Multi-nozzle ink jet printer

Abstract: In a multi-nozzle ink jet printer, an ink liquid supply system comprises a plurality of constant flow rate pumps (22). Each indivi­ dual nozzle (20) of the printer is connected to an associated one of the pumps (22). The pumps (22) may be coupled to a common actuating mechanism (30). With this arrangement, the flow rates of ink liquid through the different nozzles (20) are kept at equal values, irrespec­ tive of differences in the effective cross section of the nozzles. The pressure of the ink liquid supplied to a particular nozzle is indepen­ dent of the pressure at the other nozzles. If one of the nozzles beco­ mes clogged with dried ink, the pressure may rise to forcibly remove the dried ink from the nozzle.

Description: The present invention relates to an ink jet printer having a plurality of printing nozzles. An ink jet printer generally comprises at least one nozzle and an ink liquid supply system which supplies ink liquid to the nozzles at an appropriate pressure, so that a jet of ink liquid is ejected onto a recording medium such as a sheet of paper. An ultrasonic transducer is provided for generating ultrasonic waves in the ink liquid to assist the separation of the ink jet into a sequence of equal-sized droplets. In a typical ink jet printing system, the droplets are electrically charged when passing through a ring electrode, and afterwards the droplets pass through a pair of deflection plates. The charged droplets are selectively deflected in response to a voltage applied to the deflection plates, so that the droplets either impinge on the sheet of paper or are deflected to a beam gutter. The ink collected by the beam gutter may be recirculated to the supply system. The document US-A-4 135 197 discloses an ink jet printing device having a printing head which comprises a plurality of printing nozzles. The printing head includes an elongate chamber serving as an ink reservoir. One of the walls of the chamber is formed by an orifice plate which contains the nozzle orifices. An ink supply system is adapted to supply ink to the reservoir and to maintain a constant pressure in the reservoir, so that the ink is equally distributed on the different orifices. This known ink jet printing system has the advantage that a large number of ink jets can be generated simultaneously so that the printing speed can be considerably increased. However, a problem of reliability is encountered inasmuch as the nozzles tend to become clogged for instance with dried ink. If the nozzles are partly clogged, deviations in the effective cross sections of the nozzles result in nonuniform flow rates of ink liquid through the different nozzles, so that the quality of the printed image is impaired. In connection with a single-nozzle ink jet printer, the document US-A-4 263 602 discloses an ink liquid supply system which employs a constant flow rate pump. However, even if such a constant flow rate pump were employed in the multi-nozzle system described above, the tendency of the nozzles to become clogged would not be eliminated and the reliability of the system would still be poor. The present invention provides a multi-nozzle ink jet printer which has an improved reliability. The features of the ink jet printer are specified in the appended claims. According to the invention, the ink liquid supply system comprises a plurality of constant flow rate pumps which are associated with the individual printing nozzles. If one of the nozzles becomes choked due to the deposition of dried ink or contaminants contained in the liquid, the pressure of the liquid supplied by the pump associated with this particular nozzle is increased so that the flow rate is forcibly maintained at a value which equals the flow rate through the other nozzles. As the flow velocity of the ink liquid increases in proportion with the pressure, the material deposited in the nozzle orifice is removed so that the tendency of the nozzle to become clogged is considerably reduced. If, nevertheless, a nozzle becomes clogged, the pressure of the ink liquid will rise until the clog is forcibly removed. Since the flow rate is substantially independent of the cross section of the nozzle, the invention has the additional advantage that the manufacturing tolerances of the nozzles are less critical. Preferably, the constant flow rate pumps have an identical construction and are coupled to a common drive mechanism. The pumps may be mounted on a printing head of the printer and may be arranged directly adjacent to the associated nozzles. In this case, the overal volume of ink supply lines is reduced so that the consumption of maintenance solution for scavenging the nozzles and the supply system during inoperative periods of the printer is minimized. In order to improve the flexibility of the system and to facilitate maintenance and repair, a modular construction is preferred wherein each pump is integrated in a nozzle unit which is detachably mounted on the printing head and comprises the nozzle and associated devices such as an ultrasonic transducer, deflection plates, a beam gutter and the like. A preferred embodiment of the invention will be explained below in conjunction with the accompanying drawings in which figure 1 is a plan view of a printing head of a multi-nozzle ink jet printer; and figure 2 is a sectional view of an ink liquid supply system for one nozzle of the ink jet printer. Referring to figure 1, a printing head 10 is arranged adjacent to a platen 12 which supports a sheet of paper 16 on which the printed image is to be formed. The printing head 10 is mounted on guide rails 14 and is movable in longitudinal direction of the platen 12. A plurality of nozzle units 18 are mounted side by side on the printing head 10. Each nozzle unit 18 comprises a nozzle 20 which has its tip directed towards to platen 12 and is adapted to eject a beam of ink droplets onto the paper sheet 16. The tip of the nozzle 20 is received in a beam deflection and gutter unit 24 which is not shown in detail in the drawings. Each nozzle unit 18 further comprises an ink liquid supply pump 22 having a piston 26 which is reciprocable in longitudinal direction of the nozzle 20. The pistons 26 have piston rods 28 which are elastically biased towards a common cam 30 which extends past the rear ends of the nozzle units 18. A drive unit 32 rotates the cam 30 so that the pistons 26 of all pumps 22 are actuated synchronously. Although not shown in the drawings, the printing head 10 includes tanks and supply lines for supplying ink liquid and maintenance solution to the pumps 22 and an electronic control systems for controlling the operation of the beam deflection means in response to input printing data. When the platen 12 is rotated while the printing head 10 is held stationary, each nozzle 20 can print a line 34 on the sheet 16. When the platen 12 has accomplished one turn, the printing head 10 is shifted along the guide rails 14 by a small amount, and another line is printed immediately adjacent to the first line 34. In case of single-color printing, the printing operation is completed as soon as, after a predetermined number of turns of the platen 12, the total shift of the printing head 10 corresponds to the distance W between the nozzles 20. The structure shown in the drawings permits to arrange the nozzles 20 at comparatively small intervals, so that a high printing speed can be achieved even if the resolution and hence the line density of the printed image is very high. The system shown in figure 1 also lends itself for multi-colour printing, if the printing units are operated with ink of different colours and if the range of travel of the printing head 10 is increased in accordance with the number of colours. The ink supply pump 22 of an individual nozzle unit 18 is shown in detail in figure 2. The pump 22 is incorporated in a nozzle carrier 36 on which all members of the nozzle unit 18 are mounted. The piston 26 is slidably received in a bore 38 of the nozzle carrier 36 and defines a pressure chamber 40. The piston rod 28 is biased towards the cam 30 by means of a helical compression spring 42 which is interposed between the rear end of the nozzle carrier 36 and a spring seat 44 formed on the piston rod 28. The pressure chamber 40 is fluidly connected to the tubular nozzle 20 via a three-way cock 46. By turning the plug of the cock 46, the pressure chamber 40 can be disconnected from the nozzle 20 and connected either to an inlet port 50 for ink liquid or to an inlet port 52 for maintenance solution via another three-way cock 48. The inlet ports 50, 52 communicate with the ink reservoir and the maintenance solution tank, respectively, via supply lines which are not shown in the drawings. The plugs of the cock 46, 48 are rotated by means of actuating shafts 54 (figure 1). The actuating shafts extend through all nozzle units 18, so that the respective cocks of the nozzle units are operated synchronously. Figure 2 shows the condition of the supply pump 22 during a printing operation. The cam 30 is rotated clockwise, so that the piston 26 is moved to the right in figure 2. The ink liquid contained in the pressure chamber 40 is forced into the nozzle 20 at a constant flow rate. As the pistons 26 of all nozzle units 18 are synchronously actuated by the common cam 30, the flow rates of ink liquid through the different nozzles are equal, irrespective of differences in the nozzle cross section due to contamination or manufacturing tolerances. The normal operating pressure in the pressure chamber 40 amounts to about 30 to 40 bar. If, however, the nozzle 20 is clogged, the pressure in the corresponding pressure chamber may rise to for instance 600 bar because the force exerted by the cam 30 then concentrates on the piston associated with the clogged nozzle. As a result, there is generated a sufficiently high pressure for removing the clog. The dimension of the pressure chamber 40 and the stroke of the piston 26 are selected in such a manner that the amount of ink which can be delivered during one stroke is sufficient for a complete printing process. At the end of the compression stroke of the piston 26, the plug of the cock 46 is actuated so that the pressure chamber 40 is disconnected from the nozzle 20 and connected to the inlet port 50. During the subsequent suction stroke of the piston 26, another charge of ink liquid is sucked into the pressure chamber 40. Then, the cock 46 is returned to the position shown in figure 2 so that another printing operation may be started as soon as the sheet 16 on the platen 12 has been replaced by a new one. The purpose of disconnecting the pressure chamber 40 from the nozzle 20 during the suction stroke is to avoid that ambient air is sucked into the nozzle 20 and causes the ink contained in the nozzle to dry out. If necessary, the ink supply system can easily be scavenged with maintenance solution. To this aim, the cock 48 is actuated to disconnect the suction chamber 40 from the inlet portion 50 and connect it to the inlet portion 52 so that maintenance solution is sucked in during the suction stroke. The maintenance solution can be forced through the nozzle 20 with high pressure, so that contaminants deposited in the nozzle are efficiently removed. While the apparatus described above constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the appended claims. For instance, the nozzle units 28 may be arranged in a number of parallel rows. Furthermore, the housing of the ink supply pump need not be formed integrally with the nozzle carrier. Instead, a commercially available pump, such as a pump presently used for high pressure liquid chromatography applications may be mounted on the nozzle carrier.

Claims: An ink jet printer having a plurality of printing nozzles (20) and an ink liquid supply system (22, 30), characterized in that said ink liquid supply system comprises a plurality of constant flow rate pumps (22), each individual nozzle (20) being connected to an associated one of said pumps (22), wherein the pumps (22) are coupled to a common drive mechanism (30, 32). An ink jet printer as claimed in claim 1, wherein said pump (22) comprises a cylinder and a reciprocating piston (26) which defines a pressure chamber (40) within the cylinder, said cylinders of the pumps (22) being arranged side by side directly adjacent to the upstream ends of the corresponding nozzles (20). An ink jet printer as claimed in claim 2, wherein the cylinders of the pump (22) are arranged in a row and wherein the pistons (26) of the pumps have piston rods (28) which are engaged with a common actuating cam (30). An ink jet printer as claimed in claim 2 or 3, wherein each pump (22) has valve means (46, 48) for selectively communicating the pressure chamber (40) to the nozzle (20), an inlet port (50) for ink liquid or an inlet port (52) for maintenance solution. An inlet jet printer as claimed in claims 3 or 4, wherein said valve means (46, 48) of the different pumps (22) are actuated by a common actuating shaft (54). An ink jet printer as claimed in any of the preceding claims, wherein each pump (22) and the associated nozzle (20) form part of a nozzle unit (18) which is detachably mounted on a printing head (10).

Inventors: DERKS PAUL ANTOON HENDRIK, DERKS, PAUL ANTOON HENDRIK
Applicants: OCE NEDERLAND BV, OCE-NEDERLAND B.V.
Application year: 1986
Main classification: B41J2
IPC codes: B41J2