Ink Jet Head with Vacuum Reservoir

Technical Field

This invention relates to ink jet systems and, more particularly, to ink jet systems utilizing an ink jet head requiring a source of vacuum.

Background Art

In the Hoisington et al. Patent No. 4,788,556 issued November 29, 1988, an ink jet system is described in which dissolved air is removed from ink to avoid genera¬ tion of ink bubbles in the pressure chamber by passing the ink through elongated deaeration passages bounded by membranes which are subjected to negative air pressure. The negative air pressure is generated by an air pump remote from the ink jet head which is connected through a vacuum line to plenums containing the membranes.

This arrangement not only occupies a substantial portion of the ink jet head, producing a corresponding increase in the mass and complexity of the head struc- ture, but also requires an off-head pump and a connecting vacuum line, contributing further to the cost and com¬ plexity of the ink jet system.

Disclosure of Invention

Accordingly, it is an object of the present inven- tion to provide a new and improved ink jet system provid¬ ed with a vacuum source which overcomes the disadvantages of the prior art.

Another object of the invention is to provide an ink jet system having a vacuum source of reduced complexity and lower cost, while providing the same advantages as the prior art systems.

These and other objects of the invention are at¬ tained by providing an ink jet system in which the ink jet head is provided with a vacuum supply. In one em¬ bodiment, a sealed vacuum reservoir incorporated in the head communicates through an air-permeable, ink-imperme¬ able wall with the pressure chambers from which ink is ejected during operation of the system so as to extract dissolved air directly from the ink in the pressure cham¬ bers. The vacuum reservoir is supplied with a stored vacuum designed to maintain a sufficiently low pressure level at the air-permeable wall of the pressure chambers until the ink in an associated ink reservoir in the ink jet head has been depleted during operation of the ink jet system, after which both the ink reservoir and the vacuum reservoir may be replenished or the ink jet head may be discarded. In an alternative embodiment, the vac¬ uum supply in the ink jet head includes a vacuum pump which is actuated during operation of the ink jet system so as to maintain the vacuum at the wall of the pressure chambers at the desired level during operation of the system.

Further objects and advantages of the invention will be apparent from a reading of the following description in conjunction with the accompanying drawings, in which:

Brief Description of Drawings

Fig. 1 is a schematic plan view illustrating the ar¬ rangement of a representative ink jet system arranged in accordance with the invention;

Fig. 2 is an enlarged perspective view of a repre- sentative ink jet head arranged in accordance with the invention;

Fig. 3 is an enlarged fragmentary sectional view showing the pressure chamber region of the ink jet head shown in Fig. 2; and

Fig. 4 is a schematic elevation view of an ink jet head arranged in accordance with another representative embodiment of the invention.

Best Mode for Carrying Out the Invention In the typical embodiment of the invention schemati¬ cally illustrated in Figs. 1-3, an ink jet head 10 is mounted on a carriage 11 for reciprocating motion in the direction indicated by the arrow 12 adjacent to a sub¬ strate 13, such as a sheet of paper. The substrate 13 is supported on a platen 14 for motion perpendicular to the direction of motion of the carriage 11 and is advanced periodically or continuously in the usual manner. A drive spindle 15 driven by a motor in accordance with signals on a line 16 from a control system 17 drives the carriage 11 by means of a belt 18 which passes around a spindle 19 at the opposite end of the path of motion of the carriage. The control system 17 also transmits con¬ trol signals on a line 20 to the head 10 to control ejec¬ tion of ink drops 21 toward the substrate 13 and controls the motion of the substrate through a line 22 in the usu¬ al manner to produce an image on the substrate.

As shown in the schematic illustrations of Figs. 2 and 3, the ink jet head 10 includes an ink reservoir 23 containing a supply of ink 24 conducted through a series of ducts 25 to a corresponding series of pressure cham¬ bers 26, from which ink is ejected through a series of corresponding orifices 27 by operation of associated electromechanical transducers 28 in the usual manner. Only one of the ducts 25, pressure chambers 26 and trans- ducers 28 is visible in Figs. 2 and 3.

As described in the above-mentioned Hoisington et al. Patent No. 4,788,556, the disclosure of which is in¬ corporated herein by reference, the presence of dissolved air in the ink 24 may cause air bubbles to be generated at nucleation sites in a pressure chamber when the trans¬ ducer produces a negative air pressure in the pressure

chamber during operation of the system, and such bubbles can produce malfunctions of the ink jet system. Such dissolved air may be diffused into the ink in the pres¬ sure chamber 26 through the ink jet orifice 27 and the duct 25 with which the pressure chamber communicates.

In order to avoid the necessity for circulating ink from the pressure chamber and the orifice region to a deaeration passage remote from the pressure chamber in the manner described in Patent No. 4,788,556, the ink jet head 10 of the embodiment shown in Figs. 2 and 3 includes a vacuum supply in the form of a vacuum reservoir 29 which is in direct communication with the pressure cham¬ ber 26 through a wall 30 which is permeable to air but not permeable to ink. As shown in Fig. 3, the semiper- meable wall 30 is mounted adjacent to the transducer 28, which is made of porous material, so that the transducer provides support for the wall 30. The wall 30 preferably consists, for example, of a sheet of a polymeric material such as polypropylene or it may include a membrane made of a layer of silicone sheeting such as the Dow-Corning material designated "2174" or a layer of fluorosilicone or Teflon material, as described in the Hine et al. Pat¬ ent No. 4,940,995. If the transducer 28 is not made of porous material, it can be positioned at the side of the pressure chamber opposite the semipermeable wall 30 and that wall can be supported by a rigid, porous member so that the resilience of the wall material will not inter¬ fere with the generation of pressure pulses in the pres¬ sure chamber by the transducer 28. If the wall 30 is made of polymeric thermoplastic material such as polypropylene, the material of the wall can also serve as a bonding medium when it is heated to an appropriately high temperature to bond the wall in sealing relation to the surrounding structure of the res- ervoir.

The ink 24 in the reservoir 23 may be either an ink which is liquid at room temperature or a hot melt-type

ink which is solid at room temperature and becomes liquid at an elevated temperature to which the reservoir, pres¬ sure chamber and orifice are raised in the usual manner by appropriate heaters during operation of the ink jet system. If the ink jet head is designed to use hot melt ink, polypropylene is an especially advantageous material for the semipermeable wall 30 since it is about 100 times more permeable to air at 130°C than at 25°C. This prop¬ erty permits the vacuum reservoir 29 to have a long shelf life at room temperature, while facilitating withdrawal of air from the ink during operation at high temperature. The permeability of the wall 30 to air during opera¬ tion and the vacuum level in the reservoir 29 during the life of the ink jet head are selected so that the diffu- sion constant of air through the wall from the pressure chamber 26 to the reservoir 29 at the elevated tempera¬ ture of operation is greater than the diffusion constant of air into the ink in the pressure chamber through the orifice 27 and the passage 25 during nonprinting periods when there is no flow of ink through the pressure chamber and the orifice. During the printing operation, the presence of an acoustic field in the pressure chamber tends to prevent the formation of nucleation sites at which bubbles can be formed by dissolved air in the ink. The upper limit for the diffusion constant of the semipermeable wall 30 is selected so that the diffusion through the wall will be low enough at nonoperating tem¬ peratures of the system, such as 25°c, that the vacuum level retained in the vacuum reservoir will not fall be- low a selected minimum value during the specified shelf life of the ink jet head. If desired, such air diffusion through the wall at nonoperating temperatures may be re¬ duced by providing an impermeable cap or seal over the orifices 27 during storage which is removed when the ink jet head is mounted in the printer and by covering the orifices with an appropriate impermeable cover when the head is mounted in the printer but is not in use. The

lower limit for the diffusion constant of the wall must be high enough to permit the selected minimum vacuum lev¬ el in the reservoir to extract air from the ink in the pressure chamber faster than the rate of diffusion of air into that ink through the associated orifice and ink pas¬ sage.

In an exemplary design satisfying these criteria, a vacuum reservoir 29 communicating with 96 pressure cham¬ bers, each 1mm long by 0.1mm wide, has a total area ex- posed through the semipermeable wall 30 of 0.1cm ² . If each of the 96 orifices has a diameter 56μ and a length of 50μ, the air diffusion through the orifices is about 0.02cc/hr at a pressure difference of one atmosphere. With 96 ink passages 25, each of which has a cross-sec- tion of 0.1mm and is 1mm long, the diffusion constant through the ink passages is about 0.004cc/hr at a one- atmosphere pressure difference. With a permeable wall 30 made of polypropylene 50μ thick, the diffusion constant of the wall is about 0.31cc/hr at an operating tempera- ture of about 130°C and about 0.002cc/hr at a storage temperature of 25°C. If the vacuum reservoir 29 has an initial vacuum level and capacity such that it can take in 18cc of air while still maintaining a vacuum level above a desired minimum, the vacuum reservoir will have a shelf life at 25°C greater than one year and an operating life at 130°C of about 750 hours. With the dimensions and conditions specified above, the ink in each of the pressure chambers 26 will become deaerated sufficiently within about 3.5 seconds to avoid bubble formation at a 130°C operating temperature.

If the ink jet head 10 is intended to be reused rather than discarded after the ink in the reservoir 23 and the vacuum in the reservoir 29 have been depleted, a vacuum replenishment port 32 covered by a removable seal 33 may be provided for the vacuum reservoir 29, the ink reservoir 23 being provided with a vent 31 through which ink may be inserted into the reservoir. If desired, the

vacuu reservoir 29 could be mounted separately from the head 10 and connected thereto by a vacuum line leading to a vacuum chamber adjacent to the semipermeable member 30. In another embodiment of the invention illustrated schematically in Fig. 4, an ink jet head 34 includes a vacuum supply in the form of a vacuum reservoir 35 pro¬ vided with a vacuum pump 36 arranged to be actuated peri¬ odically during operation of the ink jet system so as to maintain a vacuum at the desired level in the reservoir 35. In the typical embodiment shown in Fig. 4, the vacu¬ um pump 36 includes a piston 37 reciprocably mounted in a chamber 38 which communicates with the atmosphere through a check valve 39 and with the vacuum reservoir through a check valve 40, the check valves being arranged so that, upon motion of the piston 37 in the direction to expand the volume of the chamber 38, the check valve 40 is opened to receive air from the vacuum reservoir 35 and, upon motion of the piston 37 in the opposite direction, the check valve 39 is opened to expel air from the cham- ber 38.

In addition, a compression spring 41 is mounted be¬ tween the piston 37 and the wall of the chamber 38 so as to urge the piston in the direction to expand the chamber 38 and a plunger 42 mounted on the piston 37 projects through the wall of the housing of the ink jet head to engage a stationary abutment 43 at one end of the recip¬ rocating motion of the head 34. If desired, the piston 37 could be replaced by a diaphragm or bellows arrange¬ ment. Moreover, the plunger 42 could, if desired, be actuated by any other periodic actuating mechanism rather than by the reciprocating motion of the ink jet head.

The structure of the head 34 is otherwise the same as that of the head 10 described above with reference to Figs. 2 and 3, except that the vacuum reservoir 35 need not have the capacity required to maintain a shelf life for a specified period as described in connection with that embodiment. Moreover, since the vacuum in the res-

ervoir 35 will not be depleted during operation of the system, the ink jet head 34 need not be replaceable, and the ink reservoir therein may be replenished periodically rather than requiring the ink jet head to be removed when the ink in the ink reservoir is depleted.

In the embodiment shown in Fig. 4, it is not neces¬ sary to actuate the vacuum pump 36 during every recipro¬ cation of the ink jet head 34, and the abutment 43 which is engaged by the plunger 42 may be interposed so as to engage the plunger only periodically during operation of the system with a frequency sufficient to assure that the vacuum in the reservoir 35 is maintained at a desired level. Moreover, the chamber 38 may serve as the entire vacuum reservoir rather than requiring a separate reser- voir 35 if the volume of that chamber is sufficient to provide a vacuum level adequate to accommodate all of the ink jet pressure chambers. As a further alternative, the vacuum pump 36 may be connected to the plenums of a de- aeration passage in the ink jet head which may be of the type described in the above-mentioned Patent No.

4,788,556, thereby eliminating the need for a remote vac¬ uum pump as described in that patent, although retaining the rest of the deaeration structure disclosed in that patent. If the head 34 is designed to be disposable, the pump 36 may be removably mounted in the head so that it can be reused in another head.

Although the invention has been described herein with reference to specific embodiments, many modifica¬ tions and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention.