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I Thank God.

It gives me lot of pleasure while sharing something. God's Grace.

I know very well the almighty is getting it done through me. This could be the reason behind my energy level of writing from 0330 hrs to 2130 hrs X 7 days a week.

I must mention here that, my better half (Prathita Athavale) is so supportive in this by not bothering me on the day to day activities. I am free to carry on my contribution with NO interruption at all. I am just wondering, how many of us are blessed in this way. God is great. Have a wonderful, peaceful day ahead, stay at home, and say Goodbye to the CORONA, a most dangerous enemy of the entire humankind on this planet.

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I believe each book as mentioned above is going to consume at least a month. Hence, I will be busy till 31 st Dec 2020. I am happy to be able to set a target for myself. Half the battle is WON. I pray GOD and friends like you to shower your blessing that will provide me energy enough to perform.

Jai Hind, Vande Mataram.

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Index

Basic Training Program Theory

Web Handling Principles

1.1. What is Web 004

1.2. How to Handle Web Efficiently 009 1.3.Web Handling Principles and Methods 010 Tension Principle of Winding

1.4.Rewinding methods and principles 010 1.5. Reason for wrinkles and remedies. 022

2.Web Roll Making History 026

3. Web Tension 037

4. Taper Tension for Rewind Control 040 5.Rewinding methods and principles 041

6.What is Slitting 060

7. Slitting, Slicing, Log roll winding, Core cutting Mini Slitting, Auto Core loader, Packing Machines

Video clips 103

8. Converting Equipment for

Pressure-Sensitive Adhesives 105

9. Selecting a Slitter/Rewinder 108 10. Slitting/Rewinding:

Why it's Integral and Proper Troubleshooting 109 11.The Unwind and Rewind Tension Controls 110 12.Innovations Make Slitting/Rewinding Smarter 113 13. Slitting/Rewinding Technology Focuses

on Increased Productivity, Sustainability, 115 14. New Technologies, Developments

in Slitting/Rewinding 117

15. Consumables required for the Slitting Process 119 16. Packaging Machines for Slit Rolls 130 17. What is Slicing ,Slicing Types and Methods 136

18. Log Rewinders 137

19. Comparison between Log Roll

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MACHINE OPERATOR , SLITTING /REWINDING ,AND CONVERTING

TECHNOLOGY

( PACKAGING MATERIAL)

Study Material

1.What is Web

Basic Training Program Theory

1. Web Handling Principles 1.1.What is Web …

A web is a long, thin, and flexible material. Common webs include foil, metal, paper, textile, plastic film, and wire. Common processes carried out on webs include coating, plating, and laminating.[1] A web is generally processed by moving over rollers. Between processing stages, webs are stored and transported as rolls also known as coils, packages and doffs. The result or use of web manufacturing is usually sheets. The primary motivation to work with webs instead of sheets is economics. Webs, being

continuous, can be made at far higher speeds and do not have the start-stop issues of discrete sheet processing. The size of the web-handling industries is unknown.

Web handling

Web handling refers to the processing of a web through a machine with maximum productivity and minimum waste.

Web Handling.

Product Machine Issues

There are many related product/machine issues to review before spending money on new slitting and winding equipment. Listed below are a few of the more prominent ones.

1. Breaking Strength

The ultimate breaking strength of the web is a very important parameter. Normally, for good web control, web tension (T) through the slitting and winding machines should run at equal or less than 10% of the yield point of the product. Because the yield point is not well defined in thermoplastic webs, it is usually taken between 1% and 3% of the ultimate strength.

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Sometimes multiple range controls are necessary when the machine is expected to operate on many different product types and thicknesses. This is because the drive control systems tend to work more accurately when the range between minimum and maximum is limited.

2. Catastrophic Shutdown

In the event of catastrophic shutdown, your machine components must be strong enough to break the entire web without damaging the rolls or transducers. The side frames, roll shafts, and bearings on the machine must be well oversized to prevent any damage during a catastrophic machine stoppage.

Some transducer manufacturers make units that can take large overloads without losing their calibration. These are the units to specify when frequent catastrophic shutdowns are expected.

The factors of safety for the rolls, shafts, bearings, and side frames are best left to the machine vendor. Although vendors may have very different philosophies on how robust your machine needs to be, your personal preference on this point should prevail.

If you are planning to operate at speeds of more than 1,000 fpm, keep in mind that a more rugged machine has better potential for high quality finished rolls. Also, you can expect lower maintenance costs and less downtime with the more rugged machine.

The downside is that the initial cost of the basic machine goes up with robust design. Most converters must make a compromise on what they want and what their budget will allow. My preference is to err on the robust side.

3. The Modulus of Elasticity

The modulus of elasticity—the ratio of web tension divided by web elongation in the elastic region of the product(s)—is an important parameter for calculating tension increases in the web. You can calculate the tension increase in the web due to elongation if you know how much elongation (draw) there is between tension isolation points, the length of web path between the isolation points, the thickness of the web, and the modulus of elasticity [see Formula (a)]. Also, you can calculate how much neck-in there will be at the operating tension level if you also know Poisson’s ratio (see Formula (b)]. The two formulas are as follows:

Formula (a)

T = (M x t) x (? L/L)

T is web tension force per unit width, M is the modulus of elasticity, t is web thickness, (? L is amount of elongation between the tension isolation points, and L is the web path length between the tension isolation points.

Formula (b)

e = - (µ x T)/ (t x M)

e is transverse neck-in in unit width/unit width, µ is Poisson’s ratio, T is machine direction tension in unit width, t is web thickness, and M is the modulus of elasticity.

4. Machine Speed

You also must consider machine speed when selecting how rugged your machine should be. Web speeds above 2,000 fpm require that the machine be more rugged in all basic structures. Vibration of critical parts at high web speed can be a limiting parameter of a light machine design. Some of these parts may vibrate at their natural frequency even though they are not moving or rotating. Serious vibration in the rewind section of a slitter at high speeds can lead to lost production through telescoped rewind rolls. Machine speed must be considered when selecting the type of roll surfaces that will work best for your product(s). High speed requires that transfer roll surfaces have a texture enough to provide relief volume for storage of entrapped boundary air under smooth surface webs to keep them tracking properly through the machine. Tracking friction is broken when the boundary air that clings to both roll and web surface lifts the web above the interlocking surface roughness of the roll.

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Formula (c) H = (0.65 x R) x (12 x µ x (V/T))2/3

H is the gap between the web and the roll, R is the radius of the transfer roll, µ is the viscosity of air at room temperature, V is velocity of the two surfaces, and T is the tension per unit width on the web.

5. Surface Roughness

Surface roughness of your products is another important parameter, because web tracking and roll winding are very much related to the height and density of the web surface asperity.

Surface asperity determines the slip characteristic of the web. The roughness limits the amount of web-to-web contact and keeps the webs from blocking (adhesion of layers) in the wound roll.

The roughness is also a significant variable in the amount of stack compression that can be obtained in the winding layers of the roll when using a contact roll. Stack compression helps improve rewind roll smoothness by permitting some reduction of diameter build-up at locations where web calliper thickness is greater than the base web.

Stack compression is also a function of the volume of entrapped boundary air. Normally, converters spend much effort trying to limit excess entrapped boundary air during the winding process. However, metered amounts of air between the wraps can be beneficial, especially on clear webs with little surface asperity.

While boundary air entrapment and asperity height and density are not related per se, they do work together during winding to give a smoother surface rewind roll through stack compression.

6. Surface Clarity

Surface clarity is an important variable in the winding process, because a very clear web is normally very smooth and can be marred by roll surfaces before and during the winding process.

For example, you must take special precautions when a contact roll is used on very clear, tacky surfaces to prevent defects known as slip pimples (sometimes referred to as slip dimples). Slip pimples are the result of very small areas of adjacent wraps blocking during stack compression. They may be generated under the contact roll during the winding process, or they can be generated after the roll has been wound. Rolling a clear web roll on a table or floor is a very good way to make slip pimples. Clear film rolls should always be supported from the core because of the potential of generating slip pimples.

Therefore, before investing in new equipment, you should review critically the technique for the offloading and handling of finished rolls from the winder.

7. Quality of the Web

The quality of the web to be wound also is significant. Calliper differences or gauge bands from the casting or extrusion areas are the bane of web winders.

Sometimes it is possible to distribute the thicker lanes over more rewind roll area by oscillating the mill roll in the transverse direction (TD) as it is unwound on the converting machine. However, in many cases, the amount of required lateral oscillation for uniform mass distribution in the rewind rolls is prohibitive because of the amount of trim that must be discarded. The period length of windup oscillation required for optimum gauge band distribution has been found to be three-quarters of the distance between standing gauge bands. The optimum speed of oscillation has been found to be about 1½ in./min.

Standing gauge bands are sometimes so severe that they result in stretched web at the gauge band areas. Baggy lanes form in the web at these points when the web is unwound from the mill roll, because the web has been stretched beyond its elastic limit.

Incoming webs that exhibit baggy lanes or edges do not yield smooth rolls even on the best winding equipment, because the baggy lanes are longer than the rest of the web. Attempting to pull the baggy lanes smooth by increasing the web tension does not work, because the web neck-in forces will narrow the flat sections of the web, and the baggy lanes will turn into machine direction (MD) wrinkles.

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8. Tension Isolation of the Web

Tension isolation of the web before and after any process is a must for ultimate control of that process. Many machines run open to the atmosphere, and they are candidates for using a vacuum roll for tension isolation. However, vacuum rolls must be well designed in order to work well. A vacuum roll that is well designed may be used on very clear webs without marking the surface, and it can be used on wet or dry webs. Vacuum rolls tend to be very reliable when properly constructed, and they usually stay very clean in the area of the web path.

Most superior designs are easy to adjust for web width changes, and some can even be adjusted for wrap angle changes without excessive downtime. Vacuum rolls should be installed with wrap angles from 90– 180 deg.

For best performance, a vacuum roll should have two or more fine-mesh (100–150) screens covering a very porous shell. The inner screen may be made of heavier wire and be more open than the outer screen. The screens must be either endless or joined by welding to form a tube.

The welded joints must be worked very smooth to prevent marking clear surface webs. Also, these screens must be fixed to the shell to prevent movement between the screens and between the screens and the shell.

Screens are necessary to distribute differential air pressure under the web and maximize the amount of tension isolation possible. The amount of tension that can be isolated on a properly designed vacuum roll can be calculated from Formula (d) below.

Formula (d)

T2 – T1 = (T1 x e (µ x K x q)) + (µ x K x? P x R x q) – T1

T1 is the low web tension, T2 is the high web tension, µ is the coefficient of friction (COF) between the web and the outside screen surface, K is the % surface area in contact with the web, ?P is the air pressure difference side to side of the web, q is the wrap angle in radians, and R is the vacuum roll radius. Formula (d) gives the amount of tension isolation capability per unit width. A larger coefficient of friction, wrap angle, and roll radius will directly increase the amount of tension isolation capability. However, maximizing K reduces the effective? P by decreasing the actual surface area that is acted on by. P.

The above formula generally describes the tension isolation of all “S” wrapped rolls, whether open to atmosphere or in a vacuum chamber. K becomes 1 and? P becomes 0 when operating on solid surface rolls in a vacuum chamber.

Nip rolls should never be used for tension isolation unless there is no other alternative, because they can damage the web, especially clear webs. Properly designed nip rolls work well in limited situations where the nip loading pressure does not have to change as different products are run on the machine. Effective tension isolation can usually be obtained at 10–15 PLI for most applications.

There is only one nip roll loading pressure that is correct for each set of nip roll shells. Nonuniform nipping pressure will result when nipping pressure is changed. Good design requires a torsion shaft between the pivot arms of the movable nip roll. This shaft should be split and coupled with a precision shaft alignment device so that the centrelines of the two rolls operate parallel in the nipped position. The torsion shaft must be flexible enough to let a large wrap occur without destroying the nipping alignment in running position.

9. Surface Profiles

Surface profiles of web handling rolls should be flat or slightly concave. And they should be textured to provide a relief reservoir for excess boundary air that clings to the web and roll surfaces during operation. The texturing should be machined into the roll surfaces in order to provide good control over the roll diameter throughout the working surface.

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The rolls should operate with a web wrap angle of 90–180 deg. They should never be used where the web temperature is high enough to lower the yield point to where the web will be elongated beyond its elastic limit. The preferred surface profiles for concave rolls are given by Formula (e).

Formula (e) Y2 = 180000 x X

The origin of Y and X is at the centre of the surface where Y represents one-half of the surface length and X represents the radius extension of the working face from the minimum radius at the origin.

10. Web Spreading

Web spreading is a major process item for consideration. Whether the web is to be slit or wound, it needs to be spread flat before the next operation. One of the methods for spreading already has been discussed in item 9 above.

Another method of spreading is the raised edge surface or (under-cut) roll. This method probably has been used for web spreading since webs were first made to run over rolls through a machine. I do not know the origin of this technique, but in all probability an operator found that wrapping two or three layers of masking tape under the web edges on the roll where wrinkles were appearing would make the wrinkles disappear.

One thing that these operators probably learned quickly was that a small increase in radius works well, but too much increase fails. I have found that a build-up of 0.007–0.010-in. radius works well on most webs. Also, only 1–1½ in. of extended radius needs to be in contact with most webs.

The method is simple and follows sound scientific principle. Because the roll radius has been increased, the web edges are pulled at a higher velocity than the rest of the web. Since the web edges are running at a higher tension than the rest of the web, they tend to draw the adjacent web toward the higher tension zones, which spreads the web. Care must be taken to keep the web edges cantered on the raised sections of the roll. Wrinkles will appear on the raised sections if the web edges can extend beyond the raised areas.

Under-cut rolls can be used on webs that are always the same width. The raised radius sections must have surface texture to provide high surface friction so that the edges can provide the TD stretching forces. Surface texture also is necessary on any tape that may be used to raise the roll radius under the web edges. Bowed rolls are positive spreading devices with a long tradition in web handling, and they are found on a great many machines. The spreading action comes from the angle of tracking friction that each web lane encounters as it moves around the roll. All points on the roll surface run perpendicular to the roll axis at each cross-section plane. When the roll axis is properly bowed in the web path, the traction vectors of the surface diverge toward the downstream direction, and these forces affect spreading.

Maximum wrap angle on a bowed roll is 90 deg. One of the limitations in application is that bowed rolls require a significant web path span. The reason for this restriction is that the length of the thread path centreline is longer than at the outside web edges in the span where the bowed roll is used. The web edges often go slack after they pass over a bowed roll. The extra length at outside edges must be flattened and the web tension profile adjusted before further processing, especially if the web is going through a slitting section.

Sometimes bowed rolls are misused to correct wrinkle problems that should be corrected by other means. For example, bowed rolls should never be used as an attempt to correct the effects of nonaligned rolls. Bowed rolls often are run with excessive bow when machine nonalignment is severe, as operators attempt to correct serious web flatness deficiencies. Often this action (excessive bow) leads to abrasion of the bowed roll elastomeric covering and the unnecessary generation of debris. Also, running a bowed roll in this fashion will degrade the roll’s surface friction with the web by causing the elastomeric surface to glaze over.

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Many concepts for web spreading have limited value. One method that you can use to test whether the spreading device is working is very simple: Insert a knife just before the roll and observe the web after the roll. The knife cut will widen if the roll is doing any spreading. This simple test may save you from buying one of these devices.

1.2.How to Handle Web Efficiently

Understanding winding, roll defects (e.g. tin-canning), and winding-related web quality (e.g., bagginess) begins with understanding how the web, winder design, and process conditions combine to create the stress and strains of the wound roll. To understand the stresses and strains within a wound roll, we must understand four things:

What determines the web tension as it makes first contact with the winding roll? How is the tension controlled from the first to final lap of the winding roll?

How do dimensional changes within a roll (e.g., compression) change roll strains and stresses? How do transverse direction variations in equipment or web properties affect roll structure? Here is the second part of a four-part series on winding fundamentals.

Part 2: Wound-on Tension Vs. Radius and Speed

The wound-on tension (WOT) will normally change for each layer of a winding roll. If applied torque and nip load are constant, the torque contribution to WOT will reduce with growing roll radius. Many winding machines include options to program a changing tension or nip set point as a function of roll radius or speed.

Winding torques and tensions may be controlled to vary with roll radius. In constant tension winding, the center torque is increase proportional to roll radius. In constant torque winding, torque is held constant and winding tension will drop inversely with roll radius. Many wound rolls are wound with a tension vs. radius controlled somewhere between the constant tension and constant torque conditions. The profile of tension vs. radius is usually called winding taper tension or tension profile.

There are many logical reasons to change winding torque or nip load during the winding process. Some of these include:

All the layers of a winding roll must transmit the applied center torque through the layer-to-layer frictional contact without slippage. The layer near the core are especially challenged in this ability to transmit torque since they have less area per layer to develop friction and they are at a mechanical disadvantage to the outer layers of a roll. Therefore, center wound, and unwound rolls need more pressure in near core layers. To achieve this near core tightness, many winding processes use higher values for winding tension and nip load when winding near core layers.

In winding smooth, non-porous products, an air layer will wind into the roll without enough pressure to compress or squeegee the entrained air. If excess air is allowed in a winding roll, the roll structure will loosen over time as the air escapes the wound roll, leading to soft roll defect (TD Buckling, sag, roll handling, or unwind telescoping).

A winding nip roller is an effective tool to greatly reduce air captured in a winding roll. The air exclusion of a winding nip is directly a function of speed and inversely a function of effective radius (which is a factor of both nip roller radius and winding roll radius). To wind a roll with constant entrained air per layer (with a goal of controlling entrained air to less than the surface roughness of the product), nip load would need to be profiled to both speed and roll radius.

The nip-induced tension (NIT) may be a function of roll radius with more NIT when roll radius is small. Entrained air will be a function of speed. Many winders, including most slitter/rewinders and all zero-speed core transfer winders must start and end the winding roll at zero zero-speed. Should winding torque and nip load be controlled as a function of speed?

Clearly, if winding torque and nip load are held constant, they will not create a constant roll structure due these speed and radius effects. However, constant roll structure is not necessarily the goal of winding. Just as a skyscraper will have different demands for the building structure of layer near the bottom vs. top, so too will winding rolls have different structure demands from core to final layer.

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taper tension vs. radius with final tension between 20% and 60% of starting tension on a linear tension or torque vs. radius curve. This approach creates a roll with near core layers that are tighter and have less entrained air and outer roll layers, while also reducing the torque transmission requirement in winding the final layers but having enough nip load to avoid shifted layers of air lubrication.

1.3.Web Handling Principles and Methods Tension Principle of Winding

When winding elastic films, web tension is the dominant principle of winding used to control roll hardness. The more tension pulled, and the more stretch put on the web before winding, the harder the wound rolls will ultimately be. The winding web tension is often determined empirically.

However, the maximum amount of web tension can also be determined by using 10% to 25% of the

material’s elastic limit. When relying only on tension to control roll density, it is important that the

winding tension be tapered smoothly as the roll diameter increases. The tension taper should be between 0 and 50%. A tension taper of 25% at full roll is common.

Nip Principle of Winding

When winding inelastic webs, nip is the dominant principle used to control roll hardness. Web tension is controlled to optimize the slitting and spreading operations. The nip controls the roll hardness by removing the boundary layer of air following the web into the winding roll. The rolling nip also induces in-wound tension into the roll. The harder the nip, the harder the winding roll will be.

The challenge for winding is to have enough nip to remove the air and wind hard, straight rolls without introducing too much in-wound tension in order to prevent roll blocking or deformation of the web over the high-caliper area.

Key considerations in applying the nip principle of winding are: o The nip must be applied where the web enters the winding roll.

o The winding film’s weight and the lay-on roll’s weight, as well as web tension, should not affect

the nip loading.

o The nip pressure should be tapered as the roll winds, to prevent “starring” and “telescoping.” o The larger the roll diameter, the more air is introduced to the nip, producing a tapered nip pressure

with a constant nip loading.

Torque Principle of Winding

Torque winding is the force induced through the center of the winding roll, which is transmitted through the web layers and tightens the inner wraps. This torque is used to produce the web tension when center

winding. Therefore, “tension” and “torque” are the same winding principle. However, when the pressure roll is driven to control the web’s tension, then the torque induced through the center of the roll can be independently controlled to adjust the winding roll’s hardness profile.

Film winding is often referred to as an art. This is because the setting and programming of the TNT will

vary depending on:

o The type and design of the winder.

o The type of web material being wound (thick/thin, extensible/non-extensible, slippery/sticky).

o The width of the rolls being wound.

o The speed of the winding operation.

There are three basic winding processes used for web materials: center winding, surface winding, and combination center-surface winding. Each process uses one or more of the TNT winding principles to build roll hardness.

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Typically, winding a product occurs many times during a converting process from the time the material is made until it is applied by the end user. Because of this, winding is extremely critical in the web handling process and must be undertaken correctly otherwise damage to the material will take place and it is virtually impossible to be corrected during the next process. However, in some cases it can be masked if previously rewound correctly.

It is like packing a parcel for shipment, if you don't pack the articles in the box correctly, when shipped it will never arrive at

its destination in the condition you want it to. Rewinding a material is like that. It appears to be easy but, if not done correctly, you will destroy the material and it will be impossible to be handled in the next process.

Material Can Be Wound in Three Basic Ways: Centre Winding, see figure 1

Surface Winding, see figure 2

Centre Surface Winding, see figure 3

There are many improvements that can be made to each of the basic winding concepts and it is important to understand some of the benefits and disadvantages of each technique. The following are views and opinions of the author in connection with the various types of winding and is provided as technical information on the various aspects of winding but does not include in depth mathematical formulas associated with the information provided. It is given as an overview for the basic understanding of the principles of rewinding paper and flexible packaging materials.

A center surface wound reel in my opinion provides the optimum control of the density of the rewinding package, however, it has a disadvantage from a loading and unloading point of view, of the finished reels, especially when automation is involved. The principle is basically the combination of a surface rewind where the surface rewind roller is driven and there is a force applied between the rewinding reel and the surface rewind roller or drum. In combination with center wind whereby a motor is fitted to the rewind core, which draws the material onto the core at a given tension depending on the torque being applied via the motor to the core, see Figure 3. The contact force, between the reel and the surface rewind roller or roller as well as the torque applied to the core itself and the initial web tension will now finally control the density of the reel and tension in the web as it is rewound.

Center Winding

A center winder could be a gap winder where only tension is used to control roll hardness. A center winder could also incorporate a lay-on or pressure roll. This winder would use both tension and nip to

control the roll’s hardness. In center winding, the spindle torque through the center of the roll provides

the web tension.

An advantage of center winding is that this process can wind softer rolls. This type of turret winder can provide quick indexing and fast cycle times. The disadvantage of center winding is the limitation of maximum roll diameter due to the torque applied through the layers of slippery webs. In addition, center winders have a higher probability of generating scrap during roll changes.

Turret center winders are:

•Best for winding soft rolls (i.e. webs with gauge bands). •Best for winding film with high tack.

•Best for winding small-diameter rolls. •Easily designed for dual-direction winding. •Able to provide adhesive less transfers.

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Centre winding is probably the most frequently used method of winding a reel whether it incorporates a differential rewind mandrel, a fixed rewind mandrel or has duplex differential rewind mandrels. However, winding using this method is restricted to relatively small, low weight reels of material typically reels up to 800 mm diameter are wound using this method.

The main and this method is typically used where high tensions are involved or very low speed because the air between the layers of material must be excluded by the tension of the web.

To overcome this problem a lay-on roller or packing roller can be

used, as shown in Figure 15 and Figure 16. In Figure 15 the packing roller is positioned at some point around the circumference of the rewinding reel and in Figure 16 the web is wrapped around the lay-on roller prior to it entering the rewinding reel.

For guidance purposes, if you wish to achieve a finished reel of a given density then using the method shown in Figure 14 you would need a tension of X Newtons per cm. To achieve the same density of reel using the method as shown in Figure 15 you would only need approximately a tension of X/2 and if you use the method shown in Figure 16 then the rewind tension would be approximately X/4. These are just guideline figures but indicate the reduction in tension that can be used to achieve the same density of reel. The reason for this is that the air is excluded by the lay-on roller and a tension is induced into the reel by means of the lay-on roller contact force – NIT (nip induced tension) and the WIT (wound in tension) is achieved without extending the material. Therefore, a less distorted reel will always be achieved with the use of a lay-on roller. I must emphasize here that this is for winding non-adhesive material.

Another advantage of using a lay-on roller in the format shown in Figure 16 is that the web enters a perfectly flat surface (the lay-on roller) just prior to it entering the rewind reel. This reduces the possibility of any creases or wrinkles taking place in the material at the point of rewind. There is sometimes an advantage when winding wide widths of material to add a parallel spreader roll within the lay-on roller assembly just prior to the web contacting the lay-on roller. This ensures that the material is wrinkle-free prior to winding and tends to create a good quality rewind package.

It is sometimes found beneficial that as the speed of the web increases the lay-on roller contact force increases ensuring the required amount of air is excluded during the rewinding process. You are not trying to totally exclude the air entrapped between the layers as it is beneficial to have some air there, but it is important not to have too much, otherwise the roll will telescope. It should be remembered that the torque to create the tension is transmitted from the center of the reel and as the reel diameter increases the torque has to be transmitted from layer to layer of the material and if you are winding a reel under a constant tension profile then the torque applied to the center of the reel is increasing as the diameter increases. This means that a screwing effect can take place to the reel, this is shown in Figure 17. Because of this, it is beneficial in many cases to wind a reel using a taper tension profile.

The two limits as far as rewind tension control is concerned are between a constant tension and a constant torque, therefore a reel that is wound with a hundred percent taper is following a constant torque profile, see Figure 18. Many people consider a hundred percent taper means that at the end of the reel you have zero tension. This is totally incorrect. The tension profile varies between a constant tension

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Torque = Force (Tension) x Radius

For a constant torque profile, you have a taper tension. Also, the limits for the diameter in the above equation is the maximum diameter capability of the machine and, therefore, in the example shown in Figure 17.

If you are only winding up to a diameter of 600mm you will see the maximum change in tension takes place between the 75 mm diameter and the 300 mm diameter. This is one reason why it is advantageous to wind material from a 152 mm (6") core as there is less diameter change taking place.

The smaller the diameter of core, the more the change that must take place at the beginning of winding a reel. It is of paramount importance that there is enough tension in the material or density of a reel at the bottom when center winding to ensure there is enough consolidation of the layers of material so they can transmit the required torque as the reel builds up in diameter. A good test for this is to do the following. Say you start to wind a reel from a 76 mm (3") core, 90 mm o/d core, once you have reached a diameter of say 150 mm stop and draw a line on the side of the core, as shown in Figure 19. Now wind the diameter up to 200 mm and extend this line. Continue this process every 100 mm up to say the maximum diameter of 600 mm and you should have a straight line. However, typically, if the reel has not been wound properly you will end up with a 'J' line which shows that the web has screwed during the winding process, see Figure 19. This, if too much, is very bad. You can also do a similar test when unwinding the material. This determines whether the reel has been wound to the correct density for the tension that is going to be used during the unwinding process.

It is very difficult to give a formula for the lay-on roller contact force and the rewind tension. As previously stated, the rewind tension or, in fact, process tension in a web should be around 25 – 30% of the elastic limit of the material, however, this is not totally cast in stone but, obviously, you would not wind a material at a tension beyond the elastic limit or even approaching it. As far as the lay-on force is concerned, this is enough force to exclude the air from the layers of material and it is normally advantageous to use a lay-on roller which is rubber covered to provide a little bit of compliance during winding.

This is because no materials are perfect and, therefore, the surface of the rewinding reel is not completely flat as more and more coatings or laminates of material takes place or even if it is printed, then there will always be some 'out of flatness' within the rewinding reel.

It is essential that the lay-on roller is a rigid assembly and does not follow the profile of the rewinding reel. If the lay-on roller is not parallel, then what will happen is that the material will track itself on the lay-on roller and give a poor-quality edge profile of the finished reel. Sometimes people believe that by allowing the lay-on roller to follow the contours of the rewinding reel, this will give a better-quality roll. This is totally incorrect. The contact force of the lay-on roller will also increase the density of a reel due to the nip induced tension and, therefore, the wound-in tension will be increased. It is also very important to ensure the 'J' line is kept to an absolute minimum because this means distortion in the material is taking place in the lower layers. However, by undertaking some simple tests, winding a few reels of material under different winding parameters, the optimum characteristics can soon be determined for the product being processed.

With modern technology, this means that these parameters can easily be stored against a suitable mnemonic for retrieval later, therefore ensuring from a QA point of view that a product can always be wound at the optimum characteristics.

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When surface winding elastic materials, web tension is the dominant winding principle. When surface winding inelastic materials, nip is the dominant winding principle. Surface-type film winders use a driven winding drum. The winding rolls are loaded against the drum and are surface wound.

The advantage of surface winding is that web tension is not supplied from torque being applied through the layers of film wrapped into the roll. The disadvantage is that air cannot be wound into the roll to minimize gauge bands and roll blocking problems.

Drum surface winders are:

o Best for winding hard rolls (i.e., protective films).

o Best in utilization of space and horsepower.

o Best for winding very large-diameter rolls.

o Best for minimizing waste during transfers.

o Less expensive.

o Mechanically simpler, with a single and smaller winding drive.

Figure .

Surface winding is probably the simplest and most basic form of winding, however, if not undertaken correctly it can cause horrendous problems to the final material. Surface winding is typically used in a slow-moving process, say maximum 100 meters per minute depending on the material being processed see Figure 2. Basically, the concept is to force a reel into contact

with a roller, which is rotating at a given surface speed. The co-efficient of friction between the material and the rewind roller as well as the contact force between the reel and the rewind roller causes the web to be rewound onto the rewinding core.

The principle of increasing the contact force will increase the roll density (it will be harder). This is something that should be borne in mind when looking at alternative methods of winding. The fact that the roll density increases as the contact force increases is the basic principle of nip induced tension or wound in tension (WIT). If the contact force was zero, then the reel would not wind at all. As the contact force increases the material is wound onto the core

and as the force increases the tension in the material increases which increases the density of the wind by creating a tighter wind. This is caused by the exclusion of air and the nip induced tension (NIT) at the nip point.

Surface Winding Tension Controls

With single rewind roller surface winding the tension in the web at the winding point is somewhat difficult to control as there is no direct means of adjusting the tension at the rewind reel itself, however, the density of the reel and to some degree the tension in

the rewinding web, can be changed by the contact force and the surface texture of the rewinding roller or rewind drum, this wound in tension is sometimes referred to (WIT).

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There will be a wave of material created in front of the roller, creating a tension in the material greater than zero, before the roller and a tension that is less than zero, which of course is not possible, so it is zero at the out feed of the roller and greater than zero at the infeed.

Therefore, creating a tension in the material due to the nip force, hence the term Nip induced Tension (NIT). If the web was held in position at the front of the roller and behind the roller you would find that at the front of the roller you would have a wave of material or excessive material and behind the roller you would have a reduction in the amount of material caused by the change in tensions or the elongation of the material.

Remember that tension relates to material elongation. This demonstrates that the tension in the web after the nip is greater than the tension before it, therefore, proving the basic principle of nip induced tension see Figure 13.

This is the principle of surface winding whereby tension in the reel is only being created by the nip induced force and the tension in the web before the rewind. Quite often, to improve this concept, the rewind roller or drum has a separate drive on it such that it induces tension into the web prior to winding taking place, by increasing the speed of the rewind roller or drum relative to the other web transport path rollers. This is an enhancement to the basic principle of surface winding, however, since surface winding is a concept that is used in many, many applications and, as previously stated at relatively low speeds, it is a concept that should be borne in mind for the following principles of winding and rewind tension control.

There is, of course, a maximum induced tension that can be created, it should be realized that the winding nip cannot induce more tension than the frictional forces available at the point of contact between the layers of material. Therefore, the top of web to back of web co-efficient of frictions determine how much elongation or tension occurs for a given winding nip force. Please remember that we are discussing non-adhesive products here as far as this is concerned as the co-efficient of frictions when processing non-adhesive materials are totally different than those for non-adhesive materials and the principle of surface winding for adhesive materials is generally unacceptable, mainly because of this.

If you think of this logically from Figure 11 where the contact roller is pressing onto the material, the material must slide on the solid steel base, otherwise the material cannot pass under the roller. Therefore, an amount of nip induced tension, or wound in tension (WIT) is dependent upon the co-efficient of friction between the layers of the material and the surface nip roller of surface winding roller. If it cannot slide you cannot induce tension from the nip roller, hence due to this principle an adhesive product, because of the high coefficient of friction cannot normally have tension induced this way.

To determine the co-efficient of friction between two layers of material, reference should be made to the British Standard BS 2782, part 8, method 824A 1996 or the ISO Standard 8295 dated 1995 which gives a detailed explanation of how to measure the co-efficient of friction between two materials.

As an indication of the effects of nip induced tension or wound-in tension, if the force of the nip roller is doubled, then you will increase the wound-in tension by approximately fifty percent under the same winding conditions. If the force of the nip roller is tripled, then the wound-in tension could be doubled by applying this nip force but, of course, remember that the limiting

factor is the co-efficient of friction between the layers of material.

Center-Surface Winding

A center-surface winder uses both center winding and surface winding processes (see Fig. 1).

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roller. However, the basic principle is that the core is driven from the center by a motor and the web is then rewound onto the core, see Figure 1. In its basic form it has the disadvantage that air has to be removed from between the rewinding layers of the material to increase the density of the package and slippage between the layers of material, by increasing the torque at the center drive or the tension in the web prior to rewinding which has the disadvantage that elongation (stretch) of the web may take place. It also means that the layers of the material as the reel builds up in diameter must transmit this torque (Tension) to the incoming web, therefore the density of the package at the start of the reel needs to be enough to do this. There are many ways, which will be described later, on how this can be achieved. Center-surface winding uses all three of the TNT principles. The web tension is controlled by the surface drive connected to the lay-on or pressure roll to optimize slitting and web spreading. The feedback from the web-tension load cells trims this drive to control constant web tension during the winding operation. The lay-on roll loading applied to the winding roll controls the nip. The torque from the center drive is programmed to produce the desired in-wound tension for the roll hardness profile desired.

The advantage of center-surface winding is that the winding tension can be controlled independently of the web tension. For high-tension applications, center-surface winders can share the tension horsepower requirements to allow small center drives. The disadvantage of center-surface winding is that the winding equipment is more expensive and complex.

Center-surface winders are:

Best for winding high-slip films to larger diameters.

Best for slitting and winding extensible films to larger diameters.

Best to significantly taper in-wound tension without affecting the film width. Able to supply in-wound tension without

stretching the web over caliper bands

Centre Surface Winding Tension Control

The principle of center surface winding is to have a drive to the center of the rewinding reel which would be under torque control in some format or another and the roll itself to be forced in contact with the rewind roller or drum. Figure 20 shows the most popular system whereby the reel is held on a pivoting arm; however, an alternative

system can be used where the rewinding reel is on a horizontal slide, which is shown in Figure 21. The horizontal slide system has the advantage that there is no influence on the contact force as the reel increases in diameter, therefore the weight increases. With modern day computing power this can be compensated for, but you need to know the density of the material being processed and this is also dependent on the density of the reel being wound. The advantage of a center surface system is that the weight of the reel does not dramatically affect the rewind roll density or the tension in the rewinding web which is the case when you are center winding.

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The principles of the 'J' line still appertain to center surface winding, however, typically you do not suffer with 'screwing' of the reel when center surface winding as the contact force excludes the air and induces nip induced tension. The torque in the center of the reel is only there to overcome the mass of the reel itself. As the speeds increase the contact force must increase and the torque in the center of the reel must increase to compensate for the masses of the reel being wound. It is the same for center surface winding as previously stated for center winding and optimizing the tension, that this must be determined empirically to suit the product being processed.

If you did a survey of all the types of machines that are manufactured and the method of winding used, center winding would dominate the converting industry. This domination is not because it is the best winding method but because it is simpler to manufacture and far easier to automate from a reel unloading point of view. Also, unless you are winding very large diameter rolls, say in excess of 800 mm diameter, or at very high speeds, say 1000 meters per minute, then center winding is probably the simpler and most efficient to operate.

You will see in the paper industry that surface winding or center surface winding will dominate this industry due to the high weight of reels and the large diameters that are wound. Typically, you are winding rolls with a 20:1 ratio from the core to the final diameter. Also, the tensions that are used are much higher than in the flexible packaging industry and the principle of center surface winding allows you to wind a reel at a higher tension or a higher density because you have the combination of the center wind and the surface contact to create the wound-in tension.

Twin Roller Surface Winding

Twin roller surface winding is typically used in the paper industry and not for processing flexible packaging materials. This technique is also often used in the textile industry. When a twin roller surface rewind is being used to process multiple webs, it is critical that the caliper of the material is good, hence the benefit of this type of winding is when processing paper material. A flexible packaging material such as polythene would not wind successfully using this principle.

To enhance the winding of a twin roller, rewind a lay-on roller can be incorporated on top of the rewinding reel which will hold the core between the two surface wind rollers during the initial stages of winding

i.e. at the core. This, of course, is the most critical part of the wind but once material has been wound onto the core, the weight of the reel itself will retain contact between the winding reel and the surface of the rewind rollers. You can see from Figure 4 and Figure 5 the two basic principles of two roller surface winders. Figure 4 shows the principle where the web is brought between the two surface winding rollers and Figure 5 shows where the web is brought around the front rewind roller and then onto the rewind core.

One added benefit of winding as shown in Figure 4 is that slitting can take place on the front rewind roller, therefore ensuring the absolute minimum web path from the slit point to the rewind reel. When winding material as shown in Figure 5 it is essential that there is a differential in the surface speed of the two rewinding rollers to ensure optimum winding of the material. When winding this way it is necessary for the surface rewind roller, that does not have the web passing round it, to revolve at a slightly faster speed than the other and, using digital drive technology, this can be electronically geared to enable optimum winding to take place.

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As previously stated, it is of paramount importance to ensure that the web starts tight on the core to prevent telescoping and dishing reels as the diameter of roll increases. The only method of transporting the web onto the reel is by the surface contact between the reel itself and the rewind rollers. This increase in speed is needed because the rear roller is driving the outside wrap of the material and the front roller is driving the layer of web underneath it. If the front roller exerts more drive than the rear roller, it will tighten the wind which is needed, particularly at the start of the process when winding onto the core. When winding material as shown in Figure 4, where the same web layer passes over both the front and the rear rewind surface rewind roll, it is typically not necessary to have a large differential speed between the two surface rewind rollers as they are both in contact with the reel and the same layer of the web. However, depending on the material being processed, you can create a bubble between the two rollers and by having the facility to adjust the speed of one surface rewind roller to the other, this bubble can be reduced as required. The amount of over speed would be determined empirically during the winding process.

The main advantage of winding using the principle of Figure 4 is accessibility to the slitting section of the web, which typically is positioned just prior to the front rewind roller or drum. This allows, if required, separation to take place by means of a spreader roll. The other advantage is accessibility of the slitting knives and ease of change from one slit width to another.

As previously explained, the use of a rider roll is beneficial to ensure surface contact at the start of the wind. If the pressure is only applied at the edges of the rewind shaft, then the contact force is typically only taking place at the edge of the reel. The wider the machine, the more this phenomenon is accentuated, therefore with the use of a rider roller positioned on the top of the reel, this will help ensure the core is held in contact with the two rewind rollers at the start of winding. By varying the speed of the front rewind roller as the diameter increases it is possible to control the roll density as required for the material being processed. As the reel diameter increases the degree of over speed can be reduced, therefore adjusting the wound-in tension created by the differential in surface rewind roller speeds and the nip forces created typically by the weight of the reel being wound. Unfortunately, as the reel diameter increases the contact force increases and therefore the nip induced tension increases, so a balance must take place between the over speed between the rewind rollers and the tension induced by the increase in weight of the rewinding roll.

When winding paper using the twin surface roller wind principle, if the tension in the roll is too high you will create excessive residual strain in the roll which can cause bursting of the roll or web breaks when unwinding. Conversely, if the reel is wound too soft you will have a situation where telescoping will take place either during the winding or unwinding of the web. You will also encounter interleaving between the layers of material from the various slit reels as all the reels are wound on a single shaft and therefore sideways control of the material is extremely difficult, hence the necessity to have control of the rewinding process.

By using a thread-up as shown in Figure 5 where the web passes between the surface rewind rollers, you have the better chance of controlling the winding tension by means of the differential speed between the two rewind rollers.

By means of the ‘Cameron’ strain test, it is possible to determine the strain in a roll

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The basic formula and mathematics associated with the various tensions induced in a web on a 2-roller winder is shown in Figure 7 and are explained as follows.

If TR was to be less than TW, VR would have to be less than VW. It therefore is logical to attempt the reduction in surface velocity of the rewinding roll by reducing its speed via the rear rewind roller to achieve this. To provide this reduction in tension

VRD would have to be made less that VFD by means of varying the speed of the rollers. Also, if VR was to be made less than VW, slippage would have to occur between the rewinding roll and the incoming web on the front rewind roller.

Consequently, the rear rewind roller would have to restrain the rewinding roll with enough force to overcome the friction force between the roll and the incoming web. Typically, for processing paper materials the co-efficient of friction between the material and the Rewind Roller should be approximately 0.3. To achieve this reduction in speed, significant forces are required in the drive system to the rollers and therefore it is of paramount importance that the motors being used have adequate capacity to provide this facility.

This bubble should not be less than approximately 1 mm in height otherwise the tension in the rewinding reel will be too great, however, the optimum bubble size has to be determined by empirical means as it is totally dependent upon the characteristics of the material and the two rewind roller surfaces. In the event of the caliper of the material varying across the width of the web then the density will vary across the rewound reels

and therefore additional adjustments may have to be made.

As well as the differential speed between the two rewind rollers, the tension in the rewinding reel on a twin roller surface rewind heavily depends on the nip induced tension caused by the weight of the reel and the actual rewind roller diameter. For a constant rewind roller diameter and a constant unwind tension see Figure 8.

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the nip force is kept constant then the resulting wound-in tension in the rewind roll decreases from the base line by approximately 70%. Tripling the diameter decreases the tension further to approximately 50%. This principle can be demonstrated as shown in Figure 9 by means of laying multiple webs of material on a flat steel surface, representing a roll of infinite diameter and attaching a spring balance to each piece of material.

If a roller is then laid onto the web, simulating the rewind roller and it is rolled along the web it will be possible to see the nip induced tension which is created by the rewind roller.

This will give an indication of the effects of roller size and the nip induced tension in the varying webs. It will be seen from the tests that the smaller diameter roller will produce more nip-induced tension, which, in effect, is the wound-in tension (WIT) in the reel.

As the surface rewind roller diameter increases this effect will reduce.

For most materials there will be a point where the effect of increasing the rewind roller diameter on wound in tension is minimal and therefore only nip force will have any significant effect on the wound-in tension.

What Is the Required Rewind Tension?

Basically, the requirement as far as rewind tension is concerned, is to create a package that can be transported to its destination and unwound without causing the final user problems. The rewind tension will vary depending on the material being processed. The following principles are based on winding non-adhesive materials (watch this space for winding non-adhesive materials). Reel density is one of the most important factors when winding non-adhesive materials and very often is not considered by the converter. The reel density is basically the weight of the reel per unit volume of the reel. By this, I mean that if you calculate the volume of the reel (that is the material less the core) and divide it into the weight of the reel (that is the material less the weight of the core), you will have a weight per unit volume, typically grams per cubic centimeter.

For example, if you have a reel which is 50 cm diameter x 50 cm wide and is wound on a core of 9 cm

o/d then the volume of the reel is ({50 + 2}2 x ∏ ) - ({9 + 2}2 x ∏ ) x 50 = 94,994 Cms³

The weight of the reel is 60kgs. and therefore, the density of the reel is 60 x 1000 + 94.994 = 631.6 grams per cubic centimeter for that product. A very useful numeric value to be noted for the material being processed.

The density of the finished reel will vary, depending upon the rewind tension and the air inclusion, which takes place during the rewinding process. Typically, a reel will telescope if there is too much air between the layers of the material (remember at this point we are talking of non-adhesive materials). Therefore, it is important to exclude air from between the layers of material (well to a certain extent) and wind the material without extending it due to excessive tension. This will create a reel that has been produced to the desired density, tension and edge profile. There are various techniques that can achieve this based on the above basic winding principles and are described in more detail below.

As a guide, the maximum rewind tension at any point during the rewinding process should never reach more than 25% - 30% of the materials elastic limit. Reference should be made to figure 10 below showing a typical stress/strain curve for a material and its elastic limit. When we talk about tension we also mean extension in the material which, as long as the winding tension ensures that the extension is well below the elastic limit, the web extension will try and return to its normal zero extension state or length, therefore during the winding process will create compressive forces which can distort the layers of material below.

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Earlier it was explained about the necessity of having tight layers at the start of the reel where the core is. Using this differential drive technique, it is possible to provide an over speed at the start of the reel, therefore inducing tension and then gradually reducing this as the reel diameter increases. This principle will then provide the optimum density of roll required for the material being processed. However, it should be remembered that the rewind tension is affected by the tension in the web upstream of the rewind as well as at the rewind.

Any tension variations upstream will be transmitted downstream and any elongation in the web that takes place upstream will affect the density and tightness of the rewound reel.

In the event of the under speed of the second rewind roller being too great, a bubble will be created between the surface rewind rollers, therefore visually it is possible to determine whether the differential in speed between the two rewind rollers is enough. Optimum winding is achieved by creating a very small bubble between the rollers.

Web Lines | Why Bigger Cores?

Published: Friday, 16 November 2012 14:13, Written by Timothy J. Walker

Bigger cores avoid many winding defects. I like to joke that the ideal wound roll is a gigantic core with one wrap on it. Think about it.

Bigger cores have fewer layers for a given length of web on the core. Fewer layers of buildup mean less magnifying of long-term thickness variations, less significant headbands, and less winding-induced bagginess. Fewer layers mean less near core pressure buildup and high-pressure defects, such as coining, core impressions, blocking, starring/spoking, and core crushing.

A smaller change from core diameter to final roll diameter means a smaller torque range requirement for a given roll and a winder with a wider tension range able to handle a great variety of products.

The greatest benefit of bigger cores is in torque transmission and associated telescoping. Near core layers have two strikes against them in the challenge of transmitting center winding torque into outer layer winding tension. First, they are at a mechanical disadvantage to the tension at the outside of the roll. Second, they have less area per layer to develop the friction needed to transmit torque. Bigger cores eliminate the near core layers and their low torque transmission capacity.

Diameter will increase, but the diameter increases in moving from a 3-in. (75-mm) to 6-in. (150-mm) inner diameter core is not much. For a 20-in. (0.5-m) diameter roller, moving from a 4-in. outer diameter to a 7-in. outer diameter (assumes a 0.5-in. core wall thickness) only increases the final diameter by 0.85 in.

Yes, I know your customer wants everything on a 3-in. core. Yes, I know that 6-in. cores cost more than 3-in. cores. Yes, I know that 6-in. shafts and core chucks cost more than 3-in. shafts and chucks. This just means the change must be justified.

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1.5. Reason for wrinkles and remedies.

How to identify the source of a web wrinkling problem and then determine the cure.

Web wrinkling during converting can come from a variety of sources--sometimes multiple sources. Thus, pinpointing the exact cause and cure is not always easy. In fact, it is often a process of trial and error.

This article addresses seven common causes of web wrinkling along with practical solutions. It should be understood up front that no two webs or web lines run the same. Likewise, no two wrinkling issues are identical. The cure for one may not be the cure for another, no matter how alike.

In addition, the causes of web wrinkling can vary greatly. Nonetheless, armed with a better understanding of some of the common causes and cures can help minimize trial and error efforts to get you back on target with your desired productivity and quality goals.

1. Poor Machine Alignment

Machine alignment is the first thing to check when experiencing web performance problems (see Figure 1). New rollers may be a waste of money if machine stands are not aligned properly. Though most machines are aligned when first installed, misalignment can develop later due to vibration, improper machine or roller maintenance, tension over time, or in colder climates, even freeze-thaw conditions that can shift plant floors.

Poor alignment between rollers ultimately can cause a wide variety of web problems, including drift, flutter, and wrinkling. Straightening out the slightest misalignment is critical to achieving a higher quality process. Though alignment services can be tedious or costly, you may save time and money over other remedies that simply do not work. Checking machine alignment is an essential starting point and a good investment.

2. Poor Roller Geometry

Sometime, a non-cylindrical roller can be the cause (see Figure 2). Imagine a racecar whose tires are larger in diameter on one side of the car than the other, so the car turns naturally inward on the oval track. Likewise, the web will “turn” if the roll is bigger on one side than the other. When the web contacts the following roll at an angle, this roll

will try to readjust the web back parallel to the roller’s centerline. This generally results in a wrinkle.

Poor roll geometry can come from poor craftsmanship in the design and/or manufacture of the roll, from excessive surface wear or from inadequate roll specifications.

3. Poor Web Quality

Poor web quality such as gauge variation or poorly wound parent rolls can cause all kinds of problems. Excessive parent roll run-out can translate into vibration and web flutter. Inconsistent or poor-quality web material can end up creating wrinkles through the whole process. No roller can totally cure that.

Most machine operators can tell when they have a bad parent roll

on the stand. Don’t accept inferior parent rolls from your upstream

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Roller deflection from higher web tensions can create tension differences between edges and inside material that can cause sagging centers and/or wrinkles.

4. Excessive Roll Deflection

Excessive roll deflection or bowing of the roll under the force or load of the web, can cause wrinkling (see Figure 3). It may not be the effect of a single roll that causes the problem but the effect of the total system.

Like so many other situations, the extent of the problem will depend greatly upon the web characteristics. Small-diameter idlers with long face lengths should raise a red flag for any roller supplier or machine designer. Roller material (steel, aluminum, or carbon composite), roller diameter, wall thickness, face length, web width, shaft sizing, and bearing location all must take load, or deflection, into consideration. To avoid deflection problems, be sure your rolls are properly specified. When ordering, you should know

the amount of web wrap (usually stated in terms such as 10 o’clock to 3 o’clock), web tension (PLI), any

nip load (PLI), web width, and maximum expected line speed. A good supplier will walk you through these questions and can even help you troubleshoot much of this over the phone.

5. Web Expansion

Changes in web conditions such as increased temperature or moisture content are likely to cause variation in the web material. For example, when a web tries to expand and it is constrained by frictional contact with a roller, wrinkles are likely to occur. In these cases, a strategically located spreader roll—one with a special surface grooving—can accommodate the variation and alleviate the tendency of the web to wrinkle.

6. The Unsupported Web

Too little web support can result in bagging and wrinkling (see Figure 4). Unsupported webs also are susceptible to other environmental factors, such as the drafts from nearby equipment or normal plant ventilation. Even an open door can cause enough draft to result in wrinkling.

Idlers typically are used to provide needed web support. The very act of wrapping a web around a roller

does increase the web’s lateral rigidity. The strength and weight of the web will have a lot to do with the

number and location of idlers needed. An applications engineer can better troubleshoot your problem when provided an elevation side view indicating rollers and location(s) where wrinkling occurs.

If wrinkling occurs on a longer web span between rollers, additional idler support may be needed. When evaluating web span, consider the width of the web. A rule of thumb is short = less than the width of your web; long = 3x the width of your web.

7. Improper Web Tension Control

Excessive web tension can wrinkle, stretch, or break the web and cause unwanted roll deflection. Excessive tension can come from too much drag on idler rolls (bearing friction, roll weight, etc.) or from too many idler rolls between drive sections. Stretched edges and slack centers on the web are common with excessive tension.

Loose webs also can result in wrinkles. Lack of enough tension can come from drive sections being out of sync or a poorly designed or calibrated tension control system.

Lighter weight rolls may be the solution for tension problems experienced during line speed changes. These idlers can supply support without excessive drag. If the problem persists once the web is up to speed, an idler designed with special free-running bearings may be needed.

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