Letterpress Printing, Flexography

On letterpress printing plates (fig. 3.1-52) the printing

areas are higher than the non-printing areas. A rightreading

negative film that is subjected to ultraviolet radiation

in contact with a carrier plate coated with a

polymerizable layer serves as the copy master for direct

letterpress printing. The polymer layer cures in the radiated

areas, unexposed areas are then washed off (see

also sec. 2.3.2). This produces a distinct relief, the side

wall slope of the halftone dot or line must be controlled

by the well-defined addition of diffusive light.

In indirect letterpress printing, the printing ink is first

transferred from the printing plate to a printing blanket

and then to the substrate. The technology is frequently

encountered in business forms printing and is

referred to as “letterset” (or “dry offset,” which is sometimes

confused with waterless offset).

The fundamental difference between direct and indirect

letterpress printing regarding film and platemaking

lies in the condition of the copy master. Since the intermediate blanket reverses the orientation, right-reading films are required for direct letterpress printing.The term right-reading always refers to a condition where the emulsion side faces the viewer.

In recent years flexographic printing has increased,

particularly in package printing (see sec. 2.3.3). Unlike

the letterpress described above, in which a relatively

hard printing form is used to print onto a substrate

with a soft impression cylinder, the flexographic printing plate is itself flexible, and it prints against a hard impression cylinder. Flexographic printing plates are produced as already described for letterpress printing; once again a right-reading negative film is used. For the production of rubber relief plates, a mold is made of a metallic relief plate into which a rubber sheet is pressed. However, these types of printing plates are made use of less and less.

Since the light-sensitive coatings used in letterpress

printing are negative-acting coatings, the radiation

should be monitored by a continuous-tone wedge. In

this way it can be checked what multiple of the basic

Fig. 3.1-52 Microphoto of a letterpress plate

exposure has been applied. The photochemical reaction

depends on this parameter. The photochemical reaction,

however, is not the only decisive factor for tonal

transfer; it is also affected by the degree of divergence

of the radiation (specular or diffuse).

In flexographic printing the tonal transfer can be determined on the inked printing plate using special

densitometers. Another measuring method is the

analysis of the microscopic image, for instance, using

the “Photoshop” software program. There are also special tone value measuring instruments on the market

that record the microscopic halftone image with a CCD

camera and analyze it automatically.

Дата добавления: 2015-11-16