WO2004054806A1

WO2004054806A1 - Rotary pad printing system for printing objects of limited flexibility

Info

Publication number: WO2004054806A1
Application number: PCT/BE2003/000220
Authority: WO
Inventors: Laurent De Volder
Original assignee: Laurent De Volder
Priority date: 2002-12-13
Filing date: 2003-12-15
Publication date: 2004-07-01
Also published as: BE1015250A6; AU2003296297A1; ES2372487T3; ATE521473T1; EP1636033A1; EP1636033B1

Abstract

Rotary pad printing system for printing objects of limited flexibility comprising a set of cylinders fixedly mounted on mutually parallel rotary shafts comprising a rotatable engraved roller and a pad printing cylinder, remarkable in that said engraved roller (11) consists of a core fixedly mounted on said shaft (11a) with at lest one removable sleeve (11b) around it on which a screen is fitted, on the one hand, and in that said pad printing cylinder (12) consists of a core (12a) fixedly mounted on the shafts with at least one removable sleeve (12b) around it with material on the outer layer thereof allowing to pick up ink from said engraved roller and to further deposit it on an object to be printed (15), wherein an engraving of the image to be printed on said object (15) is applied on one of said sleeves.

Description

Rotary pad printing system for printing objects of limited flexibility

Field of the invention

This invention relates to a rotary pad printing system for printing objects of limited flexibility, inter alia of the board, profile, ceramic tile, type etc. Under objects of limited flexibility should be classified objects which cannot be rolled up, in contrast to fully flexible objects for which this can be done.

Background of the invention

At present there are two technologies which closely relate to a printing system of the aforesaid type : traditional rotary pad printing systems for printing objects of limited flexibility, on the one hand, and traditional rotary printing systems for printing fully flexible objects, such as paper, plastic film and aluminium foil, on the other hand.

Prior art

In traditional rotary printing systems for printing objects of limited flexibility, an engraved roller 1 , which is fixed to a rotation shaft, is inked with the aid of an open inking system by rotation of the engraved roller through an ink bath 3a, after which the excess ink is scraped off with the aid of a doctor blade 3b. The desired pattem is then offset via the pad printing cylinder 2, by off-rolling, onto the print object 5. Once the image has been laid down, the pad is cleaned with the aid of an open pad clean-wipe system. Solvent is applied via a rotating cylinder 4b to the pad printing cylinder 2, after which the excess ink and the added solvent are scraped off by the doctor blade 4c.

The engraved roller 1 and the pad printing cylinder 2 are cylinders fixedly mounted on the shaft. Two combinations are possible. In the printing method known as indirect gravure, the engraved roller appears with the engraving of the desired image. The pad printing roller must in this case transfer the pattem in its entirety to the print object. In the printing method known as flexography, the engraved roller appears with a uniform engraving. This system allows the whole cylinder to ink, after which a pad printing cylinder formed specifically in the shape of the pattem only translates the desired pattern to the print object.

In traditional rotary printing systems for printing fully flexible objects of the last- named type, the engraved roller is inked and then the desired pattem is placed via the pad printing cylinder onto the print object.

In this case, the techniques of flexography or indirect gravure, or even direct gravure, can be applied. In direct gravure, the image engraved on the engraved roller is placed directly onto the print object, without an additional pad printing cylinder.

In traditional rotary printing systems, cylinders fixedly mounted on the shaft are used for the engraved roller and the pad printing cylinder. This poses a problem when these cylinders come to be replaced in connection with maintenance and resetting of the machine. For this, special tools are necessary, though resetting continues to be a time-consuming exercise. The weight of the impression cylinders herein poses a major problem. In addition, the weight is also a drawback in case of transportation, which is billed, after all, per kilogram. Transportation is consequently expensive and demands, moreover, special requirements with regard to protection of the cylinders. There are therefore drawbacks associated with the use of fixed impression cylinders.

Aim of the invention

The object of the present invention is to overcome the aforesaid drawbacks and deficiencies of the state of the art for printing objects of limited flexibility.

Summary of the invention

The main measure taken to offer a solution according to the invention is defined in the main claim. In order to eliminate the weight problem of the engraved roller, the existing impression cylinder fixedly mounted on the shaft is replaced by the combination of a core fixedly mounted on the shaft with, round said core, a removable sleeve to which the screen can be fitted. The weight of the cylinder to be removed is hereby, advantageously, significantly reduced. Analogously, in order to eliminate the weight problem of the pad printing cylinder, the pad printing cylinder fixedly mounted on the shaft is replaced by the combination of a further core fixedly mounted on the shaft with, around said core, a further removable sleeve. Where the existing system in each case consisted of one part, the system according to the invention consists of two parts: the core fixedly mounted on the shaft and the easily removable sleeve.

The sleeve system according to the invention offers the following advantages over the existing systems. The weight problem of the engraved roller is eliminated. The same applies to the pad printing cylinder. After all, in the resetting of the machine, the weight of the component to be removed, both pad printing cylinder and/or engraved roller can be reduced by up to a factor of approximately 40 in the first instance, and even further, up to a factor of approximately 200 in the second instance. This also explains, at the same time, why a special tool is no longer required when replacing the cylinders or resetting the machines. This constitutes a considerable advantage in the use of such systems, thanks to the simplification and greater flexibility which is herewith obtained by virtue of the invention.

Furthermore, the screen no longer needs to be attached to a heavy engraved roller, but to a thin sleeve which can be slipped over the core. This makes it easier to maintain and reset the machine and reduces the cost price in connection with transportation.

Analogously, the solid engraved roller is also replaced by a sleeve, preferably having a limited quantity of silicone rubber, with the same associated advantages.

The traditional inking and clean-wipe systems are referred to as open systems, because ink and/or solvent is constantly in contact with the air. These systems are therefore not fully closed. In open inking and cleaning systems, a part of the reducer evaporates as a result of the constant direct contact with the air. Although machines of this type have to satisfy the regulations regarding the presence of such substances, a large part of these toxic substances nevertheless finds its way into the environment. The installation of a closed cage with extractor unit and the use of explosion-free apparatus, motors, is the most commonly used method for this, but it still constitutes a harmful pollution of the environment. Every effort should thus be made to limit the concentration of such toxic substances as much as possible. This can only be done by providing a technical solution for these systems.

As a result of the direct contact between the ink and the air, the solvents mixed in the ink find their way into the air. The viscosity of the ink is thereby altered, which creates a problem with regard to print quality. Solvents must consequently regularly be added.

Furthermore, the use of open inking systems goes hand in hand with ink loss. The ink finds its way onto all kinds of machine components, ink being able to end up on components which subsequently come directly or indirectly into contact with the surface to be printed. The machine therefore needs to be cleaned on a very regular basis and frequently thoroughly.

Furthermore, dust and other dirt can find its way into the open inking system, resulting in print quality problems.

As a result of the inking system not being fully closed off, a fair amount of ink is spilt, thereby giving rise to higher ink consumption and hence higher production costs.

For the use of open inking systems, the minimal ink store within the system is of such a size that the system is unprofitable for small production sizes. There are therefore a number of drawbacks associated with the open inking system.

In order to eliminate the above-cited problems with regard to the open inking system, a closed inking system has been developed, as set out in the subclaims. The evaporation of the solvents and the ink loss is hereby substantially reduced.

Further advantages of the inking system according to the invention consist in the fact that it is dust-free in the sense that no dust can find its way into the ink reservoir and also that there is less evaporation of solvents, resulting in lower environmental pollution, lower use of solvents, yielding lower production costs, better control of viscosity - with improved constant viscosity - and hence of print quality, and a lesser risk of explosion, indirectly resulting in the use of cheaper electrical components, motors, and no further need for special extractor apparatus.

Nor is there any ink spillage, resulting in lower maintenance costs, a lower use of inks, yielding lower production costs, and a better control of viscosity and hence of print quality.

In addition, such a system is suitable for the launch of a small production with a minimal ink store.

An added advantage is that the doctor blade chamber can be connected to an extra ink container by means of a diaphragm pump which guarantees the supply, thereby allowing uninterrupted production of large print runs.

Ultimately, the doctor blade chamber can be provided with an overflow connected to the ink container. Thus this can always remain perfectly filled and a permanent ink circulation achieved, whereby the viscosity can be kept constant at all times, resulting in a more uniform print quality.

The closed inking system according to the invention therefore offers a number of advantages over the existing open inking systems, amongst which less environmental pollution, better constant viscosity, smaller minimal ink stores and elimination of dust problems are the most important.

Finally, since a fairly significant quantity of solvents find their way into the air through evaporation, a major environmental and safety problem arises. After all, a risk of explosion is generated. In addition, evaporation ensures a higher use of solvents and thus higher production costs. Thus there are also drawbacks with the open clean-wipe system.

In order to eliminate these problems with regard to the open clean-wipe system, a closed clean-wipe system is proposed, as set out in the further subclaims. In this respect, the closed clean-wipe system according to the invention has one significant advantage over the known open clean-wipe systems, namely that there is less evaporation of solvents, resulting in lower environmental pollution, a lower use of solvents, resulting in lower production costs, and a lesser risk of explosion, indirectly resulting in the use of cheaper electrical components, motors, and no further need for special extractor apparatus.

Such a printing system makes it additionally possible to print objects of limited flexibility at a throughput speed which can vary over a wide range: speeds in the order of 5 to 25 meters per minute are possible, just like speeds up to 300 meters per minute.

The print roller can be used to advance the print object.

Further properties and particularities of this invention are defined in the appended subclaims.

Additional details and advantages will appear from the description hereafter of some exemplary embodiments of the invention which are illustrated by means of the appended drawings.

Brief description of the drawings

Figure 1 is a diagrammatic representation of the cross-section of a classic rotary pad printing system for printing objects of limited flexibility.

Figure 2 diagrammatically represents the principle of indirect gravure. Figure 3 diagrammatically represents a principle of a flexography system. Figure 4 is the diagrammatic representation of a cross-section of the printing head based upon printing cylinders with sleeves and a closed ink and clean-wipe system according to the invention.

Figures 5a and 5b represent possible structures of the roller, respectively cylinder fixedly mounted on the shaft according to the invention.

Figure 6 is the cross-section of a sleeve structure for a sleeve with engraving according to the invention. Figure 7 is the cross-section of a sleeve structure for the sleeve with silicone according to the invention.

Figure 8 shows a diagrammatic representation of a closed ink system according to the invention.

Figure 9 shows a closed clean-wipe system according to the invention. Description

Generally speaking, the entire rotary pad printing system which is described below consists substantially of the following principal components: print roller and sleeves, a closed inking system and a closed clean-wipe system, which further ^'■> consist of the following components as shown in figure 4: core of the engraved roller 11a, sleeve with engraving 11b, core of the pad printing cylinder 12a, sleeve with silicone rubber, constituting the pad 12b, the pad taking up the ink from the engraving at 11 , a closed inking system 13, a pad printing cylinder clean-wipe system 14, and a print object 5.

The arrow designated as F represents the direction of rotation of the engraved roller, whilst the arrow designated as G represents the direction of rotation of the pad printing cylinder and arrow H represents the direction of motion of the cleaning cylinder. E represents the direction of motion of the print object.

The core of the engraved roller 11a is fixedly mounted on the rotation shaft. This is represented in figures 5a and 5b. The core can consist of a solid steel cylinder as represented in figure 5a. However, other materials are also possible, such as aluminium. This embodiment does not differ substantially from an embodiment wherein the solid core, for weight-reduction purposes, is replaced by a tubular structure of the sleeve type, which is fixedly mounted on the rotation shaft.

The diameter of the core of the engraved roller is matched to the diameter of the associated sleeve. Such a sleeve has typically a diameter of e.g. 236,3 mm with a weight of approximately 150 g. For the corresponding pad printing cylinder the weight is approximately 800 g. With the very same diameter of 236,3 mm and a printing width of 415 mm, the solid cylinders in question weight approximately 30 kg. The sleeved version thus constitutes a marked reduction in weight, both in relative and in absolute values.

Figure 6 shows the sleeve as a thin-walled, laminated tube with an engraving of the colour image 11b to be printed. The wall thickness typically measures less than 1 mm, the innermost layer of which is a thin, preferably metal, support, carrying the layer in which the engraving is made. The thickness of the sleeve is typically comprised between 0,5 and 1 mm and it is slipped over the core of the engraved roller. In order to enable this, the sleeve as a whole must be elastic-ally deformable. Clearance can thereby be created relative to the core, allowing easy fitting or removal of the sleeve.

In the outer surface of the outer layer, an engraving having the print pattern for a specific colour is made either by etching, or by engraving or by any other possible method. The material of the outer surface can be steel or any other kind of metal, or nylon or any other synthetic material. The most important selection criterion for this material is the desired wear resistance with regard to the wiping. When choosing this material, the desired production size and the associated wear and tear can advantageously be taken into account.

For wiping off the excess ink on the sleeve, a doctor blade is used. The better the doctor blade follows the cylindrical surface of the sleeve, the better the wipe. In order to accomplish this, the sleeve is as flat as possible in the longitudinal direction. In addition, a special screen is also preferably used, thereby ensuring continuous contact between sleeve and doctor blade and this for a doctor blade having the required elasticity.

The core of the impression cylinder 12a is fixedly mounted on the rotation shaft. This is represented by way of example in figures 5a and 5b. The core may consist of a solid steel cylinder as represented in figure 5a. However, other materials are also possible, such as aluminium, for example. In addition, an embodiment is also possible wherein the solid core is replaced by a sleeve-type tubular structure of the sleeve-type, which is fixedly mounted on the rotation shaft as represented on figure 5b in order to reduce the weight.

The diameter of the core of the impression cylinder is matched to the inner diameter of the associated sleeve. Such a sleeve has typically a diameter of e.g.

236,3 mm.

The sleeve with silicone rubber 12b should also be regarded as a thin-walled, laminated tube. The wall thickness typically measures a few centimeters, the innermost layer of which is a thin, preferably metal, support of typically less than 1 mm, with silicone on top as shown in figure 7. This sleeve is slipped over the core of the impression cylinder. In order to enable this, the sleeve as a whole must be elastically deformable. Clearance can thereby be created relative to the core, allowing easy removal or fitting of the sleeve.

The material of the outer surface of the outer layer is a silicone mixture, the most important criterion for the selection of the material components being the print quality. When choosing this material, the desired production size and the associated wear and tear, which is dependent on the hardness and wear- resistance of the silicone, can also advantageously be taken into account.

The composition of the silicone can further also be modified by the addition of electrically conductive materials. This allows the build-up of electrostatic charge in the machine to be averted.

The shape of this sleeve is designed such that the clean-wipe system is fully closed off. To this end, the solvents are not only scraped off from the cylindrical surface, but also from the side face.

Figure 8 shows the closed ink system 23 consisting of the following functional parts. The ink reservoir 21 contains the ink which, through direct contact, finds its way onto the screen sleeve. This can be compared with the principle of the ink bath in existing systems 3a. The reservoir 21 has an opening 28 in which the engraved roller with sleeve fits perfectly during production and along which the ink is brought onto the sleeve. This opening lies, for example, in a vertical plane.

The contact between ink and sleeve takes place in a vertical plane. As a result thereof, the inking system is advantageously provided with a pump system in order to fill the ink chamber before production and empty it after production. Otherwise, when ink chamber and sleeve are moved away from each other, in the rest state, therefore, ink would find its way into the machine, thereby necessitating time-consuming clean-wipe work.

A set of doctor blades is provided, which are fastened to the ink chamber by a specific holder 22. This doctor blade holder is designed such that the ink chamber is fully closed off when the inking system is pushed against the engraved roller.

This doctor blade holder ensures that the doctor blades are always correctly mounted, both at an optimal angle and at a suitable distance relative to the sleeve. The optimal angle is 30° relative to the tangent to the sleeve. This doctor blade holder further allows rapid replacement of the doctor blades.

A lowermost doctor blade 23 is mounted with the doctor blade holder 22 on the ink reservoir 21 , so that, along the bottom side of the opening in the ink reservoir, no ink can leak away. Beneath the inking system, a pan 29 is preferably provided to collect the minimized residual ink losses so as to prevent these from finding their way into the machine. Said doctor blade 23 consists of a thin steel plate coated with a ceramic layer, or any other combination having the same properties in terms of wiping and wear resistance.

A second doctor blade 24 wipes the excess ink from the engraved roller, so that only ink in the engraving bearing the desired image for a specific colour is left. This doctor blade 24 functionally replaces the doctor blade 1 in known systems. The second doctor blade 24 is mounted with the doctor blade holder 22 on the ink reservoir 21 , so that, along the top side of the opening 31 , no ink can leak away into the ink reservoir. The latter doctor blade 24 consists of a thin steel plate coated with a ceramic layer, or any other combination having the same properties in terms of wiping and wear resistance.

Ancillary seals 25 are provided to make the ink chamber 21 ink-tight on the side without a doctor blade in order thereby to combat ink loss. These seals 25 consist of a small layer of cellular rubber with a thickness of approximately 10 mm. Any other material which possesses the same properties in terms of sealing, wear resistance and dimensional stability or elasticity can equally be used.

Furthermore, an inlet and outlet system 26 is provided, along which the ink can be fed and evacuated. Such a system 26 allows the ink to be continuously pumped round, whereby the viscosity can be continuously controlled in order to keep it constant. To this end, a quantity of solvent is added, where necessary, to the ink. This system 26 is also used to fill the ink reservoir 21 before production and empty it after production. Otherwise, when ink chamber and sleeve are moved away from each other, in the rest state, therefore, quite a quantity of ink would find its way into the machine, thereby necessitating time-consuming clean-wipe work. The inking system can be moved in its entirety away from and toward the engraved roller with sleeve, giving rise to two states: a rest state A, far apart, and a production state B, close together.

Figure 9 shows the closed pad cleaning-wipe system intended to clean the pad printing cylinder 34 consisting of a number of functional parts. A reservoir 31 is included herein, the reservoir containing the solvents, which find their way, through direct contact, onto the pad printing cylinder with sleeve. The reservoir has an opening 32', in which the pad printing cylinder with sleeve fits perfectly during production and along which the solvents are brought onto the sleeve. This opening 32 lies in a vertical plane. Consequently, the clean-wipe system is provided with a pump system (not shown) for filling the reservoir 21 before production and emptying it after production. Otherwise, when reservoir and sleeve are moved away from each other, in the rest state, quite a quantity of solvents would find their way into the machine.

The doctor blades 33, 34 are fastened to the reservoir by a specific holder 32. This doctor blade holder 32 is designed such that the reservoir 31 is fully closed off when the clean-wipe system is pushed against the pad printing cylinder with sleeve. This doctor blade holder 32 ensures that the doctor blades 33, 34 are always correctly mounted, i.e. at an optimal angle and at an optimal distance to the sleeve. This doctor blade holder 32 allows rapid replacement of the doctor blades.

A lowermost doctor blade 33 is mounted with the doctor blade holder 32 on the reservoir 31 , so that, along the bottom side of the opening 32 in the reservoir, no solvents can leak away. Beneath the clean-wipe system there is a pan 39 to collect the minimized residual ink losses so as to prevent these from finding their way into the machine.

A second doctor blade 34 wipes the solvents off the sleeve, so that a perfectly clean sleeve can accept the print image from the screened sleeve. This doctor blade functionally replaces the known doctor blade. The second doctor blade 34 is mounted with the doctor blade holder 32 on the reservoir 31 , so that, along the top side of the opening in the reservoir, no solvents can leak away. Said doctor blades 33, 34 consist of a thin steel plate coated with a ceramic layer or any other combination having the same properties in terms of wiping and wear resistance, such as plastic, for example polyamide.

Ancillary seals 35 are provided in order to make the reservoir 31 fully seal-tight on the side with no doctor blade. These seals 35 consist of a small layer of cellular rubber or any other material possessing the same properties in terms of sealing, wear resistance and dimensional stability or elasticity. Said seals thus allow solvent loss to be combated.

Furthermore, an inlet and outlet system 36 is provided, along which the solvents can be fed and evacuated. Such a system allows the solvents to be continuously pumped round, and this in order to recycle them as long as possible. This system is also used to fill the reservoir before production and to empty it after production. Otherwise, when reservoir and sleeve are moved away from each other, in the rest state, quite a quantity of solvents would find their way into the machine.

The clean-wipe system can be moved in its entirety from and to the impression cylinder with sleeve, giving rise to the two states : the rest state A, from each other, and the production state B, against each other.

The print object 15 has to satisfy a number of conditions: the object should be bendable, yet not to the extent that it could be rolled up, for example. The object therefore has only limited flexibility. Examples of such objects are boards and plastic profiles, inclusive of MDF, aluminium or wood and ceramic tiles.

Finally, the central core of the cylinder is provided with a compressed-air connection.

The operation of the above-described system according to the invention is explained below. With this system, the assembly or disassembly of the sleeve and engraved roller or impression cylinder is effected as follows:

The fitting or removal of the sleeve proceeds in the following steps. Under normal circumstances, a sleeve is not perfectly cylindrical but slightly oval. Thus, compressed air is supplied in order to move the sleeve over the core. This produces a film of air between the sleeve and the core, whereby the cross section of the sleeve becomes almost circular. The fact that the diameter of this circle is greater than the shortest axis of the original oval shape enables the sleeve to be taken from the core or slipped over it without additional tools. Once the sleeve has been slipped fully over the core and is installed in its place (locating pin), the compressed air is shut off. Without a film of air, a firm fastening of the sleeve relative to the core is obtained, through the pinching of the oval shape. In addition, this assembly is also very precise. Any displacement during the printing is virtually impossible.

The operation of the closed inking system is as follows. The ink from the ink reservoir finds its way onto the screen or sleeve through direct contact. The ink is brought onto the sleeve along the opening in the reservoir in which the engraved roller with sleeve fits perfectly during production. This opening lies in a vertical plane.

The ink chamber is filled before production and emptied after production by means of the pump system.

The inking system is pushed against the engraved roller and, with the aid of doctor blade holders and the appropriate doctor blades, the ink chamber is fully closed off.

By virtue of these doctor blade holders, the doctor blades are rapidly replaced.

The second doctor blade in the direction of rotation of the cylinder wipes the excess ink off the engraved roller, so that only ink in the engraving containing the desired image for a specific colour is left.

The inking system is pressed against the engraved roller with an adjustable pressure force.

Via a system of pumps, the ink in the ink chamber is kept, during production, constantly above a minimum level higher than the position of the first doctor blade. The viscosity of this ink is, moreover, constantly finely adjusted, so that the correct viscosity is maintained at all times, even after extra ink has been added. Consequently, ink and reducer are pumped to the ink chamber in the correct ratio.

In order to accomplish all this, the doctor blade chamber is connected to an additional ink container by means of a diaphragm pump which guarantees the supply. The doctor blade chamber is additionally provided with an overflow connected to the ink container, whereby this is always perfectly filled and there is a permanent circulation of ink.

This pump system is also used to fill the ink reservoir before production and empty it after production.

The inking system, where appropriate, is moved in its entirety from and to the engraved roller with sleeve, giving rise to two states: far apart, the rest state A, and close together, the production state B.

The operation of the closed clean-wipe system is as follows :

The solvents find their way through direct contact onto the impression cylinder

(sleeve).

The reservoir has an opening in which the impression cylinder with sleeve fits perfectly during production and along which the solvents are brought onto the sleeve.

Since the contact between solvents and sleeve takes place in a vertical plane, the clean-wipe system should be provided with a pump system in order to fill the reservoir before production and empty it after production.

The clean-wipe system is pushed against the pad printing cylinder and, with the aid of doctor blade holders and the appropriate doctor blades, the reservoir is fully closed off.

The clean-wipe system is pressed against the pad printing cylinder with an adjustable pressure force. Via a system of ink pumps, the solvents in the reservoir are kept, during production, constantly above a minimum level higher than the position of the first doctor blade.

An inlet and outlet system is used to fill the reservoir before production and empty it after production.

An overflow centrally receives the wiped-off solvents.

As a result of this system, the reservoir is filled before production and emptied after production.

The clean-wipe system is moved in its entirety from and to the impression cylinder with sleeve, giving rise to two states: a rest state, far apart, and a production state, close together.

This particular application, a printing system based on impression cylinders with sleeves, a closed inking system and a closed clean-wipe system, has been developed for the printing of boards and other objects of limited flexibility. The present invention could also be used, subject to the necessary adjustments, for fully flexible objects, such as paper, plastic film, aluminium foil, etc.

The use of a printing machine equipped with closed ink and clean-wipe systems and sleeve-based cylinder combinations may further be mentioned in combination with an accurate electronic control, which offers a considerable amount of possibilities with respect to existing printing systems : printing objects with limited flexibility in an outstanding environment friendly manner, printing objects with limited flexibility with drawings in a plurality of colours wherein either arbitrary or deterministic patterns may be generated, printing objects with limited flexibility with drawings in a plurality of colours at a low cost owing to the small minimal ink provision, printing objects with limited flexibility at speeds in a wide range between 5 meters per minute and 300 meters per minute, quick switching times for exchanging decorations to be printed, which is very important with respect to achieving so called "just-in-time printing".

Claims

1. Rotary pad printing system for printing objects of limited flexibility comprising a set of cylinders fixedly mounted on mutually parallel rotary shafts comprising a rotatable engraved roller and a pad printing cylinder, characterized in that said engraved roller (11 ) consists of a core fixedly mounted on said shaft (11a) with at least one removable sleeve (11b) around it on which a screen is fitted, on the one hand, and in that said pad printing cylinder (12) consists of a core (12a) fixedly mounted on the shaft with at least one removable sleeve (12b) around it with material on the outer layer thereof allowing to pick up ink from said engraved roller and to further deposit it on an object to be printed (15), wherein an engraving of the image to be printed on said object (15) is applied on one of said sleeves.

2. Rotary pad printing system according to the preceding claim, characterized in that said material is a soft material, particularly a silicone mixture, in particular a silicone rubber.

3. Rotary pad printing system according to claim 1 , characterized in that said material is PVC.

4. Rotary pad printing system according to claim 1 , characterized in that said material of the outer layer is gelatine.

5. Rotary pad printing system according to one of the preceding claims, characterized in that electrically conducting materials are added to said material.

6. Rotary pad printing system according to one of the preceding claims, characterized in that the diameter of the core of each said cylinder is fitted to the diameter of the corresponding sleeve.

7. Rotary pad printing system according to one of the preceding claims, characterized in that the core has a cylindrical profile (41 ).

8. Rotary pad printing system according to one of the preceding claims, characterized in that the core has a tubular structure (42) which is fixedly mounted on the rotary shaft.

9. Rotary pad printing system according to one of the preceding claims, characterized in that the core consists of a metallic cylinder.

10. Rotary pad printing system according to claim 9, characterized in that the core consists of a cylinder made of steel or aluminium.

11. Rotary pad printing system according to one of the preceding claims, characterized in that said sleeve is realized as a thin-walled laminated tube with an engraving of the desired image with a specific colour (11 b), the inner layer whereof is a thin support with thereon the layer wherein the engraving is applied.

12. Rotary pad printing system according to the preceding claim, characterized in that said support is made of metal.

13. Rotary pad printing system according to one of the preceding claims, characterized in that the sleeve (11b) is slideable on the core (11a) of the engraved roller.

14. Rotary pad printing system according to one of the preceding claims, characterized in that the sleeve is elastically deformable as a whole. r

15. Rotary pad printing system according to one of the preceding claims, characterized in that in the outer surface of said outer layer an engraving is applied with the image to be printed for a specific colour.

16. Rotary pad printing system according to one of the preceding claims, characterized in that the material of the outer surface is metal, particularly steel.

17. Rotary pad printing system according to one of the claims 1 to 15, characterized in that the material of the outer surface is a synthetic material, in particular nylon.

18. Rotary pad printing system according to one of the claims 1 to 15, characterized in that the material of the outer surface is a ceramic material.

19. Rotary pad printing system according to one of the preceding claims, characterized in that the sleeve is substantially straight in the longitudinal direction.

20. Rotary pad printing system according to one of the preceding claims, characterized in that there is provided at least one doctor blade for scraping off the ink in excess on the sleeve and in that said screen is specially provided for a continuous contact between the sleeve and the doctor blade, wherein said doctor blade has the required elasticity.

21. Rotary pad printing system according to one of the preceding claims, characterized in that there is provided a closed ink system (13) consisting of an ink tank with ink which finds its way onto the screen by direct contact, wherein the tank (21) has an aperture (28) in which the engraved roller with sleeve fits during production and along which the ink is applied on the sleeve.

22. Rotary pad printing system according to the preceding claim, characterized in that said ink system is provided with an autonomous or manual system, in particular a pump system, to fill the ink chamber before production and to empty it after production.

23. Rotary pad printing system according to one of both preceding claims, characterized in that there is provided a set of doctor blades which are fixed to the ink chamber by means of a specific holder (22), wherein the doctor blade holder is designed in such a way that the ink chamber is completely closed off when the ink system is slided against the engraved roller.

24. Rotary pad printing system according to the preceding claim, characterized in that the doctor blade holder is provided with doctor blades which are mounted with an optimal angle and at a suitable distance relative to the sleeve.

25. Rotary pad printing system according to the preceding claim, characterized in that said angle is comprised between 30 and 35° relative to the tangent to the sleeve.

26. Rotary pad printing system according to one of the preceding claims 23 to

25, characterized in that a lowermost doctor blade (23) is mounted with the doctor blade holder (22) on the ink tank (21 ) in such a way that no ink can leak away along the bottom side of the aperture into the ink tank and by the means of which any dirt on the sleeve is prevented from getting into the ink chamber.

27. Rotary pad printing system according to one of the preceding claims 21 to

26, characterized in that there is provided a collecting container (29) under the ink system in order to collect ink losses and/or dirt rest.

28. Rotary pad printing system according to one of the preceding claims 21 to

27, characterized in that a second doctor blade (24) scrapes the excess ink off from the engraved roller in such a way that only the ink in the engraving bearing the desired image for a specific colour is left.

29. Rotary pad printing system according to one of the preceding claims 26 to

28, characterized in that said doctor blade (23) consists of a thin plate of steel coated with a ceramic layer.

30. Rotary pad printing system according to one of the preceding claims 21 to 29, characterized in that additional seals (25) are provided to seal off the ink chamber (21 ) from ink at the side having no doctor blade, thereby to prevent ink loss.

31. Rotary pad printing system according to the preceding claim, characterized in that these seals (21) consist of a layer of cellular rubber with a determined thickness.

32. Rotary pad printing system according to claim 30, characterized in that these seals (25) consist of a layer of fabric impregnated with oil, preferably felt.

33. Rotary pad printing system according to one of the preceding claims 21 to 32, characterized in that there is provided an inlet and outlet system (26) along which the ink can be fed and evacuated and by means of which (26) the ink is circulated continuously.

34. Rotary pad printing system according to one of the preceding claims, characterized in that the shape of said sleeve is designed in such a way that the clean-wipe system is completely closed off.

35. Rotary pad printing system according to the preceding claim, characterized in that the clean-wipe system is movable as a whole from and to the pad printing cylinder with sleeve with a rest state (A) apart from each other and a production state (B) close together respectively.