Abstract:
An apparatus for controlling the pressure in an ink reservoir of an ink jet printer, including a casing and a piston movable relative to the casing and defining therewith a variable-volume chamber communicating with the ink reservoir, the piston being biased to maintain a pressure difference between the variable-volume chamber and the outside, wherein the piston is biased mainly by gravitational forces.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for controlling the pressure in an ink reservoir of an ink jet printer, comprising a casing and a piston movable relative to the casing and defining therewith a variable-volume chamber communicating with the ink reservoir, the piston being biased to maintain a pressure difference between the variable-volume chamber and the outside. 
     An ink jet printer typically comprises printhead having one or more nozzle arrays and an ink reservoir from which liquid ink is supplied to the nozzles of the nozzle arrays, so that ink droplets may be ejected from the nozzles by thermal or piezoelectric action, as is generally known in the art. When the level of ink in the ink reservoir is higher than the level of the nozzles, the ink reservoir should be maintained at subatmospheric pressure in order to avoid ink from leaking out through the nozzles. Since the difference between the internal pressure in the ink reservoir and the atmospheric pressure has an influence on the process of droplet generation and hence on the quality of the printed image, it is desirable to keep this pressure difference constant. Since, however, the internal pressure in the ink reservoir may vary in response to changes in the ink volume contained therein, thermal expansion and the like, it is necessary to control the internal pressure in the ink reservoir. 
     U.S. Pat. No. 5,039,999 discloses a pressure control apparatus of type indicated above, in which a coil spring is employed for biasing the piston. U.S. Pat. No. 4,509,062 discloses another type of pressure control apparatus in which the variable-volume chamber is bounded by an elastically deformable bladder. 
     Both conventional designs have the drawback that the elastic biasing forces which maintain the pressure difference between the internal pressure in the ink reservoir and the atmosphere depend on the amount of deformation of the spring or the bladder, respectively, so that the pressure difference may still vary along with the expansion or contraction of the variable-volume chamber. 
     EP-A-0 375 383 describes a pressure control apparatus in which the variable-volume chamber is partly bounded by a rolling diaphragm. This rolling diaphragm provides a substantially linear volume/pressure characteristic, similar to that of a piston biased by a coil spring. In this apparatus, the rolling diaphragm is used only for mitigating the pressure fluctuations in response to volume changes, and the pressure is ultimately maintained constant by sucking air bubbles or liquid into the variable-volume chamber through a small orifice. Thus, this apparatus requires a rather complicated design and further has the problem that slight pressure fluctuations are induced by the air bubbles sucked into the variable-volume chamber. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a simple apparatus which maintains the internal pressure in the ink reservoir at a constant value with high accuracy. 
     According to the present invention, this object is achieved with an apparatus in which the piston is biased mainly by gravitational forces. 
     Since the gravitational forces which may be created by the weight of the piston itself or by an additional load applied thereto are constant irrespective of any changes in the volume of the variable-volume chamber, the internal pressure in the ink reservoir, or more exactly, the pressure difference between the ink reservoir and the outside, can be maintained constant with high accuracy even when the volume of the variable-volume chamber is allowed to vary within a comparatively large range. As a result, a high quality of the printed image can be achieved even with a system in which the droplet generation process is highly sensitive to the pressure drop across the nozzles, and the ingress of air into the nozzles is safely prevented. The apparatus according to the present invention is particularly useful in combination with a hot-melt ink jet printhead which is operated at elevated temperatures. 
     Preferably, the piston is connected to the walls of the casing defining the variable-volume chamber by means of a highly flexible diaphragm which provides a perfect seal for the gap between the piston and the walls of the casing without inducing any substantial friction between the piston and the casing. As a result, the frictional effects are negligible, even in the case where the internal pressure in the ink reservoir is only slightly below atmospheric pressure, e.g. in the order of 1 kPa below atmospheric pressure, and the effective pressure-sensitive area of the piston is comparatively small, wherein the gravitational forces involved in biasing the same are extremely small. 
     In a particularly preferred embodiment, the casing defining the variable-volume chamber has the form of a cylinder, and the piston is fitted therein with a small annular gap formed between the outer circumferential surface of the piston and the inner circumferential surface of the cylinder walls, and the rolling diaphragm is accommodated in this annular gap. In this way, a particularly compact construction of the apparatus is achieved, and the diaphragm is smoothly and stably guided in the cylinder without any substantial friction. Since the diaphragm is not subject to any substantial tensile stresses, it can be made extremely thin so that to will not exert any elastic forces on the piston. The pressure difference between the inside and the outside of the variable-volume chamber will help to keep the two layers of the rolling diaphragm apart, and since, when the piston is displaced, relative movement occurs only between the two layers of the diaphragm, friction is almost completely eliminated. In addition, since the diaphragm is not required to have elastic properties, the material may be optimized in view of reducing its frictional coefficient relative to itself. 
     While the printhead of an ink jet printer is generally mounted on a moving carriage, the pressure control apparatus can be mounted on a stationary frame of the printer and can be connected to the ink reservoir of the printhead through a flexible hose. Thus, the weight-biased piston will not be subject to any substantial forces of inertia. If the printhead comprises a plurality of ink reservoirs, for example in a color printer, all ink reservoirs may be connected to same pressure control apparatus. 
     Due to the constant gravitational forces acting upon the piston, the displacement of the piston depends linearly on the total air volume in the ink reservoir, the variable-volume chamber and the hose connection therebetween. In the long run, this air volume tends to slightly increase due to leakage or diffusion. In a preferred embodiment, this increase of the air volume is detected by monitoring the position of the piston, so that a reset process for evacuating the ink reservoir can be initiated automatically, when necessary. Likewise, the displacement of the piston can be used for generating a signal for automatically terminating the evacuation process when the air volume has again reached its target value. As an alternative, the variable-volume chamber is reset to a specific volume at regular intervals, for example at the end of each scan cycle of the printer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, wherein 
     FIG. 1 is a sectional view of the main components of a pressure control apparatus, with associated, components of an ink jet printer, depicted schematically; 
     FIG. 2 is a sectional view of the apparatus shown in FIG. 1 in a different operating state; and 
     FIG. 3 is a diagram of a reset mechanism for the pressure control apparatus in a printer according to a modified embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As is shown in FIG. 1, a printhead  10  of an ink jet printer is mounted on a carriage  12  that performs scan movements in a direction indicated by the double-arrow S. The printhead comprises two nozzle arrays  14  attached to the lower edge of an ink reservoir  16 . When the printer is operative, the ink can be supplied to the nozzles of the nozzle arrays  14 . The level of the liquid ink in the ink reservoir  16  is indicated by a dashed line  18 . Since this ink level is higher than the level of the nozzles in the nozzle arrays  14 , the ink in the nozzles is under a static pressure, so that ink might tend to leak out of the nozzles. For this reason, the air volume above the ink level  18  in the ink reservoir  16  is maintained at a slightly subatmospheric pressure, e.g. 1 kPa below atmospheric pressure. 
     This pressure is controlled by means of a pressure control apparatus  20  that is mounted to a stationary frame  22  of the printer and is connected to the top part of the ink reservoir  16  by a flexible hose  24 . 
     The pressure control apparatus  20  comprises a casing  26  shaped as an upright cylinder and having an open bottom. A port  28  to which the hose  24  is connected is formed in the top wall of the casing  26 . 
     A cylindrical cup-shaped piston  30  is slidably disposed in the casing  26  with the open end facing upward into the interior of the casing, so that a variable-volume chamber  32  is defined inside of the casing  26  and the piston  30 . 
     A rolling diaphragm  34  in the form of a hose or bag made of extremely thin flexible material has an end portion  36  sealingly connected to the lower edge of the circumferential wall of the casing  26 , and the other end of the diaphragm is sealingly connected to the bottom of the piston  30 . 
     The outer circumferential surface of the piston  30  and the internal wall of the casing  26  define an annular gap  40 , which accommodates the main part of the diaphragm  34 . This main part forms an outer layer  42  engaging the wall of the casing  26  and an inner layer  44  engaging the outer circumferential surface of the piston  30 . The outer and inner layers  42 ,  44  are interconnected at their top ends by a rolling rim  46 . 
     The piston  30  is biased downwardly by its own weight and thus tends to expand the variable-volume chamber  32 . Since the diaphragm  34  forms an air-tight seal between the piston and the casing  26 , the expansion of the variable-volume chamber  32  causes the pressure prevailing in this chamber and also in the ink reservoir  16  to drop below atmospheric pressure. The piston  30  therefore assumes an equilibrium position in which the gravitational forces are counterbalanced by the differential pressure acting on the bottom face of the piston. Thus the internal pressure in the ink reservoir  16  is maintained at a constant value which is determined by the weight and the cross-sectional area of the piston  30 . 
     It is important to note that the diaphragm  34  does not exert any elastic forces on the piston  30 , regardless of the displacement of the latter. Although minor elastic stresses may occur in the rolling rim  46  of the diaphragm, these forces do not bias the piston upwardly or downwardly but rather tend to center the piston on the axis of the casing  26 . 
     Due to the subatmospheric pressure in the variable-volume chamber  32 , ambient air will penetrate into the small gap between the outer and inner layers of the diaphragm  34  and will hold these layers in engagement with the walls of the casing  26  and the piston  30 , respectively. Thus, the outer and inner layers  42 ,  44  will always be separated by a slight gap so that no frictional forces between these layers will impede the axial displacement of the piston  30 . 
     When the ink level  18  in the ink reservoir  16  changes or the air above this ink level undergoes thermal expansion, the piston  30  is free to move in the casing  26 , so that the pressure in the ink reservoir will always be maintained constant. 
     In the long run, the subatmospheric pressure prevailing in the ink reservoir and the variable-volume chamber  32  may cause an ingress of air due to leakage, diffusion or the like. As a result, the piston  30  will gradually move downward, as is illustrated in FIG.  2 . This gradual downward movement of the piston should be compensated from time to time by “resetting” the piston. To this end, a vacuum pump  48  is connected to the hose  24  as is shown in FIG.  1 . When the vacuum pump  48  is operated, the ink reservoir  16  and the variable-volume chamber  32  are evacuated, so that the piston  30  will rise again. An orifice  50  limits the flow of air drawn out of the ink reservoir and the variable-volume chamber, so that the piston  30  can readily keep-up with the evacuation of air, without causing a temporary pressure drop in the ink reservoir. A pressure accumulator  52  connected between the orifice  50  and the hose  24  smoothes out the pressure fluctuations that might be caused by the vacuum pump  48 . 
     In the embodiment shown in FIG. 1, a position sensor  54 , e.g. an optical sensor, is mounted to the frame  22 . When the variable-volume chamber  32  is evacuated and the piston  30  rises to the position shown in FIG. 1, the sensor  54  will deliver a signal for switching off the vacuum pump  48 . Thus, the original position of the piston  30  can be restored automatically after an evacuation has been initiated. 
     Optionally, another position sensor  56  is provided in a lower position than the sensor  54 . When the piston  30  has been lowered to the position shown in FIG. 2, due to the ingress of air, the sensor  56  will deliver a signal for automatically initiating an evacuation process. 
     FIG. 3 illustrates a modified embodiment of a reset mechanism for resetting the pressure control apparatus  20  in regular time intervals. In this embodiment, the pressure control apparatus  20  and a plurality of printheads  10  of, for example, a color printer, are commonly mounted on the carriage  12  which moves back and forth relative to the frame  22  of the printer. The vacuum pump  48  is also mounted on the carriage  12 . Thus, the pressure control apparatus  20  can be connected to the printheads  10  and the vacuum pump  48  by rigid pipings, so that no flexible hoses are required. 
     The vacuum pump  48  comprises a cylinder  58  and a piston  60  which define a working chamber  62 . The piston  60  is movable relative to the cylinder  58  in a direction parallel with the scan direction S of the carriage  12  and comprises a plunger  64  which projects towards a portion of the frame  22 . A compression spring  66  accommodated in the working chamber  62  biases the piston  60  towards said portion of the frame, i.e. in a direction which increases the volume of the working chamber. 
     A vacuum line  68  connects the working chamber  62  of the vacuum pump to the variable volume chamber  32  of the pressure control apparatus  20  and includes a first check valve  70  which opens in the direction of the vacuum pump  48 . Another check valve  72  opens to the atmosphere and is connected to the vacuum line  68  between the first check valve  70  and the vacuum pump. 
     A third check valve  74  which also opens to the atmosphere is arranged in the top wall of the casing of the pressure control apparatus  20 . The valve member of this check valve is connected to an control rod  76  which projects downwardly into the piston of the pressure control apparatus. 
     When the printer is operating and the carriage  12  reaches an end position of its scan stroke, the plunger  64  abuts against the frame  22 , and the piston  60  is pressed inwardly against the force of the compression spring  66 . The air displaced out of the working chamber  62  is vented through the check valve  72  while the check valve  70  is closed. When the carriage  12  then performs the next scan cycle and moves away from the frame  22 , the working chamber  62  is expanded again by the force of the compression spring  66 . Under these conditions, the check valve  72  closes and the check valve  70  opens so that air is sucked out of the variable-volume chamber  32  and into the working chamber  62 . As a result, the piston  30  of the pressure control apparatus is caused to rise. 
     When the rising piston  30  reaches a certain zero-position, the bottom of the piston abuts against the end of the control rod  76  and causes the check valve  74  to open. Thus, while the compression spring  66  continues to expand, the air sucked out of the variable-volume chamber  32  is replaced by ambient air drawn-in through the open check valve  74 . As a result, the piston  30  will not rise further but will stay in the zero-position. When the compression spring  66  approaches its equilibrium position, i.e. when its biasing force approaches zero, the suction force of the vacuum pump  48  can no longer overcome the suction force caused by the weight of the piston  30 . At this instant, the check valves  70  and  74  close, and the variable-volume chamber  32  is disconnected from both the ambient air and the vacuum pump  48 , so that the vacuum pressure in the variable-volume chamber  32  is again determined only by the weight of the piston  30  which has been restored to its zero-position. Thus, the reset process is completed. 
     The reset process described above is repeated after each scan cycle of the carriage  12 , each time the plunger  64  engages the frame  22 . 
     While specific embodiments of the present invention have been described above, it will occur to a person skilled in the art that various modifications can be made without departing from the scope of the present invention.