Text 1 Basic printing techniques

There are certain principles in printing that apply regardless of the particular process that is being used. It is important, for instance, to use the right size of press for the job. Conversely, the size of the job should in general be tailored to the machine being used for printing.

The main printing processes can be defined according to the physical characteristics of the printing surfaces used. Letterpress is a 'relief process, where the image to be printed is raised above the background; this raised surface is inked by rollers and then pressed against the paper to make the impression. Lithography is 'planographic', with a flat printing surface: the image area is chemically treated so that it accepts ink and rejects water, while the non-image area is treated to accept water and reject ink. Gravure is an 'intaglio' process, with the printing image recessed into the plate and filled with liquid ink; the non-image area is wiped free of ink, so that ink is deposited on the paper only from the recessed wells.

An operation that applies to all printing processes is make-ready' — that is, all the operations that take place prior to the first good copies being produced on a press. Most modern printing techniques therefore aim to reduce the time and expense involved in make-ready.

Most printing processes require the paper to be either sheet-fed or web-fed.

In sheet-fed printing the paper, which is made in the form of reels (large rolls), is cut into sheets of a suitable size for the press being used. The 'feeder' section of the press picks up the sheets, usually with a combination of metal fingers and vacuum suckers, and feeds them through to be printed. The sheets then pass on to the delivery end of the press, still as flat sheets — folding or other finishing processes are a separate operation requiring other machinery.

With web-fed (also known as reel-fed) printing, the paper is supplied to the machine in the form of reels. The front end of the press has a reelstand, which holds the paper as it is unwound and fed through the press. The actual method of printing is the same as with a sheet-fed press, but printing can take place at much higher speeds because the machine is not slowed down by having to pick up and put down each sheet before printing the next. Most web presses also incorporate some form of finishing facility after printing. Usually this is folding, but it can also include various types of gluing, stitching and perforating to give special products for direct-mail and other purposes; here the folding section of the machine may have added equipment for gluing and perforating.

The advantages of web-fed printing are speed. Web-fed presses therefore lend themselves to long runs. The disadvantages is that the web-fed presses involve a very high initial outlay, need more time before they are ready to start printing and mostly produce only items of a fixed length.

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