GB2417924A

GB2417924A - Ultrasonic chambered doctor blade

Info

Publication number: GB2417924A
Application number: GB0420100A
Authority: GB
Inventors: David Jones
Original assignee: ALPHASONICS Ltd
Current assignee: ALPHASONICS Ltd
Priority date: 2004-09-10
Filing date: 2004-09-10
Publication date: 2006-03-15
Anticipated expiration: 2024-09-10
Also published as: GB2417924B; GB0420100D0

Abstract

A method and apparatus for the in-situ cleaning of take up rollers in chambered doctor blade flexographic printing presses using, preferably variable power, ultrasonic impulses applied directly or indirectly to a take up roller. E.g. colour station for a printing press comprising a chamber doctor blade for depositing ink onto a rotatable roller, and comprising one or more ultrasonic transducers for delivering ultrasonic impulses directly or indirectly to the rotatable roller and wherein a control means is provided for controlling the operation of the ultrasonic transducers.

Description

241 7924 Title: Flexographic printing

Description:

The present invention relates to flexographic printing and in particular to improvements in and relating to flexographic printing utilising chambered doctor blades.

Flexographic printing presses are well known in the printing art and generally comprise a colour station which dispenses a liquid ink onto a take up roller which in tunn transfers the ink to the print roller. The print roller nonnal]y has a raised pattern profile which profile is used to transfer the inked pattern to a web material (e.g. paper) that comes into contact with it.

Relatively sophisticated printing presses use a so-called "anilox" take up roller which has a central axle about which an ink transfer surface nonnally ceramic is disposed. The ink transfer surface has many thousands of cavities therein each cavity being a few microns in size that are filled with ink by a dispensing means and used to transfer ink to the print roller.

The dispensing means is usually accompanied by a doctor blade which wipes the take up roller surface for removing excess ink thereby minimising wastage. The doctor blades can have various configurations such as "reverse angle" or "trail doctoring" although it is generally accepted that a "chambered doctor blade" system gives the best results in teens of precise ink application . .

. ... . across the press width minimal splashing minimal evaporation of ink and the fastest printing speeds.

A chambered doctor blade system comprises two doctor blades arranged to contact the take up roller. The first blade is mounted at a positive angle to the roller and the second blade is mounted at a negative angle thereto. The space between the doctor blades defines a chamber into which the liquid ink is deposited.

As the take up roller rotates ink is drawn into the cavities and excess ink removed using the second doctor blade. That ink is then transferred to the print roller (whereby the cavities are emptied) before re-entering the ink chamber to be re-filled.

However the "empty" cavities contain air which becomes entrained in the ink supply causing it to "foam" or to cavitate. This causes the ink transfer rate to be adversely affected and also causes unpredictable fluctuations in the ink density.

The maximum resolution of the printing process is ultimately determined by the size of the take up roller cavities hence there is a constant desire to continuously reduce the take up roller cavity size and to increase their density i.e. the "dots per inch" or dpi value. However as the cavity sizes are reduced and the dpi value increased the cavities become more susceptible to blockages with a 2() resultant decrease in print quality. Furthermore if a layer of ink is accidentally allowed to dry in the cavities then the percentage reduction in cavity volume of a . . - . . .. - . small cavity is more greatly affected than that of a larger cavity contaminated with the same layer of dried ink.

Cleaning of take up rollers is difficult, and tune consuming. Connally, take up roller cleaning involves stopping the press (with resultant downtime) to clean it using a high pressure solvents or removal of the take up roller, immersion of the roller in an ultrasonic tank and reassembly of the press. Owing to the size of the take up roller, which in some presses can be up to 3 metres in length and 0.25 metros in diameter, the prior art solutions can be impractical and costly.

The present invention aims to provide a solution to one or snore of the above problems.

Accordingly, a first aspect of the invention provides a chamber doctor blade fonning part of a colour station for a printing press, the chamber; being adapted for at least partially filling with an ink for depositing said ink onto a rotatable roller; comprising back, end and side walls and having an open top for offering up to the roller; being adapted to substantially seal against the roller to control egress of ink therefrom in use; and having first and second doctor blades fonned integrally with the side walls; and characterised by the chamber having at least one ultrasonic transducer for delivering ultrasonic impulses, directly or indirectly to the rotatable roller and -7 - _ -e . . . . . . . . . c.

wherein a control means is provided for controlling the ultrasonic transducer or transducers.

A second aspect of the invention provides a colour station for a printing press comprising a chamber doctor blade for depositing ink onto a rotatable roller, and comprising one or more ultrasonic transducers for delivering ultrasonic Impulses directly or indirectly to the rotatable roller and wherein a control means is provided for controlling the operation of the ultrasonic transducers.

The at least one transducer may be affixed to one wall of the chamber.

The ultrasonic impulses may be applied directly to the rotatable roller or indirectly I () through the ink and/or through one or more walls of the chamber and/or through the doctor blade or blades and/or any other means that functions as a vibration conduction means.

A third aspect of the invention provides in a method of printing using a chamber doctor blade printing press, comprising a chamber doctor blade and a rotatable roller, the step of applying ultrasonic energy either directly or indirectly to the rotatable roller whilst the rotatable roller of the printing press is being rotated.

The chamber preferably comprises end walls and a back wall to which the ultrasonic transducers are affixed. The sidewalls of the chamber are at least 2() partially comprised of the doctor blades. The walls of the chamber are preferably manufactured of metal and are most preferably integrally i'orrned. The doctor . ... . blades are preferably manufactured of a resilient material, e.g. polymer or elastomer sheet/strip.

The thickness of the back wall of the chamber is preferably relatively large. The thickness of the chamber back wall is such that the mass of the wall per unit area Is many times grater than the mass of ink supported by the same unit area of back wall. Thus, if the density of the ink changes, (e.g. through foaming), the load that the ultrasonic transducer or transducers have to drive remains relatively unchanged in percentage terms. By making the chamber back wall relatively thick, the drive power and frequency of the transducers need not be tuned to each ink type and or finely adjusted during use to compensate for foaming, fluctuations in dilution etc. This makes the transducer or transducers relatively insensitive to the ink properties.

The ultrasonic transducers are driven by control means, which is preferably a drive circuit. Preferably there are a plurality of transducers or banks of transducers. The control means is preferably adapted to drive the transducer or transducers at more than one power and/or frequency. In a most preferred emhodment of the invention the control means can drive each of the transducers (where more than one transducer is present) at different operating parameters, e.g. power and/or frequency. This means that the print parameters can be adjusted on the fly, either manually or automatically, to compensate for e.g. the printed ink density being greater at one side of the take up roll to the other.

_, . --. --.-'' -me.. J. __ . . . . - In a most preferred embodiment, each transducer has at least two operating modes, namely high-frequency, low-power (HELP) and high-power, low fi^cquency (HPLF). The WFLP mode may be used to clean the take up roller during downtime, whereas the HPLF could be used during use to help prevent fouling of the take up roller cavities.

Thus, the invention provides a system for cleaning the take up rollers when the press is not in use in addition to a means for inhibiting the fouling of the take up roller in the first place. Other possible advantages of the invention include being able to compensate for differences in ink deposition across the length of the roller during printing.

A preferred embodiment of the invention shall now be described, by way of example only, with reference to the accompanying drawings, in which; Figure 1 shows a schematic cross section through the invention; Figure 2 shows a perspective view ofthe invention; 1 S Figure 3 shows a detail of the ultrasonic transducers 52 of the invention; Figure 4 shows a perspective view of the inside of a chamber according to the invention with the doctor blades removed; and Figure 5 shows a perspective view of the outside of a chamber according to the invention with the doctor blades and ink removal tubes renovcd.

Referring now to Figure 1, a printing press l 0 is generally shown comprising a colour station 12, a take up roller 14, a print roller 16 and a nip roller 18 for printing onto a web material 20. The take up roller 14 comprises a central - . . . . . . ..

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core 22, about which a ceramic layer 24 is disposed. The print roller 16 also has a core 26 about which an elastomeric print layer 28 is disposed. The print layer 28 has a relief patterns that dictates the print pattern. The nip roller 18 provides an abutment surface against which the print roller can work.

As can be seen in the insets to Figure 1, the ceramic layer 24 of the take up roller 14 has cavities 3() therein for transporting ink droplets 32 to the print roller 16.

The colour station 12 comprises a chamber 34 that is filled with ink 36 using a liquid ink feed tube 38. The chamber is formed of a back wall 40 and side walls 42. End walls (not shown) lie in the plane of the drawing at either end of the chamber 34. A pair of doctor blades 44 and 46 are used to prevent leakage of ink 36 from the chamber 34 in addition to regulating the size of the ink droplets 32 deposited into the cavities 30.

As can be seen, air bubbles 48 become entrained it the in ink 36 as the empty cavities 3() advance into the chamber in use. To mitigate this effect an ink removal tube (not shown) is also provided to constantly remove ink from the chamber to an anti foaming apparatus (also not shown) before being re-introduced into the chamber via the liquid ink feed tube 38. Thus, the ink is constantly being circulated through the anti foaming apparatus to minnnise the amount of f'oam/mcrofoam present in the chamber. Anti-f'oaning chemicals may also be added to the ink to help mitigate this effect. .

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Affixed to the back wall 40 of the chamber 34 are ultrasonic transducers 52. Each transducer comprises a piezoelectric stack of relatively standard construction. As can be seen, the thickness of the back wall 40 is relatively large.

In one specific example of the invention, the back wall 40 is manufactured of 6mm thick steel plate having a density of 8gcm3. This, if used to drive a watcr- based ink having a 10% air content, i.e. having a density of 0.9gcm3, the energy required to drive the back wall is approximately 9 times that required to drive the Ink. Thus, if the air content of the ink increases by 10%, the frequency dependent coupling efficiency of the ultrasonic transducer is relatively unaffected.

Figure 2 shows the invention in perspective. As can be seen, the chamber 34 is elongate and has a plurality of ink inlet and outlet tubes 38 & 50. Ultrasonic transducers 52 are disposed at regular intervals along the length of the chamber 34. A control means in the f'onn oi'a computer-controlled drive circuit 54 is used to individually control the operation of each transducer via the independent umbilical leads 56.

The ink inlet tube 38 is fed from an ink reservoir 58 and a de-foaming tank 60. Ink removal tubes 50 extract ink forth the chamber 34 and feed it into the de- f'oaning tank 60 to have the bubbles 48 therein extracted.

Figure 3 shows a detail of the ultrasonic transducers 52 of the invention affixed to a section of chamber back plate 40. Each transducer 52 comprises a drive 62 and ground 64 electrodes, which are connected to the control means 54.

Between each pair ol' electrodes is a stack of piezoelectric discs (e.g. Lead . . . . . .. . .. c

Zirconium-Titanate or Barium-titanate). The transducers 52 are either welded or adhered to the back wall 4() of the chamber using an adhesive.

Figure 4 shows a perspective view of the inside of a chamber according to the invention with the doctor blades removed. Finally, Figure 5 shows a perspective view of the outside of a chamber according to the invention with the doctor blades and ink remova] tubes removed.

The invention has been shown to operate relatively successfully when driven in HELP mode at 92kHz, 0-500W when the press is not in use and in LFHP mode at 54kHZ, 0-500W when the press is in use. When cleaning between print runs l-IFLP is used and the ink is removed from the chamber (as well as any wet ink from the roller) by an automated wash down system that is commonly used in the art and not described in further detail. Water and a chemical (based on Sodium Hydroxide) are introduced into the chamber. The ultra sorties drive through the water and chemical mixture to clean the roller. e

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Claims

Claims: 1. A chamber doctor blade apparatus forming part of a colour

station for a printing press, the chamber; being adapted for at least partially filling with an ink for depositing said ink onto a rotatable roller; comprising back, end and side walls and having an open top for offering up to the roller; being adapted to substantially seal against the roller to control egress of ink therefrom in use; and having first and second doctor blades formed integrally with the side walls; and characterized by the chamber having at least one ultrasonic transducer for delivering ultrasonic ] O impulses, directly or indirectly to the rotatable roller and wherein a control means is provided for controlling the ultrasonic transducer or transducers.

2. A colour station apparatus for a printing press comprising a chamber doctor blade for depositing ink onto a rotatable roller, and comprising one or more ultrasonic transducers for delivering ultrasonic impulses directly or indirectly to the rotatable roller and wherein a control means is provided for controlling the operation of the ultrasonic transducers.

3. An apparatus as claimed in claim 1 or claim 2, wherein the at least one transducer is affixed to one wall of the chamber.

4. An apparatus as claimed m clang 1, claim 2 or clang 3, wherein the ultrasonic impulses are applied to the rotatable roller via a vibration conduction means.

... . 5. An apparatus as claimed in claim 4, wherein the vibration conduction means one or more of the group comprising; the ink, one or snore walls of the chamber and the doctor blade or blades.

6. An apparatus as claimed in any of claims 1 to 5, wherein the ultrasonic transducer or transducers are aff xed to the back wall of the chamber.

7. An apparatus as claimed in any of claims 1 to 6, wherein the thickness of the wall to which the transducer or transducers are affixed is such that the mass of the wall per unit area is grater than the mass of ink supported by the same unit area of back wall.

IO 8. An apparatus as claimed in claim 7, wherein ratio of the mass of the wall per unit area to the mass of ink supported thereby per unit area is greater than 2.

9. An apparatus as claimed in any of claims I to 8, wherein the control means comprises a drive circuit.

]O. An apparatus as claimed in any of claims I to 9, wherein the control means is adapted to drive each transducer at more than one power and/or frequency.

11. An apparatus as claimed in any of claims I to 10, wherein the control means is adapted to drive each transducer at different power and/or frequency settings.

tJ 12. An apparatus as claimed in any of claims I to] I, wherein the control means is adapted to adjust the operating power and/or frequency settings of each ë e e ë eve e e e e e e ee eve e transducer whilst in use to compensate for fluctuations in printing and/or ink conditions.

13. An apparatus as claimed in any of claims 10, 1 1 or 19, wherein the power and/or fi cquency of each transducer is automatically and/or manually adjustable.

14. An apparatus as claimed din any of claims 10 to 13, wherein each transducer can operate in two nominal operating modes, namely high power low frequency mode or high frequency low power mode.

1 S. An apparatus as claimed in claim 14, wherein high frequency low power mode is used when the printer is not printing and wherein the high power, low frequency mode is used when the press is in use.

16. An apparatus as claimed in claim 15 wherein the high frequency low power mode means driving the transducers at a nominal frequency of 92000 Hertz at up to 5()0 watts or transducer banks.

17. An apparatus as claimed in claim 15 wherein the low frequency high power mode means driving the transducers at a nominal frequency of 54()00 Hertz at up to 5()0 watts per transducer bank.

]8. An apparatus as claimed in any of claims 1 to 17, wherein the take up roller comprises a central core, about which a ceramic or chromium layer is disposed.

1). An apparatus as claimed in claim 18, wherein the ceramic layer has cavities therein. ( , ,

. .. ... . A 20. An apparatus as claimed in any of claims 1 to 19, wherein the chamber further comprises an ink removal tube for removing ink from the chamber to an anti foaming means before being re-introduced the ink into the chamber.

21. An apparatus as claimed in claim 20, wherein the anti-foaming means uses ultrasonic to remove air bubbles from the ink.

22. An apparatus as claimed in either claim 20 or claim 2], wherein the anti- foaming means is adapted to mix an anti-foaming composition into the ink.

23. An apparatus as claimed in any of claims I to 21, wherein the ultrasonic transducer or transducers comprise a piezoelectric stack located between a drive and ground electrode.

24. An apparatus as claimed in claim 23, wherein the piezoelectric stack comprises a plurality of discs of piezoelectric material.

25. An apparatus as claimed in claim 23 or claim 24 wherein the piezoelectric material Is l,ead-Zirconium-Titanate.

26. An apparatus as claimed m claim 23 or claim 24 wherein the piezoelectric material is Barium-titanate.

27. An apparatus as claimed in any of claims I to 26, wherein the transducers are bolted to a wall of the chamber.

28. An apparatus as claimed in any of claims] to 26, wherein the transducers are stuck to a wall of the chamber using an adhesive.

29. A third aspect of the invention provides in a method of printing using a chamber doctor blade printing press, comprising a chamber doctor blade and a v. _ _ee ee.

rotatable roller, the step of applying ultrasonic energy either directly or indirectly to the rotatable roller whilst the rotatable roller of the printing press is being rotated.

30. An apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

31. A method substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

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