Abstract:
A dryer operable in close proximity to and in series with an inkjet printhead comprises a heat source and an air bearing structure on one side of the predetermined path and having a pressurized air inlet and an air outlet adjacent to the drying position of the receiver medium. Air flow from the air bearing structure outlet forms an air bearing for the receiver medium. A microporous filter positioned at the outlet and being adapted to convert the air flow from the outlet to a diffuse flow, the microporous filter being formed of an inner layer of very fine screen for optimum air diffusion and an outer layer of courser woven screen to add rigidity and protection from scuffing.

Description:
FIELD OF THE INVENTION 
     The present invention is related to the field of inkjet printers, and more particularly to the drying of the ink during the printing process. 
     BACKGROUND OF THE INVENTION 
     Inkjet printing is prominent because of its non-impact, low-noise characteristic, its use of plain paper, and its avoidance of toner transfers and fixing. Inkjet printing mechanisms can be categorized as either continuous or drop-on-demand. Drop-on-demand systems are generally lower cost but relatively low print speed when compared to continuous systems. In either drop-on-demand or continuous inkjet systems, it is necessary to assign a different fluid ink color to a separate printhead. Therefore, in color prints, several layers of wet ink may be deposited onto a printed medium. 
     Traditional printing presses are able to use high viscosity inks to obtain vibrant, high-density colors. However, continuous ink jet systems employ low viscosity solutions of dyes or pigments in a water solvent, and the printed colors tend to not be as vibrant and dense as with other printing systems. It is known that increasing the amount of dye or pigment applied to the paper can brighten the colors. However, this process also increases the amount water solvent applied to, and absorbed by, the paper. Absorption of water may cause a paper wrinkling effect called cockle, a wicking and spread of colors referred to as color-to-color bleed, and/or a show-through to the back side of the paper. 
     To remove water from the printed medium, continuous systems have conventionally utilized an end-of-line dryer that is similar to those used in printing presses. See for example U.S. Pat. No. 5,423,260 issued to Rockwell International Corporation in 1995, wherein the end-of-line dryer removes water from the printed medium only when all wet ink has been deposited and is at its maximum. It has been suggested to use infrared lamps or microwave radiation to preferentially heat the ink relative to the unprinted receiver media. However, tests have shown that dryers consisting of infrared lamps or microwave radiation cause a significant amount of receiver media heating to occur. 
     Further reductions in the time required between printing and drying have been realized by placing dryers between two printheads to dry the ink before significant amounts of the ink can wick into or otherwise be absorbed by the receiver media. Placement of dryers between printheads is referred to herein as “inter-station drying,” and has been disclosed in U.S. Pat. No. 6,428,160B2, issued to Xerox in 2002. Inter-station drying is effective to provide better optical density, sharper edges, less show through and reduced cockle. In multi-color systems, high-speed dryers placed between the different color printheads reduce color-to-color bleed, and enable more ink to be employed without overly wetting the receiver media. U.S. Pat. No. 5,631,685 discusses these benefits in relationship to single color printers. JP07-314661 speaks of these benefits for a multi-color inkjet printer. U.S. Pat. No. 6,428,160B2 addresses the paper scorching issues by selectively heating only the ink and not the paper. However, selective heating of the ink may create a saturated boundary layer at the ink surface. That is, as heat is directed to the newly applied ink, water evaporates rapidly from the surface of the ink, forming a thin layer of saturated air just above the ink. Therefore, it has been found necessary to include a mechanism for removing the saturated air layer just above the ink spot. 
     It has been suggested to remove the saturated air layer using a combination of convection and radiation. U.S. Pat. No. 5,261,166 discloses a dryer comprising a plurality of infrared burner units with air floatation dryer elements between the infrared units. The air floatation elements mentioned in the patent are of the Coanda type. U.S. Pat. No. 6,412,190 also employs infrared burners in conjunction with air bars. U.S. Pat. No. 6,088,930 employs alternating infrared sources and blower units. Suction nozzles are located between the infrared sources and the blower units to remove air from the blower regions. This patent discloses the concept of reflectors being placed on the opposite side of the paper from the infrared sources to reflect the radiation back at the paper. WO 88/07103 describes a dryer unit in which the lamp used for generation of infrared radiations enclosed in a box with a reflector behind the lamp and an infrared transmitting window in front of the lamp. Air is directed through the box to cool the lamp, the reflector, and the inner surface of the window. This air exits the box by way of a Coanda slot that causes the air to be directed between the window and the paper. U.S. Pat. No. 5,092,059 describes a dryer unit in which an infrared source directs infrared at the paper through a Quartz window. Coanda slots located on two sides of infrared source cause air to flow between the window and the paper to remove moist air from this space. Commonly assigned U.S. Pat. No. 6,058,621 describes a dryer in which a plurality of radiant heating bars direct radiation at photosensitive paper. Reflectors are placed behind the infrared lamps. Air flows out between the reflectors, impinging on the paper. 
     Air bearing systems allow for contact-less support of a print media, especially web-like materials. This contact-less support is sometimes crucial to ensure that the web or print is not damaged. The air bearing condition is traditionally created by deflecting the trajectories of the air molecules immediately adjacent to the print media in a direction parallel to the movement of the printed medium. The parallel movement of the air molecules thus establishes a cushion of air providing support for the printed medium. For example, U.S. Pat. No. 3,324,570 issued in 1967 teaches a float dryer developed for fabrics. A more recent adaptation of the 1967 patent, U.S. Pat. No. 5,261,166 issued to WR Grace in 1993, used a combination infrared and air flotation dryer. WR Grace uses a combination of their HI-FLOAT® air bar in combination with an infrared gas burner, INFRAWAVE® by Maxon Corporation, to create a fast dryer that removes the saturated boundary layer by impinging air upon the ink surface. The end-line dryer taught by WR Grace requires that all fluid inks be placed onto the printed media web prior to initiation of drying. 
     The patents described above utilize infrared radiation to provide the energy transfer needed for effective drying combined with air bearing features to enhance the transfer of moist air away from the paper. None of the prior art used a microporous filter air bearing design, as is the case of the present invention, but rather used either Coanda type or air bar types of air bearings. While Coanda type or air bar types of air bearings are effective to handle large air volumes and velocities, the air flow is directed toward a common point, which causes a wet image to smear at the air impingement point. It would be advantageous to allow for a diffuse and more controlled overall air flow without loosing the capacity for large air flow or volume. 
     It is an object of the present invention to provide an inter-station drying system such that the benefit of rapid drying of printed ink or other water based liquid without the creation of a saturated boundary layer issue by supplying a large volume and high velocity air flow such that air flow prevents overheating without creating additional smear, and to rapidly cool the substrate by removing any residual heat generated by the radiation source. 
     It is another object of the present invention to provide a dryer system to be used in close proximity and in series with at least one inkjet printhead or water based liquid applicator to include a source of heat, a source of air flow, and a structure in communication with the air flow that converts the air flow to a substantially diffuse flow compatible with printed, wet inks. The diffuse flow of air is such as to create a cushion of air at the surface of the receiver medium. 
     It is still another object of the present invention to arrange air sources along the printed medium and on both sides of the receiver medium in a manner to provide a contact-less receiver medium support. 
     It is yet another object of the present invention to layer the heat source and the gas source to minimize the overall length of the printing system. 
     SUMMARY OF THE INVENTION 
     According to a feature of the present invention, a dryer operable in close proximity to and in series with a water based liquid applicator such as, for example, an inkjet printhead comprises a heat source and an air bearing structure on one side of the predetermined path and having a pressurized air inlet and an air outlet adjacent to the drying position of the receiver medium. Air flow from the air bearing structure outlet forms an air bearing for the receiver medium. A microporous filter is positioned at the outlet and is adapted to convert the air flow from the outlet to a diffuse flow, the microporous filter being formed of an inner layer of very fine screen for optimum air diffusion and an outer layer of courser woven screen to add rigidity and protection from scuffing. 
     According to a preferred feature of the present invention, the heat source is radiative and is adapted to selectively heat the water based liquid rather than the receiver medium. The microporous filter is a stainless steel laminate microstructure According to another preferred feature of the present invention a second air bearing structure is provided having an outlet adjacent to the drying position on a side of the predetermined path opposed to the one side, wherein positive pressure is applied onto a first side of the receiver medium by the first-mentioned air bearing structure and onto a second side of the receiver medium by the second air bearing structure to create a contact-less support for the receiver media. 
     According to yet another preferred feature of the present invention, the heat source is adapted to emit radiation on the one side of the predetermined path; the air bearing structures are transparent to the emitted radiation; and the second air bearing structure includes a reflector adapted to reflect radiation that has passed through the receiver medium back to the receiver medium. There may be a plurality of applicators along the predetermined path, and there is a drying position between each pair of the applicators. 
     According to still another preferred feature of the present invention, a receiver support drum is provided adjacent to the drying position on a side of the predetermined path opposed to the one side to support the receiver medium at the drying position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of an inkjet printer system with an inter-station dryer system according to the present invention; 
         FIG. 2  is a schematic view of still another alternate embodiment of the present invention of  FIG. 1 ; 
         FIG. 3  is a schematic view of an alternate embodiment of the present invention showing a microstructured air bearing inter-station combination dryer; 
         FIG. 4  is a detail view of the embodiment of  FIG. 3 ; 
         FIG. 5  illustrates still another embodiment of the present invention, specifically for drying around a drum; and 
         FIG. 6  is shows an embodiment of the present invention similar to that of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     Referring now to  FIG. 1 , a first printhead  12  and a second printhead  14  are separated by an inter-station dryer  16 . While the preferred applications of the present invention are for use in drying of inkjet inks on print media, the dryers could also be useful for drying other coatings on paper and other media. The dryer illustrated is a combination of radiation sources  18  and  20 . Radiation sources  18  and  20  may be any source of radiation that selectively dries only the fluid ink without sufficiently increasing the temperature of a receiver medium  25 , such as for example near infrared lamps, microwaves, infrared radiation, etc. The two radiation sources  18  and  20  are followed respectively by air bearing structures  22  and  24 . 
     Air bearing structures  22  and  24  are opposed, respectively, by similar air bearing structures  26  and  28 . Each air bearing structure  22 ,  24 ,  26  and  28  includes an air inlet  30 , an air plenum  31 , and a microporous filter  32 . According to a feature of the present invention, it has been found that a material used to form pleated tubular filter elements as a sand filter for use in an oil and/or gas producing well, as disclosed in U.S. Pat. No. 5,411,084, is particularly suitable for use as micoporous filter  32 . Such a material is commercially available from Purolator Facet, Inc. of Greensboro, N.C., USA, and is sold under the registered trademark “POROPLATE.” While the POROPLATE material is a stainless steel material, similar microporous filters can be fabricated using other materials. More generally, microporous filter  32  has an inner layer of very fine screen for optimum air diffusion and an outer layer of courser woven screen to add rigidity and protection from scuffing. 
     Air passes through microporous filter  32  impacting the printed receiver medium  25 . This air must then flow parallel to the print media  25  to exit the gap between the print media  25  and the microporous filter  32 . The air flow produced in this manner is highly effective in removing the saturated boundary layer from the air adjacent to the print media  25 . The microporous filter based air bearings provide exceptional benefit in drying over earlier Coanda or air bar types of air bearings. First, the microporous structure ensures uniform air flow across the width of the air bearing so that drying is more consistent across the width of the dryer. Second, the diffuse nature of the air flow as it passes through the microporous filter prevents the air flow from blowing the ink around on the print media as can happen with Coanda type or air bar types of air bearings. As a result the microstructures allow for a large volume and high velocity of air output onto the printed receiver medium to improve drying without adversely affecting the print quality. 
     While the illustrated embodiment demonstrates two stations of the combined radiation and air bearing dryer, it will be understood that one or more stations may be used, depending on the application involved. Additionally, while the illustrated embodiment illustrates the air bearing structures directly opposing on either side of the printed media, the opposing air bearing structures may be offset one from the other in order to obtain a similar air bearing condition. 
       FIG. 2  shows a second preferred embodiment of the present invention wherein the housing for interstation dryer  17 , which holds radiation sources  18  and  20 , also serves as a plenum to supply air to both of the microporous filter elements  32 . In this way, the air supplied for the air bearing function can also serve to cool the reflectors of the radiation sources. 
     In a third preferred embodiment of the present invention illustrated in  FIGS. 3 and 4 , the overall length of the inter-station dryer is further decreased. A radiation source  34  is incorporated into an air bearing structure  36 . An infrared reflector  40  is integrated into air bearing structure  38 . In  FIG. 4 , radiation from radiation source  34  moves along a path  44  through the plenum  31  and the microporous filter  42  of the air bearing structure  36  to receiver medium  25  to partially dry the fluid ink without sufficiently increasing the temperature of the receiver medium. Because standard materials for a printed web are transparent to infrared radiation, much of the radiation will transmit through the receiver medium, pass through second air bearing structure  38 , plenum  31  and associated microporous filter  46  to be reflected back along a second path  52  to receiver medium  25  to complete the drying process of the fluid ink without sufficiently increasing the temperature of the receiver medium. This arrangement allows for the irradiation of both surfaces of wet ink on the printed web for a more complete and effective drying time. One skilled in the art will readily notice that microporous filters  42  of air bearing structures  36  and  38 , respectively above and below the web, must be radiation transparent. This requires that microporous filters  42  be made out of a glass or polymer that is transparent to the radiation produced by radiation source  34 . In this way, air can be directed at high volume and high velocity but in a diffuse manner at the web by microporous filter  42 , the radiation can pass through it largely unaffected. In  FIG. 4 , dashed lines indicate the direction of air flow from air inlets  30  toward and along the receiver medium  25 . Radiation follows large dotted lines  44  from radiation source  34  through microporous filters  42  to infrared reflector  40  and returns to receiver medium  25 . 
     In  FIG. 5 , a printhead  54  represents the final printhead of a series wherein inter-station dryers are positioned between the printheads. A radiation source  56  is integrated with an air bearing structure  58  having a microporous filter  60 . A web support, such as a drum  62 , consists of a radiation absorbing material. The presence of air in this embodiment is solely for removal of the saturated boundary layer since the receiver material is not supported on an air bearing. This embodiment allows for the radiation absorption by receiver medium  25  such that the bottom side of the receiver medium may be heated. The microporous filter  60  has been curved to match the curvature of drum  62  and to provide more efficient air transfer. However, the inventive contribution of the present invention is not limited to a curved structure, and may also include an array of small linear microstructures such that the desired area is covered. Likewise, while not necessary but included in the illustration as a preferred version of this embodiment, an optional radiation source  64  may be included on the side of drum  62  opposed the combined radiation and air source to increase the heating capacity of the drum and to allow the receiver medium to maintain a more constant temperature during slow print speeds. In another embodiment, one or more heater elements such as are described in U.S. Pat. No. 4,982,207, not shown, can be attached to the inside surface of the drum  62  to heat the drum. Such heaters would be used instead of the optional radiation source  64 . By heating the print media by direct contact with the heated drum  62  in combination with the radiative heating of the ink by the radiation sources  56  and the air flow produced by the air bearing structure  58 , these embodiments have enhanced drying capacity. 
     Referring to another embodiment shown in  FIG. 6 , air is supplied through an air port, and distributed by plenum  31  of air bearing structure  58  to a plurality in microporous filter elements  60 . Radiation sources  66  integrated into air bearing structure  58  direct near IR radiation at the printed media. As in  FIG. 5 , one or more heater elements such as are described in U.S. Pat. No. 4,982,207 can be attached to the inside surface of the drum  62  to heat the drum. Such heaters would be used instead of the optional radiation source  64 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, while a preferred application of the present invention is for use in drying of inkjet inks on print media, the dryers could also be useful for drying other coatings on paper and other media. 
     PARTS LIST 
       12 . first printhead 
       14 . second printhead 
       16 . inter-station dryer 
       17  air bearing structure 
       18 . radiation source 
       20 . radiation source 
       22 . air bearing structure 
       24 . air bearing structure 
       25 . print medium 
       26 . air bearing structure 
       28 . air bearing structure 
       30 . air inlets 
       32 . microporous filters 
       34 . radiation source 
       36 . air bearing structure 
       38 . air bearing structure 
       40 . infrared reflector 
       42 . microporous filter 
       44 . path 
       46 . microporous filter 
       52 . second path 
       54 . printhead 
       56 . radiation source 
       58 . air bearing structure 
       60 . microporous filter 
       62 . drum 
       64 . radiation source 
       66  radiation source