Simplified ink jet head

Abstract: A deaeration member (128) for an ink jet head comprises a tubular member made of air-permeable, ink-impermeable material and adapted to be inserted into an ink passage in an ink jet head. One end of the tubular member (128) is sealed and the other end of the tubular member being connectable to a source (132) of subatmospheric pressure. The member (128) may be inserted into a carbon pressure plate (102) of a piezoelectric ink jet head.

Description: This invention relates to a deaeration member for an ink jet head and to an ink jet head provided with an ink deaerator. US Patent 4940995 explains that in many ink jet systems ink is supplied to a chamber or passage connected to an orifice from which the ink is ejected drop by drop as a result of successive cycles of decreased and increased pressure applied to the ink in the passage. If the ink introduced into the passage contains dissolved air or other gas, decompression of the ink during reduced-pressure portions of the pressure cycle may cause the dissolved gas to form small bubbles in the ink within the passage. Repeated decompression of the ink in the chamber causes these bubbles to grow and such bubbles can produce malfunction of the ink jet apparatus. In order to overcome this difficulty the ink in the ink jet system is subjected to a reduced pressure applied through a membrane which is permeable to gas but not to ink and according to a feature of the aforesaid US patent the membrane is made of a fluorine containing sheet material which may be fluorosilicone or a fluorocarbon. The invention provides an ink jet head including a carbon pressure chamber plate that has an ink deaeration passage through which ink is supplied to the inlet passages leading to the pressure chambers. An internally supported, thin-walled tubular member made of air-permeable, ink-impermeable material connected at one end to a source of reduced pressure is inserted into the ink deaeration passage to provide a unitary ink deareating and pressure chamber carbon plate. This arrangement accomplishes the necessary deaeration of ink immediately before it is supplied to the pressure chambers with minimal space requirements and without necessitating recirculation of ink to an ink reservoir. In one aspect of the invention comprises a deaeration member as set out in claim 1 of the accompanying claims. In another aspect the invention provides an ink jet head provided with a deaerator and as set out in claim 5 of the accompanying claims. In addition to its other advantages disclosed in WO95/2563 a carbon plate is porous, preferably being about 80-90% dense, and the porosity and a vacuum source communicating with the surface of the plate can extract dissolved air from ink in the ink passages separated from the porous carbon material by an air-permeable, ink-impermeable layer. Moreover, the carbon plate may have orifice passages formed in an edge of the plate rather than in one of the sides of the plate. Since engineering grade carbon is friable, i.e., microscopic grains are readily broken away from a carbon body, it is easily shaped without producing burrs. As described in "Graphite Machinery Made Easy", EDM Today, Sept./Oct. 1993 pp. 24ff, the relative softness and lack of ductility of such carbon allows it to be cut at high feed rates with little distortion and low tool wear. These characteristics permit carbon blocks to be readily formed into components of ink jet heads by conventional or unique machining techniques. How the invention may be put into effect will now be described, by way of example only, in the accompanying drawings, in which Fig. 1 is an exploded perspective view illustrating a representative embodiment of a simplified ink jet head and deareation member in accordance with the invention; Fig. 2 is a side view illustrating a representative carbon pressure chamber plate of the type used in the arrangement shown in Fig. 1; Fig. 3 is an end view of the representative carbon pressure chamber plate shown in Fig. 2; Fig. 4 is a top view of the carbon collar block used in the arrangement shown in Fig. 1; Fig. 5 is a side view of the collar block shown in Fig. 4; Fig. 6 is a plan view showing one side of a representative carbon manifold plate of the type used in the arrangement shown in Fig 1; Fig. 7 is a plan view showing the opposite side of the manifold plate of Fig. 6; and Fig 8 is a perspective view showing a typical shaping tool for shaping arrays of ink passages in a carbon body for use in an ink jet head. In an embodiment of the invention shown in Figs 1 - 7, an ink jet head is assembled from a plurality of carbon components. A carbon plate forming one of the components may be machined by micromachining techniques from opposite sides to produce chambers and passages required for the ink jet head. The plate may be made of engineering carbon graphite, which is preferably about 80-90% dense, providing a slightly porous plate structure. The carbon plate can be machined by milling, drilling, broaching, grinding and the like, using conventional tools providing high material removal rates with minimum tool wear, to produce openings with much closer tolerances than the conventional metal plates of the type described, for example, in the Hoisington et al. Patent No. 4,835,554. Because the carbon material is friable, no burrs are produced during machining. Moreover, the coefficient of thermal expansion of the carbon graphite body is substantially the same as that of the ceramic piezoelectric material of which the piezoelectric plate 23 is made so as to reduce or substantially eliminate thermal stresses which occur between those components of the head as a result of variations in temperature. The preferred carbon material for use in forming components on ink jet heads is polycrystalline graphite, which is a mixture of small crystals of graphite sintered with amorphous carbon (lamp black). This produces an amorphous matrix of small (0.025-5µ) grains and smaller (0.005-0.2µ) pores. This material, which is different from powdered graphite and carbon fiber materials, offers many benefits, including good thermal conductivity, coefficient of thermal expansion close to ceramic piezoelectric materials, good machinability, dimensional stability and chemical inertness. The thermal properties of polycrystalline graphite (Grade DFP-1 available from POCO Graphite, Inc., Decatur, Texas) and other materials which might be considered for printheads are compared with those of lead zinc titanate (PZT) ceramic piezoelectric material in Table 1 below: Material Conductivity (W/CmK) Thermal Expansion (µ/m/degC) Modulus (x105kg/cm_) PZT 0.015 2 to 4 7 Thermoplastic (Ultem) 0.0022 56 0.15 Aluminum (6000) 1.7 23.4 7 Carbon (DFP-1) 0.75 8.4 1.1 The Ultem (as well as other thermoplastics) has both poor conductivity and a very high thermal expansion coefficient. The conductivity of aluminum is attractive, but its high thermal expansion and modulus are problems. Polycrystalline carbon offers a good combination of all three properties. A potentially prohibitive aspect of the use of carbon members for ink jet components is the forming of closely spaced arrays of pressure chambers and other multitudinous, long aspect ratio, channels with precise dimensions. Using an end mill for these features could be expected to result in excessively long machine cycle times. To overcome this problem, a desired array of adjacent channel profiles is shaped in the surface of a carbon body by a specially formed tool 95, shown in Fig. 8, in a series of repeated linear motions or "scrapes." In the tool 95 for example, an array of 64 short uniformly spaced teeth 96 may be provided at one end of the tool to cut 64 parallel pressure chambers in the surface of a carbon plate. If the tool cuts at 0.025mm per scrape, a depth of 0.15mm can be achieved in 6 scrapes, which requires only a few seconds of machine time. The tool 95 makes an array of channels equal to the width of the tool, and can make a wider array by taking repeat adjacent passes. Tool irregularities are averaged out by taking finish cuts in the reverse direction or at a one-tooth offset. Reversing the tool also allows the formation of steeper channel ends when this is required. This technique can be used to make pumping chambers, manifold passages, flow-through passages, and the like. Finally, channels of variable or tapered depth can be made as well, by raising or lowering the tool as a cut is being made. For an array of closely spaced deep, small diameter holes in a carbon body, where the depth is more than 3 times the drill diameter, drilling can become difficult and expensive. To provide an array of closely spaced holes having a diameter of, for example, 0.28mm through a carbon plate 1.75mm thick would require a great deal of time and a number of expensive drills. To form such an array in a simple manner, matching arrays of channels (which may be semi-hexagonal in shape) 0.14mm deep are formed in two matching blocks of carbon which are then bonded together with adhesive to form a single block containing an array of parallel, long 0.28mm diameter holes. This block is then sliced perpendicularly to the axes of the holes and ground flat to form the array bodies. The adhesive joint down the middle of the array body has been found to be very strong. Polycrystalline carbon is easily bonded by a variety of adhesives. Three systems are particularly compatible with carbon in hot melt ink jet head applications. The first is a simple dispensed epoxy which works well for coarse scale joints. The second is a thermoplastic sheet adhesive. Using this technique, a thin teflon (TFE) sheet compressed at elevated temperature and pressure provides a tenacious bond between flat surfaces of polycrystalline carbon. Similarly, acrylic sheet adhesives should perform well at lower temperature. In the third technique, a dilute sprayed B-stage epoxy system is applied to the surfaces to be bonded. This has been shown to be adaptable to the complex geometries of ink jet printheads, yet high in strength at elevated temperatures. There is some challenge in bonding a porous material like carbon with a spray application technique. Since excess adhesive will clog small passages, and thin layers may be drawn by capillary forces into the carbon body pores, careful control of process variables is required. In particular, the carbon pore structure must be uniform, the spray-deposited layer thickness must be small compared to the particle/pore size, and the heat cure process must be tuned to the adhesive rheology. In addition, a carbon plate is especially advantageous for ink jet heads used with hot melt ink. Because of its high thermal conductivity, the carbon plate provides excellent heat conduction from heaters mounted at relatively remote locations in the head to all of the ink passages in the head. This assures that the hot melt ink at each of the orifices is at the same temperature and therefore has the same viscosity, thereby providing good uniformity of operation throughout the length of the array of orifices. In certain high-frequency ink jet applications, the rigidity of the walls of the pressure chambers formed in the carbon plate may be less than desired, requiring a higher operating voltage for the piezoelectric actuating plate. To alleviate this, the surfaces of the pressure chambers formed in the carbon plate may be coated with a thin layer, such as 0.01 to 0.1mm thick, of a very hard ( i.e . high modulus of rigidity) material such as a carbide or nitride, e.g., silicon carbide or nitride, boron carbide or nitride, tungsten carbide or nitride, tantalum carbide or nitride, or the like. Preferably, the coating is applied by chemical vapor deposition. In this embodiment, as illustrated in the exploded view of Fig. 1, two carbon pressure plate assemblies 102, described in greater detail hereinafter, are assembled in a carbon collar 103 so that their end surfaces, which contain ink outlet passages, are aligned with corresponding openings in a manifold plate 104 to which the pressure chamber plate assemblies 102 are affixed by screws 105 extending through the manifold plate and into an adjacent pressure chamber plate 106. An orifice plate 107 has a linear array of closely spaced orifices 108 which are aligned with the ends of arrays of passages 109 (Fig. 7) in the manifold plate 104 so as to eject ink in response to selective actuation of the pressure plate assemblies. Interposed between the ends of the pressure plate assemblies 102 and the manifold plate 104 is a filter layer 110 having pores or openings slightly smaller than the orifices 108 in the orifice plate 107 so as to prevent potentially orifice-clogging solid material from reaching the orifices 108 but large enough to permit particles of solid material smaller than the size of the orifices to pass through the filter layer. This type of filter is described, for example, in the copending Moynihan et al. application for "Filter Arrangement For Ink Jet Head" Serial No. 08/231,102, filed April 22, 1994 (now US Patent 5724082). An ink reservoir 111, mounted against one side of the collar 103 has an ink supply opening 112 which supplies ink to the collar 103. As best seen in Fig. 5, a corresponding opening 113 in the collar is aligned with the opening 112 to receive ink from the reservoir 111. In addition, if hot melt ink is to be used in the ink jet head, a cartridge heater 114 is mounted in a groove 115 formed in the side of the reservoir and in a corresponding groove 116 (Fig. 5) in the side of the collar 103 and is controlled so as to maintain the ink within the assembled ink jet head at a desired temperature during operation of the system. Each pressure chamber assembly 102 includes a pressure plate 106 having arrays 117 of closely spaced pressure chambers formed on opposite sides of the plate 106 and each of those arrays is covered by a piezoelectric plate 118 of the type described with respect to Fig. 4 of our WO 95/25637, having an array of electrodes 119 arranged with respect to the array of pressure chambers 117 to change the volume of the corresponding pressure chamber in response to appropriate electrical signals. When selectively activated the piezoelectric plate 118 is mechanically distorted in shear mode in the direction toward the adjacent pressure chamber 20 to cause ejection of an ink drop from the orifices with which that pressure chamber communicates. Shear-mode operation of a piezoelectric plate is described, for example, in Fischbeck et al US Patent No 4,584,590. Such shear-mode operation does not require any electrode on the opposite site of the piezoelecxyric plate but, of desired, the carbon plate, being conductive, can be used to prove an electrode on the opposite side of the plate. The pressure chamber plate 106, which is illustrated in greater detail in Figs. 2 and 7, has a longitudinally extending opening 120 which, in the illustrated embodiment receives ink through an internal passage 123 terminating at an end surface 124 which faces the manifold plate 104 as seen in Fig. 1. As shown in Fig. 1, the surface of the manifold plate 104 facing the pressure chamber plate, has an opening 125 which receives ink from the collar passage 113 and supplies it to a groove 126 (Fig. 7) on the opposite side of the manifold plate, from which ink passes through two further openings 127 in the manifold plate to the passages 123 in the pressure plates 106 so that the ink is distributed through the longitudinal opening 120 to all of the pressure chambers in both of the arrays 117 in each plate. In order to extract dissolved air from the ink as it is being supplied to the arrays 117 of pressure chambers, a deareator 128 consisting of a tubular member 129 made of air-permeable, ink-impermeable material, such as extruded polytetrafluoroethylene having a 0.1mm wall thickness and a 1.5mm internal diameter, extends through an opening 130 in the edge of each pressure chamber plate 106 and into the longitudinal opening 120. A plug 131 closes the inner end of the tube and the end projecting out of the opening 130 in the plate 106 is connected to a vacuum source 132 supplying sufficient negative pressure, such as 0.7 atmosphere, to reduce the dissolved air content of the ink being supplied to the pressure chambers below the level at which air bubbles can form in the pressure chamber during operation of the ink jet system. In order to prevent the tube 129 from collapsing in response to application of negative pressure, a porous support, such as a rod of porous carbon or a helical wire having a diameter substantially equal to the internal diameter of the tube, is inserted into the tube. As shown in Fig. 3, the end surface 124 of the carbon plate 106 has two arrays of ink passages 133 which extend perpendicularly to the end surface 123 and each of those passages communicates internally with the adjacent end of a corresponding pressure chamber in the arrays 117. Consequently, upon actuation of one of the pressure chambers, ink is forced out of the plate 106 through a corresponding one of the passages 133. After passing through the filter layer 110, ink from each of the passages 133 is supplied through a corresponding passage 134 in an adjacent surface of the manifold plate 104 shown in Fig. 4 and, as shown in Fig. 7, the arrays of passages 109 in the opposite surface of the manifold plate extend horizontally along the surface of that plate to convey the ink supplied through the passages 134 in a lateral direction toward the center of the manifold plate. Those passages terminate in a central line 135 extending longitudinally along the manifold plate so as to be in line with the line of ink jet orifices 108 in the orifice plate 107. Although carbon is the preferred material for the manifold plate 104, especially for ink jet heads used with hot melt ink, other materials which can be formed with a sufficiently flat surface and which have a thermal expansion coefficient compatible with adjacent components may also be used. For example, steel and ceramics such as alumina and glass, in which appropriate passages can be formed by photoetching, may also be used to form the manifold plate. In a typical embodiment of the type shown in Figs. 1-7, each pressure chamber plate 106 is approximately 75mm long, 22mm wide and 2.5mm thick and each pressure chamber array 117 contains 64 pressure chamber approximately 9mm long, 1mm wide and 0.15mm deep and the manifold plate 104 is approximately 1.4mm thick. Heretofore it was believed that the total length of the descender, which is the ink path leading from the end of the pressure chamber to the orifice in the orifice plate, should be as short as possible, i.e. no more than about 1mm. Although it is clear that each descender should have a constant cross-section similar to that of the pumping chamber so that its acoustic properties do not result in undesirable reflections, and that it should be short enough that viscous flow losses are not excessive and that it should also be fluidically stiff so that pressure energy losses from the surrounding structure are not excessive, it has now been determined that, in ink jet heads made of carbon components, the descender need not be so limited in length and can consist of a plurality of passages such as those in the manifold plate and within the pressure chamber plate which total as much as 7mm in length without loss of performance. This permits greater flexibility in the design of ink jet heads in several respects. For example, the piezoelectric plate, which is quite fragile, can be spaced a significantly greater distance away from the substrate being printed by the head and the body of the ink jet array may be made thick enough to be mechanically robust and to provide good thermal uniformity. Moreover, laterally spaced pressure chambers such as those in the arrays 117 may be connected through laterally spaced passages 133 to supply ink to a single line of orifices 108 by using an arrangement of laterally directed passages 109 such as that incorporated into the manifold plate 104.

Claims: A deaeration member (128) for an ink jet head made of air-permeable, ink-impermeable material characterized in that the member is tubular and is adapted to be inserted into an ink passage (12) in an ink jet head, one end (131) of the tubular member being sealed and the other end of the tubular member being connectable to a source (132) of subatmospheric pressure. The deaeration member of claim 1, wherein the tubular member (129) is made of polytetrafluoroethylene The deaeration member of claim 1 or 2, including a porous support member disposed within the tubular member. The deaeration member of any of claims 1 -3, wherein the tubular member (120) has a 0.1mm wall thickness and a 1.5mm diameter. An ink jet head provided with an ink deaerator of an air-permeable ink-impermeable material, characterized in that the ink jet head includes a carbon pressure chamber plate (106) having an ink deaeration passage (120) for supply of ink therethrough to inlet passages leading to pressure chambers (117) and in that the deaerator comprises an internally supported, thin-walled tubular member (129) of air-permeable, ink-impermeable material located within the ink deaeration passage for connection at one end to a source (132) of reduced pressure. The head of claim 5, wherein the plate (105) is of polycrystalline graphite. The head of claim 5 or 6 where the plate is about 80%-90% dense. The head of claim 5, 6 or 7 wherein the tubular member (129) is of polytetrafluoroethylene. An ink jet head according to any of claims 5 - 8, filled with hot melt ink.

Inventors: ADAMS-BRADY DAVID, BARSS STEVEN H, BIGGS MELVIN L, GAILUS DAVID W, HINE NATHAN P, HOISINGTON PAUL A, MACKAY DIANE, MACKAY STEPHEN C, MCDONALD MARLENE M, MOYNIHAN EDWARD R, PALIFKA ROBERT G, PAULSON BRUCE A, ADAMS-BRADY, DAVID, BARSS, STEVEN H., BIGGS, MELVIN L., GAILUS, DAVID W., HINE, NATHAN P., HOISINGTON, PAUL A., MACKAY, DIANE, MACKAY, STEPHEN C., MCDONALD, MARLENE M., MOYNIHAN, EDWARD R., PALIFKA, ROBERT G., PAULSON, BRUCE A.
Applicants: SPECTRA INC, SPECTRA, INC.
Application year: 1995
Main classification: B41J2
IPC codes: B41J2, H01L41