Abstract:
The present invention relates to a sheet conveying method in use of a conveyance belt formed with a plurality of electrodes for attracting a sheet with electrostatic force. The electrodes are applied with a voltage changed along with the lapse of time for generating attraction force, and the integral value of voltage applied to one electrode has a polarity different from that of the integral value of voltage applied to another adjacent electrode.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a recording apparatus, a sheet conveying method for the recording apparatus, and a sheet attracting conveying device for the recording apparatus.  
           [0003]    2. Description of Related Art  
           [0004]    Among inkjet recording apparatuses, an inkjet recording apparatus using a full-line type recording head can record at high speed with high definition. A generally known method for attracting and conveying a conveying object is to dispose an electrically conductive electrode to the recording apparatus and create electrostatic force by applying electric charge to the electrode.  
           [0005]    A description of prior art is given hereinafter with reference to FIG. 4  and FIG.5. FIG. 4  is a cross-sectional view of a conveyance belt. The conveyance belt includes a first electrode  36   a,  a second electrode  36   b,  which are formed from electrically conductive resin respectively, a base layer  36   c,  and a surface layer. In FIG.5, a voltage supplying means includes a voltage supplying brush  51  contacting with a voltage receiving portion for supplying electric charge thereto, a voltage supplying electrode  52 , and a support member  53 . The surface layer and the voltage receiving portion  36   e  form a flat plane. Electric charge is supplied constantly from the voltage supplying brush  51  to the first electrode  36   a  for creating electrostatic force. Therefore, a suitable and a constant attraction force can be generated.  
           [0006]    However, the comb-teeth electrode of the conventional conveying apparatus, in general, being constantly applied with same voltage raises problems such as staining of an image due to ink mist attracted to the proximity of the electrode or blurring of an image due to ink droplets deviating from a targeted impact area.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of this invention to prevent mist adhering upon a sheet and to prevent impact deviation of ink droplets.  
           [0008]    In means to achieve the foregoing object, a representative structure of this invention is a sheet conveying method in use of a conveyance belt formed with a plurality of electrodes for attracting a sheet with electrostatic force, wherein the electrodes are applied with a voltage changed along with the lapse of time for generating attraction force, and wherein the integral value of voltage applied to one electrode has a polarity different from that of the integral value of voltage applied to another adjacent electrode.  
           [0009]    Adherence of ink mist upon a recording paper and impact deviation of ink droplets can be prevented since the sheet conveying method of this invention is characterized by using a conveyance belt formed with a plurality of electrodes for attracting a sheet with electrostatic force, wherein the electrodes are applied with a voltage changed along with the lapse of time for generating attraction force, and wherein the integral value of voltage applied to one electrode has a polarity different from that of the integral value of voltage applied to another adjacent electrode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The above and other objects and features of the invention are apparent to those skilled in the art from the following preferred embodiments thereof when considered in conjunction with the accompanied drawings, in which:  
         [0011]    FIG. 1  is an explanatory view showing an attraction force generating means;  
         [0012]    FIG. 2  is an explanatory view showing the positional relation among an attraction force generating means, a conveyance belt and a recording head;  
         [0013]    FIG. 3  is a top plan view showing a conveyance belt;  
         [0014]    FIG. 4  is an explanatory view showing an interior of a conveyance belt;  
         [0015]    FIG. 5  is an explanatory view showing an attraction force generating means;  
         [0016]    FIG. 6  is a view showing an overall mechanical structure of a recording apparatus; and  
         [0017]    FIG. 7  is a view showing a result of measuring the electric potential of a belt surface. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    First Embodiment  
         [0019]    FIG. 1  is an explanatory view showing a voltage supplying means of this invention. The voltage supplying means includes a voltage supplying brush  51 , a voltage supplying electrode  52 , and a support member  53 . The right side of FIG. 1  is the sheet feeding side and the left side thereof is the sheet delivery side, wherein a recording paper P (sheet) is conveyed from the right side to the left side of FIG.1. The voltage supplying brush  51  contacts with a voltage receiving portion of a belt and supplies electric voltage to the voltage receiving portion of the belt.  
         [0020]    FIG. 2  is an explanatory view showing the positional relation among a voltage supplying means, a conveyance belt serving as a sheet conveying means, and a recording head  7  serving as a recording means. A conveyed recording paper P is attracted to the belt. At the area where the paper P begins to pass beneath a recording head  7 Y, the recording paper P is attracted to the belt with a prescribed voltage. Each recording head is aligned with an interval of 2 cm, and the width of the recording head in the conveying direction is 2 cm. The length of the voltage supplying electrode  52 , which is 20 cm, is the necessary and sufficient length for supplying voltage from the beginning of the recording head  7 Y arranged most upstream to a recording head  7 K arranged most downstream.  
         [0021]    FIG. 3  is an explanatory view showing a conveyance belt  31  and an attraction force generating means  36 . FIG. 3  shows the belt when the apparatus body is seen directly from above. The attraction force generating means  36  includes the first electrodes  36   a  and the second electrodes  36   b.  The first electrode  36   a  and the second electrode  36   b  have strip-like shapes and are plurally aligned parallel to each other in the width direction of the conveyance belt  31 . The first electrodes  36   a  and the second electrodes  36   b  face to each other in a direction perpendicularly intersecting with the conveying direction of the belt to form a comb-teeth shape.  
         [0022]    A voltage receiving portion  36   e   1  of the first electrode  36   a  (first voltage receiving portion) and a voltage receiving portion  36   e   2  of the second electrode  36   a  (second voltage receiving portion), having a length longer than the width of each electrode  36   a,    36   b,  are arranged on both sides of the conveyance belt  31  with respect to a conveying direction, and thereby positioning the voltage receiving portion  36   e   1  on a right end side with respect to the conveying direction of the conveyance belt  31  and the voltage receiving portion  36   e   2  on a left side with respect to the conveying direction of the conveyance belt  31 . The attraction force generating means  36  also includes a conductive voltage supplying brush  51  for contacting with each voltage receiving portion  36   e  with a prescribed pressure.  
         [0023]    An AC voltage biased with a positive bias voltage from a high voltage electric source (not shown) is applied to the voltage receiving portion  36   e   1  via the voltage supplying brush  51 . An AC voltage biased with a negative bias voltage is applied to the voltage receiving portion  36   e   2 . It is to be noted that the voltage supplying brush  51  is preferably made of a conductive material having a volume resistivity no more than 10 5  Ohms-cm.  
         [0024]    FIG. 4  is a view for explaining the generation of attraction force by the attraction force generating means  36 . When electric voltage is applied to the first electrodes  36   a,  an electric force is created in the arrow direction, thereby forming an electricity field line. From the electric potential difference between the first electrodes  36   a  and the second electrodes  36   b,  attraction force is created at the upper side of the conveyance belt  31  allowing the recording paper P on the conveyance belt  31  to be attracted to the conveyance belt  31 . Since the surface layer  36   d  in this invention has a higher volume resistivity than that of the base layer  36   c,  the electricity field line and the sheet attraction force can be generated greater at the upper side of the conveyance belt  31 .  
         [0025]    FIG. 5  is an explanatory view of a voltage supplying means supplying electricity to a conveyance belt. The attraction force generating means  36  comprises the first electrode  36   a,  the second electrode  36   b,  formed from conductive material respectively, the base layer  36   c,  the surface layer  36   d,  the voltage receiving portion  36   e,  the voltage supplying brush  51 , the voltage supplying electrode  52 , and the support member  53 . The voltage receiving portion  36   e  is disposed to share a same plane with the surface layer  36   d.  The voltage supplying brush  51  contacts the voltage receiving portion  36   e  with a certain pressure and supplies voltage thereto. The base layer  36   c  and the surface layer  36   d,  which are formed from dielectric material, have the first electrode  36   a  and the second electrode  36   b  protectively disposed therebetween.  
         [0026]    The base layer  36   c  has a volume resistivity from 10 12  Ohms-cm to 10 17  Ohms-cm, and the surface layer  36   d  has a volume resistivity from 10 9  Ohms-cm to 10 14  Ohms-cm, in which both are formed from a synthetic resin such as polyethylene, polypropylene, polyamide, polycarbonate, PTFE (polytetrafluoroethylene), PFE (polyfluoroethylene), PVDF (polyvinylidene fluoride), polyimide, or silicone resin. The voltage receiving portion  36   e  has a volume resistivity from 10 −1  Ohms-cm to 10 5  Ohms-cm and is formed from a conductive synthetic resin comprising carbon, or a conductive paste mixed with silver or copper powder. The water repellency for the surface layer  36   d  and the top surface of the voltage receiving portion  36   e  can be further improved by applying, for example, a fluororesin process thereto.  
         [0027]    FIG. 6  is a cross-sectional view showing an overall structure of a recording apparatus of this invention. In the feeding section, a pushing plate  21  for stacking recording paper P and a feeding rotary body  22  for feeding recording paper P are attached to a base  20 , and the pushing plate  21  is rotatable around a rotation axis engaged to the base  20  and is pushed by a pushing plate spring  24  toward the feeding rotary body  22 . The pushing plate  21  has a separation pad (not shown) with a high frictional coefficient for preventing double-feed of recording paper P and a separation nail-member (not shown) for separating the recording paper P. A release cam (not shown) is provided at the base  20  for disengaging the contact between the pushing plate  21  and the feeding rotary body  22 .  
         [0028]    With this structure, during a standby mode, the release cam presses down the pushing plate  21 , thereby disengaging the contact between the pushing plate  21  and the feeding rotary body  22 . When a drive force of the conveyance roller  32 , while in this state, is transmitted to the feeding rotary body  22  and the release cam through gears or the like, the pushing plate  21  moves up to allow the feeding rotary body  22  contact with the recording paper P. The apparatus then starts sheet feeding upon picking up the recording paper P along with the rotation of the feeding rotary body  22 . The feeding rotary body  22  continues to rotate until the recording paper P is delivered to the conveyance section.  
         [0029]    The conveyance section serving to attract the recording paper P includes a conveyance belt  31  for sheet conveyance and an unshown PE (paper end) sensor. The conveyance belt  31  is driven by a driving roller  34  and is tensely wound around the conveyance roller  32  and a pressure roller  35 , which serve as a driven roller, respectively. The driving roller  34  is the driving source for a belt motor  50 .  
         [0030]    The conveyance belt  31  is formed from a synthetic resin such as polyethylene, polypropylene, polyamide, polycarbonate, PTFE (polytetrafluoroethylene), PFE (polyfluoroethylene), PVDF (polyvinylidene fluoride), polyimide, or silicone resin and is shaped as an endless belt. The voltage supplying means is earlier described in detail with reference to other drawings.  
         [0031]    The voltage supplying means applies a DC bias voltage of approximately 100V ±3 kV superimposed on an AC voltage for allowing the recording paper P to be closely attracted to the conveyance belt  31 . In tenes of the relation between the DC bias voltage and the AC voltage in such a case, the peak value of the AC voltage is preferable to be no more than 3 times of the DC bias voltage, and more preferable to be no more than the DC bias voltage since an exceedingly high AC voltage swinging greatly toward the opposite potential weakens the attraction force. The peak value of the AC voltage is preferable to be no less than ⅓ times of the DC bias voltage, and more preferable to be no less than ½ times of the DC bias voltage since an exceedingly low AC voltage weakens the effect of equalizing the surface potential of the recording paper (The explanation for a high voltage generating means and a high voltage control means will be omitted). It is to be noted that the conveyance belt is moved at a speed of 170 mm per second.  
         [0032]    The conveyance belt  31  and a pinch roller  33  driven by the conveyance roller  32  make contact in a position opposite from the conveyance roller  32 . The recording head  7  (the recording head  7 Y is for yellow, a recording head  7 M is for magenta, a recording head  7 C is for cyan, and the recording head  7 K is for black) is disposed downstream in the conveying direction of the conveyance roller  32 . The recording head  7 , which is a line-type inkjet recording head having plural nozzles arrayed in a direction perpendicularly intersecting to the conveying direction, uses a line-type head having a row of nozzles whose length is the same as or greater than sheet width. The recording head  7 , which is also a thermal type recording head, applies heat to ink with a heater or the like. The ink is subject to film boiling by the heat, and pressure change created by growing and shrinking of vapor bubbles caused by the film boiling makes the nozzles discharge the ink, thereby forming an image on the recording paper P. It is to be noted that the resolution in recording with the recording head is 600 dpi.  
         [0033]    The delivery section comprises a delivery roller  41  and a spur  42 . The recording paper P having an image formed thereto is delivered to a delivery tray  43  by being conveyed in a manner nipped between the delivery roller  41  and the spur  42 . It is to be noted that the area in which the spur contacts the recording paper P is small, and therefore, the spur  42  serving as a rotating member has no adverse effect on an ink image even when ejection of ink causes the ink image to contact with a side of the recording paper P upon which the image recorded.  
         [0034]    Numeral  38  is a cleaning roller used for cleaning the belt  31 . Numeral  39  is a de-electrifying brush used for earthing the charge remaining on the belt and for enabling easy sheet delivery. Numeral  50  is a belt motor serving as a driving source for rotating the conveyance belt.  
         [0035]    In this embodiment, a bias voltage of ±750 V superimposed on a sinusoidal wave voltage having an amplitude of 1500 V is applied to the positive electrodes and negative electrodes, respectively. The frequency of the sinusoidal wave in this case was 2500 Hz.  
         [0036]    In changing the voltage applied to the electrodes, it is preferable to change the voltage in a manner where there would be at least two or more maximum values of the surface potential while the recording head passes underneath a single electrode.  
         [0037]    In recording various images with the recording apparatus under the above settings for one hour, no irregularity was found in sheet conveyance, and the recording apparatus was able to ensure steady operation. The recording apparatus was able to record with a high quality with no mist accumulated on the surface of the recording paper P.  
         [0038]    Examples for solving the problem of images stained by ink mist attracted in the vicinity of an electrode is disclosed in Japanese Patent Laid-Open Publication No.Hei5-8392, in which an electrode is arranged contacting to a recording paper, and an electrode is arranged surrounding an ink discharging port for controlling the ink. However, the conveyance belt for the invention described in the publication is different from the belt with a comb-teeth electrode of the present invention, that is, the conveyance belt for the invention described in the publication is a type that attracts the recording paper by accumulating electrical charge on the surface of the belt.  
         [0039]    As a result of various attempts in attracting and conveying a recording paper by using a comb-teeth electrode, a waveform voltage, which is changed along with the lapse of time, is employed in this invention as the voltage applied to each electrode for generating attraction force. In one experiment, for example, an AC voltage biased with a DC bias voltage (various voltages which are combinations of a sinusoidal wave, a triangular wave, a serrate wave, a square wave, or a rectified wave of such waveforms, etc.) was supplied to the electrodes to serve as an attraction voltage allowing the positive electrode to constantly have positive electric potential and the negative electrode to constantly have negative electric potential. In consequence, the surface potential of the recording paper with respect to the attracted portion of the recording paper not only swung toward positive potential but also swung toward negative potential even at a position above the positive electrode, and swung not only toward negative potential but also toward positive potential even at a position above the negative electrode. That is, an AC voltage or the like biased with a DC bias voltage allows to change the surface potential of the recording paper and create an indefinite surface potential of the recording paper, thereby, preventing adherence of ink mist and impact deviation of ink droplets.  
         [0040]    Second Embodiment  
         [0041]    A new belt was attached to the apparatus of the first embodiment. In this embodiment, bias voltages of ±500 V, ±1000 V,and ±1500 V superimposed on sinusoidal waves voltages with amplitudes of 500 V, 1000 V, and 1500 V are applied to the positive and negative electrodes. In such a case, a conveyance experiment was performed by changing the AC frequency to 50 Hz, 100 Hz, 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz.  
         [0042]    In the experiment, FIG. 7  shows a result of measuring the surface potential of the conveyance belt when a recording paper is attracted to the conveyance belt under the conditions where bias voltage of ±500 V is superimposed on a sinusoidal wave with an amplitude of 1000 V. FIG. 7  is one example showing a result of measuring the surface potential of the belt in using the conveying method of this embodiment. The waveforms swing toward positive or negative at the portions of the electrodes where no recording paper is attached to the belt, and yet, the surface potential swing toward both positive and negative at the portions where the recording paper is attached to the belt. This restrains electrically charged mist from concentrating on a single portion of the recording paper. Therefore, concentration of mist and impact deviation of ink droplets can be prevented.  
         [0043]    In recording various images upon two thousand sheets with each of the frequencies, no irregularity was found in sheet conveyance, and the recording apparatus was able to ensure steady operation with all of the frequencies. However, when the frequency is 50 Hz, from which a value (V/f) no less than 2 can be extracted when dividing the moving speed of the conveyance belt V (mm) by the frequency of the AC voltage f (Hz), the load change upon the motor is greater in comparison with other conditions, and when the frequency is 2000 Hz, the attraction force tends to be weaker in comparison with other conditions. The attraction force also tends to be weaker in comparison with other conditions when bias voltage of ±500 V is superimposed on a sinusoidal wave with an amplitude of 500 V. The impact deviation of ink droplets, though being visible at slight portions through a microscope, is hardly visible through the naked eye when bias voltage of ±1500 V is superimposed on a sinusoidal wave with an amplitude of 500 V. Furthermore, recording of high quality is provided with no mist accumulated on the surface of the recording paper and with no blurring of image from impact deviation of ink droplets.  
         [0044]    Comparative Example  
         [0045]    In this example, the belt used in the second embodiment is attached to the apparatus in the first embodiment, in which recording and conveying were tested by applying DC voltages of ±500 V,±1000 V, ±1500 V, and ±2000 V to the positive and negative electrodes. In recording various images upon 50 sheets with each of the voltages, adherence of ink mist corresponding to the arrangement of the electrodes could be visually recognized through the naked eye in part of the images. This becomes more significant as the voltage becomes higher. When the voltage is ±1500 V or more, blurring of images from impact deviation of ink droplets could also be visually recognized through the naked eye.  
         [0046]    Other Embodiments  
         [0047]    This invention is not to be restricted to the foregoing embodiments in which the voltage receiving portion  36   e   1  of the first electrode  36   a  is disposed on one end of the conveyance belt  31  while the voltage receiving portion  36   e   2  is disposed on the other opposite end of the conveyance belt  31 . The voltage receiving portion  36   e   1  of the first electrode  36   a  and the voltage receiving portion  36   e   2  of the second electrode  36   b  can also be disposed on the same end of the conveyance belt  31 .