JP2009031749A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To maintain stable performances over a long term by suppressing image density defects, fogging, toner scattering, ghost phenomenon and the like. <P>SOLUTION: The apparatus has: a latent image carrier that carries an electrostatic latent image; a toner carrier that is disposed opposite the latent image carrier and carries the toner to be conveyed to a development region to develop the electrostatic latent image; a developer carrier that is disposed opposite the toner carrier, carries two-component developer, and supplies the toner of the two-component developer to the toner carrier; and a regulator that sets the thickness of a toner layer carried on the toner carrier to 6 μm to 15 μm and sets a difference between a half width of a first toner charge number distribution as a number distribution of the charge amount of the toner carried on the toner carrier and that of a second toner charge number distribution as a number distribution of the charge amount of the toner in the two-component developer carried on the developer carrier to 0.8 (10-10 C/m) or smaller. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine using an electrophotographic method, and more particularly, charged using a two-component developer that charges nonmagnetic toner with a magnetic carrier. The present invention relates to an image forming apparatus using a non-contact developing method in which only a toner is held on a developing roller and the latent image is developed by flying the toner onto an electrostatic latent image.

  Conventionally, development in a non-contact developing system using a one-component developer has been studied in image forming apparatuses such as copying machines, printers, facsimiles, and multifunction machines. In recent years, with the increase in printing speed, development for a high-speed image forming system, in particular, development for a one-drum color superposition system for sequentially forming a plurality of color images on a photoconductor has been studied. This one-drum color superimposing method is capable of forming a color image with little color misregistration by accurately superimposing toner on a photoconductor, and is attracting attention as a technique for improving the quality of color images.

Recently, a so-called tandem method in which a plurality of photoconductors corresponding to respective colors of toner is used to form a color image in synchronization with the feeding of the transfer member and color is superimposed on the transfer member has been attracting attention. Although this method has the advantage of being excellent in high speed, the electrophotographic process members (image forming units) for each color must be arranged side by side, resulting in an increase in size of the apparatus. On the other hand, a small tandem image forming apparatus has been proposed in which a compact image forming unit is arranged by narrowing the interval between the photosensitive members.
U.S. Pat. No. 3,929,098 JP 2003-211961 A JP 2003-21966 A JP 2001-134050 A JP-A-2005-242281 JP 2003-280357 A JP 2001-272857 A

  With regard to this tandem image forming apparatus, for example, in Patent Document 1, a developer is sent to a donor roller (developing roller) using a magnetic roller, and toner is transferred onto the donor roller to form a toner layer, thereby developing the toner. Techniques to do are disclosed. However, in this technique, toner charging control is complicated, and it is necessary to apply a high surface potential and a large development electric field to the photoreceptor.

  Further, since it is difficult to refresh the undeveloped toner on the donor roller, if a toner consumption area and a non-consumption area are generated on the donor roller, the toner adhesion state and the toner potential difference on the donor roller vary. Therefore, a phenomenon that a part of the previous developed image appears as an afterimage (ghost) during the next development, that is, a so-called history phenomenon is likely to occur.

  On the other hand, for example, in Patent Document 2 and Patent Document 3, a magnetic roller (magnetic brush roller) that holds a magnetic brush formed by using a two-component developer having a carrier and toner by a magnetic pole member fixed inside, A developing roller that forms a toner layer by rubbing with the magnetic brush, and a power source that forms an alternating electric field between the developing roller and the photosensitive member, and is exposed to the toner flying from the toner layer by the alternating electric field. A technique is disclosed in which a latent image on a body is developed to prevent occurrence of an afterimage (ghost) during development while avoiding occurrence of fog.

  Patent Document 4 uses a one-component developer, and includes a developing roller in contact with the photosensitive member and a supply roller in contact with the developing roller. The toner is supplied to the developing roller by the supply roller and is rubbed by the regulating blade. In a developing device that develops a latent image on a photosensitive member by charging to a thin layer state, a technique is disclosed in which an alternating voltage is applied to a supply roller so that both alternating voltages have the same frequency and different phases. ing.

  In this technique, from the viewpoint of difficulty in developing low density images and fine line images, or from the viewpoint of preventing density unevenness due to an increase in toner charge amount, if the developing electric field applied to the developing roller is AC, the low density image Although images and fine line images are well developed and undeveloped toner tends to be scraped off, high AC voltage causes fogging, and low AC voltage reduces the effect of scraping off undeveloped toner. I try to solve the problem.

  In order to solve the above problem, Patent Document 5 discloses that a toner layer is carried on a developing roller by rubbing with a magnetic brush formed of a two-component developer, and a first power source is provided between the developing roller and the photosensitive member. In the developing device in which the latent image on the photosensitive member is developed by causing the toner to fly from the developing roller by an alternating electric field composed of a rectangular wave formed by the second power supply, the above-described operation is performed between the magnetic roller and the developing roller. A technique is disclosed in which an alternating electric field is applied by a rectangular wave having the same frequency as that of the alternating electric field generated by the first power supply but having the opposite phase and the duty ratio reversed.

  However, in each of the above technologies, for example, when the bias voltage (AC bias) applied to the developing roller is Vslv and the bias voltage applied to the magnetic roller (magnetic brush) is Vmag, the power supply configuration for applying the bias voltage is shown in FIG. Thus, the bias voltages Vslv and Vmag are applied to the developing roller 901 and the magnetic roller 902 by the first and second bias power sources 911 and 912 (each power source is individually grounded), respectively. The potential difference between the magnetic roller 902 and the developing roller 901 is obtained as a difference between the bias voltage Vslv and the bias voltage Vmag.

  Hereinafter, for the sake of simplicity, the magnetic roller is expressed as “M”, the developing roller as “S”, and the interval between the magnetic roller and the developing roller as “between MS”.

  Considering the balance of peeling of undeveloped toner on the developing roller 901 between the MSs, toner thin layer formation, toner developability, etc., the AC bias voltage applied between the MSs is applied to the developing roller 901, for example. In the bias voltage Vslv, the duty ratio is 10 to 30%, the frequency is 4 kHz, and Vpp is 1.6 kV. On the other hand, in the bias voltage Vmag applied to the magnetic roller 902, the duty ratio is 70 to 90% in the reverse phase and the same period, the frequency is 4 kHz and Vpp is 0.3 kV. Is the best.

  In the following description, all duty ratios are expressed in percent (%).

  However, as the toner particle diameter is reduced for higher speed and higher image quality, the range in which the above balance is achieved becomes narrower. For this reason, it is difficult to secure an optimum value in consideration of durability.

  Since the potential difference between the MSs is obtained as the difference between the bias voltage Vslv and the bias voltage Vmag as described above, the potential difference between the MSs cannot be set directly, and the first and second bias power supplies 911, It is necessary to control each of the output voltages 912 so that the potential difference between the MSs becomes a desired potential difference.

  Then, the output voltages of the first and second bias power supplies 911 and 912 are related to the control of the undeveloped toner on the developing roller 901, the toner thin layer formation, and the toner developability. It is not easy to set the potential difference between the MSs to a desired potential difference while balancing the voltage suitable for the control and the potential difference between the MSs.

  Hereinafter, for the sake of simplicity, the photosensitive drum is expressed as “D”, the developing roller as “S”, and the interval between the photosensitive drum and the developing roller as “DS”.

  For example, Patent Document 6 discloses a technique for applying an AC bias voltage having a duty ratio of 10 to 50% to the developing roller. In this technique, an AC bias voltage is not applied to the magnetic roller but applied only to the developing roller. Patent Document 6 describes the relationship between the duty ratio of the AC bias voltage applied to the magnetic roller and the developing roller. No mention is made.

  Also, this development method (touch-down development method; a two-component process up to the magnetic roller, and then a one-component process in which a thin film is formed from the magnetic roller to the development roller to fly the toner) In this case, when the toner is moved from the magnetic roller to form the toner thin layer on the developing roller, the toner that is easy to move selectively (preferentially) forms the toner thin layer, After repeated print output, the toner charge quantity distribution (toner particle size distribution) in the two-component developer fluctuates greatly, causing problems such as poor image density, fogging, and toner scattering, and stable over a long period of time. It may be difficult to maintain the performance.

  In this regard, for example, Patent Document 7 discloses a technique for developing such that the toner charge amount distribution on the developing roller is different from the toner charge amount distribution in the developer on the magnetic roller.

  However, the difference in the number distribution of the charge amount between the developing roller and the magnetic roller means that the toner having a specific charge amount in the two-component developer on the magnetic roller is selectively transferred onto the developing roller. Will be. In other words, when the toner is selectively moved, the toner charge amount distribution in the two-component developer shifts to broad, and the toner thin layer formation (image formation) on the developing roller is stable over a long period of time. Operation; printing operation) becomes difficult.

  The present invention has been made in view of the above circumstances, and provides an image forming apparatus capable of suppressing image density defects, fogging, toner scattering, ghosting, and the like and maintaining stable performance over a long period of time. Objective.

An image forming apparatus according to the present invention is an image forming apparatus using a two-component developer containing toner and a carrier, and is arranged to face a latent image carrier that carries an electrostatic latent image, and the latent image carrier. A toner carrying member that carries the toner conveyed to the development area to develop the electrostatic latent image; and the toner carrying member that is disposed opposite the toner carrying member to carry the two-component developer. The developer carrying body for supplying the toner in the toner carrying body and the layer thickness of the toner carried on the toner carrying body are set to 6 μm to 15 μm, and the toner carried on the toner carrying body is charged. A first toner charge amount number distribution that is a number distribution with respect to the amount, and a second toner charge amount number distribution that is a number distribution with respect to the charge amount of the toner in the two-component developer carried on the developer carrier. The difference in half width is 0.8 And an adjustment section that adjusts to (10 ⁻¹⁰ C / m) or less, and the layer thickness of the toner carried on the toner carrying member is 6 μm to 15 μm.

According to this configuration, the image forming apparatus includes a latent image carrier that carries an electrostatic latent image, and a toner that is disposed opposite to the latent image carrier and that is conveyed to the development area to develop the electrostatic latent image. A toner carrier, a developer carrier that is disposed opposite to the toner carrier, carries the two-component developer, and supplies the toner in the two-component developer to the toner carrier, and an adjustment unit. And a first toner charge amount number distribution that is a number distribution with respect to the charge amount of the toner carried on the toner carrying member by the adjusting unit, and the two-component developer carried on the developer carrying member. The difference in half width from the second toner charge amount number distribution, which is the number distribution with respect to the toner charge amount, is set to 0.8 (10 ⁻¹⁰ C / m) or less. Furthermore, the layer thickness of the toner carried on the toner carrying member is set to 6 μm to 15 μm.

Thus, since the toner layer thickness is as thin as 6 μm to 15 μm, all the toner on the toner carrier (developing roller) can be used for development (as much as possible) or on the toner carrier. It is possible to prevent the development residual toner from returning to the latent image carrier (photosensitive drum). Further, since the half-value width difference and the peak position difference are as small as 0.8 (10 ⁻¹⁰ C / m) or less, the toner charge amount distribution in the toner thin layer on the toner carrier and the developer carrier The difference (deviation) from the toner charge quantity distribution in the two-component developer can be reduced (matched), and between the toner carrier and developer carrier (or developer carrier and latent image carrier). The selectivity of toner movement between bodies can be reduced. From these facts, it was experimentally found that it is possible to suppress image density defects, fogging, toner scattering, ghost phenomenon, and the like and to maintain stable performance over a long period of time.

Further, it is preferable that the adjustment unit sets a difference between peak positions of the first and second toner charge amount number distributions to 1.0 (10 ⁻¹⁰ C / m) or less.

According to this, since the difference between the peak positions of the first and second toner charge amount number distributions is 1.0 (10 ⁻¹⁰ C / m) or less by the adjusting unit, the toner thin layer on the toner carrier The difference (displacement) between the toner charge amount distribution in the toner and the toner charge amount distribution in the two-component developer on the developer carrier can be further reduced (matched). In addition, the selectivity of toner movement between the developer carrier (or between the developer carrier and the latent image carrier) can be reliably reduced.

  The adjusting unit preferably includes a silicon-modified urethane resin that is coated on the surface of the toner carrier with a predetermined thickness.

According to this, a silicon-modified urethane resin that is coated with a predetermined thickness on the surface of the toner carrying member is provided as the adjusting portion. As a result, the half width difference can be reduced to 0.8 (10 ⁻¹⁰ C / m) or less, and the peak position difference can be reduced to 1.0 (10 ^-10 C / m) or less can be easily realized.

  The adjustment unit includes a first bias applying unit that applies an AC bias voltage having a duty ratio of Duty (slv) to the toner carrier, and a duty ratio of Duty (mag) to the developer carrier. A duty ratio that satisfies a condition of 100 (%) − Duty (mag) <Duty (slv), wherein the duty (slv) and the Duty (mag) are included in a second bias applying unit that applies a certain AC bias voltage. It is preferable that it is set to.

According to this, the adjustment unit applies the AC bias to the toner carrier with a duty ratio that satisfies the condition of 100 (%) − Duty (mag) <Duty (slv), and the developer carrier. Including a second bias applying unit that applies an AC bias, and with a simple configuration in which an AC bias is applied to the toner carrier with a duty ratio that satisfies the condition, the adjustment unit sets the half-value width difference to 0.8. (10 ⁻¹⁰ C / m) or less, or the half-value width difference is 0.8 (10 ⁻¹⁰ C / m) or less and the peak position difference is 1.0 (10 ⁻¹⁰ C / m) or less. It is easy to do.

  Further, if the frequency of the AC bias voltage output from the first bias applying unit is f (mag) and the frequency of the AC bias voltage output from the second bias applying unit is f (slv), then f (mag) )> F (slv), and f (mag) is preferably 2.5 kHz or more.

According to this, it discovered that the effect which makes a half-value width difference 0.8 (10 < ^-10 > C / m) or less, and also a peak position difference 1.0 (10 < ^-10 > C / m) or less is acquired. .

  The second bias application unit is preferably connected in series with the first bias application unit and electrically connected to the ground via the first bias application unit.

  According to this, the first bias application unit and the second bias application unit are connected in series, and the second bias application unit is electrically connected to the ground via the first bias application unit. The bias voltage applied to the toner carrier can be superposed on the bias voltage based on the bias voltage applied to the toner carrier. As a result, the toner carrier and developer carrier (toner carrier and developer) The AC bias voltage applied between the carriers or between the developer carrier and the latent image carrier and the duty ratio can be easily adjusted individually.

According to the present invention, since the toner layer thickness is as thin as 6 μm to 15 μm, all of the toner on the toner carrier (developing roller) can be used for development (as much as possible) It is possible to prevent development residual toner on the body from returning to the latent image carrier (photosensitive drum). Further, since the half-value width difference and the peak position difference are as small as 0.8 (10 ⁻¹⁰ C / m) or less, the toner charge amount distribution in the toner thin layer on the toner carrier and the developer carrier The difference (deviation) from the toner charge quantity distribution in the two-component developer can be reduced (matched), and between the toner carrier and developer carrier (or developer carrier and latent image carrier). The selectivity of toner movement between bodies can be reduced. From these facts, it was experimentally found that it is possible to suppress image density defects, fogging, toner scattering, ghost phenomenon, and the like and to maintain stable performance over a long period of time.

(First embodiment)
A printer as an example of an image forming apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram illustrating an example of a printer according to the first embodiment. As shown in FIG. 1, a printer 1 is a so-called tandem type image forming apparatus, and an image forming unit for each color of magenta (M), cyan (C), yellow (Y), and black (K) in a printer body. 2M, 2C, 2Y, and 2K are juxtaposed.

  The image forming units 2M, 2C, 2Y, and 2K (collectively referred to as the image forming unit 2) perform color image formation (printing) on paper, and are, for example, photoconductors made of amorphous silicon (a-Si). A drum 21 (latent image carrier), a charging unit 22, an exposure unit 23, and a development unit 24 are provided around the photosensitive drum 21.

  The charging unit 22 uniformly charges the surface of the photosensitive drum 21 to a predetermined potential. The exposure unit 23 irradiates the surface of the photosensitive drum 21 with a laser beam (LED light) generated based on the image data, and forms an electrostatic latent image on the drum surface. The developing unit 24 makes the electrostatic latent image appear as a toner image by supplying toner to the electrostatic latent image formed on the photosensitive drum 21 and attaching it. In the present embodiment, the configuration of the developing unit 24 (and the photosensitive drum 21) has a main characteristic point, and details thereof will be described later.

  Below the image forming units 2M to 2K, an intermediate transfer roller 31 (primary transfer roller) and an intermediate belt (intermediate transfer belt) for performing an intermediate transfer (primary transfer) of a toner image that has become apparent on the surface of the photosensitive drum 21. An intermediate transfer unit 3 having 32 is disposed. The intermediate belt 32 is formed of a predetermined belt body, and is rotated endlessly by the driving rollers 33 to 35 in a state where the intermediate belt 32 is pressed against the photosensitive drum 21 by the intermediate transfer roller 31 disposed to face the photosensitive drums 21 of the respective colors. It is configured. The toner images of the respective colors formed on the photosensitive drum 21 are transferred and superimposed on the intermediate belt 32 that is rotated endlessly in order of magenta, cyan, yellow, and black at the same timing. As a result, a color image composed of four colors Y, M, C, and K is formed on the intermediate belt 32.

  A secondary transfer roller 36 is provided at a position facing the drive roller 35 via an intermediate belt 32 (the secondary transfer roller 36 is included in the intermediate transfer unit 3). The secondary transfer roller 36 transfers a color image on the intermediate belt 32 to a sheet by a transfer bias voltage from the control unit 100 described later.

  The printer 1 also includes a paper feeding unit 4 that feeds paper toward the image forming units 2Y to 2K. The paper feed unit 4 includes a paper feed cassette 41 that stores paper of each size, a transport path 42 that is a path through which the paper is transported, a transport roller 43 that transports the paper in the transport path 42, and the like. The paper P picked up one by one from the cassette 41 is conveyed toward the image forming units 2Y to 2K, that is, the position of the secondary transfer roller 36. Incidentally, the fixing unit 5 is provided at an appropriate position downstream of the secondary transfer roller 36 in the conveyance path 42. The fixing unit 5 fixes the toner image transferred to the paper. The fixing unit 5 includes a heat roller 51 and a pressure roller 52, melts toner on the paper by the heat of the heat roller 51, and applies pressure by the pressure roller 52 to fix the toner on the paper. The paper feed unit 4 conveys the paper P that has undergone the secondary transfer process to the fixing unit 5 and discharges the paper P that has undergone the fixing process to a paper discharge tray 44 at the top of the printer main body.

In addition, the printer 1 includes a control unit 6 at an appropriate place inside. The control unit 6 reads out and executes a ROM (Read Only Memory) that stores various control programs, a RAM (Random Access Memory) that temporarily stores data or functions as a work area, and the control program. It consists of a microcomputer and the like, and transmits and receives various control signals to and from each functional unit of the printer 1 to control operation of the entire printer 1. In this embodiment, in particular, the control unit 6 drives a first bias power supply 246 (adjustment unit, second bias application unit) and a second bias power supply 247 (adjustment unit, first bias application unit) shown in FIG. To control the bias voltage application operation (cycle, phase, V _PP , frequency and duty ratio) to the magnetic roller 244 (developer carrier) and the developing roller 245 (toner carrier).

  The printer 1 is a network I / F unit 7 that controls transmission / reception of various data to / from an information processing apparatus (external device) such as a PC connected via a network such as a LAN. An operation panel unit 8 or the like provided on the front unit functions as an input key for inputting various instructions by the user or displays predetermined information.

  Here, the developing unit 24 will be described in detail. The developing unit 24 includes a developer container 241, a stirring mixer 242, a paddle mixer 243, a magnetic roller 244, and a developing roller 245. The developer container 241 is, for example, a cartridge type storage unit that stores the developer (toner) of each color. The agitating mixer 242 agitates the developer replenished from the developer container 241. The paddle mixer 243 agitates the developer and collects the developer by scraping a magnetic brush that collects residual toner that has not been used for development on the developing roller 245.

  The magnetic roller 244 forms a toner thin layer on the developing roller 245 by generating a magnetic brush with a carrier contained in the developer by a magnet disposed therein. The developing roller 245 carries a thin toner layer and develops the electrostatic latent image on the photosensitive drum 21.

In this embodiment, a so-called two-component developer composed of a toner and a carrier is employed as the developer. The toner is a fine powder having a particle diameter of, for example, 6 to 12 μm in which additives such as a colorant, a charge control agent, and wax are dispersed in a binder resin. Here, a positively chargeable toner is used. On the other hand, the carrier is magnetic particles having a particle diameter of, for example, 60 to 200 μm such as magnetite (Fe ₃ O ₄ ), and is used for charging the toner. The carrier has a role of collecting and supplying the toner. For example, a carrier having a volume resistivity of 10 ⁶ Ωcm to 10 ¹³ Ωcm is used.

Then, at the nip between the developing roller 245 and the magnetic roller 244, the toner that is strongly electrostatically attached is peeled off by the magnetic brush and the toner necessary for development is supplied. At this time, in order to increase the contact point with the toner, it is desirable to increase the surface area of the carrier by using a carrier having a small diameter of 50 μm or less. Here, a coated ferrite carrier having a volume resistivity of 10 ¹⁰ Ωcm, a saturation magnetization of 65 emu / g, and an average particle size of 45 μm is used.

  The two-component developer in the developing unit 24 forms a magnetic brush composed of toner and carrier on the magnetic roller 244. The toner is stirred by the stirring mixer 242 and charged to an appropriate level. Then, a magnetic brush is formed on the magnetic roller 244 by the two-component developer, and comes into contact with the developing roller 245 with a constant layer thickness while being regulated by a cutting blade (not shown), and the magnetic roller 244 and the developing roller 245. Due to the potential difference | DC1-DC2 | (this potential difference is expressed as ΔV), a thin layer of toner only is formed on the developing roller 245 from the magnetic brush.

  As described above, the layer thickness of the toner carried on the surface of the developing roller 245 is controlled by a known technique such as ΔV, layer regulation by a pan blade, or the like, and is, for example, 6 μm to 15 μm.

  “DC1” indicates a DC component of the toner supply bias voltage applied to the magnetic roller 244 by the first bias power supply 246, and “DC2” indicates the development bias voltage applied to the developing roller 245 by the second bias power supply 247. DC component is shown. However, the printer 1 of the present embodiment has a configuration shown in FIG. 2, which is different from the configuration shown in FIG. 6 in that a bias voltage is applied to the magnetic roller 244 and the developing roller 245.

  A bias voltage is applied to the magnetic roller 244 by a first bias power source 246 (second bias applying unit). A bias voltage is applied to the developing roller 245 by a second bias power source 247 (first bias applying unit). In addition, the reference potential side terminal (negative electrode side terminal) of the first bias power supply 246 is connected to the output terminal of the second bias power supply 247.

  The first bias power supply 246 is a power supply circuit that applies to the magnetic roller 244 a bias voltage Vb1 obtained by adding a rectangular wave AC voltage component AC1 having a duty ratio set to Duty (mag) and a DC voltage component DC1. is there. As for Duty (mag), a time T1 in which a voltage in the direction in which the toner moves from the magnetic roller 244 to the developing roller 245 is applied and a voltage in the direction in which the toner is pulled back from the developing roller 245 to the magnetic roller 244 are applied. It is the ratio of T1 to the total time with state time T2. In the first embodiment, Duty (mag) is set to 70%, for example.

  The second bias power supply 247 applies a bias voltage Vb2 obtained by adding a rectangular wave AC voltage component AC2 having a duty ratio set to Duty (slv) and a DC voltage component DC2 to the developing roller 245. Circuit. Duty (slv) is applied with a time T3 in which a voltage in the direction in which the toner moves from the developing roller 245 to the photosensitive drum 21 is applied and a voltage in a direction in which the toner is pulled back from the photosensitive drum 21 to the developing roller 245. It is the ratio of T3 to the total time with state time T4. In the first embodiment, Duty (slv) is set to 30%, for example.

  Accordingly, the first bias power source 246 that applies a bias voltage to the magnetic roller 244 is connected to the common ground with the second bias power source 247 via the second bias power source 247 that applies a bias voltage to the developing roller 245. It is connected. The circuit configuration (wiring) is such that the ground of the first bias power supply 246 and the ground of the second bias power supply 247 become a common (one) ground.

  Then, since the second bias power supply 247 and the first bias power supply 246 are connected in series, the bias voltage Vb2 output from the second bias power supply 247 and the bias voltage Vb1 output from the first bias power supply 246 are added. And applied to the magnetic roller 244. At this time, the voltage applied between the MSs is equal to the bias voltage Vb1 output from the first bias power supply 246.

  In the case of the conventional example shown in FIG. 6, the alternating current (AC) components of the first bias power supply 911 and the second bias power supply 912 are applied to the developing roller 901 and the magnetic roller 902 so as to face each other. , MS is the difference between the output of the first bias power supply 911 and the output of the second bias power supply 912.

  Therefore, the voltage between the MSs cannot be set unless both the output voltage of the first bias power supply 911 and the output voltage of the second bias power supply 912 are controlled. On the other hand, the output voltage of the first bias power supply 911 is related to the AC bias voltage between the DSs, and also affects the peeling of the toner from the developing roller 245 and the developability of the toner on the photosensitive drum 21. Therefore, it is difficult to set the AC bias voltage between the MS (AC voltage) and the AC bias voltage between the DSs so that the respective effects are optimum, and either one of the effects is reduced and adjusted (balanced). There was a need.

  However, in the case shown in FIG. 2, the first bias power source 246 is connected to the common ground with the second bias power source 247 via the second bias power source 247, so that the bias voltage Vb2 applied to the developing roller 245 is shown. As a base, the bias voltage Vb1 is superimposed on the bias voltage Vb2, and Vb1 + Vb2 is applied to the magnetic roller 244. In other words, the adjustment reference in the first bias power supply 246 can be adjusted to the second bias power supply 247 side.

As a result, the output voltage of the first bias power supply 246 becomes the MS voltage as it is, independently of the output voltage of the second bias power supply 247, so that the AC bias voltage between the MS and DS can be easily set individually. It is possible to adjust, that is, between the MS and the DS, the period, phase, V _PP , frequency, etc. (DC voltage (Vdc), AC voltage (V _PP ), frequency (f), and duty ratio described later) Etc.) can be applied with different AC bias voltages.

  The thin toner layer on the developing roller 245 varies depending on the resistance of the developer and the rotational speed difference between the developing roller 245 and the magnetic roller 244, but can also be controlled by the potential difference ΔV. The toner layer on the developing roller 245 becomes thicker when ΔV is increased and becomes thinner when ΔV is decreased. The range of ΔV of the magnetic roller 244 and the developing roller 245 is generally about 100V to 350V.

  The charged toner is held on the developing roller 245 as a thin layer with a thickness corresponding to the potential difference ΔV between the magnetic roller 244 and the developing roller 245. Further, by applying a bias voltage in which a DC voltage and an AC voltage are superimposed between the developing roller 245 and the photosensitive drum 21, the toner flies from the developing roller 245 to the photosensitive drum 21, and the photosensitive drum 21 is moved onto the photosensitive drum 21. The electrostatic latent image is developed. In order to prevent toner scattering, an alternating voltage is applied immediately before development.

  The development residual toner on the developing roller 245 is not provided with a special device such as a scraping blade, but the magnetic brush on the magnetic roller 244 contacts the toner layer on the developing roller 245, and the brush due to the difference in peripheral speed between these rollers. It is easily recovered and replaced by the effect and the stirring effect of the screw by the paddle mixer 243.

  At this time, since the width of the magnetic brush is a width for collecting the toner on the developing roller 245, by making the width of the developing roller 245 shorter than the width of the magnetic brush, the development residual on the surface of the developing roller 245 is surely achieved. The toner uncollected area can be eliminated. As a result, the toner adhering to the developing roller sleeve (not shown) outside the magnetic brush area is eliminated (reduced), and toner scattering at both ends of the developing roller 245 can be eliminated (reduced).

  Further, the rotation speed of the magnetic roller 244 is set to, for example, 1.0 to 2.0 times the rotation speed of the developing roller 245, the toner on the developing roller 245 is collected, and an appropriate toner density is set. By supplying the developer to the developing roller 245, a uniform toner layer can be formed.

  Further, in order to maintain a uniform image density, the potential difference ΔV between the magnetic roller 244 and the developing roller 245 is eliminated at times other than the development timing (each potential is set to the same potential), so that no burden is imposed on the toner. In addition, it is effective to collect the toner on the developing roller 245 on the magnetic roller 244.

  By the way, when the a-Si photosensitive member is used as the photosensitive material of the photosensitive drum 21, the post-exposure potential on the surface has a very low potential of 20V or less. When the film thickness of the a-Si photosensitive member is reduced, the saturation charging potential is lowered, and the withstand voltage leading to dielectric breakdown is lowered. On the other hand, when the film thickness of the a-Si photosensitive member is reduced, the charge density on the surface of the photosensitive drum 21 when a latent image is formed tends to be improved, and the development performance tends to be improved.

This tendency is particularly remarkable when the film thickness is 25 μm or less, more preferably 20 μm or less, in an a-Si photoreceptor having a high dielectric constant of about 10. In this case, in the developing bias voltage, DC voltage component DC2 is 150V or less, 200V～2000V the peak-to-peak voltage _{V PP} as the AC voltage component AC2, and can be developed by setting the 1~4kHz frequency.

  Conventionally, an organic photoreceptor (OPC) is known as a photoreceptor used in an image forming apparatus. When a positively charged organic photoconductor is used as the photoconductor drum 21, the photosensitive layer thickness is set to 25 μm or more in order to reduce the residual potential to 100 V or less, and the amount of charge generation material added is increased. is important. In particular, the organic photoreceptor having a single layer structure is advantageous in that the charge generation material is added to the photosensitive layer, and therefore the sensitivity change is small even if the photosensitive layer is reduced.

Even in this case, the DC voltage component DC2 in the development bias voltage is preferably set to 400 V or less, more preferably 300 V or less, from the viewpoint of preventing a strong electric field from being applied to the toner. Furthermore, to the extent that the potential difference between the photoreceptor does not exceed 1500V, preferable from the viewpoint of setting the DC2, V _PP to prevent leakage.

  Here, the bias voltage and the duty ratio for the developing roller 245 and the magnetic roller 244 will be described. FIG. 7A is a diagram illustrating a conventional bias voltage Vslv and bias voltage Vmag for the developing roller and the magnetic roller, and a composite bias voltage waveform between the MSs of the bias voltages Vslv and Vmag.

  A waveform 921 shown in FIG. 7A indicates the bias voltage Vslv with a solid line, and the bias voltage Vmag with a broken line. A waveform 931 shown in FIG. 7A shows a voltage (combined bias voltage) between the MSs generated by the bias voltages Vslv and Vmag.

  When the conventional power supply connection configuration shown in FIG. 6 is used, in the configuration in which an AC bias having the same period, the same frequency, and the opposite phase as that of the developing roller is applied to the magnetic roller, a waveform 941 in FIG. Thus, when duty ratio (slv) ≠ 100−duty ratio (mag), the potential difference between the MSs has a waveform as indicated by reference numeral 951 in FIG. That is, a potential between the maximum bias voltage (Vmax) and the minimum bias voltage (Vmin) of the bias voltages Vslv and Vmag shown in the waveform 941 appears between the MSs.

  As a result, the voltage between the MSs becomes a stepped voltage waveform as indicated by reference numeral 952, and the effect of toner flying and pulling back is reduced.

  However, the duty ratio (slv) and the duty ratio (mag) indicate the duty ratio for the developing roller and the magnetic roller, respectively.

  Therefore, when a voltage as indicated by reference numeral 931 in FIG. 7A is applied between the MSs, it is necessary to set the duty ratio of the bias voltage Vslv so that the duty ratio corresponds to the voltage between the MSs. As a result, the toner thin layer forming time on the developing roller based on the bias voltage Vslv is shortened, and the recovery time of undeveloped toner from the developing roller is also shortened, resulting in poor efficiency.

  In the embodiment according to the present invention, the bias voltage applied between the MSs is substantially equal to the bias voltage Vb1 applied to the magnetic roller. Therefore, the toner thin layer formation time on the developing roller and the undeveloped toner from the developing roller The recovery time depends on only the bias voltage Vb1 applied to the magnetic roller.

  Here, when the duty ratio (slv) = 100 (%) − duty ratio (mag) in the conventional configuration shown in FIG. 6, the combined waveform between the MSs is Vmag as shown in FIG. 2 and the absolute value of Vslv appear, and the electric field due to this voltage acts as a force to move the toner. In the configuration of this embodiment shown in FIG. 2, the bias voltage applied between the MSs is The output voltage of the first bias power supply 246 becomes the MS voltage as it is.

  Therefore, in the configuration shown in FIG. 2, if the output voltage of the first bias power supply 246 is equal to the bias voltage Vmag in FIG. 6, the electric field for moving the toner becomes weak. Therefore, in this embodiment shown in FIG. 2, it is necessary to make Vpp (peak to peak voltage) of the output voltage of the bias voltage Vb1 output from the first bias power supply 246 larger than the bias voltage Vmag in FIG.

  In the first embodiment, such a printer 1 is configured (set) so as to satisfy the following conditions (development conditions). That is, the a-Si drum made of the a-Si photosensitive member is used as the photosensitive drum 21, the outer diameter of the photosensitive drum 21 (photosensitive drum diameter) is 30 mm, and the outer diameter of the developing roller 245 (developing roller diameter). The outer diameter of the magnetic roller 244 (magnetic roller diameter) was set to 25 mm. These peripheral speeds are as follows.

Photosensitive drum 21: 300 mm / sec
Developing roller: 450mm / sec
Magnetic roller: 675mm / sec
The developing roller 245 is made of an aluminum substrate whose surface is covered with a silicon-modified urethane resin with a predetermined thickness. Here, the coating thickness was set to 0.8 μm. The gap (separation distance) between the magnetic roller 244 and the developing roller 245 was 350 μm. Further, the bias voltages applied to the magnetic roller 244 and the developing roller 245 are as follows.

Developing roller applied bias voltage Vb2: DC voltage Vdc2 (DC2) = 300 V, AC voltage (AC2) V _PP = 1.6 kV, frequency f = 2.7 kHz, duty ratio = 30%
Magnetic roller applied bias voltage Vb1: DC voltage Vdc1 (DC1) = 400V, AC voltage (AC1) (same cycle and opposite phase as the developing roller 245) V _PP = 2.8 kV, frequency f = 2.7 kHz, duty ratio = 70%
Toner: Volume average particle size = 6.5 μm, CV value of number distribution = 23.5%
Carrier: weight average particle diameter = 45 μm, saturation magnetization = 65 emu / g
However, the saturation magnetization was obtained by measurement with a magnetic field of 79.6 kA / m (1 kOe) using “VSM-P7” manufactured by TOEI. Further, the CV value in the number distribution of the volume average particle diameter of the toner was obtained by measurement with an aperture diameter of 100 μm (measurement range: 2.0 μm to 60 μm) using “Multisizer 3” manufactured by Beckman Coulter.

  The CV (coefficient of variation) value is an index indicating the uniformity of the particle size (diameter) of the particle product (sharpness of particle size distribution), and is the ratio of the standard deviation to the average particle size. The larger the CV value, the broader the particle size distribution, and the smaller the CV value, the sharper the particle size distribution. Here, the CV value in the number distribution of the toner particle diameter is obtained by dividing the standard deviation of the toner particle diameter by the average particle diameter of the toner.

  When the printer 1 having the configuration satisfying this condition (development condition) is operated to form a toner thin layer on the developing roller 245, the toner thin layer thickness on the developing roller 245 becomes 12.5 μm. This toner thin layer thickness was measured using LASER SCAN DIAMETER LS-3100 manufactured by KEYENCE. Specifically, the diameter of the developing roller on which the toner thin layer was formed and the diameter of the developing roller on which the toner thin layer was not formed were measured, and the difference between them was calculated to calculate the toner thin layer.

At this time, as shown in FIG. 3, the full width at half maximum 302 of the toner charge number distribution 301 in the toner thin layer on the developing roller 245 is 3.2 (10 ⁻¹⁰ C / m), and the peak position 303 is 3.2. ⁽¹⁰ -10 C / m). on the other hand, the half width 312 of the toner charge number distribution 311 in the two-component developer on the magnetic roller 244 ^{3.1 (10 -10 C / m)} , the peak position 313 3.1 (10 ⁻¹⁰ C / m).

That is, the difference between the half widths (half width difference) is 0.1 (10 ⁻¹⁰ C / m), and the peak position difference (peak position difference S1) is 0.1 (10 ⁻¹⁰ C / m). Become. However, the half width is the width of the distribution when the peak height of the toner charge amount distribution is halved.

  The toner charge quantity distribution was measured using E-SPART ANALAYZER MODEL EST-3 manufactured by Hosokawa Micron. Specifically, about 1 g of the two-component developer is sampled on the developing roller 245 or from the inside of the developing unit 24, and is carried on a 90 mT magnet and disposed at the measurement position (position where the air hits) of this E-SPART ANALAYZER. The toner on the developing roller 245 is arranged so that air hits the toner, and the two-component developer and toner are measured.

At this time, the air pressure was set to 0.55 to 0.8 kgf / cm ² (= 0.055 to 0.08 Mpa), PM VOLTAGE = −0.5 kV, and FILDE VOLTAGE = 0.050 kV.

As described above, in the first embodiment, when the toner is a positively chargeable toner as described above, the selective movement of the toner is suppressed by coating the surface of the developing roller 245 with the silicon-modified urethane resin. As a result, the fluctuation of the toner particle size distribution in the two-component developer is reduced, and the half-value width difference of the toner charge amount number distribution is 0.8 (10 ⁻¹⁰ C / m) or less, or the half-value width difference is It was experimentally found that the peak position difference was 0.8 (10 ⁻¹⁰ C / m) or less and 1.0 (10 ⁻¹⁰ C / m) or less.

  This silicon-modified urethane resin contains a urethane resin component having the same polarity as that of the positively chargeable toner, so that the friction with the magnetic brush carried on the magnetic roller 244 or the toner carried on the silicon-modified urethane resin is achieved. Therefore, negative charges are not charged. For this reason, the charge amount of the toner carried on the silicon-modified urethane resin does not increase, and thus the electric adhesion force does not increase.

  Further, the releasability due to the silicon component also acts, so that the developability of toner from the developing roller 245 is greatly increased. Accordingly, the toner layer thickness formed on the developing roller 245 is set to 6 to 15 μm, and the amount of moving toner is reduced, whereby the amount of toner remaining on the developing roller 245 after development is extremely reduced due to the effect of the silicon-modified urethane resin. Therefore, an increase in the charge amount of the toner accumulated on the developing roller 245 is suppressed, and fluctuations in the toner charge amount number distribution on the developing roller 245 are reduced. As a result, the toner thin layer can be stably formed. It is possible to prevent the toner charge amount distribution of the toner on the H.245 and the toner on the magnetic roller 244 from fluctuating.

(Second Embodiment)
In the second embodiment, the printer 1 is configured (set) so as to satisfy the following conditions (development conditions). That is, the photosensitive drum 21 is an a-Si drum made of the a-Si photosensitive member, and the photosensitive drum diameter is 30 mm, the developing roller diameter is 20 mm, and the magnetic roller diameter is 25 mm. These peripheral speeds are as follows.

Photosensitive drum 21: 300 mm / sec
Developing roller: 450mm / sec
Magnetic roller: 675mm / sec
Further, the developing roller 245 is made of an aluminum base, and the surface of the aluminum base is alumite-treated (in the first embodiment, coated with silicon-modified urethane resin). The gap (separation distance) between the magnetic roller 244 and the developing roller 245 was 350 μm.

  Further, the bias voltages applied to the magnetic roller 244 and the developing roller 245 are as follows. The waveforms of the alternating voltages (AC1) and (AC2) are shown in FIG.

Developing roller applied bias voltage Vb2: DC voltage Vdc2 (DC2) = 300 V, AC voltage (AC2) V _PP = 1.6 kV, frequency f = 2.7 kHz, duty ratio = 50% (30% in the first embodiment)
Magnetic roller applied bias voltage Vb1: DC voltage Vdc1 (DC1) = 400V, AC voltage (AC1) (same cycle and opposite phase as the developing roller 245) V _PP = 2.8 kV, frequency f = 2.7 kHz, duty ratio = 65% (70% in the first embodiment)
Toner: Volume average particle size = 6.5 μm, CV value of number distribution = 23.5%
Carrier: weight average particle diameter = 45 μm, saturation magnetization = 65 emu / g
Similar to the first embodiment, the saturation magnetization was obtained by measurement with a magnetic field of 79.6 kA / m (1 kOe) using “VSM-P7” manufactured by TOEI. Further, the volume average particle diameter and the CV value of the number distribution of the toner were determined by measurement with an aperture diameter of 100 μm (measurement range: 2.0 μm to 60 μm) using “Multisizer 3” manufactured by Beckman Coulter.

  When the printer 1 having the configuration as the developing condition is operated to form a toner thin layer on the developing roller 245, the toner thin layer thickness on the developing roller 245 is 14.5 μm (1. 2 in the first embodiment). 5 μm). The toner thin layer thickness is measured using LASER SCAN DIAMETER LS-3100 manufactured by KEYENCE, and the developing roller diameter on which the toner thin layer is formed and the developing roller diameter on which the toner thin layer is not formed are the same as described above. The toner thin layer was calculated by taking the difference.

At this time, as shown in FIG. 4, the half-value width 402 of the toner charge amount distribution 401 in the toner thin layer on the developing roller 245 is 3.4 (10 ⁻¹⁰ C / m), and the peak position 403 is 2.4. ⁽¹⁰ -10 C / m). on the other hand, ^3.5 half width 412 of the toner charge number distribution 411 in the two-component developer on the magnetic roller 244 ⁽¹⁰ -10 C / m), the peak position 413 2.8 (10 ⁻¹⁰ C / m). That is, the difference between the half widths (half width difference) is 0.1 (10 ⁻¹⁰ C / m), and the peak position difference (peak position difference S2) is 0.4 (10 ⁻¹⁰ C / m). Become.

  As described above, in the second embodiment, the duty ratio (65%) of the AC bias voltage (AC voltage) applied to the magnetic roller 244, that is, the two-component developer is set to the above Duty (mag) and applied to the developing roller 245. When the duty ratio (50%) of the AC bias voltage to be applied is Duty (slv), the magnetic roller 244 is set as 100 (%) − Duty (mag) <Duty (slv) (100−65 <50). Thus, a bias condition suitable for forming a toner thin layer on the developing roller 245 and a bias condition suitable for forming a toner image on the photosensitive drum 21 from the developing roller 245 could be achieved.

Accordingly, the selective movement of the toner is suppressed, and the half-value width difference of the toner charge amount distribution is 0.8 (10 ⁻¹⁰ C / m) or less, or the half-value width difference is 0.8 ( 10 and the peak position difference ^-10 C / m) or less has been found experimentally to be below ^{1.0 (10 -10 C / m)} .

(Third embodiment)
In the second embodiment, Duty (slv) = 50% and Duty (mag) = 65%. However, the present invention is not limited to this, and the above 100 (%) − Duty (mag) <Duty (slv) is satisfied. For example, Duty (slv) = 35% and Duty (mag) = 70% may be used. The waveforms of the alternating voltages (AC1) and (AC2) in this case are shown in FIG.

Again, the half-value width difference between the toner charge number distribution to achieve ^{0.8 (10} -10 C / m) or less, or and in the half-width difference ^{0.8 (10} -10 C / m) or less It was experimentally found that the peak position difference could be 1.0 (10 ⁻¹⁰ C / m) or less.

  Results obtained when images are formed on paper by the image forming apparatuses according to the first to third embodiments are summarized in the table shown in FIG. FIG. 5 shows the experimental results obtained for the first to third embodiments as Examples 1 to 3, respectively. Moreover, the other Example which concerns on this invention is described as Examples 4-8. In this table, Comparative Examples 1 to 4 in the conventional case are also shown. A specific evaluation method of the image unevenness evaluation index A, the image density ID, and the ghost in the drawing will be described later.

  As shown in FIG. 5, an image unevenness evaluation index A was obtained by outputting up to 10,000 images with a printing rate of 6% under the conditions in the first to third embodiments. The image unevenness evaluation index A indicates that the larger the value, the larger the image unevenness. In FIG. 5, the image unevenness evaluation index A exceeding 0.15 is shown in parentheses.

  As shown in FIG. 5, in Examples 1 to 3 according to the present invention, A = 0.115, 0.120, and 0.145 (A ≦ 0.15), respectively. Also in other Examples 4 to 8, the image unevenness evaluation index A was 0.15, and good results were obtained.

  On the other hand, in Comparative Examples 1, 2, and 4, which are conventional examples, the image unevenness evaluation index A exceeds 0.15, and image unevenness can be reduced by satisfying the conditions of Examples 1 to 8 according to the present invention. Was confirmed.

  Further, an image density ID, which is an evaluation index for evaluating the image density, was obtained after printing 1000 sheets. The image density ID indicates that the larger the value, the better the image density. In FIG. 5, image density IDs less than 1.30 are shown in parentheses.

  As shown in FIG. 5, in Examples 1 to 8 according to the present invention, the value of the image density ID is larger than those of Comparative Examples 1 to 3 which are conventional examples. Thus, it was confirmed that by satisfying the conditions of Examples 1 to 8 according to the present invention, the image density after repeating printing can be maintained better than the image forming apparatus according to the conventional example.

  Further, visual evaluation was performed on a ghost in which a part of the developed image appears as an afterimage during the next development. As a result, as shown in FIG. 5, extremely good results were obtained in Examples 1, 2, 4 to 6 according to the present invention, and good results were also obtained for Examples 3, 7, and 8. On the other hand, in Comparative Examples 1, 2, and 4, which are conventional examples, good results were not obtained for ghosts.

  As described above, in Examples 1 to 8 according to the present invention, it was confirmed that generation of image unevenness and ghost images was stably reduced over a long period of time, and a high-quality image was obtained. On the other hand, in Comparative Examples 1 to 4, which are conventional examples, there is one that satisfies all the evaluation conditions that the image unevenness evaluation index A is 0.15 or less, the image density ID is 1.30 or more, and the ghost is excellent or good. Absent.

  In each of Examples 1 to 8 according to the present invention, the toner thin layer thickness is in the range of 6 μm to 15 μm (in the conventional case shown in Comparative Examples 1 to 4, the toner thin layer thickness is 15 μm or more). Become). Here, the toner layer thickness should be as thin as possible. By reducing the toner layer thickness, all the toner on the developing roller 245 is used for development (as much as possible), and the development residual toner on the developing roller 245 returns to the photosensitive drum 21. It is possible to prevent the occurrence of defects such as image density defects and fog.

In each case, Examples 1 to 8 according to the present invention in FIG. 5 satisfy the condition that the half-value width difference of the toner charge amount distribution is 0.8 (10 ⁻¹⁰ C / m) or less. The condition where the peak position difference is 1.0 (10 ⁻¹⁰ C / m) or less is also satisfied.

  This is because, as shown in FIGS. 3 and 4, the difference between the toner charge amount distribution in the toner thin layer on the developing roller 245 and the toner charge amount distribution in the two-component developer on the magnetic roller 244 is different. It shows that (deviation) is small. In the case of FIG. 3 where the difference in toner charge number distribution is small and the degree of coincidence is high, the case of FIG. 3 is preferable to the case of FIG.

  As described above, if the half-value width difference is small, or if the half-value width difference and the peak position difference are small, the toner charge amount distribution (301, 401) in the toner thin layer on the developing roller 245 and the magnetic The difference from the toner charge amount distribution (311, 411) in the two-component developer on the roller 244 is also small, and between the magnetic roller 244 and the developing roller 245 (or between the developing roller 245 and the photosensitive drum 21). Since the selectivity of toner movement can be reduced, it is possible to prevent image density defects (unevenness) and the like.

  This can be paraphrased as follows. That is, the fact that the full width at half maximum and the peak position of each toner charge quantity distribution does not shift means that there is no selective movement of toner that is easily charged and easily moved, and between the developing roller and the magnetic roller or This shows that the so-called charge-up of the toner on the developing roller is also suppressed.

  In the second and third embodiments (Examples 2 and 3 in the table), Duty (slv) and Duty (mag) are 50, 65%, 35, and 70%, respectively (Example 1). In this case, it is desirable that the duty ratios of 30 and 70% are conventional values), the duty (slv) is 35 to 65%, and the duty (mag) is 40 to 70%. However, the condition of 100 (%) − Duty (mag) <Duty (slv) is satisfied.

  Thus, by setting the Duty (slv) of the AC bias voltage applied to the developing roller 245 to 35 to 65%, the remaining amount of toner in the portion corresponding to the latent image of the toner thin layer formed on the developing roller 245 It is possible to develop to such an extent that there is almost no. Further, the duty (mag) of the AC bias voltage applied to the magnetic roller 244 is set to 40 to 70%, and the peak-to-peak voltage Vpp of the AC bias voltage is in a range where no leakage occurs between the developing roller 245 and the magnetic roller 244. In this case, the toner can be selectively moved to the developing roller 245 and the undeveloped toner on the developing roller 245 can be sufficiently pulled back, so that the required image density can be maintained over a long period of time. Therefore, the occurrence of image unevenness can be suppressed, and the occurrence of a ghost phenomenon can also be suppressed.

  The image unevenness evaluation index A was obtained from the luminance Pi at a plurality of locations on the paper on which the image was formed, using the following formulas (1) to (4). However, the luminance of the solid portion painted in black is Pmax, and the luminance of the blank paper is Pmin. This luminance was measured using a Dot Analyzer DA-6000 manufactured by Oji Scientific Instruments. In the first to third embodiments described above, a sheet is obtained using image data obtained by capturing a halftone image having a halftone dot area ratio (600 dpi) at 3000 dpi using a color scanner ES8500 manufactured by EPSON. An image was formed. The brightness Pi was measured at a plurality of locations on the paper. The Dot Analyzer DA-6000 was used for measuring the brightness Pi.

  Di = Log [(Pmax−Pi) / Pmin] (1)

A = σ _D / Da (4)
However, Pi: luminance, Di: luminance converted value to image density In the calculation of the image unevenness evaluation index A, first, the luminance data is converted into the density by the equation (1). When the density is converted, the relative density of Pi with respect to Pmax (the brightness of the solid portion painted in black) and Pmin (the brightness of the blank paper portion) is obtained, and as the density increases, the unevenness in image density becomes less visible (appears in the brightness Therefore, it is corrected by taking Log.

Next, the average value Da of Di is calculated using the formula (2). Then, the average deviation amount from the average value Da of Di is calculated as an average square deviation (Root Mean Square) σ _D using Equation (3) to calculate a so-called “fre”. Then, an image unevenness evaluation index A is calculated using Expression (4).

Further, the frequency of the AC voltage applied to the two-component developer (magnetic roller 244) by the first bias power source 246 is f (mag), and the frequency of the AC bias voltage applied to the developing roller 245 by the second bias power source 247. Where f (slv) is f (mag)> f (slv) and f (mag) is 2.5 kHz or more, the half-value width difference is 0.8 (10 ⁻¹⁰ C / m). Hereinafter, an effect of further reducing the peak position difference to 1.0 (10 ⁻¹⁰ C / m) or less is obtained.

  The image density ID was obtained as follows. First, the evaluation image shown in FIG. 10 was output. FIG. 10 shows an example of an evaluation image 120 for evaluating the image density ID. This evaluation image 120 is an image having five solid portions 121 as shown in FIG.

  Next, the image density IDs of the five solid portions 121 were measured, and the average value thereof was set as the image density ID of the main evaluation, and evaluation was performed according to the following criteria. The image density ID was measured using a Gretag Macbeth portable reflection densitometer RD-19 manufactured by Sakata Inx Engineering Co., Ltd.

  The ghost was evaluated as follows. First, the evaluation image shown in FIG. FIG. 11A and FIG. 11B are drawings for explaining ghost evaluation. FIG. 11A shows an example of an evaluation image 130 for ghost evaluation, and FIG. 11B shows an example of an output image 135 when a ghost occurs. As shown in FIG. 11A, the evaluation image 130 includes three solid portions 131 having a halftone dot area ratio of 100% and halftones having a halftone dot area ratio of 10% or 25% on the rear end side in the printing direction. It is an image including the image 132.

  Next, it is visually determined whether or not a ghost (afterimage) 133 as shown in FIG. 11B is formed in the halftone image 132 in the output image that has been output, and evaluated according to the following criteria: did.

    Excellent: Even if the halftone image 132 is a 10% halftone image, the ghost 133 is not confirmed.

    Good: A ghost 133 is slightly confirmed when the halftone image 132 is a 10% halftone image, but a ghost 133 is not confirmed when the halftone image 132 is a 25% halftone image.

    Impossible: Even if the halftone image 132 is a 25% halftone image, the ghost 133 is clearly confirmed.

As described above, according to the image forming apparatus (printer 1) of the present invention, the image forming apparatus is disposed so as to be opposed to the latent image carrier on which the electrostatic latent image is carried, and the electrostatic latent image. A toner carrying member that carries the toner conveyed to the development area to develop the toner, and the toner carrying member disposed opposite to the toner carrying member, carrying the two-component developer and supplying the toner in the two-component developer to the toner carrier. A developer carrying member and an adjusting unit. The adjusting unit makes the toner layer carried on the toner carrying member have a thickness of 6 μm to 15 μm and is carried on the toner carrying member. A first toner charge amount number distribution that is a number distribution with respect to the toner charge amount, and a second toner charge amount number distribution that is a number distribution with respect to the toner charge amount in the two-component developer carried on the developer carrier. The difference in half width from ⁻¹⁰ C / m) or less.

As described above, since the toner layer thickness is as thin as 6 μm to 15 μm, all of the toner on the toner carrier (developing roller) is used for development (as much as possible), or development on the toner carrier is performed. It is possible to prevent the residual toner from returning to the latent image carrier (photosensitive drum). Further, since the half-value width difference is as small as 0.8 (10 ⁻¹⁰ C / m) or less, the toner charge amount number distribution in the toner thin layer on the toner carrier and the two-component developer on the developer carrier. The difference (deviation) from the toner charge amount distribution in the toner can be reduced (matched), and between the toner carrier and the developer carrier (or between the developer carrier and the latent image carrier). The selectivity of toner movement can be reduced. For these reasons, it is possible to suppress image density defects, fogging, toner scattering, ghost phenomenon, and the like, and to maintain stable performance over a long period of time.

In addition, the adjustment unit causes the half-value width difference between the first and second toner charge amount number distributions to be 0.8 (10 ⁻¹⁰ C / m) or less, and in addition, the first and second toner charge amounts. since the difference between the peak position of the number distribution is to 1.0 (10 -10 C / ^m) or less, and the toner charge number distribution of the toner thin layer on the toner carrying member, a two-component developer on the developer carrying member The difference (deviation) from the toner charge quantity distribution in the developer can be made even smaller (so as to match), and between the toner carrier and the developer carrier (or the developer carrier and the latent image carrier). In this case, it is possible to more reliably reduce the toner transfer selectivity.

In addition, since the adjustment unit is a silicon-modified urethane resin that is coated on the surface of the toner carrier with a predetermined thickness, the adjustment can be performed with a simple configuration (method) in which the toner carrier is coated with the silicon-modified urethane resin. The layer thickness of the toner is 6 μm to 15 μm, and the half width difference is 0.8 (10 ⁻¹⁰ C / m) or less, or the half width difference and the peak position difference are 0.8 (10 ^−10). C / m) or less and 1.0 (10 ⁻¹⁰ C / m) or less can be easily realized.

In addition, the adjustment unit applies an AC bias voltage to the toner carrier with a duty ratio that satisfies a condition of 100 (%) − Duty (mag) <Duty (slv), and an AC bias is applied to the developer carrier. Since the second bias application unit that applies voltage is used, the toner layer thickness by the adjustment unit can be set by a simple configuration (method) in which an AC bias voltage is applied to the toner carrier with a duty ratio that satisfies the condition. 6 μm to 15 μm, and the half-value width difference is 0.8 (10 ⁻¹⁰ C / m) or less, or the half-value width difference and the peak position difference are 0.8 (10 ⁻¹⁰ C / m) or less and 0 ⁽¹⁰ -10 C / m) can be easily realized that below.

  Further, since the second bias application unit is connected in series with the first bias application unit and is electrically connected to the ground via the first bias application unit, the bias voltage applied to the developer carrier is As a result of being able to take a configuration in which the bias voltage applied to the toner carrier is superposed on the bias voltage, the toner carrier and the developer carrier (between the toner carrier and the developer carrier or the developer carrier) In addition, it is possible to easily adjust the AC bias voltage applied between the latent image carrier and the duty ratio.

1 is a schematic configuration diagram of a printer that is an example of an image forming apparatus according to a first embodiment. It is a schematic diagram for demonstrating the structure which applies a bias voltage to the magnetic roller and developing roller in the 1st-3rd embodiment. FIG. 6 is a graph showing the toner charge amount distribution in the developing roller and magnetic roller in the first embodiment. FIG. 10 is a graph showing the toner charge amount distribution in the developing roller and the magnetic roller in the second embodiment. It is the figure which put together the operation result of the printer in the 1st-3rd embodiment as a table | surface. It is a schematic diagram which shows the structure which applies a bias to the conventional magnetic roller and developing roller. (A) is a figure which shows the bias voltage Vslv applied to the conventional developing roller, the bias voltage Vmag applied to the magnetic roller, and the voltage (Vmag−Vslv) between MSs. (B) shows the voltage (Vmag−Vslv) between the bias voltage Vslv, the bias voltage Vmag, and the MS when the sum of the duty ratios of the bias voltage Vslv and the bias voltage Vmag does not reach 100% in the conventional configuration. FIG. It is a wave form diagram of AC voltage (AC1) and (AC2) in a 2nd embodiment. It is a wave form diagram of AC voltage (AC1) and (AC2) in a 3rd embodiment. It is drawing which shows an example of the evaluation image for evaluation of image density ID. (A) is drawing which shows an example of the evaluation image for ghost evaluation. (B) is a drawing showing an example of an output image when a ghost occurs.

Explanation of symbols

1 Printer (image forming device)
2, 2M, 2C, 2Y, 2K Image forming unit 21 Photosensitive drum (latent image carrier)
24 Development Unit 241 Developer Container 242 Stir Mixer 243 Paddle Mixer 244 Magnetic Roller (Developer Carrier)
245 Development roller (toner carrier)
246 First bias power supply (adjustment means)
247 Second bias power supply (adjustment means)
31 Intermediate transfer roller 32 Intermediate belt 36 Secondary transfer roller 4 Paper feed unit 5 Fixing unit 6 Control unit 7 Network I / F unit 8 Operation panel unit G Ground

Claims (6)

An image forming apparatus using a two-component developer containing a toner and a carrier,
A latent image carrier on which an electrostatic latent image is carried;
A toner carrier that is disposed opposite to the latent image carrier and carries the toner conveyed to the development region to develop the electrostatic latent image;
A developer carrier disposed opposite to the toner carrier, carrying the two-component developer, and supplying the toner in the two-component developer to the toner carrier;
While the layer thickness of the toner carried on the toner carrier is 6 μm to 15 μm,
The number distribution of the first toner charge amount number distribution that is the number distribution with respect to the charge amount of the toner carried on the toner carrier, and the number of toner with respect to the charge amount of the toner in the two-component developer carried on the developer carrier. An image forming apparatus comprising: an adjustment unit that sets a difference in half-value width from the second toner charge amount distribution as a distribution to 0.8 (10 ⁻¹⁰ C / m) or less. The adjustment unit is
The image forming apparatus according to claim 1, characterized in that the difference between the peak position of the first and second toner charge number distribution 1.0 (10 -10 C / ^m) or less. The adjustment unit is
3. The image forming apparatus according to claim 1, further comprising a silicon-modified urethane resin coated on the surface of the toner carrier with a predetermined thickness. The adjustment unit is
A first bias applying unit that applies an AC bias voltage having a duty ratio of Duty (slv) to the toner carrier;
A second bias applying unit that applies an AC bias voltage having a duty ratio of Duty (mag) to the developer carrier;
The duty (slv) and the duty (mag) are
The image forming apparatus according to claim 1, wherein the duty ratio is set to satisfy a condition of 100 (%) − Duty (mag) <Duty (slv). If the frequency of the AC bias voltage output from the first bias application unit is f (mag) and the frequency of the AC bias voltage output from the second bias application unit is f (slv), then f (mag)> The image forming apparatus according to claim 4, wherein f (slv) and f (mag) are 2.5 kHz or more. 6. The image according to claim 4, wherein the second bias application unit is connected in series with the first bias application unit and is electrically connected to the ground via the first bias application unit. 7. Forming equipment.