JP2020012964A - Developing device and image forming apparatus - Google Patents

Abstract

To prevent a variation in surface potential of a photoreceptor drum accompanying a change in environment.SOLUTION: An image forming apparatus 100 comprises: a photoreceptor drum 42; a charging roller 44; and a developing roller 431. The photoreceptor drum 42 has an electrostatic latent image formed on its peripheral surface. The charging roller 44 charges the peripheral surface of the photoreceptor drum 42. The developing roller 431 supplies a toner to the electrostatic latent image to form a toner image. The charging roller 42 and photoreceptor drum 44 are brought into contact such that in a nip area formed by the charging roller 44 and photoreceptor drum 42, the ratio of the actual contact area SR to the nip area SN (actual contact area ratio α) becomes a predetermined value or less. The nip area SN is calculated from the product of a nip width NB and a contact length NL. The contact length NL indicates the length in which the charging roller 44 is in contact with the photoreceptor drum 42 in the longitudinal direction of the charging roller 44. The actual contact area SR indicates the area in which the charging roller 44 is in contact with the photoreceptor drum 42.SELECTED DRAWING: Figure 7

Description

  The present invention relates to a developing device and an image forming device.

  In an image forming apparatus, various techniques have been disclosed for suppressing a decrease in image quality due to a change in environment or a change in linear velocity of a photosensitive drum.

  For example, the image forming apparatus described in Patent Literature 1 includes a charging device and a photosensitive drum. The charging device has a charging roller and a suspension device. The charging roller is in contact with the surface of the photosensitive drum, and forms a nip with the photosensitive drum by elastic deformation. The charging roller charges the surface of the photosensitive drum while rotating. The suspension device changes the state of the elastic deformation according to the peripheral speed of the photosensitive drum, thereby changing the area of the surface of the charging roller. The area of the charging roller surface indicates an area contributing to air discharge. Air discharge occurs between the surface of the charging roller and the surface of the photosensitive drum near the nip.

JP 2012-181407 A

  However, in the image forming apparatus described in Patent Document 1, the surface potential of the photosensitive drum fluctuates due to a change in environment. Due to the fluctuation of the surface potential, a density difference occurs in an image formed on a sheet. The “change in environment” includes a change in the temperature of the outside air and a change in the humidity of the outside air.

  The present invention has been made in view of the above problems, and has as its object to provide an image forming apparatus capable of suppressing a change in the surface potential of a photosensitive drum due to a change in environment.

  A developing device according to the present invention includes a photosensitive drum, a charging roller, and a developing roller. An electrostatic latent image is formed on the peripheral surface of the photosensitive drum. The charging roller charges the peripheral surface of the photoconductor drum. The developing roller supplies a toner to the electrostatic latent image to form a toner image. In the nip region between the charging roller and the photosensitive drum, the charging roller is brought into contact with the photosensitive drum such that the ratio of the actual contact area to the nip area is equal to or less than a predetermined value. The nip area is calculated by the product of the nip width and the contact length. The contact length indicates a length in which the charging roller contacts the photosensitive drum in the longitudinal direction of the charging roller. The actual contact area indicates an area where the charging roller and the photosensitive drum are in contact.

  An image forming apparatus according to the present invention includes a photosensitive drum, a charging roller, a developing roller, and a transfer unit. An electrostatic latent image is formed on the peripheral surface of the photosensitive drum. The charging roller charges the peripheral surface of the photoconductor drum. The developing roller supplies a toner to the electrostatic latent image to form a toner image. The transfer unit transfers the toner image to a recording medium. In the nip region between the charging roller and the photosensitive drum, the charging roller is brought into contact with the photosensitive drum such that the ratio of the actual contact area to the nip area is equal to or less than a predetermined value. The nip area is calculated by the product of the nip width and the contact length. The contact length indicates a length in which the charging roller contacts the photosensitive drum in a longitudinal direction of the charging roller. The actual contact area indicates an area where the charging roller and the photosensitive drum are in contact.

  ADVANTAGE OF THE INVENTION According to the developing device and image forming apparatus which concern on this invention, the fluctuation | variation of the surface potential of a photoconductor drum accompanying a change of an environment can be suppressed.

FIG. 1 is a diagram illustrating an example of a configuration of an image forming apparatus according to an embodiment of the present disclosure. FIG. 2 is a diagram illustrating an example of a configuration of an image forming unit according to the embodiment of the present disclosure. It is a figure showing an example of a measuring device of an actual contact area. It is a figure showing an example of a measurement result of an actual contact area. 9 is a graph showing an example of an effect of an actual contact area ratio on a change in dark potential. FIG. 3 is a conceptual diagram illustrating a phenomenon in which internal carriers of a photoconductor layer are drawn out to a charging roller. 5 is a graph illustrating an example of a relationship between an actual contact area ratio and a surface potential difference. 6 is a graph illustrating an example of a relationship between a developing bias voltage and occurrence of fogging. 4 is a graph illustrating an example of a relationship between a surface roughness of a charging roller and charging unevenness. 5 is a graph illustrating an example of a relationship between a linear velocity and a nip width. 4 is a chart showing an example of a relationship between an actual contact area ratio, a surface potential difference, and charging unevenness. 4 is a chart showing an example of a relationship between an actual contact area ratio, a light elimination light amount, and a surface potential difference. 9 is a graph illustrating an example of a relationship between a product of a nip width, an actual contact area ratio, and a light elimination light amount, and a surface potential difference.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings (FIGS. 1 to 13). In the drawings, the same or corresponding portions have the same reference characters allotted, and description thereof will not be repeated.

  First, the configuration of an image forming apparatus 100 according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a diagram illustrating a configuration of the image forming apparatus 100. The image forming apparatus 100 is a color MFP.

  As shown in FIG. 1, the image forming apparatus 100 includes an image forming unit 1, an image reading unit 2, and a document conveying unit 3. The image forming unit 1 forms an image on a sheet P. The image reading unit 2 reads an image formed on the document R and generates image information. The document transport unit 3 transports the document R to the image reading unit 2.

  The image forming unit 1 includes a feeding unit 12, a conveyance unit LP, a toner supply unit 13, an image forming unit 4, a fixing unit 16, and a discharging unit 17. The image forming unit 4 includes a transfer unit 5.

  The feeding section 12 supplies the sheet P to the transport section LP. The transport unit LP transports the sheet P to the discharge unit 17 via the transfer unit 5 and the fixing unit 16. The sheet P corresponds to an example of a “recording medium”.

  A toner container 131, a toner container 132, a toner container 133, and a toner container 134 are mounted on the toner supply unit 13. The toner container 131 stores cyan toner TN1. The toner container 132 stores magenta toner TN2. The toner container 133 stores yellow toner TN3. The toner container 134 stores black toner TN4. In the following description, each of the toner containers 131 to 134 may be collectively referred to as a toner container 130. Further, each of the toners TN1 to TN4 may be collectively referred to as a toner TN. The toner container 130 supplies the toner TN to the image forming unit 4. The image forming unit 4 forms an image on the sheet P. The configuration of the image forming unit 4 will be described later in detail with reference to FIG.

  The transfer unit 5 includes an intermediate transfer belt 54. The image forming unit 4 transfers the cyan, magenta, yellow, and black toner images onto the intermediate transfer belt 54. A plurality of color toner images are superimposed on the intermediate transfer belt 54, and an image is formed on the intermediate transfer belt 54. The transfer unit 5 transfers the image formed on the intermediate transfer belt 54 onto the paper P. As a result, an image is formed on the sheet P.

  The fixing unit 16 heats and pressurizes the sheet P, and fixes an image formed on the sheet P to the sheet P. The discharge unit 17 discharges the sheet P to the outside of the image forming apparatus 100.

  Next, the configuration of the image forming unit 4 according to the embodiment of the present invention will be described with reference to FIGS. FIG. 2 is a diagram illustrating an example of the configuration of the image forming unit 4. As shown in FIG. 2, the image forming unit 4 includes an image forming unit 4c, an image forming unit 4m, an image forming unit 4y, and an image forming unit 4k.

  Each of the image forming unit 4c, the image forming unit 4m, the image forming unit 4y, and the image forming unit 4k includes an exposing unit 41, a photosensitive drum 42, a developing unit 43, a charging roller 44, a static eliminator 45, and a cleaning blade 46. The developing unit 43 has a developing roller 431. The configuration of each of the image forming unit 4c, the image forming unit 4m, the image forming unit 4y, and the image forming unit 4k is substantially the same except for the color of the supplied toner TN. Therefore, in the following description, the configuration of the image forming unit 4c to which the cyan toner TN1 is supplied will be described, and the configurations of the image forming units 4m, 4y, and 4k other than the image forming unit 4c will be described. Description is omitted.

  The image forming unit 4c includes an exposure unit 41c (41), a photoconductor drum 42c (42), a developing unit 43c (43), a charging roller 44c (44), a static eliminator 45c (45), and a cleaning blade 46c (46). .

The charging roller 44c charges the photosensitive drum 42c to a predetermined potential. The electric resistance of the charging roller 44c is, for example, not less than 1 × 10 ⁴ Ω and not more than 1 × 10 ⁹ Ω. The exposure unit 41c irradiates the photosensitive drum 42c with laser light to expose the photosensitive drum 42c to form an electrostatic latent image on the photosensitive drum 42c. The developing unit 43c has a developing roller 431c (431). The developing roller 431c supplies the cyan toner TN1 to the photosensitive drum 42c and develops the electrostatic latent image to form a toner image. Thus, a cyan toner image is formed on the peripheral surface of the photosensitive drum 42c.

  The static eliminator 45c irradiates the photosensitive drum 42c with static elimination light to neutralize the photosensitive drum 42c. The static elimination light is, for example, laser light. The tip (upper end in FIG. 2) of the cleaning blade 46c is in sliding contact with the peripheral surface of the photosensitive drum 42c. The cyan toner TN1 remaining on the peripheral surface of the photosensitive drum 42c is removed by the sliding contact between the peripheral surface of the photosensitive drum 42c and the tip of the cleaning blade 46c.

  The transfer unit 5 transfers the toner image onto the paper P. The transfer unit 5 includes a primary transfer roller 51, a secondary transfer roller 52, a driving roller 53, an intermediate transfer belt 54, a driven roller 55, and a blade 56. The primary transfer roller 51 transfers the cyan, magenta, yellow, and black toner images from the photosensitive drum 42 to the intermediate transfer belt 54. The primary transfer roller 51 includes a primary transfer roller 51c, a primary transfer roller 51m, a primary transfer roller 51y, and a primary transfer roller 51k.

  The drive roller 53 drives the intermediate transfer belt 54. The intermediate transfer belt 54 is an endless belt stretched around the primary transfer roller 51, the driving roller 53, and the driven roller 55. The intermediate transfer belt 54 is driven to rotate counterclockwise by the drive roller 53 as indicated by arrows DR1 and DR2. The driven roller 55 is driven to rotate as the intermediate transfer belt 54 rotates. The blade 56 removes the toner TN remaining on the surface of the intermediate transfer belt 54.

  The secondary transfer roller 52 is pressed by the drive roller 53, and a nip portion NQ is formed between the secondary transfer roller 52 and the drive roller 53. The secondary transfer roller 52 transfers the toner image on the intermediate transfer belt 54 to the sheet P when the sheet P passes through the nip NQ.

  Note that the photosensitive drum 42, the charging roller 44, the developing roller 431, and the static eliminator 45 correspond to an example of a “developing device”.

  Next, the measuring device 6 for measuring the actual contact area SR between the charging roller 44 and the photosensitive drum 42 will be described with reference to FIGS. FIG. 3 is a diagram illustrating an example of the measuring device 6 for the actual contact area SR. The actual contact area SR indicates an area where the charging roller 44 and the photosensitive drum 42 are in contact. Specifically, the actual contact area SR indicates the total area of the area RED where the surface of the charging roller 44 and the surface of the photosensitive drum 42 are actually in contact. The area RED will be described later in detail with reference to FIG.

  As shown in FIG. 3, the measuring device 6 includes a prism 61, a light source 62, a light receiving unit 63, and an electronic balance 64.

  The prism 61 is formed of triangular prism glass. The prism 61 is mounted on the two charging rollers 44. A pressing force F acts on the upper end of the prism 61 downward. The prism 61 guides the light beam LB from the light source 62 to the charging roller 44. The prism 61 guides the light reflected on the lower surface of the prism 61 to the light receiving unit 63.

  The light source 62 emits the light beam LB toward the prism 61. The light source 62 includes, for example, an LED (Light Emitting Diode). That is, the light beam LB is, for example, a laser beam.

  The light receiving section 63 receives light reflected from the lower surface of the prism 61. The light receiving unit 63 includes a lens and a camera. The lens focuses the reflected light on the camera. The camera includes, for example, a CCD (Charge Coupled Device) and detects reflected light.

  In a region RED where the surface of the charging roller 44 is in contact with the surface of the prism 61, the light beam LB is irregularly reflected on the surface of the charging roller 44. Therefore, when the light beam LB is reflected on the area RED, the light receiving unit 63 does not receive the light beam LB. On the other hand, in a region RFD where the surface of the charging roller 44 does not contact the surface of the prism 61, the light beam LB is regularly reflected at the bottom surface of the prism 61. Therefore, when the light beam LB is reflected by the region RFD, the light receiving unit 63 receives the light beam LB. The region RFD will be described later in detail with reference to FIG.

  The electronic balance 64 detects the pressing force F.

  Next, a measurement result of the actual contact area SR will be described with reference to FIGS. FIG. 4 is a diagram illustrating an example of a measurement result of the actual contact area SR. In FIG. 4, the upper diagram shows an example of an image captured by the camera of the light receiving unit 63. The width NB indicates a nip width between the charging roller 44 and the prism 61. The lower diagram in FIG. 4 is an enlarged view of the area AR in the upper diagram.

  As shown in FIG. 4, the area AR includes an area RED and an area RFD. The region RED is shown in black, and the region RFD is shown in white. The region RED indicates a region where the light beam LB does not enter. That is, the region RED indicates a region where the surface of the charging roller 44 and the surface of the prism 61 are in contact. The region RFD indicates a region where the light beam LB is incident. That is, the region RFD indicates a region where the surface of the charging roller 44 and the surface of the prism 61 are not in contact.

  In the nip area between the charging roller 44 and the photosensitive drum 42, the actual contact area SR is obtained by calculating the total area of the area RED.

  The actual contact area SR increases or decreases according to the surface roughness Rz of the charging roller 44. The actual contact area SR increases as the surface roughness Rz decreases. Further, the surface roughness Rz of the charging roller 44 can be adjusted by adjusting at least one of the filler particle diameter and the content of the filler particles contained in the surface layer of the charging roller 44.

  Next, the effect of the actual contact area ratio α on the change in the dark potential DP will be described with reference to FIGS. FIG. 5 is a graph showing an example of the effect of the actual contact area ratio α on the change in the dark potential DP. The actual contact area ratio α indicates a quotient obtained by dividing the actual contact area SR by the nip area SN. The nip area SN is calculated by the product of the nip width NB and the contact length NL. The contact length NL indicates the length of contact of the charging roller 44 with the photosensitive drum 42 in the longitudinal direction of the charging roller 44.

  The horizontal axis in FIG. 5 indicates time T, and the vertical axis in FIG. 5 indicates dark potential DP. A graph G11 indicated by a solid line indicates a change in the dark potential DP when the actual contact area ratio α is small (for example, when the actual contact area ratio α is 0.1). A graph G12 shown by a broken line shows a change in the dark potential DP when the actual contact area ratio α is large (for example, when the actual contact area ratio α is 0.5).

  As shown in FIG. 5, when the actual contact area ratio α is large, the dark decay is greater than when the actual contact area ratio α is small. The dark decay indicates the amount of decay of the dark potential DP. Specifically, as the actual contact area ratio α increases, the dark decay increases. The cause will be described below.

  As the actual contact area ratio α increases, the electric field intensity at the nip between the charging roller 44 and the photosensitive drum 42 increases. In addition, as the electric field intensity at the nip is higher, the amount of the internal carrier INC drawn from the photoconductor layer 421 on the surface of the photoconductor drum 42 increases. Furthermore, the dark decay increases as the amount of internal carrier INC extracted from the photoconductor layer 421 increases. Thus, the dark decay increases as the actual contact area ratio α increases.

  Next, a phenomenon in which the internal carrier INC of the photoconductor layer 421 is drawn out to the charging roller 44 will be described with reference to FIGS. FIG. 6 is a conceptual diagram illustrating a phenomenon in which the internal carrier INC of the photoconductor layer 421 is drawn out to the charging roller 44. The internal carrier INC indicates charged particles existing inside the photoconductor layer 421.

  Since the diameter of the photoconductor drum 42 is larger than the diameter of the charging roller 44, in FIG. 6, the upper surface of the photoconductor layer 421 of the photoconductor drum 42 is illustrated as a plane for convenience.

  The photoconductor layer 421 is B-doped amorphous silicon in the embodiment of the present invention. Extraction of the internal carrier INC is more likely to occur when the photoconductor layer 421 is formed of amorphous silicon than when the photoconductor layer 421 is formed of an organic photoconductor. This is because amorphous silicon has a lower surface resistance than the organic photoreceptor, so that the internal carrier INC is easily drawn out to the surface of the photoreceptor layer 421.

  In particular, when the photoconductor layer 421 is formed of amorphous silicon having a large B-doped amount, the internal carriers INC are more likely to be drawn out. This is because the amount of the internal carrier INC increases due to the B doping.

  As described above with reference to FIGS. 2 to 6, in the embodiment of the present invention, since the photoconductor layer 421 is made of B-doped amorphous silicon, the internal carriers INC are easily extracted. Therefore, the surface potential of the photoconductor layer 421 easily fluctuates due to a change in the environment. The “change in environment” indicates a change in the temperature of the outside air and a change in the humidity of the outside air in the embodiment of the present invention. On the other hand, by reducing the actual contact area ratio α, it is possible to reduce the extraction amount of the internal carrier INC of the photoconductor layer 421. Therefore, it is possible to suppress a change in the surface potential of the photoconductor layer 421 due to a change in the environment. Therefore, the change in the density of the image formed on the paper P can be effectively reduced.

  Next, the relationship between the actual contact area ratio α and the surface potential difference ΔV1 will be described with reference to FIGS. FIG. 7 is a graph showing an example of the relationship between the actual contact area ratio α and the surface potential difference ΔV1. The horizontal axis in FIG. 7 is the actual contact area ratio α, and the vertical axis in FIG. 7 is the surface potential difference ΔV1.

  The surface potential difference ΔV1 indicates a difference between the surface potential of the photoconductor drum 42 in the HH environment and the surface potential of the photoconductor drum 42 in the LL environment. The HH environment indicates an environment in which the temperature of the outside air is higher than the reference temperature and the humidity of the outside air is higher than the reference humidity. Specifically, in the HH environment, for example, the temperature of the outside air is 32.5 ° C., and the humidity of the outside air is 80%. The LL environment indicates an environment in which the temperature of the outside air is lower than the reference temperature and the humidity of the outside air is lower than the reference humidity. In the LL environment, for example, the temperature of the outside air is 10 ° C., and the humidity of the outside air is 10%. The reference temperature is, for example, 25 ° C. The reference humidity is, for example, 30%.

  As shown in the graph G2 of FIG. 7, the larger the actual contact area ratio α, the larger the surface potential difference ΔV1. The surface potential difference ΔV1 can be corrected. However, as the surface potential difference ΔV1 increases, the correction deviation increases, and an appropriate density may not be obtained. Therefore, it is preferable that the surface potential difference ΔV1 is set to, for example, 100 V or less. In order to make the surface potential difference ΔV1 100 V or less, for example, the actual contact area ratio α may be made 60% or less.

  As described above with reference to FIGS. 2 to 7, in the embodiment of the present invention, the surface potential difference ΔV1 can be suppressed to 100 V or less by setting the actual contact area ratio α to 60% or less. Therefore, it is possible to suppress a change in the surface potential of the photosensitive drum 42 due to a change in the environment. Therefore, a change in the density of the image formed on the paper P can be reduced.

  In the embodiment of the present invention, a case where the actual contact area ratio α is set to 60% or less will be described. However, the actual contact area ratio α may be equal to or less than a predetermined value. By reducing the actual contact area SR, it is possible to reduce the extraction amount of the internal carrier INC of the photoconductor layer 421. Therefore, it is possible to suppress a change in the surface potential of the photosensitive drum 42 due to a change in the environment. Therefore, a change in the density of the image formed on the paper P can be reduced.

  Next, the relationship between the developing bias voltage VS and occurrence of fogging (fog) will be described with reference to FIGS. FIG. 8 is a graph showing an example of the relationship between the developing bias voltage VS and occurrence of fogging. The horizontal axis in FIG. 8 indicates the position PS in the width direction of the outer peripheral surface of the photosensitive drum 42. The width direction indicates the axial direction of the photosensitive drum 42. The vertical axis in FIG. 8 indicates the surface potential V of the photosensitive drum 42.

  A graph G3 shown in FIG. 8 shows a change in the surface potential V of the photosensitive drum 42 depending on the position PS in the width direction of the outer peripheral surface of the photosensitive drum 42. As shown in the graph G3, the surface potential V of the photosensitive drum 42 changes according to the position PS in the width direction of the outer peripheral surface of the photosensitive drum 42.

  There is a correlation between the surface roughness Rz of the charging roller 44 and the variation in the surface potential V of the photosensitive drum 42 in the width direction of the outer peripheral surface. Specifically, as the surface roughness Rz of the charging roller 44 increases, the variation in the surface potential V of the photosensitive drum 42 in the width direction of the outer peripheral surface increases. The charging unevenness ΔV2 indicates the magnitude of variation in the width direction of the outer peripheral surface of the surface potential V of the photosensitive drum 42.

  The uneven charging ΔV2 is evaluated as follows. That is, the surface potential V of the photosensitive drum 42 is fixed at the predetermined potential V0. The predetermined potential V0 indicates, for example, 250V. Then, the developing bias voltage VS is increased from the low bias side, and when the fogging SF of the toner TN starts to occur, the potential difference between the developing bias voltage VS and the predetermined potential V0 is measured as charging unevenness ΔV2. The developing bias voltage VS indicates a DC voltage applied between the developing roller 431 and the photosensitive drum 42 shown in FIG. As the charging unevenness ΔV2 increases, the uniform charging property of the surface potential V of the photosensitive drum 42 decreases.

  Next, the relationship between the surface roughness Rz of the charging roller 44 and the charging unevenness ΔV2 will be described with reference to FIGS. FIG. 9 is a graph illustrating an example of the relationship between the surface roughness Rz of the charging roller 44 and the charging unevenness ΔV2. The horizontal axis in FIG. 9 indicates the surface roughness Rz of the charging roller 44. The vertical axis in FIG. 9 indicates the charging unevenness ΔV2. As shown in the graph G4 of FIG. 9, the charging unevenness ΔV2 increases as the surface roughness Rz increases.

  When the photoconductor layer 421 is made of B-doped amorphous silicon, if the charging unevenness ΔV2 exceeds, for example, 20 V, the charging unevenness ΔV2 appears as image density unevenness. Further, when the surface roughness Rz is, for example, 18.0 μm, the charging unevenness ΔV2 becomes 20V. Therefore, in order to avoid the occurrence of image density unevenness, the surface roughness Rz needs to be set to, for example, 18.0 μm or less.

  As described above with reference to FIGS. 2 to 9, in the embodiment of the present invention, as the surface roughness Rz increases, the uniform chargeability decreases. As the uniform chargeability decreases, the charging unevenness ΔV2 increases. As the charging unevenness ΔV2 is larger, the toner TN is more likely to be covered. According to the experimental results, when the surface roughness Rz is, for example, 18.0 μm or less, the charging unevenness ΔV2 becomes, for example, 20 V or less, and no fogging occurs. Therefore, by setting the surface roughness Rz to, for example, 18.0 μm or less, occurrence of fogging can be suppressed.

  In addition, in the embodiment of the present invention, the surface roughness Rz is 18.0 μm or less, but the present invention is not limited to this. It is sufficient that the surface roughness Rz is equal to or less than a predetermined roughness. The value of the predetermined roughness is obtained by an experiment, as described with reference to FIGS.

  Next, the relationship between the linear velocity VL and the nip width NB will be described with reference to FIGS. FIG. 10 is a graph showing an example of the relationship between the linear velocity VL and the nip width NB. The horizontal axis in FIG. 10 indicates the linear velocity VL (mm / sec), and the vertical axis in FIG. 10 indicates the nip width NB (μm). The linear velocity VL indicates the velocity of the peripheral surface of the photosensitive drum 42 during image formation.

At point MP shown in FIG. 10, a normal image was formed. At the point MPN shown in FIG. 10, it was confirmed that the charge supply amount from the charging roller 44 to the photosensitive drum 42 was insufficient. Therefore, in order to form a normal image, it is necessary to satisfy the following expression (1) as shown in a graph G5 in FIG.
NB ≧ 1.0 × VL + 200 (1)

  As described above with reference to FIG. 2 to FIG. 10, it has been experimentally confirmed that in the embodiment of the present invention, if Expression (1) is satisfied, the shortage of the charge supply amount can be suppressed. Therefore, it is possible to suppress a change in the surface potential of the photosensitive drum 42 due to a change in the linear velocity VL. Therefore, a change in the density of the image formed on the paper P can be reduced.

In the embodiment of the present invention, the condition for forming a good image is represented by Expression (1), but the present invention is not limited to this. The following equation (2) may be satisfied.
NB ≧ C1 × VL + C2 (2)
However, each of the constant C1 and the constant C2 is set in advance by, for example, an experiment. In the embodiment of the present invention, for example, the constant C1 is 1.0 and the constant C2 is 200.

<Example 1>
Next, the relationship between the actual contact area ratio α, the surface potential difference ΔV1, and the charging unevenness ΔV2 will be described with reference to FIGS. FIG. 11 is a chart showing an example of the relationship between the actual contact area ratio α, the surface potential difference ΔV1, and the charging unevenness ΔV2. FIG. 11 shows the results of the experiment. The experimental conditions are shown below.

<Experiment conditions>
As the image forming apparatus 100, TASKalfa406ci (product name) manufactured by Kyocera Document Solutions Inc. was used. The photoconductor layer 421 was made of amorphous silicon. The linear velocity VL of the photosensitive drum 42 was 250 mm / sec. The diameter of the charging roller 44 was 12 mm, and the electric resistance of the charging roller 44 was 1.0 × 10 ⁵ Ω. The nip width NB was 470 μm.

<Evaluation items>
The surface potential difference ΔV1 was measured in the HH environment and the LL environment. In the HH environment, the temperature of the outside air was 32.5 ° C., and the humidity of the outside air was 80%. In the LL environment, the temperature of the outside air was 10 ° C., and the humidity of the outside air was 10%. Further, the charging unevenness ΔV2 was measured in an RR environment. In the RR environment, the temperature of the outside air was 25 ° C., and the humidity of the outside air was 30%.

<Experimental results>
As shown in FIG. 11, the experiment was performed at ten levels from level A to level J. At the level, the surface roughness Rz of the charging roller 44 was 1.0 μm, and the surface roughness Rz increased from the level A to the level J. At level G, the surface roughness Rz was 17.8 μm, at level H, the surface roughness Rz was 18.4 μm, and at level J, the surface roughness Rz was 28.5 μm.

  The actual contact area ratio α decreased from the level A toward the level J. For example, at the level A, the actual contact area ratio α is 98%, at the level B, the actual contact area ratio α is 72%, and at the level C, the actual contact area ratio α is 58%. .

  The surface potential difference ΔV1 decreased from the level A toward the level J. For example, at the level A, the surface potential difference ΔV1 was 114 V, at the level B, the surface potential difference ΔV1 was 108 V, and at the level C, the surface potential difference ΔV1 was 97 V.

  The charging unevenness ΔV2 increased from the level A to the level J. For example, at the level G, the charging unevenness ΔV2 was 15 V, at the level H, the charging unevenness ΔV2 was 25 V, at the level I, the charging unevenness ΔV2 was 35 V, and at the level J, the charging unevenness ΔV2 was 70 V. .

<Evaluation results>
At the levels A and B, the surface roughness Rz was 18 μm or less, and the charging unevenness ΔV2 was 20 V or less, so that the uniform chargeability was good. On the other hand, the actual contact area ratio α was 60% or more, and the surface potential difference ΔV1 was 100 V or more. Therefore, the evaluation of the level A and the level B was poor. In FIG. 11, a defect is indicated by a cross.

  At levels H, I and J, the actual contact area ratio α was 60% or less, and the surface potential difference ΔV1 was suppressed to 100 V or less. On the other hand, since the surface roughness Rz was larger than 18 μm, the charging unevenness ΔV2 was larger than 20 V, and the uniform chargeability was deteriorated. Therefore, the evaluation of the level H, the level I, and the level J was poor.

  From level C to level G, the actual contact area ratio α was 60% or less, and the surface potential difference ΔV1 was suppressed to 100 V or less. Further, the surface roughness Rz was 18 μm or less, and the charging unevenness ΔV2 was 20 V or less. Therefore, the evaluation of the levels C to G was good. In FIG. 11, good is indicated by a mark.

  As described above with reference to FIGS. 2 to 11, in the embodiment of the present invention, when the surface roughness Rz is 18.0 μm or less, the charging unevenness ΔV2 becomes 20 V or less, and no fogging occurs. found. Further, it has been found that by setting the actual contact area ratio α to 60% or less, the surface potential difference ΔV1 can be suppressed to 100 V or less, and the fluctuation of the surface potential of the photosensitive drum 42 due to a change in environment can be suppressed.

<Example 2>
Next, with reference to FIGS. 2 to 13, a relationship between the actual contact area ratio α and the charge removal light amount Q and the surface potential difference ΔV1 will be described. FIG. 12 is a chart showing an example of the relationship between the actual contact area ratio α and the charge removal light amount Q and the surface potential difference ΔV1. FIG. 12 shows the results of the experiment. The experimental conditions are shown below.

<Experiment conditions>
As the image forming apparatus 100, TASKalfa406ci (product name) manufactured by Kyocera Document Solutions Inc. was used. The photoconductor layer 421 was made of amorphous silicon. The linear velocity VL of the photosensitive drum 42 was 250 mm / sec. The diameter of the charging roller 44 was 12 mm, and the electric resistance of the charging roller 44 was 1.0 × 10 ⁵ Ω.

  As the light source of the static eliminator 45, 16 red LED chips were arranged at an interval of 15 mm along the axial direction of the photosensitive drum. The wavelength of the light emitted from the red LED chip was 780 nm.

<Evaluation items>
The surface potential difference ΔV1 was measured by changing the amount of static elimination light Q between 1 μJ / cm ^{2 and} 10 μJ / cm ² between the HH environment and the LL environment. In the HH environment, the temperature of the outside air was 32.5 ° C., and the humidity of the outside air was 80%. In the LL environment, the temperature of the outside air was 10 ° C., and the humidity of the outside air was 10%.

<Experimental results>
As shown in FIG. 12, the experiment was performed at five levels from level K to level O. For example, at the level K, the nip width NB was 674 μm, the actual contact area ratio α was 0.187, and the charge removal light amount Q was 1 μJ / cm ² . The product of the nip width NB, the actual contact area ratio α, and the light elimination light amount Q was about 126. At the level K, the surface potential difference ΔV1 was 88V.

For example, at the level M, the nip width NB was 586 μm, the actual contact area ratio α was 0.257, and the light elimination light amount Q was 5 μJ / cm ² . The product of the nip width NB, the actual contact area ratio α, and the light elimination light amount Q was about 753. At the level M, the surface potential difference ΔV1 was 95V.

For example, at the level O, the nip width NB was 586 μm, the actual contact area ratio α was 0.257, and the light elimination light amount Q was 10 μJ / cm ² . The product of the nip width NB, the actual contact area ratio α, and the charge removal light amount Q was about 1506. At the level O, the surface potential difference ΔV1 was 115V.

<Evaluation results>
At the levels K, L and M, the surface potential difference ΔV1 was suppressed to 100 V or less. Therefore, the evaluation of the level K, the level L, and the level M was good. In FIG. 12, good marks are indicated by ○.

  At levels N and O, the surface potential difference ΔV1 was greater than 100V. Therefore, the evaluation of the level N and the level O was poor. In FIG. 12, a failure is indicated by a cross.

  FIG. 13 is a graph showing an example of the relationship between the product of the nip width NB, the actual contact area ratio α, and the charge removal light amount Q, and the surface potential difference ΔV1. The horizontal axis of FIG. 13 shows the product of the nip width NB, the actual contact area ratio α, and the amount of charge removal Q. The vertical axis in FIG. 13 indicates the surface potential difference ΔV1.

  As shown in the graph G6 of FIG. 13, the surface potential difference ΔV1 increases as the product of the nip width NB, the actual contact area ratio α, and the charge removal light amount Q increases. By setting the product of the nip width NB, the actual contact area ratio α, and the amount of static elimination light Q to 800 or less, the surface potential difference ΔV1 can be made 100 V or less, and good image quality can be obtained in the HH environment and the LL environment. Was completed.

As described above with reference to FIGS. 2 to 13, it has been confirmed that in the embodiment of the present invention, if the following expression (3) is satisfied, the surface potential difference ΔV1 can be suppressed to 100 V or less. Therefore, it has been found that the fluctuation of the surface potential of the photosensitive drum 42 due to the change of the environment can be suppressed.
NB × α × Q ≦ 800 (3)

In the embodiment of the present invention, the condition for forming a normal image is represented by Expression (3), but the present invention is not limited to this. The following equation (4) should be satisfied.
NB × α × Q ≦ C3 (4)
However, the constant C3 is set in advance by, for example, an experiment. In the embodiment of the present invention, for example, the constant C3 is 800.

  The embodiment of the invention has been described with reference to the drawings. However, the present invention is not limited to the above embodiment, and can be implemented in various modes without departing from the gist thereof (for example, the following (1) to (3)). The drawings schematically show each component in order to make it easy to understand, and the thickness, length, number, etc. of each component shown in the drawings are different from actual ones for convenience of drawing. There are cases. Further, the shapes, dimensions, and the like of the respective constituent elements shown in the above-described embodiment are merely examples, and are not particularly limited, and various changes can be made without substantially departing from the configuration of the present invention.

  (1) As described with reference to FIG. 1, in the embodiment of the present invention, the image forming apparatus 100 is a color MFP, but the present invention is not limited to this. The image forming apparatus 100 may form an image on the sheet P. The image forming apparatus 100 may be, for example, a color printer. Further, the image forming apparatus 100 may be, for example, a monochrome copying machine.

  (2) As described with reference to FIGS. 2 to 6, the photoconductor layer 421 is B-doped amorphous silicon in the embodiment of the present invention, but the present invention is not limited to this. For example, the photoconductor layer 421 may be an organic photoconductor. Further, for example, the photoconductor layer 421 may be amorphous silicon.

  (3) As described with reference to FIGS. 2 to 7, in the embodiment of the present invention, a case where the actual contact area ratio α is set to 60% or less will be described. Should be fine. By reducing the actual contact area SR, the extraction amount of the internal carrier INC of the photoconductor layer 421 can be reduced. Therefore, it is possible to suppress a change in the surface potential of the photosensitive drum 42 due to a change in the environment. Therefore, a change in image density can be reduced.

  INDUSTRIAL APPLICATION This invention is applicable to the field of a developing device and an image forming apparatus.

REFERENCE SIGNS LIST 100 image forming apparatus 1 image forming unit 4 image forming unit 42, 42c, 42m, 42y, 42k photoconductor drum (part of developing device)
421 Photoconductor layer 43, 43c, 43m, 43y, 43k Developing section 431, 431c, 431m, 431y, 431k Developing roller (part of developing device)
44, 44c, 44m, 44y, 44k Charging roller (part of the developing device)
45, 45c, 45m, 45y, 45k Static eliminator (part of the developing device)
5 Transfer unit 54 Intermediate transfer belt α Actual contact area ratio SR Actual contact area SN Nip area NB Nip width NL Contact length DP Dark potential INC Internal carrier ΔV1 Surface potential difference ΔV2 Charging unevenness Rz Surface roughness VL Linear speed Q Discharge light amount

Claims (7)

A photosensitive drum on which an electrostatic latent image is formed on a peripheral surface;
A charging roller for charging the peripheral surface of the photoconductor drum,
A developing roller that supplies toner to the electrostatic latent image to form a toner image,
In the nip region between the charging roller and the photoconductor drum, the charging roller and the photoconductor drum are brought into contact with each other so that a ratio of an actual contact area to a nip area is equal to or less than a predetermined value,
The nip area is calculated by a product of a nip width and a contact length,
The contact length indicates a length at which the charging roller contacts the photosensitive drum with respect to a longitudinal direction of the charging roller,
The developing device, wherein the actual contact area indicates an area where the charging roller and the photosensitive drum are in contact. The developing device according to claim 1, wherein a nip width NB (μm) between the charging roller and the photosensitive drum satisfies Expression (A).
NB ≧ C1 × VL + C2 Equation (A)
Here, the linear velocity VL (mm / sec) indicates the velocity of the peripheral surface of the photosensitive drum. Each of the constant C1 and the constant C2 is set in advance. The image forming apparatus further includes a static eliminator that irradiates the photosensitive drum with static elimination light,
The developing device according to claim 2, wherein a light amount Q (μJ / cm ² ) of the static elimination light and the nip width NB (μm) satisfy Expression (B).
NB × α × Q ≦ C3 Formula (B)
Here, the actual contact area ratio α indicates a quotient obtained by dividing the actual contact area by the nip area. The constant C3 is set in advance. The photoconductor drum includes a photoconductor,
The developing device according to claim 1, wherein the photoconductor is B-doped amorphous silicon.   The developing device according to claim 1, wherein the predetermined value is 60%. The electric resistance of the charging roller is 1 × 10 ⁴ Ω or more and 1 × 10 ⁹ Ω or less,
The developing device according to claim 1, wherein a surface roughness (Rz) of the charging roller is equal to or less than a predetermined roughness. A photosensitive drum on which an electrostatic latent image is formed on a peripheral surface;
A charging roller for charging the peripheral surface of the photoconductor drum,
A developing roller that supplies toner to the electrostatic latent image to form a toner image;
A transfer unit for transferring the toner image to a recording medium,
In the nip region between the charging roller and the photoconductor drum, the charging roller and the photoconductor drum are brought into contact with each other so that a ratio of an actual contact area to a nip area is equal to or less than a predetermined value,
The nip area is calculated by a product of a nip width and a contact length,
The contact length indicates a length at which the charging roller contacts the photosensitive drum with respect to a longitudinal direction of the charging roller,
The image forming apparatus, wherein the actual contact area indicates an area where the charging roller is in contact with the photosensitive drum.