The right roll for wrinkle-free web processing.

The Right Roll For Wrinkle-Free Web Processing

Today's thinner films and higher line speeds can cause more trouble with wrinkles. Know your choices of spreader rolls for solving this problem.

For any web substrate to achieve optimum printing, coating, laminating and metallizing properties, it must be free of wrinkles. In today's market, processors are running thinner, more delicate substrates at faster line speeds than ever before, significantly increasing the potential for creasing and wrinkling.

To achieve smooth, taut webs, a wide variety of wrinkle-removing rollers, known as spreader or stretcher rollers, is available. As their name implies, spreader rollers are designed to pull and stretch a substrate during production to remove costly wrinkles.

Wrinkles can develop in a web for a number of reasons, but one thing is certain--the longer the web has to go, and the more processes it must go through, the greater the opportunity for wrinkles. Creases may also develop if the web has large wrap angles, if web tension varies along the processing line, if web gauge is inconsistent, if the web is exposed to static electricity or improperly aligned rollers, and if the web is tacky due to semi-dry coatings--as in production of adhesive-backed products such as tape.

The optimum location of a spreader roller varies, depending on the cause of the wrinkles. On printing presses, coaters and laminators, stretcher rollers most often are placed in the first idler position off the unwind, the idler position before the first print unit or coating or adhesive applicator, and in the last idler-roller position prior to rewinding. Spreader rollers are also utilized on extrusion lines to smooth webs prior to corona treating, winding and slitting.

Today, there are five basic families of rollers: bowed rolls, expander stretcher rolls, grooved metal rollers, rubber idler rollers, and angle-grooved, soft-rubber idler rolls. Many variations exist within each family. While there is no universal spreader roller, some processors utilize a number of different types of rollers on a single line, because the degree of wrinkles and each roller's stretching action and limitations differ.

This article will explain the five different families of spreader rollers (including newer innovations), their construction, how they function, and the advantages and limitations of each.

BOWED ROLLS

One of the oldest types of spreader roller is the bowed roller (see Fig. 1). It consists of a flexible center shaft, a series of bearings placed along the shaft, a flexible metal inner covering, and a smooth-surfaced, one-piece elastomeric outer sleeve. The bowed roller removes wrinkles by creating an ever-increasing skew or angle on the roller rotation, providing a shift in web direction from the roll's center outwards toward the ends.

The major benefit of the bowed roller, users say, is that the roller "crown" or skew can be adjusted while the line is running to shift the orientation of the web as it passes over the roller.

However, the bowed roller design alters the natural flow of the web, creating uneven tension across the face of the roller, resulting in possible drag in the processing line. This can cause the web to stretch and distort, especially with thin-gauge films.

Another limitation is that the bowed roller needs careful maintenance. Due to the continual flexing or skewing of the roller surface, and the increased tension and abrasion exerted by the web on the center of the roller, the roller covering wears more frequently. Since these rollers are often power-driven, there are also more parts that require maintenance--not to mention the cost of power required for operation.

Also, bowed rollers cannot be installed on every processing line, because they cannot be positioned in tight spaces. Bowed rollers require a specific amount of space to provide optimum performance, which depends on the application and roll design.

EXPANDER ROLLS

The second type of spreader roller is the expander roll. It originally consisted of metal or wooden slats with internal elastic bands connected to angled end plates (Fig. 2). The connection of the bands to pitched end plates caused the bands to expand as the roller rotated, pulling apart the slats to provide a spreading action. There are several variations of this roller, including some with internal mechanical mechanisms.

The next generation of expander rollers eliminated the wooden or metal slats with more substantial elastic cords; however, they are still attached to pitched end plates (Fig. 3). With these expander rollers, the substrate enters at a point where the cords are relaxed and then exits at the point of maximum cord expansion to achieve web spreading. The amount of spreading action can be adjusted by changing the angle of the roller's end plates. The direct elastic-cord and substrate contact provides better coefficient of friction and is less abrasive than the metal or wooden slats.

However, while these rollers offer adjustable spreading action, they eventually lose their effectiveness because the rubber cords do not fully recover to their original state after continual stretching. Another limitation is that at high web speeds, air enters between the elastic bands and is trapped under the web. This causes the web to float over the roller surface, negating its function. The individual cords can also cause web marking.

A new roller that operates on the same principle, but features a stretchable one-piece rubber sleeve supported by a series of disk brushes, was introduced a couple of years ago. As this roll rotates, the entire roller sleeve--as opposed to individual cords--expands and contracts to provide spreading action. The amount of spreading is controlled by two factors: the wrap or angle at which the web enters onto the roller, and the angular displacement of the end caps.

Notable advancements in this expandable-sleeve roller include a smooth, continuous surface, which doesn't produce marking or allow air to enter under the web. However, the stretching of the rubber still causes the roller to wear over time.

GROOVED METAL ROLLS

Another spreader roller is the grooved metal idler roll, which is frequently used for tracking as well. A variation on this roller consists of PTFE tape wrapped on a smooth metal roll to create a grooved surface.

The grooved metal roller has opposing, etched spiral grooves, which start at the roll's center and spiral toward the ends (Fig. 4). As the roll turns, air flow is generated, which follows the direction of the grooves along the metal surface. Since the groove pattern proceeds from the center of the roller out, the air travels in the same path, forcing web wrinkles out towards the ends of the roll. The major advantages of this roll are that it is free-turning, and existing idler rolls can be easily modified for this application.

However, as web-processing line speeds have increased, this roller has been losing effectiveness. Because the roll has a smooth surface, it has a low coefficient of friction. Subsequently, as line speeds increase, so does the air flow, causing the web to slide over without making complete contact with the roll. If the roller does not make contact with the web, it doesn't rotate, so its ability to remove wrinkles is insignificant.

The surface of this roller also has limitations for certain applications. For films, metallized, printed and coated materials, the roller's hard surface can produce marks on the substrate as it passes over the grooves.

GROOVED RUBBER ROLLERS

The same basic principle of the grooved metal roll has been applied to hard rubber rollers to improve the performance of grooved rollers. With a hard rubber surface, the depth of the grooves can be increased for greater air flow and consequently increased spreading action. Rubber also has a higher coefficient of friction, so line speeds can be increased without the web completely floating over it.

However, these rollers are still limited by line speeds. Although the web maintains contact at higher speeds than the metal roller, if speeds get too high, the roller has the same restrictions as the metal idler rollers.

ANGLE-GROOVED SOFT RUBBER

Just over a decade ago, the basic technologies of the elastic expander and spiral-grooved metal and hard-rubber idler rollers were combined to create a more effective wrinkle-eliminating roller. The key to the operation of this type of roller is that wrinkles are removed by rubber deflection rather than air flow. The two unique features of this roller are a softer rubber covering and angling of the grooves in the roll. By angling the grooves into a soft rubber surface, the roller responds to the tension of the web passing over it, deflecting the rubber grooves outward (Fig. 5). This deflection or flexing provides a spreading or "cross-stretching" action to effectively remove web wrinkles. Unlike the grooved hard-rubber or metal rollers, the soft-rubber types suffer no linespeed limitation.

Also, because the roller responds to the tension of the web, these rollers are somewhat self-adjusting. This reduces the chance of web breaks and the need for roller adjustments. An advantage of soft, grooved rubber rollers is that the rubber durometer or hardness and type of rubber can be tailored to provide the desired level of cross-stretching, as well as other characteristics, such as easy release or static elimination.

However, as with metal idler rollers, these rollers are not manually adjustable once the rubber durometer has been selected. Therefore, the parameters of the processing operation must be carefully reviewed in advance so the roller can be custom-designed for the application. Because of its non-adjustable nature, this roller type is not normally recommended for applications involving web separation after slitting.

Several years ago, this roller was modified again into an advanced version. This soft, grooved rubber roller has a patented design featuring angled grooves that increase in depth from the center of the roll to each end for improved spreading action. This greater stretching is especially beneficial for thin films under 3 mils and metallized films, which require more delicate spreading.

Currently, single- or double-grooved soft-rubber rollers are available. The type and gauge of substrate being processed dictate which roller is more effective. As a general rule of thumb, single-grooved rollers are recommended for film 5 mils or more thick, and double-grooved designs for thinner films.

As with all grooved rollers, they are easy to install because they can replace any idler roller, so no machine modification is required. Their free-turning operation also provides low maintenance.

As line speeds continue to increase and new more expensive and difficult-to-handle substrates are developed, spreader rollers with different shapes and operating features will be introduced. However, the basic spreader principle will remain the same.

To determine which roller is most appropriate for a particular application or line position, discuss with the roller manufacturer all the parameters that may limit the use of one type of roller--such as line speed, location on the line, web tension, the substrates being processed, and the degree of wrap of the substrate. Since one roller manufacturer typically does not offer all types of spreader rollers, you will want to contact several suppliers to hear how their particular products can meet your wrinkle-removing needs.

PHOTO : Fig. 1--Bowed rollers have a curved, flexible center shaft and elastomeric outer sleeve,

PHOTO : which when angled, shifts the web direction from the roll's center outward toward the ends

PHOTO : to remove wrinkles.

PHOTO : Fig. 2--The crown/slat expander roll has adjustable slats, which expand and contract to

PHOTO : change the roller diameter at various points. This creates opposing pulling forces on the

PHOTO : web to remove wrinkles.

PHOTO : Fig. 3--Elastic expander rollers have rubber or elastic cords attached to angled end

PHOTO : plates. The substrate enters onto the roller at a point where the cords are relaxed and

PHOTO : then exits at the point of maximum cord expansion to achieve web spreading.

PHOTO : Fig. 4--The scroll or grooved metal idler roll (which also can be made of hard rubber)

PHOTO : consists of opposing, etched spiral grooves that create an air flow across the roll's

PHOTO : surface to force wrinkles out toward the ends.

PHOTO : Fig. 5--Angle-grooved soft-rubber roller responds to the tension of the web passing over

PHOTO : it, causing an alteration in the web path to eliminate wrinkles. Note how the surface

PHOTO : deflects after contact with the web (dotted lines), while the groove and land widths

PHOTO : remain the same, for uniform cross-stretching.