- August 01, 2001, Duane Smith, Black Clawson Converting Machinery
What is a good roll? A slitter/winder operator will tell you that a good roll of paper is one that has the following characteristics:
It is the right shape round and proper width.
It is the right size not too big or small.
It has the right consistency not too hard or soft.
It looks good no blemishes or visual defects.
Your customers demand a roll that will run without problems. This article will discuss the factors that go into the consistent production of top quality rolls off a slitting/rewinding operation. It will focus on roll hardness and visual defects and how they can be controlled for rewinding good rolls consistently.
Roll density or hardness is probably the most important factor in determining the difference between a good and bad roll.
Rolls that are wound too soft will go out of round while winding or will go out of round when they are handled or stored. The roundness of rolls is very important in your customer's operation. When unwinding out-of-round rolls, each revolution will produce a tight and slack tension wave. These tension variations can distort the web and cause register variations in the process.
The only way to minimize the effect of these tension variations is to run the operation at a much lower speed, which greatly affects the production process.
Rolls that are wound too tight also will cause problems. Tightly wound rolls contain high inwound tension. These rolls can split open like a watermelon when dropped or squeezed by a roll-handling truck. The web will stretch and deform as these stresses are relieved while the roll cures during storage. If this inwound tension becomes too high, bursts can occur inside the wound roll. These cross-machine tension bursts are well hidden normally and cannot be detected until the roll is unwound. These bursts cause web breaks that can cause major production losses on your customer's production lines.
Since there is no such thing as a perfect web, it is the responsibility of the winder operator to make sure that these slight imperfections do not stand out in appearance and do not cause their customers process problems.
Rolls that are wound too tightly will exaggerate web defects. No web is perfectly flat or the same thickness from one side to the other. Typically webs will have slight high and low areas in the cross-machine profile where the web is thicker or thinner. If the rolls are wound too hard, the web will stretch over these thicker areas, causing bumps or ridges. As the web is stretched over these ridges, it deforms in these areas. This causes untensioned areas or baggy paper when the roll is unwound.
There also may be streaks in localized areas across the web where the web is higher in moisture. These moisture bands will cause web defects known as corrugations or rope marks.
These slight defects will not be noticeable in a wound roll if sufficient air is wound into the roll in the low or wet areas and the web is not stretched over the high areas.
Some webs, either by their formation process or by their coating or web conditioning process, have cross-machine variations of thickness or moisture too severe to be wound without exaggerating these defects. To overcome this, these webs are moved back and forth before they are slit. This is called oscillation, which randomizes these localized defects across the wound rolls.
On a slitter/rewinder, the unwind is normally oscillated. Oscillation may be either a constant speed, stop, and constant speed back, or a sin-type wave speed curve. It is important that the oscillation speed is fast enough to randomize defects and slow enough that it does not strain or wrinkle the web. The rule of thumb for the maximum oscillation speed is 1 in./min/500 fpm winding speed. For best results, the oscillation speed should vary proportional to the winding speed.
Achieving & Measuring Roll Hardness
Now that we know why roll hardness is important, let's discuss how to achieve and measure roll hardness.
As a roll of web material winds, tension builds inside the roll, which is known as inwound tension or residual stress. If these stresses become greater as the roll is wound, then the inner wraps toward the core will loosen. This is what causes the rolls to dish while winding or telescope when they are handled or being unwound in your customer's production operation.
To prevent this, the rolls should be wound tight at the core and then wound with less tightness as the roll builds in diameter. The larger the finished rewind diameters, the more critical the roll hardness profile.
Roll hardness is developed in different ways on different types of winders, but the basic principles of how to build roll hardness are always the same. To remember these principles, remember that to wind “dynamite” rolls consistently you need “TNT” (see left).
There are three basic types of winders: center winders, surface winders, and center/surface winders.
Single-Drum Surface Winders
Single-drum surface winders are the simplest and least expensive type of winders. As shown in Figure 1, these will use web tension and nip. Because of the amount of wrap that normally is around the driven winder drum, web tension is isolated from the winding roll. On nonextensible products such as heavy board, web tension has little effect on roll hardness. Basically roll hardness is affected only by the control of the nip pressure. On extensible products such as films and nonwovens, roll hardness is controlled through both web tension and nip.
The advantage of this type of surface winder is that the roll's weight does not affect the amount of nip load. The disadvantage is that the single-drum surface winder has limited control of roll density and must be a shafted operation. A single-drum winder requires much less horsepower than a center winder and typically is used as a continuous type winder for heavier-grade products and extensible nonwovens.
Pure Center Winders
Pure center winders (Figure 2) use web tension or, when combined with a pressure roll, can use nip and web tension to control roll hardness. Note that on this type of winder, torque produces web tension.
A pure center winder may be a single-position or continuous-operation turret winder and may be a shafted or shaftless operation. Often this type of winder provides flexibility of gap or contact winding to control amount of air being wound into roll and roll hardness.
Pure center winders have the ability to wind softer rolls than a single-drum winder and have better inwound tension control. Still they use only one or two of the principles to control roll hardness.
Pure center winders usually are used on lighter grades, which are wound at lighter tensions and to smaller roll diameters. They require much higher horsepower than surface winders as the tension horsepower needs to be multiplied by the ratio of the maximum speed at core to the maximum speed at full roll to obtain the horsepower required at the full roll.
Two-Drum Surface Winders
Two-drum surface winders (Figure 3) normally are used for stop/start slitting and rewinding operations and use all three principles for maximum control of roll hardness.
Typically, this type of winder has enough wrap on the drum that the web tension has little effect on the roll hardness when winding non-extensible grades. Roll hardness and profile are achieved by programmed nip control of the rider roll pressure and programmed torque control from the torque drum.
Today, these winders usually are shaftless operation, run at high speeds, and are very productive. Their disadvantage is that they typically build hard rolls as the winding roll's weight produces the winding nip. This requires high rider roll pressure at the start, which is relieved proportional to the winding roll's weight as it winds. Programmed torque control is the most effective tool in controlling roll hardness, which also requires a hard nip to transmit the torque into the roll without slippage.
Two-drum surface winders are used as high-speed slitter/rewinders for light-density grades such as nonwovens or heavier paper, board, or laminate grades that can tolerate being wound hard.
The center/surface-type winders use all three principles in controlling roll hardness and may be configured either in a continuous turret-type winder, a center/surface single-shaft “simplex,” or a dual-shaft “duplex”-type slitter/winder.
The surface drive produces the web tension, while the nip pressure of the pressure roll and the torque from the spindle drive produce the roll hardness.
By programming these nips and torques, the desired roll hardness and profile can be achieved. The nip can be controlled independent of the roll's weight. The disadvantages of winders of this type are the cost, complexity, and lower production due to the shafts required when slitting and rewinding.
Measuring Roll Hardness
The setting and programming of the tension, nip, and torque to produce the desired roll hardness will vary depending on the following:
type and design of the winder
type of web material being wound
width of the rolls being wound
speed of the winding operation.
Different web products and different applications for these products will dictate the roll hardness desired. Once the desired roll hardness profile is determined, it needs to be measured, then reproduced on a consistent basis. The measurement tools need to be hand-held and available on the winder, so an operator can check roll hardness being obtained and make adjustments accordingly to ensure roll hardness is within the acceptable range for that product.
A Smith meter is an instrument that can be used to measure the hardness profile from the core to the outer wraps of the roll. The Smith meter measures the penetration of a small needle as it is inserted in the wraps of the web along the roll's sides.
To measure the roll hardness across the outer surface of the roll, it is suggested that either a Rhometer or a Schmidt hammer be used. Both of these are impact-based devices for measuring roll hardness on a relative 0-100 scale. The Rhometer is an instrument that measures the peak deceleration of a small hammer as it strikes the outer surface of the roll.
The Schmidt hammer is an instrument that measures the rebound height of a small plunger after it strikes the outer surface of the roll.
With the computerized data acquisition systems available today, it is now possible to calculate the roll density factor (RDF) and plot the relative roll density from core to full roll as the roll winds.
The operator should have available on the winder a means to measure the roll hardness from both core to full roll and across the roll.
In conclusion, winding good rolls is the challenge that every slitter/winder operator faces.
Consistently winding good rolls depends on the consistency of bringing good material to the slitting and rewinding operation.
A winder operator's job is not to camouflage poor quality web products into shippable rolls. His or her responsibility is to handle webs with slight imperfections and to produce quality rolls that will run without problems on your customer's process to produce high quality products for their customers.
R. Duane Smith is product manager of unwind, winding, and slitting systems at Black Clawson Converting Machinery LLC, Fulton, NY. He is active in TAPPI and has been published more than a dozen times in major trade journals and magazines. He is the author of the TAPPI best-seller “Roll and Web Defect Terminology.” He can be reached at 315/593-0312; e-mail: firstname.lastname@example.org.
The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to author(s).
Tension - The Winding Web Tension
Nip - The Nip of the Pressure Roll or Drum
Torque - From the Center Drive or Torque Drum