Offset process explained in detail

Description

Offset lithography is the most widely used print process. About 40% of all print jobs are produced with offset printing. It is an indirect printing process which means that an image is transferred, or offset, from one surface to another. A printing plate mounted on a cylinder transfers the image to a rubber blanket mounted on another cylinder. The image is then transferred from the blanket cylinder to the substrate as the substrate passes between the blanket cylinder and an impression cylinder. The image on the plate is "right reading" and when the image is transferred to the blanket it becomes "wrong reading". When the image is transferred to the printing surface it becomes right reading again.

The image area and non-image area of the offset plate are on the same plane and work on the principle that oil and water do not mix. The non-image areas of the plate attract a wetting agent (fountain solution) and repel ink made from an oil base. The image areas attract the ink and repel the fountain solution.

Applications

The types of printed materials that can be produced with offset lithography are numerous and varied. Some of the items include: newspapers, magazines, books, continuous business forms, unit sets, advertising pieces, brochures, posters, greeting cards, business cards, folders, mailers, laser sheets, integrated products, coupons, and art reproductions.

Press Types

Offset presses can be put into two categories: sheet-fed and web-fed.

Sheet-fed: A sheet-fed press prints an image on single sheets of paper as they are fed individually into the press. The print quality and sheet to sheet registration is often better than web-fed printing, but it is often more economical to produce very large runs on web presses because of their higher running speeds. Sheet-fed presses can be divided into three categories: small, medium, and large sheet presses. Small Sheet-fed: The small sheet-fed press can print sheets up to 14" x 17". They are used primarily for short runs of one or two colors for such items as business forms, letterheads, and business cards and are popular for instant print companies. Medium Sheet-fed: Sheet sizes of up to 25" x 38" can be printed on a medium sheet-fed press. The presses are used for runs up to 20,000 and are common equipment for many medium and large printers. Products such as brochures, business forms, medium press runs of color work are produced with the mid-size press. Large Sheet-fed: The largest runs (usually 100,000 or more) and the most complex jobs are reserved for the large format sheet-fed presses. They can accommodate a paper size of up to 49" x 74" and they may have several printing towers so that multiple colors can be printed with one pass. Web-fed: A web-fed press prints images on a continuous web of paper fed into the press from a large roll of paper. The web of paper is then cut into individual sheets after printing or as with continuous business form applications, it is left in web form and is perforated for later separation into individual sheets. Like sheet-fed presses, web-fed presses come in many types and sizes. Some smaller web presses are capable of printing only on narrow width paper rolls and can only print one or two colors on the front side of the paper. Other web presses can handle large width webs and can print on the front and the back side of the paper in one pass through the press. There may be  8 or more printing units so that applications requiring full color on the front and back can be printed.

Press Components

Offset presses (sheet-fed and web-fed) are made up of some common components that work together to carry out the offset printing function. Some of the common components include a device for feeding paper into the press, a set of cylinders that create the printed impression on the paper, a roller train for distributing ink and for dampening non-image areas of the plate, and a system for removing the printed paper from the printing system.

Feeding System: The feeding system is the device that feeds the paper into the press. There are different types of feeding systems for sheet-fed and web-fed presses. Sheet-fed: The paper is usually stacked in a tray at the front end of the press where it is pulled into the press one sheet at a time. Vacuum devices called "sucker feet" pick up each sheet of paper from the stack. As paper is fed into the press, the tray of paper automatically raises up so that there is no interruption in the paper feed until the tray is empty. Web-fed: A mechanism called a "rollstand", which accommodates a large continuous roll of paper, is used with the web-fed system. As the paper is fed through the press, another system maintains proper tension on the paper web as the roll of paper gets smaller in the rollstand. Some presses have automatic roll changers which splice in a second roll of paper as soon as the first roll is nearly out of paper. Printing System: The printing system for offset presses contain 3 major components: the plate cylinder, blanket cylinder, and the impression cylinder. The circumference of the cylinders determine the size of the applications that can be printed on the press. For example, a press with printing cylinders of 17" in circumference is able to print applications with a depth of 17", 8 1/2", 4 1/4", and so on. For an 8 1/2" application, there would be two separate 8 1/2 inch pieces printed per revolution of the cylinders. Presses are often named for the circumference of their cylinders, such as a "17 inch press", or a "22 inch press". Plate Cylinder: The plate cylinder contains a slot or "plate gap" into which the lead edge of the plate is inserted. The plate is wrapped around the cylinder and then the tail end of the plate is inserted into the slot. The plate ends are then locked into the slot. Some sheet-fed presses utilize plates the are punched at both ends. The plate cylinder gap contains two sets of pins that the punched ends of the plate fit over. The pins are tightened do that that the plate remains stationary on the cylinder. Blanket Cylinder: The blanket cylinder is much the same as the plate cylinder except instead of holding a plate, a compressible rubber blanket is mounted on it. The blankets vary in type and thickness depending on the type of press on which it is used. Impression Cylinder: The impression cylinder is usually a seamless, hardened steel cylinder that provides a surface for the print impression to take place. The paper passes between the blanket cylinder and impression cylinder where just the right amount of squeeze between the two cylinders allows for the transfer of the image onto the paper.

Note: The gaps in the plate and blanket cylinders are "non-printable" areas. Allowances must be made with the overall image size so that the image on the plate does not extend into the plate gap when the plate is installed. The slot in the blanket cylinder, known as the "blanket gap" is usually wider than the plate gap, so even though the image may look correct on the plate, a sliver of the image may not be offset to the blanket because of its wider gap. For this reason, the image allowance is usually based on the non-printable area of the blanket cylinder. The non-printable gap is also known as the "lock-up" dimension and it varies between different types of presses.

Note: Some applications may require that the printed image be slightly larger than what can be actually printed by the press. To accommodate the larger print size, the copy may have to be split and printed on two separate printing units. This is known as an "over image" job or a "split image" and should be taken into consideration when planning a print job.

Inking System: The inking system on offset presses consists of a fountain which holds the ink and a set of rollers, known as the roller train, which distribute the ink and carry it to the printing plate. A roller within the fountain draws the ink from the fountain into the roller train where it is milled into the proper thickness. It is then brought to the final rollers in the system called the "form rollers" which apply the ink to the plate. The number and type of rollers in an inking system varies widely between different types of offset presses. A small duplicator press may have only a minimum number of rollers to supply the flow of ink to the plate as most of the applications printed on a duplicator press are very basic. A large web press used for printing complex applications in full color requires a larger number of rollers to mill the ink and several form rollers to apply the ink to the plate. The more rollers there are in an inking system, the better the ink will be distributed and the better the print quality will be. Dampening System: The dampening system consists of a set of rollers that distribute the fountain solution to the plate. The fountain solution is necessary to keep the non-image areas of the plate free of ink. As with the inking system, the dampening system consists of a fountain which holds the dampening solution, a roller within the fountain that carries the solution into the dampening rollers, and form rollers that apply the dampening solution to the plate. Like inking systems, the type of dampening system can vary greatly between different types of presses. Delivery System: Sheet-fed and web-fed presses each have different types of delivery systems, which are described below: Sheet-fed: Printed sheets are carried from the printing units of a sheet-fed press to a delivery tray. The tray has guides which assist in delivering the sheets to the proper place on the tray. A jogging system helps to keep the printed sheets in a neat stack. The tray of the delivery system automatically lowers as it is filled with printed sheets. Web-fed: The printed web is carried from the printing units to one of two types of delivery systems. A "roll-to-sheet" press has a mechanism for cutting the web into individual sheets. The sheets are then carried a short distance on belts to a delivery tray where they are automatically jogged and can be removed in predetermined increments by the press operator. Another type of web-fed delivery system is found on a "roll-to-roll" press. The printed web is carried from the printing units to a "rewind" unit where it is wound onto a spool. There are several reasons why rewinding is necessary:

A multiple part business form may require off-line collating, so each part of the form would be printed separately and wound on rolls at the press. The rolls would then be mounted on a collating machine where the individual parts would be attached together. The application may require additional features that cannot be applied at the press, so they are handled off-line on other web-fed equipment. Some types of continuous business forms are supplied to customers on rolls so that the forms will work properly through certain types of statement rendering equipment.

Note: Besides the common components described above, many offset presses have other components for applying additional finishing functions that would otherwise have to be accomplished off-line. The addition of perforations, scoring, punching, and consecutive numbering are only a few of the additional functions that can be performed. Many presses are modular in that additional printing units and finishing units can be added to the basic press in order to provide added functionality.