Patent Publication Number: US-2007122209-A1

Title: Hybrid type image forming apparatus

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
      This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2005-0114051, filed on Nov. 28, 2005, in the Korean Intellectual Property Office, the entire disclosure of which is hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an electrophotographic image forming apparatus. More particularly, the present invention relates to an image forming apparatus employing a hybrid development method.  
      2. Description of the Related Art  
      A development method of image forming apparatuses such as copying machines, printers, facsimiles, and multifunctional peripherals includes a variety of additional methods such as a dual component development method, a mono component development method, and a hybrid development method. The dual component development method uses a toner and a magnetic carrier. The mono component development method uses an insulation toner or a conductive toner. The hybrid development method comprises a non-magnetic toner charged by rubbing the non-magnetic toner against a magnetic carrier to allow only the charged toner to be attached on a developing roller and supplying the charged toner to an electrostatic latent image, thereby developing the electrostatic latent image.  
      Advantages of the dual component development method include excellent charging characteristics of the toner, long life, and a uniform beta image. Alternatively, the dual component development method has disadvantages that include a large apparatus size, a complicated structure, scattering of the toner, and attachment of the carrier onto the latent image.  
      The advantages of the mono component development method include its compact structure, excellent dot reproducibility and a background fog. The background fog means that the toner is attached on the background portion of a photosensitive body. The background fog occurs because much of toner charged at an opposite polarity exists on a developing roller. The dual component development method charges toner by mixing the toner with carrier and agitating the mixture. This reduces the possibility of generating the toner of the opposite polarity. However, the mono component development method has toner attached on a developing roller and then charges the toner by rubbing a regulating blade against the toner, so that the toner is not sufficiently charged and thus there is a high possibility that the toner of the opposite polarity is generated. Examination of an amount of charge of the toner using an E-Spart Analyzer, which is a device for measuring distribution of an amount of charge of particle by Hosokawa Micron Co., Ltd., demonstrates the existence of 10%-25% of toner with the opposite polarity.  
      The hybrid development method charges toner by mixing the toner with a carrier and agitating the mixture. A magnetic brush containing the carrier and the toner is formed on a magnetic roller. A bias moving the toner from the magnetic brush to the developing roller is applied between the magnetic roller and the developing roller. Only toner with an appropriate charged polarity caused by the bias is moved from the magnetic brush to the developing roller. Toner charged to an opposite polarity does not easily move to the developing roller. Therefore, this prevents the contamination of the background portion. Only the toner is supplied to a development region where a photosensitive body faces the developing roller. Therefore, it is possible to reduce attachment of the carrier onto a latent image or toner scattering. That is, the hybrid development method is a development method that takes advantage of the dual component development method and the mono component development method. However, the hybrid development method has a problem of a development ghost. The toner on the developing roller moves to the photosensitive body while passing through the development region where the photosensitive body faces the developing roller. After that, a sufficient amount of toner should be supplied to the developing roller so that a uniform toner layer may be formed on the developing roller. When the toner layer is not uniform, an afterimage of a previous development appears by a rotational period of the developing roller on an image developed on the photosensitive body, which is called a development ghost.  
      Accordingly, there is a need for an improved system and method for providing a hybrid type image forming apparatus capable of preventing a development ghost.  
     SUMMARY OF THE INVENTION  
      An aspect of exemplary embodiments of the present invention is to address at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of exemplary embodiments of the present invention is to provide a hybrid type image forming apparatus capable of preventing a development ghost.  
      Exemplary embodiments of the present invention also provide a hybrid type image forming apparatus capable of preventing contamination of a background portion.  
      Exemplary embodiments of the present invention also provide a hybrid type multi-color image forming apparatus capable of achieving a high quality and stable development by adopting a tri-level exposure method.  
      According to an aspect of an exemplary embodiment of the present invention, a hybrid type image forming apparatus is provided that uses a developer that contains a mixture of a toner and a carrier. An electrostatic latent image is formed on a photosensitive body. The apparatus also comprises a development unit with a magnetic roller forming a magnetic brush containing the toner and the carrier on the outer periphery of the magnetic roller. A developing roller is installed to prevent the developing roller from contacting the magnetic roller and the photosensitive body to develop the electrostatic latent image using the toner supplied from the magnetic brush, wherein the resistivity of the developer is no greater than 10 9  Ωcm.  
      The resistivity of the carrier may be no greater than 10 8  Ωcm.  
      The magnetic roller may include a rotating sleeve and a magnetic core with a plurality of magnetic poles forming the magnetic brush and arranged inside the sleeve. The plurality of magnetic poles include a pair of magnetic poles with the same polarity and facing a supply region where the magnetic roller faces the developing roller. In the supply region, movement directions of the surfaces of the sleeve and the developing roller may be similar.  
      The developing unit may further include a collision member installed in the supply region and colliding with the magnetic brush.  
      The magnetic roller may include a rotating sleeve and a magnetic core with a plurality of magnetic poles forming the magnetic brush and arranged inside the sleeve to rotate. The magnetic core may rotate in a direction opposite to a rotational direction of the sleeve. In a supply region where the developing roller faces the magnetic roller, movement directions of the surfaces of the sleeve and the developing roller may be similar.  
      A bias generating a pulsating electric field may be applied between the magnetic roller and the developing roller.  
      The hybrid type image forming apparatus may further include a charger, an exposer and a plurality of development units. The charger charges the photosensitive body, the exposer scans light on the photosensitive body and the plurality of development units contain toners of different colors, thereby printing a color image.  
      The hybrid type image forming apparatus may also include a first image forming unit, a charger, an exposer and two developing units. The first image forming unit comprises the photosensitive body, the charger charges the photosensitive body, the exposer exposes the photosensitive body in a tri-level exposure method and the two developing units containing toner of a first color and toner of a second color, respectively. The hybrid type image forming apparatus also includes a second image forming unit, a charger, an exposer, two developing units and an intermediate transfer body. The second image forming unit comprises the photosensitive body, the charger charges the photosensitive body, the exposer exposes the photosensitive body in a tri-level exposure method, and two developing units contain toner of a third color and toner of a fourth color, respectively and the intermediate transfer body transfers a toner image from the first and second image forming units, thereby printing a color image in a single-pass type.  
      The hybrid type image forming apparatus may include a charger, an exposer and four developing units. The charger charges the photosensitive body and the exposer exposes the photosensitive body in a tri-level exposure method. The four developing units contain toners of a first color, a second color, a third color, and a fourth color, respectively, thereby printing a color image in a two-pass type.  
      According to another aspect of an exemplary embodiment of the present invention, a hybrid type image forming apparatus using a developer that contains a mixture of a toner and a carrier is provided. According to an exemplary implementation, an electrostatic latent image is formed on a photosensitive body. A magnetic roller forms a magnetic brush including the toner and the carrier on the outer periphery of the magnetic roller. A developing roller is installed to prevent the developing roller from contacting the magnetic roller and the photosensitive body to develop the electrostatic latent image using the toner supplied from the magnetic brush. The magnetic roller includes a rotating sleeve, and a magnetic core with a plurality of magnetic poles forming the magnetic brush and arranged inside the sleeve. Also, the plurality of magnetic poles include a pair of magnetic poles with the same polarity and facing a supply region where the magnetic roller faces the developing roller.  
      In the supply region, movement directions of the surfaces of the sleeve and the developing roller may be similar. The image forming apparatus may further include a wire installed in the supply region and colliding with the magnetic brush. The resistivity of the carrier may be no greater than 10 8  Ωcm. A bias generating a pulsating electric field may be applied between the magnetic roller and the developing roller.  
      Other objects, advantages and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other exemplary objects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a view of an image forming apparatus according to an exemplary embodiment of the present invention;  
       FIG. 2  is a graph illustrating the resistivity of a developer versus efficiency of forming a toner layer on a developing roller according to an exemplary embodiment of the present invention;  
       FIG. 3  is a view illustrating an example of an apparatus for measuring the resistance of a developer according to an exemplary embodiment of the present invention;  
       FIG. 4  is a diagram of an equivalent electrical circuit for the apparatus illustrated in  FIG. 3 ;  
       FIG. 5  is a graph illustrating time response characteristics of a voltage in the equivalent circuit diagram of  FIG. 4 ;  
       FIG. 6  is a graph illustrating a charging time versus an amount of charge of toner according to an exemplary embodiment of the present invention;  
       FIG. 7  is a graph illustrating the resistivity of a carrier versus a toner charging time according to an exemplary embodiment of the present invention;  
       FIG. 8  is a view of an apparatus measuring the resistance of a carrier according to an exemplary embodiment of the present invention;  
       FIG. 9A  is a view illustrating an example of a development unit increasing a toner supply amount to a developing roller according to an exemplary embodiment of the present invention;  
       FIG. 9B  is a view illustrating the intensity of magnetic force in a supply region of the development unit illustrated in  FIG. 9A ;  
       FIG. 9C  is a view explaining an operation of the development unit illustrated in  FIG. 9A ;  
       FIG. 10A  is a view illustrating another example of a development unit increasing a toner supply amount to a developing roller according to an exemplary embodiment of the present invention;  
       FIG. 10B  is a view illustrating an operation of the development unit illustrated in  FIG. 10A ;  
       FIG. 11A  is a view illustrating another example of a development unit increasing a toner supply amount to a developing roller according to an exemplary embodiment of the present invention;  
       FIG. 11B  is a view illustrating an operation of the development unit illustrated in  FIG. 11A ;  
       FIG. 12  is a view of a single-pass type multi-color development unit according to an exemplary embodiment of the present invention;  
       FIG. 13  is a view illustrating the principle of a tri-level exposure method according to an exemplary embodiment of the present invention; and  
       FIG. 14  is a view of a multi-pass type multi-color development unit according to an exemplary embodiment of the present invention.  
    
    
      Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features and structures.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.  
      An exemplary embodiment of the present invention provides a hybrid type (or touchdown type) image forming apparatus capable of forming a magnetic brush consisting of a toner and a carrier on the surface of a magnetic roller. The hybrid type image forming apparatus supplies only the toner from the magnetic brush to a developing roller and moves the toner to a photosensitive body to develop an electrostatic latent image on the photosensitive body.  
       FIG. 1  is a view of a hybrid type image forming apparatus according to an exemplary embodiment of the present invention. Referring to  FIG. 1 , the apparatus includes a photosensitive body  10 , a charger  20 , an exposer  30 , a development unit  40 , a transfer unit  35 , a fixer  80  and a cleaning member  70 . The purpose of charger  20  and the exposer  30  is to form an electrostatic latent image on the photosensitive body  10 . The charger  20  may be a corona discharger or a charging roller. The exposer  30  may a laser scanning unit (LSU) illuminating a laser beam onto the photosensitive body  10 .  
      The development unit  40  includes a developing roller  425 , a magnetic roller  426 , agitation members  427  and  428 , a developer regulating member  429  and a toner hopper  421  containing toner. The developing roller  425  is disposed in a configuration so that it does not contact the photosensitive body  10  and the magnetic roller  426 . A composite power source  510  provides a development bias capable of moving the toner from the developing roller  425  to the photosensitive body  10 . A DC power source  511  provides a supply bias capable of moving the toner from the magnetic roller  426  to the developing roller  425 . According to a non-contact development method, a space (development gap) between the developing roller  425  and the photosensitive body  10  is about 150-400 μm and may be 200-300 μm. When the development gap is smaller than 150 μm, a background portion is contaminated. When the development gap is larger than 400 μm, it is difficult to move the toner to the photosensitive body  10 , so that a sufficient image density is hard to achieve. The magnetic roller  426  includes a rotating sleeve  424  and a magnetic core  423  installed within the sleeve  424  to provide a magnetic force to form a magnetic brush. A space between the magnetic roller  426  and the developing roller  425  is about 0.3-0.7 mm. A toner layer formed on the developing roller  425  may be about 0.5-1.0 mg/cm 2 . For that purpose, the space between the magnetic roller  426  and the developing roller  425  is about 0.2-0.5 mm. An average potential difference between the developing roller  425  and the magnetic roller  426  may be about 50-200V, an amount of charge of the toner may be 10-20 μC/g and a speed ratio of the magnetic roller  426  to the developing roller  425  may be about 0.5-2.0.  
      The development unit  40  contains a developer where the toner and the carrier are mixed. The toner and the carrier are agitated by rotation of the agitation members  427  and  428  and rub against each other. The toner is charged by this rubbing. Generally, it takes a certain amount of time before an amount of charge of the toner reaches a saturated value. When a new toner (not charged) is supplied from the toner hopper  421  to the development unit  40 , the new toner is agitated by the agitation members  427  and  428  and reaches the magnetic roller  426 . It is possible to finally form a toner layer charged to a saturated state on the developing roller  425 . This may be done by optimizing the composition of a material of the toner or the carrier. Alternatively, a toner layer charged to a saturated state may be formed by controlling conditions such as the structure or the number of rotations of the agitation members  427  and  428  so that the toner may be sufficiently charged while the toner reaches the magnetic roller  426 . The developer regulating member  429  regulates the magnetic brush formed at the magnetic roller  426  in a uniform manner. Only the toner is separated from the magnetic brush and moved to the developing roller  425  by the supply bias.  
      According to an exemplary implementation, the charger  20  charges the surface of the photosensitive body  10  at a uniform potential. The exposer  30  illuminates light that corresponds to image information onto the photosensitive body  10 . Accordingly, an electrostatic latent image including an image portion and a non-image portion with different potentials is formed on the surface of the photosensitive body  10 . In a supply region where the developing roller  425  and the magnetic roller  426  face each other, the toner is separated from the magnetic brush by the supply bias applied to the magnetic roller  426  and then supplied to the developing roller  425 . A uniform toner layer is formed on the outer periphery of the developing roller  425 . While the toner layer formed on the developing roller  425  passes through a development region where the photosensitive body  10  and the developing roller  425  face each other, the toner is separated from the toner layer on the developing roller  425  and attached on the image portion by the development bias. Accordingly, a visual toner image is formed on the photosensitive body  10 . The toner image is transferred to a recording medium P by a transfer electric field provided from a transfer unit  35 . A fusing unit  80  fuses the toner image onto the recording medium P using heat and pressure. The cleaning member  70  removes the remaining toner from the surface of the photosensitive body  10 .  
      To solve the problem of a development ghost, many efforts have been made in the prior art to collect a residual toner on a developing roller and to remove an after-image of an image that has been previously developed from the developing roller after passing a development region. These efforts made in the prior art include a method for collecting toner on a developing roller using a magnetic roller for collection, a method for collecting toner from the developing roller to a magnetic roller by changing the direction of an electric field formed between the developing roller and the magnetic roller and a method for collecting toner on a developing roller using a magnetic brush by rotating the developing roller and a magnetic roller in the same direction (such as, a direction in which directions of the surfaces of the two rollers move opposite to each other in a region where the two rollers face each other).  
      However, the method that uses the magnetic roller for collection requires a large-sized development apparatus. Also, the method that changes the direction of the electric field between the developing roller and the magnetic roller is difficult to apply because it is difficult to make a bias providing an optimized development condition compatible with a bias providing an optimized collecting condition. The increase in the price of a power supply also makes it difficult to apply this method. Also, the carrier has a charged polarity opposite to that of the toner. The carrier, particularly, the carrier with a small diameter may be moved to the developing roller by a bias collecting the toner from the developing roller to the magnetic roller, and this carrier may be attached back on a background portion of the photosensitive body. Since the carrier has lower electrical resistance than that of the toner, the carrier may cause transfer defect and density non-uniformity of an image due to charge leakage when an image developed on the photosensitive body is transferred to paper or an intermediate transfer medium.  
      An exemplary embodiment of the present invention solves the above problem and prevents the development ghost by supplying a sufficient amount of toner onto the developing roller  425  and forming a uniform toner layer.  
      To form the uniform toner layer, a sufficient amount of toner should be supplied to the developing roller  425  from the magnetic brush within a time period shorter than a time period for which the magnetic brush passes through the supply region. For that purpose, the relationship between conductivity of a developer and a movement rate of the toner from the magnetic roller  426  to the developing roller  425  has been considered. Charged toner is moved from the magnetic brush to the developing roller  425  in the supply region. When the resistivity of the developer is less than 10 9  Ωcm, the toner moves by an amount so that the potential of the toner layer formed on the surface of the developing roller  425  becomes a potential difference between the magnetic roller  426  and the developing roller  425 . At this point, a time period before the toner moves is shorter than a time period in which the magnetic brush passes through the supply region, so that the toner moves very fast. Therefore, a sufficient amount of the toner is supplied to the development region, which effectively prevents the development ghost. Also, even the continuous printing of a high density image facilitates the prevention of a density non-uniformity of a printed image.  
      Assuming that movement direction of the surfaces of the developing roller  425  and the magnetic roller  426  are the same, the movement speed of the surfaces is 0.3 m/s, a space between the developing roller  425  and the magnetic roller  426  is 0.5 mm, a DC potential difference between the developing roller  425  and the magnetic roller  426  is 100V, and an amount of charge of the toner is 13 μC/g, the relationship between the resistivity of the developer and a movement rate of the toner from the magnetic roller  426  to the developing roller  425  has been examined. The result of the examination is illustrated in a graph of  FIG. 2 . A toner movement rate of 100% means that a toner movement rate is saturated. Referring to  FIG. 2 , a toner movement rate of 95% is used for a reference because a 5% difference of the toner movement rate cannot be recognized on a finally printed image.  
       FIG. 3  is a view illustrating an example of an apparatus capable of measuring the resistance of a developer. Referring to  FIG. 3 , the apparatus includes a resistor  501  (whose value is Rx), a high voltage power source  502 , a voltage meter  503 , a current meter  504 , a blade  505  removing the toner on the developing roller  425  and a case  506  receiving the removed toner. While a magnetic brush  500  formed on the magnetic roller  426  contacts the developing roller  425 , the high voltage power source  502  applies a DC current between the developing roller  425  and the magnetic roller  426 . For example, when negatively charged toner is used, the high voltage power source  502  applies a voltage of about −100V. The toner contained in the developer moves to the developing roller  425 , and the toner layer is formed on the surface of the developing roller  425 . The blade  505  removes the toner layer. When the movement of the toner and the removal of the toner layer are repeated, a current that corresponds to the quantity of electric charge of the toner moved to the developing roller  425  is measured by the current meter  504 . A voltage applied between the developing roller  425  and the magnetic roller  426  is measured by the voltage meter  503 . The resistance of the developer may be calculated from the measured voltage and current. The resistivity of the developer is obtained by multiplying the resistance of the developer by an area in which the developer (magnetic brush) on the magnetic roller  426  contacts the developing roller  425  (that is, product of a length where the developer is attached on the magnetic roller  426  and a nip width of the magnetic brush) and then dividing the product by a space between the magnetic roller  426  and the developing roller  425 .  
       FIG. 4  is a diagram of an equivalent electrical circuit for the apparatus for measuring the resistance of the developer illustrated in  FIG. 3 .  FIG. 5  is a graph illustrating time response characteristics of a voltage Vd in the equivalent circuit diagram of  FIG. 4 . A method for obtaining the resistivity of the developer and the reason the developer with low resistivity is valid will be described with reference to  FIGS. 4 and 5 .  
      The developer is expressed as a parallel circuit consisting of a capacitor Cd and a resistor Rd. When a voltage E 1  is applied from the high voltage power source  502 , a response waveform of the voltage Vd measured by the voltage meter  503  may be obtained. The resistance of the resistor Rd may be calculated from a saturated voltage Vsat of this response waveform, a voltage E 1  and resistance Rx of a resistor  501  for measurement. Also, a time constant tc may be calculated from the initial slope of the response waveform. The above values may be calculated using the following equations 1.
 
 Vd=E 1 ×{Rd /( Rx+Rd )}×[ 1−exp{−   t /( Rd×Cd )}]
 
 Id =( E 1− Vd )/ Rx 
 
 Vsat=E 1× Rd /( Rd+Rx )
 
 Rd=Rx /( E 1 /Vsat− 1)
 
 Cd=tc/Rd   Equations 1
 
      The toner from the toner layer formed on the surface of the developing roller  425  is developed on the photosensitive body  10  in the development region. When the surface of the developing roller  425  reaches the supply region, the toner is moved from the magnetic brush to the developing roller  425 , so that the toner layer is recovered to the original toner layer. This process is analogous to the response experienced when the voltage E 1  is applied in the equivalent circuit of  FIG. 4 . According to the saturated voltage Vsat, the movement rate of the toner is saturated in a graph illustrated in  FIG. 2 . Referring to  FIG. 5 , a time period taken before a voltage reaches 95% of the saturated voltage Vsat is approximately three times the time constant tc. For example, the toner layer on the surface of the developing roller  425  is recovered to 95% of the original toner layer within a time period that is three times the time constant tc. When the toner layer is recovered up to 95%, the development ghost may be prevented. That is, a difference of an image density due to 5% of a recovery rate of the toner layer is not recognized from a finally printed image. Therefore, the resistance Rd of the developer is determined so that three times a time constant (tc) is shorter than the time taken until the surface of the developing roller  425  is separated after contacting the magnetic brush (that is, a time until the surface of the developing roller  425  passes through the supply region). Resistivity of the developer obtained on the basis of the above facts may be almost 10 9  Ωcm.  
      It is possible to realize the resistivity of the developer less than 10 9  Ωcm by controlling the composition of the carrier and the toner. The resistance of the developer changes depending on the electric resistance of the carrier, and the mobility and quantity of electric charge of the toner. To obtain a developer whose resistivity is 10 9  Ωcm or less, processes of manufacturing the developer while changing the above-described parameters and measuring the resistance of the manufactured developer while changing the quantity of electric charge of toner to determine an appropriate combination are repeated. A parameter with a great influence on the resistance of the developer is the electric resistance of the carrier. The electric resistance of the carrier may be controlled by changing the resistance of a wick material of the carrier or an amount of doping of a conductive material on a surface coating material. The wick material of the carrier includes ferrite, magnetite and iron.  
      Next, initial charging of the toner is considered. When the toner in the development unit  40  is consumed and an amount of the toner is reduced, a new toner is supplied. Since the newly supplied toner has not been charged, the new toner should be quickly charged. When the charging of the new toner is delayed, a slightly charged toner is used to the developing process to cause contamination of a background portion and toner scattering. A method of using a carrier of low resistance is used in order to quickly charge the toner.  
      During a supply time for which the newly supplied toner reaches the developing roller  425 , the toner should be charged by as much as a degree so that the background portion is not contaminated. For example, it is possible to set the supply time at 30 seconds on the assumption that a linear speed of the developing roller  425  is 0.3 m/s, and the length of the developing roller  425  is made to correspond to the vertical length of A 4 -sized paper in a developing device using the agitation members  427  and  428  illustrated in  FIG. 1 . In that case, a time (a reference charging time) consumed for properly charging the newly supplied toner may be within 30 seconds.  
      According to an exemplary implementation, a method for measuring a charging time will be described. A case of 100 cc is used as a developer case to mix the toner and the carrier. A 210HS-2A suction type charge measurement device by Trek Co. is used as a device to measure the quantity of electric charge. A ball-mill mixer is used as a mixer. For example, a case of mixing a carrier with a diameter of 50 μm and a toner with a diameter of 8 μm is described. First, a carrier of 50 g is uniformly put into a case laid to a side and a toner of 4 g is uniformly dispersed on the carrier. The rotation speed of the ball-mill mixer is controlled so that the case rotates thirty times per minute to agitate the carrier and the toner. When 10 seconds, 20 seconds, 30 seconds, 1 minute, and 2 minutes elapses, mixing is suspended and a developer is collected, so that the quantity of electric charge of the toner is measured using the suction type charge measurement device. A graph demonstrating a relationship between the charge of the toner and the charging time is illustrated in  FIG. 6 .  
      The amount of charge of toner required while the newly supplied toner reaches the developing roller  425  is determined with consideration of an influence of the amount of charge of the newly supplied toner on the amount of charge of the entire toner supplied to the developing roller  425 . Actually, an amount of the toner moving from the magnetic brush of the magnetic roller  426  to the developing roller  425  is about 10% of the toner contained in the magnetic brush. Therefore, about 10% of the toner contained in the magnetic brush is constantly supplied to the magnetic brush. 5% of the entire toner included in the magnetic brush has a minimal influence on a finally printed image even when the 5% of the toner is weakly charged. Therefore, 10% of the toner constantly supplied to the magnetic brush may be agitated for half of a charging time required for the toner to reach a saturated charge. Then, it is expected that about half of the 10% of the toner is charged up to the saturated charge, and the other half of the 10% of the toner is charged up to half of the saturated charge. Therefore, a reference charging time may be set to half of a time taken until the 10% toner reaches the saturated charge. Since a quantity of toner charge due to agitation within the developing unit  40  for the same charging time may be different from a quantity of toner charge due to agitation in the above-described device measuring the quantity of electric charge, a reference charging time that can be applied to an actual development unit is determined using a repeated experiment.  
      An examination of the relationship between the resistivity of the carrier and the reference charging time using several toners and carriers demonstrates that the reference charging time decreases when the resistance of the carrier is lowered. Generally, a graph illustrated in  FIG. 7  is obtained. A conclusion that the resistivity of the carrier may be 108 Ωcm or less is obtained from the above experimental results. When the resistivity is no greater than 108 Ωcm, a proper reference charging time may not be achieved depending on the type of the carriers. The graph illustrated in  FIG. 7  demonstrates a relationship between a reference charging time and carrier resistivity in a combination of a toner and a carrier where a reference charging time decreases as the resistivity of the carrier is lowered among available combinations of several toners and carriers. Therefore, the graph illustrated in  FIG. 7  may be a useful guide in determining a proper combination of a carrier and a toner, capable of quickly charging a newly supplied toner to prevent contamination of a background portion and toner scattering.  
      A method for measuring the resistivity of a carrier will be described with reference to  FIG. 8 . A carrier  600  is positioned between electrodes  601  and  602  for measurement, and a high voltage is applied to the electrodes  601  and  602  using a high voltage power source  607 . A current flowing through the electrode  602  is measured using a current meter  606 . Resistance is calculated based on the relationship between a voltage and a current. According to an exemplary implementation, an insulator  604  and a guide electrode  603  are installed around the electrode  602  for measurement. Any influence of current flowing through an inner wall of a case  605  receiving the carrier is removed using the above structure, so that correct resistance may be obtained. The resistivity of the carrier is calculated by multiplying the obtained resistance by the area of the electrode  602  for measurement and dividing the obtained value by the thickness d of the carrier. Force applied by the electrode  601  to the carrier is 0.1 kg/cm 2 . The voltage applied and the thickness d of the carrier are controlled such that the intensity of an electric field applied to the carrier is 10 3  KV/m.  
      To increase a movement rate of the toner from the magnetic roller  426  to the developing roller  425 , a supply bias where a DC current and an AC current are mixed is applied between the magnetic roller  426  and the developing roller  425 . This allows an electric field between the magnetic roller  426  and the developing roller  425  to change with respect to time. An electric field that changes with respect to time includes an alternating electric field and a pulsating electric field. The alternating field is an electric field whose direction and intensity all change with respect to time. For example, when a DC potential difference between the magnetic roller  426  and the developing roller  425  is 100V, a peak-to-peak voltage of an AC voltage may be selected to be 300V. Then, a potential difference between the magnetic roller  426  and the developing roller  425  becomes −50˜250V. Therefore, the electric field between the magnetic roller  426  and the developing roller  425  becomes the alternating electric field whose direction and intensity change with respect to time. Since the developer used in an exemplary embodiment of the present invention has a very small resistivity that is no greater than 10 9  Ωcm, a potential difference between the magnetic roller  426  and the developing roller  425  instantly increases when an AC voltage with a large amplitude is applied. This facilitates an excessive current that flows between the magnetic roller  426  and the developing roller  425 . Such an excessive current may cause disorder to a power supply. Therefore, an AC voltage is required to be set so that an excessive current does not flow. This condition is met by setting the amplitude of the AC voltage so that the electric field between the magnetic roller  426  and the developing roller  425  is a pulsating electric field whose direction does not change and whose intensity changes. For example, when a DC potential difference between the magnetic roller  426  and the developing roller  425  is 100V, a peak-to-peak voltage of an AC voltage may be selected to be 180V. Then, a potential difference between the magnetic roller  426  and the developing roller  425  becomes 10˜190V. Therefore, the electric field between the magnetic roller  426  and the developing roller  425  becomes the pulsating electric field whose direction does not change and whose intensity changes with respect to time. It is possible to improve a movement rate of the toner to the developing roller  425  using the pulsating electric field.  
      Another method for increasing a toner supply amount from the magnetic roller  426  to the developing roller  425  will be considered below.  
      Referring to  FIG. 9A , a magnetic core  423  generating a magnetic brush inside a sleeve  424  is fixedly disposed. The magnetic core  423  is polarized into a plurality of poles. A pair of magnetic poles S 1  and S 2  have the same polarity and are disposed in a region facing a supply region. The intensity of magnetic force drastically decreases between the magnetic poles S 1  and S 2  as illustrated in  FIG. 9B . A magnetic brush generated by the magnetic pole S 1  drastically collapses as illustrated in  FIG. 9C  when reaching a portion located between the magnetic poles S 1  and S 2  as the sleeve  424  rotates. The magnetic brush is then re-generated once the magnetic pole S 2  is reached. At this point, the magnetic brush that has collapsed between the portion located between the magnetic poles S 1  and S 2  moves toward the magnetic pole S 2  at a very fast speed. The speed at which the magnetic brush moves drastically decreases when the magnetic brush is re-generated by the magnetic pole S 2 . By this impulse, the toner attached on the carrier is separated form the carrier and moved to the developing roller  425  by a supply bias. According to the above construction, since the toner in the supply region is separated from the carrier by electric force formed by the supply bias and the mechanical impulse generated during the collapse and regeneration of the magnetic brush, it is possible to separate a very large amount of toner from the magnetic brush and to move the separated toner to the developing roller  425 . Therefore, a sufficient amount of toner which is greater than an amount of toner consumed in the development region where the photosensitive body  10  and the developing roller  425  face each other, may be supplied to the developing roller  425  again, so that the development ghost may be prevented.  
      Referring to  FIG. 10A , a wire  422  (collision member) may be installed between the developing roller  425  and the magnetic roller  426 . Referring to  FIG. 10B , since the magnetic brush that has collapsed between the magnetic poles S 1  and S 2  collides with the wire  422 , the toner may be more easily separated from the carrier. The wire  422  may be made of metal with high tension such as tungsten and stainless steel. The diameter of the wire may be 0.05-0.20 mm and appropriately selected with consideration of a space between the developing roller  425  and the magnetic roller  426 . For example, when the space between the developing roller  425  and the magnetic roller  426  is 0.3 mm, the wire with a diameter 0.05 mm is selected. Since the toner is easily separated from the magnetic brush, the development ghost and the non-uniform density of an image are effectively prevented when a large amount of toner is required to be supplied to the developing roller  425 , for example, when high speed printing is required or high density printing is required. The collision member is not limited to the wire  422  but any member including a mesh-shaped member may be used as long as the member is installed between the developing roller  425  and the magnetic roller  426  to collide with the magnetic brush.  
      Referring to  FIG. 11A , the magnetic core  423  installed inside the sleeve  424  is rotated. The magnetic core  423  is polarized such that N poles and S poles are alternatively located in turns with respect to each other. Referring to  FIG. 11B , the sleeve  424  rotates counterclockwise and the magnetic core  423  rotates clockwise. The carriers are attached on the surface of the sleeve  424  in the order of E-D-C-B-A in the upstream of the supply region. Since the magnetic core  423  rotates, the direction of magnetic force changes in the supply region. The carriers are attached on the surface of the sleeve  424  in the order of A-B-C-D-E in the downstream of the supply region. When the direction of the magnetic force is changed in the supply region by rotating the magnetic core  423 , the magnetic brush is turned over as illustrated by an arrow of  FIG. 11B . This allows all of the toner constituting the magnetic brush to approach the developing roller  425  and an amount of the toner moving to the developing roller  425  to increase. Therefore, a sufficient amount of toner is supplied to the developing roller  425  and a uniform toner layer may be formed.  
      In addition to an exemplary embodiment of the present invention arranging the poles of the magnetic core  423  and rotating the magnetic core  423 , the resistivity of the developer and the resistivity of the carrier may be controlled as described above. Also, a bias generating the pulsating electric field as described above may be applied as a supply bias between the developing roller  425  and the magnetic roller  426 .  
      The above development unit may be applied to a color development unit.  FIG. 12  is a view of a single-pass type multi-color development unit according to an exemplary embodiment of the present invention. The multi-color development unit according to an exemplary embodiment of the present invention includes two image forming units using a tri-level exposure method. One image forming unit includes a photosensitive body  11 , a charging roller  21 , an exposer  31 , development units  41  and  43 , a pre-transfer charger  51  and a cleaner  71 . The other image forming unit includes a photosensitive body  12 , a charging roller  22 , an exposer  32 , development units  42  and  44 , a pre-transfer charger  52  and a cleaner  72 . The development unit illustrated in  FIGS. 1, 9A ,  10 A, and  11 A may be used for the development units  41 ,  42 ,  43 , and  44 .  
      The tri-level exposure method is a method of forming three potential portions consisting of a high potential portion VH, a middle potential portion VM, and a low potential portion VL on each of the photosensitive bodies  11  and  12 . The tri-level exposure method uses one type of exposure by controlling an exposure power of the exposers  31  and  32  in three steps consisting of off, a middle power, and a full power when illuminating light on the photosensitive bodies  11  and  12  using the exposers  31  and  32 . Toners charged at different polarities are developed on the high potential portion VH and the low potential portion VL, respectively.  
      First, the image forming unit including the photosensitive body  11 , the charging roller  21 , the exposer  31 , the development units  41  and  43 , the pre-transfer charger  51 , and the cleaner  71  will be described. The surface of the photosensitive body  11  is charged using the charger  21 . For example, when negatively charging the photosensitive body  11 , the charging is performed such that the high potential portion VH has a potential of −900V.  
      Next, the photosensitive body  11  is exposed by changing the exposure power in three steps depending on a color to be printed using the exposer  31 . Referring to  FIG. 13 , when the photosensitive body  11  is negatively charged, positively charged toner is developed on the high potential portion VH. When an image signal is a signal to print a color of a positive polarity (that is, positive polarity color data is “0”), the exposure power is off and a corresponding portion on the photosensitive body  11  becomes the high potential portion VH. Negatively charged toner is developed on the low potential portion VL. When an image signal is a signal printing a color of a negative polarity (that is, negative polarity color data is “0”), the exposure power becomes the full power and a corresponding portion on the photosensitive body  11  becomes the low potential portion VL (for example, −30V). When an image signal is a white image, the exposure power becomes the middle power and a corresponding portion on the photosensitive body  11  becomes the middle potential portion VM (for example, −450V) between the high potential portion VH and the low potential portion VL.  
      Negatively charged toner  412  is then developed using the development unit  41 . In that case, a development bias with a potential  222  located between a potential  212  of the low potential portion VL and a potential  213  of the middle potential portion VM is applied to the development unit  41 . The negatively charged toner  412  is developed on the low potential portion VL. Positively charged toner  411  is developed using the development unit  43 . A development bias with a potential  221  located between a potential  211  of the high potential portion VH and a potential  213  of the middle potential portion VM is applied to the development unit  43 . The positively charged toner  411  is developed on the high potential portion VH.  
      Next, the positively charged toner  411  and the negatively charged toner  412  developed on the photosensitive body  11  are changed to one polarity using the pre-transfer charger  51 . For example, it is possible to change the polarity of the negatively charged toner  412  to a positive polarity by illuminating a positive corona using the pre-transfer charger  51 . A dual-colored toner image formed on the photosensitive body  11  is transferred to an intermediate transfer belt  60  by a negative voltage applied to a first transfer roller  61 .  
      The above-described same process is performed on the other image forming unit including the photosensitive body  12 , the charging roller  22 , the exposer  32 , the development units  42  and  44 , the pre-transfer charger  52 , and the cleaner  72 . A dual-colored toner image formed on the photosensitive body  12  is transferred to the intermediate transfer belt  60  by a negative voltage applied to a first transfer roller  62 .  
      Accordingly, four-colored toner image is formed on the intermediate transfer belt  60 . This four-colored toner image is transferred to paper P supplied from a cassette  90  through a second transfer roller  63 , and then fused on the paper P using a fusing unit  80 , so that a four-colored image may be printed. When the four colors are cyan, magenta, yellow, and black, respectively, a full color image may be obtained. Toner remaining on the photosensitive bodies  11  and  12  is removed by the cleaning members  71  and  72 . The tri-level method of dividing the potential of the photosensitive body into three potentials has approximately half of a potential range required for developing one color compared to a method with two divisions (dividing into an image portion and a non-image portion), which is used for most laser printers. Also, the charging characteristics of the photosensitive body changes depending on environment conditions (for example, temperature and humidity), or deterioration caused by constant use. Therefore, even when the photosensitive body is exposed using the same exposure power, the surface potential of the photosensitive body changes. When the potential of the high potential portion VH or the low potential portion VL changes, an amount of toner developed changes, which changes printing density. When the potential of the middle potential portion VM changes, the toner is developed on a background. This results in the contamination of the background because the toner should not be developed on the background. Particularly, since the change of the middle potential portion VM is large, it is required to stably control the potential in order to use the tri-level method.  
      Therefore, an electrophotographic apparatus using the tri-level method adapts a method of detecting a surface potential after exposure using surface potential sensors  831  and  832  and controlling the chargers  21  and  22  or the exposers  31  and  32  to stably control the potential. It is possible to print a dual-colored image by exposing the photosensitive body one time using the above-described method. Therefore, since a dual-color printing may be performed using one exposure, miniaturization of a product and cost reduction may result from using this method.  
       FIG. 14  is a view of a multi-pass type multi-color development unit using a tri-level method according to an exemplary embodiment of the present invention. The development unit includes a photosensitive body  10 , four development units  41 ,  42 ,  43 , and  44  arranged around the photosensitive body  10 , and an intermediate transfer belt  60 . First, an initial dual-colored toner image is developed on a photosensitive body  10  using development units  41  and  42 . The polarity of the developed dual-color toner image is changed to an appropriate polarity by using a pre-transfer charger  50  and is then transferred to an intermediate transfer belt  60 . Next, another dual-colored toner image is developed on the photosensitive body  10  using development units  43  and  44  and transferred to the intermediate transfer belt  60  using the same method. Accordingly, a four-colored toner image is formed on the intermediate transfer belt  60 . The four-colored toner image is transferred onto paper P by using a second transfer roller  63  and fused by using a fusing unit  80 , so that a four-colored image may be printed. When the four colors are cyan, magenta, yellow, and black, respectively, a full color image may be obtained.  
      The contamination of the background is particularly problematic in a multi-color image forming apparatus using a tri-level exposure method. However, the image forming apparatus of an exemplary embodiment of the present invention may obtain high quality printing image with almost no background contamination. Since a bias collecting the toner from the developing roller to the magnetic roller is not used, the carrier with a small diameter is not attached to the background of the photosensitive body via the developing roller. Therefore, it is possible to solve a transfer defect or density non-uniformity of an image caused by charge leakage when transferring an image developed on the photosensitive body to paper or an intermediate transfer medium using the carrier with low electric resistance.  
      Though not shown in the drawings, it is obvious to those skilled in the art that a single-pass type image forming apparatus including four photosensitive bodies, four exposers forming a dual-level electrostatic latent image consisting of a non-image portion and an image portion on the four photosensitive bodies, respectively, four development units supplying toners of different colors to the electrostatic latent image formed on the respective photosensitive bodies to develop the same may be realized. A multi-pass type image forming apparatus including one photosensitive body, one exposer and four development units may be realized. More specifically, the exposer sequentially forms dual-level electrostatic latent images consisting of a non-image portion and an image portion that correspond to image information of respective colors. The four development units sequentially supply toner of different colors to the electrostatic latent images formed on the photosensitive body to develop the same.  
      As described above, according to the hybrid type image forming apparatus of an exemplary embodiment of the present invention, it is possible to realize a small image forming apparatus that provides an image having excellent image quality without the development ghost, and background contamination. Also, it is possible to realize a color image forming apparatus using a tri-level exposure method, capable of stable printing quality.  
      While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims and their equivalents.