EP0686503A1 - Fastening device for flexible printing plates on a printing cylinder - Google Patents

Abstract

The fixing is for a stereotype with one or more print surfaces (8) attached to a backing sheet (2) with a hook-shaped strip (3) along one end to engage with a matching groove (6) in the surface of the cylinder, and tensioners (12) on the other end. The under-surface of the backing sheet has a wedge (4) which fits into an additional groove (5) of the same cross-section, situated adjacent to the hook-shaped groove and parallel to it. The wedge-shaped groove can be connected to the inside of the cylinder by a series of radial channels, while the cylinder's inner chamber can be connected to a vacuum source such as a pump which is connected to the cylinder by a rotary joint. <IMAGE>

Description

The present invention relates to a device for hanging flexible plates on a printing cylinder in a rotary printing machine.

In such machines, generally called flexo printing machines and frequently used in the cardboard industry, flexible plates are mounted on a rotating cylinder to transfer the ink onto the surface to be printed. These machines pass successively individual sheets of cardboard taken from the top of a stack through a printing station, possibly through a cutting station before arriving at a receiving station.

A major problem with such machines is the changing of plates on the printing cylinders, and more particularly the rapid mounting of printing plates so as not to unnecessarily delay the machine when changing a format of sheet or print format. A printing plate usually comprises one or more forms of rubber or plastic printing fixed on a flexible support sheet, either of thick paper or of rubberized or plastic material, so that it can be wound around the cylinder. impression.

One of the first known means for attaching a plate to a cylinder consists in stapling the sheet onto a piece of wood belonging to the cylinder. However, these staples tend, over time, to damage the backing sheet and the piece of wood. which must then be replaced regularly. In addition, if the photograph is in a bad position, it must be completely removed and repositioned, which can double or triple the dead time.

Another attachment device comprises a rigid strip at each upstream and downstream end of the plate. The upstream strip is engaged in a fixed groove of the cylinder while the downstream strip is taken in an articulated groove so as to be able to put this photograph under tension. However, the dimensions of the plate must then be fairly precise and there are also problems with the means of implementing the movable groove.

In fact, the real problem is to maintain the whole of the plate pressed against the cylinder, which does not necessarily imply a strong tensioning of this plate.

Documents FR 2 434 035 and FR 2 196 910 disclose plate cylinders, the cylindrical outer surface of which has a plurality of grooves connected by radial openings to an internal chamber of the cylinder which can be put under vacuum by a vacuum pump. One can either arrange a plurality of parallel circular grooves, or a mesh of grooves, or simply a single groove progressing helically along the cylinder. When these grooves are covered by the plate, it thus forms sealed spaces which, once placed under slight depression, press the plate against the cylinder. However, there still remains the problem of the correct fixing of the upstream and downstream edges.

The attachment device described in document FR 2 196 910 comprises two parallel side-by-side grooves formed substantially tangentially along a generatrix of the cylinder. Each groove opens on their opposite side so that one of them receives the front edge of the plate, the other receiving the downstream edge. These plate edges are held in place by means of bolts passing radially through the grooves and engaging in threaded holes in the cylinder. This device is however imperfect in that the edges are only positioned or maintained at two or three points. In addition, the insertion of the edges in the grooves can be done more or less easily depending on the thickness of the support sheet.

The attachment device described in the document FR 2 434 035 is based on the presence of protruding fixing pins of the cylinder and in which openings are made a little behind the front and downstream edges of the plate. A plate is then added between these pins to keep the edges pressed against the cylinders. Again, the accuracy of the positioning of the front edge cannot be guaranteed by simply maintaining it in a few points.

Probably the most common device, called the Matthews device, comprises a hook band of U-shaped transverse shape fixed to one end of the plate, this band coming to hang in a groove corresponding to the printing cylinder also having a U-shaped cross. A plurality of elastic tensioners are attached to the other end of the plate. Each of these tensioners ends in a U-shaped hook which can be engaged in another groove of the cylinder which also has a U-shaped transverse shape. These elastic tensioners allow the plate to be placed under high tension against the cylinder. The installation of each of the tensioners can however be tedious, they can also be replaced by a large adhesive strip directly fixing the downstream edge of the plate to the cylinder.

Operating satisfactorily, there is nevertheless a risk of loosening the tension of the downstream edge fasteners which can subsequently lead to a stall of the plate which can then fall between the ink cylinders or pass into the rest of the machine if it is rotating. If certain devices for mechanical tensioning or magnetic fixing of the plates reduce this risk, they then have the disadvantage of a tedious implementation and an adjustment of the complex ultimate position.

The object of the present invention is a device for hanging plates comprising a front band of substantially U-shaped cross section coming to engage in a hook-groove formed along a generatrix of the cylinder; as well as means for fixing and tensioning the downstream edge of the plate, which is such that a detachment of the front edge of the plate is practically impossible on the occasion of a relaxation of the tension of the means of fixing the downstream edge . This device must however always remain as simple to handle and be achievable without excessive cost.

This goal is achieved by one or more tabs attached against the face of the sheet of the plate and oriented towards the cylinder side, set back from the front hook-strip, these tabs being positioned in an additional groove of transverse profile corresponding to the profile. cleats and formed, in correspondence, parallel and close to the first hook-groove.

Then, the hook-strip can only be disengaged from its groove if the tabs have themselves previously come out of their own groove, which supposes that the support sheet of the plate has been lifted well beyond the plane tangential to the cylinder passing through the hooking device. In practice, a simple relaxation of the sheet tension causes only a few beats in the rear part of the plate but very rarely a lifting of the upstream part. Then, the cleats remaining in place also maintain the hook band.

According to a preferred embodiment, the lug groove is connected by a plurality of radial channels to an internal chamber of the cylinder which can be placed under vacuum. In addition, the plate preferably comprises at least one cleat in each of its lateral edges. Thus, the lateral tabs delimit between them an area in the groove capable of supporting a vacuum, which confirms that these tabs are held in place.

Usefully, the cleat groove is completed by a plurality of transverse grooves in an arc. Alternatively, the cleat groove is connected to a series of parallel circular grooves formed around the entire periphery of the external face of the cylinder. This arrangement makes it possible to keep the front part of the plate particularly well pressed against the level of the cleat groove.

Alternatively, the cleat groove consists of a plurality of grooves placed end-to-end and not communicating with each other. Each groove can optionally communicate with several complementary transverse grooves. In this embodiment, a cleat, generally central, occupies one of the grooves. It is thus possible to overcome the lateral tabs which facilitates the lateral positioning of the plate.

When the circular grooves are present around the entire periphery of the cylinder, a separation is preferably installed allowing these grooves to be divided into two parts: an upstream and a downstream and independently connected to the internal vacuum chamber. This embodiment makes it possible to install shorter photographs stopping at the level of the separation.

Usefully, if the sheet exceeds the separation, it is then slightly folded or embossed at its downstream edge to form a transverse cavity making it possible to increase the surface in depression, therefore the plating force. As an alternative to folding or embossing, a thin strip can be glued to the downstream edge of the plate. Alternatively, a plurality of transverse grooves can be cut on the upstream side of the partition.

According to a preferred embodiment, the transverse shape of the grooves is narrower at its mouth on the surface of the cylinder than at its bottom in the thickness of the cylinder. For example, a groove can have a trapezoidal transverse shape whose bottom has a width of between 6 mm and 8 mm while the opening is reduced between 1 mm and 3 mm, preferably 2 mm. Such grooves ensure good maintenance of plates whose support sheets are thin. In addition, the circulation of the depression is done with less pressure drop from one end to the other of the groove while limiting the leakage section for an uncovered part.

• la figure 1 est une vue en coupe longitudinale de deux clichés selon l'invention;
• la figure 2 est une vue en coupe transversale partielle du cylindre sur lequel est monté un cliché selon l'invention;
• la figure 3 est une vue identique à la figure 2 illustrant le dégagement du cliché hors du cylindre;
• la figure 4 est une vue identique à la figure 2 illustrant une rainure complémentaire;
• la figure 5 est une vue schématique du dispositif de mise en dépression des rainures;
• la figure 6 est une vue en coupe transversale du cylindre illustrant différents modes d'accrochage du bord aval d'un cliché selon l'invention;
• la figure 7 est une vue en coupe transversale d'une rainure particulièrement avantageuse pour le dispositif selon l'invention;
• la figure 8 est une vue de devant du dispositif d'accrochage de clichés; et
• la figure 9 est une vue en coupe partielle d'un autre mode d'accrochage du bord aval d'un cliché court.

The invention will be better understood from the study of embodiments taken on a non-limiting basis and illustrated in the following figures in which:

• Figure 1 is a longitudinal sectional view of two pictures according to the invention;
• Figure 2 is a partial cross-sectional view of the cylinder on which is mounted a plate according to the invention;
• Figure 3 is a view identical to Figure 2 illustrating the release of the plate from the cylinder;
• Figure 4 is a view identical to Figure 2 illustrating a complementary groove;
• Figure 5 is a schematic view of the groove depression device;
• Figure 6 is a cross-sectional view of the cylinder illustrating different modes of attachment of the downstream edge of a plate according to the invention;
• Figure 7 is a cross-sectional view of a particularly advantageous groove for the device according to the invention;
• Figure 8 is a front view of the snap device; and
• Figure 9 is a partial sectional view of another method of attachment of the downstream edge of a short shot.

As illustrated in FIG. 1, a plate 1 consists of a thin and flexible support sheet 2 of rubber or plastic, on the external face of which are stuck printing patterns 18. The front edge of the sheet is equipped of a wide hook-strip 3, the front edge of which has, seen in cross section, a U-shape. The downstream edge of the sheet 2 is arranged to receive either elastic tensioners 12 or a strip disposable adhesive 13 of adhesive tape type. More particularly, the plate comprises, in its front part, a plurality of tabs 4 glued to the "cylinder side" face and close to the hook-strip 3.

As best seen in Figures 2 and 3, the cylinder 7 comprises on the one hand a hook-bar forming a groove 6 in L provided to receive the hook-strip 3 of the plate 1. This bar is in fact fixed in the middle of a sufficiently wide longitudinal recess so as to leave free, on the rear side, a complementary groove 5 provided for the cleats 4 of the plates 1.

As can easily be understood, the positioning of the plate 1 is carried out by engaging the hook-strip 3 in the groove formed by the bar 6. The fact of subsequently winding the plate on the cylinder engages the tabs 4 in the groove 5. As soon as the plate has been wound a little on the cylinder, a slackening of the tension can no longer cause the stall due to the security provided by the cleats. Conversely, to take the picture, you have to give it a very important angle, greater than 30 °, beyond the tangential plane passing through the bar, to first release the tabs 4 and only then the hook 3.

Furthermore, and as best seen in FIG. 3, the groove 5 communicates by a radial channel 8 with the internal chamber of the cylinder 7 which, as illustrated in FIG. 5, can be put under vacuum by a vacuum pump connected to the cylinder. by a rotating joint 10.

In Figure 8, we observe how two lateral tabs 4 close the groove 5 from the edges of the plate. The space of the groove between these two tabs can then be put in depression thanks to the radial channels 8. The diameter of these channels is limited to a small value so that those of them which are not covered by the plate do not induce an excessive loss preventing the depression.

As illustrated in FIGS. 4 and 6, the plating force pressing the front edge of the plate 1 against the cylinder can be increased by arranging grooves 11 in an arc of a circle in the external surface of the cylinder 7, these grooves being in communication at right angles to the lug groove 5. The positioning of the lugs in the center and on each side of the front edge does not in any way compromise the communication between the circular grooves 11, the lug groove 5, and the channels 8.

According to another embodiment, the groove 5 consists of a plurality of individual grooves 14 placed end-to-end as illustrated in FIG. 8. Each groove 14 communicates, on the one hand, with several grooves 11 and, d 'other hand, with a radial channel 8 of depression. In this case, only a single cleat 4 is provided, generally in the middle, within one of the individual grooves 14, which facilitates the adjustment of the lateral position of the plate.

FIG. 6 illustrates a separation 16 dividing the grooves 11 into two distinct zones, the front part of a groove being supplied by the radial channels 8, the downstream part then being supplied by radial channels 15 opening into grooves 14. Then, if the dimensions of the plate are such that the downstream edge of the sheet 2 comes to cover the grooves 14, the depression can propagate until the separation 16 acting as a partition.

On the other hand, if the plate is shorter to the point of stopping only at the level of the partition 16, provision is made for a fold or embossing at its downstream edge to form a transverse cavity 17 facilitating equalization of the depression of a groove with the other allowing to notably increase the surface on which the atmospheric pressure is applied. This surface leveling can also be obtained by gluing a thin strip 19 under the downstream edge of the plate or, alternatively, by making fine transverse grooves 20 near the partition 16 as illustrated in FIG. 9.

The peripheral grooves formed on the surface of the cylinder described in documents FR 2 434 035 or FR 2 196 910 usually have a width of the order of 7 mm for a depth of 3 mm to 4 mm. By their dimensions, the support sheets 2 of plate 1 are not properly supported when they are thin, which is the case with the majority of existing plates. It is then necessary to use thick support sheets therefore more rigid, which increases the costs and the difficulty of handling. However, it is very important to maintain a uniform shape of the plate in order to obtain an optimum printing quality with a minimum of ink.

As illustrated in FIG. 7, the grooves 11 associated with the attachment device according to the invention have a very narrow transverse shape at the mouth, of the order of 2 mm and 3 to 4 times wider at its bottom, c that is to say within the thickness of the cylinder 7. Then, the penetration of the flexible support sheet 2 is practically 80 times less than before. In other words, for a even permissible insertion of the sheet, it may be 3 times thinner than before.

Many improvements can be made to this attachment device within the scope of the claims.

Claims (7)

Device for hanging flexible plates (1) on a printing cylinder in a rotary printing machine, the plates comprising one or more printing patterns (18) fixed on a support sheet (2) ending in a front hook band (3) engaging in a hook groove (6) formed along a generator of the cylinder (7) as well as fixing and tensioning means (12, 13) of the downstream edge of the plate, characterized in that one or more tabs (4) are fitted against the face of the support sheet (2) and oriented towards the side of the cylinder set back from the front hook strip (3), these tabs (4) from being positioned in an additional groove (5) of transverse profile corresponding to the profile of the cleats (4) and formed, in correspondence, parallel and close to the first hook-groove (6). Device according to claim 1, characterized in that the lug groove (5) is connected by a plurality of radial channels (8) to an internal chamber of the cylinder which can be placed under vacuum. Device according to claim 2, characterized in that the support sheet (2) of the plate (1) comprises a cleat (4) at each of its lateral edges. Device according to claim 2, characterized in that the groove (5) of lugs (4) is completed by a plurality of grooves (11) transverse in an arc of a circle. Device according to claim 2 or 4, characterized in that the groove (5) of cleats (4) consists of a plurality of grooves (14) placed end to end and not communicating with each other. Device according to Claim 4, in which the transverse grooves (11) are circular and divided into two parts by a partition (16), characterized in that the support sheet (2) is slightly folded or embossed at its downstream edge, or in that a thin strip (19) is glued under its downstream edge. Device according to one of claims 2 to 6, characterized in that the transverse shape of the grooves (5, 11, 14) is narrower at its mouth on the surface of the cylinder than at its bottom in the thickness of the cylinder.

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