Patent Publication Number: US-6220695-B1

Title: Ink-jet recording device for suppressing excess electrophoretic accumulation of charged particles

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to an ink-jet recording device, and particularly, to an electrostatic inkjet recording device in which a recording is achieved by controlling coloring particles in a pigmentary ink through the electrophoretic effect and discharging a jet of coloring particles by means of electrostatic force. 
     An example of such a recording device is shown in FIG.  1 . In FIG. 1, the ink-jet recording device comprises an ink chamber  201  filled with pigmentary ink, an electrophoretic electrode  203  for gathering the coloring particle  206  or charged toner around an ink discharge aperture  202  by means of the electrophoretic effect, and a discharge electrode  205  for discharging a jet of charged toner gathered around the ink discharge aperture toward a recording medium  204  to record on. 
     The ink chamber  201  is provided within a dielectric member  208 . The ink discharge aperture  202  is provided in the dielectric member  208  for communicating the inside and the out side of the ink chamber  201 . The discharge electrode  205  has a long and narrow form disposed in the ink discharging direction, and has its end pointed like a needle so as to facilitate electric field concentration on the end. The electrophoretic electrode  203  is fixed as one body on the side and back surfaces of the dielectric member  208 . A grounded facing electrode  214  is provided counter the ink discharge aperture  202  via the recording medium  204 . The pigmentary ink comprises petroleum organic solvent (isoparaffin) and coloring particles  206  of thermoplastic resin colored with electrification control agent or toner dispersed in the organic solvent. The toner is charged apparently positive by the zeta potential. The recording object  204  is ordinary paper. The electrophoretic electrode  203  and the discharge electrode  205  are connected to a voltage driver (not shown) for applying predetermined voltages with a polarity opposite from that of the coloring particle  206  to the electrodes  203  and  205  in predetermined timing. 
     FIG. 2 is a diagram showing the voltages applied to the electrophoretic electrode  203  and the discharge electrode  205  in printing operation of the recording device of FIG.  1 . In FIG. 2, if a constant voltage V 1  is applied to the electrophoretic electrode  203  as shown in FIG. 2A, forming an electric field in the ink chamber  201  filled with pigmentary ink, then the coloring particles  206  in the pigmentary ink moves at an electrophoretic speed toward the ink discharge aperture  202  by the action of the electric field, and eventually gathers around the ink discharge aperture  202 . If a pulse voltage V 2  with a duration of T 2  is applied to the discharge electrode  205  with the coloring particles  206  gathered around the ink discharge aperture, then a jet of coloring ink is discharged in synchronism with the pulse voltage V 2  from the ink discharge aperture  202  in the form of a flying particle group  207 , which adheres to the recording medium  204 . Subsequently, the electric field formed by the electrophoretic electrode  203  causes coloring particles  206  to be supplied to the ink discharge aperture  202 . Thus, the discharge of coloring particle is repeated, resulting in a formation of image on the recording medium  204 . 
     However, in the conventional ink-jet recording device as described above, the coloring particles  206  keep moving toward the ink discharge aperture  202  as long as the constant voltage VI shown in FIG. 2 is applied to the electrophoretic electrode  203 . If no coloring particle  206  has been discharged from the ink discharge aperture  202  for a long time, then too many coloring particles will gathering around the ink discharge aperture  202 , causing an obstruction in the ink discharge aperture  202 . This prevents stable discharging of coloring particles  202 , having adverse influence on the quality of printed images. 
     The above and other problems in the prior art are solved and advances are made by the present invention. It is an object of the invention to provide an electrostatic ink-jet recording device for providing a stable printing by preventing too many coloring particles from gathering around the ink discharge aperture. 
     SUMMARY OF THE INVENTION 
     According to the invention, an electrostatic ink-jet recording device provides stable printing by preventing too much charged toner or too many coloring particles from gathering around an ink discharge aperture even if the charged toner has not been discharged for a long time. If the charged toner is not to be discharged, the potential difference between the electrophoretic electrode and the discharge electrode is controlled so as to suppress electrophoretic movement of the charged toner toward the ink discharging aperture, or the potential difference is made zero. This prevents the accumulation of too much toner around the ink discharge aperture. If the charged toner is to be discharged, the voltage applied to the discharge electrode is kept lower than the voltage applied to the electrophoretic electrode for a predetermined period of time before discharging a jet of the charged toner. This enables sufficient charged tone to gather around the ink discharge aperture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram showing a part involved in printing in a conventional ink-jet recording device; 
     FIGS. 2A and 2B are diagrams showing the voltages applied to the electrophoretic electrode and the discharge electrode in printing operation of the recording device of FIG. 1; 
     FIG. 3 is a schematic diagram showing a relevant part of an exemplary arrangement of an ink-jet recording device according to the present invention; 
     FIG. 4 is a flow chart showing a flow of operation executed by a controller under the control of a program stored in a ROM; and 
     FIGS. 5A and 5B are diagrams showing the voltages applied to the electrophoretic electrode and the discharge electrode in printing operation of the controller unit of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, an illustrative embodiment of the invention will be described in the following. 
     FIG. 3 is a schematic diagram showing a relevant part of an exemplary arrangement of an ink-jet recording device according to the present invention. In FIG. 3, the elements denoted by the same numerals as those of FIG. 1 are identical to corresponding elements of FIG. 1, and accordingly their descriptions are omitted. 
     As shown in FIG. 3, the controller unit  100  comprises an input interface  113  for receiving printing data including print control codes from an external device (not shown), a voltage supplier  111  connected to the electrophoretic electrode  203  and the discharge electrode  205  for supplying voltages thereto, and a controller  110  connected with the input interface  113  and the electrophoretic electrode  203  via bus lines for analyzing the printing data and giving instructions to the voltage supplier  111 . Specifically, the controller  110  comprises a read only memory (ROM)  115  for storing a program for controlling the recording device, a random access memory (RAM)  119  for storing data for use in operation of the recording device, and a central processing unit (CPU)  117  for controlling operation of the recording device under the control of the program stored in ROM  115 . 
     Referring to FIGS. 4 and 5, operation of the ink-jet recording device  10  of FIG. 3 will now be described. FIG. 4 is a flow chart showing a flow of operation executed by the controller  110  under the control of a program stored in the ROM  115 . FIG. 5 is a diagram showing the voltages applied to the electrophoretic electrode  203  and the discharge electrode  205  in a printing operation of the controller unit  100  of FIG.  3 . In FIG. 4, when the ink-jet recording device  10  is activated, the controller  110  enters the flow at step  20 , and proceeds to step  22 , where the CPU  117  controls the voltage supplier  111  to apply a voltage V 1  of the same polarity as the coloring particles  206  have to the electrophoretic electrode  203  and the discharge electrode  205 . At this time, no electric field is formed between the electrophoretic electrode  203  and the discharge electrode  205 , causing no electrostatic force to act on the coloring particles  206  in the ink. In step  24 , a check is made to see if any printing data has been input. If not, the flow returns to step  24 . If the answer is YES in step  24 , the controller  110  analyzes the input printing data to see if a jet of toner is to be discharged in step  26 . If so, the controller  110  proceeds to step  28 , where the controller  110  controls the voltage supplier  111  to supply the discharge electrode  205  with a voltage lower than the voltage V 1 , e.g., 0V in this embodiment. This causes an electric field to be formed between the electrophoretic electrode  203  and the discharge electrode  205  causing the coloring particles  206  in the ink to gather around the ink discharge aperture  202  by means of the electrophoretic effect. Then, the controller  110  waits for a predetermined period of time in step  30 . If the predetermined period of time has elapsed, the controller  110  proceeds to step  32  to control the voltage supplier  111  to apply a pulse voltage of a value V 3  with a pulse width of T 2  to the discharge electrode  205  in step  32 . The predetermined period of time is preferably set for long enough time period to permit a sufficient quantity of coloring particles to gather around the ink discharge aperture  203 . Once a pulse voltage V 3  is applied to the discharge electrode  205 , an electric field is formed between the discharge electrode  205  and the facing electrode  214 , a group of coloring particles  206  are pulled apart from the top of the ink meniscus into a group of flying particle  207  to adhere to the recording medium  204  forming a dot. Subsequently, with movement of the recording medium  204 , the procedures from step  24  to step  32  are repeated resulting in a desired printing pattern on the surface of the recording medium. On the other hand, if the answer is NO in step  26 , the controller  110  controls, in step  34 , the voltage supplier  111  to supply the discharge electrode  205  with the voltage V 1  which is equal to the voltage applied to the electrophoretic electrode  203 . This stops the formation of the electric field between the electrophoretic electrode  203  and the discharge electrode  205 , and accordingly the electrophoretic movement of the coloring particles  206 , thereby preventing too many coloring particles  206  from gathering around the ink discharge aperture  202 . After completing the action of step  34 , the controller returns to step  24 . 
     As described above, stopping charged particles  206  from gathering around the ink discharge aperture  202  while coloring particles are not to be discharged prevents too many coloring particles from gathering around the ink discharge aperture, resulting in a stability of printed image without a catch in the ink discharge aperture  202 . 
     Further, if coloring particles  206  are to be discharged in printing, the discharge of coloring particles  206  is guaranteed because coloring particles are gathered around the ink discharge aperture  202  in advance. 
     The voltage applied to the electrophoretic electrode  203  is always kept a constant value V 1  even when the electrophoretic movement of coloring particles is stopped, which eliminates the need of a complicated control system for controlling the voltage applied to the electrophoretic electrode  203 . This contributes to a simplification of the control system and accordingly a reduction in cost. 
     It is noted that the voltage which is applied to the discharge electrode  205  a predetermined period of time before the discharge of coloring particles  206  has not to be necessarily zero, if it is lower than the voltage applied to the electrophoretic electrode  203 . 
     Though the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims. It is to be understood that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.