PRESSURE CHAMBER FOR INK JET SYSTEMS

Abstract: In the particular embodiments of the invention described in the specification, the pressure chamber (18) for an ink jet system is coated with a thin, conforming layer (25) of a coating material, such as epoxy, urethane, silicone, fluoroethylene or xylylene polymer material to eliminate nucleation sites in the surfaces forming the walls of the chamber and thereby inhibit formation of bubbles from dissolved air contained in ink within the chamber when the ink is subjected to reduced pressure during operation of the ink jet system.

Description: Description Pressure Chamber for Ink Jet Systems Technical Field This invention relates to ink jet systems utiliz- ing pressure chambers and, more particularly, to a new and improved ink jet system having a pressure chamber arranged to inhibit formation of air bubbles therein. Background Art In many ink jet systems, ink is supplied through a supply duct to a pressure chamber which communicates with an outlet orifice, and ink is ejected periodi¬ cally from the orifice by a rapid contraction of the volume of the compression chamber as a result of ac¬ tion by an electromechanical transducer, such as a piezoelectric element. The rapid contraction is pre¬ ceded or followed by a correspondingly rapid expansion of the chamber volume. During the expansion portion of the ink drop ejection cycle, the pressure of the ink in the pressure chamber is reduced significantly, increasing the tendency of any air dissolved in the ink within the chamber to form bubbles on the surface of the chamber. Bubbles tend to form in that manner especially at nucleation sites in the chamber such as sharp corners, minute cracks or pits, or foreign par- tides deposited on the chamber surface, where gases can be retained. Because the presence of gas bubbles within the pressure chamber prevents application of pressure to the ink in the desired manner to eject an ink drop of selected volume from the orifice at a selected time, it is important to avoid the formation of such bubbles in the pressure chamber of an ink jet system. Disclosure of Invention Accordingly, it is an object of the present in¬ vention to provide a new and improved ink jet system having a pressure chamber arranged to inhibit the formation of air bubbles. Another object of the invention is to provide a method for producing a pressure chamber for an ink jet system which is effective to inhibit the formation of air bubbles during ink jet operation. These and other objects of the invention are attained by providing an ink jet system having a pres¬ sure chamber connected to an ink jet orifice and com¬ municating with an ink supply duct in which the sur¬ face of the pressure chamber is coated with a layer of material providing a smooth, continuous, ink-wettable surface conforming to the configuration of the chamber walls. Preferably, the coating material is an organic substance which can be introduced conveniently into the chamber of an assembled ink jet system and form a conforming coating on the chamber walls which has a low affinity for dirt or solid particulate material that may be contained in the ink used in the system. Preferred materials include epoxy, silicone, urethane and fluoroethylene polymers and the like and especi- ally preferred are xylylene polymer materials. Further objects and advantages of the invention will be apparent from a reading of the following de¬ scription in conjunction with the accompanying draw¬ ings in which: Brief Description of Drawings Fig. 1 is a schematic fragmentary view in longi¬ tudinal section illustrating the arrangement of a pressure chamber and its connections to an ink jet orifice and a supply duct in a typical conventional ink jet system; and Fig. 2 is a view similar to that of Fig. 1, il¬ lustrating a representative pressure chamber for an ink jet system arranged in accordance with the inven¬ tion. Best Mode for Carrying Out the Invention In the schematic representation of a typical conventional ink jet system shown in Fig. 1, an ink jet head is conveniently assembled from a series of plate-like elements arranged in sandwich form to pro- duce a composite structure. Thus, an orifice plate 10 has an ink jet orifice 11 which communicates through aligned apertures 12, 13 and 14, respectively, in a membrane plate 15, a cavity plate 16 and a stiffener plate 17 leading to a pressure chamber 18 formed by an opening in a pressure chamber plate 19. The thickness of the pressure chamber plate 19 may be about 0.076mm, for example, and the pressure chamber 18 may be about lmm wide and about 9.5mm long. One side wall of the pressure chamber 18 is provided by the stiffener plate 17 and the opposite side wall is provided by a pie¬ zoelectric transducer 20 which moves toward or away from the plate 17 in response to electrical signals as described, for example, in the Fischbeck et al. Patent No. 4,584,590. The orifice plate 10 and the plates 15, 16 and 17 may have thicknesses of from about 0.025mm to 0.25mm each, and the apertures 12, 13 and 14 may be, for example, about 0.13mm to 0.25mm in diameter. At the other end of the pressure chamber 18, the stiffener plate 17 has an aperture 21 which may be, for example, about 0.13mm to 0.25mm in diameter, lead¬ ing to a cavity 22 in the cavity plate 16 which is connected to an ink supply duct (not shown). When the ink jet system is in operation, ink 23 fills the cav- ity 22, the aperture 21, the pressure chamber 18, the apertures 12, 13 and 14, and part of the orifice 11 in the orifice plate 10 where a meniscus is formed which normally resists any flow of ink out of the orifice. At the meniscus, however, the ink in contact with the atmosphere absorbs air and dissolved air will be dis¬ tributed through the ink in the apertures 12, 13 and 14 and into the ink in the pressure chamber 18. Thereafter, when the wall of the pressure chamber 18 formed by the piezoelectric transducer 20 moves away from the stiffener plate 17, expanding the pres¬ sure chamber to draw in ink from the cavity 22, the resulting reduction of pressure on the ink in the chamber 18 tends to produce cavitation as a result of the dissolved air, which can cause air bubbles 24 to form at nucleation sites within the chamber. Such nucleation sites may be provided by sharp discontinu- ities, such as cracks, pits or corners formed at the line of contact between adjacent plates, they may also be provided by particulate or other contamination deposited on the walls of the pressure chamber. Be¬ cause of the presence of such nucleation sites in conventional pressure chambers, there will be a tend¬ ency for bubbles to form in the pressure chamber when¬ ever air dissolved in the ink is subjected to reduced pressure during operation of the ink jet system. In accordance with the present invention, the tendency of dissolved air to form bubbles in the pres¬ sure chamber during operation of the system is sub¬ stantially eliminated by providing a coating on the surface of the pressure chamber of a material which conforms to the surface configuration of the chamber, but fills up or smooths out microscopic discontinu¬ ities, such as pits, cracks and sharp corners, in the surface of the pressure chamber walls. A typical arrangement according to the invention is shown in Fig. 2 wherein a thin, continuous coating 25 covers the walls of the chamber 18 and extends into the aper¬ tures 12, 13, 14 and 21 as well as the cavity 22. Thus, nucleation sites in the pressure chamber and adjacent regions are eliminated. To be effective for this purpose, the coating material should provide a pinhole-free, mechanically flexible coating having a clean and wettable surface, i.e.. , a surface with high surface energy. Preferably, the surface should also be nonconductive electrically. Also, to assure a smooth, continuous surface on the interior of the pressure chamber which is free of microscopic discontinuities, the surface coating should preferably be applied after the pressure cham- ber and its related connections to the ink jet orifice and the ink supply duct have been assembled. Other¬ wise, discontinuities may appear, for example, between the coatings on the surfaces of the separate plates which are assembled to form the pressure chamber and related ink ducts. Thus, the material from which the coating is made should preferably comprise a fluid such as a liquid which may be passed through the ducts and apertures into the pressure chamber to leave a thin, uniform coating on the surfaces, or a material which can be passed through the system in vapor or suspended particulate form to condense or deposit on the surfaces and coagulate or coalesce into a uniform, smooth coating. To provide the necessary electrical, mechanical and surface properties, coating materials, such as epoxy, urethane, silicone and fluoroethylene polymers and similar materials are preferred. Especially pre¬ ferred are the xylylene polymer materials, such as poly(p_-xylylene) and poly(chloro-p-xylylene) which can be produced by vaporizing the dimer form to form a vapor which polymerizes upon condensation to form a uniform conforming thin-film polymer coating having the desired electrical, mechanical and surface proper¬ ties. Since thin layers or films of xylylene polymers can be deposited from the vapor phase in a nondirec- tional manner, the pressure chamber in an ink jet system can be provided with a uniform thin conforming coating of such polymer materials after assembly of the ink jet head by exposing the ink jet system to the vapor phase of the xylylene material. To provide a thin, conforming xylylene polymer coating 25 on the walls of a pressure chamber such as the chamber 18 shown in Fig. 2, the ink jet head as¬ sembly consisting of the plates 15, 16, 17, 19 and 20, preferably with the orifice plate 10 removed, is sub¬ jected to a reduced pressure such as about 0.1 torr. The dimer form of the desired xylylene material, such as dichlorodi-p_-xylylene, which is available com¬ mercially under the name parylene D, is vaporized at about 250°C at a pressure of 1 torr and heated to about 600°C at 0.1 torr to produce the monomer form which is then applied to an ink jet head assembly maintained at about 25°C. On contact with the sur¬ faces of the ink jet assembly, the monomer condenses and polymerizes to form a continuous thin conforming coating on the surfaces. Preferably, the conforming coating on the sur¬ faces forming the pressure chamber should be from about 0.1 to about 5 microns thick and, most prefer¬ ably, between about 0.2 and about 2 microns thick. Since poly(chloro-p_-xylylene) is normally deposited from vapor at a rate of about 0.5 microns per minute at room temperature, a 2-micron-thick layer 25 can be coated on the walls of the pressure chamber 18 in about 4 minutes. Poly(p-xylylene) layers form more slowly and may require considerably more time to at- tain the same thickness under the same conditions. With a thin, continuous conforming organic coat¬ ing layer of the type described herein on the walls of a pressure chamber, nucleation sites which lead to formation of bubbles when ink containing dissolved air is subjected to reduced pressure are substantially eliminated. As a result, ink containing some dis¬ solved air can be subjected to greater pressure re¬ duction without causing bubble formation in the pres- sure chamber, or ink containing an increased amount of dissolved air can be subjected to the same pressure reduction which would otherwise produce bubbles in the pressure chamber. Consequently, the improved pressure chamber for an ink jet system according to the present invention which effectively inhibits formation of air bubbles overcomes disadvantages of present ink jet systems and permits operation of ink jet systems over a wider range of conditions. 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 vari¬ ations and modifications are included within the in- tended scope of the invention.

Claims: Claims 1. A pressure chamber for an ink jet system compris¬ ing a chamber formed by a plurality of wall seg¬ ments, first aperture means extending through a wall segment and communicating with an ink jet orifice, second aperture means extending through a wall segment and communicating with an ink supply duct, and a layer of coating material forming a smooth, continuous, impermeable coating conforming to the configuration of the wall seg¬ ments of the chamber, thereby eliminating nuclea¬ tion sites for bubble formation when ink contain¬ ing dissolved air within the chamber is subjected to a reduced pressure. 2. A pressure chamber according to Claim 1 wherein the coating on the wall segments is between about 0.1 and about 5 microns thick. 3. A pressure chamber according to Claim 2 wherein the coating on the wall segments is between about 0.2 and about 2 microns thick. 4. A pressure chamber according to Claim 1 wherein the coating is formed of a polymer material. 5. A pressure chamber according to Claim 1 wherein the coating is formed of a material selected from the group consisting of epoxy, urethane, sili¬ cone, fluoroethylene and xylylene polymer mate¬ rials. 6. A pressure chamber according to Claim 5 wherein the coating is formed of a xylylene polymer mate- rial. 7. A pressure chamber according to Claim 6 wherein the coating is formed of poly(p-xylylene) . 8. A pressure chamber according to Claim 6 wherein the coating is formed of poly(chloro-£-xylylene) . 9. A method for preparing a pressure chamber for an ink jet system comprising assembling a plurality of plate-like components to form a chamber having a plurality of wall surfaces formed with a first aperture for communication with an ink jet ori- fice and a second aperture for communication with an ink supply duct, and introducing a fluid coat¬ ing material into the chamber so as to deposit a thin, continuous, conforming coating of the mate¬ rial on the wall surfaces of the chamber. 10. A method according to Claim 9 wherein the fluid material introduced into the chamber is selected from the group consisting of epoxy, urethane, silicone, fluoroethylene and xylylene materials. 11. A method according to Claim 10 including the step of vaporizing a xylylene material and introducing the xylylene vapor into the pressure chamber and depositing a coating of xylylene polymer material on the wall surfaces of the pressure chamber. 12. A method according to Claim 11 wherein the coat- ing deposited on the chamber wall surfaces is poly(chloro-E-xylylene) . 13. A method according to Claim 11 wherein the coat¬ ing deposited on the chamber wall surfaces is poly(j≥-xylylene) .

Inventors: MOYNIHAN EDWARD R, MOYNIHAN, EDWARD, R.
Applicants: SPECTRA INC, SPECTRA, INC.
Application year: 1989
Main classification: G01D15
IPC codes: B41J2