Printing images and their methods of Production

LETTER PRESS PRINTING
1.1 THE BASIC PRINCIPLE:
Letterpress is a highly versatile printing process. It is adaptable to both short and long printing runs, from a few hundreds to several millions of printed impressions. In letterpress printing the image area of the printing surface stands in relief, raised above the surrounding non-image areas. Ink is applied to the image areas with a roller and is transferred to paper under pressure. Letterpress printing process is derived from pre-industrial art of woodcut.

1.2 PRINTING FROM TYPE:
Some letterpress printing is done directly from handset or machine – cast type. The type is positioned as it should appear on the printed sheet within a steel frame called a chase. It is held in position by wood blocks called furniture & locked up tightly by means of wedge – shaped devices called quoins. The resulting assembly of type, called a type former or from is placed on the printing press and linked, and impressions are made.
Printing from type is used chiefly for stationery, business cards, circulars, and similar small jobs where only a small quantity is to be printed. It is not feasible for longer printing runs of many thousands of copy because type metal is relatively soft and wears down quickly, while it is on press. In addition, if the type is damaged the entire composition process has to be repeated.

1.3 LETTERPRESS PLATES:
The shortest printing runs the letterpress type form is replaced by a printing plate. The starting point of the plate making process is a negative of the image on film, line art, or continuous tone originals that have been made into halftone negatives.
The most widely used processes for making original letterpress plate involve three steps:-
Reproducing the image to be printed on the surface of a mental plate
Hardening the image on the plate
Etching away the non-image area of the mental surface. The last step leaves a positive image area raised above the background area and ready for printing.
(A) PLASTIC PLATES
Photopolymer plastics are also used for letterpress plates. These light sensitive plastics harden on exposure to light. The plastic plate is exposed by shining ultraviolet light through a negative in contact with the plate. Where the light strikes the plastics, the surface becomes hard and insoluble. The portion of the surface not exposed to light remains soluble and can be washed away, leaving the printing image raised the non-image background.
(B) DUPLICATE LETTERPRESS PLATES:
They are made from type and from original engravings of illustrations. Duplicate plates can be made quickly and inexpensively from original often several sets of duplicate plates are made to permit printing on several presses at the same time. The curved plated needed for rotary presses are made as duplicate plates from flat originals.
Duplicate plates are usually stereotypes.
Electrotypes or rubber plates.

1.4 LETTERPRESS PRINTING PRESSES
There are three major types of letterpress printing presses namely:-
Platen press
Flat bed cylinder press
Rotary press
All are based on the principle of inking the raised image on the former or plate and printing a sheet of paper against the inked image. The processes differ in the way in which the former or plate is carried in the press, and in the way in which paper is brought the paper is brought into contact.
THE PLATEN PRESS PRINCIPLE
The platen press consists essentially of two flat surfaces linked together. One surface, the bed is vertical and carries the former or prinking plate. The other surface, the platen, carries the paper and opens and closes on the bed by swinging on its linge. When the platen swings open from the bed, rollers ink the plate or form, and a sheet of paper is placed on platen. The bed and platen then close, pressing the sheet against the inked plate. As the press opens again, the printed sheet is removed and a new sheet is inserted as the plate is re – inked.
Many small platens, presses are hand-fed, but larger ones are fed by machine. Platen presses are relatively small and slow. They are used mainly for printing letterhead and circulars and for other small jobs.
PRINCIPLE OF FLAT BED CYLINDER SYSTEM
On flat-bed cylinder press, the form or plate is carried on a moving horizontal bed, and the paper sheet is carried on an impression cylinder sheets of paper are fed automatically to the impression cylinder, which holds them by clamps called grippers. During printing the bed moves under the rotary cylinder, which prints the sheets by pressing it against the inked plate. The cylinder is than raised as it continues to rotate, delivering the printed sheet and picking up a blank sheet to be printed. At the same time the be moves back and is re-inked for the next impression,
PRINCIPLE OF ROTARY PRESS
On rotary presses, both the plates and the papers are carried on the curved surface of cylinders. The most commonly used plates are carved stereotype or electrotype. The plates are inked by a series or rollers as the plate cylinder rotates, and the paper is printed as it passes between the inked plate cylinder and the impression cylinder. The paper may be fed either in sheet, on a sheet fed press, or in continuous rolls, on a web fed or web press.

1.5 IMAGE IDENTIFICATIONS
Image printed by letterpress printing process could be identified by the ink squash seen around the hard edges of the characters. Also, some slight identifications from the sheet is held face down towards a single light source.