Photoengraving

Photoengraving, process of preparing illustrations for printing by transferring their images to metal plates by a combination of photography and acid etching. The resulting relief surfaces can themselves be used as printing plates, or they can serve as moulds for other plates, produced as duplicates or as curved plates for rotary presses.

Photoengraving is becoming obsolete in modern commercial printing. In the letterpress printing technique, itself now found only rarely, the process has been largely abandoned in favour of simpler platemaking methods, but some moulded-rubber flexographic printing plates still depend on photoengraving for originals.

In photoengraving, an unexposed light-sensitive material that will dissolve in a solvent is applied as a thin coating to a thick sheet of etchable metal such as magnesium or copper. The coating is then exposed to ultraviolet radiation through a high-contrast photographic film of the image to be reproduced. The film is held in contact with the coated metal plate, and ultraviolet radiation is transmitted through the transparent image areas to harden the coating underneath. The opaque non-image areas prevent radiation from penetrating and exposing those areas of the plate.

When the exposed plate is immersed in a solvent bath, the unexposed material washes away, leaving the exposed area in the form of a stencil resist, or coating, on the surface. The plate is then etched in machines that splash a mixture of dilute acid and surfactant chemicals against the plate. The chemicals dissolve the bare metal in the non-image areas without damaging or undercutting the resist-protected image areas, leaving them standing in relief.

Various modifications of the photoengraving process are used to prepare printing surfaces for letterpress, flexography, lithography, screen printing, and gravure—all of which use materials that undergo solubility changes when exposed to ultraviolet radiation. An important class of these materials is photopolymers.

Photopolymer printing plates for letterpress, flexography, and lithography do not require an acid etching but instead use a simple wash-out process and are therefore much easier to make than photoengravings. Relief plates require a coating of photopolymer that is at least as thick as the desired relief on the finished plate. This coating is exposed to an ultraviolet source through a film that polymerizes it throughout its entire thickness. The exposed plate is then washed in a solution that dissolves the unexposed photopolymer in the non-image areas. Some of these materials are washable in ordinary tap water, while others require stronger organic solvents. Once the plate has been washed and dried, it may be mounted directly on the printing press or in some cases may undergo further treatment such as baking or electroplating to enhance the hardness of the image surface.

Printing plates for lithography require a much thinner coating of photopolymer, because the image areas of a lithographic plate are on the same plane as the non-image areas. The plate is usually made of aluminium, and the hardened photopolymer coating is formulated to repel water and attract ink. The non-image areas of the plate, therefore, attract a thin layer of water that repels ink; ink then adheres only to the image areas of the plate. Lithographic platemaking is not classified as photoengraving because lithography does not use relief plates.

In a variant of the lithographic process, the non-image surface is formed of an ink-repelling material such as silicone. These plates can be printed by waterless lithography. Plate exposure by laser ablates the surface layers, revealing the image and non-image areas without requiring a washout stage.

Printing surfaces for gravure can be made by an acid-etching process similar to that for letterpress, except that the image is formed by etching below the surface in the image areas so that ink prints from these areas after it has been removed from the surface. A cylinder is coated with a layer of copper, and an exposed resist is applied followed by etching. Unlike other processes, the resist has a varying density so that the etching process will produce different depths of engraving, which will result in a varying thickness of ink film when printed. Photoengraving for gravure has largely been replaced by direct electronic engraving, in which the cylinder surface is engraved by a diamond stylus (or occasionally a laser) driven directly by a computer.

Photoengravings and photopolymer plate materials for the relief process letterpress and flexography require film negatives to expose the image areas, while gravure cylinders are normally engraved using positives so that only the sub-surface image area is left unhardened. Lithographic plates can, depending on the type of coating used, be either negative or positive working. Film exposure units, known as imagesetters, can produce film negatives or positives from digitally assembled pages containing type, graphics, and images. The use of computers to create content, and assemble pages and output them to films or plates for printing, first seen in commercial use in the 1970s, now completely dominates the industry.

Film negatives for platemaking must be highly opaque in the non-image areas, and transparent in the image areas. Image areas on the resulting plate will transfer a thin layer of ink to the paper. Since this ink layer does not vary in thickness from one part of the plate to another, processes such as lithography, letterpress, and flexography must print tiny dots that vary in size to simulate the appearance of a range of grey tones. Images reproduced in this matter are called half-tones.

Current methods of photomechanical platemaking involve replacing contact film exposure with direct exposure of the printing surfaces by a laser or LED driven by a computer. These are known as “computer-to-plate” or CTP systems. With a sufficiently powerful laser or LED, the entire surface of a lithographic plate can be exposed in a minute or so. CTP platemaking eliminates the need for expensive film and significantly reduces the time required to get a set of plates to the press. This is particularly attractive to the newspaper industry, where time is critical.