| The Graphic Arts Technical Foundation presents another in a series of audio visuals on the lithographic press. This audio visual on "The Offset Press" is presented to further an understanding of the development and function of the sheetfed lithographic offset press. |  |
| Here is an early lithographic press. In order to appreciate the function of the printing unit, let us trace its development. At first it was very simple in design and very limited in capacity. |  |
| The printing unit was made up of a flat stone--the image carrier--which was carried back and forth on the bed of the press under a fixed revolving cylinder. The paper to which the image was to be transferred was carried on the stone. The cylinder, covered with a rubber-coated blanket, was called the impression cylinder. |  |
| Lithographic stones were made from porous Bavarian Limestone upon which images were drawn, engraved, or transferred. These stones were costly and their bulk made handling difficult. |  |
| As early as Senefelder's time--around 1800-efforts were being made to replace the stone with a metal sheet. Eventually this resulted in the grained zinc plate, and was later followed by the aluminum, paper, and multi-metal plates that are used today. |  |
| In the latter part of the 19th Century, the principle of the rotary press was adapted to the lithographic process. Metal plates, which had been developed by this time, could be bent and readily mounted on a rotating plate-carrying cylinder. |  |
| Utilizing two cylinders--the plate-carrying cylinder and an impression cylinder--direct rotary lithography became the first big step toward higher press production speeds in lithography. |  |
| The solution to many of the problems of direct rotary lithography came unexpectedly. It was discovered that images accidentally printed on the rubber-covered impression cylinder and transferred, or offset, to the back of a printed sheet were superior to those made directly from the plate on the paper. |  |
| This discovery, by Ira W. Rubel around 1904, led to the development of the third cylinder--the rubber covered blanket cylinder. Now, instead of direct lithography, we have indirect or offset lithography. |  |
| To help complete the press, an inking system is needed. Shown here are the inking form rollers which apply ink directly to the plate. The form rollers are part of a complex system of inking rollers. |  |
| Since the lithography process of printing uses both ink and water, dampening rollers are needed to apply the water to the plate. The dampening rollers are positioned so that as the plate cylinder is turned the plate will first contact the dampening rollers, and then the inking rollers. |  |
| Once the plate has been properly dampened and inked, a feeder system inserts paper into the printing unit. |  |
| The paper is held by grippers as it is pressed against the inked image on the blanket cylinder. |  |
| Finally, the printed sheet is delivered out of the printing unit and piled on the delivery platform. |  |
| The model of a sheet-fed offset press shown here is typical of most offset presses. Since the beginning of offset lithography, there has been continuous development by press manufacturers toward larger and faster sheet-fed offset presses, both single- and multi-color. |  |
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This is one of many different types of modern sheet-fed offset lithographic presses in use today. There are many press manufacturers producing presses of this basic type. Presses are available in a wide range of sizes from small duplicators that print in-plant communications to large presses that produce some of the finest color printing ever produced. A modern two-color press can be viewed by clicking here. A modern four-color press can be viewed by clicking here. |
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| In summary, these three cylinders are the printing unit of the modern offset press--the plate cylinder that carries the lithographic printing plate; the rubber-covered blanket cylinder that accepts the inked image from the lithographic plate and transfers it to the paper, and the impression cylinder that carries the paper and squeezes it against the inked blanket cylinder. Reference to a fourth cylinder, called either a delivery or transfer cylinder, is a misnomer. |  |
| The End. |  |
