Text 1 Printing presses

Printing presses fulfill a central role in the print production process They transfer ink to parts of the substrate/paper during a printing process in which text and image information are reproduced. In conventional printing presses the visually perceptible contrast between the printed and non-printed image elements is created using process-specific printing plates. Since, in the case of this printing process, the text and image information stored in a printing master cannot be changed, special plates have to be made and mounted in the press for each printing job.

The impression is created in the press in the “printing nip.” This consists of two surfaces that are pressed against each other, between which are the substrate and the inked plate. Technical development has progressed from the flat to the cylindrical impression surface and from the flat to the cylindrical master.

Process-specific contact pressures or forces (force per unit of area) have to be applied to transfer the ink from the master to the paper or other substrates.

These pressures (reference values) are:

• for letterpress printing 5–15 MPa,

• for flexographic printing 0.1–0.5 MPa,

• for offset printing 0.8–2 MPa,

• for gravure printing 1.5–5 MPa.

(1 Pa = 1 N/m2)

The impression is created in the press in the “printing nip.” This consists of two surfaces that are pressed against each other,between which are the substrate and the inked plate. Technical development has progressed from the flat to the cylindrical impression surface and from the flat to the cylindrical master.

In the platen press printing method, a plate (printing surface) that is moved in a vertical direction presses the paper against the plate. This principle was applied in the Gutenberg press and then later in platen presses. This principle is still used in flat-bed die-cutters and vertical embossing machines.

Flat printing plate with an engaged impression cylinder that is rotating and a plate moving to and fro, was used in flat printing presses. Because of the rolling motion, these presses,which are also known as “automatic cylinder presses”, enable higher printing speed to be achieved in comparison to platen presses.This method is still used in die-cutting and embossing machines and also in proofing devices.

The on-going development of the printing nip with cylindrical master and impression cylinder led to the high-output rotary printing method employed in all rotary printing presses.

Since both sheet and web substrate have to be printed, sheet-fed presses and web-fed presses were developed. In the printing couple of rotary presses there is a radial flexible layer for all printing processes, and this compensates for the variations in thickness of the plate and run out of the cylinder, and also makes it possible to print on rough paper.

In letterpress printing, a cylinder jacket of several layers of paper and cardboard is mounted on the impression cylinder. Its thickness of 1.25 to 1.75 mm enables compensating radial indentation to occur. In flexographic printing, the soft, flexible rubber or photopolymer plate enables radial deformation to occur. More recent developments have resulted in harder and thinner plates (<1 mm) that are glued onto a compressible packing (film or sleeve).

In offset printing, an additional blanket cylinder is configured between the plate cylinder and the impression cylinder, on which the radial flexible blanket is mounted.

In gravure printing, a thick-walled rubber layer on the impression roller presses the paper onto the ink-filled cells of the form cylinder.