EVOLUTION OF OFFSET LITHOGRAPHY
The offset press as we know it today was invented around 1905 by Ira Rubel, a papermill owner in Nutley, New Jersey, who also used the contemporary lithographic
stone presses to print bank deposit slips. At that time, stone-based lithographic presses used a natural rubber blanket wrapped around the impression cylinder (the cylinder providing a hard backing to the paper, allowing a clear, firm impression by the plate), and the stone plate transferred the image directly to the paper. Occasionally, the feeder (which in those days was a person manually inserting paper into the press) would miss a sheet, and the plate would accidentally transfer the image to the blanket. When this happened, the next sheet through the press would print on both sides, a right-reading image on the top of the sheet transferred by the plate, and a wrong-reading image on the bottom of the sheet, transferred by the still-wet image on the blanket. Rubel noticed, as many others had, that the image transferred by the blanket was clearer and sharper than that transferred by the stone plate, and decided to build a printing press using the rubber blanket as the means of transferring the image to the paper. He joined forces with a Chicago-based printer, A.B. Sherwood, and together they created the Sherbel Syndicate. Hoping to monopolize the new printing process, they set strict rules concerning how many offset presses could be built; they decided on only one per region of the country. Twelve offset presses were built initially, but internal strife within the Sherbel Syndicate, the desire on the part of printers for more machines than the syndicate was willing to allow, and Rubel's death in 1908 ruined any chance of a monopoly succeeding. It did, however, quickly produce a competitor, who would be more successful than Sherbel: the Harris Automatic Press Company. In 1906, a Harris salesman tried to sell a lithographic stone press to the Goes Lithograph Company, a Chicago print shop. Charles Goes, head of the company, wasn't interested, but had recently been irked by Sherwood and the Sherbel Syndicate's rules, and told the Harris salesman that if they could build a press like the one Sherbel was manufacturing, he would buy one. This was reported to Charles Harris, an engineer at the Harris Company, and an offset press was designed and built. By the end of 1906, Harris shipped four offset presses; six more were shipped in 1907. Much later, in 1927, Harris bought the company that had been manufacturing Rubel's presses. Offset lithography was heralded as a triumph by many, though there were some pockets of resistance. By 1912, there were 560 offset presses in operation in the United States, and by the 1930s it was the dominant form of lithography—stone lithography had ceased to exist. Further refinements in the component parts of the press—from plates, which began to be composed of metal, to the rollers, to the blanket itself, which was made less troublesome after the invention of synthetic rubber in the 1930s—eventually made it the rapid, efficient, and overhwelmingly popular printing process that it is today. There are several basic parts or systems of an offset printing press:
INKING SYSTEM
As one would expect, 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. (See Ductor Roller.) Controlling the ink film thickness, or the amount of ink that is ultimately deposited on the plate, is a function of the form rollers, which actually contact the plate itself. A printing ink thickness of 0.2:0.4 mils (0.0002:0.0004 inch) generally produces the best results, depending on other aspects such as ink color. Too great an ink thickness transferred to the plate results in such problems as ink setoff, graininess, and dot gain, as well as ink drying problems, increased emulsification of the ink, and inaccurate color reproduction. Too thin an ink film can result in such problems as picking, hickeys, decreased color strength, decreased ink gloss, and poor ink coverage of the paper. Desired inking levels can be determined utilizing the concept of print contrast, a formula based on the printing density of solids and tints. Some printing presses have auxiliary inking system devices attached. An ink agitator helps prevent backing away from fountain, a problem experienced with certain inks that tend to thicken up readily, and need to be stirred frequently to properly transfer to the fountain roller. The agitator is a motorized cone-shaped device that moves back and forth across the ink fountain, automatically keeping the ink stirred. A fountain splitter is used occasionally in spot-color printing to separate two different colors of ink in the same fountain, enabling the two different colors to be printed at the same time, provided that the colors appear far enough apart on the substrate. Printing problems called hickeys (caused by particles of debris from dried ink skins, paper lint and dust, or other particulate matter adhering to the printing plate or blanket) can be reduced with a hickey-picking roller, a roller covered with synthetic fibers which contacts the plate and either picks off the offending particle, or surrounds it with ink, eliminating the white halo characterizing a doughnut hickey. Some presses also benefit from the installation of an ink leveler, a device that uses ultrasonic vibrations to determine the level of the ink in the fountain, and automatically triggers a pneumatic pump to add ink to the fountain when the level gets low enough. An Air Curtin is used on some presses to remove excess moisture from the inking system by directing concentrated streams of air at a particular roller, which evaporates the water being transferred through the system.
DAMPENING SYSTEM
Lithographic printing is based on the principle that oil and water do not mix readily (although a very slight amount of mixing is what makes the system work). Therefore, the dampening system of an offset press keeps non-image areas of the plate moistened so that they will repel ink. The non-image areas of a plate have been desensitized during platemaking to facilitate water receptivity. An offset press dampening solution, commonly referred to as a fountain solution, can consist entirely of water. 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 plate, 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, and an antifoaming agent to prevent the production of foam in the dampening system. Fountain solution is usually premixed by the supplier, requiring only the addition of water and alcohol by the printer. 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 consists of 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 rollers, which then transfer the solution directly to the plate. The dampening form rollers (of which there are typically two) contact 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. 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. Continuous-flow systems help eliminate some of the common dampening system problems, and allow instantaneous response to adjustments in the system.
PRINTING UNIT
The basic printing unit of an offset press 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. Many presses also contain some number of transfer cylinders, which carry the paper either to additional printing units (in multi-color presses) or to the delivery tray. The last transfer cylinder that sends the printed sheets to the delivery tray is called the delivery cylinder. Some presses, in lieu of a transfer or delivery cylinder, utilize an air-transport drum that reduces the potential for ink smearing by carrying the paper on a cushion of air. 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. There, the blanket transfers the image to the substrate. The substrate then is transferred to the delivery cylinder (if the press is a single-color press) or to a series of transfer cylinders which send it to other printing units (if it is a multi-color press). 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.
INFEED AND SHEET CONTROL
The sheet control and feeding mechanisms on an offset press are concerned with proper transfer of paper into the press, between printing units (if a multi-color press), and, finally, out of the press into a delivery tray. Consequently, the sheet control system as a whole comprises four basic sub-parts. 'Feeder Section'. At an offset press's feeder section, sheets are removed from the top of a paper pile. The feeder section consists of an adjustable pile table on which the stack of paper is placed, a sheet-separation unit (jets of air and/or a vacuum to ensure that only one sheet is sent through the press at any one time), and a feedboard (a platform or ramp on which the paper is sent to the devices that properly position the paper for printing). The most widely-used type of sheet feeder is a stream feeder, in which more than one paper sheet, traveling slower than the speed of the press, is on the feedboard at any one time, in contrast to a single-sheet feeder in which only one sheet, traveling at the speed of the press, is on the feedboard at one time. (Both stream and single-sheet feeders share the various elements of the feeder section.) The paper stock is stacked on the pile table, precisely centered to the feeder. Even slight variations on stock position can cause feeding problems, necessitating press stoppage. Some pile tables can be continuous-feeding, meaning that the press doesn't need to be stopped to reload. Configurations vary. A variation of the continuous-feed system is called a roll sheeter, in which a roll of paper is cut into sheets just prior to being sent into the press. The sheet-separation unit uses blasts of air to separate a batch of the topmost sheets, while suction feet lift the top sheet and forward it to the feedboard. The strength of the air blasts and the vacuum can be adjusted to the thickness of the stock. Brushes and fingers can also be added to the sheet-separation unit, which enhances paper control. Before transfer to the feedboard, a series of forwarding rollers and sheet detectors ensure that only the proper number of sheets are sent through at any one time. The configuration and sensitivity of the feeder rollers and the sheet detectors can be adjusted. The feedboard consists of a series of rollers, brushes, balls, and feed tapes that carry the sheet to a set of stops, ensuring that any lateral movement of the paper is compensated for. Various feedboard configurations can also ensure that foreign objects are not sent into the press, and can detect early or late sheets, preventing paper jams or misregister. 'Infeed Section'. Once a sheet is aligned properly at the head of the feedboard, it needs to be transferred by the infeed section to grippers on the impression cylinder. There are three basic configurations that accomplish this end: a swing-arm system in which a swinging gripper arm grabs the sheet of paper from the feedboard and carries it to the grippers on the impression cylinder, a rotary-drum system in which a rotating drum armed with grippers picks up the sheet from the feedboard with each rotation and transfers it to grippers on the impression cylinder, and an overfeed system in which the paper is forced by vacuum belts, or rollers, against a stop located on the impression cylinder itself. The paper buckles against this stop until impression cylinder grippers grab it. It is the careful calibration of this buckling effect that ensures proper paper alignment. The configuration of grippers and stops can be adjusted or varied, depending on the stock. 'Transfer Section'. As we saw above, sheets are moved between printing units by a transfer cylinder. (In some presses, however, the paper is transferred between printing units by transfer cylinder-powered chains. The exact configuration of the transfer system varies by press, and can involve one or three transfer cylinders of varying diameters. Each cylinder is equipped with grippers that pick up the paper from the previous cylinder (either transfer or impression) and transfer it to the next (either transfer or impression). On a perfecting press, specially-adjusted transfer cylinders flip the sheet over so that additional printing units can print on the reverse side of the paper. 'Delivery Section'. Once the printed sheet leaves the final impression cylinder, grippers on the delivery cylinder pick up the sheet and deposit it either on a moveable delivery chain, which carries the sheet to the delivery pile, or directly onto the delivery pile itself. On the delivery pile, moveable guides jog the sheets into a neat pile. As was mentioned above, unlike most other printing processes, offset lithography uses not only a metal plate, but also a rubber blanket to transfer the image to the substrate. Both of these elements of the press have certain considerations.
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 sheet 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. One problem that blankets can be subject to is 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. Each printing process also has certain ink and paper requirements which, although the exact nature is typically governed by the print job itself, achieve the best results with a minimim of printability and runnability problems.
PAPER AND INK
Offset lithography uses tackier inks than other methods, so the paper used must have high surface and internal-bonding strength. It must also have high water resistance to maintain its surface strength in the presence of water from the dampening system. The paper surface must be free of dust, lint, or other debris to keep foreign material from piling on the blanket or upsetting the press's chemical balance. If successive printings on each sheet are required, the paper must have high dimensional stability, and its equilibrium moisture content must approximate the relative humidity of the pressroom, to prevent register misalignment. Long-grain paper is also required for multicolor jobs. Various types of non-paper surfaces, such as metals, foils, plastics, etc., can also be utilized. The suitability of the offset lithographic process for printing on a wide variety of surfaces has resulted in a large number of inks available for the process. Typically, lithographic inks are more viscous and tacky than other types of inks, and since the ink film is thinner with offset printing, the pigment content must be higher. (Offset presses deposit ink films that are about half the thickness of films deposited by letterpress presses.) Inks designed for the process must contain significant amounts of water-repellent materials. Sheetfed offset presses primarily use quick-set inks, which dry rapidly without the need for additional equipment, such as drying ovens necessary for heatset inks. Some sheetfed offset presses, however, do use various radiation-curing devices, as is needed for super quick-set infrared ink, ultraviolet curing ink, and electron beam curing ink. Lithographic inks primarily set by a combination of absorption of oil-based vehicle components into the substrate, followed by oxidation and polymerization of the remaining components of the vehicle. The most important criterion for offset inks, however, is their insolubility, as they must resist bleeding in the presence of the water-based press dampening systems. Problems with the drying of offset inks that dry by oxidation include emulsification of the fountain solution into the ink. An excessive amount of dampening solution (or one with a high pH) can impede proper ink drying, and the use of papers that have a low pH also has a deleterious effect on ink-drying properties. ( Lithographic inks used for printing on metals (such as cans and other metallic packaging) contain synthetic resin varnishes that dry in high-temperature ovens.
PRESS CONFIGURATIONS
There are many different types of sheetfed offset presses, which tend to fall into one of three categories. A single-color press has one set of printing unit cylinders to print one color at a time on only one side of the sheet. Additional colors, or the second side, can be printed following reinking and the mounting of separate plates. One particular consideration of multi-color printing on a single-color press is dry trapping, or the ability of the wet ink film to lay on top of the dried ink from previous passes. A multi-color press contains more than one printing unit lined up in series, through which the press sheet passes before being sent to the delivery pile. A consideration of multi-color presses is wet trapping, or the ability of the wet ink film to lay on top of still-wet ink films deposited by previous printing units. Press sheets are transferred between printing units by the transfer cylinders mentioned above, their numbers varying according to press configuration, but which needs to comprise an odd number of transfer cylinders between units to keep the printed side of the sheet from facing the impression cylinder. A perfecting press is a press that prints at least one color on both sides of the sheet simultaneously. A convertible perfector has more than one printing unit, and uses special reversing cylinders to turn the sheet over before it contacts the second printing unit. By adjusting the transfer cylinders, this type of perfecting press functions either as a single-side multi-color press or as a two-sided perfector. A variety of perfecting press is a blanket-to-blanket press, in which the printing unit replaces the impression cylinder with a second blanket cylinder, printing both sides simultaneously and in a single printing unit. Blanket-to-blanket presses are found most commonly in web offset printing. A fourth type of press which is gradually falling into disuse is a proof press, a small, stripped-down offset press used to produce a proof from a particular plate, negative or positive. A proof press varies from a typical production press in that it lacks automated feeding, inking, and dampening systems, and most configurations feature the plate and paper mounted on flat beds, a rolling blanket cylinder being used to first pick up the inked image from the plate and then transfer it to a single sheet at a time. Proofing is now done in a variety of other ways, such as on a production press (or, often, the press that will actually be used for the job), or using special photosensitive papers, colored films, etc. Such means are known as off-press proofing, and can involve single sheets or colored overlays taped together in register. A fast-growing form of proofing is direct digital color proofing, using computer information to digitally image proof paper or film. ( A fifth type of offset press is known as a duplicator, which is a small press (typically one that prints paper sizes less than 11 x 17) without cylinder bearers used for small, quick jobs. Their simplicity and low cost had made them ideal for small copy centers. Newer machines retain the simplicity of the duplicator, but add some of the advanced automated features of full-size offset presses. A new breed of intelligent copier/printer bridges the gap between the standard offset press and the photocopier, providing the advantages of photocopying—quick turnaround time and practically no makeready—as well as acceptable quality for many applications. Many printers are supplementing their larger offset presses with IC/Ps, which enable them to use the latter for quick, single-color jobs while reserving the former for larger, multi-color jobs. Newer generations of IC/P have higher print resolutions and can print directly from computer data. Add-on finishing and binding devices enhance the speed and efficiency of the process. New technologies are also enhancing color printing, especially technologies that allow direct computer imaging (often via PostScript files) of printing plates. Digital printing, in a variety of forms, is becoming more and more common as technology allows higher quality and faster speeds than could be previously obtained.
No comments:
Post a Comment