Text 4 Offset Presses

The offset presses use the rotary principle, the cylinders roll against each other. The printing section of the press consists of three cylinders: a blanket cylinder around which the sheet of rubber is wrapped; a plate cylinder, with the metal plate wrapped around it; and the impression cylinder, which rolls and presses the paper against the blanket cylinder to create the impression.

Sheet-fed offset presses may print a single colour on only, one side of the sheet, or on both sides of the sheet in one pass through the machine.

However, there are the multi-colour machines used to print several colours in a single pass through the machine. Most often, these are four-colour machines, creating a full-colour effect by overlaying the four constituent colours yellow, magenta, cyan and black. Multi-colour presses allow the final result to be seen immediately and prevent some of the problems of loss of register.

Sheet-fed presses can print from 4,000 to 12,000 sheets per hour.

Web-fed offset presses These offset machines, which print on to reels rather than sheets of paper, are taking over from sheet-fed machines in many areas of printing.

Most web-offset presses are 'blanket-to-blanket' — that is, the web of paper runs between two blanket cylinders, so that both sides are printed simultaneously; each of the two blanket cylinders thereby acts as an impression cylinder for the other (see diagram). Speeds range from 13,000 to 50,000 impressions per hour. Many of these machines have four printing units so that they can print four-colour images.

a. Offset presses don’t perform the operations of dampening and delivering.

b. Multi-colour presses allow us to print several colours in a single pass.

c. The web-offset presses are better than sheet-fed presses in many areas of printing.

The screen printing press Many screen printing presses are manually operated and consist of a simple frame hinged to a flat surface. The equipment can be very cheap and, as such, is often used by people printing at home, There are also semi-automatic presses in which, while the screen is raised and lowered and the squeegee is pulled across the screen automatically, the material to be printed has to be inserted and removed by hand.

Automatic and hand-operated machines may have vacuum bases to help them separate the paper from the screen after printing. These consist of a flat piece of plastic laminate with regularly spaced holes connected to a vacuum pump; the vacuum suction holds the paper firmly to the base. Fully automatic presses also feed and deliver the paper (or whatever is being printed on) automatically, and some have an impression cylinder which holds the paper while the screen moves in unison and the squeegee remains stationary. These presses can attain speeds of up to 6,000 copies per hour.

For hand-operated presses used on short runs (a few hundred or even less) of items such as posters, the sheets are laid out to dry on racks. The semi-automatic and automatic machines often have drying tunnels and sometimes ultraviolet drying units. All screen printing presses are automatically operated. Hand-operated presses are used to print films.