JP5196772B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus that performs simultaneous development cleaning.

  Among electrophotographic image forming apparatuses, there is a cleanerless type image forming apparatus in which a dedicated cleaning device is eliminated. In this image forming apparatus, residual toner on the photoconductor after the transfer process is removed from the photoconductor by “development simultaneous cleaning” in the developing device.

  As disclosed in Japanese Patent Application Laid-Open No. 2004-228688, in the simultaneous development cleaning, the toner remaining on the photoreceptor after transfer (transfer residual toner) is first charged amount control in which a bias having the same polarity as the toner in the developing device is applied. Charged by means.

  Then, the transfer residual toner charged to the same polarity as the toner in the developing device by the charge amount control means is collected by the developing device due to the fog removal potential difference in the subsequent development process.

  Here, the fog removal potential difference is a potential difference (Vback) between the DC voltage applied to the developing device and the surface potential of the photoreceptor. Thereby, the transfer residual toner present on the surface of the photoreceptor (non-image portion) that should not be developed with toner is collected by the developing device. This method is advantageous in reducing the size of the image forming apparatus because there is no dedicated cleaning device.

JP 2002-99176 A JP 2003-162182 A JP 2004-21951 A Japanese Patent Laid-Open No. 10-288934 JP-A-8-190269 JP 2002-311692 A

  However, there are toners that are not normally charged even when charged on a charging member to which a bias having the same polarity as that of the toner in the developing device is charged, due to deterioration of the durability of the developer or discharge at various parts on the photoreceptor. Examples of the toner that is not normally charged include a so-called reversal toner charged with a reverse polarity and a weakly charged toner having a smaller charge amount than usual.

  Here, assuming that the charge amount of the normally charged toner is about −10 to −30 μc / g, the weakly charged toner means a charge amount of less than −10 μc / g. These exist with a continuous charge amount distribution (see FIG. 8). These normally uncharged toners are not recovered by the above-described fog removal potential difference, and are not collected by the developing device but are carried around on the photosensitive member. As a result, the toner causes a charging failure or an exposure failure, or an image defect is caused by fusing onto the photoreceptor.

  As a method for effectively removing the reversal toner on the photoconductor against such a problem, a reverse polarity bias is applied to the transfer belt to reverse the normal image formation, and the reversal toner is transferred to the transfer belt. A configuration to collect is proposed. (For example, Patent Document 2)

  However, in the method disclosed in Patent Document 2, weakly charged toner remains on the photoreceptor. Here, in the method disclosed in Patent Document 1, if the voltage applied to the charge amount control means is increased when the toner is charged, the toner is excessively charged, making it difficult to appropriately perform the simultaneous development cleaning. Become.

  In addition, when the voltage applied to the charge amount control unit is increased, toner may adhere to the charge amount control unit during charging, which may make it difficult to perform appropriate charging during subsequent image formation.

  The present invention provides an image forming apparatus in which weakly charged toner is sufficiently removed to prevent image defects caused by poor charging, poor exposure, adhesion of transfer residual toner, and the like in an image forming apparatus that performs simultaneous development cleaning. The purpose is to do.

An image forming apparatus according to the present invention is configured to charge an image bearing member that rotates by carrying an electrostatic image and a voltage having the same polarity as the regular charging polarity of the toner. A charge applying unit; a developing unit that electrostatically collects the toner charged by the charge applying unit on the image carrier while developing the electrostatic image to form a toner image; and the toner image is transferred An intermediate transfer member, and a primary transfer member that primarily transfers the toner image on the image carrier to the intermediate transfer member by applying a voltage having a polarity opposite to the normal charging polarity of the toner; A secondary transfer member for secondary transfer of the toner image from the intermediate transfer member to the recording material . The absolute value of the current amount larger than the absolute value of the current amount that flows through the primary transfer member when the toner image is primarily transferred during at least one rotation of the image carrier during non-toner image formation. after continuing the control to be applied to the primary transfer member regular having the same polarity as the charging polarity of the voltage of the toner such that the developing unit is capable of executing mode that recovered the toner on the image bearing member .

In the image forming apparatus of the present invention, at the time of non-toner image formation, the transfer residual toner is charged through the intermediate transfer body by the primary transfer member over one rotation of the image carrier to charge the transfer residual toner. The state is adjusted to a level at which electrostatic transfer to the intermediate transfer member is possible or easy. Therefore, the transfer residual toner on the entire circumference of the image carrier can be efficiently and electrostatically moved to the intermediate transfer member. The transfer residual toner is charged via the intermediate transfer member having a self-cleaning action (cleaned even if it becomes dirty) by the toner recovery means, so that the transfer residual toner does not adhere to the primary transfer member during charging.

  That is, the weakly charged toner can be sufficiently removed by effectively using the existing member surrounding the image carrier.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, what attached | subjected the same code | symbol in the same drawing or a different drawing performs the same structure or the same effect | action, The duplication description is abbreviate | omitted suitably about these.

<Embodiment 1>
FIG. 1 shows an image forming apparatus to which the present invention can be applied. The image forming apparatus shown in FIG. 1 is an electrophotographic system, an intermediate transfer body system, and a cleanerless system, and is a four-color full-color image forming apparatus having four image carriers. This figure is a schematic view corresponding to a longitudinal sectional view of the image forming apparatus as seen from the front side, that is, from the side where the user is positioned during operation.

  The configuration and operation of the image forming apparatus will be described with reference to FIG.

  The image forming apparatus shown in FIG. 1 includes four process units Pa, Pb, Pc, and Pd that form images (toner images) of colors of yellow, magenta, cyan, and black. The toner images of the respective colors formed by these process units Pa, Pb, Pc, and Pd are sequentially transferred (primary) on the intermediate transfer belt 51 as an intermediate transfer member (transferred member) at the primary transfer portion. Transferred). Thereafter, the image is transferred to the recording material P such as paper in a batch (secondary transfer) in the secondary transfer portion.

  Each of the process units Pa, Pb, Pc, and Pd is provided with a photosensitive drum (image carrier) 1. Around the photosensitive drum 1, a primary charging roller (primary charging unit) 2 and an exposure device (electrostatic latent image forming unit) 3 are arranged in almost the order along the rotation direction (arrow direction in FIG. 1). A developing device (developing device) 4, a primary transfer roller (primary transfer member) 53, and the like are disposed.

  The configuration of the process unit Pa will be described with reference to FIG. The configuration of the other three process units Pb, Pc, and Pd is the same as that of the process Pa, and a description thereof will be omitted.

(A) Photosensitive drum As shown in FIG. 2, in the present embodiment, a drum-shaped electrophotographic photosensitive member (photosensitive drum) 1 is used as an image carrier that carries an electrostatic image. FIG. 3 schematically shows the layer structure of the photosensitive drum 1 and the charging roller 2. As shown in the figure, the photosensitive drum 1 is provided with a subbing layer 1b that suppresses light interference and improves the adhesion of the upper layer on the surface of a conductive drum base (for example, an aluminum cylinder) 1a. Yes. Further, the surface of the undercoat layer 1b is configured by providing a negatively chargeable organic optical semiconductor (OPC). The OPC is composed of an inner photocharge generation layer 1c and an outer charge transport layer 1d. The photosensitive drum 1 has a diameter of 30 mm as a whole. The photosensitive drum 1 is driven to rotate in the direction of the arrow (counterclockwise) in FIG. 2 with a process speed (moving speed) of 100 mm / sec around the center support shaft by a driving means (not shown).

(B) Charging roller As shown in FIG. 3, in this embodiment, the charging roller 2 is used as a charging means for charging the surface of the photosensitive drum 1 to a predetermined polarity and potential. The charging roller 2 has a length in the longitudinal direction of 320 mm. FIG. 3 shows a layer structure of the charging roller 2. The charging roller 2 has a three-layer configuration in which a lower layer 2b, an intermediate layer 2c, and a surface layer 2d are sequentially laminated from the bottom (inside) on the outer periphery of a core metal (support member) 2a. The lower layer 2b is a foamed sponge layer for reducing charging noise, and the intermediate layer 2c is a conductive layer for obtaining a uniform resistance as the entire charging roller. The surface layer 2d is a protective layer provided to prevent leakage even if the photosensitive drum 1 has a defect such as a pinhole. The above-described cored bar 2a is formed of a stainless steel round bar having a diameter of 6 mm. The lower layer 2b is foamed EPDM in which carbon is dispersed, has a specific gravity of 0.5 g / cm ³ , a volume resistance value of 102 to 10 ⁹ Ω · cm, a layer thickness of 3.0 mm, and a length of 320 mm. The intermediate layer 2c is an NBR rubber in which carbon is dispersed, and has a volume resistance value of 10 ² to 10 ⁶ Ω · cm and a layer thickness of 700 μm. The surface layer 2d is obtained by dispersing tin oxide and carbon in a resin resin of fluorine compound, and has a volume resistance of 10 ⁷ to 10 ¹⁰ Ω · cm, a surface roughness (JIS standard 10-point average surface roughness Ra). The thickness is 5 μm and the layer thickness is 10 μm.

  As shown in FIG. 3, the charging roller 2 is in contact with a charging roller cleaning member 2f formed of a flexible cleaning film. The charging roller cleaning member 2f is arranged in parallel with the longitudinal direction of the charging roller 2 and fixed at one end to a support member 2g that reciprocates a certain amount in the longitudinal direction, and is charged on the surface near the free end side. It is arranged to form a contact nip with the roller 2. The support member 2g reciprocates a certain amount in the longitudinal direction via a gear train by a drive motor (not shown) of the printer. As a result, the surface layer 2d of the charging roller 2 is rubbed with the charging roller cleaning member 2f, and adhered contaminants (fine toner, external additives, etc.) are removed.

  In the charging roller 2, both ends of the cored bar 2a are rotatably held by bearing members (not shown). These bearing members are urged toward the photosensitive drum by a pressing spring 2e. As a result, the charging roller 2 is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force, and is driven to rotate in the direction of the arrow (clockwise) by the rotation of the photosensitive drum 1 in the direction of the arrow (counterclockwise). A pressure contact portion between the photosensitive drum 1 and the charging roller 2 is a charging portion (charging position) a.

  The charging roller 2 uniformly charges the surface of the rotating photosensitive drum 1 to a predetermined polarity and potential by applying a charging bias voltage of a predetermined condition from the power source S1 to the cored bar 2a. In the present embodiment, the charging bias voltage for the charging roller 2 is an oscillating voltage obtained by superimposing the DC voltage Vdc and the AC voltage Vac. More specifically, the DC voltage is -600V, and the AC voltage is a sine wave having a frequency f of 1000 Hz and a peak-to-peak voltage Vpp of 1400V. By applying a charging bias voltage in which these DC voltage and AC voltage are superimposed, the surface of the photosensitive drum 1 is uniformly contact-charged to −600 V (dark potential Vd).

(C) Exposure Device As shown in FIG. 2, an exposure device 3 is provided as information writing means for forming an electrostatic latent image on the surface of the charged photosensitive drum 1. In the present embodiment, a laser beam scanner that irradiates a semiconductor laser is used as the exposure apparatus 3. The exposure device 3 outputs a laser beam L modulated in response to an image signal sent from a host device such as an image reading device (not shown) to the image forming device side to expose the charged surface of the photosensitive drum 1 to an exposure unit. Laser exposure (image exposure) is performed at (exposure position) b. By this laser scanning exposure, the potential of the portion irradiated with the laser light on the surface of the photosensitive drum 1 is lowered. As a result, electrostatic latent images corresponding to image information are sequentially formed on the surface of the photosensitive drum 1.

(D) Developing Device As shown in FIG. 2, a developing device (developing device) is used as a developing unit that supplies developer (toner) to the electrostatic latent image on the photosensitive drum 1 to visualize the electrostatic latent image. The In the present embodiment, a two-component magnetic brush developing type reversal developing device is employed.

  The developing device 4 includes a developing container 4a and a nonmagnetic developing sleeve 4b. The developing sleeve 4b is rotatably disposed in the developing container 4a with a part of the outer peripheral surface thereof exposed to the outside. A magnet roller 4c is inserted into the developing sleeve 4b in a fixed state. The developing device 4 further includes a developer coating blade 4d, a developer stirring member 4f disposed on the bottom side in the developing container 4a, and a toner hopper 4g.

A two-component developer (hereinafter referred to as “developer” as appropriate) 4e in the developing container 4a is a mixture mainly composed of toner and a magnetic carrier, and is stirred by the developer stirring member 4f. The resistance of the magnetic carrier is about 10 ¹³ Ω · cm, and the particle size is about 40 μm. The toner is triboelectrically charged to a negative polarity by rubbing against the magnetic carrier.

  The developing sleeve 4b is disposed to face the surface of the photosensitive drum 1 in a state where the closest distance (S-Dgap) to the photosensitive drum 1 is maintained at 350 μm. A facing portion between the photosensitive drum 1 and the developing sleeve 4b is a developing portion c. The developing sleeve 4b is rotationally driven so that the moving direction (traveling direction) of the surface of the developing unit c is opposite to the moving direction (traveling direction) of the surface of the photosensitive drum 1. Part of the two-component developer 4e in the developing container 4a is adsorbed and held as a magnetic brush layer by the magnetic force of the magnet roller 4c on the outer peripheral surface of the developing sleeve 4b. Then, the toner is rotated and conveyed with the rotation of the developing sleeve 4b, and is stratified into a predetermined thin layer by the developer coating blade 4d. Further, as the developing sleeve rotates, the developing portion c comes into contact with the surface of the photosensitive drum 1 and rubs appropriately. A predetermined developing bias is applied to the developing sleeve 4b from the power source S2. In the present embodiment, the developing bias voltage for the developing sleeve 4b is an oscillating voltage obtained by superimposing the DC voltage Vdc and the AC voltage Vac. More specifically, the DC voltage Vdc is −450 V, and the AC voltage has a peak-to-peak voltage of 1600 V.

  The two-component developer 4e in the developing container 4a is carried on the surface of the developing sleeve 4b, is coated on a thin layer by the developer coating blade 4d, and is conveyed to the developing unit c as the developing sleeve 4b rotates. The two-component developer 4e conveyed to the developing unit c is selectively attached with toner corresponding to the electrostatic latent image on the surface of the photosensitive drum 1 by an electric field due to a developing bias. In the present embodiment, so-called reversal development is performed in which toner adheres to the exposed bright portion of the electrostatic latent image on the surface of the photosensitive drum 1. The developer that has passed through the developing portion c on the developing sleeve 4b is returned to the developer reservoir in the developing container 4a as the developing sleeve 4b rotates.

  In order to maintain the toner concentration of the two-component developer 4e in the developing container 4a within a predetermined substantially constant range, the toner concentration of the two-component developer 4e in the developing container 4a is, for example, an optical toner concentration sensor (not shown). Detected by. The toner hopper 4g is driven and controlled in accordance with the detected information. As a result, the toner in the toner hopper 4g is supplied to the two-component developer 4e in the developing container 4a. The toner supplied to the two-component developer 4e is stirred by the stirring member 4f.

(E) Intermediate Transfer Unit As shown in FIG. 1, an intermediate transfer unit 5 as a transfer unit is disposed below the photosensitive drum 1 of the process units Pa, Pb, Pc, and Pd. The intermediate transfer unit 5 includes an intermediate transfer belt 51, four primary transfer rollers (primary transfer members) 53, an intermediate transfer belt drive roller 55, a secondary transfer inner roller 56, and a secondary transfer outer roller (secondary transfer member). ) 57, a tension roller 59, an intermediate transfer belt cleaner (toner collecting means) 60, and the like.

The intermediate transfer belt 51 is configured by forming an endless dielectric resin. In this embodiment, a PI resin having a volume resistivity of 10 ⁹ Ω · cm (using a probe conforming to the JIS-K6911 method, applied voltage 100 V, applied time 60 sec, temperature 23 ° C., humidity 60% RH) and thickness t = 90 μm is used. Adopted. However, the material and thickness are not limited to those described above as long as the specific volume resistance when applying 100 V is 10 ⁸ to 10 ¹² Ω · cm.

As shown in FIG. 2, the primary transfer roller 53 is configured by covering the outer peripheral surface of a core metal having a diameter of 8 mm with a cylindrical conductive urethane sponge layer having a thickness of 4 mm. The resistance value of the primary transfer roller 53 is such that the primary transfer roller 53 is pressed against a grounded metal roller with a load of 500 g and rotated at a peripheral speed of 50 mm / sec. It is obtained from the relationship of the current measured by applying. The value was about 10 ⁵ Ω (temperature 23 ° C., humidity 60% RH). A transfer bias of +200 V is applied to the primary transfer roller 53 from the power source S3 as will be described later. As a result, the toner image on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 51 at the transfer portion d. The transfer bias applied to the transfer roller 53 by the power source S3 is controlled by the control unit 54.

(F) Fixing Device As shown in FIG. 1, a fixing device 70 as a fixing unit includes a rotatable fixing roller 71, a pressure roller 72 that rotates while being in pressure contact with the fixing roller 71, and the inside of the fixing roller 71. And a heater 73 such as a halogen lamp. In the fixing device 70, the temperature of the surface of the fixing roller 71 is adjusted by controlling the voltage to the heater 73 and the like.

  Image formation (at the time of image formation) in the above-described image forming apparatus is performed as follows. Each color toner image formed on the photosensitive drum 1 is sequentially primary-transferred onto the intermediate transfer belt 51 and then conveyed to the secondary transfer portion T as the intermediate transfer belt 51 rotates. On the other hand, by this time, the recording material P taken out from the paper feed cassette 80 is fed by the pickup roller 81 and supplied to the transport rollers 82 and 83. Further, the recording material P is conveyed in the direction of the arrow Kp along the pre-transfer conveyance guide 84 and is applied between the secondary transfer inner roller 56 and the secondary transfer outer roller 57 in the secondary transfer portion T2. The toner image is transferred from the intermediate transfer belt 51 by the next transfer bias. The transfer residual toner or the like on the intermediate transfer belt 51 is removed and collected by the intermediate transfer belt cleaner 60.

  The recording material P is separated from the secondary transfer portion T, conveyed along the post-transfer conveyance guide 85, and conveyed to the fixing device 70. When the recording material P passes between the fixing roller 71 and the pressure roller 72, the recording material P is pressed and heated at a substantially constant pressure and temperature from both the front and back surfaces. The recording material P is fixed by melting and fixing the toner image on the surface. Thereby, the four-color full-color image formation on one side of one recording material P is completed.

  Next, the configuration of the cleanerless system according to the present embodiment will be described.

  As shown in FIG. 2, the image forming apparatus of the present embodiment employs a cleanerless system. Each photosensitive drum 1 does not have a dedicated cleaning device for removing toner (transfer residual toner) remaining on the surface after the toner image is transferred onto the intermediate transfer belt 51. The transfer residual toner on the photosensitive drum 1 is carried to the developing unit c through the charging unit a and the exposure unit b as the photosensitive drum 1 rotates during the subsequent subsequent image formation. Then, simultaneous development cleaning (collection) is performed by the developing device 3.

  Since the transfer residual toner on the surface of the photosensitive drum 1 passes through the exposure portion b, the exposure process is performed on the transfer residual toner. However, since the amount of the transfer residual toner is small, there is no significant influence. However, as described above, the transfer residual toner includes a normal charge polarity, a reverse polarity toner (reverse polarity toner), and a low charge amount toner.

  These weakly charged toners and reverse polarity toners are not sufficiently cleaned (collected) simultaneously by the developing device 3. FIG. 4 shows the recovery rate in the developing device 4 with respect to the toner charge amount. In the drawing, the horizontal axis represents the toner charge amount, and the vertical axis represents the recovery rate (weight ratio) in the developing device 3. As can be seen from the figure, the recovery rate is approximately 100% for negatively charged toner having a normal polarity of charge amount of about −20 μc / g, while the charge rate is about −5 μc / g of weakly charged toner or charged toner. The reverse polarity toner of the amount of about +20 μc / g is a very low value of 25% or less.

  The inventor believes that this reason can be explained from the relationship of the force acting on the toner. FIG. 5 shows the force acting on the toner in the developing section c. Among the forces acting on the toner, the force that attracts the toner t to the surface of the photosensitive drum 1 includes the mirror image force Fg due to the charge of the toner t, the liquid crosslinking force Fb due to the contact between the toner t and the surface of the photosensitive drum 1, and the intermolecular force Fm. Can be given. On the other hand, the force for peeling off the toner from the surface of the photosensitive drum 1 is the Coulomb force Fc generated by the developing electric field of the developing device 4. The negatively charged toner t, which is normally charged, has a Coulomb force Fc = qE (q is the toner charge, due to the potential difference (fogging potential) between the surface potential of the surface of the photosensitive drum 1 of −600 V and the potential of the developing sleeve 4 b of −450 V. E is an electric field of the developing part c). This force acts from the surface of the photosensitive drum 1 toward the developing sleeve 4b. The toner t is recovered to the developing sleeve 4b side when this force exceeds the sum of the mirror image force Fg, the liquid crosslinking force Fb, and the intermolecular force Fm.

  However, since the toner charge amount is small in the weakly charged toner t, the Coulomb force becomes weak. For this reason, as shown in FIG. 6, the ratio of the action of the liquid crosslinking force Fb and the intermolecular force Fm is relatively increased. Therefore, the force acting on the toner t is the sum of the mirror image force Fg, the liquid crosslinking force Fb, and the intermolecular force Fm exceeding the Coulomb force Fc. For this reason, the weakly charged toner t passes through the developing portion c without being peeled off from the surface of the photosensitive drum 1.

  Further, as shown in FIG. 7, in the reversely charged toner t, since the toner charge amount has a reverse polarity, the Coulomb force Fc acts in the reverse direction (photosensitive drum 1 side). As for the force acting on the toner t, all of the mirror image force Fg, the liquid crosslinking force Fb, the intermolecular force Fm, and the Coulomb force Fc are directed to the surface of the photosensitive drum 1. Accordingly, the reversely charged toner t also passes through the developing portion c without being peeled off from the surface of the photosensitive drum 1.

  However, in reality, the magnetic brush layer formed on the surface of the developing sleeve 4b mechanically acts by rubbing the surface of the photosensitive drum 1, so that a part of the weakly charged toner or the reverse polarity toner is transferred to the developing device 4. It is considered that the recovery rate is about 25% or less.

  These toners that are not collected by the developing device 4 are rotated around the surface of the photosensitive drum 1. Accumulation due to durability not only causes charging failure and exposure failure, but also causes image failure by fusing to the surface of the photosensitive drum 1.

  Therefore, in the present embodiment that employs a cleanerless system, two configurations are employed to suppress the generation of weakly charged toner and reverse polarity toner. The details will be described below.

(A) Transfer Configuration In the image forming apparatus according to the present embodiment, as a configuration for reducing as much as possible the weakly charged toner having a charge amount near 0 or the positively charged toner having the opposite polarity contained in the transfer residual toner, A configuration that suppresses the discharge at the transfer portion d in FIG. 2 as much as possible is adopted.

  According to the study by the present inventor, it has been clarified that the amount of the weakly charged toner or the reversely charged positively charged toner contained in the transfer residual toner has a correlation with the setting of the transfer bias. FIG. 8 shows the charge amount distribution of the residual toner when the transfer bias applied to the primary transfer roller 53 is +200 V and +700 V in the image forming apparatus of the present embodiment. This figure is a frequency distribution table, in which the horizontal axis represents the toner charge amount, and the vertical axis represents the number of toners (existence probability). As shown in the figure, regardless of whether the transfer bias (transfer voltage) is set to +200 V or +700 V, the weakly charged toner whose charge amount is near 0 or the positively charged toner having the opposite polarity is used. Existing. However, it can be seen that the weakly charged toner and the reverse polarity toner are less in the case where the transfer bias is set to +200 V, which is lower than in the case where the transfer bias is +700 V, which is high. From this, it can be seen that setting the transfer bias as small as possible is effective in suppressing the generation of weakly charged toner and reverse polarity toner.

  On the other hand, in setting the transfer bias, it is necessary to achieve sufficient toner transfer efficiency, and the lower limit of the transfer bias is defined by the transfer efficiency. FIG. 9 shows the relationship between the transfer bias (transfer voltage) and transfer efficiency. In the figure, the horizontal axis represents transfer bias (transfer voltage), and the vertical axis represents transfer efficiency (weight ratio of toner transferred from the photosensitive drum 1 to the intermediate transfer belt 51). As shown in the figure, until the transfer voltage exceeds +100 V, the transfer efficiency increases as the transfer voltage is increased. Thereafter, it can be seen that when the transfer voltage exceeds approximately 140 V, stable transfer efficiency can be obtained. When the transfer voltage is further increased, the transfer efficiency is decreased.

  Therefore, in this embodiment, the transfer bias is kept as low as possible to suppress the generation of weakly charged toner and reverse polarity toner, while the lower limit of the transfer bias is defined from the transfer efficiency. As a result, in this embodiment, the primary transfer bias is set to +200 V as the optimum transfer bias for the cleanerless system. In this case, however, as shown in FIG. Positively charged toner will be present.

(B) Charge Control Unit In this embodiment, therefore, the charge amount of the first toner (developer) is set as shown in FIG. Control means (charge imparting member) 7 and second toner (developer) charge amount control means (charge imparting means) 8 are provided. These are disposed on the downstream side of the transfer portion d along the rotation direction of the photosensitive drum 1 and on the upstream side of the charging portion a. Further, the first toner charge amount control means 7 is disposed on the upstream side, and the second toner charge amount control means 8 is disposed on the downstream side.

In the present embodiment, the first toner charge amount control means 7 and the second toner charge amount control means 8 are brush-like members having an actual resistance of 10 ⁵ to 10 ⁷ Ω. These are set such that the brush width along the rotation direction of the photosensitive drum 1 is set to 5 mm and the brush length is set to 4 mm, and are arranged in contact with the surface of the photosensitive drum 1. A positive voltage is applied from the power source S4 to the first toner charge amount control means 7, and a negative voltage is applied from the power source S5 to the second toner charge amount control means 8.

  In FIG. 2, a contact portion (contact position) between the photosensitive drum 1 and the first toner charge amount control means 7 is indicated by e. Among the residual toners having various polarities, the toner that is charged negatively mainly is temporarily collected by the positive voltage applied to the first toner charge amount control means 7. The Then, the negative polarity toner is charged to the positive polarity by the positive polarity voltage applied to the toner charge amount control means 7, and then gradually separated and attached to the surface of the photosensitive drum 1 and conveyed. On the other hand, most of the positively charged toner and the weakly charged toner immediately after passing through the transfer portion d pass through without being collected by the first toner charge amount control means 7. Therefore, the toner conveyed downstream from the contact portion e of the first toner charge amount control means 7 is mainly a toner charged from a weak charge to a positive polarity.

  In FIG. 2, the contact portion (contact position) between the photosensitive drum 1 and the second toner charge amount control means 8 is indicated by f. Among the toners charged from the weakly charged to the positive polarity that are conveyed downstream from the contact portion e of the first toner charge amount control means 7, particularly the toner charged to the positive polarity is the second toner charge amount control means. 8 is temporarily collected by the negative polarity voltage applied to 8. Then, the negative polarity voltage applied to the toner charge amount control means 8 causes the positive polarity toner to become negative polarity, and then gradually separates and adheres to the surface of the photosensitive drum 1 and is conveyed. FIG. 10 shows changes in the toner charge amount before and after passing through the second toner charge amount control means 8. The toner before entering the second toner charge amount control means 8 was a toner charged from weakly charged to positive polarity, but after passing through the second toner charge amount control means 8, it was charged negatively. You can see that By aligning the toner with positive polarity by the first toner charge amount control means 7, the toner is easily collected by the second toner charge amount control means 8, and the toner can be more effectively aligned with negative polarity.

  In this way, the transfer residual toner whose charging polarity is aligned to the negative polarity that is the normal polarity is conveyed to the developing unit b without adhering to the charging roller 2 to which the negative charging bias voltage positioned downstream is applied, It is collected and reused in the developing device 3.

  However, according to the study of the present inventor, the toner charge amount control means 7 and 8 using the brush-like member which is the configuration of the present embodiment accumulates toner between the brushes due to durability. It was revealed that the ability to temporarily collect and recharge was reduced.

  FIG. 11 shows a change in the toner charge amount before and after passing through the second toner charge amount control means 8 whose performance has been reduced due to durability. Compared with the charge amount distribution of FIG. 10, it can be seen that there are many toners whose charge amount is close to 0, that is, weakly charged after passing. This is presumably because toner is accumulated between the brushes, so that the toner collecting ability of the brush is reduced, and some of the toner passes through the brush without being recharged. As a result, weakly charged toner and reversely charged toner that are difficult to be collected by the developing device 4 accumulate on the surface of the photosensitive drum 1 and are carried around.

  In order to remove the toner being carried around in this way, the present inventor tried to apply a bias having a polarity opposite to that during normal transfer to the primary transfer roller 53. Then, it was effective to remove the toner charged to the opposite polarity. However, this method could not sufficiently remove the weakly charged toner.

  This phenomenon will be considered with reference to FIG. This is considered to be because the weakly charged toner is superior in the liquid cross-linking force Fb and the intermolecular force Fm between the toner t and the photosensitive drum 1 over the reverse polarity transfer electric field, that is, the Coulomb force Fc. At this time, in order to increase the Coulomb force Fc, a measure to increase the transfer electric field can be considered. For example, in order to generate a coulomb force Fc equivalent to that of a normal toner with a charge amount of the toner t being 1/5 that of a normal toner, it is necessary to increase the transfer electric field by a factor of 5, which is extremely high output. High voltage power supply is required.

  Therefore, in this embodiment, a method of removing the weakly charged toner after being recharged using the primary transfer roller 53 is used. In other words, recharging of weakly charged toner enables removal by a transfer electric field. Further, according to this method, since the toner t is recharged using the primary transfer roller 53, unlike the toner charge amount control means 7 and 8, the charging ability is not lowered due to durability. The weakly charged toner can be stably removed.

  The details of the cleaning mode for removing weakly charged toner, which is a characteristic operation of the present embodiment, will be described below.

  (1) The cleaning mode is performed during a time other than normal image formation (image formation) (non-image formation). In the cleaning mode, a weakly charged toner is charged to a reverse polarity by applying a first transfer bias larger than that during normal image formation. Here, the large transfer bias is a transfer bias having a large absolute value of the current flowing through the primary transfer roller 53 of FIG. Here, when a large transfer bias is applied to the charging roller 2 or the toner charge amount control means 7 and 8 and the same charging is performed, the toner adheres to the charging roller 2 and the toner charge amount control means 7 and 8. Appropriate charging becomes difficult in later image formation. Therefore, a bias larger than that at the time of image formation is applied to the primary transfer roller 53 provided with the intermediate transfer belt cleaner 60 and contacting the photosensitive drum 1 via the self-cleaning intermediate transfer belt 51.

  As described above, in the present embodiment employing the cleanerless system, the primary transfer bias is set as small as possible in order to suppress the generation of weakly charged toner and reverse polarity toner, and the toner charge amount control means 7 , 8 are provided. As a result, the weakly charged toner and the reverse polarity toner existing on the photosensitive drum 1 can be suppressed in a minute amount as compared with the case of performing normal image formation.

  However, when image formation is performed for a long period of time, the amount of weakly charged toner or reverse polarity toner that exists on the photosensitive drum 1 increases, and charging failure or exposure failure, or toner fusion onto the photosensitive drum 1 occurs. There is a risk of wearing. For this reason, the above-mentioned weakly charged toner and reverse polarity toner are removed by periodically operating this cleaning mode.

  The operation in the cleaning mode will be described with reference to the schematic configuration diagram of the process unit Pa (Pb, Pc, Pd) shown in FIG. First, the surface of the photosensitive drum 1 is uniformly charged to −600 V by the charging roller 2. This surface potential can be set arbitrarily, but by increasing the absolute value of the voltage, it is possible to increase the transfer contrast at the transfer portion d (transfer contrast will be described later). When the charged area on the photosensitive drum 1 passes through the exposure part b, irradiation with laser light is not performed.

  Further, when the charged region passes through the developing portion c, a DC voltage of −450 V is applied as a developing bias for the developing sleeve 4b. This is a bias for suppressing the adhesion of the magnetic carrier to the surface of the photosensitive drum 1. At this time, the developing sleeve 4b need not be rotated. Further, when the charged region passes through the transfer portion d, a DC voltage of +700 V is applied as a first transfer bias for the transfer roller 53. As shown in FIG. 14, the amount of transfer current flowing is determined by the contrast voltage (transfer contrast) corresponding to the difference between the surface potential of the photosensitive drum 1 and the transfer bias. In the present embodiment, the transfer contrast in the cleaning mode is 1300 V (= 700 − (− 600)). The absolute value of the current flowing through the primary transfer roller 53 when the cleaning mode is executed is 15 μA.

  On the other hand, the transfer contrast during normal image formation according to the present embodiment is 800 V (= 200 − (− 600)). The absolute value of the current flowing through the primary transfer roller 53 during normal image formation is 9 μA.

  In this manner, when the cleaning mode is executed, a discharge is generated in the vicinity of the transfer portion d, and the weakly charged toner on the photosensitive drum 1 can be charged positively.

  FIG. 15 shows the charge amount distribution when the weakly charged toner traveling on the photosensitive drum 1 passes through the transfer portion d to which the normal transfer contrast 800V is applied, and the transfer to which the transfer contrast 1300V in the main cleaning mode is applied. The difference in charge amount distribution when passing through the portion d is shown. The distribution indicated by the solid line is the charge amount of the toner traveling on the photosensitive drum 1, and it can be seen that there are many toners near zero or weakly reversely charged. The charge amount distribution after these toners pass through the transfer portion d to which the normal transfer contrast 800V is applied is indicated by a broken line, and this distribution does not change significantly before the transfer. On the other hand, the charge amount after passing through the transfer portion d to which the transfer contrast of 1300 V is applied in this cleaning mode has a distribution that is strongly charged with reverse polarity, as indicated by a one-dot chain line. In this way, a discharge is generated by applying a transfer contrast larger than that during normal transfer, and the weakly charged toner on the photosensitive drum 1 is charged to a positive polarity.

  By performing the cleaning mode as described above, that is, the process of charging the toner to the opposite polarity over one or more rounds of the surface of the photosensitive drum 1, the weakly charged toner that moves around the surface of the photosensitive drum 1 can be almost recharged.

  (2) The positively charged toner is transferred from the photosensitive drum 1 to the intermediate transfer belt 51 by applying a second transfer bias in the opposite direction to that during normal image formation and causing a transfer current to flow.

  The positive toner on the photosensitive drum 1 generated by the above-described process is conveyed again to the transfer unit d as the photosensitive drum 1 rotates. The operation of each part of the process unit Pa (Pb, Pc, Pd) at this time will be described with reference to FIG. The surface potential of the surface (first region) of the photosensitive drum 1 after passing through the transfer portion d is charged by the transfer contrast, and is closer to the transfer bias than the surface potential of −600 V before passing through the transfer portion. This means that the surface of the photosensitive drum 1 is DC charged by the transfer bias from the transfer roller 53. FIG. 17 shows the relationship between the transfer bias to be applied and the surface potential of the photosensitive drum 1 after the photosensitive drum 1 charged with a surface potential of −600 V passes through the transfer portion d. According to the figure, as the transfer bias is increased, the surface potential of the photosensitive drum after passing through the transfer portion d becomes higher, and when the transfer bias is applied with about +780 V, the photosensitive drum becomes about 0 V. I understand that In this embodiment, a transfer bias of +700 V is applied, and the surface potential of the photosensitive drum 1 after passing through the transfer portion d is −50 V.

  Since positively charged toner is mainly present on the surface of the photosensitive drum 1, the potential of each member is prevented so that the positive toner does not adhere to each member due to a potential difference when passing through each member thereafter. Need to control. In the present embodiment, as shown in FIG. 16, +400 V is applied to the first toner charge amount control means 7 located downstream of the transfer portion d, the second toner charge amount control means 8, the charging roller. 2. The developing sleeve 4b was set not to apply a bias. As a result, the positive toner conveyed by the rotation of the photosensitive drum 1 reaches the transfer portion d again with little adhesion to each member.

  As described above, when the surface of the photosensitive drum 1 passes through each member, in order to prevent the positive toner from adhering to each member due to a potential difference, the surface potential after passing through the transfer portion d is a normal image. The negative polarity is preferably the same as the polarity applied to the charging roller 2 at the time of formation. When the potential on the surface of the photosensitive drum 1 after passing through the transfer portion d is positive, it is necessary to apply a positive bias to the charging roller 2 and the developing sleeve 4b in order to prevent adhesion of positively charged toner. There is. This is because this bias is not used in normal image formation, leading to an increase in the cost of the high-voltage power supply. From this, the upper limit is set for the “first transfer bias larger than that during normal image formation” to be applied to the transfer roller 53 in the main cleaning mode in consideration of the relationship shown in FIG.

  As described above, the second transfer bias of −650 V having a reverse polarity to the normal polarity is applied from the primary transfer roller 53 to the positive polarity toner conveyed on the photosensitive drum 1 and reaching the transfer portion d. The positive toner is transferred onto the intermediate transfer belt 51 with a transfer contrast of 700V. This will be explained with reference to FIG. 18. The Coulomb force Fc acting on the positively charged toner exceeds the sum of the liquid bridging force Fb, intermolecular force Fm and mirror image force Fg between the toner and the photosensitive drum 1. Then, the toner image is transferred from the photosensitive drum 1 onto the intermediate transfer belt 51.

  As described above, the second transfer of the reverse polarity from the primary transfer roller 53 to all the surface areas (first areas) of the photosensitive drum 1 on which the process of charging the toner with the first transfer bias is performed. Apply a bias. Thereby, the toner recharged to the positive polarity on the surface of the photosensitive drum 1 can be almost transferred onto the intermediate transfer belt 51.

  (3) The reversely charged toner on the intermediate transfer belt is cleaned.

  As shown in FIG. 1, the positive toner transferred onto the intermediate transfer belt 51 is conveyed while contacting the secondary transfer outer roller 57 of FIG. 1 as the intermediate transfer belt 51 rotates. At this time, it is preferable to apply a bias having the same polarity as the positive toner or to ground the secondary transfer outer roller 57 so that the positive toner does not adhere to the surface of the secondary transfer outer roller 57. Further, the secondary transfer outer roller 57 can be separated from the intermediate transfer belt 51. Thus, the positive toner on the intermediate transfer belt 51 is conveyed to the intermediate transfer belt cleaner 60 and removed.

  As described above, according to the present embodiment, the weakly charged toner that causes charging failure, exposure failure, toner fusion to the photosensitive drum 1, etc. is recharged to positive polarity using the transfer roller 53. Then, the photosensitive drum 1 is removed. As a result, a stable image can be formed regardless of durability.

  Examples of the operation timing in the cleaning mode include every predetermined number of printed sheets and every predetermined time. In addition, since the chargeability of the toner is greatly affected by the amount of environmental moisture, it is also effective to change the operation timing according to the amount of environmental moisture.

  Further, when the toner on the photosensitive drum is transferred to the intermediate transfer belt 51 in the process (2) of the cleaning mode, the difference between the peripheral speeds of the photosensitive drum 1 and the intermediate transfer belt 51 is about 1 to 3%. Is provided. As a result, the transferability of the recharged toner to the intermediate transfer belt 51 can be improved.

  In addition, the physical property values and bias setting values of the members used in the present embodiment are not limited to these, and can be determined as appropriate. Further, the charging unit and the transfer unit are not limited to the contact type roller, and the present invention can be implemented in a member using a corona charging system. In the present embodiment, the configuration including the toner charge control unit in contact with the photosensitive drum 1 has been described. However, even in an apparatus that does not include these, the present embodiment uses the transfer unit to charge the toner. Can be controlled.

  Further, although the present embodiment has described the image forming apparatus using the intermediate transfer system, the image forming apparatus of the direct transfer system using a recording material transport body such as a recording material transport belt or a transfer drum for carrying and transporting the recording material. However, it is possible to apply the present invention.

<Embodiment 2>
The main purpose of the present embodiment is to remove the weakly charged toner that rotates around the photosensitive drum 1 as in the first embodiment. However, unlike the first embodiment, the toner using the transfer roller 53 is used. The polarity of the toner when the toner is recharged is different. In the first embodiment, the toner is charged to the positive polarity opposite to the normal charge and then collected by the transfer unit d. In the present embodiment, the weakly charged toner is used as the primary transfer roller. 53 is charged to a regular negative polarity, and this is recovered by the developing device 4.

  Since the configuration of the image forming apparatus of the present embodiment is the same as that of the first embodiment, description thereof is omitted. The details of the cleaning mode in which the weakly charged toner is recharged and collected by the developing device 4 as a characteristic operation of the present embodiment will be described below.

  (1) A weakly charged toner is charged to a negative polarity by applying a third transfer contrast having a polarity opposite to that during normal image formation and a transfer contrast larger than that during normal image formation.

  With reference to the schematic configuration diagram of the process unit Pa (Pb, Pc, Pd) shown in FIG. 19, the operation of the cleaning mode in the present embodiment will be described. First, the surface of the photosensitive drum 1 is uniformly charged to −300 V by the charging roller 2. Although this surface potential can be set arbitrarily, in this embodiment, it is possible to increase the transfer contrast at the transfer portion d by reducing the absolute value of the voltage.

  When the charged area on the photosensitive drum 1 passes through the exposure position b, irradiation with laser light is not performed. Further, when the charged region passes through the developing portion (developing position) c, a DC voltage of −150 V is applied as a developing bias for the developing sleeve 4b. This is a bias for suppressing the adhesion of the magnetic carrier to the surface of the photosensitive drum 1. At this time, the developing sleeve 4b need not be rotated.

Furthermore, when the charged region passes through the transfer portion d, a DC voltage of −1600 V is applied as a third transfer bias for the transfer roller 53. As shown in FIG. 20, the amount of transfer current flowing is determined by the contrast voltage (transfer contrast) corresponding to the difference between the surface potential of the photosensitive drum 1 and the transfer bias. The transfer contrast in this cleaning mode is 1300 V (= (− 300 − (− 1600)) The absolute value of the amount of current flowing through the primary transfer roller 53 when the cleaning mode is executed is 15 μA.

  On the other hand, the transfer contrast is 800 V (= 200 − (− 600)) during normal image formation in the present embodiment. The absolute value of the current flowing through the primary transfer roller 53 during normal image formation is 9 μA.

  In this way, a discharge is generated in the vicinity of the transfer portion d. In addition, since the transfer bias has a negative polarity opposite to the normal polarity, the positive toner on the photosensitive drum 1 is transferred to the intermediate transfer belt 51 and the weakly charged toner on the photosensitive drum 1 is charged to the negative polarity at the same time. be able to. The charge amount distribution before and after passing through the transfer portion d shows that the toner after passing through the transfer portion is strongly charged with negative polarity as shown in FIG.

  As described above, the process of charging the toner by applying the second transfer bias to the transfer roller 53 is performed over one or more rounds (second region) of the surface of the photosensitive drum 1, thereby weakly charging the surface of the photosensitive drum 1. The toner can be almost recharged.

  (2) Collect the negatively charged toner from the photosensitive drum 1 to the developing device 4.

  The negative toner on the photosensitive drum 1 generated by the above-described process is conveyed to the developing unit c as the photosensitive drum 1 rotates. The operation of each part of the process unit at this time will be described with reference to FIG. The surface potential of the photosensitive drum 1 after passing through the transfer portion d is charged by the transfer contrast, and in this embodiment, the surface potential of the photosensitive drum 1 after passing through the transfer portion d is −1000 V with respect to the transfer bias of −1600 V. there were.

  Since the negatively charged toner exists mainly on the surface of the photosensitive drum 1, the potential of each member is controlled so that the toner does not adhere to each member due to a potential difference when passing through each member thereafter. There is a need to. In this embodiment, −1100 V is applied to the first toner charge amount control means 7, the second toner charge amount control means 8, and the charging roller 2 that are located downstream of the transfer portion d. As a result, the positive toner conveyed by the rotation of the photosensitive drum 1 reaches the developing portion c with almost no adhesion to each member.

  The negative polarity toner on the photosensitive drum 1 that has reached the developing portion c is applied to the developing sleeve 4b by applying a fog removal bias used in normal image formation to the developing device 4 like the negative transfer residual toner. Collected. Here, the fog removal bias means a fog removal potential difference Vback which is a potential difference between the DC voltage applied to the developing device 4 and the surface potential of the photosensitive drum 1. The developing bias in this embodiment is set to -850V.

  It is also effective to apply a positive fourth transfer bias to the primary transfer roller 53 in order to collect toner that has not been collected by the developing device 4 at this time.

  As described above, according to the present embodiment, the weakly charged toner that causes charging failure, exposure failure, toner fusion to the photosensitive drum 1, etc. is recharged using the transfer roller 53, and then is supplied to the developing device 4. By collecting, a stable image can be formed regardless of durability.

  The configuration of the present embodiment has a drawback that the capacity of the transfer high-voltage power supply is larger than that of the first embodiment. However, since the accompanying toner can be collected in the developing device 4, the toner is effectively used. be able to. However, in this case, in order to prevent the accompanying toner from being deteriorated (external agent is dropped from the toner, etc.), mechanical damage (peripheral speed difference at the transfer portion d, etc.) can be suppressed to the toner on the photosensitive drum 1. It is preferable to set.

<Embodiment 3>
The image forming apparatus of the present embodiment uses a recording material conveyance belt that carries and conveys a recording material.

  FIG. 23 shows an image forming apparatus according to the present embodiment. The configuration and operation of the image forming apparatus will be described with reference to FIG. The image forming apparatus according to the present embodiment includes four process units Pa, Pb, Pc, and Pd that form images (toner images) of colors of yellow, magenta, cyan, and black. The toner images of the respective colors formed by these process units Pa, Pb, Pc, and Pd are sequentially transferred at the transfer portion g to the recording material P carried and conveyed by the recording material conveyance belt 91 as a recording material conveyance body. Is done.

  Since each of the process units Pa, Pb, Pc, and Pd described above has the same configuration as that of the first embodiment, the same reference numerals are given to the members having the same configuration and the description thereof is omitted.

  Next, the recording material transport unit 9 will be described.

  As shown in FIG. 23, a recording material transport unit 9 is disposed below the photosensitive drum 1 of the process units Pa, Pb, Pc, and Pd. The recording material conveyance unit 9 includes a recording material conveyance belt 91, an adsorption roller 92, an adsorption roller facing roller 93, four transfer rollers 94, a recording material conveyance belt drive roller 95, a tension roller 96, and a recording material conveyance belt cleaner 97. doing.

The recording material conveyance belt 91 is configured by forming an endless dielectric resin. In this embodiment, a PI resin having a volume resistivity of 10 ¹² Ω · cm (using a probe conforming to JIS-K6911 method, applied voltage 1000 V, applied time 60 sec, temperature 23 ° C., humidity 60% RH) and thickness t = 90 μm is used. Adopted. However, the material and the thickness are not limited to those described above as long as the specific volume resistance when 1000 V is applied is 10 ¹² to 10 ¹⁴ Ω · cm.

  The transfer roller (transfer member) 94 is the same as the primary transfer roller 53 of the image forming apparatus of the first embodiment. As shown in parentheses in FIG. 2, a transfer bias of +1700 V is applied to the transfer roller 94 from the power source S3 as will be described later. As a result, the toner image on the photosensitive drum 1 is transferred to the recording material P carried on the transfer material conveyance belt 91 in the transfer portion g. The transfer bias applied by the power source S3 to the transfer roller 94 is controlled by the control means 98.

  Since the fixing device 70 (FIG. 1) as the fixing unit has the same configuration as that of the fixing device 70 of the first embodiment, the same reference numerals are given to members having the same components and the description thereof is omitted.

  Image formation (at the time of image formation) in the above-described image forming apparatus is performed as follows. The toner images of the respective colors formed on the photosensitive drum 1 are sequentially transferred to the recording material P carried and conveyed on the recording material conveyance belt 91.

  The recording material P accommodated in the paper feed cassette 80 is fed by the pickup roller 81 and supplied to the transport rollers 82 and 83. Further, the recording material P is transported in the arrow Kp direction along the pre-transfer transport guide 84. A voltage of −2300 V is applied to the suction roller 92 from the power source S4, and the recording material P is electrostatically applied to the recording material conveying belt 91 by the action of an electric field between the suction roller 92 and the suction roller facing roller 93 that is grounded. Adsorbed. Here, the suction roller 92 is a rubber roller, and the suction roller facing roller 93 is a metal roller.

  The recording material P to which the toner image has been transferred is separated from the recording material conveyance belt 91 and conveyed to the fixing device 70. When the recording material P passes between the fixing roller 71 and the pressure roller 72, the recording material P is pressed and heated at a substantially constant pressure and temperature from both the front and back surfaces. The recording material P is fixed by melting and fixing the toner image on the surface. Thereby, the four-color full-color image formation on one side of one recording material P is completed. Note that the toner adhering to the recording material conveyance belt 91 is collected by the recording material conveyance belt cleaner 97.

  Also in the image forming apparatus of the present embodiment, a cleanerless system is employed in the process units Pa, Pb, Pc, and Pd. Since the configuration of the cleanerless system is the same as that of the image forming apparatus according to the first embodiment, the same reference numerals are given to members having the same configuration and the description thereof is omitted. Moreover, in FIG. 13, FIG. 16, the code | symbol is attached | subjected with the parenthesis to the corresponding member in Embodiment 1. FIG.

  Also in the image forming apparatus of the present embodiment, the weakly charged toner and a part of the reverse polarity toner remaining on the photosensitive drum 1 are not collected by the developing device 4 but are entangled on the surface of the photosensitive drum 1.

  Therefore, in the present embodiment, as in the image forming apparatus of the first embodiment, two configurations for suppressing generation of weakly charged toner and reverse polarity toner are employed.

(A) Transfer Configuration In order to reduce the occurrence of weakly charged toner and reverse polarity toner, a configuration that suppresses the discharge at the transfer portion g as much as possible is adopted.

  Also in the image forming apparatus of the present embodiment, it has been clarified that the amount of weakly charged toner or reversely charged positively charged toner contained in the untransferred toner has a correlation with the setting of the transfer bias.

  Therefore, in consideration of the amount of weakly charged toner and positively charged toner of reverse polarity and transfer efficiency, the transfer bias is set to +1700 v in this embodiment. Similar to the image forming apparatus of the first embodiment, when the transfer bias is set to +1700 v, a slight amount of positively charged toner having a reverse polarity exists in the transfer residual toner.

(B) Charging Control Unit Therefore, in the present embodiment as well, in the same way as in the first embodiment, in order to make the charging polarity of the transfer residual toner the negative polarity that is the normal polarity, as shown in FIG. Toner (developer) charge amount control means 7 and second toner (developer) charge amount control means 8 are provided.

  The transfer residual toner having the negative polarity that is the normal polarity is conveyed to the developing unit c without being attached to the charging roller 2 applied to the negative polarity located downstream, and is collected by the developing device 4. It will be reused.

  However, some toner passes through the first toner (developer) charge amount control means 7 and the second toner (developer) charge amount control means 8 without being recharged. As a result, weakly charged toner or reversely charged toner that is difficult to be collected by the developing device 4 accumulates on the surface of the photosensitive drum 1 and is carried around.

  As described in the first embodiment, the reversely charged toner can be removed by applying a bias having a polarity opposite to that during normal transfer to the transfer roller 94. However, this method could not sufficiently remove the weakly charged toner.

  Therefore, in this embodiment, a method is used in which the weakly charged toner is recharged using the transfer roller 94 and then removed. In other words, recharging of weakly charged toner enables removal by a transfer electric field.

  The details of the cleaning mode for removing weakly charged toner, which is a characteristic operation of the present embodiment, will be described below.

  (1) The cleaning mode is performed in a state where there is no recording material in the transfer portion g at a time other than normal image formation (image formation) (non-image formation).

  In the cleaning mode, by applying the first transfer bias so that the absolute value of the amount of current flowing through the transfer portion g is larger than the absolute value of the amount of current flowing through the transfer portion g during normal image formation, Charge weakly charged toner to reverse polarity.

  The operation in the cleaning mode will be described with reference to the schematic configuration diagram of the process unit Pa (Pb, Pc, Pd) shown in FIG. First, the surface of the photosensitive drum 1 is uniformly charged to −600 V by the charging roller 2. Although this surface potential can be set arbitrarily, it is possible to increase the transfer contrast at the transfer portion g by increasing the absolute value of the voltage (the transfer contrast will be described later. And this photosensitive drum) When the charged area on 1 passes through the exposure part b, irradiation with laser light is not performed.

  Further, when the charged region passes through the developing portion c, a DC voltage of −450 V is applied as a developing bias for the developing sleeve 4b. This is a bias for suppressing the adhesion of the magnetic carrier to the surface of the photosensitive drum 1. At this time, the developing sleeve 4b need not be rotated. Further, when the charged region passes through the transfer portion g, a DC voltage of +1200 V is applied as a first transfer bias for the transfer roller 94.

  The amount of transfer current flowing is determined by the magnitude of the contrast voltage (transfer contrast) corresponding to the difference between the surface potential of the photosensitive drum 1 and the transfer bias, and the presence or absence of the recording material P in the transfer portion g.

  In the present embodiment, when the cleaning mode is executed, the recording material P does not exist in the transfer portion g, and the transfer contrast is set to 1800 V (= 1200 − (− 600)). At this time, a current of 15 μA in absolute value flows through the transfer roller 94. On the other hand, during normal image formation according to the present embodiment, the recording material P is present in the transfer portion g, and the transfer contrast is set to 2300 V (= 1700 − (− 600)). During image formation, a current of 9 μA in absolute value flows through the transfer roller 94.

  In this manner, in the cleaning mode, a discharge is generated in the vicinity of the transfer portion g, and the weakly charged toner on the photosensitive drum 1 can be charged positively.

  By performing the cleaning mode as described above, that is, the process of charging the toner over one or more rounds of the surface of the photosensitive drum 1, the weakly charged toner accompanying the surface of the photosensitive drum 1 can be almost recharged.

  2) The toner charged to the positive polarity is transferred from the photosensitive drum 1 to the recording material conveyance belt 91 by applying a second transfer bias in the opposite direction to that during normal image formation and causing a transfer current to flow.

  The positive toner on the photosensitive drum 1 generated by the above-described process is conveyed again to the transfer part g as the photosensitive drum 1 rotates. The operation of each part of the process unit Pa (Pb, Pc, Pd) at this time will be described with reference to FIG. The surface potential of the surface (first region) of the photosensitive drum 1 after passing through the transfer portion g is charged by the transfer contrast, and is closer to the transfer bias than the surface potential of −600 V before passing through the transfer portion. In this embodiment, a transfer bias of +1200 V is applied in the cleaning mode, and the surface potential of the photosensitive drum 1 after passing through the transfer portion g is −50 V.

  Since positively charged toner is mainly present on the surface of the photosensitive drum 1, the potential of each member is prevented so that the positive toner does not adhere to each member due to a potential difference when passing through each member thereafter. Need to control. In the present embodiment, as in the first embodiment, as shown in FIG. 16, +400 V is applied to the first toner charge amount control means 7 located downstream of the transfer portion g, and the second toner A setting is made so that no bias is applied to the charge amount control means 8, the charging roller 2, and the developing sleeve 4b. Accordingly, the positive toner conveyed by the rotation of the photosensitive drum 1 reaches the transfer portion g again with almost no adhesion to each member.

  Thus, when the surface of the photosensitive drum 1 passes through each member, in order to prevent the positive toner from adhering to each member due to a potential difference, the surface potential after passing through the transfer portion g is a normal image. The negative polarity is preferably the same as the polarity applied to the charging roller 2 at the time of formation. When the potential of the surface of the photosensitive drum 1 after passing through the transfer portion g is positive, it is necessary to apply a positive bias to the charging roller 2 and the developing sleeve 4b in order to prevent adhesion of positively charged toner. There is. This is because this bias is not used in normal image formation, leading to an increase in the cost of the high-voltage power supply.

  As described above, a second transfer bias of 1250 V having a polarity opposite to that of the normal polarity is applied from the transfer roller 94 to the positive polarity toner conveyed on the photosensitive drum 1 and reaching the transfer portion g. With the transfer contrast of 1300 V, the positive polarity toner is transferred onto the recording material conveyance belt 91.

  As described above, the second transfer bias having the reverse polarity is applied from the transfer roller 94 to all the surface areas (first areas) of the photosensitive drum 1 subjected to the process of charging the toner with the first transfer bias. Apply. Thereby, most of the recharged toner on the surface of the photosensitive drum 1 can be transferred onto the recording material conveyance belt 91.

  (3) Cleaning the reversely charged toner on the recording material conveyance belt.

  As shown in FIG. 23, the positive toner transferred onto the recording material conveyance belt 91 is conveyed to the recording material conveyance belt cleaner 97 and removed.

  As described above, according to this embodiment, a weakly charged toner that causes charging failure, exposure failure, toner fusion to the photosensitive drum 1, etc. is recharged using the transfer roller 94 and then exposed to light. Remove from drum 1. As a result, a stable image can be formed regardless of durability.

  Examples of the operation timing in the cleaning mode include every predetermined number of printed sheets (for example, every 500 sheets) and every predetermined time. In addition, since the chargeability of the toner is greatly affected by the amount of environmental moisture, it is also effective to change the operation timing according to the amount of environmental moisture.

  Further, when the toner on the photosensitive drum is transferred to the recording material conveyance belt 91 in the process (2) of the main cleaning mode, the peripheral speed between the photosensitive drum 1 and the recording material conveyance belt 91 is about 1 to 3%. Provide the difference. As a result, transferability of the recharged toner to the recording material conveyance belt 91 can be improved.

  In addition, the physical property values and bias setting values of the members used in the present embodiment are not limited to these, and can be determined as appropriate.

  Further, the charging unit and the transfer unit are not limited to the contact type roller, and the present invention can be implemented in a member using a corona charging system.

  In the present embodiment, the configuration including the toner charge control unit in contact with the photosensitive drum 1 has been described. However, even in an apparatus that does not include these, the present embodiment uses the transfer unit to charge the toner. Can be controlled.

<Embodiment 4>
Similar to the third embodiment, the main object of the present embodiment is to remove weakly charged toner that rotates around the photosensitive drum 1 in the image forming apparatus using the recording material conveyance belt 91.

  However, unlike Embodiment 3, the polarity of the toner when the toner is recharged using the transfer roller 94 is different. In the third embodiment, the toner is charged to the positive polarity opposite to the normal charge and then collected by the transfer portion g. In the present embodiment, the weakly charged toner is transferred to the transfer roller 94. It is used to be charged to a regular negative polarity and recovered by the developing device 4.

  Since the configuration of the image forming apparatus according to the present embodiment is the same as that of the second embodiment except for the portion related to the difference between the recording material conveyance belt and the intermediate transfer belt, the description thereof is omitted. The details of the cleaning mode in which the weakly charged toner is recharged and collected by the developing device 4 as a characteristic operation of the present embodiment will be described below. Further, in FIGS. 19 and 23 used for the description, the corresponding members of the second embodiment are denoted by reference numerals in parentheses.

  (1) A transfer bias having a polarity opposite to that during normal image formation is applied to the transfer roller 94.

  At this time, the transfer bias is set so that the absolute value of the current flowing through the transfer roller 94 during execution of the cleaning mode is larger than the absolute value of the current flowing through the transfer roller 94 during normal image formation. In this way, the weakly charged toner is charged to a negative polarity.

  With reference to the schematic configuration diagram of the process unit Pa (Pb, Pc, Pd) shown in FIG. 19, the operation of the cleaning mode in the present embodiment will be described. First, the surface of the photosensitive drum 1 is uniformly charged to −300 V by the charging roller 2. Although this surface potential can be set arbitrarily, in this embodiment, it is possible to increase the transfer contrast at the transfer portion g by reducing the absolute value of the voltage. When the charged area on the photosensitive drum 1 passes through the exposure part b, irradiation with laser light is not performed.

  Further, when the charged region passes through the developing portion (developing position) c, a DC voltage of −150 V is applied as a developing bias for the developing sleeve 4b. This is a bias for suppressing the adhesion of the magnetic carrier to the surface of the photosensitive drum 1. At this time, the developing sleeve 4b need not be rotated. Furthermore, when the charged region passes through the transfer portion g, a DC voltage of −2100 V is applied as a third transfer bias for the transfer roller 53.

  The amount flowing through the transfer roller 53 is determined by the magnitude of the contrast voltage (transfer contrast) corresponding to the difference between the surface potential of the photosensitive drum 1 and the transfer bias and the presence or absence of the recording material in the transfer portion g.

  In the present embodiment, when the cleaning mode is executed, the recording material P does not exist in the transfer portion g, and the transfer contrast is set to 1800 V (= (− 300) − (− 2100). A current of 15 μA in absolute value flows through the roller 94.

  On the other hand, during normal image formation according to the present embodiment, the recording material P is present in the transfer portion g, and the transfer contrast is set to 2300 V (= 1700 − (− 600)). During image formation, a current of 9 μA in absolute value flows through the transfer roller 94.

  In this manner, in the cleaning mode, a discharge is generated in the vicinity of the transfer portion g, and the weakly charged toner on the photosensitive drum 1 can be charged to a negative polarity.

  As described above, the process of charging the toner by applying the third transfer bias to the transfer roller 94 is performed over one or more rounds (second region) of the surface of the photosensitive drum 1, thereby weakly charging the surface of the photosensitive drum 1. The toner can be almost recharged.

  (2) Collect the negatively charged toner from the photosensitive drum 1 to the developing device 4.

  The negative toner on the photosensitive drum 1 generated by the above-described process is conveyed to the developing unit c as the photosensitive drum 1 rotates. The operation of each part of the process unit at this time will be described with reference to FIG. The surface potential of the photosensitive drum 1 after passing through the transfer portion g is charged by the transfer contrast, and in this embodiment, the surface potential of the photosensitive drum 1 after passing through the transfer portion g is −1000 V with respect to the transfer bias of −2100 V. there were.

  Since the negatively charged toner exists mainly on the surface of the photosensitive drum 1, the potential of each member is controlled so that the toner does not adhere to each member due to a potential difference when passing through each member thereafter. There is a need to. In this embodiment, −1100 V is applied to the first toner charge amount control means 7, the second toner charge amount control means 8, and the charging roller 2 that are located downstream of the transfer portion g. As a result, the positive toner conveyed by the rotation of the photosensitive drum 1 reaches the developing portion c with almost no adhesion to each member.

  The negative polarity toner on the photosensitive drum 1 that has reached the developing portion c is applied to the developing sleeve 4b by applying a fog removal bias used in normal image formation to the developing device 4 like the negative transfer residual toner. Collected. Here, the fog removal bias means a fog removal potential difference Vback which is a potential difference between the DC voltage applied to the developing device 4 and the surface potential of the photosensitive drum 1. The developing bias in this embodiment is set to -850V.

  It is also effective to apply a positive fourth transfer bias to the transfer roller 94 in order to recover the toner that has not been recovered by the developing device 4 at this time.

  As described above, according to the present embodiment, the weakly charged toner causing the charging failure, the exposure failure, and the fusion of the toner to the photosensitive drum 1 is recharged by using the transfer roller 94 and then applied to the developing device 4. By collecting, a stable image can be formed regardless of durability.

  The configuration of the present embodiment has a drawback that the capacity of the transfer high-voltage power supply is larger than that of the first embodiment. However, since the accompanying toner can be collected in the developing device 4, the toner is effectively used. be able to. In this case, however, the toner on the photosensitive drum 1 is set to suppress mechanical damage (such as a difference in peripheral speed at the transfer portion g) in order to prevent the accompanying toner from deteriorating (such as dropping off of the external additive from the toner). It is preferable.

  As described above, the cleaning mode of each embodiment includes the charging process for charging the transfer residual toner accumulated on the image carrier when the non-toner image is not formed, and the charged transfer residual toner. And a removal step of electrostatically removing. In the charging process, a voltage of a discharge level (usually, by a primary transfer member or a transfer member through an intermediate transfer body or a recording material transport body provided with its own toner collecting means over a rotation of one rotation of the image carrier. Apply a high voltage that is not used. Therefore, the toner adhering to the intermediate transfer member or the recording material conveyance member due to charging is quickly removed, and there is no problem in the next image formation. Then, in the removing step, a charged polarity is applied to an existing recyclable member (an intermediate transfer body or a recording material conveyance body provided with its own toner collecting means, or a developing sleeve having no problem due to toner adhesion). Apply a voltage of the opposite polarity. Accordingly, no transfer residual toner remains on the members involved in the next image formation.

1 is a diagram schematically showing a schematic configuration of an image forming apparatus according to Embodiments 1 and 2. FIG. It is a figure which shows schematic structure of a process unit. It is a figure explaining the layer structure of a photosensitive drum and a charging roller. It is a figure explaining the relationship between a toner charge amount and the collection rate by a developing device. FIG. 6 is a diagram illustrating a force acting on a normally charged toner in a developing unit. FIG. 6 is a diagram illustrating a force acting on weakly charged toner in a developing unit. FIG. 6 is a diagram illustrating a force acting on a reversely charged toner in a developing unit. FIG. 6 is a diagram illustrating a charge amount distribution of untransferred toner when transfer voltages are +200 V and +700 V. It is a figure explaining the relationship between a transfer voltage and transfer efficiency. FIG. 6 is a diagram illustrating a change in toner charge amount before and after passing through a second toner charge amount control unit. FIG. 6 is a diagram for explaining a change in toner charge amount before and after passing through a second toner charge amount control unit whose ability is reduced due to durability. FIG. 6 is a diagram illustrating a state where it is difficult to remove weakly charged toner. It is a figure explaining the cleaning operation in a process unit. It is a figure explaining contrast potential. FIG. 6 is a diagram illustrating a change in toner charge amount distribution before and after passing through a transfer portion during normal image formation and after passing through a transfer portion in a cleaning mode. FIG. 10 is a diagram for explaining the operation of the photosensitive drum and each of the peripheral units in the cleaning mode. It is a figure explaining the relationship between a primary transfer voltage and the drum surface potential after transfer. FIG. 4 is a diagram illustrating how positive toner is transferred to a transfer belt. FIG. 10 is a diagram for explaining the operation of the photosensitive drum and each of the peripheral parts in the cleaning mode of the second embodiment. 10 is a diagram illustrating a contrast potential in Embodiment 2. FIG. FIG. 6 is a diagram illustrating a change in toner charge amount distribution before and after passing through a transfer portion in a cleaning mode. FIG. 6 is a diagram for explaining the operation of the photosensitive drum and its peripheral members when conveying negative toner to the developing unit. FIG. 3 is a diagram schematically illustrating a schematic configuration of an image forming apparatus according to Embodiments 3 and 4.

Explanation of symbols

1 Image carrier (photosensitive drum)
2 Charging means (primary charging roller)
3 Electrostatic latent image forming means (exposure device)
4 Developer (Developer)
7 First toner charge amount control means 8 Charge application means (second toner charge amount control means)
51 Intermediate transfer member (intermediate transfer belt)
53 Primary transfer member (primary transfer roller)
54, 98 Control means 56 Secondary transfer inner roller 57 Secondary transfer member (secondary transfer outer roller)
60, 97 Toner recovery means (intermediate transfer belt cleaner, recording material transport belt cleaner)
91 Recording material transport body (Recording material transport belt)
94 Transfer member (transfer roller)
P Recording material

Claims (4)

An image carrier that carries and rotates an electrostatic image;
Charge applying means for charging the toner on the image carrier by applying a voltage having the same polarity as the normal charging polarity of the toner;
A developing device for electrostatically collecting the toner charged by the charge applying unit on the image carrier while developing the electrostatic image to form a toner image;
An intermediate transfer member to which a toner image is transferred;
A primary transfer member that primarily transfers the toner image on the image carrier to an intermediate transfer member by applying a voltage having a polarity opposite to the normal charging polarity of the toner;
A secondary transfer member that secondarily transfers the toner image from the intermediate transfer member to a recording material;
During the non-toner image formation, the absolute value of the current amount is larger than the absolute value of the current amount flowing through the primary transfer member when the toner image is primarily transferred during at least one rotation of the image carrier. after the bets toner having the same polarity as the charging polarity of the voltage of the normal, such as continuing the control to be applied to the primary transfer member, said developing device is feasible mode for collecting the toner on the image bearing member An image forming apparatus. An image carrier that carries and rotates an electrostatic image;
Charge applying means for charging the toner on the image carrier by applying a voltage having the same polarity as the normal charging polarity of the toner;
A developing device for electrostatically collecting the toner charged by the charge applying unit on the image carrier while developing the electrostatic image to form a toner image;
A recording material carrier for carrying and conveying the recording material;
An image having a transfer member that transfers the toner image on the image carrier to a recording material carried on the recording material conveyance body by applying a voltage having a polarity opposite to a normal charging polarity of the toner. In the forming device,
During the non-toner image formation, at least during the period in which the image carrier rotates once, when the toner image is transferred, the toner has an absolute value that is larger than the absolute value of the current flowing through the transfer member. An image forming apparatus, wherein after the control of applying a voltage having the same polarity as a normal charging polarity to the transfer member is continued , the developing unit can execute a mode for collecting toner on the image carrier. An image carrier that carries and rotates an electrostatic image;
A charging roller for applying a charging bias to charge the image carrier;
While carrying a developer including toner and carrier to the developing sleeve to develop the electrostatic image to form a toner image, collecting the toner charged by electric load applying means on the image bearing member electrostatically A developing device,
An intermediate transfer member to which a toner image is transferred;
A primary transfer member that primarily transfers the toner image on the image carrier to an intermediate transfer member by applying a voltage having a polarity opposite to the normal charging polarity of the toner;
The primary transfer member and in contact with said image bearing member between said charging roller, said for charging the toner on said image bearing member by applying the same polarity of the voltage to the normal charge polarity of the toner Charge imparting means;
A secondary transfer member that secondarily transfers the toner image from the intermediate transfer member to a recording material;
At the time of non-toner image formation, a charging bias having the same polarity as that at the time of normal image formation is applied to the charging roller to form a charging area around the image carrier,
Applying a bias for suppressing the adhesion of the magnetic carrier to the developing sleeve with respect to the charging region,
With respect to the charged area, the absolute value becomes such normal charge polarity opposite the polarity of toner in the absolute value larger amount of current than the amount of current flowing through the primary transfer member when the toner image is primarily transferred In the state where no voltage is applied to the charge applying unit, the charging roller, and the developing sleeve with respect to the charging region that has passed through the primary transfer member after the voltage is applied to the primary transfer member. A toner that transfers the toner remaining on the image carrier to the intermediate transfer member by applying a voltage having the same polarity as the normal charging polarity of the toner to the primary transfer member, and collects the toner on the intermediate transfer member An image forming apparatus capable of executing a mode of collection by a collection unit. An image carrier that carries and rotates an electrostatic image;
A charging roller for applying a charging bias to charge the image carrier;
While carrying a developer including toner and carrier to the developing sleeve to develop the electrostatic image to form a toner image, collecting the toner charged by electric load applying means on the image bearing member electrostatically A developing device,
A recording material carrier for carrying and conveying the recording material;
A transfer member that transfers the toner image on the image carrier to a recording material carried on the recording material conveyance body by applying a voltage having a reverse polarity to the normal charging polarity of the toner;
The contact with the image carrier between the transfer member and the charging roller, the charge imparted to charge the toner on the image bearing member by applying the same polarity of the voltage to the normal charge polarity of the toner And an image forming apparatus comprising:
At the time of non-toner image formation, a charging bias having the same polarity as that at the time of normal image formation is applied to the charging roller to form a charging area around the image carrier,
Applying a bias for suppressing the adhesion of the magnetic carrier to the developing sleeve with respect to the charging region,
A voltage having a polarity opposite to the normal charging polarity of the toner so that the absolute value of the amount of current larger than the absolute value of the amount of current flowing through the transfer member when the toner image is transferred to the charging region is After the voltage is applied to the transfer member, a normal charging of toner is applied to the transfer member in a state where no voltage is applied to the charge applying unit, the charging roller, and the developing sleeve with respect to the charged region that has passed through the transfer member. A mode in which the toner remaining on the image carrier is transferred to the recording material conveyance body by applying a voltage having the same polarity as that of the polarity, and is collected by a toner collecting means for collecting the toner on the recording material conveyance body is executed. An image forming apparatus capable of being used.

Images