EP0665104B2 - Method and device for printing one sheet after the other - Google Patents

Description

The present invention relates to an improved process for sheet printing with flexographic or possibly offset sheet of semi-rigid materials such as corrugated cardboard or paper in which the ink is brought to the sheet using a rubber element mounted at the periphery of a printing cylinder. It relates more particularly to a process improved to eliminate distortions in printing dots due to uncontrolled crushing of the rubber element. It also concerns a sheet-by-sheet printing installation specially designed for setting up work of the aforementioned process.

Flexographic type installations include a screened cylinder, a plate cylinder, a back pressure system and a transfer system which feeds the sheets one by one between the plate cylinder and the against pressure. The plate, which is mounted on the plate cylinder, is a rubber element which is capable of receiving the ink which is transferred to it by the screened cylinder and to apply it on the sheet to be printed during the passage of the latter in the nip between the plate cylinder and the back pressure system.

Offset type installations include a plate cylinder, a printing cylinder called blanket and a back pressure cylinder. The blanket cylinder is coated on its periphery with a rubber element which is suitable for receiving the ink transferred to it by the plate cylinder for apply it to the sheet to be printed when it passes through the nip area between the blanket and the back pressure cylinder.

In these two types of sheet-by-sheet printing process we find a rubber element which is mounted on a first printing cylinder and which is applied on the one hand to a second ink supply cylinder and on the other part, via the printing sheet, on a back pressure cylinder.

In theory to transfer the ink from the second cylinder to the rubber element of the first printing cylinder is not necessary to exert a particular pressure between these two cylinders; it is enough that the ink which is on the surface of the second cylinder is somehow licked by the outer face of the rubber element of the first printing cylinder. It is what is called the "kiss impression".

In practice it is necessary to exert this pressure because of the irregularities which may occur in the thickness of the rubber element, especially when it comes to flexographic printing plates, or lack of straightness of the outer surface of one or other of the cylinders. In effect if such pressure was not exerted, these various irregularities would translate into variations in print intensity up to the formation of unprinted areas.

The crushing of the rubber element, on the periphery of the first printing cylinder, caused by the application of this pressure, avoids such defaults.

The same applies to the application of pressure between the first impression cylinder and the back pressure cylinder, by through the printing sheet.

In order to obtain a zero slip between the different cylinders, it is by elsewhere already known to design a printing installation in which the cylinder carrying the rubber element has a smaller diameter than the diameter of the other two adjacent cylinders. This solution is recommended in the publication: 〈〈 Report of the proceedings fourteenth annual meeting and technical forum 〉〉, Flexographic Technical Association, Chicago, Ill., US, April 5-6, 1972.

Furthermore, to obtain optimal printing operation, aimed to eliminate any possible slippage between the different cylinders that apply one against the other, it is usual to adapt the settings aimed at obtaining a perfect equality between the peripheral linear speeds of the different cylinders as well as of the transfer system feeding the sheets to be printed.

Applicants have noted, however, that even when adjusting the rotation of the different cylinders so as to obtain perfect equality of peripheral linear speeds of said cylinders, they did not obtain a perfect impression. More specifically, this observation was made by examining flexographic printing points. Indeed, being a printing point which is in circular theory, it is found on the sheet printed under a substantially oval in shape, with an actually printed surface which is therefore greater than the theoretical surface, sometimes 60% greater than this surface.

The aim that the applicants have set themselves is to propose a process sheet-to-sheet printing which compensates for this abnormal point deformation while allowing to work with a crushing of the element in rubber.

This goal is perfectly achieved by the process defined in claim 1

The plaintiffs unexpectedly found that contrary to what was normally recommended it was enough to increase in certain proportions the peripheral linear speed of the second cylinder ink supply and back pressure cylinder with respect to linear speed rubber element device to reduce defect observed, up to the possibility of obtaining in practice the theoretical surface of the point printing.

The values of the gear ratios included in the range recommended, namely from 1.004 to 1.05, go far beyond what is theoretically intended to compensate for the reduction in thickness due to crushing of the rubber element. So there is a noticeable difference speed between the first printing cylinder and the other two cylinders, difference which until now would have been considered prejudicial. It's the merit of the inventors that to have been able to verify that on the contrary these differences of speeds made it possible to reduce or even eliminate a printing defect. The claimants have attempted to approach an explanation for this phenomenon. Crushing of the rubber element, when applying pressure between the first printing cylinder and the other two cylinders, causes deformation of the rubber in contact areas; more precisely when the cylinders are when stopped during the application of the force, this deformation results in the formation of two beads of material on either side of the contact zones. When the cylinders are rotating, with the same peripheral linear speed, it occurs due to the friction brought into play between the surfaces in contact a rubber creep phenomenon tending to cause a magnification of the bead located upstream of the contact zone relative to that located downstream, in the direction of rotation of the cylinders. It is this magnification which would be at the origin of the increase in the surface of the printing point.

The implementation of the process of the invention would make it possible, by increasing the peripheral linear speed of the cylinders in contact with the element in rubber, to counteract the magnification of the upstream bead by a additional deformation giving rise again to a downstream bead. We conceives, however, that an excessive increase in speed would cause a defect in the opposite direction, i.e. excessive magnification of the downstream bead and the total disappearance of the upstream bead.

What is sought by applicants is to obtain during printing a deformation of the rubber element which is similar to that which is obtained when the cylinders are stopped.

Advantageously, the cylinders being rotational drive independent of each other, the gear ratios are adjusted automatically, for a given rubber element, depending on overwriting it.

The applicants have indeed found, through multiple tests, that the gear ratios to achieve the desired effect were directly a function crushing of the rubber element, i.e. the difference in thickness in the line of tangency between the two cylinders.

Preferably, the cylinders being independently rotational each other, the gear ratio adjustment is adjusted automatically so as to obtain the minimum power consumption for the motor of the first printing cylinder.

This preferred mode of steering the gear ratios is based on a new finding made by the applicants. This finding concerns the instantaneous power consumption of cylinder drive motors. In effect if we consider that to obtain a peripheral linear speed of rotation given the motors deliver a predetermined electric current, when there is no no contact between them, this intensity increases very appreciably when the rubber element is applied to the surface of the other cylinder. When of the implementation of the inventive method, by following the instantaneous intensity of each engine, applicants have found that there is a minimum threshold for this increase in intensity and that this threshold corresponded to the optimum setting gear ratios.

It is another object of the invention to propose an installation as defined in claim 4

Preferably the installation includes means for adjusting the pressure of the second and third cylinders against the rubber element of the first printing cylinder and said adjustment means are themselves connected to the means for adjusting the rotational speeds of the cylinders so as to allow the adjustment of the gear ratios, for a rubber element given in function of the pressure values of the cylinders against each other.

Preferably at least the drive motor of the first cylinder printing device is provided with means for measuring the instantaneous intensity of said motor, said control means being themselves connected via a circuit electronic suitable for the means of adjusting the speed of rotation of the second and third cylinders.

• la figure 1 est une représentation schématique à l'arrêt du cylindre tramé et du cylindre porte-cliché,
• la figure 2 est une représentation schématique en fonctionnement normal, à vitesse linéaire périphérique identique du cylindre tramé et du cylindre porte-cliché de la figure 1,
• la figure 3 est une représentation schématique , dans les conditions de vitesse de l'invention, du cylindre tramé et du cylindre porte-cliché de la figure 2,
• la figure 4 est une représentation schématique d'une installation complète à réglage automatique pour l'impression flexographique feuille à feuille,
• et la figure 5 est une courbe montrant le rapport des vitesses de rotation en fonction de l'écrasement de l'élément en caoutchouc, à deux cadences données.

The present invention will be better understood on reading the description which will be given of an exemplary embodiment of an installation for sheet-by-sheet printing of the flexographic type, illustrated by the appended drawing in which:

• FIG. 1 is a schematic representation when the screened cylinder and the plate cylinder are stopped,
• FIG. 2 is a schematic representation in normal operation, at identical peripheral linear speed of the screened cylinder and of the plate cylinder of FIG. 1,
• FIG. 3 is a schematic representation, under the speed conditions of the invention, of the screened cylinder and the plate cylinder of FIG. 2,
• FIG. 4 is a schematic representation of a complete installation with automatic adjustment for flexographic printing sheet by sheet,
• and FIG. 5 is a curve showing the ratio of the rotational speeds as a function of the crushing of the rubber element, at two given rates.

• un cylindre porte-cliché 2 de rayon Rc à la périphérie duquel est monté un cliché 3, d'épaisseur Ec,
• un système d'encrage 4 comportant un cylindre tramé 5,
• un cylindre de contre-pression 6,
• un système de transfert 7, du type courroie crantée.

The installation 1 sheet-by-sheet printing, of the flexographic type, which is shown in FIG. 4 comprises:

• a plate cylinder 2 of radius Rc at the periphery of which a plate 3, of thickness E c , is mounted,
• an inking system 4 comprising a screened cylinder 5,
• a back pressure cylinder 6,
• a transfer system 7, of the toothed belt type.

The surface of the screened cylinder 5 comprises a multitude of cells him for receiving the ink which is brought to the rubber cylinder 8 to using a pump not shown. The screened cylinder 5 allows during its rotation of an inking of plate 3, which represents the pattern to be printed.

The transfer system 7 allows the transport of each sheet in the direction of arrow D to the nip 9 between the back pressure cylinder 6 and the plate cylinder 2.

Plate cylinder 2, screen cylinder 5, transfer system 7 and the back pressure cylinder 6 are each driven by an individual motor with electronic servo, of the brushless motor type. All of these engines are connected to an electronic circuit 10 for adjusting their respective speed.

FIGS. 1 to 3 show the behavior of plate 3 during its passage in the contact zone 11 between the screened cylinder 5 and the cylinder plate holder 2.

To overcome the possible variations of different parameters and by especially the thickness itself of the plate 3, the straightness of the two cylinders 2, 5, positioning of plate 3 on plate cylinder 2, it is necessary to reduce the distance between the outer surface of the screened cylinders and plate holder, in such a way that during the passage of the photograph 3 in the zone 11 there occurs a certain compression of said plate 3 making it possible to compensate for said irregularities and get a print across the sheet.

Plate 3 being a rubber element, this compression results by a deformation of the rubber material.

FIG. 1 shows the said deformation, when the two screened cylinders 5 and plate holder 2 were stopped. The dotted line illustrates which would be the theoretical location of plate 3 in the absence of the screened cylinder 5. We observes on both sides of the contact zone 11 of the lateral bulges 13 and 14 in which is located the excess rubber material which is expelled by the presence of the screened cylinder 5.

It is usual to provide, as a respective adjustment of the speeds of the different cylinders, than the peripheral linear speed of the screened cylinders 5 and back pressure 6, are strictly identical to the peripheral linear speed of picture 3. The purpose of this adjustment is to avoid any sliding between the different cylinders and also between the printing sheet 12 and the plate 3.

However, according to the applicants, when the linear speeds of the screened cylinder 5 and plate 3 are identical, a printing fault. This defect is characterized by an ovalization of the point printing which in theory should be circular. This ovalization can cause an increase in the area of the printed point up to 60%.

According to the characteristic of the process of the invention, this defect is corrected by significantly increasing the peripheral linear speed of the cylinder screened 5 compared to that of photograph 3. This increase is appreciably between 0.4 and 5%.

The optimal value of this speed increase is a function of a number of parameters, such as the printing rate that is required by the speed of rotation of the plate holder, the mechanical characteristics of the photo 3 and the overwriting of the photo.

If we consider that V1 is the peripheral linear speed of snapshot 3 and V2 that of the screened cylinder 5, the optimal V2 / V1 ratio can be determined by successive tests observing, for each test, the shape and surface of the point as the ratio increases beyond 1, said surface decreases then increases again. It is for this minimal surface, which should be close to the theoretical surface of the printing point, that is the optimal V2 / V1 ratio.

According to the applicants, this is explained by the creep of the material rubbery during the rotation of the cylinders.

In Figure 2, there is shown, the deformation of the photograph 3 when the speed ratio V2 / V1 is equal to 1. This deformation results in the presence of a large bead 15 of rubber material upstream of the contact zone 11 and the disappearance or quasi-disappearance of the existing bead previously downstream of said contact zone 11. This deformation would be due to the friction forces brought into play on the surface of plate 3 due to contact with the periphery of the screened cylinder 5.

We can explain the ovalization of the printing point by the increase correlative of the contact surface between the plate 3 and the screened cylinder 5.

The increase in the speed ratio V2 / V1, in accordance with the process of the invention makes it possible to correct this defect by means of an opposite deformation of the rubber mass. Indeed the relative speed increase of the surface of the screened cylinder 5 makes it possible to counteract the effect due to the friction forces between both surfaces. According to the applicants, this would result in a new deformation in the form of two beads 16, 17 located respectively on the side and other than the contact zone 11. This deformation is substantially identical to that observed when the cylinders are stopped (Figure 1).

The electronic circuit 10 is programmed to automatically control the position of the brushless motors which rotate the plate cylinder 2, the screen cylinder 5 and the back pressure cylinder 6, respectively. This control is produced so that one obtain the speed ratio in accordance with the process of the invention. This same electronic circuit 10 is connected to the system for adjusting the positioning of the different cylinders with respect to each other, which defines the pressure exerted between said cylinders during the passage of a part of the plate 3 between the screened cylinder 5 and the door cylinder -cliché 2, and on the other hand during the passage of the sheet to be printed 12 between the radiograph 3 and the back-pressure cylinder 6. This pressure, for a given radiograph 3, is characteristic of the crushing of said radiograph, c that is to say the reduction in thickness E c in the contact zone 11 (FIG. 1).

FIG. 5 shows examples of speed ratio value V2 / V1 as a function of the overwriting in millimeters of plate 3 for two printing rates of 4,000 sheets per hour and 8,000 sheets per hour.

For the first rate and for a crushing of the order of 0.1 mm, which is a classic setting, we have a speed ratio V2 / V1 which is around 1.015, an increase in the peripheral linear speed of the cylinder 1.5% raster compared to the peripheral linear speed of photograph 3.

It is possible to work with much more overwriting important, of the order of 0.3 mm while obtaining an adequate correction of the printing defect; it then suffices to respect the speed ratio V2 / V1 of around 1.034.

These values are significantly higher when the rate is increased of production.

It is therefore remarkable that thanks to the process of the invention, it is possible to work with very large crushings compared to the conventional setting, while having excellent print quality. This is an advantage considerable in that the claimants have found that the crash of the snapshot 3 had an effect on the length of the print: the more you overwrite the shot and more printing is done over a significant length. It is therefore possible make print length corrections by playing with the overwriting of the shot, while maintaining a very good print quality.

All the above indications have been given taking as references the screened cylinder 5. In fact, they are of the same order as regards the back pressure cylinder 6. The gear ratio must also be understood in the range of 1.004 and 1.05.

Preferably the servo-control carried out by the electronic circuit 10 of the rotational speeds is obtained by measuring one of the operating parameters motors for driving said cylinders. It can be the measurement of the electric current or the measurement of the motor torque. The optimal ratio of speeds is that which corresponds to the minimum of the corresponding parameter.

The control of the installation can be done automatically after a preset on the first sheets to be printed. It is enough to measure for a speed ratio of 1 the variation of the intensity or the motor torque during passage of the plate between the screened cylinder 5 and the plate cylinder 2 or still during the passage of the sheet 12 between the plate 3 and the counter cylinder - pressure 6. The speed ratios are gradually increased while continuing to measure variations in motor current or torque. The comparison of successive measurements makes it possible to determine the gear ratios which correspond at minimum variations in current or motor torque.

The present invention is not limited to the embodiment which comes to be described by way of non-exhaustive example.

Process and installation are not limited to printing flexographic type but can also be applied to offset printing.

Claims (6)

1. Sheet-by-sheet printing process, in which a sheet is printed during its passage in the nip zone between a first printing cylinder (2) which comprises on its periphery a rubber element (3) and an impression cylinder (6), the ink being supplied to the first printing cylinder (2) by a second ink supply cylinder (5), characterized in that the cylinders being provided to be driven in rotation independently of one another, in order to reduce the deformations of the printing dots, it consists in adjusting the rotation speeds of the different cylinders so as to adjust the ratios of the peripheral linear speeds of the second ink supply cylinder (5) and of the impression cylinder (6) with respect to the peripheral linear speed of the rubber element (3) of the first printing cylinder (2), to values constantly greater than 1, included within the range 1.004 to 1.05.
2. Process according to Claim 1, characterized in that the speed ratios are automatically adjusted, for a given rubber element, as a function of the crushing thereof.
3. Process according to Claim 1, characterized in that the speed ratios are adjusted automatically so as to obtain minimum electrical consumption for the drive motor of the first printing cylinder (2).
4. Installation for carrying out the sheet-by-sheet printing process of Claim 1, comprising a first printing cylinder (2) having a rubber element (3) on its periphery, a second ink supply cylinder (5) and a third impression cylinder (6), each sheet (12) to be printed being successively fed up to the nip zone (9) between the first printing cylinder (2) and the impression cylinder (6), as well as means for driving said three cylinders in rotation, characterized in that the drive means consist in three mechanically independent motors or gear motors, and in that it comprises means for adjusting the speeds of rotation of the three determined cylinders so that the ratios of the peripheral linear speeds of the second ink supply cylinder (5) and of the impression cylinder (6) with respect to the peripheral linear speed of the rubber element (3) of the first printing cylinder (2), constantly have values greater than 1, included within the range 1.004 to 1.05.
5. Installation according to Claim 4, characterized in that it comprises means for adjusting the pressure of the second and third cylinders against the rubber element of the first printing cylinder, and in that said pressure adjusting means are connected to the means for adjusting the speeds of rotation of the cylinders so as to allow adjustment of the speed ratios, for a given rubber element, as a function of the values of pressure of the cylinders against one another or via the sheet to be printed.
6. Installation according to Claim 4, characterized in that at least the motor driving the first printing cylinder (2) is provided with means for measuring the instantaneous intensity or driving torque of said motor, which are themselves connected via an appropriate electronic circuit to the means for adjusting the speed of rotation of the second and third cylinders, and in that said electronic circuit is programmed so as to adjust as speeds of rotation of the second and third cylinders, those which correspond to the minimum variations of the intensity or of the driving torque, upon passage of the rubber element (3) between the first and second cylinder, on the one hand, and between the first and third cylinder, on the other hand.

Images