JP4661863B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that charges a cylindrical photoreceptor from a roll-type charger.

  Conventionally, a corotron type or a contact charging type is used as a charging device for charging a cylindrical photosensitive member in an image forming apparatus. Here, in the contact charging type, since discharge is performed in a gap between a roll-type charger called a bias charge roll (BCR) and the photosensitive member, density pitch unevenness of the BCR / photosensitive member is likely to occur due to the gap shape. For this reason, in the conventional image forming apparatus, the charging uniformity is covered by AC charging.

  Further, in AC charging, since there are problems such as high cost of the apparatus, damage to the photoconductor and the cleaning roll, and Deletion, DC charging is also used (see, for example, Patent Documents 1 to 3).

Japanese Patent Laying-Open No. 2005-034760 JP 2001-002164 A JP 7-129055 A

  However, in the case of direct current charging compared to alternating current charging, subtle unevenness of the photoreceptor film thickness, rotational vibration, uneven resistance of the BCR, and variation in the shape directly become uneven charging. For this reason, a BCR and a photoconductor with higher accuracy than AC charging are required. Moreover, due to the sensitivity of charging, there is a problem that even the fluctuation of the BCR and the cleaning roll becomes the pressure fluctuation to the BCR, resulting in charging unevenness (density unevenness). Furthermore, even if the fluctuations are very slight for each of the photoconductor, BCR, and cleaning roll, if the pitches of the fluctuations are synchronized, the fluctuations become larger and charging unevenness (density unevenness) increases, resulting in image quality defects. The problem of becoming.

The present invention has been made to solve such problems. That is, the present invention is arranged so as to face a cylindrical photosensitive member and the outer peripheral surface of the photosensitive member, and the surface is in contact with the surface of the photosensitive member to rotate following the rotation of the photosensitive member. A roll-type charger, and a roll that is arranged to face the outer peripheral surface of the roll-type charger, and whose surface is in contact with the surface of the roll-type charger, rotates following the rotation of the roll-type charger. In the image forming apparatus comprising a mold cleaning member, the least common multiple of the outer peripheral length of the photosensitive member and the outer peripheral length of the roll-type charger, and the least common multiple of the outer peripheral length of the photosensitive member and the outer peripheral length of the roll-type cleaning member , And the least common multiple of the outer peripheral length of the roll-type charger and the outer peripheral length of the roll-type cleaning member is greater than the image forming length of the medium having the maximum size among the media on which images are formed in the image forming apparatus. Is too big As the diameter of the photoreceptor, the diameter of the diameter and the roll-type cleaning member of the roll-type charger is one that has been set, respectively.

In the present invention, the outer peripheral length of the photosensitive member is A, the outer peripheral length of the roll-type charger is B, the outer peripheral length of the roll-type cleaning member is C, and the contact portion between the photosensitive member and the roll-type charger. When the nip width at N is N, on the outer peripheral surface of the photoconductor , the positions where the positions A, B, and C overlap each other are used as the starting points. , ± (N / 2) centered on the position of each A interval, ± (N / 2) centered on the position of each B interval, and ± (N centered on the position of each C interval / 2) is an image forming apparatus in which the diameter of the photoconductor, the diameter of the roll-type charger, and the diameter of the roll-type cleaning member are set so that they do not overlap within the range of the image formation length. .

  In the present invention as described above, the period of fluctuation given to the charging of each of the photosensitive member, the roll type charger, and the roll type cleaning member within the range of the length (image forming length) in which one image is formed by the photosensitive member. Therefore, the increase in charging unevenness (density unevenness) can be suppressed.

  Therefore, according to the present invention, since the cycles of fluctuations applied to charging in each of the photosensitive member, the roll-type charger, and the roll-type cleaning member do not overlap, the fluctuations of the respective members are synchronized and the charging unevenness of the photosensitive member increases. This can be suppressed and uniform image formation can be performed.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

<Schematic configuration of image forming apparatus>
FIG. 1 is a schematic diagram illustrating a configuration example of an image forming apparatus according to the present embodiment. In FIG. 1, a cylindrical photosensitive member 1 serving as an image carrier is provided so as to be rotatable in the direction of an arrow (counterclockwise direction) in the drawing. Around the photoreceptor 1, a roll type charger 2, a laser scanner 3, a developing unit 4, a transfer unit 5, a drum cleaner 6, and a static eliminator 7 are arranged in this order according to the drum rotation direction. A fixing device 8 is disposed on the paper transport path R on the downstream side of the transfer device 5 in the paper transport direction.

  The roll charger 2 is for charging the surface of the photoconductor 1 uniformly. The roll type charger 2 is arranged to face the outer peripheral surface of the photoconductor 1 and is opposite to the photoconductor 1 following the rotation of the photoconductor 1. Rotate to. The roll-type charger 2 is provided with a roll-type cleaning member 20 so as to face the outer peripheral surface, and serves to remove toner attached to the roll-type charger 2.

  The laser scanner 3 forms an electrostatic latent image on the surface of the photoconductor 1 by exposing and scanning the surface of the photoconductor 1 according to a raster image. The developing device 4 develops the electrostatic latent image on the photoreceptor 1 into a toner image by supplying toner to the surface of the photoreceptor 1.

  The transfer unit 5 transfers the toner image on the photoreceptor 1 to the sheet by applying a charge having a polarity opposite to that of the toner from the back side of the sheet conveyed along the conveyance path R. The drum cleaner 6 removes unnecessary toner remaining on the surface of the photoreceptor 1 without being transferred to the paper. The static eliminator 7 removes unnecessary charges remaining on the surface of the photoreceptor 1. The fixing device 8 fixes the toner image on the paper by sandwiching the paper on which the toner image is transferred between the heating roller 8A and the pinch roller 8B.

<Configuration of roll-type charger>
FIG. 2 is a schematic diagram illustrating the configuration of a roll-type charger used in the image forming apparatus of the present embodiment. In this figure, the photosensitive member 1 and the roll type cleaning member 20 facing the roll type charger 2 are also shown.

  That is, the roll-type charger 2 is called a bias charge roll (BCR) and is disposed so as to face the surface of the cylindrical photoreceptor 1. The roll-type charger 2 is made of, for example, a foam member, and its shaft is urged toward the photoreceptor 1 by a spring and fixed at a constant displacement. As a result, the rotation follows the rotation of the photosensitive member 1. At this time, the length along the rotation direction at the contact portion between the roll charger 2 and the photoreceptor 1 is set as the nip width. The nip width is about 0.5 mm to 3 mm.

  A roll-type cleaning member 20 is disposed opposite to the roll charger 2 on the side opposite to the photoreceptor 1. The roll-type cleaning member 20 has a shaft biased toward the roll-type charger 2 by a spring and is fixed at a constant displacement.

  In the roll charger 2 having such a configuration, discharge is performed in a charging gap (gap past the nip position) with the surface of the photoreceptor 1 that is generated when the photoreceptor is rotated, and the photoreceptor 1 rotates. As a result, the surface of the photoreceptor 1 is sequentially charged. The charging gap is about 100 μm to 200 μm. Therefore, uneven charging (density unevenness) of the photosensitive member 1 is likely to occur due to a gap shape in the charging gap (change in gap along the axial direction at the discharge position on the photosensitive member surface).

  Here, as shown in FIG. 3, the roll charger 2 includes a direct contact type with the photoreceptor 1 and an indirect contact type. That is, in the roll charger 2 shown in FIG. 3A, the surface of the photoreceptor 1 and the surface of the roll charger 2 are in direct contact with each other, and the rotation of the photoreceptor 1 is performed as it is in the roll of the roll charger 2. It is what you tell. In the direct contact type, the direct contact portion between the roll charger 2 and the photoreceptor 1 is the nip position.

  On the other hand, in the roll charger 2 shown in FIG. 3B, the surface of the photoreceptor 1 is not in direct contact with the surface of the roll charger 2, and is a follower provided at the roll end of the roll charger 2. The roller 2a is driven in contact with the photosensitive member 1, and the roll of the roll charger 2 is rotated by this rotation. In any configuration, discharge is generated by the gap between the roll surface of the roll-type charger 2 and the surface of the photoreceptor 1, and the surface of the photoreceptor 1 is charged. In the indirect contact type, the contact portion between the driven roller 2a and the photoreceptor 1 in the roll charger 2 is the nip position.

  As described above, in the roll type charger 2, the discharge is performed by the charging gap with the surface of the photoreceptor 1, and therefore, the charging uniformity varies due to the variation in the charging gap. This charging unevenness is influenced not only by the film thickness unevenness or rotational shake of the photosensitive member 1, but also by the resistance unevenness or shape variation of the roll type charger 2, and also by the shape and rotational shake of the roll type cleaning member 20. In addition, since the influences of these individual rolls overlap, larger charging unevenness is caused.

  In the present embodiment, in such a charging configuration using the roll charger 2, the diameters of the photosensitive member 1, the roll charger 2, and the roll cleaning member 20 are set so that the influence of charging unevenness in each roll does not overlap. There is a feature in setting.

  Specifically, the charging unevenness on the surface of the photosensitive member 1 periodically generated by the rotation of the photosensitive member 1, the charging unevenness of the surface of the photosensitive member 1 periodically generated by the rotation of the roll type charger 2, and the roll type The diameter of the photoreceptor 1, the diameter of the roll charger 2, and the roll cleaning member 20 are set so that the uneven charging on the surface of the photoreceptor 1 periodically generated by the rotation of the cleaning member 20 does not overlap within the range of the image forming length. Each diameter is set. The diameter of the roll-type charger 2 referred to here is the diameter of the charging roll that is in contact with the photosensitive member 1 in the direct contact type shown in FIG. 3A, and the diameter in the indirect contact type shown in FIG. The diameter of the driven roller 2a in contact with the body 1 is meant.

  Here, the image forming length means the length of one image forming area along the rotation direction of the photosensitive member 1, and is the length along the transport direction in the largest size of the medium (paper) handled by the image forming apparatus. Say. For example, when the image forming apparatus handles paper sizes such as B4, B5, A3, and A4 defined by JIS (Japanese Industrial Standards), it refers to the length of the paper having the largest length along the transport direction.

  As an example for realizing the prevention of overlapping charging unevenness in each roll, the outer peripheral length of the photosensitive member 1 is A, the outer peripheral length of the roll charger 2 is B, and the outer peripheral length of the roll cleaning member 20 is C, where N is the nip width (see FIG. 2) at the contact portion between the photosensitive member 1 and the roll charger 2, ± (centered around the position of A on the outer peripheral surface of the photosensitive member 1. N / 2), ± (N / 2) centered on the position for each interval of B, and ± (N / 2) centered on the position for each interval of C are within the range of the image forming length. It is supposed not to be.

  That is, there is a region where uneven charging occurs on the outer periphery of each of the photoreceptor 1, the roll charger 2, and the roll cleaning member 20, and the starting point is a position where all the charging unevenness including the nip width overlaps on the photoreceptor 1. As a result, the length on the photosensitive member 1 until each of the rolls rotates and the next charging unevenness overlaps is longer than the image forming length. Thereby, the overlapping of charging unevenness in each roll including the nip width does not occur within the range of the image forming length.

  FIG. 4 is a schematic diagram for explaining the state of charging unevenness. Since the uneven charging is caused by each of the photoconductor, the roll charger, and the roll cleaning member, it periodically occurs with the rotation of the roll, and the maximum width of one charging unevenness (in the end side direction in the figure) is the nip. It is considered to be a width. In the example of charging unevenness shown in FIG. 4, for the sake of easy understanding, charging unevenness equal to the nip width is formed in a strip shape, but actually charging unevenness of various shapes occurs.

  For example, in the example shown in FIG. 4A, the charging unevenness a caused by any one of the photoconductor, the roll-type charger, and the roll-type cleaning member, and the charging unevenness b caused by the other one. Shows a state in which the two adjacently occur. That is, the charging unevenness a and the charging unevenness b do not overlap within the range of the image forming area, and are generated at different positions.

  On the other hand, in the example shown in FIG. 4B, the charging unevenness a caused by any one of the photoreceptor, the roll charger, and the roll cleaning member, and the charging unevenness b caused by the other one, Shows a state where a part overlaps. As described above, when a plurality of charging irregularities a and b generated for different rolls overlap each other within the range of the image forming area, the charging state of the overlapping portions is greatly deteriorated. Therefore, by preventing such overlapping of charging unevenness, it is possible to suppress the deterioration of charging unevenness.

  As described above, in this embodiment, the roll diameters of the photosensitive member, the roll-type charger, and the roll-type cleaning member are set in order to prevent charging unevenness from overlapping within the image forming area. In other words, charging unevenness caused by each roll of the photoconductor, roll-type charger and roll-type cleaning member is generated with periodicity for each roll, so the least common multiple of values based on the diameter of each roll is used for image formation. Each roll diameter is set to be larger than the length. At this time, in consideration of the nip width (contact length along the rotation direction of the contact portion) at the contact portion between the photoconductor and the roll-type charger, charging unevenness occurs even in a part of the nip width. Set each roll diameter so that it does not.

  As a result, the period of each charging unevenness (charging unevenness including the nip width) caused by each roll of the photosensitive member, the roll charger, and the roll cleaning member does not overlap within the range of the image forming length. It will be possible to suppress the deterioration of. Moreover, since it can be handled only by setting the roll diameter, a special mechanism is not provided, so there is no significant cost increase, and each member is not required to have higher accuracy than necessary, and image quality defects can be obtained without any secondary failure. Can improve.

<Specific example>
FIG. 5 is a diagram for explaining a specific example of the diameter of each roll of the photosensitive member, the roll charger, and the roll cleaning member. The unit of the numerical values shown in FIG. 5 is mm, and is a value when the nip width is not taken into account for easy understanding. Further, case 1 and case 2 are cases where the diameter of each roll shown in the present embodiment is not set, and case 3 is a case where the diameter of each roll shown in the present embodiment is set. In case 1, the maximum size of the sheet on which the image is formed is A4, and in case 2 and case 3, the maximum size is A3.

  Here, FIG. 6A shows an example of charging unevenness in the case of each roll diameter indicated by case 1 in FIG. In case 1 of FIG. 5, the diameter of the photosensitive member is 24 mm, and the diameter of the roll-type cleaning member is 8 mm, and these rolls are overlapped with a period of a circumferential length (about 75 mm) with a diameter of 24 mm. This overlaps within the range of the image forming length (297 mm in A4 length) of the maximum size of paper on which image formation is performed (A4 in case 1). Therefore, as shown in FIG. Unevenness occurs.

  FIG. 6B shows an example of charging unevenness in the case of each roll diameter indicated by case 2 in FIG. In case 2 of FIG. 5, the diameter of the photoconductor is 30 mm and the diameter of the roll charger is 12 mm, and these rolls are overlapped with a period of a circumferential length (about 188 mm) with a diameter of 60 mm. This overlaps within the range of the image formation length (420 mm for A3 length) of the maximum size of paper for image formation (A3 for case 2). Therefore, as shown in FIG. Unevenness occurs.

  FIG. 7 shows an example of charging unevenness in the case of each roll diameter indicated by case 3 in FIG. In case 3 shown in FIG. 5, the diameter of the photoreceptor is 30 mm, the diameter of the roll charger is 14 mm, and the diameter of the roll cleaning member is 11 mm. ) Overlap. This is larger than the maximum image formation length (A3 in case 3) for image formation (420 mm in A3 length), and therefore, as shown in FIG. 7, large charging unevenness does not occur within the range of the image formation length. become.

  As described above, in case 1 and case 2 in which each roll diameter shown in this embodiment is not set, large charging unevenness is generated in the charging region, whereas each roll diameter shown in this embodiment is set. In Case 3, it can be seen that there is no large charging unevenness in the charging region. In other words, in case 1 and case 2, charging unevenness is overlapped by a plurality of rolls and charging unevenness is increased. In case 3, charging unevenness is not overlapped by a plurality of rolls, and only charging unevenness of each roll alone is generated. Only appears, and an increase in charging unevenness can be suppressed.

  In the above specific example, the example in which the image forming length is set to the maximum size of the paper has been described. However, the length of the range in which an image is actually formed in the maximum size of the paper is set as the image forming length, and the range within the range is set. The charging unevenness may be set so as not to overlap.

<DC charging and AC charging>
In contact charging using a roll-type charger, uneven charging (density unevenness) is likely to occur due to subtle gap fluctuations between the roll of the roll-type charger and the surface of the photoreceptor. In particular, in DC charging in which a DC voltage is applied to a roll-type charger, unevenness in the film thickness of the photoreceptor, which does not cause a problem in AC charging, appears as charging unevenness. In addition, if the pitch of the charging unevenness of each member is synchronized, the unevenness becomes larger and an image quality defect occurs.

  Here, a comparison between DC charging and AC charging will be described. FIG. 8 is a diagram showing the relationship of the photosensitive member charging potential with respect to the voltage applied to the roll charger in the case of DC charging, and FIG. 9 is the photosensitive member charging potential with respect to the voltage applied to the roll charger in the case of AC charging. It is a figure which shows the relationship. In FIG. 8, the film thickness of the photoconductor is a parameter. In any case, the charge potential of the photoconductor increases in proportion to the voltage applied to the roll charger in the case of DC charging. is doing.

  On the other hand, in the case of AC charging shown in FIG. 9, although the charging potential increases in proportion to the applied voltage, the increase in the charging potential is saturated from a certain point. Therefore, in the case of AC charging, by setting the applied voltage within a range where the charging potential is saturated, fluctuations in the charging potential due to fluctuations in the applied voltage can be suppressed. On the other hand, in the case of direct current charging, fluctuations in the applied voltage greatly affect fluctuations in the charging potential.

  FIG. 10 is a diagram showing a change in the photosensitive member charging potential with respect to the photosensitive member film thickness in the case of direct current charging, and FIG. 11 is a diagram showing a change in the photosensitive member charging potential with respect to the photosensitive member film thickness in the case of alternating current charging. is there. In FIGS. 10 and 11, the measurement position of the film thickness on the photoreceptor is a parameter. In the case of DC charging shown in FIG. 10, the photosensitive member charging potential decreases as the thickness of the photosensitive member increases. On the other hand, in the case of the AC charging shown in FIG. 11, even if the film thickness of the photoconductor increases, the photoconductor charging potential does not change greatly. Therefore, in the case of AC charging, the influence on the charging potential of the photosensitive member film thickness is not significant, whereas in the case of DC charging, the change in the photosensitive member film thickness greatly affects the fluctuation of the charging potential. become.

  Therefore, in the case of DC charging, it is difficult to manage the diameter of each roll of the photosensitive member, the roll type charger and the roll type cleaning member, and the thickness of the photosensitive member, and the charging unevenness caused by each roll overlaps. As a result, large charging unevenness occurs.

  Therefore, in this embodiment, in addition to the case of AC charging, particularly in the case of DC charging, the effect of eliminating the charging unevenness and suppressing the deterioration of the uniformity of the charging potential is increased.

1 is a schematic diagram illustrating a configuration example of an image forming apparatus according to an exemplary embodiment. It is a schematic diagram explaining the structure of the roll-type charger used with the image forming apparatus of this embodiment. It is a schematic diagram explaining the form of a roll-type charger. It is a schematic diagram explaining the state of charging unevenness. It is a figure explaining the specific example of the diameter of each roll of a photoconductor, a roll-type charger, and a roll-type cleaning member. It is a schematic diagram which shows the example of the charging nonuniformity in the case of cases1 and 2. It is a schematic diagram which shows the example of the charge nonuniformity in the case of case3. It is a figure which shows the relationship of the photoreceptor charging potential with respect to the voltage applied to the roll type charger in the case of direct current charging. It is a figure which shows the relationship of the photoreceptor charging potential with respect to the voltage applied to the roll type charger in the case of alternating current charging. It is a figure which shows the change of the photoreceptor charging potential with respect to the photoreceptor film thickness in the case of DC charging. It is a figure which shows the change of the photoreceptor charging potential with respect to the photoreceptor film thickness in the case of alternating current charge.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Photoconductor, 2 ... Roll type charger, 3 ... Laser scanner, 4 ... Developing device, 5 ... Transfer device, 6 ... Drum cleaner, 7 ... Static eliminator, 8 ... Fixing device, 20 ... Roll type cleaning member

Claims (4)

A cylindrical photoreceptor,
A roll-type charger that is disposed to face the outer peripheral surface of the photoconductor, and whose surface is in contact with the surface of the photoconductor to rotate following the rotation of the photoconductor;
A roll-type cleaning member that is disposed to face the outer peripheral surface of the roll-type charger, and whose surface is in contact with the surface of the roll-type charger, and that rotates following the rotation of the roll-type charger. In the image forming apparatus,
The least common multiple of the outer peripheral length of the photosensitive member and the outer peripheral length of the roll-type charger, the least common multiple of the outer peripheral length of the photosensitive member and the outer peripheral length of the roll-type cleaning member, and the outer peripheral length of the roll-type charger The diameter of the photoconductor, so that any of the least common multiples with the outer peripheral length of the roll-type cleaning member is larger than the image forming length of a medium of the maximum size among the media on which an image is formed in the image forming apparatus, The diameter of the roll type charger and the diameter of the roll type cleaning member are each set,
The outer peripheral length of the photosensitive member is A, the outer peripheral length of the roll charger is B, the outer peripheral length of the roll cleaning member is C, and the nip width at the contact portion between the photosensitive member and the roll charger is N. if you did this,
On the outer peripheral surface of the photosensitive member, starting from a position where a position at every interval of A, a position at every interval of B, and a position at every interval of C overlap, the position at every interval of A is the center. ± (N / 2), ± (N / 2) centered on the position for each interval of B, and ± (N / 2) centered on the position for each interval of C An image forming apparatus , wherein a diameter of the photosensitive member, a diameter of the roll charger, and a diameter of the roll cleaning member are set so as not to overlap each other within a long range . The image forming apparatus according to claim 1, wherein the voltage applied to the pre-Symbol roll type charger is DC. The image forming apparatus according to claim 1, wherein the pre-Symbol-roll cleaning member is equal to or constituted by the foamed member. Before SL roll-type charger and the image forming apparatus according to claim 1, wherein said the rolled cleaning member is characterized in that it is fixed at a constant displacement.

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