Gravure Printing

The distinctive feature of gravure printing technology is the fact that the image elements are engraved into the surface of the cylinder. The non-image areas are at a constant, original level. Prior to printing, the entire printing plate (non-printing and printing elements) is inked and flooded with ink. Ink is removed from the non-image (by a wiper or blade) before printing, so that ink remains only in the cells. The ink is transferred from the cells to the printing substrate by a high printing pressure and the adhesive forces between printing substrate and ink.

Rotogravure printing is used for the economical production of long print runs.

Gravure printing forms are usually cylindrical. A special feature of industrial rotogravure printing is the fact that a whole cylinder (and no plate) is used per color separation.

This means that in a four-color press four separate cylinders have to be changed for each new job. Consequently, a company that has a lot of repeat jobs is forced to store a large number of cylinders. Depending on the printing format, gravure printing cylinders are generally rather heavy and require special conveying and handling gear systems.

It should be noted here that only variable-depth gravure printing and more particularly variable-area/variable-depth gravure are of any significance, due to their high quality. Pure variable-area (halftone) gravure is scarcely used nowadays.

Traditional, that is to say only variable-depth,

gravure printing is also increasingly losing its importance since the printing plate production is based on complicated copying and etching processes that are almost impossible to standardize. It is for this reason that variable-area/variable-depth gravure printing processes, which in industry are based on electronic/

mechanical engraving (using a stylus) of the gravure cylinder, are becoming prevalent.

There are two familiar options for the structure of the

gravure cylinder). The steel cylinder has an electroplated coating of base copper (typically about

2 mm thick), with an approximately 100 mm-thick coating of engravable (cuttable) copper applied onto it. The steel cylinder has either the directly electroplated engraving copper or a so-called ballard skin on the base

copper layer. This peeling layer is also applied by electroplating onto the base copper, over a separating layer, and the print image is engraved into it.

In screening for gravure printing, the image is broken

up into printing elements, the cells, and then the

non-printing elements, the cell walls. The cell walls

serve to guide the blade when excess ink is being

stripped. After the doctoring process, ink remains only

in the cells. If ink were to remain on the cell walls,

scumming would occur in the print, and if there were

localized blade defects, blade streaks.

The cells and cell walls can be detected. On the one hand, this cell structure can be seen in the printed product and on the other, the “flowing apart” of the individual image elements can also be seen – the individual cells are no longer visible in the impression because of “cell wall flooding.” Gravure printing cylinders for variable depth gravure printing are produced by etching.

In the electromechanical engraving the scanning drum and the form cylinder to be engraved are either connected to each other by mechanical means or by a “self-synchronous system.” A bromide film (the positive of the print image) is scanned optically

on the scanning drum with the respective gradation

and density range. A corresponding amount of light is reflected, depending on the brightness of the pixel which has already been scanned on the bromide

film and this is converted into an electric signal.

The output signals of the scanning head, the image

data, are processed electronically in the computer and

the engraving head. The computer feeds two signals

to the engraving head – the actual image signal and

the screen signal defining the screen resolution and the angular position. Both produce a modulated signal to control the engraving head.

The angular position of the screen is produced by the

superimposed motion of the engraving head (feed and

frequency) and the cylinder (circumferential speed).

Nowadays engraving is mostly done directly instead of

scanning a bromide film, with the signals for the engraving head coming from the digital database containing the printing cylinder specifications.

Engraved printing cylinders are more prone to “missing dots” (cells which do not print completely) than etched printing cylinders since the cells transfer less ink to the printing substrate due to their shape.

Electrostatic printing aids that raise the meniscus of the

ink surface in the screen cells for improved wetting of

the printing substrate are used to eliminate or prevent

“missing dots“ in rotogravure printing.

The very high costs that printing cylinder production

entails is the decisive reason why rotogravure printing

can only be used cost-effectively to print mass-circulation products (run length > 500000).

The screen resulting in gravure printing (and also in

etching) is a periodic screen. A moiré effect is often produced in multicolor printing when periodic screens are used. The moiré effect is minimized in offset and/or letterpress

printing by the screen being suitably angled for

each individual printing ink. In gravure printing a

screen angle can be simulated by elongating or compressing the cells.

The gravure cylinder is immersed in the ink, which

is contained in an ink trough. The gravure cylinder cells are completely flooded in the ink trough. Excess ink is removed by the blade, so that ink remains only in the cells and the cell walls are free of ink. Hydrodynamic

back-pressure is built up beneath the blade, which mainly depends on the contact angle of the blade, the speed of the press and the viscosity of the ink. In modern, very high-powered gravure printing presses, a steep angle of blade contact is now mostly preferred.

In multicolor gravure printing,drying must be carried

out downstream of each printing unit (drying zone)

since wet-on-wet printing, such as offset printing, is

not possible with low viscosity gravure printing inks

(typical value approximately 0.1 Pa· s).

In Pad printing an intermediate carrier is

used for the print image rather than printing directly

onto the printing substrate from the printing cylinder.

Дата добавления: 2015-11-16