JP2006098894A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress color mixing or back ground fogging by recovering by an execution of a recovery action even when developer with high electrifying capability is transferred in reverse in a developing unit which is in a lower order in developer image forming order than the developing unit. <P>SOLUTION: A control part 10 in an image forming apparatus determines whether the electrified quantity (electrifying capability) of toner in at least one developing unit among the second to fourth developing units 52C to 52BK is smaller by more than a specified quantity than the electrified quantity of the toner in the developing unit 52 which is on the upstream side. Then, under the condition that judgement is made that the above is smaller by more than the specified quantity, recovery function improvement action is executed by applying bias voltage for recovery from a bias supply unit 11 to supplying rollers 55C to 55BK of the second to fourth developing units 52C to 52BK. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus for transferring a developer image sequentially formed on an image carrier by a plurality of developing units onto a transfer material to form a multicolor image.

Conventionally, a developer image is sequentially formed on each surface of a plurality of image carriers on which electrostatic latent images are formed (one image carrier in the case of a so-called four-cycle system) by a plurality of developing units. There is known an image forming apparatus that forms a multicolor image by sequentially transferring an agent image on a transfer material such as a sheet or an intermediate transfer member and transferring the image (see Patent Document 1 below).
JP 2002-31933 A

  By the way, when a multi-color image is formed by the image forming apparatus, the image carrier having the second or subsequent order in which the developer image is transferred to the transfer material is self-attached as a waste developer due to the transfer residue. In addition to this developer, there is also a developer attached by reverse transfer.

  This reverse transfer is a developer (reversely charged developer) charged to a part of the developer image transferred from the image carrier to the transfer material with a polarity opposite to the polarity charged by the developing unit. Therefore, when the developer image is transferred to the transfer material on the second or subsequent image carrier in which the developer image is formed, the reversely charged developer is reverse to the normal transfer. Transfer from the transfer material in the direction to the image carrier.

  Here, for example, the charging ability of the developer developed by the first developing unit having the developer image formation order is higher than the charging ability of the developer developed by the second and subsequent development units (absolute value of the charge amount). In the second and subsequent development units, a so-called background fogging phenomenon may occur. In other words, in the second and subsequent development units, when a developer with high charging ability from the upper development unit is reversely transferred, the reversely transferred developer with high charge (the absolute value of the charge amount is large) It moves to the developer carrier at the nip point between the carrier and the developer carrier and accumulates on the surface of the developer carrier. For this reason, when the accumulated highly charged developer is mixed with the developer supplied by the developer, color mixing occurs. Also, due to frictional charging with the highly charged developer, a part of the developer supplied by itself changes from the difference in charging ability (difference in charge amount) to the opposite polarity to the original charging polarity, and this polarity is reversed. The developed developer adheres outside the electrostatic latent image area of the image carrier, and so-called background fogging occurs.

  The present invention has been made in view of these problems, and an object of the present invention is to perform image formation in which the influence of reverse transfer is suppressed in the above-described image forming apparatus.

  As means for achieving the above object, an image forming apparatus according to the invention of claim 1 is provided corresponding to each of an image carrier on which an electrostatic latent image is formed and three or more developers. A plurality of developing units configured to develop the electrostatic latent image on the surface of the image carrier with the developer, the developer carrying body carrying each developer. An image forming apparatus capable of sequentially forming a developer image on the image carrier by developing an electrostatic latent image by each of the developing units and transferring the developer image to a transfer material. In the plurality of development units, at least a development unit having a developer image formation order other than the highest one is provided with a collection unit that collects the developer adhering to the developer carrier at the time of non-development, One of the plurality of developing units When a developing unit develops with a developer having a higher charging ability than a developing unit having a lower developer image formation order than that, the developer recovery by the collecting means is started, or the collecting ability is usually set. It is characterized by performing a collecting operation that is sometimes raised.

The “three or more developers” may include not only developers of different colors but also developers of the same color.
The “transfer material” may be not only a recording medium such as paper or an OHP sheet, but also an intermediate transfer member such as an intermediate transfer belt or an intermediate transfer drum.
The “image forming apparatus” is not limited to a printing apparatus such as a printer (laser printer), but may be a facsimile machine or a multifunction machine having a printer function and a scanner function. Further, the development unit is not limited to a tandem system provided with an image carrier for each development unit, and a 4-cycle system in which each development unit develops a common image carrier may be used.
“Non-development” means when the development unit is not performing a development operation. For example, before and after an image formation operation, during each image formation operation on a recording medium that is sequentially performed, further development is performed. This includes the time when the agent carrier is facing a portion of the image carrier other than the electrostatic latent image forming area.
The “collecting operation” may be executed for all or a part of the developing units having a developer image formation order lower than that of the one developing unit, even if the developing unit has a collecting unit. It may be configured to.
“Developer image formation order” means, for example, in the tandem system, the order of arrangement of the development units in the relative movement direction with respect to the transfer material.

  According to a second aspect of the present invention, there is provided an image forming apparatus comprising: an image bearing member on which an electrostatic latent image is formed; and a developer carrying member that is provided corresponding to each of the three or more developers. And a plurality of development units configured to develop the electrostatic latent image on the surface of the image carrier with the developer by the developer carrier, and electrostatically In the image forming apparatus capable of sequentially forming a developer image on the image carrier by developing a latent image and transferring the developer image to a transfer material to form an image, at least of the plurality of development units. A developing unit having a developer image formation order other than the highest one is provided with a collecting means for collecting the developer adhering to the developer carrying member during non-development, and the developer image formation among the plurality of developing units. The development unit with the lowest rank When the developer is exchanged for any one of the development units, the developer starts collecting the developer by the collecting means, or executes a collecting operation to increase the collecting ability compared to the normal time. And

  According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the collecting operation is executed for each image forming operation on a plurality of recording media.

  According to a fourth aspect of the present invention, in the image forming apparatus according to the third aspect, the plurality of sheets increase each time the collection operation is executed.

  According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the collection unit sets the number of recording media on which the image formation has been performed after the start of the collection operation. The collection operation is executed until the number of sheets is reached.

According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the collecting unit removes the developer in the developer storage chamber of the developing unit during the development. And a collecting bias voltage for collecting the developer adhered on the developer carrying member is applied to the conductor during the collecting operation.
Note that the “conductor” may be of a contact type in which a DC bias voltage is applied while being in contact with the developer carrier, or an AC bias voltage is applied apart from the developer carrier. A non-contact type may be used. In addition, the conductor does not apply a bias voltage directly during development, but applies a collection bias voltage only during the collection operation, or applies a supply bias voltage during development and collects a bias voltage during collection operation. It is also possible to apply by switching to.

  According to a seventh aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, each of the collecting units moves relative to each other while contacting the developer carrying member. A movable body capable of collecting the developer adhering on the developer carrier, and in the collecting operation, the movable body is brought into contact with the developer carrier, or the movable body and the developer carrier The contact width in the moving direction of the moving body between the two is larger than that in the normal time.

  According to an eighth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, each of the collecting means rotates while contacting with each of the developer carriers, thereby the developer carrier. A rotating body capable of collecting the developer adhering to the rotating body, wherein the rotating speed of the rotating body is higher than that in the normal time during the collecting operation.

  According to a ninth aspect of the present invention, in the image forming apparatus according to any one of the first to eighth aspects, a secondary recovery means for further recovering the developer recovered by the recovery means is provided. Features.

<Invention of Claim 1>
According to this configuration, a collecting operation (provided that one developing unit among a plurality of developing units develops a developer having a higher charging ability than a developing unit having a lower developer image formation order) The recovery of the developer by the recovery means is started, or the recovery capability is increased as compared with the normal time).
As a result, even if a developer with a higher chargeability is reversely transferred to a developing unit that has a lower developer image formation order than the one developing unit described above, it is recovered by executing the recovery operation, thus suppressing color mixing and background fogging. can do.

<Invention of Claim 2>
Usually, the developer in the developing unit whose developer has been replaced has a higher charging ability than the developer in other developing units, and color mixing and background fogging due to reverse transfer are likely to occur in the other developing units. Therefore, in this configuration, the collecting operation is executed on the condition that the developer in any of the developing units has been replaced.

<Invention of Claim 3>
After the image forming operation on the recording medium is executed for a certain number of sheets, there is a case where the adverse effect on the image forming quality due to the color mixture or the background fog due to the reverse transfer may become remarkable. In this case, it is not always necessary to perform the collecting operation for each image forming operation of one recording medium. Therefore, in this configuration, the collecting operation is executed every time the image forming operation is performed on a plurality of recording media.

<Invention of Claim 4>
The developer in the developer unit whose developer has been replaced will deteriorate over time, and the difference in charging ability (difference in the absolute value of the charge amount) from the developer in other developer units will be reduced. The effect of fogging gradually decreases. Therefore, in this configuration, the number of recording media counted at the collection operation timing is increased with time.

<Invention of Claim 5>
The difference in developer charging ability between the developing units is reduced by repeating the image forming operation, and there is a case where the color mixture due to reverse transfer and the background fog do not substantially affect the image formation. Therefore, in this configuration, after the image formation on the recording medium for the set number of sheets (the number of sheets until the color mixture or background fog by reverse transfer does not substantially affect the image formation), the collecting operation is performed. Not configured.

<Invention of Claims 6-8>
As the collecting means, for example, a collecting bias voltage is applied to the conductor during the collecting operation as in claim 6, and the contact width (nip width) between the developer carrying member and the moving body as in claim 7. The structure to change and the structure to change the rotational speed of the rotary body like Claim 8 may be sufficient.

<Invention of Claim 9>
According to this configuration, since the secondary recovery means for further recovering the developer recovered by the recovery means is provided, the once recovered developer is prevented from adhering to the developer carrier again. it can.

<Embodiment 1>
A first embodiment of the present invention will be described with reference to FIGS.
1. Configuration of Color Laser Printer FIG. 1 is a side sectional view showing a schematic configuration of a color laser printer 1 as an image forming apparatus to which the present invention is applied. As shown in FIG. 1, the color laser printer 1 (image forming apparatus) of this embodiment includes a visible image forming unit 4, a paper transport belt 6, a fixing unit 8, a paper feeding unit 9, and a stacker 12. The control unit 10 and the bias supply unit 11 are provided, and toner images (developer images) of four colors corresponding to image data input from the outside are sequentially formed on the paper P (transfer material, recording medium). By doing so, a multicolor image is formed.

(1) Visible Image Forming Unit The visible image forming unit 4 is provided corresponding to four developing units 51 (51M, 51C, 51Y, 51BK) and these developing units 51M, 51C, 51Y, 51BK. 4 photosensitive drums 3 (3M, 3C, 3Y, 3BK, image carrier), four chargers 31, 32, 33, 34, and four exposure devices 41, 42, 43, 44. ing. Each of the developing units 51M, 51C, 51Y, and 51BK has magenta (M), cyan (C), yellow (Y), and black (BK) toners (“three or more developers” of the present invention). Is equivalent). That is, the alphabetical characters in the reference numerals indicating the developing units in FIG. 1 indicate the colors of the toners stored in the developing units, and are magenta (M), cyan (C), yellow (Y), black (BK). ) Development is performed on the paper P in the order.

Hereinafter, the configuration of each component will be described in detail.
(A) Photosensitive drums The four photosensitive drums 3M, 3C, 3Y, 3BK constituting the visible image forming unit 4 are all formed by substantially cylindrical members, and these four are arranged in the horizontal direction at substantially equal intervals ( In the horizontal direction of FIG. As the substantially cylindrical member of each of the photosensitive drums 3M, 3C, 3Y, and 3BK, for example, a member in which a positively chargeable photosensitive layer is formed on an aluminum base material is used. The aluminum substrate is grounded to the ground line of the color laser printer 1.

(B) Charger The four chargers 31 to 34 are so-called scorotron chargers. Among these, the detailed configuration of the charger 31 for charging the photosensitive drum 3M on which a magenta toner image is formed is as shown in FIG. 2, and faces the photosensitive drum 3M in the width direction (the depth of the drawing in the figure). And a shield case 37 in which the charging wire 36 is accommodated and the photosensitive drum 3M side is opened. By applying a high voltage to the charging wire 36, the photosensitive member is formed. The surface of the drum 3M is charged to positive polarity (for example, + 700V). The shield case 37 has a structure in which a grid 38 is provided in an open portion on the photosensitive drum 3M side. By applying a constant voltage to the grid 38, the surface of the photosensitive drum 3M is charged with the voltage of the grid 38. The potential is almost the same. Each of the chargers 32 to 34 provided to face the other photosensitive drums 3C, 3Y, and 3BK has the same configuration as that of the charger 31 shown in FIG.

(C) Exposure Device As for the four exposure devices 41 to 44, the exposure device 41 that exposes the photosensitive drum 3M on which magenta toner is formed will be described as a representative with reference to FIG. As shown in the figure, the exposure device 41 is arranged on the downstream side of the charger 31 with respect to the rotation direction of the photosensitive drum 3M (clockwise in the figure), and corresponds to one color of image data input from the outside. Laser light corresponding to (here magenta) is emitted from the light source, and the laser light is scanned by a mirror surface of a polygon mirror (not shown) that is rotationally driven by a polygon motor (not shown). Irradiate the surface. Note that most of the exposure devices 41 to 44 shown in FIGS. 1 and 2 are omitted, and only the portion from which laser light is finally emitted is shown.

  By irradiating the surface of the photosensitive drum 3M with the laser light from the exposure device 41, the surface potential of the irradiated portion is reduced (for example, reduced to + 150V), and electrostatic latent potential is applied to the surface of the photosensitive drum 3M. An image is formed. Each of the exposure units 42 to 44 disposed opposite to the other photosensitive drums 3C, 3Y, and 3BK has the same configuration as the above-described exposure unit 41, and image data input from the outside. Based on the above, laser light corresponding to the corresponding color is emitted.

(D) Developing Unit As for the four developing units 51M, 51C, 51Y, and 51BK, a developing unit 51M that develops an electrostatic latent image with magenta toner will be described as a representative with reference to FIG.

  As shown in the drawing, the developing unit 51M includes a toner hopper 54 (developer storage chamber) as a toner storage unit, a supply roller 55M as a toner supply unit and a recovery unit in a development unit case 53 that stores magenta toner. And a developing roller 52M as a developer carrier.

  Among these, the toner hopper 54 is formed as an internal space of the developing unit case 53. The toner hopper 54 is provided with an agitator 56M as a stirring means on the side close to the developing roller 52M, and contains magenta toner. Here, the toner accommodated in the toner hopper 54 is a positively charged non-magnetic one-component developer, which is manufactured by suspension polymerization or emulsion polymerization, has a substantially spherical shape, and has extremely good fluidity. .

  The supply roller 55M is disposed below the toner hopper 54, and a metal roller shaft is covered with a roller portion made of a conductive sponge member. The supply roller 55M is rotatably supported so as to rotate in a direction opposite to the developing roller 52M (counterclockwise in FIG. 2) at a nip portion facing and contacting the developing roller 52M.

  The developing roller 52M is rotatably disposed on the side of the supply roller 55M at a position facing and in contact with the supply roller 55M. The developing roller 52M is formed in a cylindrical shape using a conductive silicone rubber or the like as a base material, and a coating layer of a resin containing fluorine or a rubber material is formed on the surface thereof.

  The developing roller 52M is disposed on the downstream side of the exposure device 41 with respect to the rotation direction of the photosensitive drum 3M so as to be in contact with the photosensitive drum 3M. The developing unit 51M charges the toner to “+” (positive polarity) and supplies the toner to the developing roller 52M as a uniform thin layer. Then, at the contact portion between the developing roller 52M and the photosensitive drum 3M, the “+” (positive polarity) electrostatic latent image formed on the photosensitive drum 3M is charged to “+” (positive polarity). The toner is developed by a reversal development method to form a toner image.

  Each of the other developing units 51C, 51Y, and 51BK has the same configuration as the developing unit 51M shown in FIG. 2 except that the color of the toner to be stored is different, and contains cyan, yellow, and black toners, respectively. Yes.

(2) Paper Feed Unit The paper feed unit 9 is provided at the lowermost part of the color laser printer 1 and includes a storage tray 91 that stores the paper P and a pickup roller 92 that feeds the paper P. Then, the paper P stored in the storage tray 91 is picked up one by one from the paper feeding unit 9 by the pickup roller 92 and is sent to the paper transport belt 6 through the transport roller 99 and the like.

(3) Paper Conveying Belt Further, the paper conveying belt 6 is narrower than the width of each photosensitive drum 3, and is configured in an endless state so as to travel integrally with the paper P held on the upper surface side. And over the driven roller 63. Four transfer rollers 66, 67, 68, and 69 are provided so as to face the respective photosensitive drums 3 and sandwich the paper transport belt 6.

  The surface of the sheet conveying belt 6 facing the respective photosensitive drums 3 (hereinafter simply referred to as “the surface of the sheet conveying belt 6”) is rotated in the right direction in the drawing as shown in FIG. Move to the left in the figure. Then, the paper P sent via the transport rollers 99 and the like is sequentially transported between the photosensitive drums 3 and the surface of the paper transport belt 6 and sent to the fixing unit 8.

  The four transfer rollers 66, 67, 68, 69 are applied with voltages of a polarity (positive polarity in this example) and a reverse polarity (that is, negative polarity) in which the chargers 31 to 34 charge the photosensitive drums 3 respectively. Thus, an appropriate transfer bias is applied between the transfer rollers 66 to 69 and the photosensitive drums 3 by so-called constant current control (for example, −10 to −15 μA). With this transfer bias, the toner images formed on the respective photosensitive drums 3 are sequentially electrostatically transferred onto the paper P transported by the paper transport belt 6.

  For example, in the case of a toner image using magenta toner, a transfer bias is applied by applying a negative high voltage to the transfer roller 66 as shown in FIG. Thus, the toner image on the photosensitive drum 3M is transferred to the paper P at a position where the photosensitive drum 3M and the transfer roller 66 face each other, that is, at the transfer nip portion TP where the paper P and the photosensitive drum 3M are in contact with each other. The

  That is, by applying a transfer bias, an electric field is generated from the photosensitive drum 3M to the transfer roller 66, and the positive toner image on the photosensitive drum 3M is electrostatically transferred onto the paper P by this electric field. It is. The same applies to the transfer of the toner images on the other photosensitive drums 3C, 3Y, and 3BK. By applying a transfer bias to the respective transfer rollers 67 to 69, the corresponding color toner images are transferred onto the paper P. Are sequentially transferred. In this way, a desired multicolor image is formed by transferring the four color toner images onto the paper P in the order of colors of magenta, cyan, yellow, and black. Note that the constant current control of the transfer bias is merely an example, and the control method may be determined as appropriate, for example, constant voltage control.

  A cleaning brush 105 is provided on the surface of the paper transport belt 6 that is turned back by the driving roller 62. The cleaning brush 105 has a configuration in which a brush is provided around a substantially cylindrical member extending in the width direction of the paper conveyance belt 6, and the electrode roller 104 provided at a position facing the paper conveyance belt 6. A bias voltage for generating a predetermined potential is applied to the sheet conveying belt 6 so as to rotate while being in contact with the sheet conveying belt 6. Further, in the vicinity of the cleaning brush 105, a collection roller 106 that removes toner adhering to the cleaning brush 105 from the cleaning brush 105, and a storage box that stores toner removed from the cleaning brush 105 by the collection roller 106. 107 is provided.

(4) Fixing Unit The fixing unit 8 includes a heating roller 81 and a pressure roller 82, and the sheet P carrying a multicolor image composed of four color toner images is transferred to the heating roller 81 and the pressure roller 82. The multicolor image is fixed on the paper P by heating and pressurizing while nipping and conveying.

(5) Stacker A stacker 12 is formed on the upper surface of the color laser printer 1. The stacker 12 is provided on the paper discharge side of the fixing unit 8 and accommodates the paper P discharged from the fixing unit 8.

(6) Control Unit The control unit 10 is configured by a control device using a known CPU, and controls the overall operation of the color laser printer 1. Then, by controlling the bias supply unit 11, the developing bias (for example, 400V) to each developing roller 52 (52M, 52C, 52Y, 52BK), the transfer bias to each of the transfer rollers 66 to 69, and the electrode roller 104 are cleaned. The cleaning bias applied between the brush 105 and the voltage applied to each of the chargers 31 to 34 are controlled. The bias supply unit 11 supplies the biases and the like to a predetermined supply target according to the control signal from the control unit 10 as described above.

  In the present embodiment, as will be described later, the control unit 10 also functions as a charge amount (charging capability) estimation unit and a collection bias control unit that controls whether the collection bias is applied to the supply roller 55. To do.

  The color laser printer 1 according to the present embodiment uses the developing roller 52 and the residual toner remaining on the surface of the photosensitive drum 3 without being transferred after the transfer of the toner image from each photosensitive drum 3 onto the paper P. This is configured as a so-called simultaneous development cleaning method in which the toner is collected in the toner hopper 54 via the supply roller 55.

2. Although the exact mechanism of reverse transfer has not yet been clarified, as a result of repeated verification of the cause of reverse transfer, more specifically, the cause of reverse charging of toner, one inference was obtained. First, reverse charging is caused by discharge in the toner layer transferred onto the paper P. This discharge in the toner layer is caused by a strong electric field between the surface potentials of the photosensitive drums 3M, 3C, 3Y, and 3BK (for example, 700V) and the potentials of the transfer rollers 66 to 69 (transfer bias; for example, -1 kV). It is. As the toner of each color is sequentially transferred, the toner on the paper P is also multilayered, so that the charge amount (positive potential) of the entire toner layer increases. When this enters a strong electric field, a discharge occurs inside the toner layer, and the upper layer portion is reversely charged to a negative polarity.

  More specifically, as illustrated in FIG. 3A, a positive toner image 70 transferred onto a sheet (not shown) conveyed in the left direction in the figure by the sheet conveying belt 6 includes: The toner image (positive polarity) 71 on the photosensitive drum 3 is transferred at the transfer nip portion TP where the photosensitive drum 3 and the transfer roller 66 (67, 68, 69) face each other. As a result, a layered toner image 72 is formed as illustrated in FIG. When the toner image 72 moves away from the transfer nip portion TP, a discharge (peeling discharge) occurs inside the toner image 72 due to the influence of a strong electric field between the surface of the photosensitive drum 3 and the transfer roller 66. As shown in FIG. 3A, a reversely charged toner image 73 in which the upper layer portion is reversely charged to a polarity (negative polarity) opposite to the normal charging polarity is generated. Even if reverse charging does not occur after passing the transfer nip portion TP, reverse charging may occur at the time of entering the transfer position during the transfer of the next color. It has also been clarified through experiments that reverse charging tends to occur as the charge amount of the toner image on the transfer material increases.

  Further, in the case where the development is performed sequentially by the four development units 51M, 51C, 51Y, and 51BK corresponding to the four colors as in the present embodiment, as shown in FIG. The cyan toner by the second developing unit 51C, which is superimposed on the magenta toner by the first developing unit 51M on the paper P, is reversely transferred to 51Y. Also, cyan toner and yellow toner by the second and third developing units 51C and 51Y, which are superimposed on the magenta toner by the first developing unit 51M, and the second developing unit 51BK with respect to the fourth developing unit 51BK. The yellow toner by the third developing unit 51Y superimposed on the cyan toner by 51C is reversely transferred. It has been found that the amount of reverse transfer of the second, third cyan toner, and yellow toner superimposed on the first magenta toner to the fourth developing unit 51BK is extremely large. Note that, as in this embodiment, when using a non-magnetic one-component toner in which charge transfer occurs when the toners rub against each other, the toner with inferior charging performance is charged to a reverse polarity due to the charge transfer, which is particularly affected. large.

3. Regarding Improvement of Toner Collection Capacity (1) Configuration for Improvement of Toner Collection Capacity In this embodiment, during normal development, a bias voltage is not directly applied to each supply roller 55. The potential level is substantially the same as that of the developing roller 52. Accordingly, each supply roller 55 supplies the toner in the toner hopper 54 to the developing roller 52 at the nip portion, while scraping the toner (waste developer) adhering to the surface of the developing roller 52 at the nip portion. It functions as a collecting means for taking and collecting it in the toner hopper 54.

  On the other hand, the second to fourth development units 52C to 52BK that can be influenced by reverse transfer are more than the development bias voltage (400V) to the development units 52 facing each other at the recovery operation timing described later. A low level recovery bias voltage (for example, 250 V) is applied to the supply rollers 55C to 55BK. Unlike the present embodiment, when the toner is negatively charged, a recovery bias voltage having a level higher than the developing bias voltage is applied. With such a configuration, it is possible to improve the recovery capability of waste toner compared to normal development (hereinafter referred to as “collection capability improvement operation”).

  For this purpose, as shown in FIG. 1, the supply rollers 55 </ b> C to 55 </ b> BK of the second to fourth development units 52 </ b> C to 52 </ b> BK are electrically connected to the bias supply unit 11 via the switch 13 that is turned on and off by the control unit 10. It is connected. The switch 13 is turned on based on a control signal from the control unit 10 so that the recovery bias voltage is applied to the supply rollers 55C to 55BK at once.

(2) Configuration for Estimating Charge Level (Charging Capability) of Toner The above recovery capability improving operation is executed when there is a risk of so-called color mixing or background fogging. In each of the second to fourth developing units 52C to 52BK, whether or not color mixing or background fogging occurs is determined by whether the toner (cyan, yellow, black) of each of the second to fourth developing units 52C to 52BK is used. This is determined by the difference in charging ability with the toner of the developing unit 52 on the upstream side (the developer image formation order is higher).

  Therefore, in the present embodiment, the control unit 10 also functions as a charge amount (charge capability) estimation unit that estimates the charge amount (charge capability) of each toner of the first to fourth development units 52M to 52BK.

  Specifically, the toner charge amount of each developing unit 52 can be estimated from the value of the bias voltage applied between the photosensitive drum 3 and the developing roller 52. If it is better to correct by multiplying the charge amount by an appropriate coefficient or adding or subtracting a certain value by experiment, etc., the charging ability obtained by such correction will be obtained. It is inherently desirable to obtain a value.

  In other words, the color laser printer 1 performs printing of a predetermined color pattern at a predetermined cycle in order to prevent deterioration in image quality due to the components of each developing unit 51 and the secular change of toner. Is being calibrated. Since the bias voltage is adjusted according to the result of the calibration, the control unit 10 estimates the charge amount of the toner based on the value of the bias voltage, and optionally corrects the charge amount to charge the toner. Seeking ability. The toner charge amount estimation operation may be performed every time an image is formed on one sheet of paper P. However, the operation is not limited to this, and is performed every time a plurality of preset images are formed. May be.

(3) Recovery Capability Improvement Operation The control unit 10 controls the charge amount of toner for each developing unit 52 based on the bias voltage applied to the photosensitive drum 3 and the developing roller 52 at the time of development for each developing unit 52. (Charging ability) is estimated. Then, the control unit 10 determines that the toner charge amount in at least one of the second to fourth development units 52C to 52BK is a predetermined amount or more than the toner charge amount of the development unit 52 on the upstream side. Determine if it is small. Then, on the condition that it is determined that it is smaller than the predetermined amount, it is assumed that there is a possibility of color mixing or background fogging, and the switch 13 is turned on. As a result, a recovery bias voltage is applied from the bias supply unit 11 to the supply rollers 55C to 55BK of the second to fourth development units 52C to 52BK, thereby improving the waste toner recovery capability, thereby preventing color mixing and background fogging. can do.

(4) Recovery operation timing Even if the toner charge amount difference between the upstream developing unit 52 and the downstream developing unit 52 exceeds a predetermined amount, the image forming quality is substantially adversely affected by the color mixture or the background fog. Does not necessarily give. Therefore, in the present embodiment, the control unit 10 counts the number of sheets of paper P on which image formation has been performed from when the charge amount difference becomes equal to or greater than a predetermined amount, and forms an image on the sheet P by a preset number of sheets. After performing, it is set as the structure which performs the said recovery capability improvement operation | movement.

  More specifically, after image formation is performed on, for example, 100 sheets of paper P from the time when the charge amount difference becomes equal to or greater than a predetermined amount, the recovery capability improvement operation is executed once, and then 200 sheets of paper P are printed. 300 sheets of paper. . . When the image is formed, the collecting ability improving operation is performed. For example, after an image is formed on, for example, 1000 sheets of paper P in total, the recovery capability improvement operation is not performed thereafter.

  Here, as the toner has a higher charge amount (charging ability) such as a new toner immediately after toner replacement, the change with time tends to be larger. Accordingly, even if the charge amount difference between the upstream development unit 52 and the downstream development unit 52 is initially large, the toner having a higher charge amount has a higher deterioration rate than the toner having a lower charge amount. It is considered that the charge amount difference between the two becomes smaller with time. Therefore, in the present embodiment, as described above, the operation timing of the recovery capability improvement operation is set to 100 sheets, 200 sheets. . . The length is gradually increased. Further, as time passes, it is considered that the charge amount difference between the two becomes small enough not to adversely affect the image forming quality. Therefore, as described above, when the total number of sheets reaches a preset upper limit number (for example, 1000 sheets), the recovery capability improvement operation is not performed thereafter.

4). Effects of the present embodiment (1) According to the present embodiment, the control unit 10 determines that the toner charge amount in at least one of the second to fourth development units 52C to 52BK is on the upstream side. It is determined whether or not the toner charge amount of the developing unit 52 is smaller than a predetermined amount. Then, on the condition that it is determined that it is smaller than the predetermined amount, the recovery capability improving operation in which the recovery bias voltage is applied from the bias supply unit 11 to the supply rollers 55C to 55BK of the second to fourth development units 52C to 52BK. The configuration is to execute. As a result, in the second to fourth development units 52C to 52BK in which color mixing due to reverse transfer and background fogging may occur, the waste toner collecting ability can be enhanced, thereby preventing color mixing and background fogging.

  (2) In the present embodiment, the recovery timing improvement operation is performed at the end of image formation on 100 sheets of paper P and at the end of image formation on 200 sheets of paper P. . . The length is gradually increased. Since the charge amount difference between the upstream developing unit 52 and the downstream developing unit 52 tends to decrease with time, the recovery timing improving operation timing is gradually lengthened to perform unnecessary recovery capability improving operation. I didn't do it.

  (3) As the time further elapses, the charge amount difference between the two is considered to become small enough not to adversely affect the image formation quality. Accordingly, when the total number of sheets reaches a preset upper limit number (set number of sheets, for example, 1000 sheets), the recovery capability improvement operation is not performed thereafter.

<Embodiment 2>
5 and 6 show a second embodiment (corresponding to the inventions of claims 2 and 7). The difference from the first embodiment lies in a method for improving the recovery capability and a determination method for executing the recovery capability improvement operation, and the other points are the same as those in the first embodiment. Therefore, the same reference numerals as those in the first embodiment are given and the redundant description is omitted, and only different points will be described next.

  In the present embodiment, the collection capacity is changed by changing the nip width N (the contact width in the rotation direction of the supply roller 55) where the developing roller 52 and the supply roller 55 contact each other. Specifically, as shown in FIG. 5, each of the supply rollers 55C to 55BK has a rotation shaft 55a that can move along the opposing direction of the supply rollers 55C to 55BK and the development rollers 52C to 52BK. 53. A pressing mechanism 110 that presses both end portions of the rotation shaft 55a of each of the supply rollers 55C to 55BK in a direction toward the developing rollers 52M to 52BK is provided outside the developing unit case 53 or the developing unit 51.

  On the other hand, as shown in FIG. 6, the control unit 10 receives a detection signal from a detection sensor 111 that detects that any of the developing units 52 has replaced the toner. Then, on the condition that the control unit 10 receives this detection signal, the pressing mechanism 110 is driven at the same timing as in the first embodiment, and the supply rollers 55C to 55BK of the second to fourth developing units 51C to 51BK are driven. Is displaced from the state shown in FIG. 5A to the state shown in FIG. As a result, in the second to fourth developing units 51C to 51BK, the distance between the axes of the supply rollers 55C to 55BK and the developing rollers 52C to 52BK is reduced from d1 to d2, and accordingly, the nip width is changed from N1 to N2. It becomes wide. That is, it is possible to improve the toner collecting ability for the supply rollers 55C to 55BK and the developing rollers 52C to 52BK to rub against each other more strongly than before the collecting ability improving operation.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention, and further, within the scope not departing from the gist of the invention other than the following. Various modifications can be made.
(1) In each of the above embodiments, the development is performed in the order of magenta, cyan, yellow, and black from the upstream side, but other developer image formation order (arrangement order) may be used.

  (2) In each of the above embodiments, four-color development of magenta, cyan, yellow, and black has been described as an example. However, the present invention is not limited to this, and seven-color development in which three colors of red, green, and blue are added. Good. Furthermore, the present invention can be applied to, for example, two-color development with red toner and black toner and six-color development.

  (3) As a method for improving the collection capability, in addition to the configuration of each of the embodiments described above, the rotation speed (the number of rotations per unit time) of the supply rollers 55C to 55BK (rotating body) is set to be higher than that during normal image forming operation. The structure which makes it quick may be sufficient.

  (4) In each of the above-described embodiments, the recovery capability improving operation is performed for all of the second to fourth development units 51C to 51BK. However, the present invention is not limited to this, and for example, the third and fourth development units 51Y having a large reverse transfer amount. , 51BK, or only the fourth developing unit 51BK. Further, the collecting ability is only for the developing unit 51 in which the toner charging amount is smaller than the toner charging amount of the upstream developing unit 51 by a predetermined amount or the developing unit 51 on the downstream side of the developing unit 51 whose toner has been replaced. The structure which performs an improvement operation | movement may be sufficient.

  (5) In each of the above embodiments, as shown in FIG. 7, a secondary recovery roller 112 that contacts the supply roller 55 is provided, and the waste toner adhering to the supply roller 55 is scraped off by the secondary recovery roller 112. It is good also as a structure to collect | recover. At this time, a secondary collection bias voltage for collecting the waste toner attached to the supply roller 55 is applied to the secondary collection roller 112. More specifically, in the above embodiment, a bias voltage (for example, 100 V) lower than the recovery bias voltage for the supply roller 55 is applied.

  (6) In the above embodiment, a so-called “direct transfer method” is used in which the visible image (developer image) formed on the photosensitive drum 3 is directly transferred to the paper P using the paper P as a transfer material. However, the present invention is not limited to this. For example, an intermediate transfer member such as an intermediate transfer belt or an intermediate transfer drum is used as a transfer material, and a visible image formed on the photosensitive drum is transferred to the intermediate transfer member ( An image forming apparatus using a so-called “intermediate transfer method” in which the image is transferred (secondary transfer) from the intermediate transfer body to the paper (recording medium) after the primary transfer). Of course, the present invention can be applied not only to the tandem system but also to the 4-cycle system.

1 is a side sectional view showing a schematic configuration of a color laser printer according to Embodiment 1 of the present invention. Schematic diagram showing the configuration near the photosensitive drum Explanatory diagram for explaining the cause of reverse charging Explanatory diagram for explaining developer image formation order and ease of reverse transfer Schematic diagram for explaining the nip width in the second embodiment Side sectional view showing schematic configuration of color laser printer Schematic diagram showing a modification

Explanation of symbols

1. Color laser printer (image forming device)
3 (3M, 3C, 3Y, 3BK) ... photosensitive drum (image carrier)
10. Control unit (charge amount estimation means, recovery bias control means)
11. Bias supply unit (development bias application means, supply bias application means)
51 (51M, 51C, 51Y, 51BK) ... developing unit 52 (52M, 52C, 52Y, 52BK) ... developing roller (developer carrier)
53, 53 '... Toner hopper (developer storage chamber)
55 (55M, 55C, 55Y, 55BK) ... supply roller (collecting means, conductor, rotating body, contact body)
112 ... Secondary collection roller (secondary collection means)
N1, N2 ... Nip width (contact width)
P ... paper (transfer material)

Claims (9)

An image carrier on which an electrostatic latent image is formed;
A developer carrying member for carrying each developer provided corresponding to each of the three or more developers; and the developer carrying member transfers the electrostatic latent image on the surface of the image carrier with the developer. A plurality of development units configured to develop with,
In the image forming apparatus capable of sequentially forming a developer image on the image carrier by developing an electrostatic latent image by each developing unit, and transferring the developer image to a transfer material to form an image.
Among the plurality of development units, at least a development unit having a developer image formation order other than the highest one is provided with a collection unit that collects the developer attached on the developer carrier during non-development,
When one developing unit of the plurality of developing units develops with a developer having a higher charging capability than a developing unit having a lower developer image formation order than the developing unit, the collecting unit starts collecting the developer. Or an image forming apparatus that executes a collecting operation that increases the collecting ability more than usual. An image carrier on which an electrostatic latent image is formed;
A developer carrying member for carrying each developer provided corresponding to each of the three or more developers; and the developer carrying member transfers the electrostatic latent image on the surface of the image carrier with the developer. A plurality of development units configured to develop with,
In the image forming apparatus capable of sequentially forming a developer image on the image carrier by developing an electrostatic latent image by each developing unit, and transferring the developer image to a transfer material to form an image.
Among the plurality of development units, at least a development unit having a developer image formation order other than the highest one is provided with a collection unit that collects the developer attached on the developer carrier during non-development,
When the developer is exchanged for any one of the plurality of development units other than the development unit having the lowest developer image formation order, recovery of the developer by the recovery unit is started, or An image forming apparatus characterized in that a collecting operation is performed to increase the collecting ability more than usual. The image forming apparatus according to claim 1, wherein the collecting operation is executed for each image forming operation on a plurality of recording media. The image forming apparatus according to claim 3, wherein the plurality of sheets increases every time the collection operation is executed. 5. The collection unit according to claim 1, wherein the collection unit executes the collection operation after the collection operation is started until the number of recording media on which the image formation has been performed reaches a set number. The image forming apparatus according to any one of the above. The recovery means includes a conductor for supplying the developer in the developer storage chamber of the development unit to the developer carrier during the development, and the developer carrier for the conductor during the recovery operation. The image forming apparatus according to claim 1, wherein a recovery bias voltage for recovering the developer attached thereon is applied. Each of the collecting means is a moving body capable of collecting the developer adhering to the developer carrying body by moving relatively while in contact with each developer carrying body, and during the collecting operation The moving body is brought into contact with the developer carrying body, or the contact width in the moving direction of the moving body between the moving body and the developer carrying body is made larger than that in the normal time. The image forming apparatus according to claim 1. Each of the collecting means is a rotating body capable of collecting the developer adhering to the developer carrier by rotating while being in contact with the developer carrier, and is rotated during the collecting operation. 6. The image forming apparatus according to claim 1, wherein the rotation speed of the body is made faster than the normal time. The image forming apparatus according to claim 1, further comprising a secondary collection unit that further collects the developer collected by the collection unit.

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