The present invention relates in the first place to a screen material made of wire, intended in particular for screen printing- Such a material is known and is generally made by weaving, braiding, knitting or the like of a wire-type material.

The final screen printing material is obtained by pattern-wise masking of the holes, for example masking the holes with a light-sensitive lacquer or the like, and then pattern-wise exposure of the lacquer, following which the exposed or non-exposed parts (depending on the type of lacquer) are removed. The holes can also be masked with a lacquer and, after it has cured, the lacquer is burnt away pattern-wise by means of a laser or the like. During screen printing a printing medium, such as ink, is forced by means of a squeegee through the holes of such a screen onto a substrate to be printed.

The printing quality with these woven, knitted or braided screen materials is unsatisfactory for many applications, since the surface is relatively uneven and channels can form between the screen wires and the substrate to be printed, which gives rise to loss of contour sharpness, due to undesirable running of the printing medium over the substrate. Screen printing screens which do not have the abovementioned disadvantages are thin-walled flat metal plates or seamless thin-walled metal cylinders provided with holes. This material can be made by providing a thin metal plate or a thin-walled metal cylinder with a light- sensitive lacquer coating, which is exposed according to the desired hole pattern and developed, following which an etching operation is carried out in order to obtain the holes. This screen material can also be grown by electroplating on a conducting substrate which has been

provided suitably pattern-wise with an insulating lacquer. For this latter operation a roller to which lacquer is applied locally, or a roller provided in a pattern-wise manner with small holes in which the lacquer is applied, is used.

However, the above techniques are fairly complex, environmentally unfriendly and expensive.

The object of the present invention is to provide a screen material made of wire which has the advantageous printing properties of the last-mentioned metal screens, but which can be made simply and cheaply and from a multiplicity of materials. To this end, the screen material according to the invention is characterized in that the holes are bounded in such a way by wire pieces which are joined to each other locally that an at least virtually flat non-woven screen surface is obtained.

This screen material can be produced cheaply from a very wide variety of types of wire, and both the dimensions and the shape of the holes can be set, while an at least virtually flat screen surface is always obtained. As discussed earlier, this is desirable in particular in the case of screen printing. The method for producing the screen material is environmentally friendly, since no etching or electroforming is required. The screen material according to the invention is not restricted to screen printing, but can also be used suitably for, for example, filtration, without the screen material being affected by contamination which is difficult to remove, as in the case of woven, braided or similar screen material. The screen material according to the invention is also suitable for use as heat-resistant filters, for example for flue gas filtration, as wire skeletons for catalyst compositions, for use as embossing equipment and for all other purposes for which screen material can be used. The screen material itself can also be applied to a support, following which the holes are filled with lacquer and a screen material can be grown by means of electroplating techniques on the screen material according to the invention, in order to produce the

abovementioned thin-walled metal products provided with holes.

The wire used in the screen material according to the invention is not particularly restricted, and can be of round, oval, square, rectangular, triangular or other desired cross-section, and it could even be approximately strip-like.

In the present description, joining wires to each other "locally" should be understood as meaning both point- wise and over some distance, since - as will be explained later - a very wide variety of screen types can be obtained through selection of a pattern of joining points, i.e. points set up at a distance from each other, or groups of points set up adjacent to each other, or continuous joining over a fairly great distance.

Adjacent wire sections, each comprising two or more wire pieces, preferably extend uncrossed in substantially the same direction.

Wire pieces should be understood as those wire parts extending between two joining points. Wire sections comprise a plurality of wire pieces.

Making adjacent wire sections run uncrossed gives rise to a non-woven, at least virtually flat screen surface which has advantageous printing properties. When a part of the screen material of the above¬ mentioned embodiment according to the invention is examined more closely, wire sections running substantially parallel to each other and joined locally to each other are clearly visible. The screen holes can be formed in many ways, but the screen material is advantageously made of zigzag wire sections which are joined to each other locally at the tips. Such a material can be produced in a simple and reproducible manner, as will be discussed below. Zigzag should be understood as being a sawtooth shape, a sinusoidal shape, a flattened sawtooth shape, or an intermediate form of these.

According to another aspect of the invention the material comprises at least two groups of wire sections,

the wire sections in each group running substantially parallel, and crossing those of another group at one side, while the wire sections are joined in such a way to each other at the crossing points through fusion and/or merging that an at least virtually flat screen surface is obtained. In the case of this embodiment a non-woven material in which each group of wires extending in a particular direction lies at one side of another group of wires is obtained. This embodiment again provides a flat screen surface, as is desirable in the case of screen printing in particular.

In particular, the wires are linear in this embodiment, so that the number of holes per linear cm of screen material, and thus the dimensions of the holes, can be regulated in a simple manner. It will be clear that square openings are obtained with two groups of linear wires, while with three groups of wires triangular and even pentagonal and hexagonal openings can also be obtained. The wire material used for the screen material according to the present invention is not particularly restricted, but this material and the method of joining of the wire pieces to each other are preferably selected in such a way that the screen material finally obtained is dimensionally stable. This ensures that, for example, a rotary screen printing cylinder made from the above- mentioned screen material, or a screen plate, does not deform in an undesirable and irregular manner, thus becoming unusable or producing uneven printing results.

The wire of the screen material is advantageously made of a weldable material, and is particularly advantageously made of an electrically conducting material. Metal wire is preferred, because of its electrical conductivity and durability, since screen material made of metal wire can easily be additionally reinforced by electroplating, preferential or otherwise, or it can be coated with a corrosion-resistant metal or provided with a coating for another purpose.

The screen material according to the invention preferably has at least 15 holes per linear cm, and more

preferably has at least 50 holes per linear cm. Such screen materials are particularly suitable for screen printing.

The invention also provides a cylinder made of screen material, intended for rotary screen printing, which cylinder is characterized in that the screen material is a screen material according to the invention.

The invention further provides a method for the production of a screen material according to the invention, which method is characterized in that wire sections are laid next to one another, the wire sections are joined to each other locally, and the screen holes are then formed through deformation of the wire sections. Depending on the way in which the wires are joined, the wires are laid at a short distance from each other or against each other. For example, where soldering techniques are used, in the case of which material is fed in at the position of the joint, a certain distance between the wire parts will not present a problem, while if laser welding is used, the wire sections preferably rest against each other. In another embodiment the method for the production of a screen material according to the invention is characterized in that wire sections are suitably preshaped and then joined locally to wire sections which may or may not have been deformed. Both methods provide a screen material made of wire sections which extend uncrossed and essentially in the same direction.

The wire sections are preferably joined to each other on a support, it being particularly preferable for the support used to be a roller. A cylindrical screen material can be obtained in this way. For this purpose, one or more wires which may or may not have been pre-for ed are wound around the roller, the number of wires, the cross- sectional dimension of the wires in question and the winding method determining the pitch. Unclosed ring-shaped wire sections can also be laid on the roller and then joined locally, and the screen holes are finally formed.

In particular, the wire sections are joined to each other locally through laser welding, in which case the

laser can operate continuously or intermittently (in a pulsed manner) . Apart from welding, all kinds of other joining techniques such as soldering, gluing, merging etc. may be mentioned. The invention also provides a method for the production of a screen material according to the invention, which method is characterized in that groups of wire sections are laid in succession on a support at an angle relative to each other, the wires being laid in such a way per group that they run substantially parallel to each other at some distance from each other, following which the wire sections are joined to each other at the crossing points through fusion and/or merging.

Here again, for the production of a seamless cylinder made of screen material it is preferable to wind wires onto a roller, in which case the distance between the wires of each group, and thus the number of holes per linear cm of the screen material finally produced, can be set by setting the pitch of wires to be wound. Fusion and/or merging can be carried out under the influence of temperature increase and/or pressure increase or the like.

In the case of this method it is preferable to subject the screen material to a flattening operation, for example, planing, polishing or gauging, after production. In order to simplify the abovementioned fusion and/or merging, the wire sections of at least one group are preferably made of plastic. If desired, after completion of the screen material, such plastic wire parts can be provided with a metal layer in a manner known to those skilled in the art.

In order to give additional strength to the screen material according to the invention, or to coat it for another purpose, it is preferable to carry this out by electroplating techniques which are known to those skilled in the art. A technique suitable for this purpose is preferential growth of a metal layer.

The wire material used in the method according to the invention is preferably a hardenable or curable material. Examples are metals which can be hardened by, for

example, nitriding, or curable plastics. The above is particularly advantageous if the wire has to be wound, for example, prior to joining, so that some flexibility of the wire is desirable. The ability to be hardened or cured makes it possible to give the final product the required strength, dimensional stability, hardness or wear resistance.

The invention also provides a screen material which is obtained by subjecting a screen material according to the invention to electrolysis in an electrolysis bath, which bath contains at least one organic compound with at least one unsaturated bond which does not belong to a

I I =c-s=o I group, and has the properties of a class-two brightener, the openings in the final screen material essentially corresponding to the openings in the starting screen material. In this connection reference is made to Applicant's EP-B-0 038 104, which discloses a method for preferential growing of a metal on a screen skeleton in an electrolytic bath. With this method the growth can take place essentially at right angles to the plane of a screen. The invention further provides a wire material, comprising essentially linear wire sections which are joined to each other locally, which material is intended for use in the production of a screen material according to the invention. In the case of this wire material the screen holes have not yet been formed through deformation of the wire sections, but all wire sections lie essentially next to each other.

The invention finally provides a device for the continuous production of wire material according to the invention, at least comprising a frame with a wire support, wire feed means for feeding wire sections to the wire support, and joining means for joining the wire sections to each other locally. Such a device according to the invention can be designed in different ways.

Preferably the wire support is designed as a rotatable roller, driving means being present to rotate

said roller, so that wire can be wound onto said roller.

More preferably said roller is supported at one end only and a circumferential stop is present near said supported end. Advantageously the wire feed means comprise at least one wire reel feeding wire continuously.

Thus, wire can be wound onto the roller in such a manner, that the wire is wound onto the roller between the wire already present on the roller surface, and the circumferential stop near the supported end. Thereby, the wire present on the roller is pushed to the unsupported end. Before leaving the roller, the wire sections are joined to each other locally, e.g. by welding or the like. It will be clear, that the shape of the roller is not critical, and depends on the final application of the material produced. In the case of cilindrical screens for rotary screen printing a roller with a circular cross section is preferred, although other cross section are also feasible for other purposes. in another aspect of the device according to the invention, the wire support is designed as at least two rotatable support wheels with radially extending projections, and with a common axis, driving means being present to rotate said wheels. In this case advantageously the wire feed means comprise at least one wire reel feeding wire continuously, and whereby reciprocating means are present to guide the wire from one wheel to another, and around the projections.

The wheels are provided with radially extending projections, being spikes, pins or the like, such that the wire can be carried between the wheels in a zig-zag winding around the respective projections on the wheels. That is, the wire is guided to a wheel, bent around a projection of said wheel, guided to the other wheel and bent around a projection of the respective wheel back to the first wheel again etc. The means which guide the wire from one wheel to another need to reciprocate, whereas the wheels rotate, which movements together position the wire in a zig-zag condition between the wheels. This will be

explained in more detail with reference to the drawing. Preferably the joining means are designed as a movable laser welding device.

The wire material produced with the device according to the invention must be deformed, to open the screen holes. This can be achieved by drawing in the correct direction. In the case of a cylindrical product this opening of the holes can be advantageously achieved by a device for forming screen holes in a cylindrical product made of wire material according to the invention, at least comprising a frame with a screen-hole-opening-element, comprising a conical bore, and moveable mounting means for pulling the cylindrical product through the conical bore in the direction in which the diameter of the bore decreases, the respective components being designed in such a way, that at least a portion of the cylindrical product can rotate relative to the screen-hole-opening-element.

The invention will be explained in greater detail below with reference to the appended drawing, in which: Fig. 1 shows an embodiment of screen material according to the invention;

Fig. 2 shows the screen material according to Fig. 1 which has been subjected to additional deformation in the horizontal direction; Fig. 3 shows another embodiment of the screen material according to the invention;

Fig. 4 shows yet another embodiment of the screen material according to the invention with zigzag wires and linear wires; Fig. 5 shows yet another embodiment of the screen material according to the invention with rectangular holes; Figs. 6 a-e show examples of wire cross-sections; Fig. 7 shows a diagrammatic view of adjacent, contacting wire sections with round cross-sec ion which are welded to each other;

Fig. 8 shows a section of Fig. 7 along the line VIII-VIII;

Fig, 9 shows screen material according to the invention, obtained by forming the holes of the welded

- 10 - material according to Fig. 7;

Fig. 10 shows a diagrammatic view of wire sections with oval cross-section which have been welded to each other locally; Fig. 11 shows a section of Fig. 10 along the line XI-XI;

Fig. 12 shows screen material according to the invention which has been obtained by forming the holes of the welded material according to Fig. 10; Fig. 13 shows an embodiment of screen material according to the invention with crossing wires;

Fig. 14 shows a section of Fig. 13 along the line XIV-XIV;

Fig. 15 shows a section of Fig. 13 along the line XV-XV;

Fig. 16 shows a diagrammatic view of a device for producing a sinusoidal wire;

Fig. 17 shows a diagrammatic view of a device for winding wire around a roller and joining adjacent wires locally;

Fig. 18 shows a diagrammatic view of the winding of a plurality of wires;

Fig. 19a shows a diagrammatic view of a continuously operating device for the production of screen material according to the invention prior to the formation of the holes;

Fig. 19b shows a section of the bearing arm with wound and welded cylindrical material;

Fig. 19c shows a section of another embodiment of the bearing arm from Fig. 19a;

Fig. 20 shows a diagrammatic view of a device for forming screen holes; and

Fig. 21 shows a diagrammatic view of a continuously operating device for the production of flat screen material according to the invention prior to the formation of the holes.

Fig. 1 shows a first embodiment of a screen material according to the invention made of iron wire, with wire pieces 1 and junction points 2. A wire section 3 is

indicated by hatching. In the case of this embodiment of the screen material, wire sections 3 comprising several wire pieces 1 are joined to each other by welding at junction points 2. This embodiment can be produced by placing linear wire sections 3 next to one another, then joining them point-wise in a suitable manner, for example by laser welding, and subjecting the material thus formed to a tensile force essentially at right angles to the wire sections, in order to form the screen holes 4. Wire sections 3 which have been provided beforehand with a zigzag shape can also be welded to each other at the tips, with the result that the formation of the holes is not necessary.

Fig. 2 shows the material according to Fig. 1 with the formation of the holes being continued through drawing in the horizontal direction in the figure, so that the screen holes 4 have become lozenge-shaped.

Fig. 3 shows an embodiment of the screen material according to the invention in which the wire sections 5 are sinusoidal and are made of wire pieces 6 which bound screen holes 7. The sinusoidal wire sections 5 are joined to each other at the tips at 8.

Fig. 4 shows another embodiment of the screen material according to the invention, comprising wire sections 9 and 10 with respective wire pieces 11 and 12. The wire sections 9 are linear, and the wire sections 10 are sinusoidal. It will be clear that this embodiment cannot be produced by placing wire sections next to each other, joining them point-wise, and then forming the screen holes, but that the wire sections 10 must be provided with the abovementioned sinusoidal shape prior to being joined to the wire sections 11.

Fig. 5 shows an embodiment corresponding to Fig. 1 with wire sections 14, which comprise two types of wire pieces 15 and 16 respectively of differing lencrths which bound rectangular screen holes 17. This embodiment can be obtained by laying wire sections next to one another and joining them point-wise in a suitable pattern.

In the light of the above, it will be clear that

the shape and dimensions of the screen holes can be varied in a very wide variety of ways through selection of the wire thickness, the wire material, the position of the junction points, the aperture of the screen holes etc. The wire shape used in the screen material according to the invention is not very restricted. Figs. 6a-e show a round, square, triangular, rectangular and oval wire cross-section, all of which are suitable for use in the invention. The triangular and oval cross-sections are preferable, the latter being most preferable. Use of a triangular wire cross-section will produce a screen with an approximately conical hole shape at one side, which during use of the screen easily releases material to be screened and does not become blocked when material is supplied to the flat side, something which is also an advantage during use for screen printing. Use of an oval cross-section produces a screen material which is very similar to the screen materials obtained by electroplating and discussed in the introductory section of the description, which screen materials have particularly good printing properties.

Fig. 7 shows metal wire sections 18 placed next to one another and having weld points 19, a section of which is shown along the line VIII-VIII in Fig. 8. If the interconnected wire sections 18 from Fig. 7 are deformed in the direction indicated by the arrows, the structure indicated diagrammatically in Fig. 9 is obtained, with screen holes 20. It will be clear that the shape of the holes can be suitably varied, as was discussed earlier. Fig. 10 shows diagrammatically wire sections 21 situated adjacent to each other and interconnected over some length at 22.

Fig. 11 shows a sectional view along the line XI-XI in Fig. 10, in which it can be seen clearly that the wire sections have an oval cross-section.

Fig. 12 shows a preferred embodiment of screen material according to the invention, which is obtained by subjecting the wire material according to Fig. 10 to a tensile force in the direction of the arrows, so that the

screen holes are formed. These hexagonal screen holes, in particular in combination with an oval wire cross-section, have very advantageous characteristics for screen printing, since hexagonal holes approach the traditional shape of screen printing stencils, and optimum distribution of screen holes is also obtained over the surface, with additional strength compared with differently shaped holes. The wire pieces 21' are joined to each other over a short length by weld points. It will be clear that a smaller number of weld points situated further apart or a continuous weld can also be used. A wire piece 21' comprises a wire section going out from the centre of a weld to the centre of an adjoining weld in the same wire section 21. If the hexagonal structure from Fig. 12 is stretched (opened) further, an essentially rectangular hole shape will be obtained, the holes being arranged in a staggered manner (brickwork structure) .

For the joining of metal wires, a soldering medium such as tin, lead, lead-tin, brass, silver or the like can also be fed in during the welding, with or without a flux. Of course, a corresponding technique can also be used for joining wires made of other materials.

Fig. 13 shows another embodiment of screen material according to the invention, showing two groups of wire sections 24 and 25 which are joined to each other at the crossing points through fusion and/or merging, through which screen holes 26 are formed.

Figs. 14 and 15 show the sectional view along the line XIV-XIV and the line XV-XV. As is clear, a screen material with an essentially flat screen surface is also obtained in the case of this embodiment.

The joining of such crossing wires can be carried out by techniques which are known to those skilled in the art. For example, the wires can merge with each other at the crossing points through welding or the like, or through the application of pressure locally at raised temperature or otherwise, i.e. by setting up the groups of wire sections 24 and 25 so that they cross each other and then compressing them between heated or unheated pressure plates

or the like. A quartz lamp can also be used for forming connections at the crossing points through fusion and/or merging.

The material of the wire used for the screen material according to the invention is not very limited, but in particular for the production of screen printing material it is preferable to use metal wire, since the latter is easily joined locally by welding by means of, for example, a laser, and any curves made in advance in metal wire are dimensionally stable. Examples of suitable metals are: iron, steel, copper, nickel, chromium etc.

In the embodiment shown in Fig. 13 it is preferable for at least one of the groups to be made of plastic, so that the compound is simplified through merging at the position of the crossing points.

The screen material according to the invention can be subjected to additional coating steps after its production, for example in order to strengthen it. For instance, after its surface has been made conducting if desired, in the case of plastic wire, the screen material can be plated with a suitable metal in a known manner.

After production and before possible metallization, the material can also be subjected to a flattening operation, in order to remove uneven points. Examples are planing, polishing, gauging etc.

Fig. 16 shows diagrammatically a device with two gear wheels 27 and 28, between which a wire 29 is conveyed by way of guide rollers 30 and 31. A zigzag-shaped wire is obtained by driving the abovementioned gear wheels 27, 28 in the direction indicated. If the wire is a metal wire, it may if desired be subjected to a heat treatment after the deformation operation, in order to give the wire dimensional stability, as a result of the hardening properties of the chosen material. Fig. 17 shows diagrammatically a device for the production of screen material according to the invention, in which a wire 33 is wound around a roller 32. The roller is rotated in the direction of the arrow, and the wire 33 is drawn off a reel 34. During the winding, the reel 34 can

be moved parallel to the roller 32 in the direction of winding. Reference number 35 indicates a moving laser device which locally welds the wire 33 wound around the roller 32. The laser device advantageously comprises positioning means which can comprise a tracer for determining the mutual position of the weld points in relation to the wire sections, on the basis of which the position of the laser can be set by means of suitable laser movement means. After the roller 32 has been provided with wire 33 over its full length and local welding has been carried out, the material thus produced is removed from the roller 32 and subjected to a tensile force in the lengthwise direction, as a result of which the length of the cylinder increases, the diameter decreases, and the screen holes are formed. It will be clear that the reduction in diameter or the increase in length of the screen cylinder and the hole shape and hole size depend on the mutual position and dimensions of the junctions, the wire diameter and the degree of deformation. If the wire 33 is conveyed through the device of Fig. 16 prior to winding, the final material need not be subjected to a deformation treatment. The mutual positioning of the tips of the already wound wire relative to the wire yet to be wound can be regulated by regulating the wire tension in a suitable manner in the inlet during winding.

Fig. 18 shows the same roller 32, winding here taking place simultaneously with a plurality of wires 33, of the same or different type and shape, as a result of which the surface of the roller can be covered with wire more quickly at the same speed of rotation. During winding involving a plurality of wires in this way, the winding pitch will increase for each individual wire.

The result of this is that when the screen holes are formed in the screen material removed from the roller 32 after welding, by a tensile force in the lengthwise direction, the material will have an inherent twist. This means that during drawing at the ends of the screen cylinder, at least one of the attachment components must be

set up in a rotatable manner.

Fig. 19a shows diagrammatically a device for the continuous production of screen material according to the invention. It contains a reel 34 from which wire 33 is unreeled. Although one reel is shown, it is, of course, possible, as discussed earlier, for a plurality of wires to be wound simultaneously.

Reference number 35 shows a drive mechanism of a mandrel 36, onto which the wire 33 is wound. The wire 33 is guided through a wire feed control unit 37 to the mandrel 36. The wire 33 is guided between a stationary stop 38 and the already wound material, with the result that during the winding the already wound material is forced to the left in the figure. During the abovementioned movement, the wire is welded locally by means of a laser 40 set up in a fixed position. Many types of lasers are suitable for this purpose. Depending on the wire pitch, the winding speed, the type of weld and the weld pattern, the laser can operate continuously or intermittently. The mandrel 36 used is preferably interchangeable, so that cylinders 39 of different diameters, with the same number of standard measurements (pattern repeat measurements) and cross-section shapes, can be produced. Different cross-section shapes should be understood as meaning oval, triangular, square etc. cross-sections.

Reference number 41 indicates a movable trestle, which by means of a bearing arm 42 serves to support the material 39 moving from the roller 36.

The bearing arm 42 of the trestle 41 is preferably in the form of a freely rotating roller, and more preferably the bearing arm is designed in such a way that it has several freely rotatable rollers disposed around the inner periphery of the cylinder produced, so that the cylinder material produced can be supported at several points. Fig. 19b shows a section of the bearing arm 42 from Fig. 19a with cylindrical material 39 which has been produced. Fig. 19c shows a preferred embodiment of the bearing arm 42 in section, which has four supporting rollers 44 for supporting the material 39. The mutual

position of the rollers advantageously is adjustable.

With the device according to the invention the pitch can be regulated by, for example, placing a ring- shaped stop with an oval cross-section at a suitable angle around the mandrel.

Reference number 43 indicates diagrammatically a cutting device which also moves along and serves to cut off a certain length of material after it has formed. After cutting, said cylindrical material can be moved by means of the trestle 41. Thereafter, the arm of another trestle can be placed in the material 39 moving off the roller 36. The supporting arm 42 on the trestle 41 can be suitably adjusted in height, for example by means of a screw spindle 45 or the like. Lengths of the cylindrical material to be obtained can also be separated from each other in another way. For example, if the laser is interrupted for some time after a certain period of operation of the device, a number of windings of wire are not welded. This number can be selected in such a way that at a suitable moment the cylindrical material already produced can be removed from the mandrel, in which case the wound wire material not welded is drawn as a coil and can be severed. The advantage of this is that only one wire need be cut through, as against the complete severing of a cylinder.

Fig. 20 shows diagrammatically a device for forming screen holes, comprising a forcing device 46 as a screen- hole-opening element. With this device 46 the material 39 in which screen holes are not yet formed is forced, as shown diagrammatically at 47, in the direction of the arrow, as a result of which the screen holes are formed in the material 39, as indicated at 48. It will be clear that the drawing section (not shown) by means of which the material 39 is moved through the forcing device or the forcing device itself must be set up in a rotatable manner, since a certain degree of rotation, depending on the method of winding the wire (or wires) , will occur during the formation of the screen holes. It may be advantageous during the formation of the holes by means of the forcing

device 46 to set up a support element in the product, in order to be able to carry out this operation in a reproducible manner.

If in a production process large numbers of wire cylinders whose holes have not yet been formed are being produced, in successive wire cylinders the screen holes can be formed to a gradually increasing extent, which means that successively cylinders with slightly decreasing diameter are produced and can be pushed into one another. The transportation and storage thereof is thus greatly simplified. The screen holes in the desired final form can then be made in these cylinders at a desired time. The same applies for wire material wound in the lengthwise direction of a roller, where the diameter increases and the length decreases during formation of the screen holes.

In the case of cylindrical screen material the wire can also be wound in the lengthwise direction of a roller, in which case projections, for example, such as pins, can be present around the periphery at the ends of the roller, so that the wire can be wound to and fro in the lengthwise direction. For winding in the lengthwise direction, the wire can also be wound so as to cross over the end faces of the roller. When such a cylinder made of wire material is welded locally, during formation of the screen holes a diameter increase and a length reduction of the cylinder will be the result, contrasting with what has been described above. For this operation it is possible to use, for example, an expandable core, or the wound wire material can be moved in the opposite direction over the forcing device 46 from Fig. 20, in order to form the screen holes. For the production of the embodiment of the screen material according to Fig. 13 in cylindrical form, a wire can be wound at such a pitch in one direction around a support, as shown, for example, in Fig. 17 or 18, that wire sections disposed at a distance from each other are obtained, following which the same wire may if desired be wound over the first group of wire sections in the opposite direction with corresponding desired pitch. Furthermore, several groups of wires can be wound, following which the

wires are joined to each other at the crossing points by means of suitable techniques known to those skilled in the art, such as welding or pressing at elevated temperature through fusion and/or merging. By means of the method described above, screen material according to the invention can be produced with a very wide range of numbers of holes per linear cm and of hole shapes, while flat screens can also be produced in a corresponding manner. These can be produced directly or by cutting open cylinders according to the invention in the lengthwise direction.

It is also pointed out that although essentially symmetrical screen materials are described above, the invention is not limited thereto, since any desired deformed wires can be joined to each other in any desired way, so that flat screen material is obtained with any desired shape of holes and any desired dimensions, which can be advantageous for certain applications. The so-called moire effects in particular are suppressed in this case. Screen material can also be produced in flat form by laying wire sections which may consist of one and the same wire next to each other on a flat support, then joining them locally, and subsequently forming the screen holes. Finally fig. 21 shows a diagrammatic view of a continuously operating device for the production of flat screen material according to the invention, prior to the formation of the holes. Said device comprises at least two rotatable support wheels 50, 51 with a common axis 52 and radially extending projections designed as pins 53. These pins 53 are shown in an enlarged view A in fig. 21.

The wheels support endless belts 55 and 56 provided with openings 57 into which the pins 53 fit. It will be clear that the belts are endless belts and guided over an additional pair of driving wheels (not shown) , also comprising pins or the like.

A guide bar 58 is present forming the reciprocating means to guide a wire 60 from wheel 51 to wheel 50 and back, around the respective pins 53. Thus, a zig-zag movement of the wire is provided. The driving means of said

guide pin are not shown for clarity reasons.

The wire 60 is unreeled from a reel 61 provided with a slip-coupling 62 to provide the necessary wire tension. 65 schematically shows a laser welding device which is moveable along a slit 66 in guide means 67. In the laser welding device a laser beam 68 is passed through a focussing optic 69 and impacts on the zig-zag wire material at a focal point 70. Very close to said focal point 70 a wedge 71 is present, which is shown in the enlarged cross- section B. Said wedge 71 can be moved in the direction of the arrow 72 to move the wire section 73 against the wire material 74 already locally welded in the direction of arrow 78. Said wedge 71 is provided with drive means (not shown) to follow the movement of the focal point of the laser welding device, assuring that at the focal point the wires are positioned against each other for accurate welding.

The wire material 75 being locally welded is supported by the driving belts 55 and 56 and further by an additional conveyor belt 76 driven by a drive roller 77 and another corresponding drive roller, not shown.

For the purpose of clarity only the important details are shown in fig. 21 and of course many modifications can be made.

The drive belts 55 and 56 do not only drive the wheels 50 and 51, and support the wire material 75, but also assure that the welded wire material 75 is removed from the pins 53. Preferably means are present between the guide bar and the reel 61 to compensate for differences in length of the wire 60 fed to the pins of the wheels, due to the reciprocating movement of the guide bar 58. This can be achieved by moving the reel 61 together with the guide bar 58, or by spring loaded wheels or the like.

After manufacture of the wire material 75 the screen holes can be opened by suitable deformation of the material as discussed with reference to earlier figures.