Printing – Intaglio – Rotary
Reexamination Certificate
2000-05-24
2003-06-17
Eickholt, Eugene H. (Department: 2854)
Printing
Intaglio
Rotary
C118S621000
Reexamination Certificate
active
06578478
ABSTRACT:
FIELD OF APPLICATION OF THE INVENTION
The present invention relates to an arrangement for transferring an electrostatic charge within a gravure and flexographic printing unit in order to improve the print quality by polarizing the drops of printing ink on the printing plate cylinder. In the gravure printing unit, the electrostatic charge is applied to the outer circumference of an impression roller, from which it flows away toward the outer circumference of the printing plate cylinder. In the flexographic printing unit, the electrostatic charge is applied to the printing plate cylinder, from which it flows away both toward the substrate transfer roll and toward the back-pressure cylinder. Under the influence of an applied electric field, the ink molecules in the dimples in the printing plate cylinder (gravure printing) or those on the surface of the printing plate cylinder (flexographic printing) are polarized, and the ink droplets experience overall an increase in volume. A flowing electric current is picked up in order to supply the energy needed for the polarization work. As a consequence of the polarization, the ink droplets are attracted by the printing material and, moreover, the transfer of the ink droplets to the printing material led past is promoted by their increase in volume.
Thus, in gravure printing it is ensured to a significantly greater extent that the dimples in the printing plate cylinder are emptied satisfactorily, that is to say the printing ink is applied to the printing material. In flexographic printing, the electrostatic charge has the effect that the printing ink is transferred better from the substrate transfer roll to the printing plate cylinder and on to the printing material. Such arrangements are also referred to as “electrostatic printing aids”; they are used to achieve a full reflection density at all tonal levels and to avoid so-called “missing dots”. The problem of “missing dots” occurs in particular with rough printing materials, for example paper webs, having corresponding irregularities.
PRIOR ART
Electrostatic printing aids of the generic type relevant here have been known for decades (see, for example DE-A-27 09 254; EP-A-0 761 458).
FIGS. 1A and 1D
, in conjunction with
FIG. 1C
, show a two-roll system in a gravure printing unit having a multi-layer impression roller
1
—but here already having three layers, according to the invention—the printing plate cylinder
2
and the printing material
4
led between the two over the deflection roll
3
. Arranged above the impression roller
1
is a rod-like voltage electrode
5
extending over its entire length. The ink doctor
6
for wiping off excessively applied ink from the printing plate cylinder
2
is indicated. The inking roller and the ink return are situated in an ink trough
7
, but are not shown. The voltage electrode
5
is connected to a high-voltage source
8
. The circumference of the three-layer impression roller
1
has, on the outside, a semiconductor layer
10
and, underneath the latter, a highly conductive layer
11
. Located underneath the highly conductive layer
11
, as an electrical insulation from the impression roller core
13
, is an insulating layer
12
.
FIG. 1B
shows a three-roller system which, differing from the above-described two-roller system, has a supporting roll
9
, which is preferably electrically insulated, additionally arranged above the multi-layer impression roll
1
. Here, the voltage electrode
5
is positioned to the side of the multi-layer impression roller
1
.
FIG. 1E
, with the electric circuit diagram of the two or three-roller system according to
FIGS. 1A
to
1
D, illustrates the current flow within the electrostatic arrangements. From the high-voltage source
8
, a DC voltage U is fed to the voltage electrode
5
, and the voltage electrode
5
has the internal resistance R
1
. The air gap S—normally of the order of magnitude of about 5 mm to 30 mm—existing between the voltage electrode
5
and impression roller
1
, represents the resistance R
2
. The upper semiconductor layer
10
and the highly conductive layer
11
form the resistances R
3
, R
4
. The grounded insulation layer
12
acts as an extra large resistance R
5
. From the highly conductive layer
11
, the current flows through the semiconductor layer
10
which is arranged underneath and which here forms the resistance R
6
, and onward through the printing material
4
, which represents the resistance R
7
. The grounded printing plate cylinder
2
has virtually the resistance value R
8
=0 .
According to Kirchhoff's law of current distribution, the main proportion of the electric current takes the path of lowest resistance via the highly conductive layer
11
, while a small fraction flows directly to the printing material
4
via the semiconductor layer
10
. Finally, there is a voltage drop &Dgr;U between the lower semiconductor layer
10
and the ground E which constitutes the so-called nip voltage, which is critical for the polarization of the ink droplets in the dimples in the printing plate cylinder
2
. The current I flows to the ground connection E, starting from the voltage electrode
5
.
In order to apply the ink droplets from the dimples as completely and uniformly as possible over the entire width of the printing material—the web widths can nowadays exceed 3 m—sufficient energy has to be supplied, and the current flow has to be distributed uniformly over the entire impression roller width. In order to satisfy this requirement, the length of the voltage electrode has hitherto depended on the maximum usable width of the printing plate cylinder or of the impression roller, so that a charge distribution which is homogeneous in the impression area is ensured on said printing plate cylinder or impression roller (see DE-A-27 09 254, p. 11, lines 21ff.; OLLECH, Bernd: Tiefdruck—Grundlagen und Verfahrensschritte der modernen Tiefdrucktechnik [gravure printing principles and process steps in modern gravure printing technology], Polygraph Verlag Frankfurt am Main, second edition 1993, p. 343,
FIG. 15.49
; company publications from Eltex-Elektrostatik GmbH, Weil am Rhein, Germany, “ESA-DIREKT—Eine neue Dimension der elektrostatischen Druckhilfe” [ESA-DIREKT—a new dimension in electrostatic printing aids], publication no.: WP-d/e/f-9043-90/7-20,
FIG. 17
; and “eltex-Handbuch der Elektrostatischen Disziplin” [eltex-handbook of the electrostatic discipline], publication no.: Üp-d-0002-93/12-100, p.32, printing assistance, Figure top right) As a result, use is made of voltage electrodes over 3 m in length. Good print qualities are achieved with such voltage electrodes. However, the drawbacks are the relatively quick contamination of the exposed voltage electrodes, which lead to considerable losses in terms of their effectiveness and ultimately to complete failure, so that the print quality rapidly deteriorates.
In order to obtain the function of the electrostatic arrangements equipped in this way, a contaminated voltage electrode has to be disassembled, cleaned and installed again. This requires a number of personnel, leads to extensive losses in terms of machine downtime and is therefore often delayed in order not to threaten existing delivery times for the printed products.
In order to eliminate the abovementioned disadvantages, the consequence has been that electrostatic printing aids have been developed where, instead of by means of a long, fitted bar electrode, the current was introduced into the rotating shaft of the impression roller (see, for example, DE-A-28 10 452). Then, although the problem of a voluminous voltage electrode had been eliminated, and servicing had therefore been made easier, there nevertheless remains the requirement for frequent cleaning; added to this, however, was an increased outlay in insulating the core of the impression roller with respect to the printing machine.
In the further development, encapsulated electrostatic printing aids were developed, where the current was introduced via the impression rol
Eickholt Eugene H.
Selitto Behr & Kim
Spengler Electronic AG
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