Printing Techniques

I

Introduction

Printing Techniques, several different ways in which printing may be accomplished, such as lithography, letterpress, flexography, gravure, and screen printing. All of these printing techniques use simple mechanisms for rapidly applying colorants to substrates such as paper or plastic to form multiple reproductions of original images for mass distribution.

Most presses are capable of printing multiple colours in a single pass through the press. Colour printing takes two forms:

Spot colour printing, using custom mixed inks to reproduce specific colours. Spot colour printing is widely used in package printing, where large areas of uniform colour are common.

Process colour printing, using four transparent inks—cyan (greenish-blue), magenta (bluish-red), yellow, and black—printed one on top of another in varying amounts. Colour photographs and other artwork can be faithfully reproduced by this method. Very occasionally the set of process colours is extended by adding other colours such as green or orange. Some high-value printing may also incorporate spot colours as well as process colour printing.

Most modern printing presses transfer ink from a cylindrical printing surface to moving sheets or rolls of substrate. Presses that print on reels (or webs) of paper can achieve speeds of 600-900 m (2,000-3,000 ft) per minute. Presses that print on sheets are generally slower than web presses but can print on thicker substrates, such as board and even sheet metal.

Since the 1960s, technological advances in optics, electronics, and computing have had a profound effect on the manufacture of printing surfaces. Light-sensitive materials such as diazonium resins and photopolymers make it possible to produce durable printing surfaces electronically rather than mechanically. Printing surfaces are imaged by computer-controlled laser beams or diamond styluses. Pages generated on computer systems are thus transferred directly to printing surfaces without any intermediate steps.

II

Lithography

By far the most important and versatile printing process today is offset lithography. The underlying principles were established at the end of the 18th century by a German map inspector, Aloys Senefelder, who was experimenting with methods of producing limestone relief printing surfaces using an acid etching process. Senefelder found that a wet limestone surface would repel an oil-based printing ink, and that an image drawn on the surface with a grease pencil would repel water and attract ink. Any drawing on the stone surface could be reproduced by bringing a damp sheet of paper into contact with the freshly inked image. This cycle could be repeated several hundred times before the drawing could no longer be faithfully reproduced.

The process, called chemical printing by Senefelder, quickly became a popular art medium because it enabled artists to produce multiple copies of freehand drawings. By the late 19th century, multiple stones were being used to transfer as many as 30 separate colours to a single sheet of paper to produce exquisite colour lithographs that resembled fine watercolour paintings. Modern colour lithography uses only four inks for a vast range of natural colours.

A

The Offset Principle

In the early part of the 20th century, it was discovered that ink could be transferred from a printing surface to an intermediate rubber surface and then to paper. The rubber intermediate, called a blanket, can transfer ink to paper and to a wide variety of materials that cannot be printed on directly, including plastics and metals. The compressible blanket conforms to the texture of the surface to be printed, and as the lithographic plate is planographic (flat) there is no local distortion as would be caused by a relief or sub-relief printing surface. As a result, offset lithographic printing produces sharp, distortion-free printing.

B

Offset Lithography Today

The function of the original stone printing surface is now served by thin aluminium plates, although other materials, such as stainless steel and plastic, can also be used. The plates are wrapped around the circumference of the printing cylinder and make direct contact with the rubber blanket cylinder. A roller train carries ink and damping solution to the plate surface. The ink is transferred first to the blanket cylinder and then to the paper.

Lithographic plates are relatively inexpensive to manufacture, and this fact has contributed greatly to the success of the process. Aluminium plate materials have a thin surface coating of light-sensitive material, such as a photopolymer, that undergoes a solubility change when exposed to an intense source of light. Images are transferred to the surface by exposing the light-sensitive plate surface by a laser or LED.

Modern offset lithographic presses range in size from small sheet-fed machines—used for small jobs such as brochures and newsletters—to massive web presses capable of printing millions of copies of magazines, catalogues, and packaging materials in full colour. No other process has such a broad range of applications.

Sheet-fed presses print sheets of paper or board up to a metre or more in width. A press will have anything from one to six or more printing units, each printing a separate colour on to the sheet. Some presses are capable of tumbling the sheet over while inside the press, making it possible to print on both sides in the same pass. Ink is often allowed to dry without assistance, although some presses are fitted with equipment to accelerate drying and allow faster processing of the printed sheets.

Web-fed presses are designed either to print on newsprint or similar materials, or on more high-quality coated paper. The former are known as cold-set presses, as no special drying equipment is required. In the latter case the ink must be dried quickly to prevent marking and rubbing as the wet ink smears on parts of the press. Heat-set presses dry the ink after printing by first heating it to remove the more volatile components, and then passing it over a chill roll, which causes the ink to set (rather like putting jelly in a fridge).