JP2011095440A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus and an image forming method eliminating charge-up of a developing roller, thereby providing desired image density. <P>SOLUTION: The image forming apparatus (1) includes a carrier and a toner, supplies the toner to an image carrier, and develops a latent image, the apparatus also includes: a magnetic roller (50) forming a magnetic brush of the carrier and the toner; a developing roller (60) having on its surface a resin layer in which a resistance controlling agent is dispersed and forming a layer of the toner transferred from the magnetic brush; and a control means (90) carrying out a refreshing operation during the formation of no image, the operation including varying a bias applied to the magnetic roller so as to prevent the toner from being transferred from the magnetic brush to the developing roller, and reducing an absolute value of a DC component in a bias applied to the developing roller to develop a latent image on the image carrier with the toner on the developing roller, to be smaller than an absolute value applied to the developing roller when an image is formed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image forming method using a two-component developer having a carrier and a toner.

In this type of image forming apparatus, a toner image is developed on the surface of an image carrier on which an electrostatic latent image is formed, for example, a photosensitive drum, and the toner image is transferred and fixed on a sheet.
Here, this image forming apparatus includes a so-called hybrid development method. Specifically, this method is a combination of the two-component development method and the one-component development method, and a non-magnetic toner is charged using a magnetic carrier, and only this toner is supplied to the drum in a non-contact manner. A toner image is developed on the surface.

  A technique using this method is disclosed (for example, see Patent Document 1), which includes a developing roller facing the drum and a magnetic roller facing the developing roller. The magnetic roller includes a carrier and toner. A magnetic brush is formed. The magnetic brush forms a toner-only layer on the developing roller, and scrapes and collects the toner layer after development.

In addition, an anodized coating layer is formed on the outer peripheral surface of the aluminum base tube on the developing roller. For example, in order to improve developability, the interval between the developing roller and the drum is reduced, or a developing bias is applied. Even if the value of the AC component is increased, leakage of the applied potential can be prevented.
On the other hand, a technique of a developing roller having a coating layer of urethane resin or the like on the surface is also disclosed (for example, see Patent Document 2).

This is because the surface charge increases with the aging of the toner and adheres to the surface of the developing roller. Note that this phenomenon becomes more prominent when a toner having a small particle diameter is used to achieve high image quality and high definition in recent years.
In other words, this developing roller is formed with a coating layer of urethane resin or the like on its surface to prevent toner sticking, and further, a resistance adjusting agent such as carbon black is dispersed to reduce the volume resistance value or surface resistance of the roller surface. The value is adjusted to prevent leakage of the applied potential.

JP 2004-318092 A Japanese Patent Laid-Open No. 11-249414

  However, in order to achieve further increase in the development speed, it is necessary to apply the AC component of the development bias to the developing roller with a larger value to widen the nip area between the developing roller and the drum. This is to further compensate for the decrease in the nip time between the developing roller and the drum. Therefore, it is desirable to select a developing roller having a high surface resistance.

On the other hand, if the surface resistance is simply increased by adjusting with a resistance adjusting agent as described above and the urethane resin coating layer is used, charges are accumulated on this surface (developing roller charge-up) during continuous printing, and a desired image density can be obtained. There is no problem.
Specifically, the density of the solid image gradually decreases. This is because the potential difference between the magnetic roller and the developing roller becomes small, and it becomes difficult for a desired amount of toner to go to the developing roller. On the other hand, the density of the half image increases as the potential difference between the developing roller and the image carrier increases because the toner moves toward the image carrier.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus and an image forming method capable of solving the above-described problems, eliminating charge-up of a developing roller, and obtaining a desired image density.

A first invention for achieving the above object is an image forming apparatus having a carrier and toner, supplying the toner to an image carrier and developing the latent image.
A magnetic roller that forms a magnetic brush made of a carrier and toner; a developing roller that has a resin layer having a resistance adjusting agent dispersed on the surface thereof, and forms a layer of toner transferred from the magnetic brush; During non-image formation, the bias applied to the magnetic roller is changed so that the toner is not transferred from the magnetic brush to the developing roller, and is applied to the developing roller to develop the latent image on the image carrier with the toner of the developing roller. And a control means for performing a refresh operation for making the absolute value of the DC component of the bias smaller than the absolute value of the value applied to the developing roller during image formation.

  According to the first invention, a two-component developer comprising a carrier and a toner is used, and a magnetic brush is formed by the developer on the magnetic roller, and this brush forms a toner layer on the developing roller. The toner image is developed by supplying the toner to the image carrier. On the other hand, this brush scrapes off and collects the toner layer of the developing roller after development.

Here, in order to achieve further speeding up of development as described above, it is desirable to select a structure having a high surface resistance. However, if the surface resistance is simply increased by providing a resin layer, during continuous image formation, There is a problem that charges accumulate on the surface (charge up of the developing roller) and a desired image density cannot be obtained.
However, in the control means of the present invention, the staying charge removal control (refresh control) for the developing roller is performed.

Specifically, the absolute value of the direct current component of the bias applied to the developing roller during non-image formation is made smaller than the absolute value of the value applied to the developing roller during image formation. Thereby, the electric charge accumulated on the surface of the developing roller can be removed.
Further, when reducing the DC component of the bias applied to the developing roller, it is necessary to prevent the fog toner from flying from the developing roller to the image carrier. Therefore, the bias applied to the magnetic roller is also changed so that the toner is not transferred from the magnetic roller to the developing roller so that the toner is not carried on the developing roller.

As a result of the refresh control based on these bias changes, the development roller refreshes the charge charge, that is, a desired image density is obtained by eliminating the charge-up of the development roller, while achieving a higher development speed. Can also be achieved.
According to a second aspect of the present invention, in the configuration of the first aspect of the present invention, the apparatus further comprises detection means for detecting the temperature and humidity of the apparatus, and the control means changes the start timing of the refresh operation based on the detection result by the detection means. It is characterized by doing.

  According to the second invention, in addition to the action of the first invention, the above-described charge-up of the developing roller becomes more significant as the amount of moisture in the atmosphere decreases, for example, and the amount of moisture in the atmosphere increases. It becomes difficult to occur. Therefore, if the start time at the time of non-image formation, that is, the start time of the refresh operation is set in accordance with the temperature and humidity of the apparatus, the refresh control can be performed at the time necessary for eliminating the charge-up of the developing roller.

A third invention is characterized in that, in the configuration of the second invention, the control means advances the start time of the refresh operation as the absolute moisture amount detected by the detection means decreases.
According to the third invention, in addition to the action of the second invention, if the start time of the refresh operation is advanced as the amount of moisture in the atmosphere decreases, the developing roller can achieve a desired image density, that is, an allowable value. It can be surely kept within the concentration range.

According to a fourth aspect of the present invention, in the configurations of the first to third aspects, the toner includes an external additive, and the control means advances the start timing of the refresh operation as the total number of printed sheets of the apparatus increases. It is characterized by that.
According to the fourth invention, in addition to the effects of the first to third inventions, the external additive contained in the toner adheres to the developing roller as the total number of printed sheets of the apparatus increases, and the developing roller Will increase the surface resistance. Therefore, if the start time of the refresh operation is advanced as the total number of printed sheets increases, the developing roller can reliably fall within the desired image density, that is, the allowable density range.

  According to a fifth aspect of the invention, in the configuration of the second aspect of the invention, the control means applies the DC component of the bias to the developing roller based on the detection result by the detection means, and the magnetic roller corresponding to this application period. The application period of the direct current component of the bias is changed.

  According to the fifth invention, in addition to the operation of the second invention, the above-described charge-up of the developing roller becomes more significant as the moisture content in the atmosphere decreases, for example, and the moisture content in the atmosphere increases. It becomes difficult to occur. Therefore, if the application period of the DC component of each bias is set according to the temperature and humidity of the apparatus, the refresh control can be performed in a period necessary for eliminating the charge-up.

  According to a sixth aspect of the present invention, in the configuration of the second or fifth aspect, the control unit is configured to apply the bias DC component applied to the developing roller based on the detection result of the detection unit, and the magnetism associated with the application. The application value of the DC component of the bias to the roller is changed.

  According to the sixth invention, in addition to the effects of the second and fifth inventions, the above-described charge-up of the developing roller becomes more significant as the amount of moisture in the atmosphere decreases, for example. When the amount increases, it becomes difficult to occur. Therefore, if the magnitude of the applied value of the DC component of each bias is set according to the temperature and humidity of the apparatus, the refresh control can be performed in an amount necessary for eliminating the charge-up.

According to a seventh invention, in the configuration of the fifth or sixth invention, the control means lengthens the application period of the DC component of each bias as the absolute moisture amount detected by the detection means decreases. Alternatively, the application value of the DC component of each bias is made smaller.
According to the seventh invention, in addition to the effects of the fifth and sixth inventions, the application period of the DC component of the bias of both the developing roller and the magnetic roller is lengthened as the moisture content in the atmosphere decreases. Thus, the electric charge can be surely removed.

Or, as the amount of moisture in the atmosphere decreases, if the magnitude of the DC component of the bias of both the developing roller and the magnetic roller is greatly reduced, the speed of removing the charge can be increased, and non-image formation for eliminating the charge-up The period will not be long.
According to an eighth aspect of the invention, in the configurations of the fifth to seventh aspects of the invention, the toner contains an external additive, and the control means controls the DC component of each bias as the total number of printed sheets increases. Each of the application periods is lengthened, or the application value of the DC component of each bias is made smaller.

According to the eighth invention, in addition to the effects of the fifth to seventh inventions, the external additive contained in the toner increases the surface resistance of the developing roller, but the total number of printed sheets increases. Accordingly, if the application period of the DC component of the bias of both the developing roller and the magnetic roller is lengthened, the charge can be surely extracted.
Or, as the total number of printed sheets increases, if the magnitude of the DC component of the bias of both the developing roller and the magnetic roller is greatly reduced, the speed of removing the charge can be increased, and the non-image formation period for eliminating the charge-up is also increased. It won't be long.

A ninth aspect of the invention is an image forming method in which a toner is charged using a carrier, and the latent image is developed by supplying the toner to an image carrier.
A magnetic roller for forming a magnetic brush made of carrier and toner, and a developing roller for forming a layer of toner transferred from the magnetic brush, the surface of which has a resin layer in which a resistance adjusting agent is dispersed. A step of determining whether or not it is during non-image formation, and a step of changing a bias value applied to the magnetic roller by the magnetic brush so as to collect toner from the developing roller at the time of non-image formation. And the absolute value of the DC component of the bias applied to the developing roller for developing the latent image on the image carrier with the toner of the developing roller is greater than the absolute value of the value applied to the developing roller during image formation. The bias value applied to the magnetic roller is substantially equal to the bias value applied to the developing roller, the bias value applied to the developing roller, and Composed of a step of returning each bias value applied to the magnetic roller to the value at the time of image formation.

According to the ninth aspect of the invention, the control of the staying charge from the developing roller is controlled. Specifically, when it is determined that no image is formed, first, the bias value applied to the magnetic roller is changed, and the magnetic brush collects the toner from the developing roller.
Subsequently, the absolute value of the DC component of the bias applied to the developing roller is made smaller than the value applied to the developing roller during image formation. Thereby, the electric charge accumulated on the surface of the developing roller can be removed. At the same time, when reducing the DC component of the bias applied to the developing roller, the bias value applied to the magnetic roller is set to the bias value applied to the developing roller so that toner is not transferred from the magnetic roller to the developing roller. The developing roller is in a state in which toner is not carried on the developing roller.

Thereafter, the bias value applied to the developing roller and the bias value applied to the magnetic roller are returned to the values at the time of image formation, and image formation is resumed.
As a result of these bias changes, the development roller can surely obtain a desired image density by refreshing the charge charge while achieving higher speed of development, and the image carrier can achieve further stabilization of image quality.

  According to the present invention, charge charge refresh control is performed on the developing roller, and it is possible to provide an image forming apparatus and an image forming method capable of eliminating the charge-up of the roller and obtaining a desired image density.

1 is a schematic configuration diagram of an image forming apparatus in the present embodiment. It is sectional drawing of the image development apparatus of FIG. It is a schematic diagram from the developing device of FIG. 1 to a drum. It is a flowchart of the refresh control by the controller of FIG. 3 is a timing chart of refresh control by the controller of FIG. 1. 5 is a table for explaining a transition interval to the refresh control in FIG. 4. It is a characteristic view of the developing rollers A and B used in this experiment. FIG. 8 is a relationship diagram between the number of continuous prints by the developing rollers A and B of FIG. 7 and density transition. FIG. 8 is a relationship diagram between the number of continuous prints by the developing roller B of FIG. 7 and density transition. FIG. 8 is a relationship diagram between the number of continuously printed sheets and density transition when refresh control is performed on the developing roller B of FIG. 7.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an image forming apparatus 1 according to the present embodiment. The apparatus 1 has a color printing function, and a sheet of paper is stored in a stacked state inside the paper feed cassette 3.
Also, in the apparatus 1, a registration roller pair 24, an image forming unit 4, and a transfer unit 71 are sequentially arranged on the downstream side in the sheet conveyance direction from the cassette 3.

  The image forming unit 4 includes a photosensitive drum (image carrier) 5 (FIGS. 2 and 3). For example, in the tandem apparatus 1, drums 5 corresponding to yellow, magenta, cyan, and black are arranged in parallel. . These drums 5 are rotatably installed. For example, one drum 5 is driven counterclockwise around its rotation axis by a drive motor (not shown) as shown in FIG. Further, a charger, an exposure unit 20 of FIG. 1 and a developing device 7 are provided at appropriate positions around each drum 5.

The charger uniformly charges the surface of the corresponding drum 5, and the exposure unit 20 irradiates the drum 5 with laser light. Note that an LED may be used for the exposure unit 20.
Further, a fixing unit 72 and a paper discharge tray are arranged in order on the downstream side of the transfer unit 71 in the paper transport direction.

  When the apparatus 1 performs printing, the sheets from the cassette 3 are separated and sent one by one. The fed sheet reaches the registration roller pair 24. The registration roller pair 24 feeds the sheet toward the transfer unit 71 while correcting the oblique feeding of the sheet and measuring the image transfer timing with the toner image formed by the image forming unit 4.

On the other hand, reference numeral 80 in FIG. 1 is an input port, and this input port 80 is configured so that image data to be printed can be received from the outside or a scanner. This image data is data in which various images such as characters, codes, figures, symbols, diagrams, and patterns are converted into data. Based on this data, the controller (control means) 90 controls the laser beam irradiation of the exposure unit 20.
The controller 90 is an element that functions as a computer, and is electrically connected to the above-described image forming unit 4 and the like, and has hardware resources such as a CPU and a memory.

The controller 90 executes a predetermined program using this hardware resource. As a result, an electrostatic latent image is formed on the surface of the drum 5 through charging and exposure. The developing device 7 electrostatically attaches yellow, magenta, cyan, and black toners to the surface of each drum 5.
Subsequently, the toner images formed on the respective surfaces of the drum 5 are sequentially transferred onto, for example, an intermediate transfer belt, and are combined as a toner image for one page.

When the sheet passes between the intermediate transfer belt and the transfer unit 71, the toner image transferred to the belt is secondarily transferred to the sheet, and the sheet is conveyed toward the fixing unit 72. The paper on which the toner image is fixed by being heated and pressed by the fixing unit 72 is discharged to a paper discharge tray and stacked in the height direction.
Here, the developing device 7 of the present embodiment employs a method combining a two-component developing method and a one-component developing method.

  That is, nonmagnetic toner is positively charged using a magnetic carrier, for example, and only the charged toner is caused to fly toward the drum 5, and a toner image is developed on the drum 5 by a non-contact developing method. More specifically, the developing device 7 of the present embodiment has a housing 30 shown in FIG. 2, and the housing 30 extends upward from the opening 32 facing the drum 5. The peripheral wall 34 is provided, and an end wall 36 formed at the upper end portion of the peripheral wall 34.

  Below the end wall 36, the above-mentioned carrier and toner are accommodated, and an agitating mixer 40 and a paddle mixer 44 are arranged, respectively. The shafts 42 and 46 of the mixers 40 and 44 are rotatably supported with respect to the housing 30, and the agitating mixer 40 is rotated counterclockwise in FIG. When rotating around, the carrier and toner are agitated to charge the toner. These carriers and toner are conveyed to the magnetic roller 50 via the mixer 44.

  The magnetic roller 50 is disposed below the mixer 44, and this roller 50 is also rotatably supported with respect to the housing 30. Specifically, the magnetic roller 50 has a shaft 52 at the center, and a tubular brush-side sleeve 54 is fitted to the outer periphery of the shaft 52 via a bearing. The shaft 52 is fixed, and when only the sleeve 54 is rotated by a drive motor (not shown), the sleeve 54 rotates clockwise in FIG.

  Inside the brush-side sleeve 54, a plurality of (for example, five) magnets 56 are fixed to the shaft 52 at a predetermined interval on the outer peripheral side of the shaft 52 (in FIG. 2, only the pole positions of the magnets are shown). The magnetic brush is formed by attracting the carrier and toner from the mixer 44. Specifically, the magnetic brush is formed in a region having the magnet 56 on the outer peripheral surface of the sleeve 54, and conveys the carrier and the toner downward as the sleeve 54 rotates.

  The thickness of the magnetic brush is regulated by the blade 70. The blade 70 of the present embodiment is disposed in a substantially vertical direction so as to face the sleeve 54, and is provided along the rotation axis of the sleeve 54. When a predetermined direct current (DC) bias is applied, the magnetic brush regulated to a predetermined thickness conveys the toner toward the developing roller 60 (FIG. 3). On the other hand, this magnetic brush scrapes off the developed toner layer from the developing roller 60 by switching the bias between the papers during continuous printing.

  As described above, in the magnetic brush, the toner layer firmly attached is peeled off from the developing roller 60 by a mechanical force, and new toner necessary for development is supplied to the developing roller 60. According to this magnetic brush, it is not necessary to provide a special device such as a scraping blade or a roller for the developed toner, and no direct stress is applied to the toner or the like.

The magnetic brush described above is in contact with the toner layer of the developing roller 60, and the toner is produced by the brush effect due to the difference in rotational speed between the rollers 50 and 60 and the replacement of the developer of the magnetic brush by the stirring by the mixers 40 and 44. Is easy to collect and transport.
The developing roller 60 is disposed in the vicinity of the opening 32 below the magnetic roller 50.

  The developing roller 60 is also rotatably supported by the housing 30, and its shaft 62 is arranged substantially parallel to the rotation axis of the magnetic roller 50. A tubular developing side sleeve 64 is also provided on the outer periphery of the shaft 62. Are fitted through bearings. The shaft 62 is fixed, and when only the sleeve 64 is rotated by a drive motor (not shown), the sleeve 64 rotates clockwise in FIG.

Here, in the developing roller 60 of the present embodiment, a resin coating layer is formed on the surface of the sleeve 64.
Specifically, the developing-side sleeve 64 of this embodiment has an aluminum tube, an alumite coating layer having a predetermined thickness is formed on the outer peripheral surface, and a resin coating layer is formed on the surface thereof. Has been. The resin of this embodiment is a urethane resin, and is applied to the upper side of the alumite coating layer with a predetermined thickness.

The urethane resin contains a conductive material (resistance adjusting agent) such as carbon black, and the volume resistance value and the surface resistance value of the sleeve 64 are adjusted.
Due to the potential difference between the magnetic roller 50 and the developing roller 60, a thin layer of only the toner transferred from the magnetic roller 50 is formed. Subsequently, when a developing bias, that is, an alternating current (AC) bias superimposed with a predetermined DC bias is applied, the toner on the developing roller 60 flies toward the drum 5 (FIG. 3). As a result, a toner image is developed on the surface of the drum 5.

By the way, the above-described controller 90 can perform a refresh operation by changing the bias in order to refresh the charge charges in the developing side sleeve 64.
Specifically, as illustrated in FIG. 1, the controller 90 includes a transition determination unit 92 and a refresh operation control unit 94, and the transition determination unit 92 detects, for example, a temperature or humidity outside the apparatus 1. / Based on the signal from the humidity sensor (detection means) 82, the start time of the refresh operation is determined, and the determination result is output to the refresh operation control unit 94.

Subsequently, the refresh operation control unit 94 removes charges accumulated on the surface of the developing sleeve 64.
Referring to FIG. 4, a flowchart of the refresh operation by the controller 90 is shown. Hereinafter, the operation of the image forming apparatus 1 configured as described above according to the present invention will be described.

In step S401 in the figure, the controller 90 starts continuous printing based on the signal from the input port 80. That is, as shown in the timing chart of FIG. 5, the controller 90 sets the bias DC component at the time of image formation to 370 (V) for the magnetic roller 50 and the bias at the time of image formation to the developing roller 60, for example. The DC component Vs ₀ is set to 70 (V), for example. In step S401, a transition counter described later is set.

Next, in step S402 in FIG. 4, the transition determination unit 92 determines whether the number of transitions to the refresh operation has reached a predetermined set number.
Specifically, the start timing of this refresh operation is set according to the detection result of the temperature / humidity sensor 82. This is because the degree of change in image density due to charge-up is affected by these temperatures and humidity.

That is, when the temperature and humidity outside the apparatus 1 are high, the charge accumulated in the sleeve 64 is easily released into the air, but when the temperature and humidity are low and the absolute moisture amount is reduced, the charge is released into the air. This is because the resistance of the resin coating layer tends to be high.
Therefore, as shown in FIG. 6, when the temperature t and the humidity h are high, the continuous printing interval is lengthened, that is, the period until the refresh operation is started is delayed, and a large number of printed sheets can be obtained. Then, the operation proceeds to the refresh operation. For example, when the temperature t is 20 ° C. and the humidity h is 40%, 60 sheets are set, and when the humidity h is 60%, 100 sheets are set.

  On the other hand, when the temperature t and the humidity h are low, for example, when the temperature t is 10 ° C. and the humidity h is 15%, the continuous printing execution interval is t <15, and h It is set to 5 sheets corresponding to <25, and the interval of continuous printing is shortened compared to when the temperature t and humidity h are high, that is, the period until the refresh operation is started is shortened. When the number of printed sheets (5 sheets in this case) is obtained, the operation proceeds to the refresh operation.

That is, in step S402 in FIG. 4, the transition determination unit 92 determines whether or not the number of transitions to the refresh operation has reached the set number in FIG. 6, and when this number has been reached, that is, when it is determined YES In step S403, continuous printing is interrupted.
Then, the refresh operation control unit 94 executes the refresh operation. Specifically, the bias DC component Vs of the developing roller 60 is lowered from 70 (V), which is the value Vs ₀ at the time of image formation, and then returned to the value at the time of image formation. On the other hand, the bias DC component Vm of the magnetic roller 50 is lowered step by step from the value 370 (V) at the time of image formation, and then returned to the value at the time of image formation.

  More specifically, the refresh operation control unit 94 first maintains the bias DC component Vm of the magnetic roller 50 by lowering it from 370 (V) to 70 (V) as shown in I of FIG. This is to equalize the value of the bias DC component Vs of the developing roller 60 and collect the toner on the developing side sleeve 64 to the magnetic roller 50. During this period I, the developing roller 60 is rotated once. The bias DC component Vm may be smaller than the bias DC component Vs.

Next, the refresh operation control unit 94 maintains the bias DC component Vs of the developing roller 60 by reducing it from 70 (V) to 0 (V) as shown in II of FIG. This is because the charge on the developing side sleeve 64 is released.
At the same time, in II, the bias DC component Vm of the magnetic roller 50 is also lowered from 70 (V) to 0 (V) and maintained. This is because the toner is not transferred from the magnetic brush toward the developing side sleeve 64, and the developing roller 60 is rotated once for the period II.

Subsequently, as shown in III of FIG. 5, the refresh operation control unit 94 raises and maintains the bias DC component Vs of the developing roller 60 to 70 (V) which is the value Vs ₀ at the time of image formation. The bias DC component Vm of the magnetic roller 50 is also raised to 370 (V), which is a value at the time of image formation, at once and maintained. This is because a toner layer is formed on the developing side sleeve 64, and this period III is also carried out for one rotation of the developing roller 60.

  Then, in step S404 in FIG. 4, the refresh operation control unit 94 determines whether or not the bias change is completed. When the period III described above has elapsed, that is, when it is determined YES, the process proceeds to step S405, where While resuming printing, the transition determination unit 92 resets the transition counter and exits a series of routines.

In step S402, when the number of sheets to be refreshed has not yet reached the set number, such as 1 to 4, for example, the process proceeds to step S406. In this case, the shift determination unit 92 increases the shift counter by one and then returns to step S402. Thereafter, when the set number of sheets (for example, five) is reached, the process proceeds to step S403.
By the way, in the above embodiment, the period II for releasing the charge on the surface of the developing side sleeve 64 is carried out for one rotation of the developing roller 60. The length of this period is detected by the temperature / humidity sensor 82. You may set according to a result.

Specifically, the refresh operation control unit 94 applies the bias DC component Vs of the developing roller 60 that has been lowered to 0 (V) as the absolute moisture amount detected by the sensor 82 decreases, and is also 0 (V ), The application period of the bias DC component Vm of the magnetic roller 50 lowered can be extended.
On the other hand, in the above embodiment, the bias DC component Vs of the developing roller 60 in the case of II is lowered to 0 (V).

However, considering that the charge on the surface of the developing sleeve 64 is released, the bias DC component Vs in the case of II only needs to be smaller than 70 (V) which is the value Vs ₀ at the time of image formation.
Further, the value of the bias DC component Vs can be set according to the detection result of the temperature / humidity sensor 82. That is, as the absolute water content detected by the sensor 82 decreases, the value of the bias DC component Vs of the developing roller 60 lowered to 0 (V) or the magnetic roller 50 also lowered to 0 (V). The value of the bias DC component Vm may be made smaller. Furthermore, in addition to the positively charged toner described above, negatively charged toner may be used.

In summary, the bias DC component Vs of the developing roller 60 in II may be a positive value or a negative value, and the absolute value of this bias DC component Vs is the absolute value of 70 (V), which is the value Vs ₀ at the time of image formation. Smaller than that. Note that the bias DC component Vm of the magnetic roller 50 at this time is matched with the reduced bias DC component Vs or is further reduced.

  As described above, according to this embodiment, the two-component developer composed of the carrier and the toner is used, and the magnetic roller 50 is formed with the magnetic brush by the developer. A thin layer is formed. The toner image is developed by supplying the toner to the drum 5. On the other hand, this brush scrapes off and collects the toner layer of the developing roller 60 after development.

  Here, in order to achieve further increase in the speed of the developing device 7, it is desirable to select a structure having a high surface resistance, that is, a high leakage generation voltage. However, by providing a urethane resin coating layer, the surface resistance is improved. Is simply increased, there is a problem that charges are accumulated on the surface of the developing sleeve 64 during continuous printing, and a desired image density cannot be obtained.

However, in the controller 90 of this embodiment, the staying charge removal control (refresh operation) for the developing roller 60 is performed.
Specifically, in II of FIG. 5 described above, the absolute value of the DC component Vs of the bias applied to the developing roller 60 is made smaller than the value Vs ₀ applied to the developing roller during image formation. Thereby, the electric charge accumulated on the surface of the developing sleeve 64 can be removed.

  Further, when the bias DC component Vs applied to the developing roller 60 is reduced, it is necessary to prevent the fog toner from flying from the developing roller 60 to the drum 5. Therefore, the magnitude of the bias DC component Vm applied to the magnetic roller 50 is made equal to the bias DC component Vs applied to the developing roller 60 in the above II so that the toner is not transferred from the magnetic roller 50 to the developing roller 60. Then, the toner on the developing roller 60 is collected, and the toner is not carried on the roller 60.

As a result of performing these refresh operations, a desired image density (particularly, suppression of an increase in half density) can be obtained by eliminating the charge-up of the developing roller 60 while increasing the speed of the developing device 7. Stabilization can also be achieved.
In addition, the above-described charge-up of the developing roller 60 becomes more significant as the amount of moisture in the atmosphere decreases, for example, and is less likely to occur when the amount of moisture in the atmosphere increases. Therefore, if the start time of the refresh operation is set according to the temperature and humidity of the apparatus 1, the refresh operation can be performed at a time necessary for eliminating the charge-up of the developing roller 60.

Furthermore, if the start time of the refresh operation is advanced as the amount of moisture in the atmosphere decreases, the developing roller 60 can surely fall within a desired image density, that is, an allowable density range described later.
These points will be described in detail. 7 to 10 show results of experiments in which a refreshing operation is performed by adopting a structure having a high surface resistance for the developing roller.

The experimental conditions are as image forming conditions: the peripheral speed of the drum 5 is 250 mm / sec, the surface potential of the drum 5 is a dark potential V ₀ of 250 V, and a light potential V _L of 20 V. Is 25 μC / g, and the particle diameter (average particle diameter) of the toner is 7.0 μm (1 μm = 1.0 × 10 ⁻⁶ m).
Further, the diameter of the magnetic roller 50 used in the experiment is φ16 mm, the diameter of the developing roller 60 is φ16 mm, and the diameter of the drum 5 is φ30 mm.

The gap between the developing roller 60 and the drum 5 is set to 0.1 mm, two types of developing rollers A and B having different surface resistances are prepared, and continuous printing of a half image (25% density) is performed. Image density was evaluated.
First, these developing rollers A and B have a urethane resin coating layer (thickness 15 μm) in which carbon is dispersed for 5 hours. However, the developing roller A has a carbon addition rate of 7 wt%. The addition rate of carbon is 4 wt%.

That is, as shown in FIG. 7, the surface resistance of the developing roller B is higher than that of the developing roller A, and the voltage at which leakage can occur in the developing roller B is about 200 V higher than that of the developing roller A.
Here, when the half image is continuously printed in an environment of the apparatus 1 at a temperature of 10 ° C. and a humidity of 15% (during drying), even if the image density reaches 30 sheets, the developing roller A has an allowable density range. (The range surrounded by the upper dotted line and the lower dotted line in FIG. 8: Δ0.1). Accordingly, the developing roller A need not be subjected to a refresh operation.

  On the other hand, with the developing roller B, the image density falls within the allowable density range up to 1 to 4 sheets, but the charge gradually accumulates on the surface of the developing-side sleeve 64 during that time, and the allowable density range already exists after 5 sheets. (The upper dotted line in FIG. 8) is exceeded. Note that when continuous printing of solid images was performed, the image density decreased, and after 5 sheets, the allowable density range (Δ0.1) was already exceeded. That is, it is necessary to perform a refresh operation on the developing roller B having a high surface resistance.

  On the other hand, as shown in FIG. 9, even with the developing roller B, when the half image is continuously printed in the environment of the apparatus 1 at a temperature of 23 ° C. and a humidity of 50% (at room temperature), the image density is increased. Is within the allowable density range even when the number reaches 30 sheets. In other words, it can be understood that the refresh operation is not particularly required even for the developing roller B at room temperature.

Therefore, if the start timing of the refresh operation for the developing roller B is set so as to be advanced as it approaches the environment at the time of drying, a decrease in image density of several sheets from the initial stage of continuous printing shown in FIG. 8 can be suppressed.
More specifically, at the time of the drying (for example, the temperature is 10 ° C. and the humidity is 15%), when five continuous prints are obtained, the operation proceeds to the refresh operation (FIG. 6).

That is, in the developing roller B in this environment, when 5 sheets are reached in FIG. 5, the continuous printing is interrupted to become a non-image (between sheets), and the above-described refresh operation (I, II, III) is performed. To do. As a result, the electric charge accumulated on the surface of the developing side sleeve 64 is gradually removed.
Then, after this refresh operation is completed, continuous printing is resumed, and when the number of prints after the restart reaches 5, the refresh operation is started again.

Thereafter, when the refresh operation and the continuous printing of 5 sheets are repeated, even if the developing roller B is used during drying, the image density reaches 30 sheets as in the developing roller A as shown in FIG. However, it can be seen that it falls within the allowable density range (Δ0.1).
Further, the above-described charge-up of the developing roller 60 becomes more significant as the amount of moisture in the atmosphere decreases, for example, and is less likely to occur when the amount of moisture in the atmosphere increases. Therefore, if the application period II of each of the bias DC components Vs and Vm is set according to the temperature and humidity of the device 1, the refresh operation can be performed with a length necessary for eliminating the charge-up.

Furthermore, if the magnitudes of the applied values of the bias DC components Vs and Vm are also set according to the temperature and humidity of the apparatus 1, the refresh operation can be performed at a depth necessary for eliminating the charge-up.
Further, as the amount of moisture in the atmosphere decreases, the charge can be surely removed by making the application period II of each bias DC component Vs, Vm longer than one rotation of the developing roller.

Furthermore, if the magnitudes of the bias DC components Vs and Vm in II are greatly reduced as compared with the example of FIG. 5, the speed of removing the charge can be increased, and the paper interval for eliminating the charge-up is not increased.
The present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the claims.

  For example, in the above-described embodiment, the description has been given of suppressing the decrease in image density of several sheets from the initial stage of continuous printing. However, the toner contains an external additive, and the external additive adheres to the developing roller 60 as the total number of printed sheets of the apparatus 1 increases, and increases the surface resistance of the developing roller 60. In view of this point, the controller 90 may advance the start time of the refresh operation as the total number of printed sheets of the apparatus 1 increases.

Also in this case, the developing roller 60 can surely fall within the desired image density, that is, the above-described allowable density range.
Then, as the total number of printed sheets of the apparatus 1 increases, if the application period II of each of the bias DC components Vs and Vm is made longer than one rotation of the developing roller, the charge can be surely removed.

Furthermore, if the magnitudes of the bias DC components Vs and Vm in II are greatly reduced as compared with the example of FIG. 5, the speed of removing the charge can be increased, and the paper interval for eliminating the charge-up is not increased.
Furthermore, in the above-described embodiment, an example in which the image forming apparatus is embodied in a printer is shown. However, the image forming apparatus of the present invention can naturally be applied to a multifunction machine, a copier, a facsimile machine, and the like.

1 Printer (image forming device)
5 Photosensitive drum (image carrier)
7 Developing device 50 Magnetic roller 60 Developing roller 82 Temperature / humidity sensor (detection means)
90 controller (control means)

Claims (9)

An image forming apparatus having a carrier and a toner, supplying the toner to an image carrier and developing the latent image,
A magnetic roller forming a magnetic brush with the carrier and toner;
A developing roller having a resin layer in which a resistance adjusting agent is dispersed on the surface, and forming a layer of toner transferred from the magnetic brush;
In order to develop a latent image on the image carrier with toner of the developing roller while changing a bias applied to the magnetic roller so that toner is not transferred from the magnetic brush to the developing roller during non-image formation. Control means for performing a refresh operation for making the absolute value of the direct current component of the bias applied to the developing roller smaller than the absolute value of the value applied to the developing roller during image formation. Image forming apparatus. The image forming apparatus according to claim 1,
It further comprises detection means for detecting the temperature and humidity of this device,
The image forming apparatus, wherein the control unit changes a start time of the refresh operation based on a detection result by the detection unit. The image forming apparatus according to claim 2,
The image forming apparatus according to claim 1, wherein the control unit advances the start time of the refresh operation as the absolute water content detected by the detection unit decreases. The image forming apparatus according to any one of claims 1 to 3,
The toner contains an external additive;
The image forming apparatus according to claim 1, wherein the control means advances the start time of the refresh operation as the total number of printed sheets of the apparatus increases. The image forming apparatus according to claim 2,
The control unit changes a bias DC component application period to the developing roller and a bias DC component application period to the magnetic roller corresponding to the application period based on a detection result of the detection unit. An image forming apparatus. The image forming apparatus according to claim 2, wherein:
The control means changes the applied value of the DC component of the bias to the developing roller and the applied value of the DC component of the bias to the magnetic roller according to the application based on the detection result by the detecting means. An image forming apparatus. The image forming apparatus according to claim 5, wherein:
The control means lengthens the application period of the DC component of each bias as the absolute moisture amount detected by the detection means decreases, or increases the application value of the DC component of each bias. An image forming apparatus that is reduced in size. The image forming apparatus according to any one of claims 5 to 7,
The toner contains an external additive;
As the total number of printed sheets of the apparatus increases, the control means lengthens the application period of the DC component of each bias, or decreases the application value of the DC component of each bias. An image forming apparatus. An image forming method of charging a toner using a carrier, supplying the toner to an image carrier and developing the latent image,
A magnetic roller for forming a magnetic brush made of the carrier and toner, and a developing roller for forming a layer of toner transferred from the magnetic brush, the surface of which has a resin layer in which a resistance adjusting agent is dispersed. Has
Determining whether it is during non-image formation;
A step of changing a bias value applied to the magnetic roller so that the magnetic brush collects toner from the developing roller during the non-image formation;
Of the bias applied to the developing roller for developing the latent image on the image carrier with the toner of the developing roller, the absolute value of the direct current component is obtained from the absolute value of the value applied to the developing roller during image formation. And making the bias value applied to the magnetic roller substantially equal to the bias value applied to the developing roller;
An image forming method comprising the steps of returning the bias value applied to the developing roller and the bias value applied to the magnetic roller to the values at the time of image formation, respectively.

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