Web Offset Lithography

Unlike sheetfed offset lithography, web offset lithography prints at high speeds on large rolls—or webs—of paper, often on both sides of the paper simultanously, using in-line printing units and in-line finishing systems. Web presses are bulky and expensive, but are widely used for many large-volume applications, most notably newspapers. The basic principle of web offset lithography is the same as that for the sheetfed variety. This article will concern itself primarily with the specific differences of web offset printing versus sheetfed printing. For a general discussion of offset lithography, see Lithography and Offset Lithography.

EVOLUTION OF WEB PRINTING

In the late 1700s, Alois Senefelder had invented the concept of lithography, and lithographic stone printing began in earnest, not long afterward. Advancements such as steam-powered presses and later the rotary press enhanced the process. In the 1850s, the newspaper industry was booming. A need arose for high-speed printing, and in 1856 the first perfecting press was invented, which allowed for the simultaneous printing on both sides of the paper. A second distinguishing feature of this press was that it printed on a continuous roll of paper. And so was born web printing. Subsequent finishing devices—such as folders—increased the capability of the process while remaining a limitation to the speeds achieved. Still lithography languished as primarily an artistic medium rather than a commercially viable means of printing. (Most presses were still letterpress presses.) In the early twentieth century, the accidental discovery that a rubber blanket transferred images to paper more efficiently and with greater quality than lithographic stones (the "offset" in offset lithography) gave the printing process the impetus it needed for wide commercial acceptance.

Technical problems hampered the development of offset lithography until the 1940s, when lithographic platemaking (replacing litho stones with metal plates) was finally perfected. Taking advantage of heatset inks and drying systems originally developed for letterpress presses, web offset lithography by the 1960s was expanding rapidly. The improvement of inks, blankets, plates, and other aspects of the process boosted growth tremendously. In the 1960s, many newspapers began switching from letterpress to lithographic presses, and more and more books began being printed on webfed lithographic presses. The quality and popularity of web offset lithography has continued unabated, and today web offset presses are used for everything from newspapers to periodicals to books to business forms. And offset lithography in general is the most widely used printing process in the world today.

INKING SYSTEM

The inking system primarily functions to transfer ink from the ink fountain to the printing plate, to add enough force to break the thick, viscous ink down into a thinner, more workable ink film (see Thixotropy), and to prevent the existence of faint reproductions of the printing image on the ink rollers (see Mechanical Ghosting). Offset inking systems can comprise as many as ten (or perhaps more) different rollers, which include ductor rollers, oscillators, distributors or vibrators (collectively called intermediate rollers), and form rollers. The entire series of rollers is called the roller train. The amount of ink that is sent to the rest of the press can be controlled either by adjusting the ink fountain's fountain keys, which vary the amount of ink transferred to the fountain roller—the roller that contacts the first roller in the train—or by adjusting the dwell—the amount of time that the first ductor roller contacts the fountain roller. Alternately, some newspaper presses utilize an eccentric roller or cam located beneath the fountain blade. This device adjusts the gap between the fountain blade and the fountain roller, and can be opened or closed by a lever.

Another form of ink-film control device is borrowed from flexography: the anilox roller, a metal roller covered with very small pits or depressions which transfer controlled, metered amounts of ink to the remainder of the roller train. An attached doctor blade is used to scrape excess ink from the surface of the anilox roller. The anilox system is especially useful for newspaper printing, which requires highly fluid inks. The amount of ink being sent toward the plate can be more easily metered using the anilox set-up. It also eliminates the need for fountain keys, or other such ink-flow control mechanisms.

The ductor roller alternately contacts the fountain roller and the first roller in the train. Controlling the speed at which the ductor moves back and forth, coupled with the ratios of the sizes and rotations of the other rollers, helps unify the ink flow to the plate.

The ductor roller is an intermittent inking roller. Some press configurations utilize continuous-feed systems, employing metal rollers, or rollers with relief patterns on them, allowing only a predetermined surface area of a roller to transfer ink. However, these alternate systems are often inadequate for providing the lateral ink-flow control that may be desired and/or required. A solution has been devised in the concept of the Aller undulating ductor roller, a segmented ductor roller in which each segment is mounted slightly off-center in relation to the others. During inking, some segments are picking up ink from the fountain roller, while others are transferring ink to the adjacent oscillator.

Controlling the thickness of the printed ink film , or the amount of ink that is ultimately deposited on the plate, is a function of the form rollers, which are the ones which actually contact the printing plate itself. Web presses can have anywhere from one to four form rollers. (See Form Roller.)

Like sheetfed offset presses, web presses also use some number of intermediate rollers between the ductor roller and the form rollers. (See Inking System: Offset Lithography.)

DAMPENING SYSTEM

A web offset press's dampening solution, commonly referred to as a fountain solution, can, like that for sheetfed presses, consist entirely of water, but as this is generally ineffective for long print runs, synthetic desensitizers are added to the fountain solution to maximize the ink repellency of the non-image areas of the plate. The substances added to the fountain solution depend on the type of ink being used, but typically a fountain solution consists of water, an acid or base (depending on the desired pH of the solution), a gum (such as gum arabic to desensitize the non-image plate regions), a corrosion inhibitor to prevent the solution from reacting with the metallic parts of the press for eg., plate cylider, an alcohol-based wetting agent to reduce the fountain solution's surface tension, a drying stimulator which increases the effectiveness of the ink drier, a fungicide to prevent the growth of mildew, fungi, and bacteria in the delivery pipes and trays, and an antifoaming agent to prevent the production of foam in the dampening tray / pan. Fountain solution is usually premixed by the supplier, requiring only the addition of water and alcohol by the printer. (See Fountain Solution.)

A typical dampening system closely resembles an inking system, but since water doesn't need to be worked the way ink does, fewer rollers are used. The dampening system includes a water pan, which holds the fountain solution. A fountain pan roller rotates in the water pan and transfers the solution to the rest of the dampening system. A ductor roller alternately contacts the fountain pan roller and an oscillator roller. The oscillator roller, like that used in the inking system, moves laterally across the press to even out the distribution of solution, then transfers the solution to the form roller, which then transfers the solution directly to the plate. The dampening form roller (most presses use only one, but some use two) contacts the plate before the ink form rollers, desensitizing the non-image areas of the plate before the application of the ink. As with ink, the amount of water flowing to the plate must be carefully metered, which can be accomplished either by adjusting the rate at which the fountain pan roller turns, the length of time the ductor roller contacts the fountain pan roller, or by using water stops, a set of squeegees, tabs, or rollers that press against the fountain pan roller and squeeze excess water off it. The pressure of the water stops can be controlled, the highest pressure squeezing the greatest amount of water off the fountain pan roller and transferring the least amount of fountain solution through the press. As with inking rollers, the proper roller setting also needs to be made to transfer the proper amount of fountain solution to the plate.

One particular alternate dampening system device is known as a flap roller, or flapper, which is a fountain roller convered with canvas flaps that transfer the solution to the oscillator directly (eliminating the need for a ductor roller). Adjusting the rotation of the flapper regulates the amount of water flowing toward the plate. An advantage of this system is that thanks to the elimination of the ductor roller, ductor shock is also eliminated. A drawback, however, is that the water flow cannot be controlled laterally across the press. Another common configuration of intermittent-flow system uses a bristle-covered roller containing flicker blades in contact with the fountain roller. The pressure between the two rollers flexes the bristles as fountain solution is applied. As the bristles rotate they "flick" fountain solution at the adjacent oscillator (which is not in contact with the flicker roller).

The dampening system described above is known as a conventional dampening system or an intermittent-flow dampening system, as the movement of the ductor roller doesn't allow the continuous flow of fountain solution through the press. Continuous-flow dampening systems, which can transfer the fountain solution either through the inking system (called an inker-feed system), directly to the plate (called a plate-feed system), or both through the inking system and directly to the plate, do away with the ductor roller and provide, as their name indicates, a continuous flow of fountain solution. One configuation of continuous-flow system uses spray nozzles to direct jets of fountain solution onto the oscillator.

Continuous-flow systems help eliminate some of the common dampening system problems, and allow instantaneous response to adjustments in the system. (See Dampening System.)

PRINTING UNIT

The basic printing unit of an offset press—be it sheetfed or webfed—contains three (or sometimes more) cylinders: a plate cylinder, to which the plate is attached, a blanket cylinder, to which the offset blanket is attached, and an impression cylinder, which carries the paper through the printing unit and provides a hard backing against which the blanket can impress an image on the paper.

The arrangement of the cylinders can vary depending on the press, so long as the proper cylinders are adjacent to each other. The plate cylinder is typically the topmost cylinder, and is in contact with the inking and dampening form rollers. The plate contains a right-reading image that is inked by the inking system. As the plate cylinder rotates, it is brought into contact with the blanket cylinder beneath it. (The blanket cylinder can be backed away from the plate cylinder, to facilitate removal and adjustment of the plate.) The inked image areas of the plate transfer the image to the rubber blanket, the image now being wrong-reading. The paper (or other substrate) is carried by the feeder system to the impression cylinder, and is sent through the nip between the blanket and impression cylinders.

Unlike sheetfed presses, web presses more often than not are set up for perfecting of some kind or other. On a general perfecting press, two complete printing units (along with two complete sets of inking and damepening rollers) are placed in a line, one unit printing one side, followed by the second unit printing the second side. Other types use a blanket-to-blanket printing unit, in which the blanket cylinders of two printing units run in contact, allowing for simultaneous perfecting. Another variation to the printing unit is the common impression cylinder, a large diameter imporession cylinder that transports the substrate from one printing unit to another, often used in multi-color printing to lay down successive colors in one press pass. (See Press Configurations below.)

As with the various rollers of the inking and dampening systems, proper cylinder setting and maintenance are important to ensure the proper fidelity of the printed image. Various adjustments can also be made to the cylinders, depending on the print job. An important aspect of offset printing is proper packing, or height adjustment, of the plate and blanket cylinders, which has important consequences in terms of print quality. (See Packing.) (See also Plate Cylinder, Blanket Cylinder, Impression Cylinder, and Transfer Cylinder.)

PLATES

The process of lithographic platemaking ensures that the image areas of a plate are lipophilic (or oleophilic) and hydrophobic (in other words, receptive to oil—namely, ink—and repellent to water) while the non-image areas are lipophobic (or oleophobic) and hydrophilic (in other words, repellent to oil and receptive to water). These chemical principles ensure that the ink will only adhere to the image areas of the plate. On the press, the water-receptive coating on the non-image areas must be increased, a process called desensitization. A gum, such as gum arabic, is typically used to accomplish this. Gum additives in the press fountain solution ensure that the plate remains desensitized over the course of the pressrun. (See Plate: Offset Lithography.) Before mounting, plates are inspected for scratches and other defects, and occasionally minor corrections can be made to the plate. Unwanted specks or other small, undesirable image areas can be erased, then treated with etch and gum; voids in solid image areas can be filled in by scratching the plate, then applying ink to the area. Such plate doctoring should only be performed on the most minor of defects. At this point, the degree of packing should be determined. (See Packing.) Mounting is performed according to the press manufacturer's recommendations, and the image lay, or position of the printed image on the paper, determined. At this point, the plate may need to be repositioned, to keep the image centered, or in register with previously-printed images on the paper. Properly formulated and applied fountain solution will more often than not obviate the need for continual plate maintenance while on press. Extended press shutdowns, however, such as overnight, should not occur while the plate is still inked. Gum that dries on the image areas causes plate blinding.

Plates can be subject to two primary defects: scumming and blinding, which are ink receptivity in non-image areas and ink-repellency in image areas, respectively. (See Scumming and Plate Blinding.) Most plate problems are caused by improper formulation or application of fountain solution.

BLANKETS

An offset blanket is commonly made of a synthetic rubber, and the most important feature of the blanket is its release capability, or the ease with which it will let go of the web passing through the impression nip. Release is primarily a function of the smoothness of the surface of the blanket, though things like ink tack, impression nip squeeze, and the surface characteristics of the paper also play a part. Printers often have found hard blankets to provide the best release, although empirical research has not borne this out. (Problems relating to the accurate measurement of blanket hardness no doubt contribute to this lack of evidence.) A blanket's smash resistance (its ability to return to its normal thickness after being subjected to printing pressure) and its durability (its ability to withstand the force of printing pressures over time) are also of importance. Whether a blanket is a conventional or compressible blanket is also important. Prior to mounting, the packing requirements of the blanket should be determined. (See Packing.) One problem that blankets can be subject to is a blanket smash, or a small, depression in the blanket surface, typically caused by a foreign object passing through the impression nip. Blanket smashes can cause various printing defects, so it is typical to replace the blanket with a spare, or repair the blanket before continuing with it. Any blanket that has more grievous problems, such as rips or tears, should be replaced immediately. Blankets may also experience piling, the accumulation of paper fibers or coating particles, which can produce printing defects such as hickeys. (See Blanket.)

INFEED AND WEB CONTROL

Just prior to the first printing unit is all the infeed equipment. Unlike sheetfed presses, web presses need to be concerned with the proper tension of the paper web throughout the press, not only to ensure image quality, but also to avoid web breaks, which can bottleneck any printing operation.

'Infeed Unit'. The paper roll itself is mounted on the roll stand, commonly located in a straight line with the rest of the press. In some configuations, however, especially where space is an issue, the roll stand can be located to one side of the press and the web is turned at a right angle into the press. Some roll stands are located below the press on a separate level of the building housing the pressroom. The most basic roll stand configuration is a single-roll stand which, as its name indicates, holds only a single roll. (There are also multiple-roll stands—the double-roll stand being the mopst common—which hold more than one roll at a time, and allow for the feeding of multiple webs through the press at any one time.) A roll is held securely onto the roll stand by means of a shaft passing through the hollow core of the roll.

The speed at which the roll unwinds into the press is reguilated by a series of rollers located between the roll itself and tghe first printing unit. The most important of these rollers is the dancer roller (also called a floating roller). A dancer roller is free to move up or down, or backward or forward, during the press run. Essentially, the dancer creates a downward path for the paper as it comes off the roll and then sends it upward again. If the amount of paper unwinding from the roll just before the dancer roller is the same as the amount of paper feeding into the press just after the dancer roller, then the dancer is kept in a "normal" position. If these two amounts of paper differ, the loop formed by the paper path will increase or decrease in size, and the the dancer moves up or down, simultaneously releasing or applying a brake (which can be electromechanical, hydraulic, pneumatic, or magnetic) on the roll stand. This effectively keeps the web tension constant during infeed. A second dancer roller may also be added just before the first printing unit as a means of further controlling web tension.

If web tension during infeed were the only consideration, a roll stand and dancer roller would be the only infeed devices needed. However, the web must be flat and taut as it enters the printing unit. Consequently, additional "metering" rollers are located between the dancer and the printing unit. These rollers (two of which are driven by gears connected to the press motor) work to keep the web speed close to that of the press speed (tolerances often need to be within ±0.3% of press speed).

Web tension and drawing speed metering are important considerations, as variations in web tension throughout the press can cause registration problems, slurring, doubling, and other printing defects and problems as well as web breaks. Although the configuration of the infeed system accurately controls web speed and tension, it cannot compensate for variations in paper structure. Consequently, paper itself needs to be manufactured within very close tolerances.

'Splicing'. It is very rare that a print run will be completed just prior to the exhaustion of a paper roll. Changing rolls manually results in downtime and wastage that is always undesirable. Consequently, press infeed sections are equipped with continuous-roll feeding devices—splicers, also known as pasters—which automatically splice a fresh roll to an expiring one. There are two types of splicers.

A flying splicer splices an expiring roll to a fresh roll "on the fly," or while the press is running at full speed. A photoelectric cell detects when the diameter of a paper roll is as small as eight inches, and the splicing process is initiated. The pasting device is moved from a loading position to a splicing position, and the new roll is accelerated to press speed. When the roll reaches G inch, the actual splicing process begins. (Any paper left on the roll when the diameter is under G inch is considered core waste and is not used.) The leading edge of the new roll (or roll nose), containing an adhesive is pressed against the expiring roll under pressure, where the adhesive sets, joining the two rolls together. (Photoelectric eyes and other detectors monitor the position of the roll nose accurately). During the attaching of the roll nose to the expiring roll, it is important that pressure be applied along the entire length of the splice, otherwise a loose adhesive-coated edge may be left exposed, causing jams or other problems in the press. Beneath the pressure roller, a knife severs the expiring roll with enough of a tail to completely cover the adhesive. A two-arm splicer (which holds one roll in addition to the running roll) or a three-arm splicer (which holds two rolls in addition to the running roll) may be used. After splicing, the expired roll can be removed, and a fresh roll installed to await the next splicing sequence.

A zero-speed splicer joins two rolls while they are both stationary, but while the press is still running. As a paper roll starts expiring, a festoon—or a collapsible set of rollers—expands to store a reserve of paper from the expiring roll. This allows the expiring roll to be stopped, paper being drawn only from that within the festoon. When the expiring roll is stopped, the new roll is spliced on and brought up to press speed. The paper stored in the festoon is replenished, and it is expanded to its full height. The expired roll is removed, and a fresh roll is added to await the next splicing sequence.

In some cases, a splice needs to be cut to a certain shape. The shape is dictated by the configuation of the paster, and splice patterns are not interchangeable from paster to paster. Consequently, when using a particular paster, it is important to prepare a splice template to ensure that the required shape is accurate for each splice. And, needless to say, the new roll must be spliced on perfectly straight, otherwise feeding problems and web breaks will occur.

'Web Tension'. As was mentioned earlier, maintaining a consistent and proper web tension is important not only for reasons of image quality, but also for proper mechanical operation of the press. Web jams and breaks cause undesirable downtime and paper wastage.

Throughout the press, tension is controlled by rollers at certain points, the region between these points being known as tension spans. It is the goal of the tension points to produce even tension throughout the spans. This is a function of several different variables.

The first is web, or surface, speed, or the speed at which the web is travelling through the press. This should remain constant, but the rollers used to effect this affect it in different ways. The composition of the rollers affects the amount of slippage the web experiences (for example, rubber-covered rollers can unpredictably alter the speeds of webs passing through their nips), and the diameters of rollers and cylinders themselves (i.e., blanket packing, taping chill rolls, etc.) affect their speed of rotation and the speed they impart to the web. Surface speed is also affected by slippage in any of these rollers. The degree to which the web slips in the nips of rollers cannot always be predicted.

Another problem affecting tension is the fact that paper is not a dimensionally stable material. The elasticity of a paper causes it to stretch in one or both of its dimensions when under tension. When tension is released, the paper may return to its original dimensions, although there is often some degree of permanent stretch. What this means is that registration may be difficult if paper stretches and contracts from one printing unit to another. The moisture content of a paper also causes it to expand and/or contract, a particular concern in lithographic printing due to the use of a water-based fountain solution. In web printing, the use of high-temperature dryers also affects the dimensions of a paper.

When paper rolls are wound at the paper mill before shipping, a great deal of pressure is used to produce tightly-wound rolls. As a result, the paper tends to "relax" when unwound into a press. This causes it to expand somewhat. When this occurs in the infeed section it may be acceptable, but if it occurs between or within printing units, it can cause registration problems. Therefore, the paper is often given a long lead-in to the printing unit during infeed (called festooning, an additional use of the festoon also used for splicing). This allows the paper to relax and recover as much as it needs to before entering the printing units.

Tnesion tends to decrease through a press, caused by, among other things, the printing units. Moisture picked up from the dampening system causes elongation and affects tension unpredictably, and blanket pull (a function of the impressioin pressure generated during printing) also works to increase tension. Consequently, blanket pull needs to be monitiored and kept constant throughout the pressrun.

The longest tension span on a web press is between the last printing unit and the chill rolls, where the drying mechanism is. As a result of an unsupported span and the heat of the dryer, wrinkles can form on the web. Problems at the drying and cooling end of the press can be reduced by creating no-slip nips between the chill rolls.

DRYERS AND CHILL ROLLS

Although most newspaper printing uses quicket inks and inks that dry by absorption of the vehicle into the substrate, modern web offset printing is still dominated by heatset inks, inks that require the application of high temperatures to dry. As a result, web presses that utilize such inks need a dryer and chill rolls attached after the last printing section.

'Dryers'. Depending upon the inks used and the capacity of the dryer, the temperature to which the printed web is subjected may be as high as 600ºF. And even though a single portion of the web may spend less than a second in the dryer, it will still exit with a temperature of up to 330ºF. Since blanket-to-blanket presses print on both sides of the web simultaneously, such presses must utilize floating dryers, or dryers through which the web can pass without being touched by a surface, so as not to smear the ink. An open-flame dryer (also known as a direct-impingement dryer) is the oldest type of dryer, and simply generates a high-temperature zone through which the web passes. Alternately, a high-velocity hot-air dryer uses nozzles to direct hot air at the web. Such devices also include exhaust vents to pipe out the solvent fumes generated during the drying of solvent-based heatset inks. The fumes are then carried to a combustion chamber where the solvent is safely burned away. Some dryers use a combination of the open-flame and hot-air configurations, often employing medium-velocity jets of heated air. The aerodynamics of the moving web cause a thin layer of air to move along with it. In open-flame dryers, this layer of air becomes saturated with solvent from the ink, and more heat becomes necessary to remove it all, which reduces the efficiency of the drying process. Solvent-saturated air which is carried out of the dryer can condense back onto the web and/or the chill rolls. This will cause the ink to resoften, marking the chill rolls and resulting in marks being transferred to subsequent portions of the web. The high-velocity dryers, however, add turbulence to the air in the dryer, which prevents solvent from saturating the air surrounding the web. Another particular problem that can be encountered during drying, especially during the early uses of high-velocity hot-air dryers, is the production of ripples in the web which then get pressed into wrinkles by the chill rolls. A solution to this problem is the use of spaced air nozzles to produce a controlled rippling of the web. This controlled rippling has the effect of imparting to the web a greater dimensional stability than if it had been left to its own devices.

'Chill Rolls'. Heatset inks not only require heat to set, buit also cooling. (Such inks work by evaporating the volatile solvent from the ink in the dryer and then cooling down the residual vehicle where it can set by polymerization.) Thus, after drying, the web is passed between the chill rolls, a set of moving steel drums through which cold water is pumped. Early designs of chill rolls filled the entire interior of the rollers with cold water, but the cooler portions of the liquid remained near the middle of the roll, causing the ends of the rollers to gradually become heated. This resulted in inconsistent cooling across the web. Newer designs use baffle plates or, more often, a thin shell around the surface of the roller and an empty center to keep the surface of the roller equally cool over its entire length. The speed at which the chill rolls, as was indicated earlier, can be adjusted over a wide range, so as to control web tension through this longest tension span. One particular problem which can be encountered in the chill rolls is slippage of the web as it passes through the nip. Such slippage can cause ink from the web to mark the rolls, which can then be transferred back to the web. Attention needs to be paid to the plumbing of the chill rolls; mineral deposits from hard water or other sources can decrease the cooling efficiency of the rollers.

IN-LINE FINISHING EQUIPMENT

Although most complex and high-volume binding and finishing operations are best handled at a bindery, many web presses have in-line finishing devices, which eliminate the time and expense of rewinding a printed job and sending it out to a bindery. However, in-line equipment does have the disdvantage of shutting down the entire press when one particular stage is shut down.

'Folders'. Even if no other finishing operations are involved, most web presses are designed to deliver folded sheets, or signatures. They accomplish this by having one (or more) configurations of folding devices attached to the delivery end of the press. There are three primary types of f

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