WO2005002860A1

WO2005002860A1 - Ink jet printer with ink pressure control

Info

Publication number: WO2005002860A1
Application number: PCT/NL2004/000498
Authority: WO
Inventors: Bernardus Petrus Paulus Morskate
Original assignee: Osiris Technology B.V.
Priority date: 2003-07-08
Filing date: 2004-07-08
Publication date: 2005-01-13
Also published as: EP1641622A1; ES2297447T3; DE602004010125D1; EP1641622B1; ATE378185T1; DE602004010125T2

Abstract

An Ink jet printer, for instance for printing textile, paper, packagings such as food packages, for instance cardboard boxes, food cans, milk packs or the like, has an ink circuit which comprises: at least one supply container (1) for ink; a first pump (6) connecting thereto via a first conduit (62); at least one electrically controlled ink jet head (12) connecting thereto via a second conduit (73, 74); a buffer vessel (75) which is arranged in the second conduit and of which the infeed (33) is located higher than the outfeed (34) and the ink level (35) is situated at a height between the height of the infeed and the height of the outfeed; an air tank (10) connecting above the ink level to the buffer vessel; and pressure adjustment (21) means for adjusting the air pressure in the air tank and thereby holding the height of the ink level constant within chosen tolerances.

Description

INK JET PRINTER WITH INK PRESSURE CONTROL

The invention relates to an ink jet printer, for instance for printing textile, paper, packagings such as food packages, for instance cardboard boxes, food cans, milk packs or the like, which ink jet printer has an ink circuit which comprises: at least one supply container for ink; a first pump connecting thereto via a first conduit; at least one electrically controlled ink jet head connecting thereto via a second conduit. The described ink circuit is adapted to feed ink to the ink jet head under precisely controlled conditions. The ink jet head is adapted to deflect generated ink droplets, for instance by electrostatic means, in a manner such that through control from a central control unit each generated ink droplet is delivered to a precisely determined location on the substrate for printing. The invention also relates to a continuous ink jet printer which is embodied such that the ink jet heads generate ink droplets continuously. The droplets which must not get onto the substrate, for instance paper, textile or tin, are collected and fed back to the ink circuit . A properly functioning ink circuit must satisfy three important requirements. (1) The ink must be fed in the correct manner to the or each ink jet head. (2) In the case of a continuous ink jet printer the ink must be fed back again from the heads to the ink circuit in the correct manner. (3) The ink circuit must be adapted such that it enables the user to change colour in economically responsible manner. The above requirements are further elucidated below- Sub (1) : The ink must be fed to the ink jet heads rapidly, at the correct pressure and without contaminations and air inclusions. The speed at which the droplets leave the jets, the so-called jet speed, is directly determined by the pressure at which the ink is supplied. If this speed does not correspond exactly with the nominal value, this then has a negative effect on the print image. For printing of textile it is possible to envisage a pressure for instance in the order of 3.5 bar, which is held constant with an inaccuracy of a maximum of ± 0.03%. Because the required pressure depends among other factors on the viscosity of the ink and the pressure of the outside air, the pressure must be very precisely adjustable. On a printer with multiple heads it is further essential that all functionally corresponding parallel conduits have exactly the same flow resistance, therefore the same diameters, the same lengths and the same number of similarly formed bends. It is of further importance for the ink to be free of air inclusions. If an air bubble enters the ink jet head, this can then stop the process of ink droplet generation or contaminate the heads, whereby ink leaks onto the substrate for printing. Sub (2) : The ink which is not used for printing must be fed back again to the ink circuit so that it can be reused. However, because the ink is generated and fed back again in the droplet form, there occurs a more or less considerable foam forming, depending on the type of ink. This foam contains air bubbles which may not be fed to the heads. The ink circuit must therefore be able to eliminate the foam formation. Sub (3) : Most ink jet printers are adapted to print in only one colour, i.e. black. For the printing of patterns however, it is necessary to change colour very regularly. This means in practice that the ink circuit with the ink jet heads must be flushed out with the new colour until all ink in the ink circuit is completely replaced by the new colour. In order to have this process take place as economically as possible, the following aspects are important: * The volume of the ink circuit must be as small as possible.

* The system may not have any blind conduits or dead spaces.

* Mixing of the two colours of ink must be limited as much as possible.

* The accumulation of contaminating ink behind and in valves must be prevented. * It is important to recover as much as possible of the ink used for the first time.

* The process must be completed as quickly as possible, for instance within two minutes.

* The pressure of the system must be kept as stable as possible, for instance within ± 0.03%.

The ink jet printer according to the invention to be specified hereinbelow is capable of realizing the above stated objectives. As will become apparent below, use is made of a pump and an air buffer. The pump is used to bring to pressure and transport the ink. If however only a pump is used, a number of problems occur:

* In order to produce the correct constant pressure the pump must be adjustable in regulating manner or use must be made of a bypass valve. In order to control the pump a sensor is needed which measures the pressure produced by the pump. As far as applicant is aware, there are no absolute sensors possessing the desired accuracy.

* Practically every type of pump produces pulsations with values greater than the desired pressure stability of ± 0.03%.

* The different ink jet heads can be switched on and switched off. This means a jump in the ink flow rate. However, the pump needs a little time to adjust its rotation speed and output, whereby a pressure fluctuation is created. In order to prevent these problems, the invention provides an ink jet printer which is able to completely eliminate the pulsations of the pump, to compensate jumps in the flow rate, to hold the pressure constant within extremely close tolerances and which can eliminate air inclusions. In respect of the above objectives, the invention provides an ink jet printer of the type stated in the preamble, which printer is characterized by: a buffer vessel which is arranged in the second conduit and of which the infeed is located higher than the outfeed and the -ink level is situated at a height between the height of the infeed and the height of the outfeed; an air tank connecting above the ink level to the buffer vessel; and pressure adjustment means for adjusting the air pressure in the air tank by means of a control unit (CPU) and thereby adjusting, in particular holding constant within chosen tolerances, the height of the ink level in the buffer vessel. The air tank can for instance have a volume of five litres. The volume of the buffer vessel is not of any technical significance per se; the buffer vessel can be considerably smaller than the air tank. The basis of the invention is a control which adjusts the pump in continuously regulating manner such that the ink level in the buffer vessel always remains the same within for instance ± 1 mm. As is known, 10 m water column corresponds to 1 bar of pressure. The pressure in the ink circuit can thus remain the same within + 100 μbar. If the pressure tank is now brought to pressure from an external pressure source, the pump will begin to work increasingly harder to compensate the increased flow rate. The pressure in the ink circuit is proportional to the jet speed. By measuring the jet speed the air pressure in the tank can be set to exactly the correct pressure. The air tank operates as a spring with a very low spring constant. The pulsations in the supplied ink caused by the pump can expand in the buffer vessel without this being transmitted to the ink in the buffer vessel, since the top part of the space in the buffer vessel is in direct communication with the air tank. If sudden flow rate fluctuations now occur, this can be accommodated with a minimal pressure change by causing the ink in the buffer vessel to rise or fall slightly. The buffer is to at least some extent also able to allow the escape of an air inclusion on the ink surface. Even more refined and very effective means will be specified hereinbelow for eliminating air inclusions and air bubbles. On the basis of the foregoing, a determined embodiment is characterized by measuring means for measuring the height of said ink level, which measuring means control the first pump via the CPU. Measuring the level of the ink is not simple because many standard techniques are easily contaminated or are sensitive to a possible foam layer and air inclusions. In order to enable regulating adjustment of the air tank to the correct pressure value at all times, the ink jet printer can have the feature according to the invention that the pressure adjusting means are adapted, under the control of the control unit, to admit air into the air tank at a chosen flow rate or to allow the escape of air from the air tank at a chosen flow rate. A continuously operating ink jet printer is characterized according to the invention by ink feedback means added to the or each ink jet head, and comprising a third conduit and a second pump connected thereto for carrying excess ink selectively to a desired container. Feedback of the ink takes place by means of a vacuum pump. The above described embodiment can have the feature in a variant that the desired container comprises a waste ink container for waste ink. The printer can also have the feature that the desired container comprises the or each supply container. A practical embodiment can include a combination of the latter two aspects, in which case the ink jet printer is characterized by first switching means for switching between the waste ink container and the or a chosen supply container. In order to allow a colour change which is easy, reliable and economically responsible, while the above stated requirements are met, the ink jet printer according to the invention can be characterized by at least two supply containers, to each of which supply containers there connects a first conduit, each of which conduits connects to the respective inlets of a selector valve, the outlets of which together connect to the first pump. For a continuously operating ink jet printer this latter embodiment can have the special feature that the third conduit branches into two branch conduits, one for each supply container, which can each be connected to a desired container. According to a particular aspect of the invention, the ink jet printer has the special feature that a filter is arranged in the second conduit between the outfeed and the at least one ink jet head. Contamination possibly occurring in the long term, and thereby even blockage of the ink jet heads, can be prevented effectively herewith. The filter is placed as closely as possible to the heads. According to a very important aspect of the invention, the ink jet printer has the special feature that a device for removing air from the fed-back ink is arranged in the third conduit, which device comprises: a closed tank, in which the first part of the third conduit debouches at the top and in which the second part of the third conduit debouches at the bottom, and to which a suction pump connects at the top for creating an underpressure in the tank such that in the tank the ink supplied through the first part of the conduit has air inclusions removed, and the ink carried through the second part of the third conduit to the pump is free of air inclusions. Thus is achieved that the return ink, which flows through the third conduit and is fed back to the at least one supply container, has air inclusions removed. These would otherwise have a very adverse effect on the print quality of the device. According to yet another aspect of the invention, the ink jet printer has the special feature that the number of ink jet heads amounts to more than one and the second conduit branches into a number of sub-conduits which each connect to a respective ink jet head and which all have substantially identical flow parameters for the ink, such as diameter, length and flow resistance. Hereby is achieved that the ink fed to all heads is processed under exactly the same conditions. This is essential for the very high print quality that is sought after and of which the ink jet printer according to the invention is capable. It is further essential that the pressure at which the ink is fed to the or each ink jet head remains constant within extremely close tolerances in accordance with a preselected value. In this respect the ink jet printer according to the invention can have the following aspects. A first aspect is that level measuring means for measuring the ink level are added to the buffer vessel, which level measuring means feed level signals to the CPU, on the basis of -which the pressure adjustment means set the air pressure in the air tank. A second aspect is that the level measuring means comprise a level sensor of the type with a float and a position sensor co-acting therewith, for instance a magnet present on the float and a magnetostrictive sensor disposed fixedly relative to the buffer vessel. A third aspect is that pressure measuring means are added to the buffer vessel, which pressure measuring means feed pressure signals to the CPU, on the basis of which the pressure adjustment means set the air pressure in the air tank. The invention will now be elucidated with reference to the annexed drawings. In the drawings: Fig. 1 shows highly schematically an ink circuit of an ink jet printer according to an exemplary embodiment of the invention; Fig. 2 is a perspective view of an alternating valve system forming part of the block diagram of Fig. 1; Fig. 3 shows a vertical longitudinal section along line III-III in Fig. 2 through two co-acting selector valves; and Fig. 4 is a cut-away perspective view of a filter. The same components are designated in all the figures with the same reference numerals. Reference is made first of all to Fig. 1. In order to set the ink circuit into operation a number of operations are successively performed to fill and vent the circuit and to have the ink jets spray properly without a supply container for ink being contaminated with ink of another colour, a flushing liquid or other service liquid. Under normal conditions however, the ink circuit is always filled with a liquid. Fig. 1 shows on the left-hand side two supply container systems 60, 61. These comprise two carriages 78 and 79 respectively which in a manner to be described below are movable reciprocally as according to arrows 80 by means of actuators 19. Supply containers 1, 76 for ink are placed on carriages 79. Placed in each supply container 1, 76 is a first conduit 5 which carries a non-return valve 8 on its underside at or very close to the bottom of the relevant supply container. When conduit 5 with non-return valve 8 is removed from the ink, this valve closes or remains closed, whereby air inclusions cannot occur. Ink can be pumped up through conduit 5 by means of a pump 6 controlled by a central control unit or CPU 36. The choice between supply container 1 or 76 is determined by the mutually exclusive open or closed positions of two selector valves 56, 57. These will be discussed later with reference to Fig. 2 and 3. In this embodiment the pump 6 is a gear pump, although in principle it can be of any suitable type provided it generates a sufficient pressure and a sufficient ink flow rate. The ink is fed via a second conduit 73 to a buffer vessel 75 which communicates at its top with an air tank 10 via a conduit 66. The pressure of the air at the top in buffer vessel 75 can be measured using a pressure sensor 11. In accordance with the understanding at the moment of filing of this patent application, the pressure sensor is preferably of the Keller 35X type, but can be of any suitable and technically equivalent type. The pressure measurement signals from pressure sensor 11 are fed to a CPU 36 via a line 69. The rotation speed of pump 6 can be adjusted in regulating manner on the basis of control by an electrical line 67 by CPU 36 such that the ink level 35 in buffer vessel 75 retains the correct value at all times within a given tolerance. According to the invention this value can be held constant within a few μm. Use is made for this purpose of a level sensor, comprising a float 50 which bears a magnet, which magnet co-acts with a magnetostrictive sensor (not shown) designated with reference numeral 20. Use is successfully being made at this moment of a sensor of the Balluff BTL-5 type. Other technically equivalent sensors can also be applied. The level signals generated by this sensor 20 are likewise fed to CPU 36. As shown in Fig. 1, a conduit 74 connects to buffer vessel 75 for draining ink therefrom. As the figure clearly shows, conduit 74 is connected for this purpose to vessel 75 via an outlet 34, this below the ink level 35. Accommodated in conduit 74 is a filter 53, an example of which is shown in more detail in Fig. 4. Via this conduit 74, therefore downstream of buffer vessel 75, which in co-action with an air conduit 10 holds the ink pressure constant, eliminates the pulsations of pump 6 and can intercept possible air inclusions, the ink flows under pressure of for instance 3.5 bar to a schematically designated first manifold 72, and from here, via conduits (not drawn) which rheologically are substantially identical, to ink jet heads 12. Air tank 10 connects via an air line 66 to the top side of buffer vessel 75 above the ink level 35. In the ink jet heads, particularly in the case of a continuous ink jet printer, the unused ink is collected again, this by suction. The return ink coming from heads 12 is drawn off via a manifold by means of a suction line 14 which debouches in an underpressure separator 51 which removes air inclusions from the return ink. To this end the separator 51 comprises a closed tank 77 which is connected at its top side to an air suction pump 4 and on its underside via a conduit 15 to an ink suction pump 52, in this embodiment a hose pump, which feeds the return ink, with air now entirely removed, via a conduit

55 to the inlets of two outfeed selector valves 58, 59. Similarly to the above mentioned infeed selector valves

56 and 57, these are of the Bϋrkert 2031 type in the present embodiment. These valves can also be replaced by valves which are technically at least equivalent. The great advantage of the used valves is that at switchover they have a practically negligible dead volume. This is of essential importance for the invention in preventing unintended effects such as colour mixing and the like. It is noted that valves 56, 57, 58, 59 in alternating valve system 7 are of the type which are controlled by means of compressed air. This compresses air is supplied selectively by means of provisions (not shown) under electrical control via an electrical line 65 from CPU 36. The outlets of outfeed selector valves 58, 59 are connected to conduits 63, 64 respectively which during normal operation debouch above a supply container for the purpose of allowing return ink to flow back therein or, if desired, for draining return ink into a waste cup 54 which, via a drain conduit 16, can drain the return ink, which may for instance be mixed with another colour, or a flushing liquid or other service liquid, to a waste container (not drawn) . In Fig. 1 the carriage 79 which carries supply container 76 is displaced by means of actuator 19 fully to the right as according to arrow 80, where the end of return conduit 64 debouches above container 76. Carriage 78 carrying supply container 1 is displaced fully to the left as according to arrow 80 by the relevant actuator 19, i.e. into a position in which the opening of return conduit 63 debouches above a waste cup 54. In this position the return ink or other return liquid can be drained to a container (not shown) for waste liquid. In this latter position the whole ink circuit can for instance be flushed out until all conduits and valves are fully filled with "new" ink, i.e. ink of another colour (or for cleaning purposes are flushed out with a cleaning liquid) . When this is achieved, actuator 19 can again displace carriage 78 to the right until the operating position is reached once again. In this position the unused return ink is poured back again into the relevant supply -container 1. Owing to the described embodiment "old" ink can never remain in conduits 63, 64, otherwise than with the use of usual valves which always have a determined volume downstream of the switching part. When the ink circuit is well vented, it is easily possible to change colour by switching the valves in alternating valve system 7 by means of CPU 36. A proportional valve 21 which is connected to a compressed air line 83 provides under the control of CPU 36 by means of an electric line 70 for respectively feed of air to air tank 10 or discharge therefrom, depending on the requirement determined by CPU 36 on the basis of the signals generated by sensors 11 and 20. Fig. 2 shows the alternating valve system 7 in perspective view. This view shows that valves 56, 57 and 58, 59 respectively are connected pairwise to each other as a unit. The alternating valve system 7 also carries buffer vessel 75 having on the top side the Keller 35X pressure sensor and on the underside the Balluff BTL-5 magnetostrictive level sensor. Following the comprehensive description above of the block diagram of Fig. 1, it is possible in respect of Fig. 2 to suffice with the comment that a very simple construction is obtained with the shown arrangement, in which the alternating valve system 7 is combined with buffer vessel 75 with attachments. For the sake of clarity in the drawing a number of conduits are shown symbolically as arrows. These are the above described conduits 5, 55, 62, 66 and 74. Fig. 3 shows schematically a longitudinal sectional view of the construction of the valve pair 56, 57. The two valves are identical and of the Bϋrkert 2031 type. These valves have a number of special characteristics. Switch-over takes place gradually in a number of seconds in order to prevent fluctuations in the pressure. The flow surface is substantially the same in every position such that pump 6 is always subjected to the same resistance, also during simultaneous switch-over of the valves of one set. The valves further have practically no spaces with stationary ink which could contaminate the ink flowing by. Each valve comprises a single-action compressed air cylinder 84 (only drawn for valve 56) , in which a piston 85 is reciprocally movable in sealing manner, and a return spring. Via a piston rod 86 the piston operates a rubber-elastic valve element 87 with a membrane 88. In the closed position of the valve the membrane has the spherical, downward directed closing position drawn for valve 56, in which it closes openings 89, 90 in a hollow spherical valve seat. In this situation the identical valve element 91 of valve 57 is retracted and has the shown open position in which it mutually connects openings 92 and 90. As described above, control takes place such that the operation of the two valves takes place in exactly complementary manner. Fig. 4 finally shows the internal structure of filter 53. As will be apparent from the drawing, the filter is of the candle type. The ink fed in on the left-hand side in the drawing is distributed under pressure over the length of filter element 93 and, as indicated symbolically with an arrow 94, pressed out over the whole surface area of candle 93 and then relinquished via the part of the second conduit 74 shown on the right-hand side in Fig. 4. The structure according to the invention makes possible an ink jet printer with an extremely high printing accuracy. The pressure under which the ink is fed via conduit 74 to ink jet heads 12 can for instance be held constant with a tolerance of ± 0.005%. This guarantees an extremely high printing accuracy.

Claims

Sch/svk/Osiris-4pCLAIMS

1. Ink jet printer, for instance for printing textile, paper, packagings such as food packages, for instance cardboard boxes, food cans, milk packs or the like, which ink jet printer has an ink circuit which comprises : at least one supply container (1) for ink; a first pump (6) connecting thereto via a first conduit (62) ; at least one electrically controlled ink jet head (12) connecting thereto via a second conduit (73, 74); characterized by: a buffer vessel (75) which is arranged in the second conduit (73, 74) and of which the infeed (33) is located higher than the outfeed (34) and the ink level (35) is situated at a height between the height of the ' infeed (33) and the height of the outfeed (34); an air tank (10) connecting above the ink level (35) to the buffer vessel (75) ; and pressure adjustment means (21) for adjusting the air pressure in the air tank (10) by means of a control unit (CPU) (36) and thereby adjusting, in particular holding constant within chosen tolerances, the height of the ink level (35) in the buffer vessel (75) .

2. Ink jet printer as claimed in claim 1, characterized by measuring means (20, 50) for measuring the height of said ink level (35), which measuring means (20, 50) control the first pump (6) via the CPU (36) .

3. Ink jet printer as claimed in claim 1, characterized in that the pressure adjusting means (21) are adapted, under the control of the CPU (36) , to admit air into the air tank (10) at a chosen flow rate or to allow the escape of air from the air tank (10) at a chosen flow rate.

4. Ink jet printer as claimed in claim 1, characterized by ink feedback means added to the or each ink jet head (12) and comprising a third conduit (14, 15, 55) and a second pump (52) connected thereto for carrying excess ink selectively to a desired container (1, 76) .

5. Ink jet printer as claimed in claim 4, characterized in that the desired container comprises a waste ink container for waste ink.

6. Ink jet printer as claimed in claim 4, characterized in that the desired container comprises the or each supply container (1) .

7. Ink jet printer as claimed in claims 5 and 6, characterized by first switching means (19) for switching over between the waste ink container and the or a chosen supply container (1) .

8. Ink jet printer as claimed in any of the foregoing claims, characterized by at least two supply containers (1, 76), to each of which supply containers (1, 76) there connects a first conduit (5), each of which conduits (5) connects to the respective inlets of a selector valve (56, 57), the outlets of which together connect to the first pump (6) .

9. Ink jet printer as claimed in claims 4 and 8, characterized in that the third conduit (14, 15, 55) branches into two branch conduits (63, 64), one for each supply container (1) , which can each be connected to a desired container.

10. Ink jet printer as claimed in claim 1, wherein a filter (53) is arranged in the second conduit (74) between the outfeed and the at least one ink jet head (12).

11. Ink jet printer as claimed in claim 4, wherein a device (51) for removing air from the fed-back ink is arranged in the third conduit (14, 15, 55) , which device (51) comprises: a closed tank (77), in which the first part (14) of the third conduit debouches at the top and in which the second part (15) of the third conduit debouches at the bottom, and to which a suction pump (4) connects at the top for creating an underpressure in the tank (77) such that in the tank (77) the ink supplied through the first part (14) of the conduit has air inclusions removed and the ink carried through the second part (15) of the third conduit to the pump (52) is free of air inclusions.

12. Ink jet printer as claimed in claim 1, wherein the number of ink jet heads amounts to more than one and the second conduit (74) branches into a number of sub- conduits which each connect to a respective ink jet head and all have substantially identical flow parameters for the ink, such as diameter, length and flow resistance.

13. Ink jet printer as claimed in claim 1, wherein level measuring means (20, 50) for measuring the ink level (53) are added to the buffer vessel (75) , which level measuring means (20, 50) feed level signals to the CPU (36) , on the basis of which the pressure adjustment means (21) set the air pressure in the air tank (10) .

14. Ink jet printer as claimed in claim 13, wherein the level measuring means (20, 50) comprise a level sensor (20, 50) of the type with a float (50) and a position sensor (20) co-acting therewith, for instance a magnet present on the float and a magnetostrictive sensor disposed fixedly relative to the buffer vessel.

15. Ink jet printer as claimed in claim 1, wherein pressure measuring means (11) are added to the buffer vessel (75) , which pressure measuring means (11) feed pressure signals to the CPU (36), on the basis of which the pressure adjustment means (21) set the air pressure in the air tank (10) .