity of a winder that runs at full speed is mainly affected by stop times, which can be minimized by automation. The most commonly automated functions are: - Slitter positioning - Reel spool ejecting - Reel inserting - Reel splicing - Core gluing - Core inserting - Web cutting - Set ejecting - Tail gluing or taping. These functions can be combined into automatic reel and set change sequences and, with complete automation, become a continuous winding operation (Fig. 45). As the name implies, continuous winding changes the batch winding process into a continuous process where the operator only needs to monitor the winder operation. All sequences occur automatically until there are no more parent reels available to wind into sets. Figure 45. Continuous winding. 2.4.3.3 Two-drum winding parameters A two-drum winder uses the winding parameters of tension, nipload, and torque or preferably the winding force (Fig. 46). The term winding force is preferred because it is more general and descriptive than torque or torque differential. The pitfall with the term torque is that it is not applicable when winders of different geometry, drum diameter, or inertia are compared. The concept winding force, being tangential load exerted by the second drum on the wound roll, is independent on the winder type.Tension is controlled by the unwind brake generator or mechanical brake with feedback from load cells usually located below one of the guide or

sectional rolls. Using tension, the web is spread and flattened on the slitter table rolls for slitting. Tension also gives the basic strain to the incoming web at the windup. Figure 46. Control of two-drum winding parameters. The main functions of the first winding nip are to prevent air entrapment and control roll tightness. Nipload of the winding drums is a result of the weight of the set and the rider roll load. At the beginning of a set, there is not enough weight from the paper rolls and the rider roll defines the nipload. When the set weight increases, the rider roll load is typically decreased. The end diameter of the set and the density of the paper roll define the nipload at the end of winding. At that phase, the rider roll only rides on top of the set for safety reasons. Thus, the nipload curve makes a smooth alteration from the start value defined by the rider roll to the end value defined by the roll weight (Fig. 47). Figure 47. Two-drum winder nipload. On a conventional two-drum winder, the nipload increases uncontrollably when the roll diameter increases. The rider roll can add, but not substract, load. This high nipload at large roll diameters can cause roll defects like crepe wrinkles, bursts, bags, and corrugations in  printing grade paper rolls when caliper and basis weight profile variations accumulate. Thus, a conventional two-drum winder is not recommended for news, SC, LWC, and other relatively thin grades which are nipload sensitive. The third parameter of a two-drum winder is winding force, which is the force after the first nip, controlled by the front drum torque. Winding force can also be interpreted as a tension after the first nip. Increasing winding force tightens the roll. On a two-drum winder, increasing the tension and winding force decreases the effect of the winding nip, so the probability of roll defects due to high nipload can be reduced by increasing these two roll buildup parameters. 2.4.3.4 Two-drum winders with a soft nip cover drum Soft nip covers have been introduced to overcome the problems related to high nipload in a two-drum winder. Usually the front drum is covered with a softer material (polymer). The soft cover makes the nip wider. The peak pressure and penetration of the drum into the roll is reduced. However, to optimize the effect of the soft roll, the modulus of elasticity and Poisson ratio must be matched to the corresponding values of the paper roll. To widen the nip, the modulus of elasticity of the soft cover should be less than or equal to that of the paper roll. The material properties have to be carefully designed for elasticity and endurance. Another effect of the soft cover on the front drum is the change in weight distribution on the drums. The rear

drum is slightly relieved when the soft cover is depressed by the roll, and the weight of the set therefore moves slightly toward the front drum. 2.4.3.5 Two-drum winder with air relief A two-drum winder can be equipped with devices that maintain overpressure between the winding drums (Fig. 48). The supporting area increases when the roll diameter increases; thus, with only a low pressure, air relieving is effective. With pressure < 10 kPa (0.1 bar), the nipload of a newsprint roll (Fig. 49) can be held within a safe range (below 4 kN/m). Air relief systems are best suited for porous paper grades where air entrapment will not cause air bags at the winding drums. Figure 48. Two-drum winder with air relief. Figure 49. Nipload of a winder with air relief. Figure 50. Two-drum winder with belt support. 2.4.3.6 Belt-supported winding This member of the two-drum winder family is based on winding geometry (Fig. 50) where the weight of the roll set is partially transferred to a belt bed as the roll diameter increases. This reduces the nipload at the rear drum to the level where no nip-induced defects exist and air entrapment is still prevented. The nipload of a belt-supported winder can be automatically controlled for various roll densities by belt tension. Thus, a belt-supported winder is easy to run with various paper grades. With belt-supported winding, the nipload is not the main roll  buildup tool. Low nipload together with the high capability of winding force allows the most effective use of traction from the belt drums. The winding force affects all sheets equally and maintains equal roll hardness through the whole web width. The advantageous relationship  between a powerful winding force and a low nipload allows large roll diameters with minimal nip-induced defects. 2.4.3.7 Variable geometry winder The variable geometry winder in connection with the articulating rider roll is a modification of the two-drum winder to extend the range of application into areas of newsprint, SC, and LWC  jumbo rolls. Equipped with compliant nips on the drums and rider roll, it also accommodates cores of different sizes. The variable geometry describes the pivoted front drum which moves away from the back roll asthe roll builds up. An asymmetric design with pivoting rider roll  beam with drums of different diameters reduces the opportunity for roll vibration (Fig. 51). Figure 51. Variable geometry winder.

Figure 52. Crepe wrinkle. 2.4.3.8 Winding challenges of two-drum winders The most often encountered winding defects produced by a two-drum winder are crepe wrinkles (Fig. 52) or bursts which are related to caliper/basis weight profile variations and high niploads. These can exist on a conventional two-drum winder due to the uncontrollable niploads at large roll diameters. Local high nipload combined with local low tension results in internal slippage below the roll surface and crepe wrinkle buildup. A low paper coefficient of friction increases the probability of crepes occurring. The only proven remedy is to change the winder type to modified two-drum with a rebuild, or to replace it with a multistation winder. Statistically, the situation can be improved by increasing web tension and winding force or with agents that increase the coefficient of friction of the paper. Corrugations (Fig. 53) are also nip-induced defects due to a poor caliper profile. Rolls are  built-up on local high peaks of caliper. This results in diameter variation across the roll width and different web draw and shear stresses on the roll surface. The cure for corrugations is to go from nip-controlled winding to tension and winding force-controlled winding which will strain the web more evenly. Figure 53. Corrugation Dished rolls (Fig. 54) can result from inadequate web separation. The paper under tension contracts between the unwind and windup. When the paper is wound into a roll again, the tension is relieved and each sheet in the roll becomes wider. If there is not enough web separation, the rolls in the middle of the set push each other outward while new layers are wound in the original position. This makes the edge rolls dished while the middle rolls remain straight. Figure 54. Dished roll. Core eccentricity and roll bouncing are related problems on two-drum winders. When caliper  profile variation results in slightly different roll diameters in a set, the rolls rotate with different angular speeds. This leads to frictional forces between roll edges. This force makes rolls bounce or rotate in a non-coaxial manner, resulting in eccentric cores and roll bouncing and even roll throw-outs (rolls uncontrollably ejecting from the windup). This phenomenon can be reduced by decreasing the frictional force by cutting cores straight and even and reducing the friction with oil or low friction spacers between cores. The axial force should be controlled by proper core chuck positioning and the correct core length. The core quality should relate to the roll hardness so cores do not elongate during winding.

Winder vibrations on a two-drum winder are related to the resonance frequency of the system, which consists of the winding drums, the set of rolls, and the rider roll system. Vibrations might also be due to unbalanced winding drums. The latter can be reduced with stiff drums manufactured to high precision. The resonance frequency is excited at the multiple of the roll rotation speed. During acceleration, the winder runs through these resonance zones so quickly that excess vibration is rarely seen. At full speed, however, when the roll angular speed slowly decreases with the diameter, vibration can occur at these frequencies. An advanced control system controls the winder speed so that the resonance zones of the set angular speed are avoided, i.e., the speed is automatically dropped through these zones quickly (Fig. 55). However, this control leads to small losses in productivity due to reduced winder speeds. Winder vibrations exist mainly when running vibration prone bulky high friction grades. Figure 55. Avoiding vibration zones with speed control.