GRAVURE PRINTING UNIT HAVING HIGH OPERATING SAFETY

Abstract

 A printing unit (1) for gravure printing a sheet - or reel-like material (74), comprising an impression roller (10) comprising an impression roller body (11) and an impression roller sleeve (30) fitted, or capable of being fitted, onto said impression roller body (11), said impression roller body (11) defining an impression roller rotation axis (12) and an impression roller body side surface (13); said impression roller sleeve (30) having a tubular shape defining an outer sleeve side surface (31) and an inner sleeve side surface (33), said impression roller sleeve (30) comprising an outer layer (32) made of an elastomeric material delimited by said outer sleeve side surface (31); a checking apparatus (80) for measuring the electrical insulation resistance of said impression roller sleeve (30) between said inner sleeve side surface (33) and said outer sleeve side surface (31), said checking apparatus (80) being permanently associated with said printing unit (1), and comprising: an electrical contact element (81); a movement device (82) for moving said electrical contact element (81) between a measuring position, in which the electrical contact element (81) is in contact with the outer sleeve side surface (31), and a rest position, in which said electrical contact element (81) is not in contact with, and spaced apart from, the outer sleeve side surface (31); an electrical circuit (90) connected to said electrical contact element (81) and capable of being connected to said inner sleeve side surface (33), when said sleeve (30) is fitted onto said impression roller body (13), so that when the electrical contact element (81) is in contact with the outer sleeve side surface (31), said electrical circuit (90) is closed through said impression roller sleeve (30) between said inner sleeve side surface (33) and said outer sleeve side surface (31); said electrical circuit (90) being configured to provide a measured electrical insulation resistance value of the sleeve (30) between said inner sleeve side surface (33) and said outer sleeve side surface (31).

Description

Field of the invention

[0001] The present invention relates to a printing unit for gravure printing, comprising an impression roller pressed against a printing roller, with the interposition of a sheet or reel material to be printed therebetween. Both the impression roller and the printing roller rotate about the axis thereof, dragging the material therebetween during printing. The impression roller is coated with a coating layer, or sleeve made of an elastomeric material, which is charged electrostatically by an electrostatic printing assistance apparatus.

Background

[0002] Gravure printing is a direct printing technique for printing a sheet or reel material.

[0003] Such a technique comprises an impression roller, which is pressed against a printing roller, and these both rotate about respective rotation axes, which are generally parallel to each other. The material to be printed is interposed therebetween and dragged by the rotation of the impression roller and the printing roller.

[0004] The printing roller has a generally cylindrical outer surface, with a plurality of cavities or cells, corresponding to a figure to be printed onto the material and adapted to receive therein respective amounts of ink to be transferred onto the surface of the material.

[0005] The ink is generally liquid and contained in an ink tank, in which a portion of the surface of the printing roller is immersed, so that the outer surface of the printing roller can be continuously wet by the ink during the rotation of the same.

[0006] The excess ink on the outer surface of the printing roller is generally removed by the use of a blade, or a doctor blade, arranged so as to wipe such an outer surface of the printing roller and remove the ink remaining outside the cavities or cells.

[0007] In this way, only the cavities, or cells, contain the ink, while the rest of the surface of the printing roller is cleaned by the doctor blade.

[0008] The side surface of the impression roller is coated by an elastomeric material layer, or sleeve, to favor a uniform pressing of the material to be printed against the printing roller.

[0009] In order to facilitate the transfer of the ink, the technique is known of supplying the printing unit with an electrostatic printing assistance apparatus associated with the impression roller and having the function of distributing an electrostatic charge onto the outer side surface of the elastomeric sleeve of the impression roller.

[0010] The presence of such a surface charge has the purpose of generating an electric field between the impression roller and the printing roller, through the sheet or reel material, to be printed, which is interposed therebetween.

[0011] Such an electric field contributes to polarizing the ink contained in the cavities, or cells, of the printing roller, to attract it to the material to be printed, significantly improving the cover of ink in the printed zones and, thus, the resulting printing quality, which is thus more detailed and precise.

[0012] The elastomeric material of the sleeve sometimes comprises a plurality of conductive particles in order to make such a material semi-conductive or conductive, depending on the type of electrostatic printing assistance apparatus.

[0013] However, the intensity of electrostatic charge at play in the gravure printing is of a very high value, thus it represents a critical safety aspect because, if it is not controlled, it could trigger a fire or an explosion.

[0014] Thus, the insulation resistance of the sleeve must always be kept at values close to optimum insulation resistance values, to avoid dangerous excess accumulations of charge on the impression roller and to ensure that the excess electrostatic charge can easily be discharged onto the ground.

[0015] In order to keep such an insulation resistance under control, it must be checked every time before powering or activating the electrostatic printing assistance apparatus.

[0016] The insulation resistance value of the sleeve must be controlled also as a function of obtaining a high printing quality, since non-optimum insulation resistance values would have a negative influence on the value of the electric field at the sheet - or reel material to be printed, and therefore the ink would not be polarized optimally for its complete transfer onto the material.

[0017] In fact, if the insulation resistance were smaller than an optimum predetermined value, the electrostatic charge distributed on the outer side surface of the semi-conductive sleeve by means of the electrostatic printing assistance apparatus, would tend to disperse quickly onto the ground, excessively reducing the amount of electrostatic charge, which can be deposited on the sleeve, thus deteriorating the printing quality.

[0018] Whereas, if the insulation resistance were greater than such a value, the semi-conductive sleeve would charge excessively and take on electrical properties similar to those of a condenser, causing serious risks of forming an electric arc between the impression roller and other conductive parts of the printing machine, or working environment.

[0019] The need is thus felt to perform the aforesaid check before switching on the printing machine, and therefore each time before activating the electrostatic printing assistance apparatus.

[0020] The need is thus felt to provide a gravure printing unit capable of ensuring high operating safety of the elastomeric sleeve of the impression roller, and simultaneously high printing quality by means of an automatic control of the features of the sleeve before every activation of the electrostatic printing assistance apparatus.

Summary of the Invention

[0021] It is the object of the present invention to devise and provide a printing unit for gravure printing, which allows the above needs to be met, and to overcome, at least partially, the drawbacks stated above with reference to the prior art.

[0022] In particular it is an object of the present invention to provide a gravure printing unit capable of ensuring high operating safety of the elastomeric sleeve of the impression roller, and simultaneously high printing quality by means of an automatic control of the features of the sleeve before every activation of the electrostatic printing assistance apparatus.

[0023] It is a further object of the present invention to provide a printing unit for gravure printing, which also ensures a high control of the operating safety of the unit itself, besides ensuring a high printing quality.

[0024] It is another object of the present invention to provide a printing unit capable of checking the conditions, to allow the prevention of an excessive accumulation of electrostatic charge on the surface of the impression roller sleeve.

[0025] It is another object of the present invention to provide a printing unit capable of checking the aforesaid safety conditions, to allow the prevention of an excessive accumulation of electrostatic charge on the surface of the impression roller, before each activation of the electrostatic printing assistance apparatus.

[0026] It is a further object of the present invention to enable such a checking operation to be carried out automatically.

[0027] These and further objects and advantages are achieved by a printing unit for gravure printing according to claim 1.

[0028] Further objects, solutions and advantages are present in the embodiments described and claimed below in the dependent claims.

Brief description of the drawings

[0029] The invention will be shown below with the description of some embodiments thereof, given by way of non-limiting examples, with reference to the appended drawings, in which:

• figure 1 shows a diagrammatic view of a printing unit according to the invention, comprising an electrostatic printing assistance apparatus of the non-contacting, or top loading type, and in which the checking apparatus is in a measuring position;
• figure 2 shows a diagrammatic view of the printing unit in figure 1, in which the checking apparatus is in a rest position;
• figure 3 shows a perspective view of the printing unit in figure 1, in which the checking apparatus is in a measuring position;
• figure 4 shows a perspective view of the printing unit in figure 2, in which the checking apparatus is in a rest position;
• figure 5 shows a diagrammatic longitudinal section view of an impression roller of the printing unit in figure 1, for an electrostatic printing assistance apparatus of the non-contacting, or top loading type;
• figure 6 shows a perspective view of a printing unit according to the invention, comprising an electrostatic printing assistance apparatus of the direct charging type, in which the charge transmitter is arranged axially outwards with respect to the impression roller, and in which the checking apparatus is in the rest position.

Description of preferred embodiments

[0030] With reference to the figures, a printing unit for gravure printing a sheet or reel material, according to the invention is globally indicated by reference numeral 1.

[0031] The material 74 for gravure printing can be, for example, wound onto a reel at the inlet to said printing unit, or at the inlet to a plurality of said printing units arranged in sequence for making successive prints on the same material.

[0032] According to one embodiment, the material 74 comprises a film, or layer, made of a polymeric material, for example, is a film made of a polymeric material.

[0033] According to one embodiment, the material 74 comprises a layer of paper, and/or a layer of fabric and/or a metal layer, for example, the material 74 is a sheet of paper, or fabric or a metal sheet.

[0034] The printing unit 1 comprises an impression roller 10, comprising an impression roller body 11 and an impression roller sleeve 30 fitted, or capable of being fitted, onto said impression roller body 11.

[0035] The impression roller body 11 defines an impression roller rotation axis 12 and an impression roller body side surface 13.

[0036] The impression roller sleeve 30 has a tubular shape defining an outer sleeve side surface 31 and an inner sleeve side surface 33, said impression roller sleeve 30 comprising an outer layer 32 made of an elastomeric material delimited by said outer sleeve side surface 31.

[0037] The printing unit 1 comprises a checking apparatus 80 for measuring the electrical insulation resistance of said impression roller sleeve 30 between said inner sleeve side surface 33 and said outer sleeve side surface 31, said checking apparatus 80 being permanently associated with said printing unit 1.

[0038] The checking apparatus 80 comprises an electrical contact element 81 and a movement device 82 for moving said electrical contact element 81 between a measuring position, in which the electrical contact element 81 is in contact with the outer sleeve side surface 31, and a rest position, in which said electrical contact element 81 is not in contact with, and spaced apart from, the outer sleeve side surface 31.

[0039] According to an embodiment, the movement device 82 comprises at least one actuator 83 selected from: a pneumatic actuator, an electro-pneumatic actuator, a hydraulic actuator, an electromagnetic actuator, a mechanical actuator.

[0040] According to an embodiment, the aforesaid actuator 83 is of the linear type.

[0041] Alternatively, the aforesaid actuator 83 could be rotating or rotating-translating.

[0042] According to an embodiment, the electrical contact element 81 comprises a rotating contact electrode 84, for example, a wheel, made of a conducting material, adapted to be rotated in contact with the outer side surface 31 of the sleeve by means of rotating the impression roller 10. In this way, an insulation resistance check of the sleeve 30 can be carried out along an annular strip of the same.

[0043] According to an embodiment, the measuring device 82 comprises a support element 85 sliding between a first position at said measuring position and a second position at said rest position, said electrical contact element 81, in particular, said rotating contact element 84, or said wheel, being rotatably engaged, preferably free to rotate, at a free end of said support element 85.

[0044] According to an embodiment, the printing unit 1 comprises a support upright 60, said impression roller 10 being rotatably engaged to said support upright 60, wherein said checking apparatus 80 of the electrical insulation resistance, is constrained, or fixed, to said support upright 60.

[0045] According to an embodiment, the movement device 82, preferably the actuator 83, is constrained, or fixed, to said support upright 60.

[0046] According to an embodiment, said checking apparatus 80 of the electrical insulation resistance, in particular the movement device 82, in particular the actuator 83, is integrally fixed to the support upright 60, preferably by means of a support bracket 61 fixed to the upright 60.

[0047] According to an embodiment, the support bracket 61 is made of a metal sheet, preferably of a folded sheet.

[0048] The checking apparatus 80 further comprises an electrical circuit 90 connected to said electrical contact element 81 and capable of being connected to said inner sleeve side surface 33, when said sleeve 30 is fitted onto said impression roller body 13, so that when the electrical contact element 81 is in contact with the outer sleeve side surface 31, said electrical circuit 90 is closed through said impression roller sleeve 30 between said inner sleeve side surface 33 and said outer sleeve side surface 31.

[0049] The electrical circuit 90 is configured to provide a measured electrical insulation resistance value of the sleeve 30 between said inner sleeve side surface 33 and said outer sleeve side surface 31.

[0050] According to an embodiment, the electrical circuit 90 comprises an inlet for an electrical voltage source 91 and an electronic control unit 92 adapted to measure the electric current which travels through said electrical circuit 90 and adapted to provide said measured electrical insulation resistance value of the sleeve 30 between said inner sleeve side surface and said outer sleeve side surface 31 as a function of said measured electric current.

[0051] In particular, Ohm's law is used to measure the electrical insulation resistance between the inner sleeve side surface 33 and the outer sleeve side surface 31: V = R x I, where V is the electric potential difference between the inner sleeve side surface 33 and the outer sleeve side surface 31, supplied by the electrical voltage source 91, I is the current measured along the electrical circuit 90 by means of the electronic control unit 92, and R is the insulation resistance value between the inner sleeve side surface 33 and the outer sleeve side surface 31.

[0052] According to an embodiment, the electronic control unit 92 comprises comparison means configured to compare said measured electrical insulation resistance value with a predetermined electrical insulation resistance reference value of the sleeve 30.

[0053] According to an embodiment the printing unit 1 comprises an electrostatic printing assistance apparatus 50 associated with said impression roller 10 and configured to transmit an electrostatic charge to said outer sleeve side surface 31.

[0054] According to an embodiment, the electronic control unit 92 comprises activation means configured to provide an inhibition signal for inhibiting the activation of said electrostatic printing assistance apparatus 50 when the measured electrical insulation resistance value of the sleeve 30 differs from the predetermined electrical insulation resistance reference value of the sleeve 30.

[0055] Furthermore, such activation means are configured to provide an authorization signal to allow the activation of said electrostatic printing assistance apparatus 50 when the measured electrical insulation resistance value of the sleeve 30 is equal, or substantially equal, to the predetermined electrical insulation resistance reference value of the sleeve 30.

[0056] According to an embodiment, the electrical circuit 90 is connected to said inner sleeve side surface 33, by means of an electrical connection between said electrical circuit 90 and the impression roller body side surface 13, when said sleeve is fitted onto said impression roller body 11, with said inner sleeve side surface 33 in electrical contact with said impression roller body side surface 13.

[0057] According to an embodiment, the electrical circuit 90 is capable of being connected, or connected to said inner sleeve side surface 33 by means of an electrical ground connection 100.

[0058] This electrical ground connection 95 allows a potential excess electrostatic charge distributed on the outer sleeve side surface 31 to be discharged, or dispersed, onto the ground.

[0059] According to an embodiment, the elastomeric material of the outer sleeve layer 32 has a hardness from 50 to 95 Shore A (ShA), preferably from 70 to 95 Shore A (ShA), even more preferably of about 75 Shore A.

[0060] According to an embodiment, the elastomeric material comprises a material selected from synthetic rubber and natural rubber.

[0061] Such an elastomeric material may comprise a plurality of conductive particles dispersed therein, to give such a material a value of conductivity, and thus of a predetermined electrical resistance, based on the type of electrostatic printing assistance apparatus used.

[0062] Thus, the sleeve 30 has an electrical surface resistance value capable of being measured between two different points of said outer side surface 31 of said sleeve, and an insulation resistance value capable of being measured between said outer side surface 31 of said sleeve and the inner side surface 33 of said sleeve.

[0063] The electrical surface resistance value can be measured, for example, by means of a pad provided with strips of leather soaked in water and arranged around the outer surface of the sleeve, at a distance of 110 mm from one another, to which a voltage of 500 Volts is applied and between which the resulting voltage is measured.

[0064] According to an embodiment, the electrostatic printing assistance apparatus 50 comprises a non-contacting, or top loading, electrostatic charge transmitter 51, preferably an electrostatic charge bar 51, configured to transmit an amount of electrostatic charge to the outer side surface 31 of the sleeve 30 from the outside of the impression roller, preferably radially to the impression roller, preventing an electrical contact with said outer side surface 31 of said sleeve 30.

[0065] In the case of a non-contacting electrostatic charge transmitter, the outer sleeve layer 32 made of an elastomeric material is semi-conductive, and the predetermined electrical insulation resistance reference value of the sleeve 30 is preferably comprised between 0.85 GOhm and 1.15 GOhm, preferably about 1 GOhm.

[0066] 1 GOhm is understood to mean a resistance value of 1,000,000,000 Ohm, or 1x10⁹ Ohm.

[0067] According to an embodiment, in the case of a non-contacting or top loading electrostatic charge transmitter 51, the sleeve 30 can comprise an inner sleeve layer 98 made of an electrically insulating material, delimited by said inner sleeve side surface 33.

[0068] In the case of a non-contacting or top loading electrostatic charge transmitter 51, said inner sleeve side surface 33 can comprise, according to one embodiment, an electrically conductive surface portion 96, electrically connected to said outer sleeve layer 32, for example, by means of an electrical resistance element 97, so that when the sleeve 30 is fitted onto the impression roller body 10, said electrically conductive surface portion 96 is in electrical contact with a corresponding conductive portion of said impression roller body side surface 13, said conductive portion of said impression roller body side surface 13 being connected to said electrical circuit 90.

[0069] According to an embodiment, said electrically conductive surface portion 96 is the radially inner surface of a conductive annular element 96' coaxial to the tubular sleeve 30, preferably incorporated in said inner sleeve layer 98.

[0070] According to an embodiment, the electrically conductive surface portion 96 is flush with said inner sleeve side surface 33.

[0071] According to one embodiment, the conductive annular element is made of carbon.

[0072] In the case of a non-contacting, or top loading electrostatic charge transmitter 51, the impression roller body side surface 13, or at least a conductive portion of said impression roller body side surface 13, is connected to ground by means of a ground connection 95.

[0073] Such a ground connection 95, may be part of the electrical circuit 90.

[0074] The semi-conductivity of the sleeve 30 allows the charges distributed on the outer sleeve side surface 31, to cross the thickness of the sleeve 30 from the outer sleeve side surface 31 towards the inner sleeve side surface 33, through the impression roller body side surface 13, and then downwards through the electrical ground connection 95.

[0075] In the case of a non-contacting, or top loading electrostatic charge transmitter 51, the surface resistance is preferably comprised between 0.5 Mohm and 15 Mohm, at a temperature comprised between about 30°C to 40°C.

[0076] According to an embodiment, the electrostatic charge bar 51 comprises a high voltage electrostatic charge generator, powered by low voltage, preferably at about 24 Volt.

[0077] According to an embodiment, such an electrostatic charge generator is integrated in said electrostatic charge bar 51.

[0078] In this way the electrical power wires connected to the electrostatic charge bar are powered by low voltage, thus avoiding the risks relating to the operating safety of the machine, which would be caused, instead, by the high voltage.

[0079] According to an embodiment, the electrostatic printing assistance apparatus 50 and the checking apparatus 80, are both fixed to the support upright 60, preferably by means of a common support bracket 61.

[0080] Preferably, the electrostatic charge bar 51 and the actuator 83 are both fixed to the support upright 60, preferably by means of a common support bracket 61.

[0081] According to an embodiment, the electrical insulation resistance checking apparatus 80, preferably the movement device 82, preferably the actuator 83, is constrained, or fixed, to said charge bar 51.

[0082] According to an embodiment, the electrostatic printing assistance apparatus 50 comprises a plurality of charge electrodes adapted to provide a predetermined voltage and to be crossed by a predetermined current during use.

[0083] According to another embodiment, the electrostatic printing assistance apparatus 50 comprises an electrostatic charge transmitter of the direct charging type 53, configured to transmit an amount of electrostatic charge to the outer side surface 31 of the sleeve 30 from the inside of the impression roller 10 towards the outside of the impression roller 10, preferably radially, through said sleeve 30, by means of a direct electrical connection between the electrostatic charge transmitter and the impression roller side surface 13.

[0084] In the case of a direct-charging-type electrostatic charge transmitter 53, the outer sleeve layer 32 made of an elastomeric material is preferably conductive, and the predetermined electrical insulation resistance reference value of the sleeve 30 is preferably comprised between 0.001 Ohm and 0.1 Ohm, preferably about 0.01 Ohm.

[0085] According to an embodiment, the sleeve 30 comprises an inner sleeve layer made of an electrically conducting material, delimited by said inner sleeve side surface 33, so that when the sleeve 30 is fitted onto the impression roller body 11, said inner sleeve layer puts the outer sleeve layer 32 in electrical contact with the impression roller body side surface 11.

[0086] In the case of a direct-charging-type electrostatic charge transmitter 53, the surface resistance value is comprised between 0.5 Mohm and 2 Mohm, at a temperature comprised between about 30°C and 40°C.

[0087] In the case of a direct-charging-type electrostatic charge transmitter 53, according to one preferred embodiment, it comprises an electrostatic charge generator, preferably integrated in the impression roller 10, or arranged axially in contact with said impression roller 10, or arranged at an axial end of the impression roller 10.

[0088] According to an embodiment, the electrostatic charge generator of the electrostatic charge transmitter of the direct-charging type 53 is adapted to be powered by low voltage, preferably at about 24 Volt.

[0089] In this way the electrical power wires connected to the electrostatic charge generator integrated in the impression roller are powered by low voltage, thus avoiding the risks relating to the operating safety of the machine, which could be caused, instead, by high voltage.

[0090] According to an embodiment, the printing unit 1 comprises a pair of antistatic bars 70, of which a first antistatic bar 71 is arranged at the inlet, and a second antistatic bar 72 is arranged at the outlet, with respect to the impression roller 10 along a feeding direction 73 of a material 74 to be printed.

[0091] In other words, the first antistatic bar 71 and the second antistatic bar 72 are arranged on opposite sides to the impression roller 10.

[0092] According to an embodiment, each of said first antistatic bar 71 and said second antistatic bar 72 comprises a plurality of electrodes adapted to face the material 74 and an electrostatic charge generator connected to said electrodes for remove electrostatic charges on said material 74 by means of said electrodes.

[0093] Each of said first antistatic bar and said second antistatic bar, or said generator, are configured to provide said electrodes with an alternating sign voltage over time, so as to remove both positive and negative electrostatic charges from said material 74.

[0094] Preferably, the electrodes of said plurality are in the shaper of metal tips.

[0095] According to an embodiment, the electrodes of said plurality are arranged on two parallel rows extending along a main direction of development of said first antistatic bar 71 and of said second antistatic bar 72.

[0096] According to an embodiment, the first antistatic bar 71 and the second antistatic bar 72 are powered by low voltage, preferably at about 24 Volt.

[0097] Preferably, the first antistatic bar 71 and the second antistatic bar 72 each comprise a respective electrostatic charge generator therein.

[0098] According to an embodiment, the printing unit 1 comprises at least one tensioning roller 65, 66, arranged on at least one side to the impression roller 10, adapted to press on the material to be printed, so as to keep it tense.

[0099] Preferably, the printing unit 1 comprises two tensioning rollers 65, 66, arranged on opposite sides to the impression roller 10, adapted to press on the material to be printed, so as to keep it tense.

[0100] According to an embodiment, a first roller 65 of said two guide rollers is interposed between the first antistatic bar 71 and the impression roller 10, and a second roller 66 of said two guide rollers is interposed between the impression roller 10 and the second antistatic bar 72.

[0101] According to an embodiment, each of said at least one tensioning roller 65, 66, or each of said two tensioning rollers 65, 66, has a rotation axis parallel to the rotation axis of the impression roller 10.

[0102] According to an embodiment, the printing unit 1 comprises a printing roller 20, defining an outer side surface 21 of the printing roller and a rotation axis 22 of the printing roller.

[0103] The outer side surface 21 of the printing roller has a plurality of cavities, or small cells, having such a shape and arrangement as to mainly form a figure to be printed on the sheet or reel material 74.

[0104] The cavities, or small cells, are adapted to house therein an amount of ink 24, to be transferred onto the material 74, by means of pressure of the impression roller 10 against the printing roller 20.

[0105] According to an embodiment, the printing unit comprises a container tank 23 for ink 24, arranged so that the outer side surface 21 of the printing roller 20 can be wet by the ink 24 contained in the container tank 23, and so that the printing roller 20 may rotate about the rotation axis thereof 22.

[0106] According to an embodiment, the printing unit 1 comprises movement means for moving the impression roller 10 and for pressing it against the printing roller 20.

[0107] In order to meet contingent needs, those skilled in the art can make changes, adaptations, and replacements of elements with others which are functionally equivalent to the embodiments of the device described above, without thus departing from the scope of the following claims.

[0108] Each of the features described as belonging to an embodiment can be made irrespective of the other described embodiments.

[0109] The figures are not necessarily on scale.

[0110] All features described herein can be combined according to any combination except for combinations in which at least some of such features are mutually exclusive.

Claims

1. A printing unit (1) for gravure printing a sheet or reel material (74), comprising:

- an impression roller (10) comprising an impression roller body (11) and an impression roller sleeve (30) fitted, or capable of being fitted, onto said impression roller body (11),
said impression roller body (11) defining an impression roller rotation axis (12) and an impression roller body side surface (13);
said impression roller sleeve (30) having a tubular shape defining an outer sleeve side surface (31) and an inner sleeve side surface (33), said impression roller sleeve (30) comprising an outer layer (32) made of elastomeric material delimited by said outer sleeve side surface (31) ;

- a checking apparatus (80) for measuring the electrical insulation resistance of said impression roller (30) between said inner sleeve side surface (33) and said outer sleeve side surface (31), said checking apparatus (80) being permanently associated with said printing unit (1), and comprising:

- an electrical contact element (81);

- a movement device (82) for moving said electrical contact element (81) between a measuring position, in which the electrical contact element (81) is in contact with the outer sleeve side surface (31), and a rest position, in which said electrical contact element (81) is not in contact with, and spaced apart from, the outer sleeve side surface (31) ;

- an electrical circuit (90) connected to said electrical contact element (81) and capable of being connected to said inner sleeve side surface (33), when said sleeve (30) is fitted onto said impression roller body (13), so that when the electrical contact element (81) is in contact with the outer sleeve side surface (31), said electrical circuit (90) is closed through said impression roller sleeve (30) between said inner sleeve side surface (33) and said outer sleeve side surface (31); said electrical circuit (90) being configured to provide a measured electrical insulation resistance value of the sleeve (30) between said inner sleeve side surface (33) and said outer sleeve side surface (31).

2. A printing unit (1) according to claim 1, wherein said electrical circuit (90) comprises an inlet for an electrical voltage source (91) and an electronic control unit (92) adapted to measure the electric current which travels through said electrical circuit (90) and adapted to provide said measured electrical insulation resistance value of the sleeve (30) between said inner sleeve side surface and said outer sleeve side surface (31) as a function of said measured electric current.

3. A printing unit (1) according to claim 2, wherein the electronic control unit (92) comprises comparison means configured to compare said measured electrical insulation resistance value with a predetermined electrical insulation resistance reference value of the sleeve (30).

4. A printing unit (1) according to claim 3, comprising an electrostatic printing assistance apparatus (50) associated with said impression roller (10) and configured to transmit an electrostatic charge to said outer sleeve side surface (31),
wherein said electronic control unit (92) comprises activation means configured to provide an inhibition signal for inhibiting the activation of said electrostatic printing assistance apparatus (50) when the measured electrical insulation resistance value of the sleeve (30) differs from the predetermined electrical insulation resistance reference value of the sleeve (30), and wherein said activation means are configured to provide an authorization signal to allow the activation of said electrostatic printing assistance apparatus (50) when the measured electrical insulation resistance value of the sleeve (30) is equal, or substantially equal, to the predetermined electrical insulation resistance reference value of the sleeve (30).

5. A printing unit (1) according to at least one preceding claim, wherein said electrical circuit (90) is connected to said inner sleeve side surface (33), by means of an electrical connection between said electrical circuit (90) and the impression roller body side surface (13), when said sleeve is fitted onto said impression roller body (11), with said inner sleeve side surface (33) in electrical contact with said impression roller body side surface (13).

6. A printing unit (1) according to at least one preceding claim, wherein the electrical circuit (90) is connected to said inner sleeve side surface (33) by means of an electrical ground connection (100).

7. A printing unit (1) according to at least one claim from 4 to 6, wherein the electrostatic printing assistance apparatus (50) comprises a non-contacting, or top loading, electrostatic charge bar (51), configured to transmit an amount of electrostatic charge to the outer side surface (31) of the sleeve (30) from the exterior of the impression roller, thus avoiding an electrical contact with said outer side surface (31) of said sleeve (30) .

8. A printing unit (1) according to claim 7, wherein the outer sleeve layer (32) made of elastomeric material is semi-conductive, and wherein the predetermined electrical insulation resistance reference value of the sleeve (30) is between 0.85 GOhm and 1.15 GOhm, preferably about 1 GOhm.

9. A printing unit (1) according to claim 8, wherein the sleeve (30) comprises an inner sleeve layer (98) made of electrically insulating material, delimited by said inner sleeve side surface (33), said inner sleeve side surface (33) comprising an electrically conductive surface portion (96) electrically connected to said outer sleeve layer (32) so that, when the sleeve (30) is fitted onto the impression roller body (10), said electrically conductive surface portion (96) is in electrical contact with a corresponding conductive portion of said impression roller body side surface (13), said conductive portion of said impression roller body side surface (13) being connected to said electrical circuit (90).

10. A printing unit (1) according to at least one claim from 1 to 6, wherein the electrostatic printing assistance apparatus (50) comprises an electrostatic charge transmitter (53) of the direct charging type, configured to transmit an amount of electrostatic charge to the outer side surface (31) of the sleeve (30) from the interior of the impression roller (10) towards the exterior of the impression roller (10), through said sleeve (30), by means of a direct electrical connection between the electrostatic charge transmitter and the impression roller side surface (13).

11. A printing unit (1) according to claim 10, wherein the outer sleeve layer (32) made of elastomeric material is conductive, and wherein the predetermined electrical insulation resistance reference value of the sleeve (30) is between 0.001 Ohm and 0.1 Ohm, preferably about 0.01 Ohm.

12. A printing unit (1) according to claim 11, wherein the sleeve (30) comprises an inner sleeve layer made of electrically conducting material, delimited by said inner sleeve side surface (33), so that when the sleeve (30) is fitted onto the impression roller body (11), said inner sleeve layer puts the outer sleeve layer (32) in electrical contact with the impression roller body side surface (11).

Drawing