Background of the Invention

The present invention relates to ink jet print¬ ers, and, more particularly, to ink jet printers which utilize the so-called drop-on-demand method of operation.

Non-impact printers have recently become very popular due to their quiet operation resulting from the absence of mechanical printing elements impacting on record media during printing. Among such printers, ink jet ^' printers are particularly important as they permit high speed recording on plain untreated paper.

Various ink jet printing methods have been developed over the past years. In the so-called con¬ tinuous ink jet method, such as disclosed in U.S. Patent No. 3,596,275, the ink is delivered under pressure to nozzles in a print head to produce a continuous jet of ink emitted ^' through each nozzle. The ink jet is separated by vibration into a stream of droplets which are charged, and the flying droplets are either allowed to impact on a record medium or are electrostatically deflected for collection in a gutter for subsequent re- circulation.

A second method, known as the electrostatic method, is disclosed, for example, in U.S. Patent No. 3,060,429. In this method the ink in the nozzles is under zero pressure or low positive pressure, and the droplets are generated by electrostatic pull and caused to fly between two pairs of deflecting electrodes ar¬ ranged to control the direction of flight of the droplets and their deposition in desired positions on the record medium.

A third method, which is known as the drop-on- demand method, is described, for example, in U.S. Patent No. 4,125,845. The droplets in this method are emitted under the control of an electronic character generator by means of volume displacement brought about in an ink

chamber or channel by means of energization of a piezo¬ electric element. The volume displacement generates a pressure wave which propagates to the nozzles causing the ejection of ink droplets. The drop-on-demand method has several advan¬ tages over the other above-mentioned methods. Ink jet printers using this method have a simpler structure re¬ quiring neither deflecting means for controlling the flight of the droplets nor the provision of an ink recovery system. Multiple-nozzle print heads using this method are simple and compact and are relatively easy to manufacture.

The drop-on-demand method requires that under quiescent conditions there is an appropriate under- pressure, i.e. negative pressure, in the ink chamber or reservoir, in order to retain the ink in the nozzle until such time that it is to be ejected. The amount of the underpressure is critical. With too small an underpressure, or with a positive pressure, ink tends to escape through the nozzles. On the other hand, with too high an underpressure, air may be sucked in through the nozzles under quiescent conditions. The required underpressure may be obtained gravitationally by lowering the ink reservoir so that the ink surface level therein is below the level of the nozzles. However, such po¬ sitioning of the ink reservoir may not always be easily achieved, as it may require complex changes in the design of the ink jet printer or its print head. More¬ over, it cannot be achieved in hand-held ink jet print- ers which must be capable of being tilted to print on a variety of objects such as parcels, packets, envelopes, _sheets, or the like.

U.K. Patent No. 2,063,175 describes an ink jet printer in which ink within an ink container is maintained at a pressure lower than atmospheric pressure so that the ink container can be held above the nozzle without ink leaking out of the nozzle. This is achieved by the elasticity of the container which tends to assume a

shape of maximum volume. For expanding the ink con¬ tainer, the described embodiments rely on springs, bel¬ lows and similar mechanical means and this will reduce life expectancy as a result of wear of such elements. Also, such elements tend to be rather bulky and cannot be easily used in small and compact hand-held ink jet printers.

Disclosure of the Invention

It is an object of the invention to provide an ink jet printer of the drop-on-demand type in which the required underpressure is automatically obtained by simple means requiring no mechanical moving parts, and which enables the ink jet printer to be constructed as a small and compact printer. According to the invention, there is provided an ink jet printer including a print head having at least one piezoelectric driving element selectively energizable to cause ejection of a droplet of ink through a nozzle associated therewith, and an ink reservoir for supplying ink to said print head, characterized by capillary means connecting the interior of the ink reservoir with the ambient atmosphere, with one end of said capillary means being so arranged as to be immersed in operation in ink contained in the reservoir, the reservoir being air-tight in operation apart from the provision of said capillary means whereby an under¬ pressure is maintained in respect of the ink in said nozzle, any increase in underpressure as ink droplets are ejected being inhibited by virtue of the intake of air into the reservoir via said capillary means.

The present invention finds particular use in hand-held ink jet printers for which the drop-on-demand method is particularly well suited by virtue of its simplicity and low energy consumption and by virtue of the fact that it makes possible the construction of a self-contained and compact unit.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic part-sectional repre¬ sentation of a hand-held printer according to the inven¬ tion;

Fig. 2 is a partial side view of the lower part of the printer of Fig. 1, taken along line 2-2 of Fig. 1, with the housing partly broken away to expose details of the printer within the housing;

Fig. 3 is a schematic part-sectional repre¬ sentation of an ink jet printer similar to that of Fig. 2, in which the capillary means is differently disposed;

Fig. 4 is a partial side view of the printer of Fig. 3, as seen along line 4-4 of Fig. 3;

Fig. 5 is a simplified block diagram showing means for controlling the operation of the printers of Figs. 1 to 4; and

Fig. 6 is a diagram helpful in explaining the manner in which a steady underpressure is maintained in the printers of Figs. 1 to 4.

Best Mode for Carrying Out the Invention Referring to the drawings, there is shown in

Figs. 1 and 2 a hand-held ink jet printer 90 and there is shown in Figs. 3 and 4 a hand-held ink jet printer 92. Each of the printers 90 and 92 is arranged to be moved by hand along a record medium 110 to print alpha- numerical characters thereon by selective actuation or energization of piezoelectric crystal drive elements 106 of a print head 96, the elements 106 being arranged to bring about ejection of droplets of ink through respective nozzles 102 associated therewith. The ener- gization of the piezoelectric elements 106 is controlled in a well-known conventional manner, and will be briefly described later with reference to Fig. 5.

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Each of the printers 90 and 92 includes a pen- shaped casing or housing 94 which is cylindrical with a tapered lower end in which is housed the print head 96. The print head 96 incorporates seven ink channels 98 each extending between an ink supply manifold 100 and the corresponding nozzle 102 formed in a nozzle plate 104, the nozzles 102 being arranged in a straight line. Each of the ink channels 98 includes a segment surrounded by a respective piezoelectric element 106. In order to enable the elements 106 to be accommodated in the casing 94, alternate ones of the ink channels 98 are disposed in> two divergent planes, as illustrated best in Fig. 2.

The operative elements of the print head 96, including the ink channels 98, the manifold 100, nozzle plate 104, and piezoelectric elements 106, can be em¬ bedded or potted in a suitably shaped block of plastic material to be fitted into the casing 94, although such block of plastic material has been omitted in the draw¬ ings for purposes of clarity. The casing 94 of each printer 90 or 92 has rotatably mounted thereon adjacent its lower end a wheel 108 to permit rolling movement of the printer over a record medium 110 and to maintain the required spacing between the nozzle plate 104 and the record medium 110. The wheel 108 also serves as a timing wheel and is dis¬ posed in operative relationship with an optical sensing device 112 mounted in the casing 94 adjacent the wheel 108. The sensing device 112 is arranged to sense radi¬ ally extending, equally spaced lines or markings 113 (Fig. 2) on the side of the wheel 108. Although not illustrated in detail, the device 112 includes an LED and phototransistor arranged in a conventional fashion to generate in operation a series of timing pulses which are utilized to control the operation of the piezoelec- trie elements 106. Electrical leads 114 for the piezo¬ electric elements 106 and the sensing device 112 are carried by an electrical cable 116 which extends along the interior of the casing 94.

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As shown schematically in Fig. 5, the print head 96 and sensing device 112 of each of the printers 90 and 92 are connected via the cable 116 and conven¬ tional driver circuits 68 to a computer or data proces- sing equipment and associated printer control, that provide data and character signals in response to the timing signals from the sensing device 112, in order to control the driver circuits 68 to print the desired characters. The use of a movement sensing device, such as the device 112, in conjunction with a computer to control a hand-held ink jet printer is known in the art. For example, reference can be had to U.S. Patent No. 3,656,169, issued to T. Kashio. Briefly, however, the timing pulses generated by the device 112 are delivered to the computer, indicating the movement of the printer 90 or 92. In response to the timing pulses, the com¬ puter and conventional character generating circuitry can control the driver circuits to selectively energize the piezoelectric elements 106. Since the energization of the piezoelectric elements 106 is brought about in response to a timing pulse generated by the sensing device 112, a uniform print will always be produced, at whatever speed the printer is moved over the record medium 110. During the printing of a character on the record medium 110, each of the columns of dot positions making up the character is printed by energization of a selected one, or selected ones, of the piezoelectric elements 106.

Referring to Figs. 1 and 2, an ink reservoir 118 containing ink 40 is housed in the casing 94 above the print head 96 of the printer 90. A capillary pas¬ sage or tube 120 is mounted in the ink reservoir 118, the tube 120 being open to the ambient atmosphere at its upper end and extending towards the bottom of the ink reservoir 118 so that in operation the lower end of the tube 120 is immersed in the ink 40 contained in the reservoir 118. It should be understood that the filled

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reservoir 118 is air-tight in operation, apart from the . provision of the capillary tube 120.

Referring to Figs. 3 and 4, a capillary passage or tube 122 is incorporated in the printer 92. In this case, the lower end 123 of the tube 122 is open to the ambient atmosphere, the tube 122 extending through the body of the print head 96 with the upper end of the tube 122 being disposed inside an ink reservoir 124 housed in the casing 94 above the print head 96. The upper end of the tube 122 is positioned adjacent the bottom of the reservoir 124 and is immersed in operation in ink 40 contained in the reservoir 124. It should be understood that the filled reservoir 124 is air-tight in operation, apart from the provision of the capillary tube 122.

In operation of the printer 90 (Figs. 1 and 2) and the printer 92 (Figs. 3 and 4), it is necessary to set initially the proper value of the underpressure at the nozzles 102 by draining or ejecting a small amount of ink from the reservoir 118 or 124. Once this value is set, the underpressure will be automatically maintained at this value by virtue of the provision of the capillary tube 120 or 122. When the underpressure tends to increase due to ink droplets being ejected through the print head nozzles 102, air bubbles enter through the capillary tube 120 or 122 into the air space of the reservoir 118 or 124 thereby preventing an actual increase in underpressure.

In each of the printers 90 and 92, the required underpressure in respect of the ink in the nozzles 102 relative to the ambient atmospheric pressure is the result of the combination of the capillary forces of the ink in the nozzles 102 and capillary tube 120 or 122, the hydrostatic pressure of the ink when the device is in its working position and the pneumatic underpres¬ sure in the air space in the ink reservoir 118 or 124. The capillary force or capillarity of the ink at the capillary tube 120 or 122 depends on the inner diameter

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of the tube, the surface tension of the ink and the adhesion between ink and tube.

It is, of course, important, that the opening of the tubes 120 or 122 within the reservoirs 118 or 124 be below the ink level, and thus immersed, since it is the capillarity of the ink and tubes that resists the intake of air through the tube and thus maintains the desired underpressure. It is additionally important that the inner diameter of the tubes 120 or 122 be suf- ficiently small for this same reason. The tubes 120 and 122 can be made of various suitable types of mater¬ ial, such as glass or nickel. For commonly available inks, a glass tube having an inner diameter of approxi¬ mately .3 to 1.2 millimetres, or a nickel tube having an inner diameter of approximately .15 to .9 millimetres, would be suitable.

Fig. 6 is a diagram showing the underpressure regulation in the ink supply system of each of the printers 90 and 92 shown in Figs. 1 through 4 as a function of time. Before printing is commenced, and during a preliminary period A, the interior of the reser¬ voir 118 or 124 may be at normal atmospheric pressure, as indicated at point (1). Ink is then removed through the nozzles 102 of the printer 90 or 92, such as by energi- zation of the elements 106, thereby reducing the pressure in the ink reservoir 118 or 124 until the required under¬ pressure, as determined by the capillary tube 120 or 122, is obtained. This is shown at point (2) of the diagram and would normally be between 2 and 5 centimetres of water below atmospheric pressure, with the diameter of the nozzles 102 being approximately .07 millimetres. At this point, the preliminary period A terminates and the period B during which printing may be done commences. The ink jet print head 96 will normally operate with the underpressure at the level indicated at point (2) until the ink within the reservoir 118 or 124 is de¬ pleted.

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In operation of the printer 90 or 92, the underpressure is maintained at the required level becaus air bubbles enter into the reservoir 118 or 124 through the capillary tube 120 or 122 as soon as the underpres- sure tends to increase as ink is ejected. Since the air bubbles entering into the reservoir 118 or 124 each have an almost negligible volume as compared with the volume of air in the reservoir, the underpressure may be considered as constant, and is represented by a straight line in the diagram.

It is thus seen that pressure regulation in an ink jet printer according to the invention is achieved by the provision of simple and cost effective means incorporated in the printer. This renders the invention particularly suitable for use in small and compact hand¬ held ink jet printers operating according to the drop- on-demand principle.