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Part 16 - Printing

Early in the 19th century, Germany had few industries and investors so, in 1804,Friedrich Gottlob Koenig travelled to England to pursue his dream of building a steam driven press. In London, he met watchmaker Andreas Friedrich Bauer and together they completed their first machine in April 1812. 

 On 29 November 1814, John Walter, worried about angering his employees, secretly used one of these high-speed, steam-powered printing presses to print an edition of The Times newspaper in London. The press could print both sides of the paper at the same time and at a rate of 1,100 sheets per hour, far faster than existing printing presses.

Koenig returned to Germany in August 1817 after disagreeing with a partner, Thomas Bensley, who wanted sole rights to the new machine. He chose an abandoned monastery in Würzburg for his new factory and the company Koenig & Bauer AG is still operating there 200 years after its founding.

Blueprints were developed by Sir John Herschel in 1842, as a way to quickly make copies of engineering drawings. This were widely used, in construction and industry, for more than a century (until the introduction of the diazo whiteprint process and xerographic photocopiers). The process used a light source to expose a light-sensitive chemical coating on a sheet of paper in contact with an original drawing. The light passed through the drawing except where it was blocked by the pencil lines. The blueprint was then washed with water to remove the unexposed chemical leaving white lines on a blue background.

The American inventor Richard March Hoe patented a rotary drum printing press in 1847 where the type was set in the form of a cylinder. It was first used to print the New York Sun. By 1870 his improved rotary press could print on both sides of a continuous roll of paper five miles long, which passed through the press at the rate of 800 feet (240 m) a minute. The roll was then cut into sheets and folded producing 18,000 papers an hour. 

Letterpress printing was the most common method of printing text, from its invention by Johannes Gutenberg until the second half of the 20th century. The movable type was composed and clamped into the bed of a press where it was inked before the paper was pressed against the type. It was largely supplanted by offset printing where the inked image was transferred (or "offset") from a plate to a rubber blanket which was then pressed against the paper.

There are four main printing processes.

1.  With relief printing, ink is applied only to the raised surface of the printing plate, no ink is supplied to the recessed areas. A printing press is not necessary as the back of the paper can be rubbed or pressed by hand with a simple tool. Letterpress, flexography, rubber stamp, potato printing, woodcut, metalcut, relief etching and linocut are all examples of relief printing.

Flexography can be used to print on almost any surface, including plastic and metallic films and paper. It is also suitable for printing large areas of solid colour, flexible packaging, labels and newspapers and for non-porous material and irregular surfaces.

2.   With Intaglio printing, the ink is wiped off the raised surface leaving ink only in the recesses. This requires considerable pressure to be applied by the press to force the paper into the recesses containing the ink. Examples include; gravure, steel die engraving, etching and drypoint. With gravure and rotogravure printing, small depressions (pits) in the surface of the printing plate form the image. These are filled with ink, and excess ink is scraped off the surface. The depth of the depression determines the amount of ink printed with each dot, the image is less bright with smaller depressions. A rubber-covered roller presses the paper onto the surface of the plate with sufficient force to contact the ink. Printing cylinders are usually made from steel that is copper plated and may be plated with chromium. The image depressions are produced by diamond engraving; chemical etching, or laser ablation.

Gravure printing is used for long runs of high-quality magazines and mail-order catalogues, packaging, fabric and wallpaper printing. It is also used for postage stamps, currency and decorative plastic laminates. An early method of printing black and white photographs in newspapers used a grid dots of various sizes (halftoning).

3. With the planographic techniques, such as lithography, there are no recesses, the ink is held only by chemically treated parts of the surface required to print the image. Examples include; offset lithography, collotype, and screen-less printing.

The lithographic process, used with offset printing, has a flat (planographic) image carrier. The non-printing area is coated with a film of water while the oil-based ink is supplied from rollers. The ink does not stick to the water film.  Currently, most books and newspapers are printed using offset lithography.

4. With porous printing, the ink is forced through a fine mesh screen. Areas not printed are blocked so that the ink does not penetrate the mesh. Examples include; silk screen printing and stencil duplication.

Other printing techniques include:

Pad printing, for complex three-dimensional surfaces.

Inkjet and laser printing for variable data on letters or coupons.

Thermal printing for airline baggage tags and supermarket price labels. 3D printing to make objects built up by many thin layers of material.

3D Printing