The invention relates in the first place to a screen cylinder, comprising a smooth metal base cylinder and a metal outer cylinder provided with perforations, said cylinders being in tight contact with each other. Such a screen cylinder is known from European Patent Application 85200621.2 (Publication No. 0,160,341) of Applicants.

The above-mentioned patent application describes a process and a device for the production of a screen roller in which a thin-walled metal screen printing stencil is disposed around the periphery of a smooth cylindrical roller.

Fitting takes place by first sealing off with a sealing agent the perforations of the screen printing stencil to be used and then, by means of a compressed medium, sliding the sealed cylinder thus formed onto the smooth roller, which for that purpose is provided, at the end where the sliding-on operation begins, with outflow apertures for the compressed medium used. Such screen rollers are used, for example, as ink transfer rollers in printing systems.

Such a roller, which is therefore provided with a large number of cavities on the surface, is essentially brought into contact with the ink to be used, which is in a reservoir and fills the cavities on this contact. The excess .ink is scraped off the surface of the screen roller using a doctor knife, so that ink only remains in the surface cavities of the roller.

The roller thus filled with ink is then brought into contact, at a point which in the direction of

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rotation follows the doctor knife, with a substrate which has to be provided with a screen-type ink coating such as, for example, the rubber blanket of an offset machine or the surface of a flexographic roller used. The screen roller can also be used for the application of an ink or printing medium coating on the surface of a moving paper or textile web.

During use of the screen roller described above the base cylinder and the outer cylinder are in tight contact with each other, but are not connected to each other.

Such an arrangement has the disadvantage that, where inks with microscopically finely divided pigments are used, in the course of time a pigment accumulation can still take place between the dams of the outer cylinder and the smooth metal base cylinder.

When the ink type is changed, such pigment residues coming from an ink used earlier can lead to pollution of the ink used on a subsequent occasion, which is . undesirable.

Moreover, in the event of a heavy load, the outer cylinder can shift relative to the base cylinder, so- called "running".

The object of the present invention is to produce a soluti-θn to the above-mentioned problems, which is characterized in that in the above-described screen cylinder the base cylinder and the outer cylinder are connected to each other by means of a soldered joint.

As will be indicated more clearly here, it appears possible according to the invention to apply a solder selectively on the inside of the dams of the outer cylinder and on the outside of the base cylinder and, after bringing the base cylinder and the outer cylinder into intimate contact with each other, to activate said solder through a suitable temperature treatment in such

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a way that a soldered joint is produced between the smooth base cylinder and the perforated outer cylinder.

The soldered joint thus formed acts as an absolute seal, so that there is no longer any question of any form of pigment accumulation between the base cylinder and the outer cylinder.

Another advantage found is that through the provision of a soldered joint between the base cylinder and the outer cylinder any "running" of the outer cylinder relative to the base cylinder, such as that which could occur during heavy mechanical loading of the screen roller, can no longer occur.

Another advantage is that the soldered joint forms an electrically conducting connection between the base cylinder and the outer cylinder, which means that - as will be discussed later - certain electrochemical operations can be used on the assembly of base cylinder and outer cylinder.

In particular, the soldered joint in the screen cylinder according to the invention is a tin or a tin/lead soldered joint.

In the screen cylinder according to the invention the base cylinder is expediently selected from the group comprising a solid metal cylinder, a thick-walled hollow cylinder and a thin-walled hollow cylinder.

Depending on the type of the base cylinder, the screen cylinder according to the invention will be suitable for certain applications.

If the base cylinder is a solid metal cylinder, the screen cylinder formed therewith will be extremely suitable as an ink transfer roller.

However, such a screen cylinder can also be used as a basis for the formation of an electroforming mould.

For the formation of metal rotary screen printing stencils, the starting point is a cylindrical roller in

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the surface of which cavities are formed, the distribution of the cavities corresponding to the perforation pattern in the rotary screen printing stencil to be formed. The cavities in the roller are filled with an electrically insulating resin which is hardened. The roller thus provided with a pattern of insulating places is placed, for the formation of a rotary screen printing stencil, as the cathode in a bath for the electrodeposition of metal, following which metal is deposited on the roller. The deposited metal will be deposited on the metal dams which lie between the electrically insulating places and will possibly spread over a part of the electrically insulating places. On completion of the electrolytic process, the deposit thus formed can be removed from the cylindrical mould, thereby producing a rotary screen printing stencil.

Therefore, if in the screen cylinder according to the present invention the base cylinder is in the form of a solid metal cylinder, such a screen cylinder, after filling of the cavities in the surface thereof with an electrically insulating resin, will be very serviceable as a mould for forming seamless metal screen printing stencils, for example made of nickel» If the base cylinder is a hollow thick-walled cylinder, after suitable clamping of such a cylinder round a shaft by means of suitable end plates, the screen cylinder will be usable as an ink transfer roller. In this case also, on filling of the perforations the screen cylinder will be usable as a basic mould for forming, for example, rotary screen printing stencils.

If the base cylinder is a thin-walled hollow cylinder, the screen cylinder formed will have to be fitted on a supporting cylinder before it can be used.

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The fitting of such a relatively thin-walled cylindrical object round a supporting cylinder is known per se, and for carrying this out you are referred to European Patent Application 85200621.2 of Applicants, mentioned above.

In this form also, such a screen cylinder can be used again as an ink transfer roller; for use of the screen cylinder as a basis for forming an electrolysis mould, an electrically conducting connection must be formed between the cathode of the power source to be used and the screen cylinder. If the carrier cylinder is a metal cylinder, the electrically conducting connection can be brought about between the carrier cylinder and the screen cylinder, while the cathode pole of the direct current apparatus used is also connected to, for example, the shaft of the carrier cylinder.

For fitting of the outer cylinder around the base cylinder, if the base cylinder is a solid or thick- walled hollow cylinder, use can be .made of the process described in the European Patent Application with publication number 0,160,340 mentioned above, i.e. temporary sealing off of the perforations of the outer cylinder and sliding it round the base cylinder by means of a compressed medium. If the base cylinder is a hollow thin-walled cylinder such as, for example, a thin-walled nickel sleeve, the base cylinder can be brought into the so-called kidney shape, following which the outer cylinder can be slid around it.

When the base cylinder is released from being held in a kidney shape, it resumes its original cylindrical shape, and the outer surface of the base cylinder comes to rest against the inner surface of the outer cylinder.

In all cases the internal diameter of the outer cylinder will be slightly smaller than the external diameter of the base cylinder; however, the elongation

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which the outer cylinder must exhibit for contact must lie within the range of elasticity of the outer cylinder.

The invention also relates to a process for the production of a screen cylinder in which a smooth metal base cylinder and a metal outer cylinder provided with perforations are brought into tight contact with each other.

Such a process is according to the invention characterized in that, prior to the assembly of the screen cylinder, the inner surface of the outer cylinder and/or the outer surface of the base cylinder are provided with one or more metal layers which by themselves or in combination can form a soldered joint, and in that after bringing the base cylinder and the outer cylinder into contact with each other the assembly thus formed is subjected -to a heat treatment at a temperature which lies little above the melting point of the soldered joint of the layer(s). The surface of the outer cylinder and/or the outer surface of the base cylinder can be provided with metal layers for the formation of a solderable intermediate layer. For the formation of solderable layers, a single layer of adequate thickness on one of the elements of the screen cylinder is conceivable.

A metal layer can also be applied to each of the elements of the screen cylinder to be formed.

The metal layer(s) can comprise a single metal or a combination of metals. A metal layer can also be applied to both the base cylinder and the outer cylinder, the metals of the two layers being different, but in combination being capable of forming a soldered joint. The metal or the combination of metals must, however, be such that a soldered joint is formed at a temperature which does not exceed 250°C.

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In particular, prior to the formation of the assembly of base cylinder and outer cylinder, at least one of the metal layers applied is coated with a flux compound. Such flux compounds are generally known and are used to obtain an improved flow behaviour of the solder layer compounds. Such flux compounds can simply be applied from a solution, by means of, for example, a roller application system, to the layers applied for the formation of a soldered joint; the flux layer compound can, however, also be applied by spraying or brushing. The solder layer materials used can be, for example, of tin or lead/tin; other solder compounds can also be used. If the material of the outer cylinder is nickel, for practical purposes the upper limit of the solder temperature is set at 250 C.

The soldering fluxes to be used are generally powdered or liquid compounds containing certain metal salts, resin components and wetting agents. The action of such a flux is a reducing or conditioning action at the soldering temperature. For a good adhesion of the soldered joint layer both to the outer cylinder and to the base cylinder, good degreasing of both elements prior to the application of the soldered metal layer, combined with cleaning and activation, is very desirable. Very good adhesion can also be ensured by applying a so-called copper or nickel strike; this is an electroplated copper or nickel layer with a maximum thickness of 1 micron. The heat treatment to be carried out is preferably carried out in an oven in which there is a protective gas atmosphere which can be, for example, an inert gas atmosphere or a slightly reducing gas atmosphere.

If the base cylinder is a so-called sleeve, i.e. a relatively thin-walled metal cylinder, for carrying out

the heat treatment the assembly of .sleeve and outer cylinder in the form of a rotary screen printing stencil is clamped on a cylinder, for example in the manner described in European patent publication 1,160,341 mentioned earlier, with an internal heating facility. The cylinder with the screen cylinder assembly is then placed in a radiation oven which is provided with, for example, quartz radiators.

After adding additional flux if necessary, the programme for forming the soldered joint is gone through, with a _. rotective gas or a gas with slightly reducing action being present in the oven as indicated.

After formation of the soldered joint, the assembly of base cylinder and outer cylinder is expediently cooled and washed for the removal of any excess flux present.

Cooling takes place very advantageously to a temperature of approximately 35 C, at which temperature the washing operation is carried out. During the carrying out of the process, metal layers applied by electrodeposition can be used for the application of the layer(s) which have to form the subsequent soldered joint. Use can also be made of electroless deposition of metal layers, while another possibility is the application of a layer in which suitable metal in powder form is incorporated in a binder. The binder in such a case can expediently contain a flux compound.

A heating oven is expediently used for the application of the heat treatment; other methods of heat supply such as induction heating, laser treatment etc. are also possible.

If the base cylinder is a so-called sleeve, a suitable carrier roller must be selected for the final use of the screen cylinder, in which case it can be a

rigid roller on which the sleeve with the stencil around it can be slid using compressed air, while in the art other tensionable rollers are also available. A major advantage of the screen cylinder according to the present invention lies in the fact that through a suitable choice of the perforated outer cylinder it is possible to obtain a cavity volume per unit area which can as desired deviate from the cavity volume per unit area which can .be obtained in the conventional screen cylinder rollers.

The conventional screen cylinder rollers are ■ provided with their surface cavities by means of mechanical working with a milling wheel or by etching. As is also the case for the screen rollers which are described in Applicants' European Patent Application 85200621.2 and for the screen cylinders of the present application, it is, for example, possible to achieve cavity volumes per unit area which are substantially greater than the cavity volumes for screen rollers which are manufactured in the conventional manner.

The invention will now be explained with reference to the drawing, in which:

- Fig. 1 shows schematically a cross-section through a part of the wall of a screen cylinder in which the base cylinder is formed by a thick-walled hollow cylinder;

- Fig. 2 shows in cross-section of a part of the wall of a screen cylinder in which the base cylinder is a thin- walled hollow cylinder and the screen cylinder is fitted on a thick-walled hollow cylinder; and - Fig. 3 shows a cross-section through a part of the wall of a screen cylinder according to Fig. 1, which is used as a mould for forming rotary screen printing stencils through electroforming;

- Fig. 4 shows schematically the combining of a thin- walled base cylinder and a thin-walled outer cylinder.

Fig. 1 shows the base cylinder 1 which is made of, for example, thick-walled copper. The outer cylinder, is indicated by 2 and is formed by a rotary screen printing stencil with a circumference of 64 cm, a length of 2 metres, and a thickness of approximately 100 micrometres, and is made of nickel.

A tin layer is applied by electrodeposition both to the base cylinder 1 and to the inside of the stencil 2, both layers being approximately 15 micrometres thick. Prior to the application of the two tin layers, a so-called copper strike with a maximum thickness of 1 micrometre is used for improvement of the adhesion of the subsequent soldered joint layer.

Through treatment of the assembly of base cylinder and outer cylinder 2, a soldered joint layer 4 is formed; the heat treatment took place at a temperature of approximately 250°C, the assembly of base cylinder and outer cylinder being at this temperature following heating up for approx. 5 minutes; the oven used had a nitrogen atmosphere.

After formation of the soldered joint, the screen cylinder formed was cooled down to approx. 35°C, after which it was washed with water to remove the xcess of flux applied. Fig. 2 shows the base cylinder formed by a seamless nickel sleeve with a wall thickness of approx. 200 micrometres, a circumference of 64 micrometres, and a length of approx. 2 . The outer cylinder is a rotary screen printing stencil with a fineness which can be selected as desired, and which in particular can lie between 100 and 250 mesh (mesh = number of holes per linear inch), and the fineness of which in general can lie between 60 and 500 mesh.

After formation of the screen cylinder, it is fitted by means of the air slide-on technique discussed

earlier on the thick-walled hollow carrier cylinder 3. It can be seen from Fig. 3 that a base cylinder 1 is provided with an outer cylinder 2 in the form of a seamless metal rotary screen printing stencil. A soldered joint layer 4, which ensures adhesion of the outer cylinder 2 to the base cylinder, is formed by carrying out a heat treatment. Reference number 5 indicates that the cavities in the outer cylinder are filled up with an insulating resin material; the surface of the resin material lies flush with the top sides of the dams of the screen material 2. If such a cylinder is inserted as the cathode in, for example, an electroplating nickel bath, metal is deposited on the dams of the outer cylinder 2 and, depending on the duration of electrolysis, will grow over the insulating points 5. Through suitable pretreatment of such a mould for example by applying chromium to the dams of the outer cylinder 2, a metal deposit such as a nickel deposit deposited on the mould can be removed from the mould using suitable equipment for the purpose as a result of the process conditions used during the growing operation.

Fig. 4 shows the method of combining a thin-walled base cylinder 1 and a thin-walled outer cylinder 2. The base cylinder, such as a nickel sleeve, is brought into the so-called kidney shape for the purpose. At great lengths of the outer cylinder the kidney shape is brought about and maintained by means of equipment suitable for the purpose. It is also pointed out that the screen cylinders formed according to the invention can be subjected to further treatments if desired by the user for achieving desired end properties of the screen cylinder. For instance, the finished screen cylinder can be subjected to an additional electrodeposition operation in order to provide the surface thereof with a

wear-resistant layer such as, for example, a tin/nickel layer, which is useful for reducing the wear as a result of the action of a doctor knife during use of the cylinder as an inking roller. Such a screen cylinder can also be subjected to an etching operation to change or to emphasize a particular perforation form and/or, assuming a particular cavity volume per unit area, to increase it further.

The screen cylinder can also be given a larger cavity volume per unit area by coating the top side of the dams of the screen with a metal which etches more slowly, such as, e.g., Cu on an Ni screen, or is not etched ^' . In that case the dam height is retained, and so is the dam width at the top; the dam flanks are, however, strongly etched away, as a result of which a large cavity volume is obtained. Through maintaining the dam width, the tool life of the screen cylinder will, however, remain unchanged. The etching operation described above can, of course, also be carried out before the outer cylinder is provided with a solder or solder constituent. If the screen cylinder is to be used as a mould, as described earlier, for forming, for example, a screen printing stencil through electroforming, the dam width can be reduced if desired at the top by etching, as a result of which in connection with the specifications of the stencils to be formed an additional influencing parameter is obtained.

Through use of in particular the screen cylinder in which the base cylinder is a thin-walled seamless metal cylinder and the outer cylinder is a seamless metal rotary screen printing stencil, a very rapid exchanging facility is obtained, which makes it possible to vary as desired the characteristic of a screen roller to be used by simply exchanging the screen cylinder fitted on the carrier roller.

The screen cylinder according to the invention can be split again into the base cylinder and the outer cylinder by heating to a temperature which lies above the soldering temperature; the base cylinder is then available again for re-use.