Abstract:
An inkjet device ensuring reliable and stable ink ejection is disclosed. A plurality of ejection electrodes are arranged in a housing having an ink chamber containing ink including toner particles and a gate electrode plate is placed at a predetermined distance from the ejection electrodes. The gate electrode plate has a slit formed such that the ejection electrodes are directed to the slit and an ink drain coupled to the slit for draining ink from the slit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inkjet recording apparatus which is capable of ejecting particulate matter such as pigment matter and toner matter by making use of an electric field, and more particularly to an improved arrangement of the inkjet recording apparatus. 
     2. Description of the Related Art 
     There has recently been a growing interest in non-impact recording methods, because noise while recording is extremely small to such a degree that it can be neglected. Particularly, inkjet recording methods are extremely effective in that they are structurally simple and that they can perform high-speed recording directly onto ordinary medium. As one of the inkjet recording methods, there is an electrostatic inkjet recording method. 
     The electrostatic inkjet recording apparatus generally has an electrostatic inkjet head and a counter electrode which is disposed behind the recording medium to form an electric field. The electrostatic inkjet head has an ink chamber which temporarily stores ink containing toner particles and a plurality of ejection electrodes formed near the end of the ink chamber and directed toward the counter electrode. The ink near the front end of the ejection electrode forms a concave meniscus due to its surface tension, and consequently, the ink is supplied to the front end of the ejection electrode. If positive voltage relative to the counter electrode is supplied to a certain ejection electrode of the head, then the particulate matter in ink will be moved toward the front end of that ejection electrode by the electric field generated between the ejection electrode and the counter electrode. When the coulomb force due to the electric field between the ejection electrode and the counter electrode considerably exceeds the surface tension of the ink liquid, the particulate matter reaching the front end of the ejection electrode is jetted toward the counter electrode as an agglomeration of particulate matter having a small quantity of liquid, and consequently, the jetted agglomeration adheres to the surface of the recording medium. Thus, by applying pulses of positive voltage to a desired ejection electrode, agglomerations of particulate matter are jetted in sequence from the front end of the ejection electrode, and printing is performed. An inkjet head like this is disclosed, for example, in Japan Laid-Open Patent Publication No. 60-228162. 
     As another conventional example, there has been disclosed an electrostatic inkjet head having a gate electrode provided in front of an ink electrode in Japan Laid-Open Patent Publication No. 1-165452. The gate electrode has an opening or slit through which ink droplets are jetted. Since the distance between the gate electrode and the ink electrode is relatively short, ink ejection occurs when applying a lower driving voltage to the ink electrode. 
     However, when the inkjet head starts moving, vibrations from head movement cause the ink in the ink chamber to flow from the nozzle of the inkjet head to outside, which eventually forms an ink bridge between the nozzle and the opening of the gate electrode. There are cases where the opening of the gate electrode is blocked with the ink bridge and ink ejection becomes impossible, resulting in deteriorated printing quality. Even in the case of no ink bridge, the jetted ink is left in meniscus form in the opening of the gate electrode, which may also causes impossible ink ejection. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an inkjet head that can perform ink ejection with reliability and stability. 
     Another object of the present invention is to provide a novel arrangement of an inkjet head that can effectively remove remaining ink from an opening in front of an ejection electrode. 
     According to the present invention, an inkjet head is provided with a plurality of ejection electrodes arranged in an ink chamber containing ink including particulate matter. The inkjet head is further provided with a front end plate that is laced at a predetermined distance from the ejection electrodes. The front end plate has a slit formed such that the ejection electrodes are directed to the slit and an ink drain coupled to the slit for draining ink from the slit. 
     The ink drain may has at least one drain slit formed in the front end plate and a width of the drain slit may be smaller than that of the slit so that capillary action occurs. 
     Further, the ink drain may include at least one drain slit formed in the front end plate and an ink absorber provided to the front end plate at a position corresponding to the drain slit. 
     The front end plate may be a conductive plate to which a predetermined voltage is applied to generate a voltage difference causing ink ejection of an ejection electrode when the ejection electrode is driven. 
     Since the ink drain is coupled to the slit to drain ink from the slit, even though an ink bridge is formed between the ejection electrodes and the front end plate due to vibrations or the like, the ink is immediately removed from the slit through the ink drain. Therefore, reliable and stable ink ejection can be achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages will become apparent from the following detailed description when read in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a part-exploded perspective view showing the schematic constitution of an inkjet recording apparatus according to an embodiment of the present invention; 
     FIG. 2 is a plan view showing a gate electrode used in the inkjet recording apparatus as shown in FIG. 1; 
     FIG. 3 is a diagram showing an ink circulating system for supplying ink to the embodiment; 
     FIG. 4 is a cross-sectional view of the inkjet recording apparatus for explanation of advantages of the present invention; 
     FIG. 5 is a cross-sectional view of the inkjet recording apparatus for explanation of advantages of the present invention; and 
     FIG. 6 is a plan view showing a gate electrode used in the inkjet recording apparatus according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an inkjet head  10  is comprised of a housing  101  that is provided with an ink supply port  102  and an ink discharge port  103  on the top and bottom thereof. An array of ejection electrodes  104  is provided within the ink chamber of the housing  101  such that the front ends of the ejection electrodes  104  protrude through the nozzle formed in the front surface of the hosing  101 . Each ejection electrode ejects the particulate matter from the protruding end thereof when a driving voltage is applied thereto. 
     The housing  101  is further provided with arms  105  each having a predetermined length extending in the ink-ejection direction and the arms  105  has a gate electrode  106  fixed thereto. The gate electrode  106  is shaped like a plate and has an ejection slit  107  and drain slits  108  and  109  cut through the plate thereof. The gate electrode  106  is placed at a predetermined distance from the front ends of the ejection electrodes  104  such that the particulate matter ejected from the ejection electrodes  104  passes through the ejection slit  107 . The gate electrode  106  is a conductive plate made of metal. The gate electrode  106  further has a pair of ink absorbers  110  and  111  fixed on the back thereof corresponding respectively to the drain slits  108  and  109 . In other words, the drain slits  108  and  109  forms an ink absorbing means with the ink absorbers  110  and  111 . The details of the gate electrode  106  will be described hereinafter. 
     Referring to FIG. 2, the gate electrode  106  has the ejection slit  107  extending in the direction of the array of the ejection electrodes  104  so that ink droplets including particulate matter ejected from the ejection electrodes  104  pass through the ejection slit  107 . The gate electrode  106  has the drain slits  108  and  109  extending to end openings  112  and  113 , respectively. The respective ink absorbers  110  and  111  are provided at the positions of the drain slits  108  and  109 . The ejection slit  107  is coupled to the drain slits  108  and  109  at the ends thereof. In other words, a single bent slit is formed with the ejection slit  107  and the drain slits  108  and  109  which are coupled to each other. 
     The width W D  of the drain slit  108  and  109  is much smaller than the width W S  of the ejection slit  107 . As described before, the width W S  of the ejection slit  107  is designed to allow an ejected ink droplet to pass through the ejection slit  107 . The drain slit  108  and  109  are designed to drain the remaining ink from the ejection slit  107 . More specifically, the width W D  of the drain slit  108  and  109  is determined so that the capillary action occurs. Therefore, even when an ink bridge is formed and some ink remain in the ejection slit  107 , the remaining ink is drained from the ejection slit  107  and flows into the absorbers  110  and  111  through the drain slit  108  and  109  by capillary action. 
     The whole shape of the slits  107 - 109  is not limited to that as shown in FIG. 2 as long as the remaining ink is drained from the ejection slit  107 . Three or more drain slits may be formed in the gate electrode and an absorber for each drain slit may be provided on the back of the gate electrode  106 . 
     The ink absorbers  110  and  111  are made of material having the property of absorbing ink. Further, it is possible to provide the ink absorbers  110  and  111  with ink suction means to enhance ink draining. 
     Referring to FIG. 3, an ink reservoir  201  containing ink  202  is connected to the ink supply port  102  through an ink supply line  203  and an ink supply pump  204  and is further connected to the ink discharge port  103  through an ink discharge line  205  and an ink discharge pump  206 . The insulating ink including charged toner may be used as the ink  202 . 
     Referring to FIG. 4, an ink chamber  301  is formed within the housing  101  made of an insulating material and the ink  202  is supplied into the chamber  301  through the ink supply port  102  and the ink reducing in toner concentration is discharged from the chamber  301  through the ink discharge port  103 . Within the chamber  301  a substrate  302  made of an insulator is provided and has an array of needle-like ejection electrodes  104  formed thereon. Further, an electrophoresis electrode  303  is provided at the rear end of the upper half of the chamber  301 . The ejection electrodes  101  are covered with an insulating film and are provided in the chamber  301  such that the front ends of the ejection electrodes  104  protrude through a nozzle  304  formed in the front surface of the hosing  101 . 
     In the case where the chamber  301  is filled with the ink  202  supplied from the ink reservoir  201  and a predetermined positive voltage higher than the voltage of the gate electrode  106  is applied to the electrophoresis electrode  303 , an electric field is generated in the chamber  301 . The electric field moves the particulate matter such as toner particles toward the front ends of the ejection electrodes  104  due to the electrophoresis phenomenon and then the meniscuses are formed around the ejection electrodes  104 , respectively. 
     In general, the ink ejection from an ejection electrode requires that a voltage difference between the ejection electrode and the gate electrode  106  is equal to or greater than a predetermined threshold value. If the voltage difference is smaller than the threshold value, the ink ejection from that ejection electrode cannot occur. Therefore, by controlling the voltage difference between each ejection electrode and the gate electrode  106 , the ejection electrodes  104  selectively eject ink particles. Since the ejected ink is almost composed of toner particles, the ink flowing into the lower half of the chamber  301  through the ejection electrodes  104  reduces in toner concentration and it is discharged to the ink discharge port  103 . 
     Referring to FIG. 5, since the meniscuses are formed around the ejection electrodes  104 , vibrations from head movement cause the ink to flow from the nozzle  304  to outside. The overflowing ink forms an ink bridge  401  between the nozzle  304  and the ejection slit  107  of the gate electrode  106 . Since the gate electrode  106  has the drain slits  108  and  109  coupled to the ejection slit  107 , the ink of the ink bridge  401  immediately flows into the drain slits  108  and  109 . Therefore, the ink bridge  401  is drained from the ejection slit  107  and is then absorbed by the ink absorber  110  and  111 . 
     As described before, the whole shape of the slits  107 - 109  is not limited to that as shown in FIG. 2 as long as the remaining ink is drained from the ejection slit  107 . Another shape may be formed in the gate electrode  106  as shown in FIG.  6 . 
     Referring to FIG. 6, the ejection slit  107  is coupled to a plurality of drain slits  401  that are spaced at regular intervals in the longitude of the ejection electrode  107  with each drain slit extending in a downward direction. As in the case of Sig.  2 , the width W D  of each drain slit  401  is determined so that the capillary action occurs. Each of the drain slits  401  has a bend forming a first portion directly coupled to the ejection slit  107  and a second portion. The first portion extends on the skew with respect to the ejection slit  107 . The second portion extends in the direction normal to the ejection slit  107 . Further, an absorber  401  is placed on the back of the gate electrode  106  such that the second portions of the drain slits  401  are covered with a part of the absorber  402 . 
     Since a plurality of drain slits  401  are spaced at regular intervals with each extending in a downward direction, the remaining ink is efficiently drained from the ejection slit  107 . Further, it is preferable that each of the ejection electrodes  104  is placed at the position between two adjacent drain slits  401  to enhance ink draining. 
     While the invention has been described with reference to specific embodiments thereof, it will be appreciated by those skilled in the art that numerous variations, modifications, and any combination of the first and second embodiments are possible, and accordingly, all such variations, modifications, and combinations are to be regarded as being within the scope of the invention.