Sheet-Fed Offset Lithography Essay, Research Paper
Sheet-Fed Offset Lithography
Without printing, where would the world be? Edward George Bulwer-Lytton wrote, The pen is mightier than the sword. But he assumed that his penmanship could be widely distributed and studied (Adams 7). Printing creates a means for information to be recorded and passed down from generation to generation. Prior to the invention of printing, all information had to be conveyed verbally and survived only as long as people could remember. Printing created the power to document information and pass it onto younger generations. Printing is the ability to reproduce graphic symbols and messages in large quantities. With thanks to the invention of printing, we are able to have our ideas mass-produced and keep the world informed (Adams 3). Since printing is such an essential segment of the society that we live in, it is vital that the public has a general understanding of the history, concepts, and processes that involve printing.
The invention of printing and a practical moveable type is credited to Johann Gutenberg around 1454. This invention earned Gutenberg the title of father of printing (Adams 2). The process of printing that Gutenberg invented is relief printing, or printing from a raised surface. Even though this process of printing is no longer commonly used, it is still the foundation of present day offset lithography.
Lithography was invented by a German playwright who was invented with the printing process to try and lower the cost of printing his plays. This man was Alois Senefelder and the year was 1789. Senedelder experimented with many different ways of printing, but found the best and cheapest way was to etch the image he did not wish to print on a stone, leaving a raised image of the desired area. This raised area could then have ink applied and paper pressed to it and the image would be created. This is an adaptation of the relief printing process. Senedelder named his invention of printing from an etched stone lithography, based on Greek words lithos, meaning stone, and graphein, meaning to write. The lithography process of printing developed a reputation for quality and fine arts through the Currier and Ives Company that operated in America from 1835 to 1895 (Adams 289).
Printing must first start with a sample of what is to be reproduced. With today s technology, this sample, or copy, is almost exclusively created with a computer. While it can be made by hand, a computer is the easiest way to fashion a copy. Specialized software can help in creating a product to be printed. Desktop publishing software brings together many elements that are helpful at doing this. This software works well with type fonts and images and can help integrate them into a pleasant looking sample to be printed. If just text is desired, word processing software can be used to input and edit the transcript. Word processing software can easily check for spelling and grammar errors and can effortlessly change the appearance of the type. Once the desired copy is created, it must then be printed on a computer printer in black and white to be moved onto the next process (Dennis 57).
Once a preferred copy of what is to be printed is made, that copy must be photographed and made into a negative. This is done in the same way that a regular photograph is obtained, but only on a larger scale. The copy is place in front of a large camera and held on a copy board. The camera is then loaded with film and exposed to the copy. The film is then developed in a darkroom by a chemical process similar to that of developing camera prints. This developing must be done in a darkroom with special red lights to protect the light sensitive film from being exposed and ruined. The size of the developed negative is usually the same size as that of the copy that was photographed. The finished negative is named such because it is the exact opposite as the copy that was photographed. Where the text was on the copy, the negative is transparent, and where the copy paper was white, the negative is black and opaque. Once the negative is developed, it is ready to be made into a flat (Adams 219).
Stripping is the process of applying the negative onto a piece of paper called a masking sheet, or flat. To do this, a table with a glass top and a source of light below it is used. The masking sheet is squared up on the light table and the negative is positioned under the flat where it is to be printed. The negative can be easily seen as a result of the light from the light table shining through the top. Once the negative is in the desired position, holes are cut in the masking sheet where the image area is located. To keep the negative from moving, tape is applied to the cut edges of the flat and onto the negative. If the negative is not stripped straight onto the masking sheet, an uneven product will be printed. To cover up areas that are not desired for the printing, a red tape is applied to these areas. This red tape will prevent the covered image from being reproduced. Once the negative is correctly stripped onto the masking sheet, it is ready to make a plate (Dennis 278).
Lithography printing plates are made from a thin sheet of aluminum and coated with a light sensitive material that hardens when it comes in contact with bright light. The masking sheet and negative are aligned on top of the plate in a plate exposure unit. The plate exposure unit has a glass top under which the flat and plate are aligned. Once the plat and the masking sheet and in the correct position, a vacuum pump is used to hold the glass to the unit and keep the plate and flat from moving. Again, special care must be taken to line up the plate and masking sheet so as to have a copy that is straight when printed. The plate exposure unit then exposes the flat to a bright light that is often a pulsed xenon light. This light exposes the plate only where the negative was transparent. To protect the rest of the plate from being exposed, the flat is made from a yellow paper that does not allow the xenon light to pass through. The light that passed through the negative and onto the plate will harden the light sensitive material on the plate and create the image. (Adams 293).
To remove the unwanted material from the plate, it must also be chemically developed. A developing solution is applied to the plate to soften the areas that were not exposed by the light. A cleaning pad is then used to remove the undesired material from the plate leaving the image area that was hardened by the bright light from the plate maker. This image should be the same as the copy that was produced from the computer and photographed. After the plate is developed, it is then washed and a protective coating is applied to keep the aluminum from oxidizing and ruining the plate. If the plate is acceptable enough to print, the printing press is the next step (Dennis 310).
Offset printing uses the principle that the material that is being printing never comes in contact with etched image area on the plate. The plate is mounted on a round cylinder called the plate cylinder. Once the plate is on a printing press, ink and water are applied to the plate. The water is repelled by the etched surface and the ink is repelled by the water on the plate. This phenomenon that an oil based ink will not mix with water is a major fundamental of the offset process. The inked printing plate is then pressed against a rubber blanket cylinder and the image is transferred and appears as a reverse, or wrong reading, of the image. Once the ink is applied to the blanket cylinder from the plate, the rubber blanket then presses a sheet of paper against another cylinder called an impression cylinder. The pressure that the blanket cylinder forces the paper to the impression cylinder transfers the ink to the paper. The image that appears on the paper is the same as the plate and as the copy that was created with the computer (Adams 316).
There are several systems that work independently from each other in a printing press. The first is the printing system that was described above using the plate cylinder, the blanket cylinder, and the impression cylinder to create the desired image on the paper. The inking system is the unit that applies the ink to the printing plate. The goal of the inking unit is to apply this ink uniformly over the surface of the plate so that one spot does not have more ink than another. To do this, a series of rollers is used to smooth the ink out and make a consistent thickness of ink on the rollers. The ink is stored in reservoir in an ink fountain and is distributed to the rest of the inking system by the doctor roller. The distribution rollers smooth the ink out and make it even across all rollers. They do this by oscillating, or moving from side to side, as well as rolling between other rollers. The form rollers receive a thin layer of ink from the distribution rollers and apply that ink to the printing plate in an even coat.
In order for the ink to only be applied to correct area, a thin layer of water must be applied to the plate first. This is done by the dampening system. The water solution, or fountain solution as it is commonly called, that is used in the dampening system consists water and gum arabic. The water fountain stores the excess fountain solution that is not being used. The water fountain roller turns in the water fountain and distributes the dampening solution evenly across the entire dampening system. The ductor roller picks up the water solution from the fountain roller and dispenses it to the distribution rollers. The distribution rollers then supply the plate with the necessary amount of water. The size of the press often influences how many rollers are in the dampening system, but the process is the same, regardless of size (Dennis 346).
The feeding system is designed to supply the printing press with the paper that is to be printed on. The feeding system uses an air and vacuum system to feed the sheet. The air is used to blow the sheets of the paper apart and the vacuum system is used to pick up the front of the sheet. The paper is picked and transferred to the register table. The register table is a precise element of the feeding system that makes sure that every sheet of paper that enters the press is in the same location before entering the printing system. To do this, printing presses have a set of grippers on the register table to grasp and position each sheet of paper to the same location. Another set of grippers grasps the front edge of the paper and move it to the impression cylinder (Dennis 345).
The delivery system of the printing press is where the sheets of paper emerge after they have been printed. The deliver system must be carefully set up so that the sheets are placed onto an orderly pile. There are many different delivery systems that all perform the same tasks of handling the sheet of paper after it has been printed. Some smaller printing machines have delivery unit as simple as a tray to catch the paper as it emerges from the press. Other larger presses have a stacker unit that contains many complicated parts to make sure that each sheet is precisely aligned with the stack below it. Many delivery units on larger presses and even on some smaller machines have a chain delivery system that uses a gripper that holds the paper as it comes off of the impression unit and pulls it along a chain. Once the paper is in the right location, the press mechanically drops the paper onto the stack of printed matter (Dennis 348).
There are many adjustments that must be made on a lithography printing press before any printing can be done. In the inking system, the amount of pressure that the form rollers press on the plate must be set by a certified mechanic. The operator must select how much ink is to be put into the press from the ink fountain by setting screws that operate teeth to allow ink to reach the rollers. In the dampening system, the form rollers must again be set to the correct amount of pressure pressing on the plate. The operator can also control how much water is being put into the dampening system by setting how long the ductor roller is in contact with the fountain roller. The printing system has many intricate controls that need to be set by a press mechanic. But the operator also has many controls concerning the printing system. The position of the plate can be moved slightly and angled if the need arises to help straighten the printed image. The pressure that the blanket cylinder presses against the impression cylinder and the paper also must be set. The amount of pressure depends on the thickness of the paper that is being printed. Where the blanket cylinder contacts the paper also can be changed. This setting is referred to as registration. If the image is to high on the printed page, the blanket cylinder needs to moved so that it does not contact the paper as early and therefore results in a lower image. Registration is also controlled by the feeding system. The registration board controls the lateral, or side-to-side movement of the paper and has to be adjusted to conform to different sizes of paper. The feeding system can also change the lateral movement of the image by moving the entire stack of paper. The delivery system must be synchronize with the rhythm of the press to receive the paper from the impression cylinder at the correct time and distribute it to the printed pile accurately. All of the systems of the lithographic printing press must be perfectly set up and harmonized for printing to be accomplished successively (Adams 353).
After the setup is complete and the press is operating, the operator must carefully check and recheck all systems to ensure that a quality product is being produced. The operator must inspect the feeding system to make certain that the paper is feeding correctly and straight. The operator must examine the delivery unit to verify that the paper is stacking properly. The inking system must be checked for the appropriate amount of ink is entering the press and is being applied evenly and uniformly. The operator must also make sure that there is a sufficient amount of ink in the ink fountain. The dampening system must be examined to see if the right amount of water is being applied to the plate. The major amount of inspection must be done to the printed sheet. The operator will randomly pull a finished copy from the delivery end and measure the image for straightness and registration. From this sample, the operator can also look for even ink distribution, excessive water application, and feeding problems. As a result of the complexity of the printing press operation, a good lithographic press operator has years of experience and a good eye for mistakes and blemishes. A good press operator also knows the machine that he or she is working with and can troubleshoot problems easily (Dennis 351).
A unique ability on larger offset lithography presses is the capability to print more than one color on the same sheet of paper while only traveling through the press once. These presses are called multicolor presses. They are often referred to by the number of colors that are possible to print on one sheet, such as a six color press that can print six colors. Multicolor units are more common on larger offset presses, but they also available on some small offset presses. Multicolor presses save considerable amounts of money over single-color presses. Multicolor printing on single-color presses requires the paper to be run through the press, as many times as there are colors to print. After each press run, the entire inking and dampening system is thoroughly cleaned and the next color is inked up. Another money waster in multicolor printing on single-color presses is the waste sheets that are created. For each printing run, setup sheets must be printed to ensure that the ink and water systems are correctly adjusted. On a multicolor press those setup sheets are less because all the colors are being printed on the same run (DeJidas 5).
On multicolor presses, each color unit has its own inking system and dampening system, as well as its own plate, blanket, and impression cylinders. Multicolor presses have a common feeder and deliver system at the beginning and end of the press. The most common multicolor presses have the different color units arranged in tandem with a transfer cylinder between the units to transfer the paper to the next printing unit. On some smaller multicolor offset presses, a common blanket and impression cylinder are used. This commonality of cylinders lowers the possibility of registration error when transferring the paper from one impression cylinder to another (DeJidas 7).
Another possible option on larger lithographic offset presses is a perfecting press. Most sheetfed presses can only print one side of the paper in a single press pass. If a two sided printing job is desired, the entire paper stack would have to be turned over and run through the press a second time. This flipping and rerunning of paper creates a greater possibly of error and registration and mistakes. A perfecting press is a press that can print on both sides of a sheet of paper in a single press run. A convertible perfecting press has special reversing cylinders that tumble the paper between printing units so that the opposite side can be printed. Another type of perfecting press is called a blanket-to-blanket press. This press uses two blankets that are in contact with each other to accomplish the printing. In a blanket-to-blanket press, no impression cylinder is need because one blanket is the impression cylinder for the other (DeJidas 9).
In the United States today, lithography contributes to over sixty percent of the commercially printed products. As a result of the diversity of the lithographic offset machines and the wide variety of uses of the lithographic process, offset printing will continue to grow. Contributing to this growth is the demand for higher quality printing and more need for printed products. And as this demand increases, sheetfed offset lithographic printing will continue to lead the pack into the future, and remain there for years to come.
Adams, J. Michael, David D. Faux, and Lloyd J. Rieber. Printing Technology. New York: Delmar Publishers, 1996.
DeJidas, Lloyd P., and Thomas M. Destree. Sheetfed Offset Press Operation. Pittsburgh: Graphic Arts Technical Foundation Press, 1998.
Dennis, Dr. Ervin A., Dr. Olusegun Odesina, and Dr. Daniel G. Wilson. Lithographic Technology in Transition. New York: Delmar Publishers, 1997.