Patent Publication Number: US-6220696-B1

Title: Simple electrostatic ink jet printing head having low cost

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an electrostatic ink jet printing head, and more particularly, to an electrostatic ink jet printing head which adheres toner particulates to a recording medium. 
     DESCRIPTION OF THE RELATED ART 
     A non-impact printing method has recently become interesting because the non-impact printing method makes negligable undesired sounds when printing. An ink jet printing method as the non-impact printing method can, in high speed, directly print to a sheet of plain paper by a simple mechanism. An electrostatic ink jet printing method is known as the ink jet printing method. The electrostatic ink jet printing method comprises printing electrodes and a counter electrode. When the printing electrodes are selectively supplied with printing pulse voltages, electric fields are caused between the printing electrodes and the counter electrode. A sheet of recording paper is positioned on the counter electrode. A small amount of coloring material such as ink is filed by electrostatic forces of the electric fields and adhered to the recording paper. 
     In the manner which will later be described in more detail, first, second, and third conventional electrostatic ink jet printing heads are known. The first conventional electrostatic ink jet printing head comprises a substrate and a covering member. The covering member has an internal surface defining, in cooperation with the substrate, an ink receiving space which receives ink containing coloring particulates. A plurality of printing electrodes are, in parallel, positioned in the ink receiving space. Front ends of the substrate and the covering member form an ink ejecting nozzle of a slit type. Each of the printing electrodes is formed in the shape of a needle. At a time of printing, the printing electrodes are selectively supplied with printing pulse voltages. 
     A counter electrode is positioned to counter the printing electrodes. A sheet of printing paper is positioned between the printing electrodes and the counter electrode so that the printing paper is in contact with the counter electrode. When the printing electrodes are supplied with printing pulse voltages, electric fields occur between the printing electrodes and the counter electrode. In this event, the electric field is concentrated at a front edge of the printing electrode. Thereby, electric charges in the ink are stored at a vicinity of the front edge of the printing electrode. 
     The second conventional electrostatic ink jet printing head is described in Japanese Unexamined Patent Prepublication (koukai) No. 228162/1985. The second conventional electrostatic ink jet printing head comprises a substrate, a covering member, printing electrodes, protruding portions, and reinforcement boards. The protruding portions are formed on the front end of the substrate so that the protruding portions correspond to front edges of the printing electrodes. The protruding portions give the ink meniscus preformed irregularities to concentrate the electric field at the vicinities of the front edges of the printing electrodes. 
     The third conventional electrostatic ink jet printing head comprises a substrate, a covering member, printing electrodes, and meniscus forming members. The meniscus forming members are positioned to correspond to the printing electrodes. The printing electrodes are formed by sputtering conductive material such as chromium to the whole surface of the substrate and by patterning the conductive material by photolithography. The meniscus forming members are formed by laminating a photosensitivity macromolecular film on the substrate and by patterning the photosensitivity macromolecular film by photolithography. 
     However, since the above conventional electrostatic ink jet printing heads comprise printing electrodes which are formed by sputtering conductive material such as chromium to the whole surface of the substrate and by patterning the conductive material by photolithography, and since the above conventional electrostatic ink jet printing heads comprise meniscus forming members that are formed by laminating a photosensitivity macromolecular film on the substrate and by patterning the photosensitivity macromolecular film by photolithography, the above conventional electrostatic ink jet printing heads have a complex manufacturing process and a high cost. In addition, since, in the above conventional electrostatic ink jet printing heads, the meniscus forming members are made of the photosensitivity macromolecular film, the thickness of the meniscus forming members is limited by a thickness of the photosensitivity macromolecular film. As a result, in the above conventional electrostatic ink jet printing heads, only a limited amount of ink can be ejected. 
     Referring to FIGS. 1,  2 ,  3 ,  4 , and  5 , first, second, and third conventional electrostatic ink jet printing heads will be described in order to gain a better understanding of this invention. 
     In FIG. 1, the first conventional electrostatic ink jet printing head comprises a substrate or base board  1  and a covering member  2 . The covering member  2  has an internal surface defining, in cooperation with the substrate  1 , an ink receiving space  3  which receives ink containing coloring particles. A plurality of printing electrodes  4  are, in parallel, positioned in the ink receiving space  3 . Front ends of the substrate  1  and the covering member  2  form an ink ejecting nozzle of a slit type. Each of the printing electrodes  4  is formed in a shape of a needle. At a time of printing, the printing electrodes are selectively supplied with printing pulse voltages. 
     On the other hand, a counter electrode  5  is positioned to counter the printing electrodes  4 . A sheet of printing paper  6  is positioned between the printing electrodes  4  and the counter electrode  5  so that the printing paper  6  is in contact with the counter electrode  5 . When the printing electrodes are supplied with printing pulse voltages, electric fields occur between the printing electrodes  4  and the counter electrode  5 . In this event, the electric field is concentrated at a front edge of the printing electrode  4 . Thereby, electric charges in the ink are stored at a vicinity of the front edge of the printing electrode  4 . 
     Processes of storing the electric charges in the ink are different due to kinds of the ink used. In case of conductive ink, storing the electric charges is due to electrostatic induction. Also, in case of dielectric ink, storing the electric charges is due to polarization. In addition, in case of ink having colored particles which are dispersed in the ink, storing the electric charges is due to inherent charges of the coloring particles that are caused by zeta potential. 
     In any case, the ink or the coloring particles in the ink are, by Coulomb&#39;s force which acts the stored electric charges, strained in a direction of the counter electrode  5 , namely, the printing paper  6 . When Coulomb&#39;s force is stronger than surface tension of the ink, a small amount of the ink is flied to be adhered on the printing paper  6 . In this case, the printing pulse voltages which are supplied to the printing electrodes are appropriately controlled in response to printing images. 
     However, since electric conductivity and dielectric constant of the ink for use in printing is greater than electric conductivity and dielectric constant of air, a location at which the electric charges are concentrated is not determined by only position of the printing electrode  4 , but is also influenced by a state of an ink meniscus at the ink ejecting nozzle. Namely, although it is expected that the ink meniscus is homogeneous in a longitudinal direction of the ink ejecting nozzle, minute irregularities of the ink meniscus are really caused due to machining accuracies of the ink ejecting nozzle, due to vibrations the ink meniscus after ejecting the ink, and due to natural fluctuations of the ink meniscus. 
     In this case, the electric field is more concentrated at minute convex portions of the ink meniscus in the vicinity of the printing electrode  4  due to conductive or dielectric properties of the ink. When the ink meniscus once starts to deform by Coulomb&#39;s force, the electric field is more concentrated. As a result, since the minute irregularities of the ink meniscus are clearly present in an initial state, it is impossible to accurately control the location from which the ink flies. In other words, when one of the printing electrodes  4  is supplied with the printing pulse voltage, the ink flies from a location which does not correspond to the one of the printing electrodes  4 . This causes deterioration of printing quality. 
     Shown in FIG. 2, for resolving the above disadvantage, the second conventional electrostatic ink jet printing head as described in Japanese Unexamined Patent Prepublication (koukai) No. 228162/1985. The second conventional electrostatic ink jet printing head comprises the substrate  1 , the covering member  2 , the printing electrodes  4 , protruding portions  7 , and reinforcement boards  8 . The protruding portions  7  are formed on the front end of the substrate  1  so that the protruding portions  7  correspond to front edges of the printing electrodes  4 . The protruding portions  7  give the ink meniscus preformed irregularities to concentrate the electric field at the vicinities of the front edges of the printing electrodes  4 . 
     In FIGS. 3,  4 , and  5 , the third conventional electrostatic ink jet printing head comprises the substrate  1 , the covering member  2 , the printing electrodes  4 , and meniscus forming members  9 . The meniscus forming members  9  are positioned to correspond with the printing electrodes  4 . 
     The printing electrodes  4  are formed by sputtering conductive material such as chromium to the whole surface of the substrate  1  and by patterning the conductive material by photolithography. The printing electrodes  4  are positioned with, for example, a space of 300 dpi (dots per inch), namely, about 85 micron meter left between two adjacent ones of the printing electrodes  4 . The printing electrodes  4  are connected to a driver (not shown) and are selectively supplied with printing pulse voltages. 
     The meniscus forming members  9  are formed by laminating a photosensitivity macromolecular film on the substrate land by patterning the photosensitivity macromolecular film by photolithography. Each of the meniscus forming members  9  has a front end which is slightly retreated from the front edge of the printing electrode  4 . The photosensitivity macromolecular film has a thickness of 30 micron meter. Each of the meniscus forming members  9  has a width of about 30 micron meter. The covering member  2  is attached on the meniscus forming members  9 . The covering member  2  has a front end which is slightly retreated from the front end of the meniscus forming members  9 . The covering member  2  is made of insulating material and has an ink supply port  10  and an ink exhaust port  11 . Each of the meniscus forming members  9  has a front end which is slightly protruded from the ink ejecting nozzle. 
     However, since the above conventional electrostatic ink jet printing heads comprise the printing electrodes  4  which are formed by sputtering conductive material such as chromium to the whole surface of the substrate  1  and by patterning the conductive material by photolithography, and since the above conventional electrostatic ink jet printing heads comprise the meniscus forming members  9  are formed by laminating a photosensitivity macromolecular film on the substrate  1  and by patterning the photosensitivity macromolecular film by photolithography, the above conventional electrostatic ink jet printing heads have a complex manufacturing process and a high cost. In addition, since, in the above conventional electrostatic ink jet printing heads, the meniscus forming members  9  are made of the photosensitivity macromolecular film, a thickness of the meniscus forming members  9  are limited by a thickness of the photosensitivity macromolecular film. As a result, in the above conventional electrostatic ink jet printing heads, an amount of ink to be ejected is limited. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a electrostatic ink jet printing head which has a simple manufacturing process and a low cost. 
     It is another object of this invention to provide a electrostatic ink jet printing head in which the amount of ink to be ejected is not limited. 
     Other objects of this invention will become clear as the description proceeds. 
     According to the first aspect of this invention, there is provided an electrostatic ink jet printing head comprising: 
     a substrate made of electrically insulating material and having a principal surface: 
     a base film made of electrically insulating material and overlaying the principal surface; 
     a plurality of printing electrodes formed on the base film with a predetermined space left between two adjacent ones of the printing electrodes, the printing electrodes having electrically insulating coating films which coat surfaces of the printing electrodes, respectively, each of the printing electrodes having a main electrode portion formed on the base film and a protruding electrode portion which protrudes from the base film; and 
     a covering member made of insulating material and covering the main electrode portion, the covering member having an internal surface defining, in cooperation with the main electrode portions and the base film, an ink receiving space which receives ink containing toner particles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view of the first conventional electrostatic ink jet printing head; 
     FIG. 2 is a schematic perspective view of the second conventional electrostatic ink jet printing head; 
     FIG. 3 is a schematic perspective view of the third conventional electrostatic ink jet printing head; 
     FIG. 4 is a schematic plan view of the third conventional electrostatic ink jet printing head illustrated in FIG. 3; 
     FIG. 5 is a section view taken along a line A—A in FIG. 4; 
     FIG. 6 is a schematic perspective view of an electrostatic ink jet printing head according to the first embodiment of this invention; 
     FIG. 7 is a schematic plan view of the electrostatic ink jet printing head illustrated in FIG. 6; 
     FIG. 8 is a section view taken along a line B—B in FIG. 7; 
     FIG. 9 is a schematic plan view for use in describing a method of manufacturing printing electrodes of the electrostatic ink jet printing head illustrated in FIG. 6; 
     FIG. 10 is a schematic perspective view of an electrostatic ink jet printing head according to the second embodiment of this invention; 
     FIG. 11 is a schematic plan view of the electrostatic ink jet printing head illustrated in FIG. 10; 
     FIG. 12 is a section view taken along a line C—C in FIG. 1 1 ; 
     FIG. 13 is a section view taken along a line D—D in FIG. 11; 
     FIG. 14 is a schematic perspective view of an  30  electrostatic ink jet printing head according to the third embodiment of this invention; 
     FIG. 15 is a schematic plan view of the electrostatic ink jet printing head illustrated in FIG. 14; 
     FIG. 16 is a section view taken along a line E—E in  5 FIG. 15; 
     FIG. 17 is a section view taken along a line F—F in FIG. 15; 
     FIG. 18 is a schematic plan view of the electrostatic ink jet printing head according to a fourth embodiment of this invention; 
     FIG. 19 is a section view taken along a line G—G in FIG. 18; 
     FIG. 20 is a section view taken along a line H—H in FIG. 18; and 
     FIGS. 21A. 21B and  21 C are waveform graphs for use in describing an operate of the electrostatic ink jet printing head illustrated in FIG.  18 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 6,  7 ,  8 , and  9 , the description will proceed to an electrostatic ink jet printing head according to a first embodiment of this invention. Similar parts are designated by like reference numerals. 
     In FIG. 6, the electrostatic ink jet printing head comprises the substrate  1 , a base film  12 , the printing electrodes  13 , and the covering member  2 . The substrate  1  is made of insulating material such as plastic and has a principal surface. The base film  12  is made of insulating material such as polyimide and overlays the principal surface of the substrate. The base film  12  has a thickness of about 50 micron meter. The printing electrodes  13  are formed on the base film  12  with a predetermined space left between two adjacent ones of the printing electrodes  13  by pattern metal plating conductive material such as copper. Each of the printing electrodes  13  has a thickness of about 20 to 30 micron meter. The printing electrodes  13  are positioned with a space of 300 dpi, namely, 85 micron meter left between two adjacent ones of the printing electrodes  13 . Each of the printing electrodes  13  has a main electrode portion  13   a  formed on the base film  12  and a protruding electrode portion  13   b  which protrudes from the base film  12 . The protruding electrode portion  13   b  has a length of about 80 to 100 micronmeter. Each of the printing electrodes  13  has an insulating coating film  13   c  which coat surfaces of the printing electrodes  13 . The insulating coating film  13   c  has a homogeneous thickness of 10 micron meter or less. Each of the printing electrodes  13  has a shape of a tape type. Particularly, each of the printing electrodes  13  is formed by a tape automated bonding (TAB) tape which is made by tape automated bonding (TAB). The insulating coating film  13   c  is formed by means of chemical vapor deposition (CVD) method which uses parylene resin. 
     The covering member  2  is made of insulating material and covers the main electrode portions  13   a  of the printing electrodes  13 . The covering member  2  has an internal surface defining, in cooperation with the main electrode portions and the base film, the ink receiving space  3  which receives ink  14  containing coloring thermoplastic particles, namely, toner particles. The covering member  2  has the ink supply port and the ink exhaust port (not shown). The ink  14  is filled in the ink receiving space  3  through the ink supply port  10 . The covering member  2  forms an ink ejecting nozzle  15  of a slit type in cooperation with the base film  12  and the printing electrodes  13 . The ink meniscus  16  is formed in the vicinity of the ink ejecting nozzle  15 . The ink supply port  10  is connected to an ink tank (not shown) through a tube. The ink  14  in the ink receiving space  3  is supplied with a negative pressure of about 1 cmH 2 O and is circulated under pressure. The ink  14  consists of petroleum organic solvent (such as isoparaffin), the toner particles, and charge controlling material which are dispersed in the petroleum organic solvent. The toner particles has an electric charge of positive polarity due to zeta potential. 
     In FIGS. 7 and 8, the ink  14 , by a surface tension, forms the ink meniscus  16  in the vicinity of the ink ejecting nozzle  15 . Since the ink  14  in the ink receiving space  3  is supplied with a negative pressure, and since the printing electrodes  13  have the protruding electrode portions  13   b  which protrude from the base film  12  and the covering member  2 , the ink meniscus  16  has a concave shape which faces below and obliquely in case of viewing the ink meniscus  16  from the side. In addition, since each of the printing electrodes  13  has a protruding electrode portion  13  which protrudes from the ink ejecting nozzle  15 , the ink meniscus  16  corresponds the printing electrodes  13 . Therefore, when an optional one of the printing electrodes  13  is supplied with the printing pulse voltage, the electric field is concentrated at an end of the ink meniscus  16  which is formed in the protruding electrode portion  13   b  of the printing electrode  13 . As a result, the toner particles charged in the ink  14  are, by the electric field, strained from the end of the ink meniscus  16  that is protruded to fly, as an agglomeration of the toner particles, in a direction of the counter electrode (not shown), namely, the recording paper. Next, the agglomeration of the toner particles is, as a printing dot, adhered onto the recording paper and is heated to be fixed. 
     Referring to FIG. 9, the description will proceed to a method of making the printing electrodes that uses the TAB tape. A film  17  of the TAB is prepared. The film  17  is made of, for example, polyimide and has sprocket holes  18  at both ends. Next, flash metal plating is formed on the film  17 . Next, a dry film is laminated on the film  17 . Next, a pattern is formed by exposing and developing the dry film. Next, metal plating of, for example, such as copper is formed on the pattern. Through holes  19  are formed by etching the film  17 . Next, a resist film is taken away from the film  17  and a finished metal plating is formed on the film  17 . Next, an insulating coating film is, by the CVD method, formed on a necessary portion of the film  17 . The protruding electrode portions  13   b  of the printing electrodes  13  are formed in the through holes  19 . 
     Referring to FIG. 10, the description will proceed to an electrostatic ink jet printing head according to a second embodiment of this invention. Similar parts are designated by like reference numerals. 
     The electrostatic ink jet printing head further  10  comprises a migration electrode  20 . The migration electrode  20  is positioned in the ink receiving space  3  so that the migration electrode  20  is in contact with the ink  14 . The migration electrode  20  is supplied with a migration control voltage which has the same polarity as an electric potential of the toner particles. The migration electrode  20 , due to electric migration phenomena, migrates the toner particles in the ink  14  to the vicinity of the protruding electrode portions  13   b  of the printing electrodes  13 . 
     Referring to FIGS. 11,  12 , and  13 , the description will proceed to an electrostatic ink jet printing head according to the third embodiment of this invention. Similar parts are designated by like reference numerals. The electrostatic ink jet printing head comprises the covering member  2  having a front end  2   a  which is in contact with the printing electrodes  13 . In this case, spaces among the printing electrodes  13  and the front end  2   a  of the covering member  2  forms passages which pass the ink  14 . The electrostatic ink jet printing head may comprise the migration electrode  20 . 
     Referring to FIGS. 14,  15 ,  16 , and  17 , the description will proceed to an electrostatic ink jet printing head according to the fourth embodiment of this invention. Similar parts are designated by like reference nurmerals. 
     The electrostatic ink jet printing head comprises the substrate  1 , the printing electrodes  13 , the base film  12 , passage control electrodes  21 , and the covering member  2 . 
     The printing electrodes  13  are formed on the principal surface with a predetermined space left between two adjacent ones of the printing electrodes  13 . The base film  12  overlays the main electrode portions  13   a  of the printing electrodes  13  and the principal surface. The passage control electrodes  21  are formed on the base film  12  with a predetermined space left between two adjacent ones of the passage control electrodes  21 . Each of the passage control electrodes  21  is positioned between adjacent ones of the printing electrodes  13 . The passage control electrodes  21  are formed by a method which is similar to the method of forming the printing electrodes  13 . Each of the passage control electrodes  21  has a thickness of 20 to 30 micron meter. The passage control electrodes  21  are positioned apart with a space of 300 dpi, namely, about 85 micron meter left between two adjacent ones of the passage control electrodes  21 . The covering member  2  covers the passage control electrodes  21 . The covering member has the internal surface defining, in cooperation with the passage control electrodes and the base film  12 , the ink receiving space  3  which receives the ink  14  containing toner particles. 
     The passage control electrodes  21  are supplied with passage control pulse voltages when the printing electrodes  13  are selectively supplied with the printing pulse voltages. 
     Each of the passage control pulse voltages has the same polarlity as the printing pulse voltage and a level lower than a printing level of the printing pulse voltage. Also, each of the passage control pulse voltages may have a different polarlity from the printing pulse voltage. When the passage control electrodes  21  are supplied with passage control pulse voltages, passage control electric fields are caused in vicinities of the passage control electrodes  21 . The passage control electric fields prevent the toner particulates in the vicinities of the passage control electrodes  21  from moving to the printing electrodes  13 . In addition, the electrostatic ink jet printing head may comprise the migration electrode  20 . Also, the electrostatic ink jet printing head may comprise the covering member  2  having the front end  2   a  which is contact with the passage control electrodes  21 . 
     Referring to FIGS. 18,  19 ,  20 , and  21 , the description will proceed to an electrostatic ink jet printing head according to the fifth embodiment of this invention. Similar parts are designated by like reference numerals. 
     In FIGS. 18,  19 , and  20 , the electrostatic ink jet printing head comprises a substrate  1 , printing electrodes  13 , a base film  12 , passage control electrodes  21 , and a covering member  2 . The passage control electrodes  21  are formed on the base film  12  with a predetermined space left between two adjacent ones of the passage control electrodes  21  so that each of the passage control electrodes  21  is positioned right above one of the printing electrodes  13 . Each of the passage control electrodes  21  has a insulating coating film  21  a which coats the passage control electrodes  21 . 
     When one of the printing electrodes  13  is supplied with printing pulse voltage Vp as shown in FIG. 21A, ones of the passage control electrodes  21  except one of the passage control electrodes  21  that is positioned right above the one of the printing electrodes  13  are supplied with the passage control pulse voltages Vc as shown in FIGS. 21B and 21C. Likewise, when one of the printing electrodes  13  is supplied with printing pulse voltage Vp as shown in FIG. 21B, ones of the passage control electrodes  21  except one of the passage control electrodes  21  that is positioned right above the one of the printing electrodes  13  are supplied with the passage control pulse voltages Vc as shown in FIGS. 21A and 21C. Also, when one of the printing electrodes  13  is supplied with printing pulse voltage Vp as shown in FIG. 21 C, ones of the passage control electrodes  21  except one of the passage control electrodes  21  that is positioned right above the one of the printing electrodes  13  are supplied with the passage control pulse voltages Vc as shown in FIGS. 21A and 21B. 
     Each of the passage control pulse voltages has the different polarlity from the printing pulse voltage. Also, each of the passage control pulse voltages may have the same polarlity as the printing pulse voltage and a low level lower than a printing level of the printing pulse voltage. When the passage control electrodes  21  are supplied with the passage control pulse voltages, passage control electric fields are caused in vicinity of the passage control electrodes  21 . The passage control electric fields prevent the toner particles in the vicinity of the passage control electrodes  21  from moving to the printing electrodes  13 . 
     In addition, the electrostatic ink jet printing head may comprise the migration electrode  20 . Also, the electrostatic ink jet printing head may comprise the covering member  2  having the front end  2   a  which is contact with the passage control electrodes  21 .