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OFFSET LITHOGRAPHY

Offset printing — or offset lithography, as the process is officially called — is the most popular method in commercial printing. The offset lithography process was invented, accidentally, in 1906, but it wasn’t until the 1950s that it became the most popular form of printing. By then, offset plates, inks and paper had improved and the lower cost and better image quality, as well as the four-color process option, helped offset printing dominate the market.

In offset printing, ink is transferred (offset) from the printing plate to a rubber blanket and then onto the paper making it an “indirect” printing process. It reduces plate wear and permits a finer detail on a greater variety of papers. Lithography refers to the general method of using plates whose image areas attract ink while the nonimage areas repel it.

Most offset presses are sheetfed with the paper coming in sheets which are fed through the press. For especially large print runs, such as magazines, web presses are used, which utilize webs or rolls of paper.

Offset printing is the most efficient process for long runs.

LETTERPRESS

Even though today’s offset presses are capable of turning out up to 13,000 sheets per hour, printing eight colors and coating in a single pass, the much slower process of letterpress printing is experiencing a revival.

A few hundred years ago, letterpress printing was the printing form of choice — and the only viable option. In the letterpress process, ink is spread over metal type or plates and directly transferred onto paper and imprinted under the pure pressure of the press.

Feeding only a fraction of the sheets (500–2,500 per hour), it offers a distinctive look and feel that offset printing simply can’t achieve. As the printed areas are literarily pressed into the paper, letters and images achieve a distinct, heavy, debossed look. It is perfect for projects that call for a unique touch, like announcements, invitations, business cards, stationery and any printing that requires an elegant and stylish impression.

Ink rollers apply ink directly to the type and the type comes into direct contact with the page, applying much heavier ink coverage to the printed piece. This gives the colors of a letterpress printed piece a more vivid look, but also prevents the effect of process colors as they appear in offset printing.

Successful letterpress printing requires the careful coordination of designer and printer in the design and paper selection to achieve successful results.

GRAVURE

Gravure printing is characteristically used for long run, high quality printing producing a sharp, fine image. The number of gravure printing plants in the U.S. is significantly lower than other printing processes. This is due, in part, to the cost of presses and components. Set-up of a print job is much more expensive as a single gravure cylinder will cost around $5000 versus around $15 for a lithographic plate. However, the gravure cylinder has a long service life and will yield a very large number of impressions without degradation.

Gravure printing is an example of intaglio printing. It uses a depressed or sunken surface for the image. The image areas consist of honey comb shaped cells or wells that are etched or engraved into a copper cylinder. The unetched areas of the cylinder represent the non-image or unprinted areas. The cylinder rotates in a bath of ink called the ink pan.

As the cylinder turns, the excess ink is wiped off the cylinder by a flexible steel doctor blade. The ink remaining in the recessed cells forms the image by direct transfer to the substrate (paper or other material) as it passes between the plate cylinder and the impression cylinder.

SCREEN PRINTING

When it comes to screen printing, most of us think of T-shirts. But this is only one of many applications. Screen printing is probably the most versatile of all the printing processes, as it can be used to print on a wide variety of surfaces. From paper to plastics, to glass to numerous two- and three-dimensional surfaces, no matter what shape or thickness, screen printing will get the job done.

A screen is made of a piece of porous material and stretched tightly over a frame. Areas of the screen are blocked off with a non-permeable material — creating a stencil — which is a positive of the image to be printed; that is, the open spaces are where the ink will appear. Placing the frame on top of the print surface, ink is then pushed through the open areas in the screen by a squeegee (rubber blade) and onto the paper or fabric below.

Screen printing is a one-color-at-a-time process. So before the next color can be applied, the printed product will be moved into the drying oven or through the UV curing system on a conveyor belt to ensure that the first layer of ink is completely dry.

You can recognize a screen printed product by its thick, raised ink and, in some cases, even by an impression of the mesh screen on the ink.