016 Platemaking for Offset Lithography
Copyright 1988 by Thomas E. Gray

Rev 08/90

Offset lithography has evolved into the dominant printing process in the United States. This video cassette deals with plates for this method of printing.

LITHOS GRAPHOS

The original plate material for lithographic printing was a fine-grain limestone, which accounts for the early name attached to the printing method. The Greek term for stone was "lithos." Their term for writing was "graphos." The name "lithography" means stone writing.

High quality printing was done using limestone as an image carrier for printing, and stone lithography is still utilized today as an artist's medium. Some old established lithographic printing companies still maintain their litho stone inventory.

These stones are the property of Courier Journal Lithographing in Louisville, Kentucky.

Preparation of the litho stone originally required that an image be drawn in reverse on the stone. Pressure applied to paper placed on the inked image on the stone caused the ink to transfer to paper, producing a print.

The need to draw directly on stone presented some difficulty, so a method was devised whereby materials could be hand-prepared or even printed on a transfer paper that was coated with a gummy or gelatinous layer by lithography, copperplate engravings or wood cuts. The transfer paper would release the images prepared in this manner onto the litho stone when it was soaked with water.

Experimentation with light sensitive materials led to the development of a method to produce photo-lithographic images on stone in 1852. Asphaltum (Bitumen of Judea) was mixed with ether, applied to the stone and dried. A paper negative was used for an exposure to a light source. This hardened the asphaltum in the exposed areas. Unexposed asphaltum could be removed with ether, leaving a photographically produced plate for printing. This method was not too successful.

Three years later, in 1855, Alphonse Poitevin, the inventor of collotype printing, developed a bichromated albumin light-sensitive coating that was successfully used until the introduction of presensitized plates over ninety years later.

Two types of light-sensitive plate coatings were in general use prior to the year 1945. They were the bichromated albumin plate coating developed by Alphonse Poitevin in 1855, and a deep-etch plate coating patented by Reverend Hannibal Goodwin in 1881.

ZINCOGRAPHY--1859

The next evolution in platemaking for lithography occurred in 1859, when zinc was introduced as a metal printing plate, a substitute for litho stone. Zincography was the term used to refer to printing from zinc plates.

ALGRAPHY --1891

Thirty-two years later, in 1891, aluminum was introduced as a plate material. Printing done with aluminum plates was called algraphy.

REQUIREMENTS FOR OFFSET PLATES

The requirements for today's offset lithographic printing plate are that it be capable of carrying a faithful reproduction of the image to be printed; that it accept and retain moistening solution readily and evenly in the non-image areas, and ink in the image areas; that it cause a minimum number of plate-related problems for the press operator; that it have a level surface of uniform thickness; and that it be durable enough to last as long as it is supposed to last.

Today, a variety of plates and coatings are available to the offset printer, produced by many manufacturers for the wide range of printing done by the printing industry.

Plates for offset printing may be categorized as direct image, electrostatic, diffusion transfer, photographic contact plates, and photographic projection plates.

PLATES FOR OFFSET PRINTING

DIRECT IMAGE PLATES

Direct image paper plates are plates can have an image that is hand-drawn with printing-ink-receptive drawing inks and pencils. They can also be typed on, using a computer or typewriter with a carbon ribbon. The non-image areas of the plate must be pre-moistened or etched before the plate can be run on the printing press.

It is also possible to use other printing methods to pre-print an image on this type of plate. The plate is then run on an offset press to reproduce the pre-printed image. Direct image plates are manufactured for short runs of up to 100 copies, and for longer runs of up to 5000 copies.

Artists sometimes draw directly on metal plates to produce art prints.

ELECTROSTATIC PLATES

Electrostatic plates are produced the same way copies are made on a copying machine.
The difference is that the plate material is designed to withstand the moisture applied by the offset press. In this process, an exposure to light coupled with an electrostatic charge causes pigment to be deposited in the image area. It is adhered to the offset plate with heat or solvent.

Like the paper plate, the electrostatic plate must be pre-moistened or etched prior to being attached to the plate cylinder of the offset press. These plates are widely used in the quick copy industry. Several different electrostatic plate-producing systems exist with some variation in the method employed to produce an image on the plate.

DIFFUSION TRANSFER PLATES

Diffusion transfer materials are available that allow an original document, such as a paste-up or typed page, to be exposed and processed to produce a positive copy of the original. The process requires a negative image sheet and a receiver sheet. The negative image sheet must be exposed and fed into a diffusion transfer processor, in contact with the receiver sheet.

During processing, the image sheet serves as a chemical mask to prevent image formation in non-image areas, while allowing a positive image to form in unmasked areas. This technology is being used to produce printing plates.

PHOTOGRAPHIC CONTACT PLATES

Most of the plates used in medium and large printing plants have light sensitive coatings that are exposed in contact with photographic film. Photographic plates may be classified as deep-etch, bi-metal, tri-metal, pre-sensitized standard and waterless, continuous-tone, projection, or wipe-on.

Many photographic plates are made from thin sheets of metal. Some are made from paper or plastic, while others are metal foil laminated to paper. Plates range in thickness from three thousandths of an inch to two hundredths of an inch, and are produced in sizes from ten by fifteen inches to fifty-nine by seventy-eight inches. Aluminum is the metal commonly used for metal offset plates.

The differences in photographic printing plates are in the composition of the light sensitive coating, the number of copies that can be produced with the plate, the method of sensitizing the plate, the type of plate coating, the exposure required, and the material used for the plate. A sensitive coating can be applied to one side or to both sides of the plate. There are advantages and disadvantages to each method.

DEEP-ETCH, BI-METAL
AND TRI-METAL PLATES

Deep-etch plates are coated and processed in the printing plant. Bi-metal and tri-metal plates may be coated and produced in the printing plant, or they may be pre-sensitized.
These plates have been used because of their capacity for long press runs and fine printing quality. Advancements in pre-sensitized plate technology have led to a decline in their use. They are now used only in those printing plants that are set up to produce them economically and efficiently, whose production requirements demand their use.

PRE-SENSITIZED PLATES

A pre-sensitized plate is one which has a light-sensitive coating applied during the manufacturing process. Pre-sensitized offset plates were produced in the United States as early as 1949. They were introduced in Germany in 1941. Many different companies now produce a variety of pre-sensitized plates.

Plate manufacturers produce several types of plates, based on their marketing strategy and the need of the printer for short-run, medium-run or long-run plates.
There is a pre-sensitized waterless plate being developed that does not require moisture during the press run. Success in running this plate is dependent upon the nature of the plate. It has an ink-repellent silicone rubber coating in non-printing areas, and an ink-receptive image area. These examples were printed from a waterless plate using a 300 line halftone screen.

Pre-sensitized plates developed with water have been introduced to respond to environmental concerns about chemicals. One plate on the market can print unscreened photographs using continuous tone film materials.

PHOTOGRAPHIC PROJECTION PLATES

Projection-type pre-sensitized plates can be exposed directly to the paste-up or other copy, bypassing the step of producing a film negative. Recently introduced non-metal projection plates have reproduction quality equal to metal and last up to twenty thousand impressions.

WIPE-ON PLATES

The plate coating for wipe-on plates is mixed with sensitizer just prior to coating the plate. The coating may be wiped on the plate. In some cases a roller applicator is recommended. Once the coating is dry, exposure and processing procedures are similar to those for pre-sensitized plates.

Pre-sensitized plates and plates sensitized in the printing plant may be additive plates, requiring a developer to build up the exposed image area. They may be subtractive, requiring that portions of the sensitive coating be removed from the printing plate. The exposed part of the subtractive plate will form a durable printing image, and the unexposed portion of the coating is removed from non-image areas. Negative-working plates require film negatives for the plate exposure, while positive-working plates require film positives. Plates may be either additive or subtractive negative working plates or they may be additive or subtractive positive working plates.

PLATE EXPOSURE

Recommended safelights should be used when working with unexposed plates.

The light source for exposing plates can be a single point source ultra-violet light, such as a carbon arc, a mercury vapor lamp or pulse xenon lamp. Other lights will expose plates, but are not as efficient.

The blanket of the platemaker should be kept in good condition, and the glass on the vacuum frame should be kept scrupulously clean. The platemaker should provide even support for the plate and flat during the exposure.

The vacuum gauge should show an accurate reading of twenty-two to twenty-five psi throughout the exposure of the plate. Adequate vacuum minimizes spot and edge halation on halftones and screen tints. When the vacuum pump on the platemaker is turned on, ample time should be allowed to insure good contact between plate and flat before the lights are turned on to expose the plate.

Steps should be taken to maintain precise register between the flat and plate during exposure. Several manufacturers market devices which are designed to insure good register during the platemaking process.
The exposure required by the plate will vary according to the type of plate, distance of the light source from the plate, type of light, and intensity of the exposing light. The distance between the light source and the plate should not be less than the diagonal of the largest plate to be exposed.

The best exposure for a particular light source and plate can be determined by a series of test exposures, followed by a series of tests on the press. Use of a platemaker's stepped gray scale for negative or positive film flats will help the platemaker arrive at the correct exposure. Most plate manufacturers suggest a specific step on the gray scale as an indicator of the proper exposure for their plates.

The GATF plate sensitivity guide serves as an industry standard for determining plate exposure. On this scale, there is a density increase of 0.15 per step.

Image development to a solid step five or six using the negative guide is a good starting point for judging exposure, while development to an open step three is an indicator of a good exposure using the positive guide.

Industry organizations and film and plate manufacturers produce control devices to be included on a plate to make it possible to evaluate such things as under-cutting, spread, dot gain or loss, registration, and evenness of ink coverage.

PLATE PROCESSING

After a plate is exposed, it should be processed according to the manufacturer's recommendations. Hand processing is shown here for the purpose of demonstrating what happens to plates during processing. Automated machine processing is widely used in the printing industry.

Negative plates are plates that are produced with film negatives. Shown here is the processing of a negative additive plate. The entire plate has a light sensitive coating. Light that passes through clear areas of the film exposes and hardens the coating. A desensitizer removes unexposed coating, and a developer builds up or adds to the image in the exposed areas.

Negative subtractive plates are exposed like negative additive plates. The exposed portions of the subtractive plate do not require a build up of the printing image. The coating that is hardened through exposure to light remains on the plate. Plate developer removes the unexposed parts of the sensitized plate coating, leaving the image area on the plate to accept ink.

Unless there is a specific reason to use other plates, the negative working plate is the type used, because it is cheaper and easier to produce.

Positive subtractive plates are produced with film positives. The film positive image prevents the exposing light from striking the plate in the image area. The image areas, where light has been blocked, are left on the plate for printing. Parts of the sensitive coating that have been struck by light are removed during plate development.
Plate chemistry consists of a developer for the image area, a desensitizer to remove the sensitive coating from unexposed parts of the plate, and a protective coating for the surface of the plate. Sometimes the developer and desensitizer are combined into one solution.

Careful plate handling is important if good printing results are to be obtained. It is important to avoid scratching, bending, kinking or placing finger-prints on offset printing plates.
The package of unexposed plates should be kept closed. Plates saved for re-use should be treated according to the manufacturer's recommendations and stored in a cool, dry atmosphere. If left unprotected, the aluminum surface will oxidize and its printing properties will be impaired.

Offset printing plates can be modified using hand techniques after they have been exposed and developed. Alterations may require honing a scratch in a non-printing area of the plate or using lacquer to fill in a solid. It is possible to remove an image and add sensitizer to an area of a plate, then expose and develop a new image in that area. However, the time required to do major plate alterations should be considered. It may be more expensive than the cost of producing a new plate.

The plate is a critical element in all printing processes. It should be made carefully, handled properly, and preserved and protected when it is being stored for later use.