JP2008261902A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus and an image forming method by which the quantity of an external additive on a photoreceptor can be stabilized without consuming a large quantity of toner and without applying burden on a cleaner. <P>SOLUTION: The image forming apparatus sets a power source 26 such that when image formation is not carried out and at least one of integrated values in a sub-scanning direction in each of areas in the main scanning direction of image data exceeds a predetermined value, a difference between the potential of the center of the area and the surface potential of the photoreceptor has a value shifted in the opposite direction to the normal charging polarity of developer relative to the difference in a latent image portion during image formation and, in addition, a quantity of developer supplied to the photoreceptor 11 is smaller than a quantity of developer supplied to the latent image portion on the photoreceptor 11 during the image formation and such that when none of the integrated values of the image data of the latent image is not greater than the predetermined value, a bias voltage is not applied. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus and an image forming method, such as a copying machine, a printer, a FAX, and the like, and a multifunction machine of these. More specifically, the present invention relates to an image forming apparatus and an image forming method using a developer containing an external additive.

  Conventionally, in an image forming apparatus using a developer containing an external additive, it is known that the external additive remains on the photoreceptor to some extent. This is because even if a cleaning device such as a blade or brush is provided, an external additive much smaller than the toner cannot be completely removed. It is also known that the amount of external additive remaining on the photoreceptor affects the toner adhesion. Therefore, control is usually performed so that a good image can be obtained on the assumption that a certain amount of external additive is present on the photoreceptor.

  However, for example, after continuously printing images whose BW ratio is significantly different depending on the part of the paper in the main scanning direction, there is a considerable difference in the amount of external additive remaining depending on the location of the photoconductor There is. In an image formed in a state where the amount of the external additive on the photoreceptor is locally increased or decreased, image quality deterioration such as density unevenness or voids may occur. In particular, it has been found that such image degradation is likely to occur in another image formed immediately after the same image is repeatedly formed.

Therefore, conventionally, when the amount of the external additive on the photoconductor greatly decreases or becomes uneven, a relatively large amount of toner is discarded on the photoconductor. This stabilized the amount of external additive on the photoreceptor. For example, in the image forming apparatus described in Patent Document 1, if it is determined that an image with a low printing ratio has been printed, the forced toner consumption is executed. Then, toner is adhered to, for example, an area between sheets on the photosensitive member.
JP 2003-263027 A

  However, in the image forming apparatus described in Patent Document 1, a large amount of toner is used and discarded due to the forced toner consumption. Therefore, there is a problem that the amount of toner consumption increases. Furthermore, there is a problem that a burden is imposed on the cleaner device for cleaning the toner discarded on the photosensitive member.

  The present invention has been made to solve the problems of the conventional image forming apparatus described above. That is, an object of the present invention is to provide an image forming apparatus and an image forming method capable of stabilizing the amount of the external additive on the photosensitive member without consuming a large amount of toner and without placing a burden on the cleaner device. Is to provide.

  The image forming apparatus of the present invention, which has been made for the purpose of solving this problem, has an image carrier, a latent image forming unit for forming a latent image on the image carrier based on image data, and a charged developer. The image forming apparatus includes a developer carrying body that is conveyed toward a developing region facing the image carrying body through a gap, and a bias power supply unit that applies a bias voltage including an AC component to the developer carrying body. The bias power supply unit sets the bias voltage at the time of image formation to a value corresponding to an amplitude of an AC component equal to or greater than 9 V / μm, and applies a developer to the latent image on the image carrier to develop the first setting. At the time of non-image formation, if at least one of the integrated values in the sub-scanning direction of the image data of the latent image formed by the latent image forming unit for each location in the main scanning direction exceeds a predetermined value Is the amplitude of the AC component of the bias voltage 9 V / μm or more, and the difference between the area center potential of the bias voltage and the surface potential of the image carrier is different from the normal charging polarity of the developer with respect to the difference in the latent image portion during image formation. The value is shifted in the opposite direction, and the second setting is such that the amount of developer supplied to the image carrier is less than the amount of developer supplied to the latent image portion of the image carrier during image formation. The bias voltage is not applied when there is no image data of the latent image formed by the forming unit that exceeds a predetermined value.

  According to the image forming apparatus of the present invention, the developer carrying member carries and conveys the charged developer, and a bias voltage is applied by the bias power supply unit. Here, the bias voltage at the time of image formation is equal to or greater than the value corresponding to the amplitude of the AC component of 9 V / μm, and the developer is applied to the latent image on the image carrier for development. Therefore, most of the developer placed on the background portion is collected on the developer carrying member, and the developer placed on the image portion on which the latent image is formed adheres to the image carrying member as it is. Here, the developer is assumed that an external additive is added to the toner.

  In addition, if there is a location where the integrated value in the sub-scanning direction for each location in the main scanning direction of the image data of the latent image formed by the latent image forming unit exceeds a predetermined value, the external additive at that location. Is consumed a lot. As a result, the remaining amount of the external additive on the photoreceptor becomes small. If there is even one such location, during non-image formation, the difference between the area center potential of the bias voltage and the surface potential of the image carrier is less than the difference in the latent image portion during image formation. The value is shifted in the direction opposite to the normal charging polarity of the developer. Further, the amount of developer supplied to the image carrier is set to be smaller than the amount of developer supplied to the latent image portion of the image carrier during image formation. However, even in this case, the amplitude of the AC component of the bias voltage is not less than a value corresponding to 9 V / μm. As a result, the external additive moves to the photosensitive member side, and the toner remains on the developer carrying member. Accordingly, the external additive is supplied to the photoreceptor.

  In the present invention, the alternating current component is one in which positive and negative voltages are alternately repeated regardless of the waveform. For example, a sine wave, a rectangular wave, a triangular wave, and the like are included. In the present invention, when the external additive is moved during non-image formation, the supply amount of the developer is sufficiently smaller than the supply amount of the developer to the latent image at the time of image formation, and is about 1/100 or less. It becomes.

  Furthermore, in the present invention, it is desirable that the bias power supply unit applies the bias voltage according to the second setting during non-image formation only in the case where a predetermined number or more of the same image is continuously formed. Even if an image in which at least one of the integrated values in the sub-scanning direction for each location in the main scanning direction exceeds a predetermined value, the amount of the external additive is large if it is not continuously formed in a predetermined number or more. There is no effect. Since the amount of external additive is limited, it is not desirable to move the external additive more than necessary. In this way, it is possible to prevent the external additive from being moved more than necessary. The predetermined number may be about 20, for example.

  In the present invention, it is desirable that the predetermined value is 50% of the total number of dots in the sub-scanning direction. In an image in which the integrated value of the image data in the sub-scanning direction is less than 50% of the total number of dots in the sub-scanning direction, there is no possibility that the amount of the external additive becomes non-uniform even if it is repeatedly printed. Thus, by doing so, it is possible to prevent the external additive from being moved more than necessary.

  Further, according to the present invention, it is desirable that the bias power supply unit increase the amplitude of the AC component of the bias voltage during non-image formation compared to during image formation. In this way, the area center potential of the bias voltage can be adjusted to an appropriate value. Furthermore, other conditions may be set.

  Further, in the present invention, it is desirable that the bias power supply unit has a smaller developing-side duty ratio of the AC component of the bias voltage during non-image formation than during image formation. In this way, the area center potential of the bias voltage can be adjusted to an appropriate value. Furthermore, other conditions may be set.

  Further, according to the present invention, the bias power supply unit calculates the absolute value of the difference between the DC component of the bias voltage and the surface potential of the image carrier during non-image formation, and calculates the absolute value of the DC component of the bias voltage during image formation and the latent image carrier. It is desirable to make it smaller than the absolute value of the difference from the surface potential of the image portion. In this way, the area center potential of the bias voltage can be adjusted to an appropriate value. Furthermore, other conditions may be set.

  Furthermore, in the present invention, the latent image forming unit charges the surface of the image carrier at a surface potential different from that at the time of image formation during non-image formation, so that the DC component of the bias voltage and the surface potential of the image carrier are The absolute value of the difference may be made smaller than the absolute value of the difference between the DC component of the bias voltage at the time of image formation and the surface potential of the latent image portion of the image carrier. Even in this case, the relationship between the area center potential of the bias voltage and the surface potential of the image carrier can be easily adjusted so as to fall within an appropriate range.

Furthermore, in the present invention, the bias power supply unit sets the DC component of the bias voltage to a voltage different from that at the time of image formation during non-image formation, thereby reducing the difference between the DC component of the bias voltage and the surface potential of the image carrier. The absolute value may be made smaller than the absolute value of the difference between the DC component of the bias voltage during image formation and the surface potential of the latent image portion of the image carrier. Even in this case, the relationship between the area center potential of the bias voltage and the surface potential of the image carrier can be easily adjusted so as to fall within an appropriate range.
A charged non-magnetic one-component developer is carried on a developer carrying member facing the image carrier through a gap and conveyed toward the developing region, and a bias voltage containing an AC component is applied to the developer carrying member. Apply

  Furthermore, the present invention relates to an image forming method for forming an image by developing a latent image on an image carrier, wherein the bias voltage at the time of image formation is greater than or equal to a value corresponding to an amplitude of an AC component of 9 V / μm. The first setting for developing the latent image on the image bearing member with developer is used. During non-image formation, the image data of the latent image includes the integrated value in the sub-scanning direction for each location in the main scanning direction. When at least one exceeds a predetermined value, the amplitude of the AC component of the bias voltage is not less than a value corresponding to 9 V / μm, and the difference between the area center potential of the bias voltage and the surface potential of the image carrier is , The difference in the latent image portion at the time of image formation is a value shifted in the direction opposite to the normal charging polarity of the developer, and the amount of developer supplied to the image carrier is the image carrier at the time of image formation Less than the amount of developer supplied to the latent image area The second setting is also applied to an image forming method in which the bias voltage is not applied when there is no latent image data that exceeds a predetermined value.

  According to the image forming apparatus and the image forming method of the present invention, it is possible to stabilize the amount of the external additive on the photosensitive member without consuming a large amount of toner and placing a burden on the cleaner device.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for embodying the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, the present invention is applied to an electrophotographic image forming apparatus.

  The image forming apparatus 1 according to this embodiment includes a photoconductor 11, a developing device 12, and a hopper 13 as shown in FIG. Further, around the photoconductor 11, a charging device 14 that charges the surface of the photoconductor 11 to a predetermined potential, an exposure device 15 that forms a latent image by irradiating the surface of the photoconductor 11 with laser light, and a photoconductor A cleaner 16 that collects the developer remaining on 11 is also provided. Although the image forming apparatus 1 in which the hopper 13 is provided separately from the developing device 12 is illustrated here, the present invention can also be applied to a single-use image forming apparatus in which the hopper is integrated.

  As shown in FIG. 1, the developing device 12 includes a developing roller 21, a supply roller 22, a stirring roller 23, a conveying roller 24, and a regulating member 25. Furthermore, a power supply 26 for applying a developing bias voltage to the developing roller 21 is provided. And it has the control part 27 which controls the charging device 14 and the power supply 26. FIG. Further, a conveying member 31 and a stirring member 32 are provided in the hopper 13. A conveyance path 33 for conveying the developer is provided between the hopper 13 and the developing device 12. In this embodiment, as the developer, a negatively chargeable non-magnetic one-component developer including a toner as a colorant and an external additive for assisting the adhesion performance thereof is used. In this embodiment, the developing roller 21 is not in contact with the photoconductor 11 and is arranged in parallel via a predetermined gap.

  The developing device 12 contains an appropriate amount of developer, is used for image formation, and is appropriately stirred by the stirring roller 23. A new developer is accommodated in the hopper 13 and stirred by a rotating stirring member 32 as indicated by an arrow in FIG. When the developer in the developing device 12 is used and decreases, the developer is supplied from the hopper 13 to the developing device 12. At that time, the developer is supplied to the developing device 12 by the transport member 31 through the transport path 33. In the developing device 12, the toner is transported in the depth direction (the axial direction of the photoconductor 11) by the transport roller 24.

  At the time of image formation, as indicated by arrows in FIG. 1, the photoconductor 11, the supply roller 22, and the developing roller 21 are rotated. Then, the developer is supplied to the surface of the developing roller 21 by the supply roller 22. As the developing roller 21 rotates, the supplied developer is regulated by the regulating member 25 and charged.

  On the other hand, the surface of the photoconductor 11 is uniformly negatively charged by the charging device 14 and is partially exposed by the exposure device 15 to form an electrostatic latent image on the surface. In the image forming apparatus 1 according to the present embodiment, a portion that is charged by the charging device 14 and not exposed is a background portion. This background portion is a portion to which toner should not adhere, and corresponds to a blank paper portion in the image. On the other hand, the portion exposed by the exposure device 15 is an image portion. That is, it is a portion to which toner should adhere, and corresponds to a colored portion in the image.

  Thus, the portion of the photoconductor 11 on which the electrostatic latent image is formed is further rotated and then faces the developing device 12. At this time, a developing bias voltage is applied to the developing roller 21 by a power source 26. As this developing bias voltage, a superimposed voltage of a DC component and an AC component is used. Here, a rectangular waveform as shown by a thick line in FIG. 2 is used. A range of the developing roller 21 that faces the developing area of the photoconductor 11 corresponds to the developing area.

In addition, each code | symbol in FIG. 2 represents the following.
GND: Ground (potential 0V)
Vi: Image portion potential (of the surface potential of the photoconductor 11, the potential of the exposed portion)
Vb: background portion potential (of the surface potential of the photoconductor 11, the potential of the unexposed portion)
Vdc: DC component of developing bias voltage Vpp: Peak-to-peak voltage of AC component of developing bias voltage Duty: Ratio of developing side in one wavelength of developing bias voltage Vav: Center potential of developing bias voltage (from Vdc, Vpp, Duty) Calculation)

  At this time, the surface potential of the photoconductor 11 is different between the image portion and the background portion. The surface potential of the background portion is a negative potential (Vb) in a state of being charged by the charging device 14. Further, the surface potential of the image portion is a positive potential (Vi) from the background portion. Here, when a developing bias voltage as shown in FIG. 2 is applied to the developing roller 21, the negatively charged developer receives an electrostatic attractive force by the electric field. A negatively charged developer basically receives an upward force in FIG.

  Here, the developing electric field includes an AC component having an amplitude of 9 V / μm or more, and is set so that the toner can jump over the gap between the photoconductor 11 and the developing roller 21. This amplitude is obtained by dividing the peak-to-peak voltage (Vpp) of the AC component of the developing bias voltage by the gap (closest distance) between the developing roller 21 and the photosensitive member 11. For example, in an image forming apparatus in which the closest distance between the developing roller 21 and the photoconductor 11 is 135 μm, when Vpp = 1300 (V), the amplitude is about 9.6 V / μm. Note that the upper limit value of the amplitude is a value at which no discharge occurs between the developing roller 21 and the photosensitive member 11, and varies depending on the DC component (Vdc) of the developing bias voltage.

  Note that the development bias voltage is not always the same, and is controlled by the control unit 27 in accordance with, for example, the image density setting. For example, the DC component (Vdc) of the development bias voltage, the peak-to-peak voltage (Vpp) of the AC component of the development bias voltage, and the ratio (Duty) of the development side in one wavelength of the development bias voltage are changed according to the conditions. It has been made possible.

Here, examples of the surface potential of the photoconductor 11 and the developing bias voltage during image formation in the image forming apparatus 1 are as follows.
Photoconductor 11: Vi = −50 (V)
Vb = −450 (V)
Development bias voltage: Vdc = −320 (V)
Vpp = 1400 (V)
Duty = 38 (%)
Vav = −152 (V)

  As shown in FIG. 2, the development bias voltage is alternately repeated with a positive voltage and a negative voltage. Therefore, when the developing bias voltage is a negative voltage, the developer moves from the developing roller 21 to the photoconductor 11. At that time, the majority of the moved toner adheres to the image portion of the photoreceptor 11 and develops the electrostatic latent image. When the developing bias voltage is a positive voltage, mainly the toner in the background portion is collected from the photoconductor 11 to the developing roller 21. In this manner, the toner adheres to the image portion of the photoconductor 11 and is developed, and the background portion is recovered without the toner being attached.

  Here, in the image forming apparatus 1 of the present embodiment, the control unit 27 sets the developing bias voltage to be applied according to the content of the image forming instruction. Normally, the development bias voltage as described above is applied at the time of image formation, and the development bias voltage is not applied at the time of non-image formation such as between sheets. On the other hand, when it is determined that the amount of the external additive on the photoconductor 11 is uneven, it is desirable to supply the external additive to the photoconductor 11 using the non-image forming time. Therefore, the control unit 27 first determines whether or not the amount of the external additive on the photoconductor 11 is uneven.

  The non-uniformity in the amount of the external additive on the photoconductor 11 occurs when an image having an image portion at a large rate in the sub-scanning direction is continuously formed. In the experiment of the present inventor, when an image having 50% or more printed dots in the sub-scanning direction is continuously formed at a predetermined location in the main scanning direction, the amount of the external additive is uneven. There has occurred. Therefore, only when image formation that satisfies this condition is performed, a developing bias voltage for supplying an external additive for supplying the external additive to the photoconductor 11 is applied during non-image formation.

  Next, a development bias voltage setting process at the time of non-image formation for determining the setting of the development bias voltage at the time of non-image formation will be described with reference to the flowchart of FIG. This process is executed when the image forming apparatus 1 receives an image formation instruction from the user. Then, the normal developing bias voltage as described above is applied by the power source 26 at the time of image formation. During non-image formation, the developing bias voltage is applied based on the setting of the developing bias voltage determined in this process.

  When this process is started, it is first determined whether or not the received image formation instruction is a continuous formation process of the same image (S101). For example, the instruction is to print 50 sheets of the same image data or to copy 100 sheets of the same document. Here, for example, in the case of an instruction to form 20 or more images, Yes is determined in S101.

  If it is determined that the image is continuously formed (S101: Yes), then, an integrated value in the sub-scanning direction is obtained for each dot in the main scanning direction of the image data (S102). That is, the number of colored dots in the image data is totaled for each predetermined position in the axial direction of the photoconductor 11. Next, it is determined whether any one of the integrated values calculated in S102 exceeds a predetermined value (S103). For example, the predetermined value is 50%.

  That is, if there is at least one colored dot ratio exceeding 50% with respect to the total number of dots in the sub-scanning direction for each location in the main scanning direction, it is determined Yes in S103. For example, it is determined as Yes in an image in which a solid portion continuous in the sub-scanning direction is included in a part of the main scanning direction. This is because in such an image, unevenness tends to occur in the amount of the external additive on the photoconductor 11. Here, not only continuous solid images, but all images including a predetermined number or more of colored dots at the same position in the main scanning direction are targets. Even if it is a colored part.

  If it is determined Yes in S103, the amount of the external additive on the photoreceptor 11 may be uneven. Therefore, in this case, it is desirable to apply a developing bias voltage for supplying an external additive during non-image formation. Accordingly, a setting is made to apply a developing bias voltage for supplying an external additive during non-image formation (S104).

  With this setting, the power supply 26 applies a normal developing bias voltage during image formation, and applies a developing bias voltage for supplying an external additive during non-image formation such as between sheets. The developing bias voltage for supplying the external additive will be described later. In this state, image formation is performed, and it is determined whether the designated number of continuous image formations has been completed (S105). If the continuous image formation is not completed, the image formation is continued with the setting.

  On the other hand, if it is determined in S101 that it is not continuous image formation (S101: No), or if it is determined in S103 that all integrated values in the sub-scanning direction are less than or equal to a predetermined value (S103: No). Alternatively, when image formation of the same image is completed (S105: Yes), the normal developing bias voltage is set. That is, it is set not to apply the developing bias voltage during non-image formation (S106). With this setting, a normal development bias voltage is applied during image formation, and no development bias voltage is applied during non-image formation. With this setting, image formation is executed. Above, description of the flowchart of FIG. 3 is complete | finished.

Next, the developing bias voltage for supplying the external additive will be described. The developing bias voltage for supplying the external additive in this embodiment is a superimposed voltage of a direct current component and an alternating current component as in the normal case. Furthermore, the amplitude of the AC component corresponds to 9 V / μm or more. That is, the toner is set to jump over the gap between the photoconductor 11 and the developing roller 21 as in the image formation. Then, one or more of the direct current component Vdc, peak-to-peak voltage Vpp, and Duty are changed during non-image formation as compared with normal image formation. As a result, the area center potential Vav is naturally changed. Each value after change is indicated by adding r to the end of the original notation. That is,
Vdcr: DC component of developing bias voltage when supplying external additive Vppr: Peak-to-peak voltage of AC component of developing bias voltage when supplying external additive Dutyr: Development side in one wavelength of developing bias voltage when supplying external additive Ratio Vavr: the area center potential of the developing bias voltage when supplying the external additive.

  Further, the potential of the photoconductor 11 at this time is set only by the relationship with the developing bias voltage. That is, the value itself may be any value, and therefore exposure may or may not be performed after charging by the charging device 14. Hereinafter, the surface potential of the photoconductor 11 in this state is expressed as Vpc. Vpc is not necessarily equal to Vb or Vi at the time of image formation, and is set according to a predetermined condition.

In the external additive supply development of this embodiment, the difference between the area center potential Vavr of the bias voltage and the surface potential Vpc of the image carrier is more positive than the difference between the surface potential Vi of the latent image portion during image formation ( The reverse of the charging polarity of the developer). In the example of the negatively chargeable developer, the relationship represented by the following formula 1 is satisfied.
Vavr−Vpc> Vav−Vi (Formula 1)
Further, the developer supply amount to the photoconductor 11 at this time is set to be smaller than the developer supply amount to the latent image portion of the photoconductor 11 at the time of image formation.

In order to satisfy such a relationship, it is assumed that the developing bias voltage for supplying the external additive satisfies, for example, any one of the following conditions as compared with the developing bias voltage at the normal time.
1. The peak-to-peak voltage Vppr of the AC component is made larger than that during image formation.
2. The duty ratio is made smaller than that during image formation.
3. The absolute value of the difference between the DC component Vdcr and the surface potential Vpc of the photoconductor 11 is made smaller than the absolute value of the difference between the DC component Vdc and the surface potential Vi of the image portion of the photoconductor 11 during image formation.

  Alternatively, the area center potential Vavr may satisfy Formula 1 by combining one or more of the above 1 to 3 and combining a plurality of conditions. In this case, a plurality of conditions different from the above 1 to 3 may be added. In order to satisfy the above condition 3, the DC component of the developing bias voltage may be changed, or the surface potential of the photoconductor 11 may be changed. Or you may change both.

  By applying such a developing bias voltage, the external additive is separated from the toner and supplied to the photoreceptor 11. The external additive is adhered to the toner by a mechanical action. This adhesion force is not so great, and when the toner reciprocates between the photosensitive member 11 and the developing roller 21, it often comes off due to an external force due to collision of the toner with the photosensitive member 11 or the like. The charge amount of the external additive itself is large, and the external additive separated from the toner and alone becomes attached to the photoconductor 11 by an electrical action. This adhesion force is relatively large and is hardly recovered even by the developing bias voltage recovery side.

  Then, at a place where the supply of the external additive is excessive, it is scraped off by the cleaner 16. As a result, the amount of the external additive on the photoconductor 11 becomes uniform with relatively no unevenness. On the other hand, most of the toner is recovered to the developing roller 21 by the recovery side of the developing bias voltage. Therefore, it remains on the developing roller 21 and is not wasted.

  Next, an example of the image forming apparatus 1 according to this embodiment will be described. The present inventor conducted the following experiment in order to confirm that by applying the developing bias voltage for supplying the external additive at the time of non-image formation, it is possible to prevent the image quality from being deteriorated such as a void in the image or density unevenness. . The testing machine used was a magiccolor 2400 manufactured by konicaminolta. Here, the diameter of the photoconductor 11 is φ30 (mm), the diameter of the developing roller 21 is φ16 (mm), and the closest distance between the photoconductor 11 and the developing roller 21 is 135 (μm).

The experiment was performed according to the following procedure.
1) 50 sheets of white paper were developed.
2) One hundred images of five types of patterns A to E shown in FIG. 4 were continuously printed.
3) Vertical line images were printed and checked for the presence of voids.
4) A 2 on 2 off halftone image was printed and examined for the presence or absence of uneven image density.
Note that the order of 3) and 4) may be reversed.
Further, when printing each pattern in 2), the experiment was conducted for two ways of applying a development bias voltage: “with development bias for supplying external additive” and “without development bias for supplying external additive”. It was. That is, an experiment was performed on a total of 10 images with 5 types of images × 2 types of biases.

  Here, “with development bias for supplying external additive” means that a normal development bias voltage (Vdc, Vpp, Duty, Vav) is applied during image formation, and development for supplying external additive during non-image formation between sheets. A bias voltage (Vdcr, Vppr, Dutyr, Vavr, photoconductor potential Vpc) is applied. “No external additive supply development bias” means that a normal development bias voltage (Vdc, Vpp, Duty, Vav) is applied during image formation, and no development bias voltage is applied during non-image formation between sheets. It is.

In this experiment, the following was used as a developing bias voltage for supplying external additives.
Example 1: Vpc = -50 (V)
Vdcr = −320 (V)
Vppr = 1600 (V)
Dutyr = 25 (%)
Vavr = 80 (V)
However, the surface potential of the photoconductor 11 was fully exposed in the development area with a background portion of -450V and an image portion of -50V.

The images of patterns A to E shown in FIG. 4 are as follows. In addition, this pattern is passed in the horizontal direction in the figure. That is, the sheet passing direction is the left-right direction in the figure, and this direction is the sub-scanning direction. The vertical direction in the figure is the main scanning direction.
A: Black solid on the whole surface B: Black solid on the upper half of the figure (50% of the area in the main scanning direction is black solid)
C: Black band extending in the sheet passing direction over the entire paper surface, 50% of the area in the main scanning direction is black solid D: C, and the area occupied by the black solid in the paper passing direction is 50%. E: The area occupied by the black solid in the paper direction is 40%.

Evaluation was performed visually for both voids and uneven density. The results are shown in Table 1 below. The evaluation criteria were as follows for both void and density unevenness.
○ If there is no void or density unevenness in the entire development area
If only a small part is missing or uneven density is observed, it is △ if the degree is very slight
If some parts are missing or uneven density is found ×

  In both cases where a developing bias voltage for supplying an external additive was applied, neither voids nor density unevenness occurred. On the other hand, in the case where the developing bias voltage for supplying the external additive was not applied, there was a case where the image pattern was lost or density unevenness occurred. In the solid pattern A, a void occurred. In patterns B and C of 50% in the main scanning direction and 100% in the sub-scanning direction, a slight void occurred and density unevenness occurred. In the pattern D of 50% in the main scanning direction and 50% in the sub-scanning direction, no void occurred, but density unevenness occurred. On the other hand, in the pattern E of 50% in the main scanning direction and 40% in the sub-scanning direction, neither omission nor density unevenness occurred.

Further, as Example 2, the same experiment was conducted with the following developing bias voltage for supplying the external additive. Conditions other than the developing bias voltage were the same as those in Example 1 above. As a result, it was confirmed that the same result as in Table 1 of Example 1 was obtained.
Example 2: Vpc = -50 (V)
Vdcr = −320 (V)
Vppr = 1300 (V)
Dutyr = 25 (%)
Vavr = 5 (V)

  From this, it can be confirmed that in the image forming apparatus of this embodiment, it is sufficient to apply the developing bias voltage for supplying the external additive only when there is a colored portion of 50% or more at the same position in the main scanning direction. It was. Since the content of the external additive is determined, it is not preferable to always perform the external additive supply process. Only when an instruction to continuously print a predetermined number or more of images having a predetermined number or more of colored portions at the same position in the main scanning direction is received, a developing bias voltage for supplying an external additive may be applied. In other cases, it is assumed that only a normal development bias voltage is applied during image formation, and no development bias voltage is applied during non-image formation.

  As described above in detail, according to the image forming apparatus 1 of the present embodiment, the image data is checked only when continuous formation of the same image is instructed. If the image data includes a portion where the total number of colored dots in the sub-scanning direction exceeds a predetermined value, the developing bias voltage for supplying the external additive is set. That is, a developing bias voltage for supplying an external additive is applied during non-image formation. When this developing bias voltage is applied, only the developer moves to the photoreceptor 11 and the toner hardly moves. Therefore, the amount of the external additive on the photosensitive member can be stabilized without consuming a large amount of toner and without placing a burden on the cleaner device.

In addition, this form is only a mere illustration and does not limit this invention at all. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof.
For example, predetermined values such as the number of continuously formed identical images and the integrated value in the sub-scanning direction vary depending on the model of the image forming apparatus, the size of the formed image, and the like, and are not limited to the above examples. . Further, the control may be performed by a value of (integrated value in the sub-scanning direction × number of continuously formed images of the same image).
The present invention can be applied to an image forming apparatus using any developer as long as the developer includes an external additive. For example, the present invention can be applied to a monochrome or color printer, copier, FAX, or the like.

1 is a cross-sectional view showing a main part of an image forming apparatus according to the present embodiment. It is a voltage waveform diagram which shows the example of a developing bias voltage. It is a flowchart which shows an external additive supply process. It is explanatory drawing which shows the example of the image for an experiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 11 Photoconductor 12 Developing apparatus 14 Charging apparatus 15 Exposure apparatus 21 Developing roller 26 Power supply 27 Control part

Claims (9)

An image carrier, a latent image forming unit that forms a latent image on the image carrier based on image data, and a developing region that carries a charged developer and faces the image carrier via a gap. In the image forming apparatus having a developer carrying body that is conveyed toward and a bias power supply unit that applies a bias voltage including an AC component to the developer carrying body, the bias power supply unit includes:
The bias voltage during image formation is
The amplitude of the AC component is greater than or equal to 9 V / μm,
A first setting for developing the latent image on the image carrier by applying a developer;
During non-image formation,
When at least one of the accumulated values in the sub-scanning direction for each location in the main scanning direction of the image data of the latent image formed by the latent image forming unit exceeds a predetermined value,
The amplitude of the AC component of the bias voltage is greater than or equal to 9 V / μm,
The difference between the area center potential of the bias voltage and the surface potential of the image carrier is a value shifted in the direction opposite to the normal charging polarity of the developer with respect to the difference in the latent image portion during image formation,
A second setting in which the amount of developer supplied to the image carrier is less than the amount of developer supplied to the latent image portion of the image carrier during image formation;
A bias voltage is not applied when there is no image data of the latent image formed by the latent image forming unit that exceeds the predetermined value among the integrated values. Forming equipment. The image forming apparatus according to claim 1, wherein the bias power supply unit is
An image forming apparatus, wherein the bias voltage is applied according to the second setting at the time of non-image formation only in the case where a predetermined number or more of the same image is continuously formed. The image forming apparatus according to claim 1,
The image forming apparatus according to claim 1, wherein the predetermined value is 50% of the total number of dots in the sub-scanning direction. The image forming apparatus according to claim 1, wherein the bias power supply unit is configured to perform non-image formation.
An image forming apparatus, wherein an amplitude of an alternating current component of a bias voltage is made larger than that during image formation. The image forming apparatus according to claim 1, wherein the bias power supply unit is configured to perform non-image formation.
An image forming apparatus characterized in that a developing-side duty ratio of an AC component of a bias voltage is made smaller than that during image formation. The image forming apparatus according to claim 1, wherein the bias power supply unit is configured to perform non-image formation.
The absolute value of the difference between the DC component of the bias voltage and the surface potential of the image carrier is smaller than the absolute value of the difference between the DC component of the bias voltage during image formation and the surface potential of the latent image portion of the image carrier. An image forming apparatus. The image forming apparatus according to claim 4,
The absolute value of the difference between the DC component of the bias voltage and the surface potential of the image carrier during non-image formation is the difference between the DC component of the bias voltage during image formation and the surface potential of the latent image portion of the image carrier. When making it smaller than the absolute value of
The latent image forming unit charges the surface of the image carrier to a surface potential different from that during image formation. The image forming apparatus according to claim 4, wherein the bias power supply unit is configured to perform non-image formation.
The absolute value of the difference between the DC component of the bias voltage and the surface potential of the image carrier is smaller than the absolute value of the difference between the DC component of the bias voltage during image formation and the surface potential of the latent image portion of the image carrier. In doing so,
An image forming apparatus characterized in that a DC component of a bias voltage is set to a voltage different from that at the time of image formation. A charged non-magnetic one-component developer is carried on a developer carrying member opposed to the image carrier through a gap and conveyed toward the developing region, and a bias voltage containing an AC component is applied to the developer carrying member. In an image forming method of forming an image by applying and developing a latent image on an image carrier,
The bias voltage during image formation is
The amplitude of the AC component is greater than or equal to 9 V / μm,
The first setting is to apply a developer to the latent image on the image carrier and develop it,
During non-image formation,
When at least one of the accumulated values in the sub-scanning direction for each location in the main scanning direction of the image data of the latent image exceeds a predetermined value,
The amplitude of the AC component of the bias voltage is greater than or equal to 9 V / μm,
The difference between the area center potential of the bias voltage and the surface potential of the image carrier is a value shifted in the direction opposite to the normal charging polarity of the developer with respect to the difference in the latent image portion during image formation.
A second setting in which the amount of developer supplied to the image carrier is less than the amount of developer supplied to the latent image portion of the image carrier during image formation;
A bias voltage is not applied when there is no latent image data that exceeds the predetermined value among the integrated values.

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