PAPER INDUSTRY WEB (PIW)

WOUND ROLL STRUCTURE

PRIMARY FUNCTION OF A WINDER

Whatever a winder must do, the most important is to produce good salable shipping rolls. Roll structure is probably a good starting point to understand what a winder is, what it does and how it functions. First, keep in mind, every roll that is wound on a winder and shipped to an end user will be supported on it's core to be unwound. The projected support area of a core is relatively small when compared to the weight of the roll. An understanding of roll structure is necessary to appreciate why this is important.

SHIPPING ROLL REQUIREMENTS

Shipping roll requirements vary greatly. The critical grades are printing papers: News, supered news, lightweight coated grades such as roto and offset papers and many specialty papers. In the most basic definition an acceptable shipping roll must be hard enough at the core to support itself during unwinding, just hard enough at the OD to be handled and shipped without damage and a smooth transition in wound in tension (or hardness) from the core to the OD. This simple definition can be embellished with all sorts of requirements such as straight sided, uniform cross machine hardness, number of splices, annular rings, etc. For now, let's stick with this simple description of a good shipping roll.

UNWINDING A SHIPPING ROLL

When a roll is supported on it's core to be unwound, the complete weight of the roll is supported on the projected area of the core. Reducing the pressure to zero on the bottom side of the core will cause the paper to loose grip on the core. An internal gear effect occurs as the core and the roll rotates, in this case the core being the pinion of the internal gear.

When the roll is unwound, chucks are inserted in the ends of the core to support the roll and braking is applied at the core or surface depending on the converting machinery. This provides tension in the sheet so it may be controlled in the converting process. As the roll is unwound, the core is held back but the roll tends to rotate freely due to inertia. This causes an internal gearing effect between the core and the roll body (the paper on the core). This internal gearing causes layer to layer slippage within the roll generally starting down near the core and working outward. The amount of internal gearing is Dependent on many factors. The structure of the roll and surface of the sheet are the biggest contributing factor.

If you would like to try the "core move test" all you need is a roll of toilet tissue. Scribe straight lines on one of the roll faces with a magic marker. Place the roll on a surface such as a table or desk with the core parallel to the surface. Rest your hand firmly on top of the roll and roll your test roll away from you. The scribed lines should deform as in the diagram. The effect can be reversed by rotating the roll back towards you until the lines return to the original shape. Continuing rolling the test roll towards you will reverse the deformation in the opposite direction.

The interesting part of this test is that using different brands of toilet tissue will produce varying patterns of deformation depending on the tightness of the roll and the coefficient of friction of the paper. Some brands and tissues with embossing may not deform very much if at all due to a significantly higher coefficient of friction. Factors such as paper characteristics, roll hardness and coefficient of friction have a similar effect on this test as on our paper mill rolls.

WHY TWO DIFFERENT WINDER TYPES?

At the risk of getting ahead of the program but before discussing wound roll structure, it may be helpful to understand the significance of two major type winders being used as paper machine winders. The two drum winder while being more efficient from the standpoint of production, is limited in it's ability to produce high quality shipping rolls when processing larger size rolls of dense coated and supercalendered papers.

The limiting factor when winding on a two drum winder is it's nip mechanics. Just the weight of the paper roll winding in a two drum winder causes an ever increasing hardness in the rewinding roll. This is further aggravated by non-uniform paper. Due to nip mechanics, the two drum winder has a tendency to wind tighter on the outer layers of the roll. The limitation of a two drum winder is reached when cross machine paper breaks (bursts) occur in the outer layers of the winding roll and all remedies at correction have been explored and the problem persists. At that point it is time to consider a duplex winder.

The duplex winder in essence reverses the process. The duplex winder rarely will generate bursts and roll defects in the outer layers of the roll. The duplex winder, like the two drum winder, also has mechanical limitations in relation to roll structuring. The limits are dependent on many factors that will be discussed in future articles. When the duplex winder mechanical limits are exceeded, the roll burst and defect problems will occur in the layers of paper near the core.

WHAT'S IN A ROLL OF PAPER?

When a roll is wound in a winder there are many contributing factors that help or hinder the roll structure. No matter how well the roll is structured, regardless of the type of winder being used, there are varying pressures and tensions within the roll. If one very carefully observes the layers of paper in a wound roll, varying bands of tension may be observed. The best way to observe this is to view what is called a "starred" roll. J. David Pfeiffer, in his studies of the internal pressures in a wound roll used the starred roll to illustrate non-uniformity of layers of paper in a wound roll. He noted the layers in positive tension on the outer inches of the roll appear significantly different then the buckled layers farther down in the roll that are in negative tension. The positive tension band on the outside of the roll is caused by a "hoop stress"-the outer layers exerting pressure towards the center of the roll to hold the roll together. It is interesting to understand that as paper is wound off the roll, the positive tension band or hoop remains although of a lesser dimension as the roll is unwound.

When the roll is wound on the winder, tension is applied to the sheet by the unwind stand. This tension is referred as the wound-in tension. Even though the backstand tension remains at a constant level, the wound in tension increases as the roll diameter increase. This particular roll was wound on a two drum surface winder. The increase in wound-in tension is attributed to the nip pressure between the paper and the winding drums. If this roll had been wound on a centerwind winder without nip at the same level of backstand tension, the roll would be so soft it would not support it's own weight. Tension levels two to two and half times would be required on the centerwind to reach a comparable hardness. The same is generally true of non-surface winding such as a duplex winder or duplex with centerwind.

The tension left inside a roll after winding is referred to as residual tension. The positive tension band on the outside of the roll are the high tension layers, sometimes referred to as the "hoop" that contains the pressure inside the roll.

Examining the pressure inside the roll, the compressive pressure to approximates the wound in tension profile.

SHIPPING ROLL QUALITY

Probably the next, and most obvious question at this point is what does the preferred roll structure look like? To be able to answer that, an awareness of industry problems is required. The subject of shipping roll quality is a very involved topic requiring technical knowledge, know how, luck and perhaps a share of witchcraft. In recent years it involves a greater share of technical knowledge and know how. It is far to broad a subject to treat with any depth in a limited discussion. Good roll quality can be viewed from several perspectives including the papermaker, the end user and the machinery builder. These perspectives do not necessarily agree at all times.

BREAKS/100 ROLLS

One of the important criteria for good running shipping rolls by the printer is the number of breaks at the printing press in the shipping rolls received from the papermaker. This is expressed in BREAKS/100 ROLLS. In some grades, quality has improved to the point that the term BREAKS/1,000 ROLLS is the criteria.

There are many reasons for breaks in a shipping roll, most considered to be roll structure defects. The main defect causing breaks in the printing press is an internal "burst" in the shipping roll or a "stressed" area that does not need much encouragement to burst during the converting process. These bursts are generally caused in areas of the shipping roll where the paper is wound to the point that the ultimate strength of the sheet is exceeded causing a break.

CROSS MACHINE DIRECTION BURST

This burst is identified by a break in the sheet across the roll face. This burst is generally found in the outer few inches of the roll that is wound to tight (hard) on a two drum winder. The burst can be across the full face of the roll or a partial burst just enough to break down the web under stress.

MACHINE DIRECTION BURST

This burst is generally identified by a break in the shipping roll in the direction of unwinding. There are several reasons for a burst of this description. If there is a large variation in caliper of the sheet in the machine direction, the continual piling up of the thick caliper area will make a hard or soft "ring" in the roll referred to as a corrugation. The area adjacent to the hard ridge is smaller in diameter and can entrain air during winding. The air lubricated section of the roll has more layer to layer slippage (due to lower coefficient of friction) and will slip more than the high caliper area and cause a "shearing" in the machine direction. This type burst is most often found when their is a large variation in caliper over a short dimension across the roll face.

If the winder is operating on a sheet with a caliper of .003" and there is a caliper variation of 5%, it hardly seems like much variation-.00015". When considering how many layers of paper are in a shipping roll, that small number becomes increasingly important. In a 40" diameter roll of newsprint, the 5% variation in caliper represents 3/8" on the radius of the roll. If there is an area 5" wide with that tolerance and each layer of paper winds the extra thickness, the 5" wide portion of the roll becomes 3/4" larger in diameter than the rest of the roll.

Another machine direction burst can be caused by entrained air in wide rolls-rolls that may have a reasonably uniform caliper. As the roll is wound, the roll entrains air which bleeds out the ends of the roll being wound. The wider the roll, the farther the air must travel to evacuate and the less air is removed from the roll. Depending on sheet characteristics, the layers at the center of the roll may be air lubricated and the layers at the edges of the roll have intimate contact. Again, unequal layer to layer slippage can cause a "shear" in the machine direction of the web. This defect is referred to as an air shear burst and is normally located 8 to 10 inches from either edge of the roll.

Still another burst is called a "core burst" due to its proximity to the core. This type burst is associated with duplex winding and is caused by over stressing the paper wound near the core. The over stressing can be caused by "internal gearing" of the core area in relation to the outer section of the roll, or just very heavy nip in the area immediately over the chuck that supports the roll during winding and unwinding. This burst or wrinkle is normally located very close to the core but has been identified as far as 10" from the core in some cases. This defect is normally within a couple inches of the roll edge and can exhibit a diagonal pattern as opposed to a straight across or in line burst. This burst may be accompanied by an accordion wrinkle (called a crepe wrinkle) adjacent to the rupture. The heavy weight of the roll in the projected area of the core chuck of the printing press will promote this type burst during unwinding at the printer although in many cases the burst is in the roll when it leaves the mill.

NON STRUCTURE BURSTS

There are many reasons for breaks in the pressroom, too many to cover here. Some are non-winder related that can be caused by poor splicing techniques, calendar cuts, glue on the roll or shipping damage to name a few. A break in the printing press means a lot of lost production to the printer while the press is being cleaned up and rethreaded or a new roll loaded in the press unwind. Mills work extremely hard in sheet formation, winding and handling to keep breaks/100 rolls to a minimum.

PAPER DEFECTS

A paper defect involves any characteristic of the sheet that inhibits good runnability or quality at the winder or end users equipment.

ROLL STRUCTURE DEFECTS

These are defect that occur before the roll leaves the mill. Many end users have their own roll specifications that have to be met by papermakers. User specification can involve, tolerance on width and/or diameter, number of mill splices in a roll, straight sided (dish), or any number of conditions the shipping rolls must meet in addition to the detail specifications to meet an individual users needs.

There are a great number of identified known roll defects. Edge cracks, tension bursts, air shear bursts, bags, corrugations, crepe wrinkles, dishing, core bursts, offsets, turnovers, cuts,, interweaving, to name a few. Some categories have several sub-categories There are 9 different burst defects, 12 different wrinkle defects, 10 different crack and cut defects. It is reasonable to expect there are considerably more than a hundred known roll defects.

PAPER VARIATIONS AND ROLL STRUCTURE

Poor roll structure can have many causes in a winding system. A major cause of structure problems is the paper itself. With today's paper making technology, the papermaker can produce a reasonably level sheet-but not perfect. Unfortunately, a winder is not always tolerant to cross machine variations in the sheet. It doesn't take much of a thickness variation to cause serious problems in the winding process.

Many mills producing coated grades, particularly wide LWC rolls, will routinely monitor rolls with a RHO Meter or other measuring instrument as a quality control check before the rolls leave the winder area. If a roll exhibits a poor hardness profile it will be routed to the reclaim winder for closer inspection. There is a possibility the reclaim winder can oscillate the unwind stand to spread a corrugation over a wider area than the shipping roll winder, expanding the area of the corrugation to a more acceptable width. If not, the roll may be cut down in width and salvaged in that manner.

TNT (Tension-Nip-Torque)

A winder uses several tools to control the roll structure of the shipping roll. The tools are popularly dubbed TNT which stands for Tension, Nip and Torque. For information on TNT click here.

Luigi Bagnato--Paper Industry Web

CREDITS

References:

J. David Pfeiffer, Internal Pressures in a Wound Roll of Paper
Kenneth G. Frye, Winding variables and Their Effect on Roll hardness and Roll Quality