Lithographic Presses

Lithographic offset presses are usually classified in terms of size, number of printing units (colors), sheet or web fed and whether they perfect (print on both sides of the substrate on a single pass through the press). Size refers to the maximum sheet width that may pass through the press. Generally measured in the across-the-cylinder direction, i.e. parallel to the cylinder axis.

The actual image size is determined both by the plate or blanket width (whichever is smaller) and the plate or blanket length (which are limited by the cylinder diameter). A press smaller than about 12"x18" without bearers (hardened metal rings attached to the end of the cylinder or cylinder journal) is called a small offset press but more often a duplicator. Duplicators started out as office machines. They are simple, but can be efficient presses. They are also very good demonstrating the offset process.

Larger presses generally have bearers at the ends of the plate cylinder in addition to larger sheet sizes. The Heidelberg KORD presses in our small pressroom have bearers, while the A B Dick and other duplicators do not. As a result the cylinders need to be packed to achieve proper impression pressure. A sheetfed offset lithographic press is one that feeds and prints on individual sheets of paper (or other substrate) using the offset method. A webfed offset lithographic press is one which prints on a continuous web or ribbon of paper fed from a roll and threaded through the press.

Sheetfed presses can sustain 10,000-12,000 impressions per hour. Webfed presses can reach speeds up to 4 times faster than sheetfed (almost 50,000 impressions per hour). This translates to web speeds of over 3000 fpm. Significant degrees of automation are present on newer presses be they sheet or web. The number of colors, or more strictly the number of units, describes the number of independent printing units, each with its own inking and dampening system, which are present on the press. Each unit is essentially a single color press.

For sheetfed presses the units are connected by one or more transfer cylinders to transport sheets between stations. Must be an odd number to assure that proper side is printed. Each unit can print a different color, so that the number of colors is determined by how many separate colors can be printed anywhere on the sheet with one pass through the press. Hence, the number of colors is generally the same as the number of units.

More colors than the number of units can be obtained if split ink fountains are utilized to print spot colors on widely separated parts of the page. Some multicolor presses employ a “semiopen design” consisting of a single impression cylinder with two pairs of plate and blanket cylinders. Semiopen units can be strung together. An 8-color semiopen sheetfed press has 4 semiopen units.

Most sheetfed presses print on only one side of the sheet on a single pass. A second pass is required to print the other side. However, an important class of press is that of perfecting presses (or perfectors), which can print on both sides of a sheet or web during a single pass through the press. The most common perfector is a convertible perfector which has special reversing cylinders that tumble the paper end for end between printing units so that the other side may be printed.

These usually can be set to print two colors on a single side in addition to a single color on each side. This is illustrated on the right and on the next slide.

Another type of perfector is the blanket-to-blanket perfector, in which each blanket acts as the impression cylinder for the other. These are generally employed on web presses.