GB2091641A - Damping and Inking in Printing Machines - Google Patents

Abstract

An arrangement for metering the moistening agent for offset printing machines, which arrangement comprises a moistening mechanism, in use normally being inked up, which consists of at least two rollers 12, 14 and a metering roller 13 (or a pressure roller system) for the moistening agent, the ink film in the moistening mechanism, that is saturated with moistening agent, being subjected, between the two rollers 12, 14, to a shear gradient by means of positively induced different peripheral speeds (slip) between these two rollers. A blowing device can be arranged to the rear of the moistening-agent application zone but forwardly of the offset cylinder, and can be directed towards the stereo cylinder 6, as shown by arrow A. <IMAGE>

Description

SPECIFICATION Improvements Relating to Printing Machines The present invention relates to offset printing machines, and in particular to an arrangement for metering the moistening agent used in such machines.

In the prior art, the transfer of the moistening agent to the picture-free (i.e. the ink-free) areas of a plate is achieved in different ways, i.e. either by means of application rollers of the mostening mechanism which are separated from the inking mechanism, are free from ink and are resiliently coated to accept moisture, or by means of application rollers of the moistening mechanism, which are separated from the inking mechanism, are inked up and wetted or cross-linked with moistening agent and are resiliently coated to accept ink. The transfer of moistening agent can also be carried out by means of the application rollers of a combined inking and moistening mechanism which are linked up and moistened or cross-linked with a moistening agent and are resiliently coated to accept ink.Less frequently, use is made of systems which apply the moistening agent to the plate without contact therewith, i.e. with the help, for example, of a moist vapour which is applied to cooled cylinders.

The moistening agent is metered by means of moistening-agent rollers or vibrator rollers which accept inking agents and which intermittently supply moistening agent to the inking means of the combined inking and moistening mechanism.

For the purpose of metering the moistening agent, use is also often made of ductor rollers which accept moistening agents, are pressed against ink-supply metering rollers and usually turn relative to each other at a variable reduced speed.

Continuously follower rollers which accept ink and meter a film of moistening agent in the zone of pressure between, for example, a container roller and a metering roller, are adequate for many printing requirements.

The maintenance of the balance between the inking and moistening agent (i.e. the cross-linking of the ink in the inking mechanism with the smallest possible quantity of moistening agent that enables the picture-free areas to be kept free of ink on the plate) is the actual function of the moisture-metering installation; the original object therefore no longer applies, i.e. that of wetting the plate and thus keeping it free of ink in the picturefree areas by using the effect whereby ink and water repel each other, so that the inking mechanism is kept free from moistening agent.

In contrast with existing views, the film of moistening agent divides upon each contact of an ink-supply roller with a moistened or a moistened and inked-up roller, i.e. with a picture-carrying or picture-free area of a plate.

Measurements of the thickness of the layer of moistening agent on the plate in the case of a separate print show that the inking agent and the moistening agent do not constitute appreciably separated layers on the rollers and that the distribution of the moistening agent cannot be divided into "inner and surface water" even in terms of a broad interpretation. In the case of a prototype inking and moistening mechanism system (Rotaprint R 35rand by pressing a container roller, which moves without slip relatively to the container roller, a film of moistening agent is dispensed which permits a separate print in a safe zone between the water galley-proof limit (too much moistening agent) and the smearing limit (too little moistening agent).By blowing cool or heated air on to the inking-moistening agent layer immediately beyond the metering position, the thickness of the layer of moistening agent-measured on the plate following the last inking roller-can be reduced only very slightly. On the other hand, by blowing air on to the layer of moistening agent on the plate at a point beyond the inking roller and the apparatus for measuring the thickness of the layer of moistening agent, it is possible to alter the layer of moistening agent to such an extent that with the setting for metering the moistening agent unchanged, tinting of the plate takes place (i.e. because of the lack of moistening agent, the picture-free areas of the plate also transfer some of the ink).

A shear gradient due to slip of the ink, crosslinked with moistening agent, between the application roller and the container roller of the moistening mechanism of a Rotaprint R 35 machine reduces, in the case of otherwise unchanged metering of the moistening agent and because of the pressure between the container roller and the metering roller, the thickness of the layer of moistening agent is dependent upon the slip setting, until tinting of the plate occurs.

The combination of a suitable device for metering the moistening agent and a shear gradient due to slip, and/or with a blowing device for drying off the plate offers many advantages.

The use of a shear gradient due to slip is known in various applications, for example from DE--OS 29 16047,DE-AS 1611 208,DE- OS 28 12 998 and DE-AS 28 32 172. In these instances however, the slip is provided between an ink-free and an ink-supply roller of the equipment for supplying the moistening agent, for example, between an ink-free container roller made of chromium and an ink-supply rubber or plastics roller, in which system the axes of the two rollers may also be set slightly obliquely relative to each other.

The present invention seeks to improve the known means for metering the moistening agent (container roller with a metering roller or with a vibrator roller etc. by making the moistening agent supply more variable and at the same time more accurate than has been hitherto possible.

According to the invention, there is provided an arrangement for metering the moistening agent for an offset printing machine having an inksupply moistening mechanism comprising at least two ink-supply rollers and a standard metering or vibrator-roller means for a moistening agent, the arrangement being such that the ink film in the moistening mechanism, that is saturated with moistening agent, is subjected, between the two ink-supply rollers of the moistening mechanism, to a shear gradient by means of positively induced different peripheral speeds (slip) between these two rollers. In addition, the blowing device, directed towards the stereo cylinder is also of importance.It thus becomes possible to print very closeiy to the tint limit if required, i.e. to print in such a way that practically no moistening agent passes from the plate on to the rubber cloth and therefore on to the paper, this being of advantage particularly in the case of moisture-sensitive papers.

The advantages and disadvantages of known moistening mechanisms including metering means are set forth particularly clearly in an extract in FOGRA-Forschungsbericht 3216 of May 1976, page 9, these results having been compared with the present invention within the Applicant's works (see attached appendix). It has been found that, on the basis of all investigatory criteria, the subject-matter of the present invention is superior to, or at least equates with, the known moistening mechanisms.

The conventional vibrator moistening mechanism exhibits particularly marked fluctuations in the moistening agent from sheet to sheet and over the width of the plates. The three alcohol moistening mechanisms require means for cooling the moistening agent and continuous topping up of the alcohol component for ensuring high quality of printing when single editions are run off (i.e. in the case of large impressions). The ink-supply metering roller, which is pressed against the chromium ductor and is driven at a differing speed, tends towards an annular construction, which negatively influences the supply of water. Furthermore, means for cooling the moistening agent are used in order to stabilise the balance between inking and moistening.In contrast with the above-discussed prior art, the present invention is based on the knowledge that ink and water cross-link with each other, and only moistening agent is supplied to the moistening mechanism during separate printing and is discharged towards the plate, whereas practically no ink-exchange takes place and yet the thickness of the film of moistening agent can be altered by a shear gradient (slip).

Figure 1 of the accompanying drawings shows a general view of an offset printing machine in which the arrangement of the present invention can be used. The machine comprises an offset cylinder 20 and counter-pressure cylinder 21 between which sheets pass on their way from an input reception table 22 to an output reception table 23. After passing between the cylinders 20 and 21, the sheets pass through a forward marking device 24, drawing marking device 25, infeed roller mechanism 26 and suction pad device 27. Reference 28 shows a preliminary stacking table.

Figure 2 shows, on a larger scale, the moistening and inking mechanism of the offset printing machine illustrated in Figure 1. The rubber-cloth cylinder and the counter-pressure cylinder have been omitted from the drawing, since only the moistening mechanism, together with the stereo or pattern cylinder, designated by the numeral 6, is of real importance.

The duct roller is shown at 1 in Figure 1. In the known manner, an ink vibrator roller 2 can intermittently receive an ink strip from the roller 1 with the aid of an appropriate swinging movement, so as to transfer the ink strip to an intermediate roller 3. The ink strip is transferred from this intermediate roller 3 to a rubbing roller 4 of the inking mechanism, which latter roller, in the known manner, swings to and fro in the axial direction during its rotation.

A number of smoothing rollers 16 are mounted to the rear of the rubbing roller 4 in the direction of rotation of an ink-applying roller 5. Provided to the rear of the last smoothing roller 1 6 is a pair of jockey rollers 11, 11', the upper jockey roller 11' being adapted to be swung about the lower jockey roller 11 in order to establish a cleaning and inking connection with the moistening mechanism at the commencement and termination of printing.

A rubbing roller 15, associated with the moistening mechanism, is provided on that side of the ink-applying roller 5 that is opposite the rubbing roller 4. The rubbing roller 1 5 is spaced from the ink-applying roller 5 but is close to the pair of jockey rollers 11, r. Although the moistening mechanism also supplies ink, its main function is to apply a moistening agent.

The moistening mechanism will now be described. With the aid of a moisture-box roller 12, the moistening mechanism picks up moistening agent from a container and transfers it to the moisture-applying roller 14, and the quantity of moistening agent dispensed, in the construction illustrated, is determined by a metering roller 13 which likewise rolls around the moisture-box roller 12. By means of relatively firm pressure of the metering roller 13 against the moisture-box roller 12, it is possible, in the known manner, to adjust the quantity of moistening agent picked up by the moisture-box roller down to a specific lower limit below which it.does not fall even as a result of increased pressure, unless other steps are taken.

The moisture-applying roller 14 is arranged to roll on the stereo cylinder 6 at a position immediately in front of the ink-appiying roller 5 as seen in the direction of rotation of the stereo cylinder, and during the normal printing process only the rubbing roller 1 5 acts on the moistureapplying roller 14.As previously mentioned, the rubbing roller 1 5 is always at a slight distance from the ink-applying roller 5, but at the same time it is so close to the pair of jockey rollers 11, t that, for the purpose of the cleaning and inking-up process, connection between the inking mechanism and the moistening mechanism can be established at the commencement and termination of printing by the pair of jockey rollers 1 1, 11' being swung about the axis of the roller 11 towards the rubbing roller 1 5 so that the jockey roller 11' moves into cdntact with the rubbing roller 15, as shown in dotted lines.

Through this contact, the moistening mechanism is inked up when printing begins, and the entire inking mechanism is thereby cleaned again when printing ends.

At that side of the moisture-applying roller 14 that is opposite the stereo cylinder 6, a plurality of smoothing rollers 20 are provided on the moisture-applying roller 14. The ink-applying and moisture-applying rollers 5 and 14 respectively are made of similar material and their construction is known per se. The same applies as regards the ink-rubbing roller 4 and the moisture rubbing roller 1 5 which turn in opposite directions.

The entire moistening mechanism is designed to accept ink and, as previously mentioned, it is inked up. The metering roller may be made of resilient pore-free material for example, this material having a Shore hardness of between 25 and SOC and a peak-to-valley depth Rz of 6 20 ,am, whereas the moisture-box roller and the moisture rubbing roller may be coated with hard rubber or an equivalent hard plastics material, and the moisture-applying roller is coated with a resilient pore-free material having a Shore hardness of 30 to 45 and a peak-to-valley depth Rz6Z5 ym. The smoothing rollers 20 are made of a material that accepts ink or are coated with such a material, and they have a Shore hardness of 80 and a peak-to-valley depth of 6 25 ym (1,um=10-6 metres).

Within the framework of the present invention it is immaterial how the moistening agent is extracted from the moisture box. It is not essential that, as provided for in the described embodiment, a metering roller should be used, in conjunction with a moisture-box roller, for effecting this operation. A vibrator roller, for example, could be used for supplying the moistening agent.

However, it is important that a shear gradient due to slip should occur between at least two ink supply rollers of the moistening mechanism. This can be achieved, for example, by the use of two ink-supply rollers of the moistening mechanism that are shape-locked i.e. by way of toothed wheels, for example, and are so driven that a slip/shear gradient occurs at their line of contact so that the ink film, containing the moistening agent, is extended on the roller that is more remote from the moistening-box roller. In the embodiment described, this may be achieved by driving the moisture-box roller 12 and the moisture-applying roller 14 in a positive manner at peripheral speeds which always differ from each other. This shear gradient due to slip may, however be set at various levels by means of a suitable form of the driving connection between these two rollers.The shear gradient may also be produced by variable elongation slip on the part of an elastic roller.

It has already been mentioned that only by means of a moisture-box roller in conjunction with a metering roller is it possible not to fall below a specific quantity of transferred moistening agent, even when heavy pressure is applied. However, by providing a shear gradient between the moisture-box roller and the moisture-applying roller (in the example described), it becomes possible to reduce the quantity of moistening agent still further.Thus, by means of the shear gradient or slip, it is possible, during transfer, between these two rollers, of the ink cross-linked with the moistening agent (which is embedded therein), to reduce the thickness of the layer of moistening agent on the application roller in dependence upon the slip setting, despite the act that metering of moistening agent cannot be further lessened by increasing pressure between the container roller and the metering roller, said reduction proceeding until tinting occurs on the plate.

It is not possible at present to explain precisely the mechanism whereby no flow of ink occurs in the moistening unit, but it is probable that flow of moistening agent takes place, and the moistening agent can be metered by means of a shear gradient due to slip at the layers of ink and moistening agent, which are not separated from each other, metering taking place without buildup of ink and/or moistening agent.

This form of construction of the moisteningagent metering means can be extended by the provision of a conventional blowing device which acts on the stereo cylinder in the direction of rotation forwardly of the offset cylinder and which further dries off the plate. This offers the possibility of further reducing, almost to zero, the moistening agent present at the picture-free areas of the plate, with the result that the effect of the moistening agent is no longer noticeabie in offset printing on moisture-sensitive paper. The position of the blowing device and the direction in which it acts are indicated in Figure 2 by means of an arrow A. The blowing device is of the kind previously used, so that its construction does not require to be described in detail.

Thickness measurements on the layer of moistening agent on the plate in the case of a separate print produced on an offset printing machine based on the form of construction herein described has shown that the ink and the moistening agent on the rollers of the moistening mechanism do not occur in widely separated layers, i.e. that the moistening agent cannot be divided into "inner and surface water", even in the broad sense of these terms. In the form of construction herein described, a film of moistening agent is metered by pressing together the container roller and the metering roller which accept ink and roll about each other without slip, and this film is itself sufficient to permit a separate print in a safe zone between the water galley-proof limit and the smear limit.In this connection it is found that, by blowing air on to the layers of ink and moistening agent directly following the metering position, the thickness of the layer of moistening agent, measured on the plate following the last ink-applying roller, can be only very slightly reduced with the aid of cool or warmed air. On the other hand, as previously mentioned, the layer of moistening agent can be altered to such an extent, by blowing air on to the plate to the rear of the ink-applying roller, that, despite an unaltered setting for the metering of moistening agent, the layer of moistening agent on the container roller and the dosing roller can be reduced to such an extent that tinting of the plate takes place.

Even when sufficient accurate metering of the moistening agent is possible between the water galley-proof limit and the smearing limit by the use of known moistening-agent metering devices, the moistening-agent metering range can still be varied by the use of the present invention and can also be brought nearer to the tinting range.

Claims (9)

Claims

1. An arrangement for metering the moistening agent for an offset printing machine having an ink-supply moistening mechanism comprising at least two ink-supply rollers and a standard metering or vibrator-roller means for a moistening agent, the arrangement being such that the ink film in the moistening mechanism, that is saturated with moistening agent, is subjected, between the two ink-supply rollers of the moistening mechanism, to a shear gradient by means of positively induced different peripheral speeds (slip) between these two rollers.

2. An arrangement according to claim 1 wherein the slip occurs between at least one selected roller-pair, coated to accept ink, of a combined inking and moistening mechanism.

3. An arrangement according to either one of claims 1 or 2, wherein, in each case, at least one of the rollers that slip relative to each other and are coated to accept ink, has an elastic covering.

4. An arrangement according to any one of the preceding claims wherein the slip between mutually contacting rollers has a substantially constant value as a result of a shape-locking or frictional drive between these rollers.

5. An arrangement according to any one of the preceding claims wherein the slip between the two rollers, coated to accept ink of the moistening mechanism can be set at various levels.

6. An arrangement according to any one of the preceding claims further comprising a blowing device which is arranged to the rear of the moistening-agent application zone, but forwardly of the offset cylinder, and is directed towards the stereo cylinder.

7. An arrangement according to claim 6 wherein the amount of air from the blowing device is adjustable.

8. An arrangement according to claim 7 wherein the air from the blowing device can be varied to be hot or cold.

9. An arrangement for metering the moistening agent for an offset printing machine, substantially as hereinbefore described with reference to the accompanying drawings.