An ink jet printer and an ink supply system for the same

Abstract: An ink jet printer including an ink-supply system comprising: a lower ink container (100), an upper ink container (104), a liquid droplet deposition apparatus (107) with nozzles wherein means (102) are provided for circulating ink from the lower container (100) to the upper container (104) and from the upper container through the liquid droplet deposition apparatus (107) back in to the lower container (100) characterised in that - the upper container (104), having a bottom (110) and side walls (111) and containing ink (101) forming an ink surface (116), has a first compartment (I) with an inlet (114) for ink wherein the inlet (114) contains means (114a) for avoiding directional flow of the ink towards the ink surface (116) and a second compartment (II), separated from the first (I) by a partition (112) placed between the first (I) and second (II) compartments so that the two compartments (I,II) communicate only in a lower part of the upper container (104).

Description: FIELD OF THE INVENTION This invention relates to droplet deposition apparatus and especially to printheads in ink jet printers. In particular it relates to a system for providing liquid ink at proper static (hydraulic) pressure to a series of printheads in a drop-on-demand ink jet printer. BACKGROUND OF THE INVENTION In ink-jet liquid droplet deposition apparatus, it is important that the proper static (hydraulic) pressure, typically a small negative static (hydraulic) pressure, is achieved at each ink jet nozzle to avoid drool of the ink. In commercial ink jet printing applications, it is advantageous to provide a printing apparatus having a large printing area to permit imaging of a large image on a target surface without multiple passes of the surface past the printing head. This may be accomplished by providing a number of liquid droplet deposition apparatus (printheads) vertically stacked or "stitched" together. Vertically stacked liquid droplet deposition apparatus arrangements, however, present special problems associated with the control of static (hydraulic) pressure, and their commercial advantages have heretofore been limited by increasing costs or complexity relating to ink storage and delivery. In, e.g., WO-A-97 44194 a solution to this problem has been disclosed. In that document an ink-supply system is disclosed for providing ink in an apparatus having a first set of nozzles at a higher elevation than a second set of nozzles, comprising: a first compartment in fluid flow communication with the first set of nozzles, said first compartment being configured to allow the egress of liquid to maintain the level of liquid therein at a desired fill height; a second compartment in fluid flow communication with the second set of nozzles, said second compartment being configured to allow the egress of liquid to maintain the level of liquid in the second compartment at a desired fill height; and a reservoir for supplying liquid to the first and second compartments. A pump circulates the ink. Although the ink supply system disclosed in WO-A-97 44194 does performs well for maintaining the desired level of static (hydraulic) pressure at each liquid droplet deposition apparatus, there is still need, as disclosed in that document, for a degassing device to evacuate the air from the ink that is circulated by a pump. The inclusion of this device makes the construction quite expensive. The problem of air inclusion in ink supply systems of ink jet printers can be addressed as disclosed in EP-A-916 502 . In this disclosure a quite complicated combination of ink tanks, suction pumps and valves is provided to degas the ink and to keep the static (hydraulic) pressure at the nozzles constant. To achieve this latter property, an ink sensor is used for sensing the height of the ink in one of the ink tanks. Patent application US 6,007,193 by Kashimura discloses a simpler ink circulation system wherein a hydrostatic height difference between the ink surfaces of two ink tanks creates an ink flow through a printhead, and wherein means are provided to degas the ink during circulation. The degassing means however are specifically designed for use with hot melt ink jet inks and are less or not at all suitable for lower viscosity inks, being the majority of ink jet inks on the market. It has been recognised that when, circulating ink through an ink supply system to the nozzles of the liquid droplet deposition apparatus, not only inclusion of air is possible but also the inclusion of dust particles. When such particles arrive at the nozzles, they can become attached to the nozzle and eventually block one or more nozzles in the liquid droplet deposition apparatus. This has a detrimental effect on printing quality. Thus it is still desirable to provide an ink supply system that is simple to construct, that has a small number of moving parts and wherein the static (hydraulic) pressure at the nozzles of the liquid droplet deposition apparatus is kept constant and wherein gas and/or dust inclusion is prevented OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention to provide an ink supply system for an ink jet printer that is simple to construct, that has a small number of moving parts and wherein the static (hydraulic) pressure at the nozzles of the liquid droplet deposition apparatus is kept constant and wherein gas and/or dust inclusion are prevented. It is a further object of the invention to provide an ink supply system for a "piezo" ink jet printer wherein the ink supply is circulated and that the flow of ink proceeds with very low or no pulsations so that the ink ejection by exerting "piezo" pressure on the ink (i.e. by applying a pulse to the ink) is not perturbed by pulsations in the ink due to the circulation of the ink. Further objects and advantages of the invention will become clear from the detailed description herein after. The objects of the invention are realised by providing an ink jet printer according to claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows schematically an ink jet printer and an ink supply system therefore Figure 2 shows schemattically a possible construction of an upper container for use in an ink jet printer and an ink supply system according to this invention. Figure 3 shows an other possible construction of an upper container for use in an ink jet printer and an ink supply system according to this invention. Figure 4 shows schematically a possible construction of an upper container for use in an ink jet printer and an ink supply system according to this invention serving two rows of printheads. Figure 5 shows a schematic front view of a possible construction of a lower container incorporating a "baby container" for use in an ink jet printer and an ink supply system according to this invention Figure 6 shows a schematic cross sectional view along line W,W' of figure 5 of a possible construction of a lower container for use in an ink jet printer and an ink supply system according to this invention. Figure 7 shows schematically a printer incorporating an upper and lower container of this invention. Figure 8 shows schematically the position of "purging" holes in partitions and overflow pipes that reach to said bottom wall of the upper and lower container and in the "baby container". DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows very schematically an ink jet printer and an ink supply system therefore. From a lower container (100) containing ink (101) the ink is brought to an upper container (104) through ducts (103) by a pump (102). The upper container (104) has an overflow opening (105) for maintaining the ink level in said container at a constant level, by a duct (106) the overflowing ink is returned to the lower container (100). The difference in height (ΔH) between the ink level in the upper container and the liquid droplet deposition apparatus determines the static (hydraulic) pressure in the liquid droplet deposition apparatus. By adjusting this difference in height (ΔH), drool is avoided. From the upper container (104) ink is brought through a duct (103a) to liquid droplet deposition apparatus (107) wherein the ink is circulated, part of the ink is image wise ejected (108) to an image receiver (not shown) and the remaining ink is brought back to said lower container through a duct (103b). In the supply system a container with ink (109) is provided for replenishing the ink used during image wise ejection. An ink supply system as described above can be used with any liquid droplet deposition apparatus know in the art. Such an ink supply system, wherein ink is circulated can very advantageously be used in an ink jet printer comprising one or multi-channel pulsed droplet deposition apparatus in which piezoelectric actuator means are used for droplet ejection. According to this art, an ink channel connects an ink reservoir to an ejection nozzle. Piezo electric transducers adjacent to the channel respond to a voltage impulse to generate a pressure pulse in the ink and eject ink droplet from the nozzle. Such multi-channel pulsed droplet deposition apparatus have been disclosed in, e.g., US-A-3 946 398, US-A-3 683 212, US-A-3 747 120, US-A-4 525 728, US-A-4 549 191 and US-A-4 584 590 and IBM Technical Disclosure Bulletin Vol. 23 No. 10 March 1981. In EP-A-278 589 multi-channel pulsed droplet deposition apparatus improved over the mentioned disclosures have been shown, also in EP-A-277 703, EP-A-278 590 and US-A-5 855 713 multi-channel pulsed droplet deposition apparatus are disclosed. Although in figure 1 only one liquid droplet deposition apparatus (printhead) is shown being served from an upper container, it is possible to have multiple liquid droplet deposition apparatus served by a single upper container. In this case, since the difference in height (ΔH) between the ink level in the upper container and the liquid droplet deposition apparatus determines the static (hydraulic) pressure in the liquid droplet deposition apparatus, all apparatus must be placed at the same ΔH from the level of the ink in the upper container. Thus several liquid droplet deposition apparatus can be placed on a single row a the same height relative to the level of ink in the upper container and be served by a single upper container. In any ink jet printer having an ink circulation circuit described in figure 1 and any printhead know in the art, it is important that the level of the ink in the upper container (104) does not fluctuate but also that the surface of the ink stays level during operation of the printer. Even fluctuations (pulsations) caused by the pump that is used to circulate the ink have to be avoided, since even these small wave-like fluctuations of the ink surface in the upper container do change the difference in height (ΔH) between the level of the ink and the printhead and can thus change the size of the droplet and/or the speed of ejection, when strong waves are produced on the surface of the ink in the upper container, even drool at the nozzles of the printhead can occasionally occur. When using the an ink circulation circuit described in figure 1 then the flow of ink has to proceed with very low or no pulsations so that the ink ejection by exerting "piezo" pressure on the ink (i.e. by applying a pulse to the ink) is not perturbed by pulsations in the ink due to the circulation of the ink. To achieve an unperturbed surface of the ink in the upper container and to alleviate the pulsations of the pump used to circulate the ink as shown in figure 1, the construction of the upper container plays an important role. It proved possible to avoid the problems cited above, simply by adapting the mechanical construction of the upper container without needing any moving parts. Basically the construction boils down to introducing in the container a labyrinth forcing the ink to pass the labyrinth close to the bottom of the container so has to leave the air or gas entrapped in the ink the possibility to escape towards the surface of the ink in the container. A possible embodiment of such an upper container is shown in figure 2. The upper container (104), having a bottom wall (110) and side walls (111), has an overflow opening (105) located at an height, h1, above the bottom wall (110) of the container, so that the ink in the container reaches a constant level at an height, h1, above the bottom wall (110). A duct (106) conducts the ink back to the lower container (not shown). The container (104) is divided in two compartments, an inlet compartment (I) and a print/overflow compartment (II), by a partition (112). This partition is placed so that the upper end of it extends to an height, h5, above the bottom wall and so that h5 > h1. Close to the bottom wall, this partition has an opening with height, h4, so that both compartments are in communication with each other. The height, h4, is preferably equal to or smaller than 0.3 x h1, more preferably equal to or smaller than 0.15 x h1. In the figure 2 the opening with height, h4, between the partition (112) and the bottom wall is placed directly on the bottom wall (110), this is a preferred embodiment, but the opening can be made in the partition (112) at some distance away from the bottom wall. The height of the opening and the exact placement are chosen so that the ink is forced to leave compartment one in the lower part of the container. A height lower than 0.5 x h5 can serve the purposes of this invention. In the inlet compartment (I) an inlet (114) for ink is present, preferably this inlet is capped (114a) and has side openings (114b), so that the inflowing ink is forced downwards and side wards and can not disturb the surface (116) of the ink. This inlet extends in the inlet compartment (I) to an height, h2, chosen to be smaller than the height, h1. Preferably, h2 ≤ 0.9 x h1, more preferably h2 ≤ 0.75 x h1. In the implementation shown in the figure 2 the capping of the inlet is realised by using a pipe that is closed at one end. The person skilled in the art will however appreciate that other forms of capping are even suitable as long as the inflowing ink is forced downwards and side wards and can not disturb the surface (116) of the ink. In the print/overflow compartment (II) also. a partition (113) is present. This partition has an height, h3, smaller than h1 and is fixed on the bottom wall (110) of the container (104). Preferably h3 ≤ 0.85 x h1, more preferably h3 ≤ 0.5 x h1. In the print/overflow compartment an outlet (115) is present for conducting the ink to the printhead (multi-channel pulsed droplet deposition apparatus) which is not shown in figure 2. This outlet is placed behind the partition (113) in the print/overflow compartment, i.e., farther away from the inlet compartment than the partition (113). The partition (112) between the two compartments and the partition (113) in the print/outlet compartment form a kind of labyrinth. In operation the ink enters the inlet compartment (I) and is forced to flow downwards, by doing so air bubbles that may be entrapped in the ink will mount to the surface (116) of the ink instead of staying with the ink that is forced down, thus by the mechanical construction of the upper container air, that may be entrapped in the ink, is evacuated from the ink. Since the inlet is capped, the force of the stream of ink is directed away from the surface of the ink and thus leaves the surface (116) undisturbed. This means that it is possible to use any type of pump for circulating the ink, because the variations in the force of the ink due to pulsations of the pump are, due to the construction of the upper container (104), of no consequence for the surface of the ink, which stays level. The consequence is that the height, h1, stays constant and that thus almost laminar flow is preserved, the size and the speed of the droplets ejected in the printhead remains constant. However in a preferred embodiment of the invention, a pump with low pulsation is used, e.g., screw pumps or centrifugal pumps. The latter type of pump is especially preferred. Although the construction of the upper container as shown in figure 2 does as explained solve the problems of prior art ink supply systems, i.e., pulsation dampening, air evacuation, occasional drool avoidance with simple constructional measures, it showed that a further improvement could be realised by the embodiment shown in figure 3. The upper container is now divided in three compartments, an inlet compartment (I), a print compartment (II) and an outlet compartment (III). Basically the inlet compartment (I) and the print compartment (II) are construed in the same manner as explained when discussing figure 2 and present the same advantages. Therefore this discussion will not be repeated here. The outer wall (111) shown in figure 2 as farthest away from the inlet compartment has now become an inner wall (117) separating the print compartment from the outlet compartment. The height of that inner wall (117), h1, is lower than the height of the outer walls and thus ink can flow over the edge of the inner wall in the outlet compartment (III). The side of the inner wall (117) facing the outlet compartment can, when so desired, be treated so as to be easily wetted by the overflowing ink, which can make the ink flow in the outlet compartment almost as a laminar flow, thus avoiding any air inclusion during overflow. It is desired in an ink jet printer according to this invention to accurately balance the wettability of the surfaces contacting the ink, so that the occurrence of film forming at the surface of the ink, where the ink surface contacts the wall and partitions in the container, is minimised and that the laminar flow of the ink along some surfaces of the container is not impaired. It may in a printer of this invention thus be necessary to have some walls and partitions - or parts of them - treated so that the wettability by the ink is poor for avoiding excessive drying of the ink and to have some walls and partitions - or parts of them - treated so that the wettability by the ink is rather good for increasing the laminarity of the ink flow. In the outlet compartment an outlet (105) is present and has an height, h6, so that the ink has to accumulate first to a certain extent in the outlet compartment before it leaves the compartment. This again helps to avoid possible air inclusions in the ink that is re-circulated through a duct (106) back to the lower container (not shown). Thus in the second embodiment of the invention only by the mechanical construction of the upper container several advantages are realised: pulsation dampening, air evacuation, avoidance of changes in size and/or speed of the ejected ink droplets and avoidance of air intake during overflow. When in a vertical printer it is desired to have two or more rows of liquid droplet deposition apparatus (printheads) one above the other, then it is possible to use a separate upper container as described above for each of the rows, wherein the height of the ink in the container is adapted to the respective position of the rows of printheads. It is also possible to adapt the construction of a single upper container (104) to accommodate two or more rows of printheads. In figure 4 , this construction is schematically shown for 2 rows of printheads located at a vertical distance from each other. In the figure, for sake of clarity only one printhead of each row is shown. The upper container (104), shown in figure 4, comprises 4 compartments (I, II, Ia and IIa). The compartments I and II are identical to the compartments described when discussing figure 2, and an outlet (115) in compartment II brings the ink to a printhead of the first row (107) from where the ink is returned in the lower container (not shown). Thus compartment I and II serve printhead of the first row. The ink from compartment II flows over a partition (117) in to compartment Ia. The height (h1) of that partition (117) determines the level of ink in compartment I and II and thus, as explained earlier the static (hydraulic) pressure at the nozzles of the printhead (107). Compartment Ia and IIa are via an outlet (115a) connected to a printhead (107a) of the second row. Compartment Ia is an inlet compartment, receiving ink because ink of compartment II flows over the partition between compartments II and Ia. In the latter compartment a partition (118) is present, with an height h13. Between compartment Ia and IIa a partition (119) is present that has an height, h15, and that leaves an opening between the bottom wall (110) of the upper container, said opening having an height, h14. Compartment IIa is a print/outlet compartment that via an outlet (115a) provides ink to a printhead of the second row, from where the ink is returned in the lower container (not shown). This compartment also comprises an outlet (105a) located at an height, h11, above the bottom wall (110) of the container. Via a duct (106) the ink is re-circulated to the lower container (not shown). The rows of printhead structures are vertically displaced over an height h PH , the difference between h1 and h11 is adjusted so that h PH = h1-h11, so that in the upper and lower row of printheads the same static (hydraulic) pressure is present. Since basically the construction of the compartments Ia and IIa is equal to the construction of the compartments I and II, the advantages explained when discussing figure 2 and 3 - where the upper container served only one row of printheads - are present for the two rows of printheads. When it is desired to use more than two rows of printheads, the person skilled in the art can easily appreciate that by adding compartments, as described, in pairs to the upper container, an upper container can be constructed that can serve three or more rows of printheads while preserving all advantages that have been explained. The advantages of the invention, realised by the construction of the upper container for ink as per this invention, are obtained when using any ink known in the art and circulating this ink at any speed customary in the art of ink jet printing. The advantages are however very prominent when the ink jet printer uses an ink with a viscosity between 5 and 20 mPas (both limits included) and when that ink is circulated at a speed between 0.1 mm/sec and 50 mm/sec (both limits included). The advantages realised by using, in an ink jet printer including an ink-supply wherein ink is circulated from a lower container to an upper container and from that upper container through the liquid droplet deposition apparatus back in to the lower container, an upper container as per this invention, can also be realised by adapting the construction of the lower container. Basically it is possible to construct the lower container so that the ink that returns in it from the printheads (liquid droplet ejection apparatus) and from the overflow (105) of the upper container does not disturb the surface of the ink in the lower container, entering the container via an almost laminar flow. Further the construction has to be adapted so that air that may be entrapped in the ink during the circulation can easily be separated from the ink without the need for special degassing devices. Again this is achieved by introducing in the lower container a labyrinth forcing the ink to pass the labyrinth close to the bottom of the container so has to leave the air or gas entrapped in the ink the possibility to escape towards the surface of the ink in the container. In figure 5 a front view of an embodiment of such a lower container is schematically shown, the front wall is shown as being transparent. The lower container (100) having a bottom wall (110) and side walls (111) is divided in two compartments, I and II. In compartment I ink (101) is present with an ink surface (116a). A "baby container" (120) is present in compartment I and dips in to the ink (101). The ink in compartment I has a surface at an height, h35, above the bottom wall of the lower container, and the "baby container" is immersed in the ink so that the bottom wall of it is placed at an height, h36, above the bottom wall of the lower container. The height, h36, is chosen such that h36 ≥ 0.6 x h35, preferably so that h36 ≥ 0.8 x h35. The ink, leaving the upper container (104) via overflow (105, 105a), enters the "baby container" via a duct (106). Also ink entering the circulation circuit from a container (109) with ink (101), for replenishing the ink used during image wise ejection, enters in the "baby container" from a duct (106a). The construction of the "baby container" will be explained further on. The first compartment (I) is separated from the second compartment (II) by a partition (122) having an height, h31. This height, h31 is chosen such that 0.25 x h31 ≤ h35 ≤ 0.95 x h31, preferably so that 0.4 x h31 ≤ h35 ≤ 0.75 x h31. The first compartment further comprises an outlet (124) for bringing the ink again in the circulation circuit. In the second compartment, a further partition (121) is present having an height, h33 chosen such that h33 > h31. At the bottom of the second compartment, the partition (121) leaves an opening with height, h34. The height, h34, is preferably equal to or smaller than 0.3 x h31, more preferably equal to or smaller than 0.15 x h31. In the figure 5 the opening with height, h34, between the partition (121) and the bottom wall is placed directly on the bottom wall (110), this is a preferred embodiment, but an opening in the partition (121) at some distance away from the bottom wall can serve the purposes of this invention. In fact when the height of the opening and the exact placement are chosen so that the ink is forced to flow from the inlet (123) to the lower part of the container, i.e. at an height lower than 0.5 x h31, the purposes of the invention are also served. The ink re-circulating from the printhead (not shown) enters the second compartment via an inlet (123) situated in the side wall (111) at an height, h32. This height, h32, is chosen such that 0.25 mm ≤ h31 - h32 ≤ 50 mm, preferably so that 0.5 mm ≤ h31 - h32 ≤ 30 mm. The placement of the inlet (123) quite close to the surface (116b) of the ink in compartment II, makes it possible for air that possibly gets entrapped in the returning ink, to easily be evacuated via that surface (116b). In operation the ink, returning from the printhead, is forced down upon entering the second compartment (II) and has to pass underneath the partition (121). This also increases the separation of the liquid and the air that is possibly entrapped in it. The ink overflows then the partition (122) between the two compartments and enters the first compartment. The flow of the ink along the side of the partition (122) facing the first compartment is quasi laminar and any air not yet separated from the ink can escape from the ink on that face. When the lower container is used in a printer with more than one row of printheads, it is possible to let the ink, returning from each row of printheads, enter the lower container in the same compartment (II) or that several compartments (II) be added to the lower container. Figure 6 is a cross-section of the lower container along the line W-W' in figure 5. It shows the lower container (100) and the "baby container" (120) in it. The "baby container" with side walls (111a) and a bottom wall (110a) dips in to the ink (101). The ink has a surface at an height, h35, above the bottom wall of the lower container and the "baby container" is immersed in the ink so that the bottom wall (110a) of it is placed at an height, h36, above the bottom wall of the lower container. The height, h36, is chosen such that h36 ≥ 0.6 x h35, preferably so that h36 ≥ 0.8 x h35. The ink, leaving the upper container (104) via overflow (105, 105a), enters the "baby container" via a duct (106). Also ink entering the circulation circuit from a container (109) with ink (101), for replenishing the ink used during image wise ejection, enters in the "baby container" from a duct (106a). The baby container is also compartmentalised, an has two compartments, (Ib and IIb) separated from each other by a partition (126), with an height, h37 above the bottom wall of the "baby container". The ink, leaving the upper container (104) via overflow (105, 105a), enters the "baby container" via a duct (106) in the first compartment (Ib). Also ink entering the circulation circuit from a container (109) with ink (101), for replenishing the ink used during image wise ejection, enters in the "baby container" from a duct (106a) in the first compartment (Ib). The ducts extend under the surface (116c) of the ink in the baby container, so that the ink has no free fall. Both ducts (106 and 106a) enter the "baby container" at an height, h39 above the bottom wall of the "baby container". This height, h39, is chosen such that 0.25 mm ≤ h37 h39 ≤ 30 mm, preferably so that 0.5 mm ≤ h37 h39 ≤ 15 mm. The placement of the ducts (106 and 106a) quite close to the surface (116c) of the ink in compartment Ib, makes it possible for air that possibly gets entrapped in the returning ink, to easily be evacuated via that surface (116c). In the first compartment (Ib) of the "baby container", a partition (125) is placed , having an height, h38, measured from the bottom wall of the "baby container" said height , h38 being such that h38 > h37. At the bottom of the partition (125) an opening with height, h42 is left. The height, h42, is preferably equal to or smaller than 0.3 x h37, more preferably equal to or smaller than 0.15 x h37. In the figure 6 the opening with height, h42, between the partition (125) and the bottom wall is placed directly on the bottom wall (110a), this is a preferred embodiment, but an opening in the partition (125) at some distance away from the bottom wall can serve the purposes of this invention. In fact when the height of the opening and the exact placement are chosen so that the ink is forced to flow from the ducts (106 and 106a) to the lower part of the "baby container", i.e. at an height lower than 0.5 x h37, the purposes of the invention are also served. From the first compartment (Ib) the ink over flows partition (126) between the two compartments in to compartment IIb, this compartment of the baby container is separated from compartment I of the lower container by a partition (128), having an height h40, measured from the bottom wall of the "baby container". Also in this compartment of the "baby container", a partition (127) is placed , having an height, h41, measured from the bottom wall of the "baby container" said height , h41 being such that h41 > h40. At the bottom of the partition (125) an opening with height, h43 is left. The height, h43, is preferably equal to or smaller than 0.3 x h40, more preferably equal to or smaller than 0.15 x h40. In the figure 6 the opening with height, h43, between the partition (127) and the bottom wall is placed directly on the bottom wall (110a), this is a preferred embodiment, but an opening in the partition (127) at some distance away from the bottom wall can serve the purposes of this invention. In fact when the height of the opening and the exact placement are chosen so that the ink is forced to flow from the ducts (106 and 106a) to the lower part of the "baby container", i.e. at an height lower than 0.5 x h40, the purposes of the invention are also served. The ink overflows the partition (128) in to the ink (101) in compartment I of the lower container. It is clear that it is possible to implement a "baby container" with more than two compartments or with only one compartment as long as also in the "baby container" a kind of labyrinth is present forcing the ink to flow along the bottom of the "baby container" and leaving any air, that was possible entrapped in the ink, time and room to escape to the surface of the ink in the "baby container". The placement of inlet (123) in the lower container (100) close to the surface (116b) of the ink contained in compartment II and the placement of the ducts (106 and 106a) in the "baby container" (120) close to the surface (116c) of the ink contained in compartment Ib of the "baby container" has not only the advantage of allowing easy air evacuation during operation of the printer, but has the great advantage that when starting the printer and thus starting the circulation of ink in empty ducts, the air is easily evacuated from the ducts and other parts, because the air is pushed away by the ink and can readily escape in the lower container. Although the advantages of the invention: pulsation dampening, air evacuation, avoidance of changes in size and/or speed of the ejected ink droplets and avoidance of air intake during overflow, can be realised by using either an upper container or a lower container according to this invention, in a most preferred embodiment of an ink jet printer wherein ink is circulated both an upper container and a lower container according to this invention are shown. In figure 7 a schematic view of such a printer, with two rows of printheads is shown. Basically it comprises the elements as discussed in figure 1. The upper container and the lower container both being containers according to this invention. While both the upper and lower container may be open containers, it is preferred that both containers are capped by a lid (104a and 100a). These lids are most beneficially air tight so that no external air can enter the containers. This has the advantage that the air above the ink, in both containers, is saturated by the solvent of the ink and that evaporation of the solvent out of the ink is minimised and even prevented. Both containers are connected by a duct (129) in the respective lids so that the air in the upper container and the air in the lower container are in communication. By doing so the air/ink volume in both containers is kept almost constant and when an ink level changes in one of the containers, air from the otter container is sucked to the first container or is pushed out of the first container. E.g., when the ink level raises in the lower container, air is pushed from the lower container to the upper one through the duct (129). Ink jet printer shown in figure 7, further comprises a inlet (130), equipped with a cock (130a) for compressed air. By bringing compressed air in the upper container, through the duct (130) it is possible to purge the printheads (107, 107a). It is advantageous that, when purging with compressed air is envisaged, the duct (129) between the upper and lower container is present for pressure equalisation. When this duct is present the equilibrium in the printer is not disturbed when purging with compressed air, because the pressure in the lower and upper container are, due to the presence of the duct (129), is equalised. As seen from figure 7, it is possible to add conditioning means, a heater (132) and a cooler (133) for the ink, to the lower container. The placement of the conditioning means in the lower container has advantages over the placement at other places of the circulation circuit. In the lower container all ink comes together : overflow from the upper container, ink returning from the printheads and ink from the supply vessel (109) for replenishment, thus all the ink is conditioned at once. The amount of ink in the lower container is rather large, so that there is a buffer capacity and the conditioning means do not have to cope with peaks of very cold ink or peaks of very hot ink. In the circulation circuit a filter is placed for cleaning the ink and for avoiding that dust and other particles would reach the printhead where dust can block some nozzles and thus deteriorate the print quality. Filtration of the ink is known from, e.g., WO-A-00 21755. It was found that, when using a filter it is best to have an oversized filter, which makes it possible to use a centrifugal pump for circulating the ink. It has been explained in this document that the use of a pump with low pulsations is preferred for circulating the ink. When using an oversized filter, the pressure drop in the filter is low and thus a pump delivering low pressure on the ink can be used. This has then further advantages with respect to the thermal and mechanical stress exerted on the ink. An oversized filter, for use in this invention is a filter that, when using ink with a viscosity between 5 to 20 mPas and a circulation speed of 3 to 5 l/min has a filter surface between 0.25 and 2 m 2 , preferably between 0.5 and 1 m 2 . For easy cleaning of the ink circuit, it is preferred that the upper container, lower container and "baby container" can be emptied without special difficulties. This is achieved by having in the upper container small holes (134) in the partitions that reach to the bottom wall of that container, (these are partitions 113, 117,118 in the upper container, see figures 3 and 4) and in the duct (106) also at the bottom of the upper container. In the lower container there is provided a small hole in the partition (120) separating compartment II from compartment I, near the bottom of the lower container. In the "baby container" there are provided holes in the partition separating compartment Ib from compartment IIb and in the bottom wall of the "baby container" in compartment IIb. In figure 8 the placement of the holes is shown : figure 8a in the upper container showing an enlargement along the bottom of the container as shown in figure 4, figure 8b in the lower container showing an enlargement along the bottom of the container as shown in figure 5 and figure 8c in the baby.container, showing an enlargement along the bottom of the "baby container" as shown in figure 6. In the figure 8 only the pertinent parts are numbered and explained, for the other elements and the numbering of them reference is made to the figures 4, 5 and 6 respectively. The holes (134) have to be dimensioned that so as not to disturb the circulation of the ink in the ink supply system because of too much ink leaking away and so as not to get (too) easily clogged. It was found that with inks with a viscosity between 5 and 20 mPas, both limits included, the holes (134) could have an area between 5 and 15 mm 2 , both limits included. Parts list 100 Lower container 101 Ink 102 Pump 103, 103a, 103b, 106, 106a Ducts 104 Upper container 105, 105a Outlets for overflowing ink 107 Printhead (droplet ejection apparatus) 108 Ejected ink 109 Ink reservoir 110 Bottom wall of a container 111 Side wall of a container 112, 113, 117, 118, 119, 121, 122, 125, 126, 127, 128 Partitions 114 Inlet 114a Cap on inlet 114b Side holes in inlet 115, 115a Outlets to printhead 116, 116a, 116b, 116 c surface of the ink 120 "baby container" in lower container 129 Air duct 130 Duct for compressed air 131 Filter 132 Heater 133 Cooler 134 Holes

Claims: An ink jet printer including an ink supply system comprising an ink serving container (104) and an ink return container (100), at least one printhead (107), means (103a, 103b) for circulating ink from said serving container through said at least one printhead into said return container by gravity, means (102) for circulating ink from said return container back to said serving container, characterized in that : a specific container selected from the group of said serving container and said return container comprises a first compartment and a second compartment communicating with the first one for containing ink having a first ink surface at a first height (h1, h31, h37) with reference to a bottom of said container; said first compartment comprises an ink inlet; said second compartment comprises an ink outlet, the second compartment is separated from said first compartment by a partition (112, 121, 125) for extending above (h5, h33, h38) said first ink surface; said partition has an opening, wherein a height of said opening and a position of said opening are chosen so as to force the ink to flow from said first compartment through a lower part of said partition to said second compartment, i.e. said height of said opening is smaller than half said first height of said first ink surface (h1, h31, h37). An ink jet printer according to claim 1, wherein said ink inlet contains means for avoiding directional flow of incoming ink towards said ink surface. An ink jet printer according to any one of the previous claims, wherein said second compartment comprises a further partition (113) having a further height (h3) with reference to said bottom of said specific container, wherein said further height is smaller than said first height of said first ink surface and wherein said further partition is positioned upstream the ink outlet of said second compartment. An ink jet printer according to any one of the previous claims, wherein said second compartment comprises means (105, 117) for maintaining said ink surface at a desired height and to permit excess ink to overflow from said second compartment. An ink jet printer according to claim 4, wherein said specific container is said serving container and wherein said first and second compartments constitute a first serving set of two communicating ink compartments and wherein said ink outlet is a first ink outlet for serving ink to a first printhead (107), and wherein: said serving container further comprises a second serving set of two communicating ink compartments (Ia-IIa) for containing ink having a second ink surface at a second height (h11) lower than said first height, with reference to the bottom of said serving container; said first and second serving set of two communicating ink compartments are arranged to have excess ink from said first serving set (I-II) to overflow into said second serving set (Ia-IIa) ; said second serving set of two communicating ink compartments has a second ink outlet for serving ink to a second printhead (107a); said second serving set of two communicating ink compartments has a second partition (119) for separating said two compartments of said second serving set and for extending above said second ink surface, said second partition having a second opening, wherein a height of said second opening and a position of said second opening are chosen so as to force the ink to flow from a first compartment of said second serving set through a lower part of said second partition to a second compartment of said second serving set, i.e. said height of said second opening is smaller than half said second height of said second ink surface. An ink jet printer according to claim 4, wherein said specific container is said return container and wherein said first and second compartments constitute a first return set of two communicating ink compartments and wherein said ink inlet is a first ink inlet for returning ink from a printhead (107), and wherein said return container further comprises: a second return set of two communicating ink compartments having an ink inlet (106) for returning excess ink from said serving container; a third return set of two communicating ink compartments having an ink inlet (106a) for replenishing ink from a supply vessel (109). An ink jet printer according to any one of the previous claims, wherein said return container comprises means (132, 133) for conditioning the ink. An ink jet printer according to any one of the previous claims, wherein said serving container and said return container each comprise an air tight lid (104a, 100) and are connected to each other by an air duct (129) for pressure equalization. An ink jet printer according to any one of the previous claims, wherein inner parts of said serving container and said return container are treated with a wetting-adjusting coating. An ink jet printer according to any one of the previous claims, wherein a plurality of partitions of said specific container that reach to the bottom of said specific container have holes near the bottom for emptying said specific container, said holes having an area between 5 and 15 mm 2 , both limits included.

Inventors: PIRON ERIC, VAN GEYTE ANDRE, VERHOEST BART, WOUTERS PAUL, PIRON, ERIC., VAN GEYTE, ANDRE., VERHOEST, BART., WOUTERS, PAUL., PIRON, ERIC. C/O AGFA-GEVAERT N.V., Van Geyte, André. c/o Agfa-Gevaert N.V., VERHOEST, BART. C/O AGFA-GEVAERT N.V., WOUTERS, PAUL. C/O AGFA-GEVAERT N.V.
Applicants: AGFA GEVAERT, AGFA-GEVAERT
Application year: 2000
Main classification: B41J2
IPC codes: B41J2