Offset Printing

On an offset printing plate (fig. 3.1-47) the printing and

non-printing areas are virtually planar.

The platemaking process using positive film and an

appropriately designed positive-acting plate is called

positive platemaking. The light-sensitive coating of the

plates is usually composed in such a way that during

processing all exposed areas are removed, as a result of

which the printing plate is ink-repellant in those areas

(see also fig. 2.1-4).

Accordingly, negative platemaking is the name given

to platemaking processes using negative films.Printing

plates with a negative-acting light-sensitive coating are

used for this purpose. During processing the coating is

removed from all areas that have not been sufficiently

radiated. Radiation curing takes place in the ink-accepting

areas.

Before the printing plates for the individual colors

are exposed or irradiated, they are first punched for fixing

the film/master copy. Very often the plate is also

prepared with appropriate register holes for mounting

it in the printing press. Alternatively, this can be done

after processing, then the punching tool can be aligned

(possibly automatically) to the register marks.

In conventional prepress a lithographic printing plate

may be produced by projection or contact exposure.

Projection exposure systems (reflection or transmission)

are used in black-and-white book printing and in

newspaper printing (as well as in screen printing),with

less stringent requirements on quality.A projection imagesetter

works on the same principle as an epidiascope

(for reflection copies) or as a slide projector (for transmission

copies). The page-size master copy in the form

of paste-up on paper or of film (fig. 3.1-48) is projected

onto a computer-controlled traversable printing

plate; consequently sheet assembly takes place during

the exposure of the printing plate.

With the step-and-repeat copying machine (fig.

3.1-49), a complete assembled sheet/plate-size film is

not required.With this machine the pages of film inserted

in a film magazine are exposed according to a

preset program onto the printing plate by the contact

exposure method.

For exposure in a vacuum frame (fig. 3.1-50), a film in

the full format of the printing plate is required. The

film/sheet assembly is fixed on the plate in accurate register

with register pins and placed in the vacuum frame.

It is located between a flexible blanket and a glass plate.

The space in between is evacuated with the result that

the air pressure presses the blanket against the glass

plate, thus ensuring satisfactory contact between assembly

and plate. This is followed by exposure with UV

radiation.

The photochemically reactive layer of the printing

plate reacts to the actinic light of the radiation source.

To ensure good processing performance a minimum

energy per area must act on the printing plate. As with

Fig. 3.1-48

Projection exposure system (transmission technology) for offset and

screen printing platemaking (Proditec Projektionssysteme)

Fig. 3.1-49 Step-and-repeat copying machine (Krause-Biagosch)

film producing, the timing of the exposure process depends

on the radiation source and the plate material.

The radiation is mostly generated by a burner with a

metal halide lamp. The radiation comprises parallel

and a variable portion of diffusely oriented rays. The

latter can be increased considerably by interpositioning

a dispersion sheet/film. This is used in positive

platemaking to prevent film edges and dust showing on

the plate. Undercutting might be an unwanted effect

occurring where too much irradiation reaches below

the blackened parts of the film (see also sec. 3.1.5).

Processing (wet/chemical) is done manually in its

simplest form, but preferably in an immersion tray or

a plate processor. This is an integrated device, in which

the printing plates pass through the developing, rinsing,

and preserving stations.

Before preservation, the plate can be checked for

errors and corrected manually if necessary. For a socalled

“ minus correction ” the unwanted printing areas

are etched with a deletion fluid, which is applied with

a correction pen or a brush. The “ plus correction” is

more difficult. Only very simple changes can be made,

such as filling in holes and reverse characters in solid

areas. To do so correction varnish is applied to the areas

concerned after any already existing preservative

layer has been washed off.

The correction and preservation stages, as well as baking,

form part of the after-treatment. In the preservation

process, the plate is covered with a thin solution of

“ gum arabic ” or similar chemical, which gives the nonprinting

areas storage-resistant hydrophilic properties.

The hardness of the light-sensitive coating is increased

during baking, and higher print run stability is achieved

as a result.

For correct exposure the following requirements

must be met:

• The entire tone value range that is usual for the

printing process must be transferred to the printing

plate.

• The tonal change from the film copy to the printing

plate should lie within narrow tolerances.

For positive plates the control of the plate is done in accordance

with a microline patch on a control wedge. The

finest microlines shown on the printing plate should

normally belong to the 12 µm or 15 µm steps (for business

forms: 20 µm). An example is shown in figure

3.1-51 [3.1-13]. Especially the continuous-tone patch, in

conjunction with the microline patches, is used as a

means of control for the negative copy. The following

specifications and/or standards give more detailed rules:

• standardization of the offset printing technology in

accordance with BVDM/FOGRA (see sec. 13.2.3),

[3.1-14],

Fig. 3.1-50 Vacuum frame with inserted dispersion sheet (Sack)

• standardization of multicolor newspaper printing

[3.1-15],

• standardization in the printing of business forms

(see sec. 13.2.3), [13.2-28],

• DIN 16620 Part 2 (see sec. 14.4) or the subsequent

ISO standard,

• ISO 12218 for offset printing in general

(see sec. 14.4), [3.1-16].

Measurement of the tone values on the printing plate

is not necessary if they can be determined from the pertinent

FOGRA copy table once the microline reading

is known (fig. 3.1-51). For conventional plate materials

with a diazo layer, the copy table corresponds to the

microscopic results. This is not always the case for digitally

imaged plates (computer to plate).

As a result of the intended slight undercutting of the

blackened sections of film there will be a decrease in

tone value from film to printing form with positive

platemaking and a correspondingly large increase in

tone value with negative platemaking. The tone value

difference is approximately 3% in the middle tone. This

difference is usually taken into account in prepress. It has

also been considered in the process standards for lithographic

printing (ISO 12647 Part 2; see sec. 14.4), and for

newspaper printing (ISO 12647 Part 3; see sec. 14.4).

For film-dependent platemaking the exposure

process is controlled by means of a “light integrator.”

This is a device that automatically calculates the product

of radiation intensity and time during the exposure

process. The radiation is terminated as soon as the set

value has been reached.

The coating of the printing plate in accordance with

the image is a special platemaking process used for small

formats in offset printing.The image on the film or paper

original is transferred to the plate (very often it is

made of a material similar to paper having suitable offset

properties) by means of toner as in an electrophotographic

printer. After the toner image has been fixed,

the plate can be used for simple printing work, mostly

for single-color jobs or together with spot colors.

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