Selecting the Proper Web Handling Equipment

By Bob Pasquale

OVERVIEW:

When specifying, designing, building and operating a web processing line much time and focus are spent on the unwinding, winding, coating, drying, laminating and embossing equipment. However, in many cases the web handling and conveying equipment that is associated with these processing lines are overlooked. In this presentation we will discuss these “other” sections such as pull rolls, accumulators, heating/cooling roll sections and web turns/flips. We will review when they may be required; how to select/design them; what their capabilities, design features and advantages/disadvantages are; how they are controlled; and how they are integrated into the processing line to enhance web handling.

INTRODUCTION:

A key requirement of every web processing line is the ability to properly handle and convey the web among the various machine sections. Though much time is spent focusing on the unwinding, winding, coating, drying, laminating and embossing, very little is discussed regarding the equipment sections that are required to allow for the proper handling and conveying of the web from section to section.

The proper handling and conveying of the web covers many areas including assuring that proper web tension is provided and maintained into, out of and between the various equipment sections; that the web is provided to the next section and removed from the previous section at the proper speed/rate; and that the web is delivered to the next section at the required temperature. Additionally, in certain web processing lines there is the need to change the orientation of the web, whether by turning it at right angles to the main web direction or by inverting it during processing.

In this paper we will look at the equipment that is required to allow for the above to occur. We will break the equipment down into four major categories which are:

Pull Rolls Heating and Cooling Rolls Accumulators Web Turns and Web Flips

We will discuss the need for and uses of each of these equipment categories, the factors and considerations that go into the design and selection of the equipment, and the pros and cons of the associated selections.

PULL ROLLS:

Pull rolls are typically defined as a driven roll or series of rolls that are used to assist in conveying the web through the process line. They are one of the most basic equipment sections and can be found in virtually every web processing line. Their design can take one of many forms, with the selection often determined by the associated web handling requirements.

Uses:

Though the need to use pull rolls in a web line varies with the process requirements, it is typically for one of the following reasons:

To allow for the web to be conveyed at a set speed – In many operations a pull roll is used to establish the speed of the process. In these systems the pull roll is driven by a motor and drive that rotates the roll at a determined surface speed, with this speed being used to control how fast the web is conveyed through the process line.

To allow for the web to be conveyed between sections under a controlled tension – In certain processes the web may need to travel over a long distance between process points. This travel distance may result in difficulty in maintaining the web at a tension that allows for the proper handling. In these cases a pull roll may be added to the web path to allow for the tension in the web to be maintained at the proper levels. An example of this is a web that needs to be maintained at a low tension so not to be distorted and which must pass about several idler rolls in order to be conveyed between the various equipment sections. In this case the drag of the idler rolls may result in an increase in the web tension to a level that will distort it. The addition of intermediate pull rolls in this area would allow for the web to be conveyed without the distortion.

To allow for the tension to be changed from one equipment section to another – Often is the case where the tension in the web needs to be different for two consecutive process points. In these cases the addition of a pull roll section allows for this change in tension to occur. An example of this would be between a coater and a dryer, where the web tension may need to be at a certain level for the proper application of the coating and at a different level for the proper handling of the web while drying.

To allow for the isolation of one equipment section from another – In certain applications a pull roll may be added to a web line to allow for the isolation of process steps. An example of this is immediately after an automatic unwind. Preventing the web tension upset that may occur during the automatic splicing process from being seen by the downstream equipment is important and can be handled by the inclusion of a pull roll.

To allow for the feed of web to/removal of web from a process point/operation – There may be areas in a process where web may need to be fed to or removed from a process point. In these cases a pull roll can perform this task. An example of this is at the exit of an unwind accumulator where a pull roll is used to feed web to the process at a steady rate even while the unwind is stopped for zero speed splicing. Another example of this is at the entry of a sheeter, where the web to it is fed continuously but where the web is intermittently stopped at the sheeter for cutting and then fed at a higher speed to allow for positioning of the new sheet.

Pull Roll Types and Selection:

In order to assist in conveying the web, the pull roll must impart a force (tension) on it. This force, which is provided by the motor and drive set that is attached to the pull rolls, is calculated as follows:

F₁/F₂=eᶬᶱ

Where:

F₁ = the force (tension) on the web prior to the pull roll

F₂ = the force (tension) on the web after the pull roll

ɱ = the coefficient of friction between the pull roll and the web

θ = the wrap angle on the pull roll(s)

Therefore, as shown by the above formula, the amount of force that is required by the pull roll to assist in conveying the web can be varied by either varying the wrap angle or the coefficient of friction between the web and the roll.

There are several different designs for pull rolls, each of which offer certain advantages and disadvantages.

The most basic is often referred to as an “omega” wrap pull roll. In this design a single driven pull roll is wrapped by the web (Figure 1 and Photo 1). This system offers the least expensive solution for a pull roll. Additionally, the pull roll only contacts one side of the web, which for certain applications is helpful.

However, because the web wraps only a single driven roll, the amount of wrap angle that can be applied is limited and therefore so too is the amount of force that can be applied.

FIGURE 1 – OMEGA WRAP PULL ROLL PHOTO 1 – OMEGA WRAP PULL ROLL

An “S-wrap” pull roll is one in which two driven rolls are used, with the web passing about them in an “S” configuration (Figure 2 and Photo 2). Because it features two (2) driven rolls that are both contacting the web, the cumulative wrap angle and therefore the force is greatly increased. However, because of the two (2) rolls this system is more expensive than the “omega” wrap pull roll and results in both sides of the web being contacted by pull rolls, which may be undesirable.

FIGURE 2 – S-WRAP PULL ROLL PHOTO 2 – S-WRAP PULL ROLL

The surface finish of the pull rolls has an impact on the ability to impart a force on the web. As the coefficient of friction between the web and the roll increases so too does the force. The most basic surface finish for a pull roll is steel, either with or without chrome. This offers the least roll wear and therefore the greatest lifetime and lowest maintenance. However, this also typically offers the lowest coefficient of friction. As an alternative the pull rolls can be provided with a rubber covering, offering a higher coefficient of friction. However, these rolls tend to wear faster and certain rubbers may not be compatible with certain webs/processes. Another alternative is to treat the pull rolls’ surface with a friction enhancing coating. However, like rubber, these coatings tend to wear faster and certain compounds are not compatible with certain webs and processes.

Another way to enhance the pull rolls’ ability to apply a force to the web is by adding a nip roll (Figure 3 and Photo 3). This enhances the frictional force between the web and the roll, particularly at higher

speeds where it can iron out any entrapped air that would act to reduce the frictional forces. However, for certain webs/processes, the use of a nip roll on the web/product is not an option.

FIGURE 3 – PULL ROLL WITH NIP PHOTO 3 – PULL ROLL WITH NIP ROLL

A final means to increase the force generated by a pull roll is by using a vacuum roll (Photo 4). In this design the pull roll is designed so that air is drawn through the rolls’ surface, pinning the web to it. In essence this design greatly enhances the coefficient of friction between the roll and the web. This type of pull roll is especially effective in areas where the available wrap angle on the roll is limited and/or where only one side of the web can be touched, for example between a coater and the entrance of a dryer. Though these assemblies do offer advantages over conventional pull rolls, they are very expensive to purchase and operate and require a high level of maintenance.

PHOTO 4 – VACUUM PULL ROLL

Regarding the drive of the pull rolls, it is typically controlled one of two ways, with the process determining which is appropriate. In certain applications it operates as a speed regulated drive, with the drive setting the motor’s speed so that the pull roll run at a set surface speed. This is typically the case when the pull roll is being used to establish the speed of the process or when it is being used to feed a section such as a sheeter on a stop and go basis. In most other applications the drive operates in a tension regulated

mode, with a force transducer roll being located in the web path to measure the web’s tension and supply this information to the control system so that the drive automatically adjusts the motor’s speed and therefore the pull rolls’ speed to maintain the tension at a set point/level. This is particularly the case when the pull roll is being used to convey the web between two points while maintaining the tension level or when it is being used to change the web tension level between two points.

HEATING/COOLING ROLLS:

Many processing lines include heating and/or cooling rolls to change the temperature of the web before entering the next process point. These rolls, which are typically driven, can also serve as pull rolls, performing many of the functions are previously described.

Uses:

There are many applications where a roll or series of rolls are used to heat or cool a web. Examples of these include:

Heating a web prior to applying a coating (Photo 5), such as in wax coating where the web is brought up to temperature prior to the hot wax being applied, and then cooling it after coating.

Heating a web prior to embossing so that the web reaches its softening temperature to allow the pattern to be applied to the web, and then cooling it after embossing so that the pattern is retained.

Heating a web prior to laminating, such as in thermal lamination where the webs are brought up to temperature prior to combining, and then cooling it after embossing to maintain the lamination.

Cooling a web after a coating is dried (Photo 6), such as when a hot air dryer is used to drive off either solvent or water.

PHOTO 5 – PRE-COATING HEAT ROLLS PHOTO 6 –POST DRYING COOLING

Roll Sizing and Design:

Properly sized heating and cooling rolls are designed to allow for the web’s temperature to be changed from that at the entry to the rolls to a desired exit temperature. Selection of the number and size of the rolls is based on calculations that are performed based on the operating conditions. In order to perform the calculations the following information is required:

the web’s specific heat the web’s melt (or ignition) temperature

the web’s temperature entering the rolls the web’s target temperature exiting the rolls the web’s thermal conductivity the target operating speed the web’s width the web’s mass throughput (or thickness and density)

In addition to calculating the number of rolls and their size, the above information can also be used to determine the web wrap angle on each roll, the temperature of the rolls’ heating or cooling fluid (and therefore the rolls’ surface temperature) and how much heating or cooling fluid is required for each roll.

Regarding the temperature of the heating fluid, the hotter it is, the quicker the web can be heated resulting in fewer/smaller rolls. However, there are practical limits to how hot the fluid can be. For example, if the fluid and therefore the rolls’ surface is too hot, the web may melt during line stops. The same applies to cooling rolls, where using water that is too cool may result in condensation on the rolls’ surface, resulting in potential damage to the web.

The internal design of the rolls is critical to proper heating and cooling of the web, especially for larger diameter rolls and wider web widths. A properly designed roll should include dual shells with spiraled flow channels between them (Photo 7), allowing for precise control of the fluid’s flow path. This design allows for control of:

the fluid’s speed, maximizing the amount of heating or cooling it can deliver the amount of time it takes for the fluid to cross the roll face, allowing for the cross roll

temperature variation to be minimized

PHOTO 7 – HEAT TRANSFER ROLL WITH INTERNAL SPIRALS SHOWN(Photo Courtesy of Webex, Inc.)

It is important to note that the roll’s internal design is based on a particular fluid flow rate, allowing for the above two conditions to be maximized. Varying the flow rate from the designed parameter will adversely alter both the roll’s heating/cooling efficiency and the cross roll temperature profile. Therefore, in order to change the desired temperature of the web exiting the heating or cooling rolls, one should adjust the temperature of the heating or cooling fluid, not the flow rate.

Lastly, regarding heating rolls, there are several methods available for providing the heat to the roll, each of which offers advantages and disadvantages. For certain applications water can be used as the heat transfer fluid, offering the advantages of being a readily available, environmentally friendly fluid that allows for precise temperature control. However, it does take a significant amount of time to change its temperature and it is limited in its maximum operating temperature. Hot oil offers the ability to be heated to much higher temperatures than water while still allowing for precise temperature control. However, it too takes a significant amount of time to change its temperature and is not a very environmentally friendly

fluid. Steam offers certain advantages including the ability to heat to fairly high temperatures and the ability to have rapid temperature changes (by adjusting the pressure). However, temperature control is not the most accurate and steam systems can often require a high degree of maintenance. Electrically heated rolls offer certain advantages including high temperature capability, very accurate temperature control, including across the roll face, the ability to zone the cross web temperature and the ability to heat the rolls without the need to handle any fluids. However, they do have certain disadvantages including taking a very long time to change temperature and being very expensive to purchase.

ACCUMULATORS:

Accumulators are provided in web processing lines where there is a need to store in process web. The size of the accumulator is typically determined by the amount of storage/time required to carry out a particular operation.

Uses:

Web accumulators can be found at any point on a processing line where the web accumulation is required. Typical examples of this are:

Immediately after an unwind to allow for a zero speed splice to be performed while the process continues to run. In this case the unwind is run in an over-speed mode filling the accumulator prior to the splice being performed, with the web stored in it being used to feed the process while the unwind is stopped. This is very common in operations such as performing a sewn splice or a manual butt splice.

Immediately prior to a winder to allow for zero speed transfers to be performed while the process continues to run. In this case the accumulator is empty prior to the transfer occurring and fills while the zero speed transfer occurs, allowing process to continue without interruption. Once the transfer is completed the winder is run in an over-speed mode allowing the accumulator to be emptied.

In the middle of the process where certain operations need to be performed on an intermittent basis. For example the operation may have an area for the removal of bad web sections, with the web needing to be stopped for the removal and splicing to be performed. In this case accumulators are used to allow for the process to continue to run around the area for removal and splicing.

Accumulator Design Considerations and Features:

There are several factors that need to be considered in the design of the accumulator. The first of these is to determine the amount of storage that is required to allow for the zero speed operation to occur. Once this is determined, one must select the number of rolls and the accumulator travel that are required in order to provide the needed storage. These two decisions are often based on the amount of space that is available for the accumulator to be installed as well as how it will be shipped to the site.

Typical accumulators are vertically orientated, with either a series of fixed rolls at the bottom and a series of rolls located in a moving carriage above them (Photo 8), in which case threading is performed at the bottom of the accumulator, or a series of fixed rolls at the top, with a series of rolls mounted in a moving carriage located beneath them, in which case threading is performed at the top. The threading can either be performed by wrapping the web around each individual roll or, if the accumulator features moving rolls that pass through the fixed rolls (Photo 9), by passing the web in a straight line through the gap between the upper and lower rolls. Not only does the “straight through” design make threading easier, when empty it allows for the web to run through the accumulator without contacting the rolls.

The handling of the web in the accumulator is of extreme importance. It must be kept under controlled tension not only when it is full but also during filling and emptying. The same system that is used to control the tension is used to move the traveling rolls up and down. This can be accomplished in one of

several fashions. The most basic is by using mechanical counter weights to move the carriage of rolls as well as provide the web tension. Though it is the least expensive method, this system does not offer precise tension control nor does it offer the ability to control and adjust the tension automatically. As an alternative a system can be provided that uses pneumatic cylinders for the movement of the rolls as well as the generation of tension (Photo 8). This system offers the advantage of the ability to control and adjust the tension automatically but does have the disadvantages of cylinder stiction and hysteresis in the pneumatic system. Another way of controlling the movement of the carriage and the tension in the web is through the use of a motor and screw system (Photo 9). In this design a drive is used to control the motor and therefore the carriage position as a true closed loop tension control device.

PHOTO 8 – ACCUMULATOR WITH PHOTO 9 – ACCUMULATOR WITH DRIVEN

PNEUMATIC LOADING CARRIAGE AND STRAIGHT THROUGH LACING

Of consideration in the design of the accumulator is the feed of the web into and out of it. It is typical to locate pull rolls, similar to those previously discussed, on the entry and/or exit side of the accumulator not only to deliver the web to and from it but also to isolate it from the rest of the process.

WEB TURNS AND WEB FLIPS:

Often is the case that a web needs to either be turned at 90° to the process line or it needs to be inverted (turned over). In order to perform this a web turn assembly or web flip assembly is required.

Uses:

Web turn assemblies can be found at any point in a processing line where turning the web at 90° is required. Examples of this are as follows:

In cases where the web line is too long to fit in a building a web turn can be used to allow for the line to be configured with a 90° turn in it, thereby shortening its length in a single direction.

In cases where additional webs are being introduced to or removed from the primary web path and there is not a convenient location to incorporate the unwind or winder, a 90° web turn can be used to allow for the unwind or winder to be positioned adjacent to the main web line.

Web flip assemblies can also be found at any point in a processing line where turning the web over is required. Examples of this are as follows:

In a processing line that features two coating heads, with certain processes requiring that both coatings be applied to the same side of the web and in other processes one coating is applied to each side of the web. In these cases a web flip can be used in one of the above instances and bypassed in the other so that both scenarios can be run without reconfiguring of the coaters.

In a process where the machine is built on two levels with the web running in opposite directions on each level and where the same side needs to be facing up on each level.

Design Requirements and Features of Web Turns and Web Flips:

There are significant similarities between the design of a web turn and a web flip.

In the web turn a single bar is positioned at a 45° angle to the entry and exit web paths, which are at 90° to each other (Photo 10). As the web enters the section it wraps the bar 180° and exits it.

PHOTO 10 – WEB TURN ASSEMBLY

A web flip is in essence two (2) web turns where the two (2) bars are positioned at 90° to each other, forming an “X” (Photo 11). The web enters the assembly at a 45° angle to the first bar that it contacts, wrapping this bar 180° and exiting it at 90° to the initial web path. It then contacts one or more idler rolls which turns the web 180°, sending it toward the second bar, which is at a 45° angle to the web. The web wraps this bar 180° and exits it parallel to the initial incoming web path, traveling in the same direction except with the side that was facing down when entering the assembly now facing up (Figure 4).

A major design consideration in the turn bar in either the web turn or the web flip is the speed of the web with respect to the surface of the roll. As the web wraps the bar from entry to exit point its angle with respect to the center axis of the roll changes from 45° to 90° and back to 45°. As this change in angle occurs so to does the velocity of the web with respect to the surface of the roll in the rolling direction. Therefore, there is no speed at which the turn bar can rotate that will satisfy the web’s speed with respect to it. As a result there are three different approaches that can be employed to address this. The first is to make the turn bar a non-rotating shaft with the web dragging over its surface. This results in an increase in tension as well as the possibility of scratching the web’s surface. The second approach is to design the turn bar as a rotating roll. In this case though the roll surface will turn at some speed with respect to the web, there will still be points at which the web will drag over it again resulting in an increase in tension and possible scratching of the web’s surface. The third approach is to provide the turn bar with a hole pattern drilled in its surface through which air is passed. This air forms a boundary layer between the web and the bar. As the pressure of the air is increased, the boundary layer goes from one which “greases” the web to bar interface, allowing for the web to more freely slip around the bar with minimal contact and friction, to one where the web is “floated” above the surface of the bar with no contact

between the surfaces. The float height of the web over the bar can be calculated using the air pressure and the web tension.

PHOTO 11 – WEB FLIP FIGURE 4 – WEB FLIP

A drawback of using a web turn or web flip is the ability to control the position of the web in the cross machine direction. This is particularly true for systems where the web is floated about the turn bar. In these cases it is common to incorporate an automatic guide system on the web as it exits the turn bar or web flip.

CONCLUSSION:

As I hope the above demonstrated, there are several equipment sections in addition to the unwinding, winding, coating, drying, laminating and embossing equipment that need to be considered in the specifying, designing, building and operating of a web processing line. There are many items that need to be considered when integrating these sections into the processing line.

BIOGRAPHY:

Bob Pasquale is one of the founders and principals of New Era Converting Machinery, where he serves as President. He holds a degree in Mechanical Engineering from Stevens Institute of Technology and has worked in the web converting industry since 1985. He is the holder of several patents in the industry. Bob can be reached at bob.pasquale@neweraconverting.com.

REFERENCES:

Satas, Donatas, Web Processing and Converting Technology and Equipment, New York, New York: Van Nostrand Reinhold Company, 1984