HK1143115B - Screen printing press and method - Google Patents

Abstract

There is described a sheet-fed or web-fed screen printing press, especially for the production of banknotes and like securities, comprising an impression cylinder (2a), a first screen printing unit (2b) cooperating with the impression cylinder (2a) for applying a first ink pattern (A) onto successive sheets or web portions, at least a second screen printing unit (2c) cooperating with the impression cylinder (2a) for applying a second ink pattern (B) onto the successive sheets or web portions, which second screen printing unit (2c) is located downstream of the first screen printing unit (2b) with respect to a direction of rotation of the impression cylinder (2a), and an intermediate drying unit (10) located between the first and second screen printing units (2b, 2c). The intermediate drying unit (10) is adapted to perform superficial drying of the first ink pattern (A) applied by the first screen printing unit (2b) before application of the second ink pattern (B) by the second screen printing unit (2c), which superficial drying involves only the drying of an outer superficial portion (S) of the first ink pattern (A). Preferably, the second screen printing unit (2c) is adapted to apply the second ink pattern (B) in close proximity to the first ink pattern (A) or at least in a partially overlapping manner onto the first ink pattern (A).

Description

Screen printing machine and method

Technical Field

The present invention relates generally to a paper-fed or web-fed screen printing press, particularly for the production of banknotes and similar securities, of the type comprising: an impression cylinder; a first screen printing unit cooperating with the impression cylinder to apply a first ink pattern onto the continuous or web portion; at least a second screen printing unit cooperating with the impression cylinder to apply a second ink pattern onto the continuous or web portion, said second screen printing unit being located downstream of the first screen printing unit with respect to the direction of rotation of the impression cylinder; and an intermediate drying unit located between the first screen printing unit and the second screen printing unit.

The invention also relates to a method for performing screen printing of a continuous paper or web portion, in particular for producing banknotes and similar securities on a screen printing machine of the above-mentioned type.

Background

Printing machines of the above-mentioned type (except provided with an intermediate drying unit) are already known, for example, in the international application No. wo 97/34767 filed in the name of the present applicant in the field. Printing machines of this type are known under the trade nameSold by the applicant. Further details regarding screen printers can be found in european patent applications EP 0723864, EP 0769376 and in international applications WO 97/29912, WO 03/093013, WO2004/096545 and WO 2005/102699.

In the context of the production of security documents, such as banknotes, screen printing (also known as "silk-screen printing" or "silk-screen printing") is particularly adopted to print optically variable patterns on documents, including so-called iridescent patterns (iridescents) and security patterns(optically variable ink) Pattern (Is a registered trademark of SICPA Holding SA, switzerland). Such patterns are printed using ink or paint containing special pigments or flakes that produce optically variable effects.

According to international application No. wo 97/34767, the first and second screen printing units are designed to apply respective ink patterns in a non-overlapping manner, the printer being arranged so that the second screen printing unit does not come into contact with the pattern previously applied by the first screen printing unit.

According to a first embodiment disclosed in international application No. wo 97/34767, the second screen printing unit comprises a stencil cylinder (stencil cylinder) which exhibits a more complex configuration than that of the first screen printing unit with raised and lowered surface portions which are non-printing portions and which coincide with the position of the pattern applied by the first screen printing unit. Depending on the actual configuration of the stencil cylinders of the second screen printing unit, the squeegee arrangement (sector blade arrangement) of the second screen printing unit also needs to be adjusted in its shape and/or in the way it is operated.

According to another embodiment disclosed in international application No. wo 97/34767, the chablon cylinder of the second screen printing unit is designed to be elastically deformable, and the annular portion is arranged to prevent deformation of the chablon cylinder at selected positions corresponding to the positions of the ink patterns applied by the first screen printing unit. Furthermore, the squeegee arrangement of the second screen printing unit comprises a comb shaped squeegee blade adapted to cooperate only with the deformable position of the stencil cylinder.

Non-overlapping applications of ink patterns using the above known printer configuration are always considered necessary in order to avoid ink contamination problems and to ensure good print quality. Thus, with the machine configuration of international application No. wo 97/34767, it is not possible to apply patterns made of different inks in close proximity to each other or even in an overlapping manner. This constitutes a limitation on the construction of these machines, which is expected to be overcome.

International application No. wo2005/000585, also in the name of the present applicant, discloses a screen printing machine of the above-mentioned type, which is also suitable for performing the orientation of magnetically-orientable pigments (pigments) contained in the printing ink by applying a magnetic field to the just-printed ink pattern before drying it. According to one embodiment discussed in international application No. wo2005/000585, it is envisaged to apply two ink patterns one on top of the other using a first screen printing unit and a second screen printing unit, and to perform drying of the first ink pattern applied by the first screen printing unit before applying the second ink pattern with the second screen printing unit. Drying may be performed using a UV lamp or any other equivalent drying system. However, international application No. wo2005/000585 is completely silent about how the intermediate drying unit is actually integrated into the screen printing machine.

The use of an intermediate drying unit as briefly suggested in international application No. wo2005/000585 is therefore not actually put into practice by the applicant. It is in fact considered that the close proximity of the two screen printing units in the above-mentioned prior art machine configuration would make it impossible to perform a correct drying of the first ink pattern applied by the first screen printing unit before the application of the second ink pattern by the second screen printing unit located downstream.

Summary of The Invention

It is therefore a general object of the present invention to provide an improved screen printing machine of the type comprising an impression cylinder, at least two screen printing units cooperating with the impression cylinder, and an intermediate drying unit located between the first and second screen printing units.

Another object of the invention is to provide a screen printing machine of the above-mentioned type which is capable of applying at least two different ink patterns in close proximity to each other or in an at least partially overlapping manner.

Another object of the invention is to provide a solution that is easy and cost-effective to implement in practice.

Another object of the invention is to provide a solution that ensures that there is no contamination between the inks applied by the first and second screen printing units, or that there is no degradation in the print quality.

These objects are achieved thanks to the solution defined below.

According to the present invention, the intermediate drying unit is adapted to perform surface drying of the first ink pattern applied by the first screen printing unit, which involves drying only an outer surface portion of the first ink pattern, before the second ink pattern is applied by the second screen printing unit.

Thanks to this solution, a correct integration of the intermediate drying unit in the screen printing machine can be ensured, and it is also possible to greatly simplify the construction of the second screen printing unit. In fact, it is no longer necessary to design and/or operate the second screen printing unit in the manner taught in international application WO 97/34767. In the context of the present invention, two screen printing units can therefore and advantageously make use of similar stencil cylinders and squeegee arrangements, which greatly simplifies the machine structure and reduces the manufacturing and operating costs of the printing press.

According to a preferred embodiment, the second screen printing unit is adapted to apply the second ink pattern next to or at least in a partially overlapping manner on the first ink pattern.

It should again be understood that, prior to the present invention, it has been thought that the close proximity of the two screen printing units in the above-described prior machine configuration made it impossible to perform the correct drying of the first ink pattern applied by the first screen printing unit before the application of the second ink pattern by the second, downstream-located, screen printing unit. However, tests carried out by the applicant have shown that this is a prejudice and that the provision of an intermediate drying unit between the first screen printing unit and the second screen printing unit brings about the hitherto unexpected advantage of being sufficient to dry the surface portion of the first ink pattern applied by the first screen printing unit, rather than the entire first ink pattern, before the second ink pattern is applied by the second screen printing unit. In particular, it has been observed that the surface drying of the first ink pattern is sufficient to avoid any contamination of the ink and to ensure a high print quality.

According to a preferred embodiment of the invention, the drying unit is a UV unit for emitting UV radiation, which unit is particularly suitable for drying UV-cured inks and paints.

In an environment in which UV radiation is used, the intermediate drying unit is advantageously housed, for example to avoid interference with the first and second screen printing units, which can be caused by the propagation of the UV radiation emitted by the intermediate drying unit. Other means may be provided to ensure that the operator operating the printing press is also properly protected from UV radiation emitted by the intermediate UV drying unit.

The intermediate drying unit may comprise one or more flash lamps, such as xenon flash tubes.

The intermediate drying unit may be operated with a flashing frequency of 40Hz to 50Hz for a paper processing speed of 10 '000 to 12' 000 sheets per hour.

The paper-feeding or web-feeding screen printing machine may further include a lamp housing which may surround the intermediate drying unit so as to prevent the propagation of UV radiation and may avoid interference with the first screen printing unit and the second screen printing unit.

The first and second screen printing units and the intermediate drying unit may be housed in a machine frame comprising a protective member which may be used to protect an operator from the transmission of UV radiation.

The first and second screen printing units may each comprise a chablon cylinder, which may present a cylindrical periphery without any discontinuities.

The paper-feeding or web-feeding screen printing machine may further include at least one final drying unit, which may be disposed downstream of the second screen printing unit, so that the first and second ink patterns applied by the first and second screen printing units may be finally dried.

The invention also comprises a method for performing screen printing of a continuous paper or web portion, in particular for producing banknotes and similar securities on a screen printing machine of the above-mentioned type, wherein the method comprises the steps of: surface drying of the first ink pattern applied by the first screen printing unit is performed before the second ink pattern is applied by the second screen printing unit, the surface drying involving drying only an outer surface portion of the first ink pattern. Advantageously, the method may further comprise the step of: the final drying of the first and second ink patterns is performed downstream of the second printing unit.

Further advantageous embodiments of the invention are discussed below.

Brief Description of Drawings

Further characteristics and advantages of the invention will become better apparent from reading the following detailed description of embodiments of the invention, which are presented by way of non-limiting example only and are illustrated by the accompanying drawings, in which:

FIG. 1 is a schematic side view of a screen printing machine according to an embodiment of the present invention;

fig. 2 is a schematic perspective view of a possible configuration of a screen printing unit of the printing press of fig. 1;

fig. 3 is a schematic partial sectional view of the screen printing unit of fig. 2 with its squeegee;

FIG. 4 is a schematic side view of a preferred embodiment of an intermediate drying unit;

FIGS. 5a to 5d are schematic diagrams illustrating a drying process of an ink pattern applied using the printer of the present invention; and

fig. 6a and 6b are two views illustrating possible examples of ink patterns that can be applied using the printer of the present invention.

Detailed description of the invention

The invention will be described hereinafter in the context of a paper-fed screen printing machine for printing security papers, in particular banknotes. The general construction of the screen printing machine is similar to that described in international application WO 97/34767. However, the invention is equally applicable to web-fed screen printing presses.

Within the scope of the present invention, it is understood that the expression "screen printing" is equivalent to "screen printing" or "screen printing" also commonly used in the art. "screen printing" generally refers to a printing process in which ink is applied using a so-called stencil provided with a pattern of apertures corresponding to the pattern of ink to be applied, the ink being forced through the apertures of the stencil using a squeegee arrangement (such as that discussed in international application No. wo 2004/096545).

Within the scope of the present invention, the expression "drying" will be understood to mean equivalent to a drying process in which the liquid solvent component is evaporated (this drying process is typically caused by thermal or infrared radiation), and to a curing process in which the polymer component is hardened by crosslinking of the polymer chains (this curing process is typically caused by UV radiation).

An embodiment of a paper-fed screen printing machine according to the invention is shown in fig. 1. The screen printing machine comprises a feed station 1, which feed station 1 is used to feed a continuous paper to a screen printing group 2, in which screen printing group 2 an ink pattern is applied to the paper. In this example, the printing group 2 includes impression cylinders 2a cooperating with first and second screen printing units 2b and 2c, which first and second screen printing units 2b and 2c are placed in series along the printing path of the paper. Once processed in the printing group 2, the freshly printed sheets are transported by a conveyor system (conveyor system)3 to a delivery station 4, which delivery station 4 comprises a plurality of delivery pile units (delivery pile units), in this example three. The conveyor system 3 is typically an endless chain conveyor system comprising a plurality of spaced-apart gripper bars (not shown in fig. 1) extending transversely to the paper transport direction, each gripper bar comprising gripping means for holding an edge of the paper.

Preferably, also located along the path of the conveyor system 3 between the printing group 2 and the delivery station 4 are drying units 5, 6. These drying units 5, 6 are used to perform the final drying and curing of the screen printed ink pattern. The drying unit 5 is preferably a thermal drying unit for applying thermal energy to the paper, and the drying unit 6 is preferably a UV dryer for subjecting the paper to UV radiation. The combination of the drying units 5, 6 ensures sufficient drying and curing of the applied ink pattern and ensures that the brightness and optically variable effect of the screen printed features is maximized and sustained. One or both of the drying units 5, 6 may be mounted on the machine, depending on the manufacturing requirements.

According to the present invention, an intermediate drying unit 10 is positioned between the first screen printing unit 2b and the second screen printing unit 2 c. The intermediate drying unit 10 is adapted to dry the surface portion of the first ink pattern applied by the first screen printing unit 2b before the second ink pattern is applied by the second screen printing unit 2 c. In the context of the present invention, it is understood that the surface drying performed by the intermediate drying unit 10 involves only drying the outer surface portion of the first ink pattern applied by the first screen printing unit 2 b.

Tests carried out by the applicant have shown that surface drying of the ink layer deposited by the first screen printing unit 2b is sufficient to avoid the problem of ink contamination and to ensure a high printing quality. In other words, it is not necessary to completely dry or cure the ink pattern applied by the first screen printing unit 2b, which means that the energy requirement of the intermediate drying unit 10 can be kept low.

In this context, the intermediate drying unit 10 may advantageously comprise one or more pulsed lamps designed to produce a flash of light at a certain frequency f, which may optionally be adjusted as a function of the transport speed of the printed sheets. In this regard, the screen printing machine of the present invention is suitable for processing paper at speeds of about 10 '000 to 12' 000 sheets per hour (sph), or lower. In this case, it has been found suitable to operate an intermediate drying unit of the type comprising a pulsed lamp to produce flashes at a frequency of about 40Hz to 50Hz (and pulse duration of about less than one millisecond) when the printing press is operated at 10 '000-12' 000sph, totaling about 10 to 20 flashes per sheet. An energy level of about 15-20kJ/s has been found to be suitable for this application.

Thanks to the intermediate drying unit 10, the two screen printing units 2b and 2c can therefore and advantageously make use of similar stencil cylinders and squeegee arrangements.

An example of a suitable screen printing unit arrangement for the units 2b and 2c is illustrated in fig. 2 and 3. As illustrated in fig. 2, the units 2b and 2c may comprise a chablon cylinder 20, which chablon cylinder 20 has a continuous cylinder surface, i.e. has a cylindrical outer circumference without any discontinuities. The ends of the chablon cylinder 20 are advantageously mounted on two head members (not referenced) which are mounted for rotation between two bearing means 21 of the type comprising rotatable jaw members, as taught in international application No. wo 03/093013 in the name of the present applicant.

It will be appreciated that, in operation, the chablon cylinder 20 is caused to rotate (in a clockwise direction in fig. 1), while the squeegee 25 (not shown in fig. 2) located inside the chablon cylinder 20 (see fig. 3) is caused not to rotate, but rather pushes the ink I through the holes provided in the surface of the chablon cylinder 20, which holes are defined as a function of the design of the ink pattern to be applied. The squeegee 25 can be designed in the manner taught in international application No. wo2004/096545, wherein the integral grooves are designed to improve the flow of ink over the squeegee 25.

The squeegee 25 is advantageously connected to a known control mechanism (not shown) so as to selectively retract the squeegee 25 from the inner periphery of the reticle cylinder 20 when the squeegee 25 passes in front of the cylinder pits of the impression cylinder 2 a. A detailed description of such a control mechanism can be found in european patent application No. ep 0723864. In the case of printing on a web, such a control mechanism may be omitted.

Fig. 4 is a schematic side view of a preferred embodiment of the intermediate drying unit 10. As schematically illustrated, the drying unit 10 preferably comprises a radiation source 11 and a reflector 12 designed to subject the paper P printed and held by the impression cylinder 2a to drying radiation. The intermediate drying unit 10 is preferably a UV type dryer for emitting ultraviolet radiation. In this environment, the radiation source 11 may be a flash tube containing an inert gas, preferably xenon, which is advantageously cooled by a cooling liquid. Xenon flash tubes are particularly suitable because they exhibit satisfactory light/electrical energy generation, especially due to low infrared radiation (typically below 10%) and very low ozone emission. For the purposes of this application, the radiation source 11 may be designed to emit in the UV-A range, i.e. at a wavelength of about 315nm to 400 nm.

A light shield 100 is preferably also provided, which light shield 100 surrounds the intermediate drying unit 10 in order to prevent the propagation of UV radiation and to avoid interference with the first and second screen printing units 2b, 2c, i.e. to prevent the start of the drying or curing process of the ink in the ink supply of the screen printing units 2b, 2 c. The mask 100 is also preferably used for safety reasons, i.e. to prevent UV radiation from reaching the printer operating the machine.

As a further safety measure, the first and second screen printing units 2b, 2c and the intermediate drying unit 10 can advantageously be housed in a machine frame comprising protective members 201, 202, which protective members 201, 202 serve to protect the operator from the propagation of UV radiation.

With reference to fig. 5a to 5d, we will briefly describe the drying process achieved using the printing press of the present invention. In fig. 5a to 5d, the ink pattern applied by the first screen printing unit 2b is denoted by reference letter a. The second ink pattern applied by the second screen printing unit 2c is not shown in fig. 5a to 5 d. As already mentioned, it is understood that once the surface drying of the first ink pattern a has been performed, this second ink pattern may be applied anywhere on the surface of the paper P (or web portion), even next to the first ink pattern a or in a manner at least partially overlapping the first ink pattern a.

Fig. 5a schematically illustrates the first ink pattern a after it is applied onto a sheet of paper P by the first screen printing unit 2 b. The surface portion S of the ink pattern a is dried by the intermediate drying unit 10, as illustrated in fig. 5 b. The surface portion S is sufficiently dried to enable contact with the upper surface of the ink pattern a (in particular, with the surface of the chablon cylinder of the second screen printing unit 2 c). As illustrated in fig. 5c, the liquid solvent component contained in the screen printing ink is absorbed by the paper P. The process starts already when the ink pattern a is applied on the surface of the paper P and is further enhanced by the thermal drying unit 5 arranged in the paper transport path of the printing press. As illustrated in fig. 5d, the ink pattern a is completely dried and cured after it passes through the UV drying unit 6.

Due to the above-described printer configuration, and in contrast to the known configuration taught in international application No. wo 97/34767, it is now possible to print two different ink patterns in close proximity to each other (e.g. in close proximity to each other) or in an at least partially overlapping manner. Possible examples of patterns that can be printed using the above-described printer are schematically shown in fig. 6a and 6 b.

Fig. 6a schematically illustrates an example in which a paper P (which generally exhibits a set of security imprints arranged in rows and columns) is provided with a first ink pattern a and a second ink pattern B applied one next to the other. Fig. 6B schematically illustrates an example in which a similar paper P is provided with a first ink pattern a and a second ink pattern B which partially overlap. In this case, the second ink pattern B may be advantageously printed using a translucent ink or in such a way as to be translucent and enable the underlying portion of the first ink pattern a to be seen. In this way, a more complex security feature can be created that combines the properties of two different screen-printing inks.

It will of course be understood that the particular shape and location of the ink pattern A, B shown in fig. 6a, 6b is merely illustrative. In addition, one or both of ink patterns A, B may be printed on top of a background (e.g., offset background) and/or overprinted or embossed (e.g., by gravure printing). Examples of security features that combine screen printed features with features printed by other printing processes such as offset and/or intaglio printing are known in the art, for example from international application No. wo 2004/071781.

Various modifications and/or improvements may be made to the above-described embodiments without departing from the scope of the invention as defined by the appended claims. For example, as already mentioned, although the invention has been described in the context of a printing press suitable for paper printing, the invention is equally applicable to printing on a continuous web of material. Furthermore, the printing press may comprise more than two consecutive screen printing units. In this case, it would be advantageous to position an intermediate drying unit between each pair of successive screen printing units.

As a further refinement, it is conceivable to print one or both ink patterns using screen-printing inks comprising magnetically orientable flakes. In this case, the magnetic sheets may be oriented using magnetic rollers, as taught in international application No. wo2005/000585 a1 in the name of the present applicant.

Claims (14)

1. A screen printing machine for printing a continuous web portion of a single sheet or continuous web, the screen printing machine comprising:

an impression cylinder (2 a);

a first screen printing unit (2b) cooperating with said impression cylinder (2a) to apply a first ink pattern (a) onto said single sheet or said continuous web portion;

at least a second screen printing unit (2c) cooperating with said impression cylinder (2a) to apply a second ink pattern (B) onto said single sheet or continuous web portion, said second screen printing unit (2c) being located downstream of said first screen printing unit (2B) with respect to the direction of rotation of said impression cylinder (2 a); and

an intermediate drying unit (10) located between the first screen printing unit (2b) and the second screen printing unit (2c), the intermediate drying unit (10) being a UV unit for emitting UV radiation,

wherein the intermediate drying unit (10) is adapted to perform a surface drying of the first ink pattern (A) applied by the first screen printing unit (2B) prior to the application of the second ink pattern (B) by the second screen printing unit (2c), the surface drying involving only drying an outer surface portion (S) of the first ink pattern (A),

and wherein the screen printing machine further comprises a lamp enclosure (100), said lamp enclosure (100) enclosing said intermediate drying unit (10) so as to prevent the propagation of UV radiation and avoid interference with said first screen printing unit (2b) and said second screen printing unit (2 c).

2. The screen printing machine according to claim 1, wherein the second screen printing unit (2c) is adapted to apply the second ink pattern (B) immediately adjacent to the first ink pattern (a) or at least in a partially overlapping manner onto the first ink pattern (a).

3. The screen printing machine according to claim 1, wherein the intermediate drying unit (10) comprises one or more flash lamps.

4. The screen printing machine of claim 3, wherein the flash is a xenon flash tube.

5. The screen printing machine according to claim 3, wherein the intermediate drying unit (10) is operated with a flashing frequency of 40Hz to 50Hz for a paper processing speed of 10 '000 to 12' 000 sheets per hour.

6. The screen printing machine according to any one of claims 3 to 5, wherein the first and second screen printing units (2b, 2c) and the intermediate drying unit (10) are housed in a machine frame comprising a protective member (201, 202), the protective member (201, 202) being for protecting an operator from the propagation of UV radiation.

7. The screen printing machine according to any one of claims 1 to 5, wherein the first screen printing unit (2b) and the second screen printing unit (2c) each comprise a chablon cylinder (20), the chablon cylinder (20) presenting a cylindrical periphery without any discontinuities.

8. The screen printing machine according to any of claims 1 to 5, further comprising at least one final drying unit (5, 6), the at least one final drying unit (5, 6) being arranged downstream of the second screen printing unit (2c) in order to finally dry the first ink pattern (A) and the second ink pattern (B) applied by the first screen printing unit (2B) and the second screen printing unit (2 c).

9. The screen printing machine of any of claims 1 to 5, for use in the production of securities.

10. The screen printing machine of claim 9, wherein the security is a banknote.

11. A method for performing screen printing of a continuous web portion of a single sheet or continuous web on a screen printing press, the screen printing press comprising: an impression cylinder (2 a); a first screen printing unit (2b) cooperating with said impression cylinder (2a) to apply a first ink pattern (a) onto said single sheet or said continuous web portion; and at least a second screen printing unit (2c) cooperating with said impression cylinder (2a) to apply a second ink pattern (B) onto said single sheet or continuous web portion, said second screen printing unit (2c) being located downstream of said first screen printing unit (2B) with respect to the direction of rotation of said impression cylinder (2a),

the method comprises the following steps: performing surface drying of the first ink pattern (A) applied by the first screen printing unit (2B) prior to the application of the second ink pattern (B) by the second screen printing unit (2c), the surface drying involving drying only an outer surface portion (S) of the first ink pattern (A) with UV radiation emitted by an intermediate drying unit (10) located between the first screen printing unit (2B) and the second screen printing unit (2c),

and wherein the method further comprises the steps of: covering the ambient light from the intermediate drying unit (10) and preventing the propagation of UV radiation and avoiding interference with the first screen printing unit (2b) and the second screen printing unit (2 c).

12. The method of claim 11, further comprising the step of: the final drying of the first ink pattern (a) and the second ink pattern (B) is performed downstream of the second screen printing unit (2 c).

13. The method according to claim 11 or 12 for producing securities.

14. The method of claim 13, wherein the security is a banknote.