ION PROJECTION PRINTER HEAD

Abstract: A fluid assisted ion-projection printer head comprising a one-piece body including a generally U-shaped cavity (44), to which is mated a generally featureless, planar, conductive member (42) which forms a closure for the major portion of the cavity opening and defines an ion generation chamber and a cavity exit region therewith. At least one wall of the cavity adjacent the exit region is electrically conductive. A conductive wire (40), supported on the body, extends along the cavity and is located closer to the one wall and to the conductive member than to any of the other walls of the cavity.

Description: This invention relates to a low-cost, easily manufactured, highly efficient, fluid-­ assisted ion projection printer head. The head comprises a one-piece conductive body which can be easily cast and which mates with a substantially flat electroconductive plate. In US-A-4 463 363 and US-A-4 524 371 there is disclosed an ion generation chamber through which air is moved for entraining ions generated therein and for transporting them through an exit channel including an ion modulation region for subsequent deposition upon a latent image receptor. In US-A-4 463 363, the entire exit channel, including the modulation region, forms a straight path extending from the ion generation chamber to the image receptor. In US-A- 4 524 371, the improvements over the ′363 structure resides in the exit channel defining a bent path through which the ions flow, in order to allow the ion modulation control elements to be fabricated upon a planar substrate. In both of these patents the ion generation chamber is formed as a substantially cylindrical cavity within which the corona wire is centrally located. It was believed that the cylindrical configuration was necessary in order to obtain a stable corona discharge from the corona wire. The high electrical fields established between the axially mounted corona wire, maintained at several thousand volts d.c., and the equidistant conductive walls of the cavity, were expected to cause arcing to any portion of the cavity walls which were non-smooth, or to any corners therein where electrical lines of force would be concentrated. However, it is extremely expensive to construct a head having the cylindrical cavity therein, since such a construction requires the head to be made up of two precisely-mating parts. Since the two parts must be properly aligned and must acurately fit together, dimensional tolerances are critical. Furthermore, the correct inlet and outlet openings leading to and from the cavity had to be accurately controlled in order to avoid non-uniformities in corona current output. It appeared to be inevitable that the cost of the printing heads would be high because of these stringent manufacturing requirements. Therefore, it is a primary object of the present invention to provide a fluid jet assisted ion projection printer head design which would be easily manufactured at low cost. Fortuitously it was discovered that a one-piece configuration, which is inherently easier and less expensive to manufacture, was also more efficient in its delivery of corona current. The present invention may be carried out in one form by providing a fluid flow assisted ion projection printer head including a body defining an elongated cavity therein, within which a conductive wire is supported. The cavity encloses the wire on three sides and one of the sides comprises an electrically conductive wall. An opening in the body passes through one of the walls of the cavity for introducing a transport fluid. The major portion of the cavity opening is closed by a planar electrically conductive plate against which a second planar member, supporting electronic control elements, is held and is separated therefrom by an intermediate dielectric member. The wire is located closer to the conductive wall and the conductive plate than to any of the other walls of the cavity for concentrating the major portion of electrical field between the wire and these elements, as opposed to any other portions of the cavity walls, when the wire is connected to a source of electrical potential. The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a partial sectional elevation view showing a known fluid assisted ion projection printer head; Figure 2 is a perspective view showing an ion projection printer head of the present invention; Figure 3 is a sectional elevation view of the head shown in Figure 2; Figure 4 is an enlarged sectional elevation view, showing the ion-generation cavity; Figure 5 is a further enlarged sectional elevation view showing the electrical lines of force in the corona-generation area of the printer head, and Figure 6 is an enlarged sectional elevation view, similar to that of Figure 5, showing modifications in the corona generation area of the printer head. There is shown in Figure 1 a fluid flow assisted ion projection printer head 10 of the form described in US-A-4 463 363 and 4 524 371. Within the housing 10 is an ion generation region including an electrically conductive cylindrical 12, a corona wire 14 extending substantially coaxially in the cavity to which a high potential source (not shown) is connected. A source of reference potential (also not shown) is connected to the housing. Fluid transport material, such as air, is delivered into the cavity 12 through an axially extending inlet channel 16, from a suitable source, schematically represented by tube 18. An axially extending exit channel 20 conducts the transport fluid and the ions entrained therein from the corona cavity 12 to the exterior of the printing head 10 via a bent path comprising a cavity exit region 22 and an ion modulation region 24. The ions allowed to leave the printer head come under the influence of an electrically conductive acceleration electrode 26 which attracts them in order that they may be deposited upon the surface of dielectric layer 28 coated thereon. A high potential electrical source (not shown), on the order of several thousand volts d.c., of a sign opposite to that of the corona potential, is connected to the acceleration electrode. Typically, the diameter of the ion generation cavity 12 has been on the order of 3.125 mm. Considering the Figure 1 structure at that scale, it should be apparent that, in order for the cavity exit region 22 to be relatively short, so as to control the ions in the ion modulation region 24, the thickness of the housing walls adjacent the cavity exit channel, identified as areas "a" and "b" would be exceedingly thin, and thereby lead to severe manufacturing limitations. Further reduction of the cavity diameter will exacerbate this problem. Additionally, since the head 10 can only be practically made and assembled in two halves, it will be apparent that accurate alignment and spacing thereof, in order to create a symmetrical cavity and the proper gap dimensions, for inlet and exit channel, will add substantially to manufacturing costs. The present invention is based upon the desire to reduce manufacturing costs by designing a fluid assisted ion projection printer head made in one piece, to which a planar, featureless, cover plate may be simply attached. Surprisingly, the result of this design effort yielded a printer head with significantly higher output current, which brought with it other advantages. Turning now to Figures 2 through 6, there is illustrated the printer 30 comprising a casting of electrically conductive material. The head is made of stainless steel but any electroconductive material would be satisfactory, as long as it will not be affected by extended exposure to a corona discharge. The upper portion of the printer head comprises a plenum chamber 32 to which is secured a fluid delivery casing 34. An entrance channel 36 receives the low pressure fluid (preferably air) from the plenum chamber and delivers it to the ion generation cavity 38. The entrance channel should have a large enough cross-sectional area to ensure that the pressure drop therethrough will be small. Cavity 38 has a generally U-shaped cross-section, with its three sides surrounding a corona wire 40. Suitable wire mounting supports are provided at opposite ends of the housing for mounting the wire at a predetermined location within the cavity. By mounting the wire ends on eccentric supports, relative to the housing, some limited adjustment of the wire location is made possible. A planar conductive plate 42, typically 0.3 mm thick, closes the major portion of the U-shaped cavity, forming an ion generation chamber 44 and leaving a cavity exit region 46 between the end of the conductive plate and the adjacent wall 48. Although a head of this construction is also formed of two parts, only one has features thereon and the other is featureless. Therefore, the cost of manufacturing, to enable assembly to tight tolerances, is greatly minimized. A planar substrate 50, typically 1 mm thick, upon which the electronic control elements are supported, is held adjacent the conductive plate 42 by an elongated spring clip 52. The spring clip 52 extends substantially across the head and is held in place by a mounting end 54 secured upon a rod 56 which spans the head from end-to-end in side plates 58 (only one shown). A force applying end 60 of the spring clip, urges the planar substrate 50 and the conductive plate 42 against the head body. The spring clip 52 should exert sufficient force to flatten irregularities in both the subdstrate 50 and the conductive plate 42 in order to ensure a uniform ion current output from end-to-end across the head. We have found that a force of about one kg works satisfactorily. A pair of extensions on the side plates form wiping shoes 62 (only one shown) which ride upon the outboard edges of the image receptor 62 so that the proper spacing is established between the head and the image receptor. When properly positioned on the head, by means of suitable locating lugs (not shown), the conductive plate 42 and the substrate 50 are each cantilever mounted so that they define, in conjunction with the head, an exit channel 66 including the cavity exit region 46 (about 0.25 mm long) and an ion modulation region 68 (about 0.50 mm long). Air flow through the head is generally represented by the arrows in Figure 2, which illustrate the entry of air through the fluid delivery casing 34 and the plenum chamber 32, into the ion generation chamber 44 through entrance channel 36 and out of the ion generation chamber through exit channel 66. In Figure 4 the features of the ion generation chamber 44 are most readily observable. In this enlarged view, it can be seen that two layers are interposed between the planar substrate 50 and the conductive plate 42. Preferably the substrate is a large area marking chip comprising a glass plate upon which are integrally fabricated thin film modulating electrodes, conductive traces and transistors. All the thin film elements are represented by layer 70. An insulation layer 72 overcoats the thin film layer to isolate it electrically from the conductive plate. In Figures 5 and 6, a further enlargement of a portion of the ion generation chamber 44 more clearly illustrates the corona generation area. Placement of the corona wire 40 is preferably about the same distance from the cavity wall 48 and from the conductive plate 42, and close to these chamber walls than to the remaining cavity walls. We have found that such an orientation will yield higher corona output currents than heretofore made possible with a cylindrical ion generation chamber of comparable size. The width "w" across the cavith 38 is also about 3.125 mm but the wire 40 is spaced only about 0.625 mm from each of the conductive walls 48 and 42 (i.e., less than half the distance between the wire and the walls of the conventional cylindrical chamber). In Figure 5 there is shown equipotential lines and electrical lines of force between the corona wire and these adjacent conductive walls. It can be seen that the great bulk of the ions will flow to the adjacent walls, although the cavity walls remote from the wire will attract some ions. However, it is only those ions following the lines of force into the cavity exit region 46, and those in close proximity, which will be driven out of the ion generation chamber 44. Therefore, it should be understood that it would be possible to fabricate the printer head of an insulation material, as long as the cavity wall 48 is made conductive and is suitably connected to a reference potential (such as ground). If the head is made insulating, the ion flow to the remote cavity walls will accumulate thereon. However, by spacing the wire much closer to the conductive walls than to the insulating walls, relatively few ions will flow to the insulating walls, charge build-up is minimized, and arcing to those walls is prevented. Proposed modifications to the printer head are shown in dotted lines in Figure 6. The corona wire 40 may be adjustably mounted for optimizing the ion current output within the zone of adjustment identified as area "c". Also, the exit channel 66 may be altered to improve the fluid characteristics. To this end, the corners 74 and 76 of cavity wall 48 and conductive plate 42 respectively may be broken off, as indicated by the dotted lines. The sharp corners create sharp curves in the fluid flow path, which generate a substantial hydrodynamic loss. With the corners broken off, the hydrodynamic loss will be decreased and it would be possible to utilize a smaller, less expensive, air blower. The printer head configuration is more efficient than the known cylindrical configuration, primarily because of the placement of the corona wire close to the chamber walls adjacent to the exit channel. Clearly the improved efficiency allows the same parameters of operation to be employed, with a resultant increase in ion output current. Alternatively, the higher efficiency has brought with it the ability to modify other printer head parameters, to the advantage of the printing process. Since the printing process, as presently practised, does not require the higher ion output current, it became possible to lower the output current to that previously obtainable with the cylindrical construction. By lowering the output current from the printer head, it was possible to lower the air pressure rewuirement, enabling the use of a smaller, less expensive, quieter blower. The lower flow rate of the smaller blower will cause the ions to spend more time in the ion modulation zone, allowing the lower control voltage to be imposed upon the modulation electrodes. It has been demonstrated than the thin film amorphous silicon field effect transistors on the substrate have a longer life when operated at a lower voltage. Thus, the increased efficiency also increases the life of the large area control chip.

Claims: 1. A fluid flow assisted ion-projection printer head comprising: a body (30) having in it an elongated cavity (44); a conductive wire (40) supported on the body and extending along the cavity, the wire being enclosed on three sides by the walls of the cavity, with one of the walls being electrically conductive; an entrance channel (36) defined in the body, through one of the walls, for introducing an ion-transport fluid into the cavity; a substantially-planar, electrically-conductive plate (42) forming a closure for the major portion of the open mouth of the cavity, thereby forming a first portion (66) of an exit channel between the end of the plate and the said one wall for providing a path for the removal of transport fluid from the cavity, a substantially-planar member (70) supporting electronic control elements, the planar member being held against the planar conductive plate and separated therefrom by an intermediated dielectric member (72), the planar member including a cantilevered portion spaced from the body for defining with it an extension (68) of the exit channel, and wherein the wire is located closer to the said one wall and to the planar conductive plate to any of the other walls of the cavity. 2. The printer head as claimed in claim 1, in which the body is made in one piece. 3. The printer head as claimed in claim 1 or 2, in which the body is made of an electroconductive material. 4. The printer head as claimed in any preceding claim, in which resilient means is provided for applying force to urge the planar member and the conductive plate against the body. 5. The printer head as claimed in any preceding claim, comprising adjustable mounting means for the ends of the conductive wire, for allowing it to be repositioned relative to the said one wall and to the planar conductive plate. 6. The printer head as claimed in any preceding claims, including spacer means on the body for establishing the distance of the printer head from a receptor surface. 7. The printer head a claimed in any preceding claim, in which the shape and/or dimensions of the exit channel are chosen to give it a desired impedance to the flow of ion-­ transport fluid through it.

Inventors: SANDER, GERHARD K., SHERIDAN, NICHOLAS K.
Applicants: XEROX CORPORATION
Application year: 1986
Main classification: G03G15
IPC codes: B41J2, G03G15