Abstract:
The present invention provides a direct printing apparatus which prevents image noise from generating due to adhesion of toner to a spacer and enables to form a good image even if the apparatus is operated for a long period. The direct printing apparatus comprises a bearing member  30  for bearing printing particles  38  thereon, the printing particles  38  being charged to a predetermined polarity, a backing electrode  44  opposed to the bearing member  38,  and a printing head  50  disposed between the bearing member  30  and the backing electrode  44.  The printing head  50  has a plurality of apertures  56  through which the printing particles  38  can propel and a plurality of electrodes  68, 70  disposed around the plurality of apertures  56.  The printing particles  38  are directly deposited on a print medium  8  which is conveyed between the backing electrode  44  and the printing head  50.  A positioning spacer  90  is provided between the bearing member  30  and the printing head  50  so that the surface of the bearing member  30  comes into contact with the spacer  90.  At least a part of the spacer  90  which comes into contact with the bearing member  30  is made of a material which is apt to be worn by the printing particles  38.

Description:
This application is based on application No. H10-61063 filed in Japan on Dec. 22, 1997, the content of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a direct printing apparatus for use in a color or monochrome copying machine, printer, facsimile and composite thereof. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,477,250 issued on Dec. 19, 1995 discloses a direct printing apparatus. In the direct printing apparatus, four printing stations are disposed along a sheet conveying direction. Each printing station comprises a toner carrier retaining toner on its outer periphery, a backing electrode opposed to the toner carrier and a printing head disposed between the toner carrier and the backing electrode, the printing head having a plurality of apertures and a plurality of electrodes surrounding each aperture. The backing electrode of each printing station is electrically connected to a power source, thereby between the toner carrier and the backing electrode is formed an electric field for attracting the toner on the toner carrier and propelling it toward the backing electrode through the apertures of the printing head. Between the printing head and the backing electrode in each printing station is formed a passage for a sheet. 
     When an ON voltage is applied to the electrode of the printing head in the printing station, the toner attracting force due to the electric field between the toner carrier and the backing electrode propels the toner on the toner carrier through the apertures toward the backing electrode and adheres it to the sheet. When an OFF voltage is applied to the electrode of the printing head, the toner attracting force does not affect the toner on the toner carrier, whereby the toner is never propelled. Thus, when ON and OFF voltage applied to the electrode of the printing head are controlled on the basis of a desired image signal, an image corresponding to the image signal is printed on the sheet. 
     In the aforementioned direct printing apparatus, a distance between the printing head and the toner carrier affects the flying distance of the toner. Thus, the distance between the printing head and the toner carrier necessitates an allowance of approximately 10 μm, thereby high accuracy is required. Conventionally, for example, in Japanese patent Laid-open publication 6-297753, as means for positioning the printing head and the toner carrier (developing roller) to ensure the accuracy of the position, there has been provided a spacer made of resin between the printing head and the developing roller such that the spacer comes into contact with the developing roller. 
     However, the aforementioned direct printing apparatus has the following disadvantage. Since the spacer comes into contact with the developing roller, the toner particles enter and accumulate in the contact portion therebetween. Thus, the heat due to the long time operation of the apparatus causes the accumulated toner particles to gradually deteriorate and melt to adhere to the surface of the spacer. Then, the adhered toner provide noise to the thin uniform layer of toner particles formed on the outer periphery of the developing roller and disturb the uniform layer, whereby noise appear on the printed image. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been accomplished to solve the aforementioned disadvantages of the prior arts. An object of the present invention is to provide a direct printing apparatus which is possible to form fine image without causing noise due to the adhesion of the toner to the spacer for long time operation. 
     In order to achieve the aforementioned object, according to the present invention, there is provided a direct printing apparatus which comprises a bearing member for bearing printing particles thereon, the printing particles being charged to a predetermined polarity, a backing electrode  44  opposed to the bearing member, and a printing head disposed between the bearing member and the backing electrode, the printing head having a plurality of apertures through which the printing particles can propel and a plurality of electrodes disposed around the plurality of apertures, whereby the printing particles are directly deposited on a print medium which is conveyed between the backing electrode and the printing head, wherein: 
     a positioning spacer is provided between the bearing member and the printing head so that the surface of the bearing member comes into contact with the spacer; and 
     at least a part of the spacer which comes into contact with the bearing member is made of a material which is apt to be worn by the printing particles. 
     Preferably, the part of the spacer which comes into contact with the bearing member may be made of such a material that maximum wearing depth per unit moving distance of the bearing member is more than 2.0×10 −3  μm/m. 
     In the direct printing apparatus of the present invention having such construction as described above, since the contact part of the spacer with the bearing member is made of such material that is apt to be worn by the printing particles, the spacer is worn away by the printing particles. Thus, the toner particles neither accumulate on the contact part nor adhere to the surface of the spacer. 
     Preferably, the bearing member may comprise an endless sleeve for bearing the printing particles thereon and a drive roller having outer diameter smaller than the inner diameter of the sleeve and being disposed in the sleeve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and advantages of the present invention will be become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which 
     FIG. 1 a schematic cross-sectional side elevational view of a first embodiment of a tandem type direct printing apparatus of the present invention; 
     FIG. 2 is a cross-sectional side elevational view of a printing station; 
     FIG. 3 is an enlarged fragmentary plane view of a printing head; and 
     FIG. 4 is a enlarged fragmentary cross-sectional view of the printing head, developing roller and backing electrode taken along a line IV—IV in FIG. 3; 
     FIG. 5 is an enlarged fragmentary cross-sectional view of the spa r and the developing roller during printing operation; 
     FIG. 6 is an enlarged fragmentary cross-sectional view showing a variation of the first embodiment of the tandem type direct printing apparatus; and 
     FIG. 7 is an enlarged fragmentary cross-sectional view of a second embodiment of a tandem type direct printing apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings and, in particular, to FIG. 1, there is shown a direct printing device, generally indicated by reference numeral  2 , according to the first embodiment of the present invention. The printing device  2  has a sheet feed station generally indicated by reference numeral  4 . The sheet feed station  4  includes a cassette  6  in which a number of sheets  8  or plain papers are stacked. A sheet feed roller  10  is mounted for rotation above the cassette  6  so that it can frictionally contact with the top sheet  8 , thereby the feed roller  10  can feed the top sheet  8  into the direct printing device  2  as it rotates. A pair of timing rollers  12  are arranged adjacent to the sheet feed roller  10 , for supplying the sheet  8  fed from the cassette  6  through a sheet passage  14  indicated by a dotted line into a printing station, generally indicated by reference numeral  16 , where a printing material is deposited on the sheet to form an image thereon. Further, the printing device  2  includes a fusing station  18  for fusing and permanently fixing the image of printing material on the sheet  8 , and a final stack station  20  for catching the sheets  8  on which the image has been fixed. 
     Referring to FIG. 2, the printing station  16  comprises a developing device generally indicated by reference numeral  24  above the sheet passage  14 . The developing device  24  comprises a container  26  which has an opening  28  confronting the sheet passage  14 . Adjacent the opening  28 , a developing roller  30  is provided. The developing roller  30  comprises a sleeve  30   a  as a bearing member of printing particles according to the present invention and a drive roller  30   b.  The sleeve  30   a  has an endless or cylindrical shape having a thickness of 0.15 mm and a diameter of 20 mm and is made of flexible and conductive material such as nickel, nylon or so. The drive roller  30   b  is contained in the sleeve  30   a  and supported for rotation in a direction indicated by an arrow  32 . The outer diameter of the drive roller  30   b  is smaller than the inner diameter of the sleeve  30   a  so that the sleeve  30   a  is formed with a slack  31  as shown in FIG.  4 . The slack  31  comes into contact with a spacer  90  that will be explained hereinafter. The drive roller  30   b  is made of conductive material and is electrically connected to the earth. Alternatively, the sleeve  30   a  can be electrically connected to the earth. A blade  36 , preferably made from a plate of elastic material such as rubber or stainless steel, is disposed in contact with the sleeve  30   a.    
     The container  26  accommodates printing particles, i.e., toner particles  38 . In this embodiment, the toner particles having a volume mean particle size of 8μ and capable of being charged with negative polarity are used. 
     Disposed under the developing device  24 , beyond the sheet passage  14 , is an electrode mechanism generally indicated by reference numeral  40  which includes a support  42  made of electrically insulative material and a backing electrode  44  made of electrically conductive material. The backing electrode  44  is electrically connected to a direct power supply  46  which supplies a voltage of predetermined polarity (positive polarity in this embodiment) so that the backing electrode  44  is provided with, for example, a voltage of +1200 volts. Thus, between the backing electrode  44  and the developing roller  30  are formed an electric field E that the negatively charged toner particles  38  on the developing roller  30  are electrically attracted to the backing electrode  44 . 
     Fixed between the developing device  24  and the electrode mechanism  40  and above the sheet passage  14  is a printing head generally indicated by reference numeral  50 . Preferably, the printing head  50  is made from a flexible printed circuit board  52 , having a thickness of about 50 to 150 micrometers. As shown in FIG. 2, a portion of the printing head  50  located in a printing zone where the developing roller  30  confronts the backing electrode  44  includes a plurality of apertures  56  having a diameter of about 25 to 200 micrometers which is substantially larger than an average diameter (about several micrometers to a dozen micrometers) of the toner particles  38 . 
     In this embodiment, as best shown in FIG. 3, the apertures  56  are formed on equally spaced three parallel lines  58 ,  60  and  62  each extending in a direction indicated by reference numeral  64  which is parallel to an axis of the developing roller  30  and perpendicular to a direction indicated by reference numeral  66  along which the sheet  8  will be transported, ensuring the printing head  50  with a resolution of 600 dpi. The apertures  56  on the lines  58 ,  60  and  62  are formed at regular intervals of D, e.g., 127 micrometers, and the apertures  56 ( 56   a ) and  56 ( 56   c ) on the lines  58  and  62  are shifted by the distance D/N to the opposite directions with respect the apertures  56 ( 56   b ) on the central line  60 , respectively, so that, when viewed from the sheet transporting direction  66 , the apertures  56  appear to be equally spaced. Note that the number N represents the number of line rows and is “3” in this embodiment, however, the number N as well as the interval D can be determined depending upon the required resolution of the print head. 
     The flexible printed circuit board  52 , as shown in FIG. 4, further includes therein doughnut-like first and second electrodes  68  and  70  each of which surrounds the apertures  56 . The first electrode  68  is disposed on one side opposing the developing roller  30  while the second electrode  70  is on the other side opposing the backing electrode  44 . 
     The first electrode  68  is electrically communicated with a driver  72  through a printed wire  74  and the second electrode  70  is electrically communicated with a driver  76  through a printed wire  78 , so that the drivers  72  and  76  can transmit image signals to the first and second electrodes  68  and  70 , respectively. The drivers  72  and  76  are in turn electrically communicated with a controller  80  that feeds out data of image to be reproduced by the printing device  2 . 
     The image signals to be transmitted to the first and second electrodes  68  and  70  consist of a DC component constantly applied to the first and second electrodes  68 ,  70  and a pulse component applied to the first and second electrodes  68 ,  70  in response to the image data from the controller  80  for forming dots on the sheet  8 . 
     In the concrete, in this embodiment, for the first electrode  68 , the base voltage V 1 (B) is about −50 volts, and the pulse voltage V 1 (P) is about +300 volts. For the second electrode  70 , the base voltage V 2 (B) is about −100 volts and the pulse voltage V 2 (P) is about +200 volts. 
     Between the developing roller  30  and the printing head  50  is disposed a spacer  90 . The spacer  90 , as shown in FIG. 4, is positioned at the upper side of the printing head  50  opposing to the developing roller  30 . At a position opposing to the portion in which the apertures  56  of the printing head  50  is formed, the spacer  90  is formed with a slit  92  extending to the main scanning direction (perpendicular to the surface of the drawing). The slack  31  of the sleeve  30   a  of the developing roller  30  comes into contact with the spacer  90  so that the slack  31  is opposed to the slit  92  in a flat condition. Thus, the distance S between the sleeve  30   a  and the printing head  50  is held stable even if the drive roller  30   b  has an eccentricity or looseness. 
     In this embodiment, the spacer  90  is made of a material which is apt to be worn by the toner particles  38 , such as polyethylene terephthalate, fluoroplastic or the likes. In other words, the spacer  90  is made of a softer material than the toner particles  38 . Particularly, the spacer  90  is made of such a material that, as shown in FIG. 5, maximum wearing depth L (μm) per unit moving distance (m) of the developing roller  30  rotating with the toner particles  38  born thereon is more than 2.0×10 −3  μm/m. 
     Having described the construction of the printing device  2 , its operation will now be described. 
     As shown in FIG. 2, in the printing station  16 , the drive roller  30   b  of the developing roller  30  rotates in the direction indicated by the arrow  32 , allowing the sleeve  30   a  to rotate in the same direction. The toner particles  38  are deposited on the sleeve  30   a  and then transported into a contact region of the blade  36  and the sleeve  30   a  where the toner particles  38  are provided with triboelectric negative charge by the frictional contact of the blade  36 . Thereby, as shown in FIG. 4, incremental peripheral portions of the developing roller  30  which has passed through the contact region bear a thin layer of charged toner particles  38 . 
     The slack  31  of the sleeve  30   a  of the developing roller  30  comes into contact with the spacer  50 , whereby the slack  31  is opposed to the slit  92  in a flat condition. Thus, the distance S between the sleeve  30   a  and the printing head  50  is held stable even if the drive roller  30   b  has an eccentricity or looseness. 
     The sleeve  30   a,  with the toner particles  38  born thereon, of the developing roller  30  rotates in a condition that it comes into contact with the spacer  90  via the toner particles  38 , whereby a load due to the contact is applied to the spacer  90 . In the conventional apparatus, the toner particles  38  are accumulated in the contact portion. In the present embodiment, on the other hand, since the spacer  90  is made of a material which is apt to be worn by the toner particles  38 , the toner particles  38  reach the slit  92  of the spacer  90  while wearing away the spacer  90 . Thus, as shown in FIG. 5, the toner particles  38  never accumulate in the contact portion, preventing the toner particles from adhering to the surface of the spacer. 
     In the printing head  50 , the first and second electrodes  68  and  70  are constantly biased to the base voltage V 1 (B) of about −50 volts and V 2 (B) of about −100 volts. Therefore, the negatively charge toner particle  38  on the sleeve  30   a  of the developing roller  30  electrically repels against the first and second electrodes  68  and  70  and therefore stays on the sleeve  30   a  without propelling toward the aperture  56 . 
     The controller  80  outputs the image data corresponding to an image to be reproduced to the drivers  72  and  76 . In response to the image data, the drivers  72  and  76  supplies the respective voltages V 1 (P) of about +300 volts and V 2 (P) of about +200 volts to the pairs of first and second electrodes  68  and  70 . As a result, the toner particles  38  on the portions of the sleeve  30   a  confronting the biased electrodes are electrically attracted by the first and second electrodes  68  and  70 . This energizes a number of toner particles  38  to propel by the attraction force of the backing electrode  44  into the opposing aperture  56 . 
     When the toner particles  38  have reached respective positions adjacent to the first and second electrodes  68  and  70 , the voltages to be applied to the first and second electrodes  68  and  70  are changed from the pulse voltages V 1 (P) and V 2 (P) to base voltages V 1 (B) and V 2 (B), at respective timings. As a result, the toner particles  38  in the aperture  56  are then forced radially inwardly by the repelling force from the first and second electrodes  68  and  70  applied with the base voltages V 1 (B) and V 2 (B), respectively, and then converged into a mass. The converged mass of the toner particles  38  are then deposited on the sheet  8  which is moving past the printing zone  54 , thereby forming a layer of the toner particles on the sheet  8 . The aforementioned second electrode  70  is provided mainly for the purpose of converging the mass of the toner particles  38 . Therefore, the second electrode  70  can be excluded if necessary. The second electrode  70  may be a shape divided from the doughnut-like shape to control the flying direction of the mass of the toner particles  38 . 
     Subsequently, the sheet  8  to which the image consists of the layers of the toner particles  38  is formed is transported in the fusing station  18  where the layers of the toner particles  38  are fused and permanently fixed on the sheet  8  and finally fed out onto the final stack station or catch tray  20 . 
     Alternatively, the spacer  90  in the direct printing apparatus  2  of the aforementioned first embodiment may have a plate-like shape as shown in FIG.  6  and may be disposed such that it comes into contact with only the sleeve  30   a  of the developing roller  30 . 
     FIG. 7 shows a direct printing apparatus, generally indicated by reference numeral  2 , according to the second embodiment of the present invention. The second embodiment is different from the first embodiment in that the printing head  50  and the spacer  90  constitute a flexible printed circuit board  52  which is bent and disposed along the slack  31  of the sleeve  30   a  of the developing roller  30 . 
     In the direct printing apparatus  2  of the second embodiment, the melting of the toner particles  38  and the adhesion thereof to the spacer  90  can be prevented in the same manner as the first embodiment. Furthermore, a distance between the sleeve  30   a  of the developing roller  30  and the printing head  50  can be kept constant over the whole range, enabling to propel the toner particles  38  in more stable condition. 
     Although the aforementioned embodiments were explained as to a monochrome type of direct printing apparatus having a single developing device, the present invention is also applicable to a tandem type of color direct printing apparatus in which a plurality of printing stations are disposed in a sheet moving direction. 
     In the shown embodiments, although the spacer  90  itself is made of a material which is apt to be worn by the toner particles  38 , the spacer  90  may be made of conventional material and coated with such a material that is apt to be worn by the toner particles. In the case that the spacer  90  is made of resin, it may be formed by two color injection molding. 
     In the aforementioned embodiments, although the printing station in the above embodiments is a type of one component system using only the toner particles  38 , a type of two components system using both toner and carrier may be also applicable. 
     In the aforementioned embodiments, although the printing particles bearing means is a type comprising a hard roller and a flexible sleeve, a type of double rollers may be also applicable. 
     In the aforementioned embodiments, although the electrodes (apertures) of the printing head  50  are provided in three lines along the longitudinal direction of the a developing roller  30 , they may be provided in at least one line. In the case of a plurality of lines, the pitch of the apertures  56  can be set based on the required resolution. 
     Although the present invention has been fully described by way of the examples with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications otherwise depart from the spirit and scope of the present invention, they should be construed as being included therein. 
     EXPERIMENTAL EXAMPLE 
     In order to certify the result of the direct printing apparatus according to the present invention, the inventor made an experiment as explained hereinafter. In this experiment, the apparatus of the second embodiment as shown in FIG. 7 was used. A chart having an image ratio of 5% was continuously printed under the following conditions. Existence and nonexistence of image noise due to the toner adhesion to the surface of the spacer was confirmed. The wearing amount of the spacer after printing 3000 sheets (sleeve moving distance: 4400 m) was measured. Table 1 shows the results. 
     Set condition of the apparatus: 
     System velocity; 
     38 mm/sec 
     Distance between developing roller and printing head; 
     80μ 
     Aperture diameter; 
     100μ 
     Total number of apertures; 
     2480 dot (A4 width) disposed in 6 lines 
     Electric potential of developing roller; 
     Vr=0 (volt) 
     Electric potential of control electrode; 
     Vb=350 (volt) at printing time 
     Vw=0 (volt) at non-printing time 
     Electric potential of backing electrode; 
     VBE=1300 (volt) 
     1 line printing time; 
     T total =Tb(Vb applying time)+Tw(Vw applying time) 
     Where, 
     Tb=700 μsec 
     Tw=1530 μsec 
     Toner: 
     Volume mean particle size; 
     8μ (negatively chargeable toner) 
     Printing station: 
     Developing device; 
     Single component type 
     Drive roller; 
     Conductive EPDM 
     Diameter 38 mm 
     Nickel sleeve; 
     Resistance 1×10E6 Ω.m 
     Diameter 40 mm 
     Circumferential velocity of roller 72 mm/sec 
     Samples: 
     Sample A; aramid 
     Sample B; PET 
     Sample C; fluoroplastic (conductive type) 
     Sample D; fluoroplastic (insulative type) 
     Where, coating thickness is 0.05 mm. 
     Line pressure of the developing roller to the printing head: 
     P=2 gf/mm 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Maximum 
                   
                   
               
               
                   
                 Wearing Amount 
                 Image 
                 Wearing Amount per Unit 
               
               
                   
                 (μm) 
                 Noise 
                 Moving Distance (μm/m) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Sample A 
                  3 
                 X 
                 0.00068 
               
               
                 Sample B 
                 16 
                 ◯ 
                 0.0036 
               
               
                 Sample C 
                 25 
                 ◯ 
                 0.0057 
               
               
                 Sample D 
                 40 
                 ◯ 
                 0.0091 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, in the case of sample A having large hardness relatively to the toner particles  38 , it was confirmed that the maximum wearing amount (depth) per unit moving distance of the developing roller was small, that the toner particles 38 were melted and adhered to the spacer  90 , and that an image noise was generated on the printed sheet  8 . On the other hand, in the case of sample B, c and D having small hardness, it was confirmed that the maximum wearing amount (depth) per unit moving distance was large and that no image noise was generated on the printed sheet  8 , resulting in no problem.