GB2263442A

GB2263442A - Avoidance of contamination during start-up in continuous-jet ink-jet printers.

Info

Publication number: GB2263442A
Application number: GB9301209A
Authority: GB
Inventors: Hans-Georg Schindler
Original assignee: Agfa Gevaert AG
Current assignee: Agfa Gevaert AG
Priority date: 1992-01-23
Filing date: 1993-01-22
Publication date: 1993-07-28
Also published as: DE4201778A1; GB9301209D0; DE4201778C2; CH686941A5

Description

2263442 continuously operating fluid jet printer and process for operating

the printer The invention is based on a process for operating a continuously operating fluid jet printer, having a nozzle for generating a fluid jet, at least one charging electrode for splitting the jet into individual droplets and for the controlled supercharging of the individual droplets, a device for controlling the charging electrode, a device for generating an electric field in which the droplets are deflected according to their charge, and a device for collecting droplets not required.

is Fluid jet printers of this type develop a very high printing speed and can therefore be used for printing fast-running belts. This predestines them for use in photographic printers, in order for example to print on the reverse side of pictures of the prints produced. Continuously operating ink jet printers have been commercially available for a long time and also operate to the satisfaction of the user.

The nozzle generates a modulated ink jet, which passes through the gap of a charging electrode and is separated in the fields of the electrode into individually charged droplets. After leaving the charging electrode, the droplets fly through an electric field generated by two deflecting electrodes. Here, they are brought on to a flight trajectory which then makes them hit the target surface in the required pattern. When the printer is switched on, the jet must first build up before the droplets can reach the target being aimed at. This problem can best be explained with the example of a water hose. Here too, when the water tap is turned on the jet first has to build up, and during this phase it waters a line between the tip of the hose and the final target. This means that in the print head, while the jet is building up, the ink, which dries very fast due to the high printing speed, does not take the path provided, but contaminates working parts of the print head. The same applies in reverse when the printer is switched off. After the printer has been switched on and off several times, in particular the passages between the electrodes are sufficiently clogged that the jet can no longer pass through. This then means that the ink sprays in all directions and contaminates the whole surrounding area. As printers of this type have hitherto been used above all on production belts which operate 24 hours a day, this problem was not particularly serious. If the belt was switched off once in a while, for is servicing for example, the whole print head was removed and cleaned in solvent, so that the narrow passages were clear and clean again by the time the belt was restarted. Particularly when used in a photographic printer, where pauses during which the printer is switched off occur much more frequently, the repeated removal of the print head for cleaning is not tolerable, particularly as the operating personnel are not specialised technicians.

Consequently, it was the object of the invention so to operate and form a known, continuously operating fluid jet printer that the amount of contamination during switching on and off of the printer can be at least drastically reduced, and the intervals between cleanings can be extended.

Accordingly the invention provides a process for operating a continuously operating fluid jet printer, having a nozzle for generating a fluid jet, at least one charging electrode for splitting the jet into individual droplets and for the controlled supercharging of the individual droplets, a device for controlling the charging electrode, a device for generating an electric field in which the droplets are

2 deflected according to their charge, and a device for collecting droplets not required, wherein the working parts of the printer are removed from the range of the jet on starting and stopping before building up and running down of the jet, without thereby interrupting the path of the jet.

The invention also provides a device for carrying out the above process, whereby working parts of the printer are so formed that they can be removed from the range of the jet on starting and stopping before building up and running down of the jet, without thereby interrupting the path of the jet.

Moreover, the invention provides a process for operating a continuously operating fluid jet printer, having a nozzle for generating a fluid jet, at least one charging electrode for splitting the jet into individual droplets and for the controlled supercharging of the individual droplets, a device for controlling the charging electrode, a device for generating an electric field in which the droplets are deflected according to their charge, and a device for collecting droplets not required, wherein to build up and run down the jet during starting and stopping of the printer, a second fluid is used. The invention also provides a device for carrying out the above process whereby a device for supplying the nozzle with a first and a second fluid is provided, as well as a device for control, which so controls the supply of the two fluids that only the second fluid is supplied to the nozzle during building up and running down of the jet.

The invention yet further provides a continuously operating fluid jet printer, having a nozzle for generating a fluid jet, at least one charging electrode for splitting the jet into individual droplets and for the controlled supercharging of the individual droplets, a device for 3 controlling the charging electrode, a device for generating an electric field in which the droplets are deflected according to their charge, and a device for collecting droplets not required, wherein a drive is provided which moves the nozzle and/or the device for collecting the droplets not required in such a manner that the distance between the two working parts is reduced.

Printers are already known which are so conceived that cleaning of the print head is easier to carry out. To this end, the charging electrode and one of the deflecting electrodes are so mounted that they can be pivoted into an area in which the space conditions permit easy cleaning. According to the invention, the electrodes are pivoted before starting of the printer so that, during building up of the jet, they are not in an area in which there is any risk of contamination. As soon as the jet is steady, the electrodes are pivoted bac k and the printing operation proper can begin. The same process takes place when the printer is switched off. The electrodes remain in their operating position for as long as the jet is steady and the droplets are aimed at their target. Only when the electrodes have been removed from the region of contamination is the printer switched off. Preferably, the pivoting operation is carried out automatically by means of a drive. The drive may act on a support on which the working parts subject to a risk of contamination are mounted.

In a second embodiment of the process, the jet is built up with a second fluid, in particular with a solvent. The same applies to running down of the jet. The transition between solvent and ink - or in reverse order, from ink to solvent during running down - is preferably carried out continuously. in this process the electrodes are only moistened with quickly evaporating solvent, which leaves virtually no residue. A printer operating according to 4 this process has both a supply line for the solvent and a control for adjusting the transition between the two fluids.

In the process or device described so far, sometimes, both during starting and stopping of the printer, droplets whose direction cannot yet be reliably controlled may leave the discharge aperture and contaminate or damage the surface to be printed or - in the case of a photographic printer - the objective or the shutter device, for example. This applies both to ink and solvent droplets. In order to remedy this problem, a shutter device is preferably provided which closes the discharge aperture while the jet is building up or running down. In order that the shutter device comes into action when the printer is switched off by an unforeseen event, e.g. a power failure, it is so conceived that it is actively held open and is in the closed state in its rest position. The shutter device can be equipped with means for collecting the fluid landing thereon and diverting it to a collecting tank or recycling it to the supply tank.

In a third embodiment, this shutter device is not necessary, as during building up or running down of the jet, neither ink nor solvent can be discharged. To this end, during these phases, the device for collecting the droplets not required, or the nozzle itself, is moved in such a manner that the distance between the two working parts at least decreases. Preferably, the device for collecting the droplets not required is moved until the receiving aperture abuts the nozzle aperture. After starting of the jet, the collecting device then moves back into its predetermined position. When the printer is switched off, the process is carried out in reverse. In this case, the collecting device is first moved to the nozzle and then the jet is shut off. In order to clean the whole device, flushing with solvent can still be carried out in this position. During the break in operation, the collecting device can then remain in this position until the next time the printer is started.

Further details and advantages of the invention will appear from the subclaims in combination with the description of embodiments, which are explained with the aid of the drawings, in which:

Fig. 1 shows a first embodiment with working parts capable of pivoting down, Fig. 2, a second embodiment in which the jet is built up with solvent, and is Fig. 3, a third embodiment with a displaceable droplet collecting device.

The Figures only show the working parts of a print head for a continuously operating ink jet printer, e.g. for use in a photographic printer. In Fig. 1, the nozzle I has an inlet 2 for the ink, an inlet 3 for the solvent, and an outlet 4. The charging electrode 5 is connected to a control device 6. The charging electrode 5 is mounted, together with a deflecting electrode 7, on a support 8, which is pivotable about the axis 9. The second deflecting electrode 10 is fixed in position on the housing in a manner not shown. A pneumatic piston-cylinder unit 12, whose free end 13 is also fixed firmly to the housing, is connected to the support 8 via the shaft ii. The collecting device 14 for droplets not required is formed as a U-tube and is connected to a vessel in a manner not shown. The shutter device 15 can be moved in the direction of the arrow A, so that the fluid droplets 16, 17, 18 cannot leave the housing. The shutter device is biased in its open position by means of the spring 20 against a support 19 rigidly 6 connected to the housing. It can be kept in this position magnetically, for example, via a solenoid.

A modulated fluid jet 21 is generated in the nozzle I in a known manner, e.g. by means of a piezo-crystal. In order to keep the viscosity of the ink constant, according to its state, solvent is added via the line 3. Surplus solvent and surplus ink are recycled to a tank via the line 4. The modulated jet 21 enters the gap 22 of the charging electrode 5 and is there split into individual drops 23. These drops then have a specific charge, which is adjustable via the control 6. The droplets 23 then pass through an electric field, which is generated by the deflecting electrodes 7, 10. In this field, they are brought on to a flight trajectory 24, 25 corresponding to their charge. The droplets not required for the printing process are referenced 16. They are either neutral or have a specific charge, so that they can be collected by the U-shaped tube 14. These droplets are returned to the tank via a connection not shown.

To switch on the printer, the support 8 is in the position shown in broken lines. Hence the sensitive working parts to be protected from contamination are retracted beneath the screening plate 26. The shutter device 15 is in the closed position so that the discharge aperture for the jet is not passable. When the printer is switched on and the jet consequently builds up, the droplets can only contaminate the inner wall of the housing, but not the sensitive working parts. As deposits on the housing inner wall do not adversely affect the function of the printer, cleaning at relatively infrequent intervals is sufficient. As soon as the jet is steady and reaches into the aperture of the U-tube 14, the pneumatic cylinder 12 is actuated and the support 8 is moved into its position shown in solid lines. The charging electrode 5 and the deflecting electrode 7 are then in their predetermined position. At 7 the same time, a further drive is.actuated and the shutter device 15 is pulled up. It is actively kept in this position. The printer is now ready to print and the droplets can be modified via the control 6 and the charging electrode 5 in such a manner that they reach their predetermined target 17, 18 on the surface 34 to be printed.

Before switching off the printer, the pneumatic cylinder 12 is again actuated, so that the support 8 with the deflecting electrode 7 and the charging electrode 5 is moved into its position shown in broken lines. Thus the sensitive parts are again protected under the screening plate 26. At the same time the catch for the shutter device 15 is released, so that said shutter device moves into its closed position. Now the jet can be switched off without the contamination of important parts or the falling of misdirected droplets on to the surface 34 to be printed. The printer can remain in this state until it is switched on again.

is The embodiment according to Fig. 2 is substantially more simply constructed. For the operation of this device, the control 27 which controls the supply of ink and solvent is critical. Otherwise the parts with the same function as in the example according to Fig. 1 are provided with the same reference numbers.

To switch on the printer, in this case the ink feed is completely closed by the control 27, whilst the feed for the solvent is fully opened. In order to prevent contamination by the solvent on the print medium, in this embodiment also the shutter device 15 is closed during building up of the jet. As the jet is only built up from solvent, which evaporates quickly and leaves no residue, the sensitive working parts do not have to be pivoted out of the range of the jet. As soon as the jet is steady, the 8 n shutter device 15 is opened again and the electronic device 27 slowly closes the solvent feed, whilst the ink feed is opened to the same degree. This operation ends when the desired mixture ratio with the corresponding viscosity of the fluid has been set. Then the printer is ready to print. During switching off, the process is carried out in reverse order. The shutter device is moved into its closed position and while the ink feed is being throttled, the solvent feed is opened. only when the jet consists of pure solvent is it shut off. In this case also the printer can remain in this state until it is switched on again.

In the embodiment according to Fig. 3, there is no risk of contamination either in the housing nor on the working parts. In this device, no shutter device is needed to protect the surface to be printed from contamination. In this case, the U-shaped collecting device 14 is movable to and fro between its positions shown in solid and broken lines respectively. The drive consists of a spindle 28 and a motor 29. The spindle 28 acts on the thrust element 30 to which the U- shaped tube 14 is fixed.

When the printer is switched on, the collecting device 14 together with the thrust element 30 is in the position shown in broken lines. The aperture 31 of the tube 14 is at the end 32 of the nozzle 1, so that there is a fluid-tight connection between these two parts. In this manner, the surrounding area is fully screened during building up of the jet and is therefore protected from contamination. After switching on of the jet, the U-tube 14 is slowly retracted into its position shown in solid lines via the spindle drive 28, 29, 30. In order to ensure a steady build-up of the jet, the displacement speed of the U-tube must be less than the flight speed of the droplets.

As soon as the collecting device 14 reaches its predetermined position the printer.is ready.

9 Before switching off, the collecting device 14 again moves slowly back as far as its position shown in broken lines, so that as the jet runs down no important working parts are contaminated. It is particularly advantageous if, after switching off the jet, the whole device is flushed with solvent via the nozzle, the U-tube 14 and the hose 33. Manual cleaning of the printer is then virtually only necessary in exceptional cases.

obviously it would also be possible to make the collecting device stationary and to provide it with a telescopic tube. In this case, the same advantages would be gained as in the embodiment according to Fig. 3.

claims 1. Process for operating a continuously operating fluid jet printer, having a nozzle for generating a fluid jet, at least one charging electrode for splitting the jet into individual droplets and for the controlled supercharging of the individual droplets, a device for controlling the charging electrode, a device for generating an electric field in which the droplets are deflected according to their charge, and a device for collecting droplets not required, wherein the working parts of the printer are removed from the range of the jet on starting and stopping before building up and running down of the jet, without thereby interrupting the path of the jet.

2.

Device for carrying out the process according to claim 1, whereby working parts of the printer are so formed that they can be removed from the range of the jet on starting and stopping before building up and running down of the jet, without thereby interrupting the path of the jet.

3. Device according to claim 2, wherein the working parts are mounted on a common support, which is pivotable via a drive.

4. Device according to claim 3, wherein the charging electrode and a deflecting electrode are mounted on the support.

Process for operating a continuously operating fluid jet printer, having a nozzle for generating a fluid jet, at least one charging electrode for splitting the jet into individual droplets and for the controlled 11 supercharging of the individual droplets, a device for controlling the charging electrode, a device for generating an electric field in which the droplets are deflected according to their charge, and a device for collecting droplets not required, wherein to build up and run down the jet during starting and stopping of the printer, a second fluid is used.

is Process according to claim 5, wherein a solvent is used as a second fluid.

Process according to claim 5 or 6, wherein the transition from one fluid to the other is carried out continuously.

Device for carrying out the process according to claim 5, whereby a device for supplying the nozzle with a first and a second fluid is provided, as well as a device for control, which so controls the supply of the two fluids that only the second fluid is supplied to the nozzle during building up and running down of the jet.

9. Device according to claim 2 or claim 8, further comprising a device for closing the discharge aperture for the droplets while the jet is being built up or run down during starting and stopping of the printer.

10. Device according to claim 9, wherein the shutter device closes the discharge aperture in its rest position and has to b6 actively held open.

11. Continuously operating fluid jet printer, having a nozzle for generating a fluid jet, at least one charging electrode for splitting the jet into individual droplets and for the controlled supercharging of the individual droplets, a device for 12 controlling the charging electrode, a device for generating an electric field in which the droplets are deflected according to their charge, and a device for collecting droplets not required, wherein a drive is provided which moves the nozzle and/or the device for collecting the droplets not required in such a manner that the distance between the two working parts is reduced.

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