EP0056623B1 - Rotary flexoprinting and indirect rotogravure printing process - Google Patents

Abstract

In a rotative printing process, especially in flexographic printing and indirect gravure (plate) printing, heat is supplied to the solvent containing printing ink so as to evaporate a portion of the solvent and ensure a reliable emptying of the screen-roller cups or the gravure printing cylinder recesses, whereupon, upon cooling the droplets of ink, there is again obtained a higher viscosity which is advantageous for effecting a print with sharp contours and stable forms.

Description

The invention relates to a rotary flexographic or indirect gravure printing process.

In flexographic printing and indirect gravure printing, a relatively low-viscosity ink provided with a high proportion of volatile solvents is transferred to the printing form and from there to the printing medium using suitable aids. The demands to be placed on the color in order to achieve an economical driving style with high speeds and high print quality contradict each other, which is why the known solutions represent a compromise. So z. B. the ink is kept low-viscosity by a high proportion of volatile solvents to facilitate the distribution until it is applied to the print carrier, to favor the emptying of the wells on the gravure cylinder or in flexographic printing on the anilox roller and the drying paths and the drying energy required afterwards to keep it within reasonable limits. However, the low viscosity also has the consequence that the ink droplets removed from the cups on the gravure cylinder or anilox roller during the process cannot be transferred with sharp contours and dimensionally stable, particularly in flexographic printing, so that blurring occurs due to dot gain. In addition, the need to avoid low-viscosity paints makes it more difficult to replace highly volatile solvents with difficult-to-evaporate liquid media, e.g. B. water.

The invention is based on the object of designing a rotary flexographic or indirect gravure printing process in such a way that, despite the high printing speed, a sharp-contoured, dimensionally stable dot with as little dot gain as possible is obtained on the printing medium.

This object is achieved by the features in the characterizing part of claim 1. A paint with a low proportion of liquid components can be used in this process. When the printing ink is heated, the solvent contained in the ink is at least partially evaporated. This makes the paint more viscous, but this effect is partially compensated for by the heating of the solid content in the paint. When the printing ink is heated in the grid cells, this effect is further compensated for by the fact that the solvent expelled by the heating presses against the wall of the grid cells and promotes the removal of the ink, so that the emptying of the grid cells is not impaired overall. The subsequent cooling of the color when transferred to the print medium, the ink droplets contracts, with a very sharp grid ^p oint achieved due to reduced solvent content. The dot gain can be reduced to a minimum, so that it is possible to work with a screen fineness that is not possible in flexographic and indirect gravure printing.

A further advantage of the process according to the invention is that, instead of volatile solvents, it is possible to use liquid media which are difficult to evaporate in the paint composition and which, moreover, can be non-flammable or hardly flammable. For example, water colors can be used in the printing process, with the heating of the printing ink during the printing process at least partially evaporating the liquid components, so that a relatively dry ink gets onto the print medium, while in the previous distribution process sufficient liquid components in the color are included, which ensure easy filling and emptying of the grid cells. This also promotes drying of the finished print.

Although it is known to heat the printing ink in various printing processes, none of these known processes gives an indication of the problem of dot gain described above and its solution. It is known from DE-A-2 635 226 to use solvent-free printing inks that are solid at room temperature in gravure and flexographic printing, which are already melted in the ink fountains of the printing device and applied in the liquefied state. Correspondingly, US-A-2322445 teaches to use a color substance which is solid at room temperature in the gravure printing or offset method and to make it viscous by heating in order to be able to carry out the color transport and the color distribution. The aim is to avoid the difficulties of drying normal printing inks on the printing medium. If such printing inks, which are solid at room temperature, cool from the molten state back to room temperature, the printing ink solidifies without separate drying.

From US-A-2 711 132 it is known in connection with a direct gravure printing method to heat the doctor blade in order to further dilute the liquid printing ink and to be able to introduce it better into the cups on the surface of the gravure cylinder. However, the gravure cylinder is cooled with the screen cups, which is intended to improve the ink transfer to the print carrier. Such cooling, however, is in contrast to the subject of the invention, since in the latter the cylinder having the grid cups is not cooled but heated.

Subclaims 2 to 4 represent advantageous developments of the method according to the invention.

• Fig. 1 schematisch ein Flexodruckfarbwerk,
• Fig. 2 ein Farbwerk für indirekten Tiedruck,
• Fig. 3 in einem Teilschnitt einer Rasterwalze oder eines Tiefdruckzylinders ein Näpfchen mit darin befindlichem Farbtröpfchen, und
• Fig. 4 einen schematischen Schnitt durch eine beheizbare Raster- oder Tiefdruckwalze.

For example, embodiments of the invention are described below with reference to the drawing tion explained in more detail. It shows

• 1 shows schematically a flexographic printing ink unit,
• 2 an inking unit for indirect printing,
• Fig. 3 in a partial section of an anilox roller or an intaglio cylinder a well with ink droplets located therein, and
• Fig. 4 shows a schematic section through a heatable anilox or gravure roller.

1 and 2, 1 denotes an ink tray in which there is printing ink. In the flexographic printing according to FIG. 1, the printing ink is taken up from the ink tray by means of an immersion roller 2, while in indirect gravure printing according to FIG. 2 the ink is fed to the printing unit through an ink application nozzle 10 and the ink tray 1 is only used to catch excess ink. During the flexographic printing, the printing ink is transferred from the dipping roller to an anilox roller 3, over the surface of which wells 11 (FIG. 3) are formed, which receive the ink. The surface of the anilox roller 3 is scraped off by means of a doctor blade 4 in such a way that there is no ink on the webs between the cells 11. The same process takes place in the indirect gravure printing method according to FIG. 2 on the cylinder 3, which is at least partially provided with raster cups and is referred to below as an anilox roller.

In flexographic printing (FIG. 1), the ink droplets 12 located in the wells 11 must be transferred to a printing plate 6 mounted on a printing plate cylinder 5, from which the printing ink is transferred directly to the printing medium 8. In indirect gravure printing (FIG. 2), ink droplets 12, similar to flexographic printing, are first transferred from the cups 11 of the anilox roller 3 to the transfer cylinder 5 ', which applies them to the print carrier 8. 7 in the figures is an impression cylinder.

The anilox rollers 3 provided with the cups 11 are heatable, so that the ink droplet 12 is heated at least in the area of the bearing surfaces on the walls of a cup 11 to such an extent that the droplet 12 easily and completely detaches from the cup when the surface 13 of the Droplet comes into contact with the cylinder 5 'or the pressure plate 6.

The anilox rollers 3 can be heated by electrical heating elements which are installed in the roller body near the surface and are controlled by temperature sensors so that a predetermined temperature is constantly maintained.

According to another embodiment, the heated rollers can be designed as a double-jacket roller or hollow cylinder through which a heated liquid medium flows, for example thermal oil or water, the temperature of which is controlled as a function of the surface temperature of the anilox roller 3.

As an embodiment, which is characterized on the one hand by low design effort for the supply of heat and low susceptibility to faults, on the other hand ensures a good and as uniform as possible heat distribution, a hollow anilox roller 3 according to FIG. 4 is provided which z. B. is completely or partially filled with thermal oil 14. In the illustrated embodiment, a partial filling is provided so that an air space 15 remains, which is provided for pressure equalization and mixes the thermal oil 14 during rotation, the wetting of the entire inner surface of the jacket 17 being ensured. To heat the liquid 14, axially extending electrical heating elements 16 are arranged, which are controlled by temperature sensors, not shown.

The ink droplets 12 located in the etched cells 11 are supplied with heat by the heated surface of the roller, whereby the viscosity of the solids content of the ink droplets is reduced while liquid components of the ink are evaporated. Only the droplet surface 13 is exposed to the ambient temperature and therefore retains a higher viscosity. This in turn causes a certain adhesive and shape stabilizing effect during the subsequent transfer to the pressure plate 6 of the plate roller 5. The low viscosity in the contact area of the droplet 12 due to the supply of heat results in good and almost complete removal of the ink droplet from the well, the ink droplet after Loosening it releases its heat back to the colder environment and becomes more viscous. As a result, sharp-edged single points transferred with little growth are achieved with a low content of liquid components that are still to be removed.

The cooling of the paint droplets on the cylinders 5 can be accelerated by additional cooling, for example by blowing the cylinders 5 with cooling air. However, it is also possible to keep the cylinders 5 at a correspondingly low surface temperature by means of a cooling medium flowing through the cylinders.

If, instead of a color with a low proportion of liquid components, a printing ink is used which, for reasons of better transport and distribution of the color, initially has a relatively high content of liquid components, for example a water color, then part of the liquid components is evaporated by the supply of heat , while at the same time the solids content becomes more liquid due to the heat supply. In the case of a change in the viscosity of the solids content due to the supply of heat, this is a reversible effect by cooling, while in the other case a permanent effect or a higher viscosity is achieved when liquid constituents are evaporated.

The printing ink used is usually filled in canisters and has room temperature. For easy transport to the anilox roller 3 and to ensure a uniform distribution and possibly to mitigate too large a temperature difference between the roller surface and the ink, it may be expedient to preheat the ink by means of device parts located in front of the rollers 3 and / or in the ink can. All contact surfaces with which the printing ink is in contact come into consideration for this heat supply. It can be pipelines, the ink tray 1 or other distribution and / or transmission elements.

The surface temperature of the heated rollers or the heating of the printing ink depends on the type of ink used, its viscosity and temperature, the technical characteristics of the printing press, the running speed and the like. It can vary widely and can be determined empirically in a simple manner for the respective conditions. Instead of or in addition to the heat supply described by heating device parts, the printing ink can also be heated by radiation, for example by means of infrared or microwaves. The surface of the anilox rollers 3 is preferably heated here. In particular in connection with heating by infrared, in which the surface 13 of the ink droplets 12 also has a lower viscosity, it can be advantageous to cool the cylinder 5, 5 ′ or the print carrier 8 itself in order to ensure that the surface 13 of the Color droplets to achieve an adhesive and shape stabilizing effect.

Since when using a printing ink with low liquid constituents and a relatively high viscosity compared to the known printing inks, a lower viscosity only has to be present for emptying the wells 11, while it has a higher viscosity on the printing medium 8 or already on the rollers 5, 5 ' cooling of the print carrier 8 can be considered before entering the printing unit.

Claims (4)

1. Rotative flexographic printing process or rotative indirect gravure printing process, respectively, characterized by the fact that the solvent containing ink is heated only after filling of the screen cups of the screen roller or of the gravure printing cylinder, respectively, for at least a partial expelling of the solvent and thereafter it is transferred to the printing carrier, the ink being allowed to cool during transfer.

2. Method according to claim 1, characterized by the fact that the ink is heated by heating the screen roller or the gravure printing cylinder, re spectively.

3. Method according to claim 1 or 2, characterized by the fact that the ink is heated by irradiation from the outside.

4. Method according to claims 1 to 3, characterized by the fact that a cooling device is used for cooling the ink during transfer to the printing carrier.

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