ELEMANS NORBERTUS FRANCISCUS J (NL)
JONKERS THOMAS MARIA (NL)
WEPEREN KARST JAN VAN (NL)
ELEMANS NORBERTUS FRANCISCUS J (NL)
JONKERS THOMAS MARIA (NL)
NL8802144A | 1990-03-16 | |||
EP0737637A1 | 1996-10-16 | |||
GB2271744A | 1994-04-27 |
1. | Printing forme (1; 21) for use in the printing of substrates, which printing forme (1; 21) comprises a thinwalled cylindrical support (2; 22) with openings (3,5; 23), which openings (3,5; 23) are separated by dykes, the printing forme (1; 21) being provided with detectable indicator means (7,7'; 26) for indicating the position of the printing forme (1; 21), characterized in that the indicator means (7,7' ; 26,27) can be read continuously with the aid of a suitable detection device. |
2. | Printing forme according to claim 1, characterized in that the indicator means (7,7'; 26,27) can be read inductively, capacitively, optically, electrically or acoustically. |
3. | Printing forme according to one of the preceding claims, characterized in that the indicator means (7,7' ; 26,27) comprise a detectable element which is selected from the group consisting of material in which magnetism can be induced, optical material, dielectric material or an acoustic element. |
4. | Printing forme according to one of the preceding claims, characterized in that the indicator means (7) are provided on the circumference of the cylindrical support (2), in the vicinity of its two ends. |
5. | Printing forme according to claim 4, characterized in that an additional continuously readable indicator means (7') is provided on the circumference of the cylindrical support (2), which additional indicator means is aligned with the indicator means (7) in the vicinity of the ends of the support (2). |
6. | Printing forme according to claim 5, characterized in that the additional indicator means (7') is provided approximately halfway along the length of the cylindrical support (2). |
7. | Printing forme according to one of the preceding claims 13, characterized in that the indicator means comprise a detectable element (27) which is in wire or strip form and extends over the length of the printing forme. |
8. | Printing forme according to one of the preceding claims, characterized in that the support (2; 22) is made from plastics material, preferably fibrereinforced plastics material. |
9. | Printing forme according to one of the preceding claims, characterized in that there are a plurality of indicator means (7, 7'; 26, 27) distributed over the circumference of the support (2; 22). |
10. | Rotary screenprinting device provided with at least one printing station, characterized in that the printing station is provided with detection means for detecting the indicator means of a printing forme (1; 21) according to one of the preceding claims, and means for determining the position of the printing forme (1; 21) with respect to a fixed point (A) on the printing device. |
11. | Gluing device for attaching end rings to a cylindrical printing forme, comprising a stand for supporting and rotating the printing forme (1) according to one of the preceding claims, which device is also provided with detection means for detecting the indicator means (7,7') of a printing forme, and means for determining the position of the printing forme with respect to the stationary structure. |
12. | Printing forme for use in the printing of substrates, which printing forme (21) comprises a thinwalled cylindrical support (22), and which support has at least one surface layer (25) which can be engraved and/or processed by laser, and provided with detectable indicator means (26,27) for indicating the position of the printing forme (21), in particular indicator means which can be read continuously with the aid of a suitable detection device. |
13. | Engraving device for making a design which is to be printed in the engravable surface layer (25) of a printing forme (21) according to claim 1 by means of engraving and/or perforation with the aid of highenergy radiation, comprising a source for the generation of highenergy radiation and directing means therefor, and a stand for supporting and rotating the printing forme (21), which device is also provided with detection means for detecting the indicator means (26, 27) of the printing forme (21), and means for determining the position of the printing forme (21) with respect to the stationary structure. |
A printing screen-a thin-walled cylinder in which a regular pattern of openings is provided, separated by the walls of the dykes-is known in the field, crossmarks being provided in the vicinity of the left-hand and right-hand ends of the cylinder, which crossmarks, prior to the printing, are read with the aid of a laser in order to preset the position of the printing screen in the screen- printing device used. A metal printing screen may be formed, for example, by electroforming. The printer usually provides the printing screen with a layer of photoresist or resin, which is then exposed and developed using light in accordance with a design which is to be printed, and the exposed or unexposed parts of the layer of photoresist or resin are removed, depending on whether a positive or negative photoresist is used. After the remaining layer of photoresist has been stoved (hardened), the result is a stencil, i. e. a printing screen provided with a design which is defined by the exposed (printing) openings. A stencil of this type is ready for use in screen-printing.
In another known printing forme, there is no light- sensitive photoresist used as a coating layer on the printing screen, but rather the design is made in a suitable layer of resin or plastic by means of engraving (down to the printing screen). This engraving can be carried out with the aid of a laser, such as a CO2 laser. In terms of its structure, a stencil produced in this way is similar to the stencil in which the photoresist technique is used.
Another known printing forme for screen-printing is
produced by the electrodeposition (electroforming) of metals on a mould which bears a pattern of the printing openings to be formed.
After sufficient metal has grown on, the printing forme obtained in this way is removed from the mould. In this way, a printing forme with printing openings which therefore does not include a coating layer covering the said printing openings is formed in one step. A printing forme produced in such a way is referred to in the field as a"galvano".
Furthermore, a plastic printing forme which comprises a cylindrical or plate-like support in which the design to be printed, i. e. the printing openings, are made directly by means of perforation is known in the field. Like the electroformed printing forme, this printing forme is known as a"galvano", even though there is actually no electrodeposition of metals. The perforations may be made, for example, using a laser, as described, for example, in NL-A-8802144.
One of the problems of cylindrical printing formes is that the exact position of the printing forme in rotation is not known.
This applies both when the design is being made, for example during exposure or during engraving, but also during the printing process itself. This position can only be approximately established with the aid of the so-called picopoint, a small protrusion on an end ring, which end rings are used in metal stencils. The accuracy of this approximation deteriorates further if an end ring of this type is not correctly positioned, for example has been glued in a skew position to the stencil or has become twisted. In addition, play in the drive of the rotating stencil causes further deviation and therefore inaccuracy in the position of the stencil. Correction of deviations of this type (placing in register) is carried out by hand.
Furthermore, during printing itself a stencil becomes arched, partly as a result of sagging, which may be considerable for a stencil length of up to 5 m. The printer attempts to make and keep the mean deviation, the sum of all the abovementioned deviations, as low as possible, so that the register is as accurate as possible.
Also, the use of the abovementioned laser-readable crossmarks does not allow the position of the printing forme to be determined during printing, since on the one hand these marks, during
printing, become dirty and covered by printing ink or paste or residues thereof, so that they can no longer be detected by a laser, and on the other hand these marks and the laser used are only suitable for allowing preliminary positioning prior to printing.
Moreover, detection for this preliminary positioning is carried out on the top side (i. e. the point which is furthest away from the substrate) of the printing forme. However, deviations at the top side are not definitively representative of deviations at the bottom side.
The object of the present invention is to allow more accurate determination of the position of a printing forme during rotation, in particular during printing and processing, such as engraving or when end rings are being glued in, thus facilitating placing the printing forme in register if necessary.
In the printing forme according to the invention, the indicator means can be read continuously with the aid of a suitable detection device. When using the printing forme according to the invention, the indicator means, which unambiguously denote one position of the printing forme, can be detected continuously, in particular also during rotation. By comparing this position to a fixed point, for example of a printing station, the exposure unit or the engraving device, it is possible to determine or calculate the curvature angle and therefore the rotation of the printing forme. If the precise position of the printing forme has thus become known, placing the printing forme manually in register (both in terms of length, width and diagonally) can be facilitated or rendered altogether superfluous. This position determination can be carried out both before printing and during printing, so that further adjustment (placing in register), if necessary, is possible. It has been found that one revolution is sufficient to accurately position the printing forme according to the invention.
The term printing forme comprises the types described above, namely printing screen, stencil and galvano, made from either metal or plastic.
The indicator means can advantageously be read inductively, capacitively, optically, electrically or acoustically and preferably comprise a detectable element which is selected from
the group consisting of material in which magnetism can be induced, optical material, dielectric material or an acoustic element. The printing forme is preferably made from a (fibre-reinforced) plastics material which has no effect on the detection means for the detection of the indicator means. If a plastic printing forme is used, the detection of indicator means which are in the form of small magnets or other magnetic materials is possible with the aid of Hall sensors or by an inductive route, in which case a static reader head is used.
Examples of materials in which magnetism can be induced include steel, iron and audio tape. Examples of a suitable dielectric include aluminium and a plastics material other than that from which the cylindrical support is made. If acoustic technology is used for the determination of position, ultrasonic sound waves are preferably used.
According to a preferred embodiment of the printing forme according to the invention, the indicator means are provided on the circumference of the cylindrical support, in the vicinity of its two ends. If these indicator means lie on a line which is parallel to the longitudinal centre axis of the support, no prior calibrations are required. More preferably, an additional indicator means is provided on the circumference of the cylindrical support, advantageously in the centre thereof, which additional indicator means is aligned with the indicator means in the vicinity of the ends of the support. This combination of indicator means at both ends and in the centre which lie on a line parallel to the centre axis of the printing forme makes it easy also to detect the sagging (arching) if a measurement is carried out in the vicinity of the printing point, i. e. that position of the printing forme in which the indicator means lie close to the point of contact between the printing forme and the substrate which is to be printed. It is thus possible for the register to be adjusted automatically in the event of a varying stencil load and resultant sagging.
According to another preferred embodiment, the indicator means comprise a detectable element which is in wire or strip form, extends over the length of the printing forme and is advantageously arranged parallel to the longitudinal axis of the printing forme. A
printing forme of this type can be shortened to any desired length without its detectability being lost. The printing forme may be provided with a plurality of wire or strip elements which are distributed over the circumference. An example is 4 wires, so that the position can be detected four times per revolution.
The dimensions of the indicator means should be such that they do not interfere with the design to be made. In an embodiment according to the invention, for this purpose indicator means are provided at a plurality of points on the circumference of the cylindrical support. When making the design, some of these indicator means will be removed. However, after the design has been made it is known which indicator means have been removed at which position. The detection system can be (automatically) compensated for this.
The indicator means described above are able to withstand a moist environment and are relatively insensitive to contamination and other dirt.
The invention also relates to a rotary screen-printing device which comprises at least one printing station, the printing device being provided with detection means for detecting the indicator means of a printing forme according to the invention, and means for determining the position of the printing forme with respect to a fixed point on the printing device.
The invention can also be employed in other methods and devices in which it is important to know the exact position of a printing forme or a precursor thereof. One example is a gluing device for attaching end rings to a stencil as defined in claim 10. An example of a precursor of a printing forme is a cylindrical support which has a surface layer which can be engraved, for example using high-energy radiation, for example from a laser, as described in claim 11. An engraving device intended for this purpose forms the subject of claim 12. In these applications, the same advantages as those described above are achieved, and the same indicator means and detection devices as those described above can be used.
The invention is explained below with reference to the appended drawing, in which: Figure 1 shows a diagrammatic, perspective view of a
nickel stencil according to the invention; and Figure 2 shows a diagrammatic, perspective view of a plastic galvano.
The figure shows a cylindrical stencil made from nickel, bearing reference numeral 1, comprising a thin-walled support 2 in the form of a printing screen in which there are printing openings 3 which define a design 4 which is to be printed, in addition to other screen openings 5 which are covered with a hardened layer of photoresist 6. The printing screen has been produced by electroforming, as is well known in the prior art. The design is made by means of conventional techniques using photoresist. On the circumference of the support 2 there are small magnets, at both ends (7) and in the centre (7'). These magnets 7 and 7'lie on a line which is parallel to the longitudinal centre axis of the printing forme 1. The magnets 7,7'can be detected with the aid of suitable sensors (not shown), and in this way the position B of these magnets 7,7'with respect to a fixed point A can be established and the printing forme can be automatically brought into register. The presence of the magnet 7'in the centre between the magnets 7 allows sagging of the printing forme 1 with respect to the magnets 7 to be determined. This sagging can only be detected if the magnets 7 and 7' are situated at the bottom (close to a substrate (not shown) which is passed along the bottom of the stencil).
If the rotary screen-printing device which is used for printing has a position-detection system, end rings (not shown) can be glued into the printing forme in any suitable way. If the rotary screen-printing device is not provided with a position-detection system, the correct position of the end rings can be determined with the aid of the magnets 7 and 7'prior to the gluing, in order to correct any deviations.
Fig. 2 shows a"galvano"21 which has been produced from a plastic which can be processed by laser and comprises a thin cylindrical support 22 with printing openings 23 which define a design 24 which is to be printed. The openings 23 are made in the outer surface 25 of the support 22 by means of engraving and perforation using high-energy radiation, for example from a CO2
laser. In this case too, indicator means 26 are provided at the same positions as in the stencil shown in Fig. 1, in order to allow positioning during the engraving and perforation itself as well as during printing. Furthermore, a detection wire 27 which extends over the length of the galvano 21 and runs parallel to the longitudinal axis, is provided as indicator means.
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