PRINTING MACHINES The different styles of printing machines are too numerous to detail. In America they are called "presses" and in Great Britain (with the exception of proving presses) they are called "ma chines." They may, however, be classified as platens, automatics, cylinders and rotaries. Many firms adopt standard makes of ma chines to suit their requirements. Thelarger number of machines in general use print on one side of the sheet at a time, but in offices where magazines and books are produced "perfecting" machines are employed. These machines have two type beds and two im pression cylinders, thus when one side of the sheet is printed, it is mechanically reversed and the other side receives an impression from another set of plates or type. In colour printing there are machines for printing a number of colours before delivery of the sheets. The usual method in multi-colour printing, is to print two colours before the sheet is delivered. There are two impression cylinders and two beds each with its respective forme of type or blocks. When the sheet receives its first impression it is trans ferred to another cylinder and the second colour is superimposed on the first. Thus, in reproducing pictures in four colours the sheets go through the machine twice before the work is completed.
Colour printing has been attempted by placing machines "tan dem wise," i.e., end on to each other, the sheets being carried from one machine to another by means of a travelling conveyor and automatically placing the sheets into the lays on the respective machines. Four colours have been printed by this method. The most effective multi-colour printing method, however, is that done on rotary presses, where a sheet is taken by means of grippers round a large impression cylinder and receives an impression from four sets of plates, each inked with a different colour.
On the hand press the type bed runs on two rails and is pro pelled under the platen by means of a rounce consisting of a handle and a pulley to which belts are attached ; the ends of the belts are secured to the ends of the bed, and by turning the handle the bed is moved to and fro as the belts wind and unwind round the pulley; while to obtain the necessary pressure the platen is de pressed by means of a lever, which when pulled forward acts upon a toggle joint (inclined pieces of metal) bringing it into a per pendicular position. This forces the platen down and presses the
sheet on to the inked forme. On the bar being released the platen is lifted to its normal position by springs. Attached to the end of the type bed a hinged frame is fixed and covered with parchment or fine linen ; this is called the "tympan," and it is on this that the making ready is fixed and the sheet of paper secured by means of "lays" (pins being usually used). The paper is kept from being soiled by a "frisket" (another hinged frame fixed on to the top of the tympan and covered with paper with parts cut out correspond ing to the area of the type which has to be printed). The portions of paper remaining on the frisket keep the sheet from being marked by the furniture or chase which is liable to become inked when rolling a forme by hand. Two hundred and fifty impressions an hour are considered a good production on this type of press.
Light platens are fast-running machines suitable for general job work, and are invariably actuated by what is known as the "clam shell" movement, which brings the platen on which the packing and "making-ready" are fastened, and on which the sheet to be printed is placed to guides, into a position where the sheet can be pressed against the forme, which is held in a perpendicular position. The forme is automatically inked by two or three rollers passing over the surface, the ink being fed by a roller from a "duct" on to a rotating "disc," which assists in the distribution of the ink before the inking rollers take up a supply from the disc. The platen and the bed are brought together by means of rods working on eccentrics. By moving a lever acting upon the eccen tric the platen is thrown back, thus preventing the forme from printing by keeping the platen from reaching the forme. This style of machine can be worked by treadle, although motive power is now usually employed. The feeder stands in front of the machine, lifts a sheet of paper with his right hand, places it on to lays on the platen which rises and prints the sheet. On the return motion of the platen the feeder removes the printed sheet with his left hand and places it on a table, at the same time placing another sheet on to the platen ready for printing. The average size of a sheet printed on these machines is io in. by 15 in. and the speed is approximately 1,500 impressions per hour.