Image forming device

Abstract

To provide an image forming apparatus that can reduce fatigue due to transfer exposure to a photoreceptor drum.SOLUTION: An image forming apparatus passes a sheet 31 between a photoreceptor drum 21 and a transfer roller 23 arranged opposite in the longitudinal direction, and applies voltage between the photoreceptor drum 21 and the transfer roller 23 to electrify the sheet 31, thereby forming an image. The image forming apparatus includes an electrifying unit 26 that is provided on the periphery of the photoreceptor drum 21 and electrifies the photoreceptor drum 21, and a transfer roller 23 that is provided on the periphery of the photoreceptor drum 21 and on the upstream side in the rotation direction of the photoreceptor drum. The electrifying unit 26 includes a screen grid electrode 36 that is provided between a saw teeth electrode and the photoreceptor drum 21. In the longitudinal direction of the photoreceptor drum 21, the opening ratio of a non-paper feed area of the screen grid electrode 36 adjacent to an end of the sheet is changed.SELECTED DRAWING: Figure 2A

Description

[Technical Field]

[0001] The present invention relates to a forming apparatus having a charging unit that reduces fatigue of a photosensitive drum due to exposure to radiation during transfer. [Background technology]

[0002] A conventional image forming apparatus that controls the surface potential of an image carrier (photosensitive drum) using a corona charger is disclosed in Japanese Patent Laid-Open No. 2018-189751 (Patent Document 1). According to Patent Document 1, the image forming apparatus includes a rotatable image carrier, a discharge wire, a grid electrode, a shield, and a corona charger that charges the image carrier, and the grid electrode has openings through which ions can pass and non-openings through which ions cannot pass, and in the width direction substantially perpendicular to the rotation axis of the image carrier, more non-openings are provided downstream of the center in the rotation direction of the image carrier than upstream of the center, thereby improving the convergence of the surface potential of the image carrier by the corona charger. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Publication No. 2018-189751 Summary of the Invention [Problem to be solved by the invention]

[0004] In conventional image forming devices, a uniform charge is applied by scorotron charging to prevent a drop in potential of the photosensitive drum due to exposure to transfer radiation after transfer, but the exposure amount is high at the edges of the paper and the outside, making it difficult to ensure a uniform charging potential on the photosensitive drum. Also, in conventional image forming devices, during continuous printing operations, the inflow of transfer current concentrates in the non-paper passing areas in the longitudinal direction of the photosensitive drum, causing a drop in potential of the photosensitive drum, resulting in image fogging in the areas where this drop in potential occurs and staining of the edge of the paper.

[0005] The present invention has been made to solve the above-mentioned problems, and aims to provide an image forming apparatus that can reduce fatigue caused by exposure of the photosensitive drum to transfer radiation. [Means for solving the problem]

[0006] The image forming apparatus of the present invention forms an image on a sheet of paper by passing the sheet between a photosensitive drum and a transfer roller arranged opposite each other in the longitudinal direction and applying a voltage between the photosensitive drum and the transfer roller. The image forming apparatus includes a charging unit provided around the photosensitive drum to charge the photosensitive drum, and a transfer roller provided around the photosensitive drum and upstream in the direction of rotation of the photosensitive drum, the charging unit including a sawtooth electrode and a screen grid electrode provided between the sawtooth electrode and the photosensitive drum, and the aperture ratio of a non-sheet-passing area of ​​the screen grid electrode adjacent to the edge of the sheet of paper is changed in the longitudinal direction of the photosensitive drum.

[0007] Preferably, the aperture ratio of the screen grid electrode is higher at the edge of the paper than at other parts, and the aperture ratio gradually changes with increasing distance from the edge of the paper outward in the axial direction of the photosensitive drum.

[0008] More preferably, the aperture ratio of the screen grid electrode is set to be higher on the upstream side in the rotation direction of the photosensitive drum than on the downstream side.

[0009] It is preferable that the aperture ratio of the screen grid electrode gradually decreases from the upstream side to the downstream side in the rotation direction of the photosensitive drum.

[0010] The aperture ratio of the screen grid electrode may be varied by changing the mesh spacing that constitutes the screen grid electrode.

[0011] The aperture ratio of the screen grid electrode may be varied by changing the grid mesh width that constitutes the screen grid electrode. [Effects of the Invention]

[0012] According to this invention, it is possible to change the aperture ratio of the non-paper passing area of ​​the screen grid electrode adjacent to the paper edge in the longitudinal direction of the photosensitive drum, thereby increasing the amount of charge supplied to the area where the potential has dropped due to transfer exposure, thereby reducing fatigue of the photosensitive drum due to transfer exposure.

[0013] The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments, which proceeds with reference to the accompanying drawings. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic diagram illustrating the configuration of a transfer unit according to the present invention, which has a configuration for reducing fatigue of a photosensitive drum due to exposure to transfer radiation. [Figure 2A] 10A and 10B are diagrams illustrating charging units in which the aperture ratio of the screen grid electrode is changed. [Figure 2B] 10A and 10B are diagrams illustrating charging units in which the aperture ratio of the screen grid electrode is changed. [Figure 3] 10A and 10B are diagrams illustrating a case where the aperture ratio of the screen grid electrode is changed in stages from the upstream side to the downstream side in the rotation direction of the photosensitive drum. [Figure 4] 10A and 10B are diagrams illustrating a case where the aperture ratio of the screen grid electrode is changed only on the upstream side in the rotation direction of the photosensitive drum. [Figure 5A] 10A and 10B are diagrams showing the case where the aperture ratio of the screen grid electrode is changed by changing the grid mesh interval and grid mesh width. [Figure 5B] 10A and 10B are diagrams showing the case where the aperture ratio of the screen grid electrode is changed by changing the grid mesh interval and grid mesh width. [Figure 6] 10A and 10B are diagrams showing the effect of varying the aperture ratio of the screen grid electrode. DETAILED DESCRIPTION OF THE INVENTION

[0015] An image forming apparatus according to an embodiment of the present invention will be described below with reference to the drawings. Fig. 1 is a diagram showing the periphery of a transfer unit 20 of an image forming apparatus to which the present invention is applied, and shows a state in which toner 32 is supplied to paper 31 via a photosensitive drum 21 and a transfer roller 23.

[0016] 1, transfer unit 20 of the image forming apparatus according to this embodiment includes photosensitive drum 21, which is grounded and on which a toner image is formed, developing roller 22, which is disposed adjacent to photosensitive drum 21 and supplies toner 32, transfer roller 23, which is disposed in contact with photosensitive drum 21 and rotates together with photosensitive drum 21 to pinch and transport paper 31, thereby transferring the toner image on photosensitive drum 21 to paper 31, and paper guide 30, which is provided below transfer roller 23 and guides paper 31. A cleaning blade 29 is provided on the outer periphery of photosensitive drum 21.

[0017] The transfer roller 23 is connected to a positive power supply 25. The cleaning pad 24 that contacts the transfer roller 23 is connected to a negative power supply of the same polarity as the charge polarity of the toner 32.

[0018] In the transfer section 20, a de-electrification lamp 28 that removes static electricity from the photosensitive drum 21, a charging unit 26 that charges the photosensitive drum 21, a developing roller 22 that supplies toner 32, a transfer roller 23, and a cleaning blade 29 that collects the toner 32 are arranged circumferentially on the photosensitive drum 21 in this order from upstream to downstream.

[0019] Next, a more detailed description will be given. Fig. 2A is an enlarged view of the charging unit 26 provided with a screen grid electrode 36 according to this embodiment and the corresponding edge 33 of the paper, and Fig. 2B is a further enlarged view of the area surrounded by an ellipse in Fig. 2A. Referring to Fig. 2A, the charging unit 26 includes a sawtooth electrode 35 and a screen grid electrode 36, and the screen grid electrode 36 is provided between the sawtooth electrode 35 and the photosensitive drum 21 in the tangential direction of the photosensitive drum 21. The screen grid electrode 36 is provided to uniformly charge the photosensitive drum 21 and has a certain aperture ratio.

[0020] 2A, the charging unit 26 and the photosensitive drum 21 have approximately the same width, the transfer roller 23 has a width narrower than the charging unit 26 and the photosensitive drum 21, and the paper 31 onto which the image is transferred has a width narrower than the transfer roller 23. A drop in the potential of the photosensitive drum 21 occurs at an end 33 in the width direction of the paper 31, as shown here.

[0021] 2A, an image 37 of the surface potential on the conventional photosensitive drum 21 is shown at the bottom of the figure. Referring to the image 37 of the surface potential on the photosensitive drum 21 shown at the bottom, the surface potential on the photosensitive drum 21 remains constant in the area where the paper 31 passes, but drops sharply at the edge 33 of the paper and recovers once the paper leaves the edge 33. This is because the edge 33 of the paper is most exposed to positive charges for transfer, and therefore a significant drop in the potential of the photosensitive drum 21 occurs.

[0022] In this embodiment, the aperture ratio of the screen grid electrode 36 of the charging unit 26 at the edge 33 of the paper is changed to reduce the amount of potential drop due to the inflow of transfer current and ensure a uniform charging potential on the photosensitive drum 21. That is, as shown in FIG. 2A , the aperture ratio of the screen grid electrode 36 is changed only in an area including a non-paper passing area 34 adjacent to the edge 33 of the paper, as shown by the ellipse in the figure, centered on the edge 33 of the paper. This configuration reduces the amount of transfer radiation exposed to the photosensitive drum 21 and ensures the drum potential of the photosensitive drum 21. Note that the aperture ratio of the screen grid electrode 36 here refers to the ratio of the openings formed in the screen grid electrode 36 to the total area of ​​the screen grid electrode 36.

[0023] The aperture ratio of the screen grid electrode 36 is set higher at the edge 33 of the paper than at other areas, and the aperture ratio at other areas is not changed significantly from the paper passing area, allowing the aperture ratio to be varied in stages. This prevents image defects (carrier buildup, etc.) caused by sudden potential correction, while also making it possible to address image contamination at the edge of the paper. In other words, the potential is controlled to prevent only the potential drop at the edge 33 of the paper as much as possible.

[0024] [First embodiment] This will be explained in more detail below. As shown in Figures 2A and 2B, the width of the screen grid electrode 36 of the charging unit 26 is varied in terms of the aperture ratio at the edge 33 in the width direction of the paper 31. Here, as shown in Figure 2B, positions are indicated by A, B, C, and D with the edge 33 of the paper as the center, and the aperture ratio is set in the order A > B > C > D depending on the position. In other words, as shown in Table 1, the width of the screen grid electrode 36 is set to be greatest at the edge 33 in the width direction of the paper 31 and to become successively smaller on both sides.

[0025] [Table 1]

[0026] Table 1 shows that the aperture ratio of the screen grid electrode 36 is normally 80%, but this is changed depending on the position; for example, at position A, the aperture ratio is increased by 10% from the normal ratio to 90%.

[0027] By setting the width of the screen grid electrode 36 in this manner, a drop in the potential of the photosensitive drum 21 does not occur.

[0028] That is, in this embodiment, the aperture ratio of the screen grid electrode 36 of the charging unit 26 at the edge 33 of the paper is changed to reduce the amount of potential drop due to the inflow of transfer current, thereby ensuring a uniform charging potential on the photosensitive drum 21.

[0029] Specifically, the aperture ratio of the screen grid electrode 36 is set to 90% at 33, and the aperture ratio of the portions other than the edge 33 of the paper is varied in stages from the edge 33 of the paper.

[0030] Here, the aperture ratio is set as shown in Table 1, but it is not limited to this. It is sufficient to set the aperture ratio of the screen grid electrode 36 at the portion where the potential drop at the edge 33 of the paper occurs most significantly high, and the variable range of the aperture ratio may be about 1 to 10%.

[0031] [Second embodiment] Next, another embodiment will be described. Fig. 3 is a plan view of a screen grid electrode 36 according to another embodiment, taken in the tangential direction of the photosensitive drum 21. In this embodiment, the screen grid electrode 36 has trapezoidal portions at both longitudinal ends. Here, as described in Fig. 1, the charging unit 26 including the screen grid electrode 36 is disposed between the upstream static elimination lamp 28 and the downstream developing roller 22. As shown in Fig. 3, the screen grid electrode 36 is divided into three portions, upstream, midstream, and downstream, by the upper bases of the trapezoidal portions.

[0032] Referring to FIG. 3, the aperture ratio of the screen grid electrode 36 relative to the photosensitive drum 21 is linearly changed so that it is higher on the discharge lamp 28 side (upstream side) and lower on the developing roller 22 side (downstream side) across the midstream side.

[0033] Since the charged potential changes depending on the width of the opening, the potential of the photosensitive drum 21 can be raised in a short time by increasing the opening ratio on the upstream side relative to the photosensitive drum 21. However, if the opening ratio remains large, the potential will be lowered too much, so the opening ratio on the downstream side is lowered to lower the excessively high potential and make it uniform with other locations.

[0034] That is, in this embodiment, when the photosensitive drum 21 passes the charging unit 26, the opening ratio of the screen grid electrode 36 is set high at the beginning of charging to increase the power of applying charge to the photosensitive drum 21, and an opening variable pattern is used to achieve uniform charging on the downstream side. Specifically, the opening ratio of the screen grid electrode 36 is set high at about 90% on the upstream side of charging, and the opening ratio is gradually reduced to about 80% as the photosensitive drum 21 moves downstream.

[0035] Next, a further embodiment will be described. Fig. 4 shows a diagram of this further embodiment. Referring to Fig. 4, here, the aperture ratio of the screen grid electrode 36 relative to the photosensitive drum 21 is higher on the upstream side and lower on the downstream side, as in the previous embodiment, but only the upstream side is made higher, and the midstream and downstream sides have normal aperture ratios. Furthermore, the aperture ratio does not change linearly as in the previous embodiment, but is constant.

[0036] That is, in this embodiment, the aperture ratio on the upstream side is increased to about 90%, and the aperture ratio of the remaining screen grid electrodes 36 is not varied but remains normal. In particular, in engines with high process speeds, it is difficult to secure a sufficient charge on the photosensitive drum 21, so the improvement in the charging property of the photosensitive drum 21 according to this embodiment is effective.

[0037] Next, another embodiment will be described. FIGS. 5A and 5B are diagrams illustrating yet another embodiment. FIG. 5A is a diagram illustrating the entire screen grid electrode 36, and FIG. 5B is an enlarged view of the circled portion in FIG. 5A. In FIG. 5B, "a" indicates the grid mesh width of the screen grid electrode 36, and "b" indicates the grid mesh spacing of the screen grid electrode 36. Referring to FIGS. 5A and 5B, two cases are shown in which the opening ratio of the screen grid electrode 36 is varied while the grid mesh spacing b of the screen grid electrode 36 is constant, and varied while the grid mesh spacing b is constant. The grid mesh width a, grid mesh spacing b, and opening ratio are as shown in Tables 2 and 3.

[0038] [Table 2]

[0039] [Table 3]

[0040] That is, in this embodiment, the opening ratio of the screen grid electrode 36 of the charging unit 26 is varied when the grid mesh interval b of the screen grid electrode 36 is changed and when the grid mesh width a of the screen grid electrode 36 is changed, thereby stabilizing the potential of the photosensitive drum 21.

[0041] As described above, the aperture ratio can be set to a desired ratio by varying either the grid mesh interval b or the grid mesh width a of the screen grid electrode 36.

[0042] Next, the effect of making the aperture ratio of the screen grid electrode 36 variable will be described.

[0043] Figure 6 is a diagram illustrating problems with conventional image forming apparatuses and the effects of the configuration according to an embodiment of the present invention. The upper part of Figure 6 shows the positional relationship between photosensitive drum 21, paper 31, and transfer roller 23, the center part shows a conventional image 40 of the charge potential of the photosensitive drum when paper is passing through and an image 41 of the charge potential of the photosensitive drum after charging, and the lower part shows an image 42 of the charge potential of the photosensitive drum when paper is passing through and an image 43 of the charge potential of the photosensitive drum after charging, in the case of the configuration according to this embodiment.

[0044] As shown in the center of Figure 6, in the past, the charged potential of the photosensitive drum was low at both ends of the paper, and the charged potential of the photosensitive drum after charging also decreased only in those areas. However, as shown in the bottom of Figure 6, in this embodiment, only this decreased area 45 can be charged more strongly than other areas, so the charged potential of the photosensitive drum after charging becomes flat, as shown by 43.

[0045] In the above embodiment, the case where a discharge lamp is provided has been described, but this is not limited to this. If a discharge lamp is not provided, the influence of transfer exposure becomes greater, so the present invention can also be applied to a case where a discharge lamp is not provided.

[0046] The present invention can be implemented in various other forms without departing from the spirit or main features thereof. Therefore, the above-described embodiments are merely examples and should not be interpreted as being limiting. All modifications and variations within the scope of the claims of the present invention are within the scope of the present invention. [Industrial Applicability]

[0047] The present invention is useful as an image forming apparatus because it can ensure uniform charging characteristics on the photosensitive drum. [Explanation of symbols]

[0048] 20 Transfer section 21 Photosensitive drum 22 Developing roller 23 Transfer roller 24 cleaning pads 25 plus power supply 26 Charging unit 28 Static elimination lamp 29 Cleaning blade 30 Paper guide 31 Paper 32 Toner 33 Paper Edge 34 Non-paper passing area 35 Sawtooth electrode 36 Screen grid electrode 37 Image of surface potential 40 Image of the charge potential of the photosensitive drum after conventional paper feed 41 Image of the charge potential of the photosensitive drum after conventional charging 42 Image of the charged potential of the photosensitive drum after paper has passed according to an embodiment of the present invention 43 Image of the charge potential of the photosensitive drum after charging according to an embodiment of the present invention 45 Highly charged area

Claims

1. An image forming apparatus that forms an image on a sheet of paper by passing the sheet between a photosensitive drum and a transfer roller that are arranged opposite to each other in a longitudinal direction and applying a voltage between the photosensitive drum and the transfer roller, a charging unit provided around the photosensitive drum and configured to charge the photosensitive drum; a transfer roller provided around the photosensitive drum downstream of the charging unit in the rotation direction of the photosensitive drum, the charging unit includes a sawtooth electrode and a screen grid electrode having a plurality of openings and provided between the sawtooth electrode and the photosensitive drum; an image forming apparatus, wherein the screen grid electrode has a higher aperture ratio per unit area in a non-paper passing region facing the region where the photosensitive drum abuts against the edge of the paper than in an axially inner region of the photosensitive drum than in the non-paper passing region, and the aperture ratio gradually decreases from the edge of the paper toward the outer side of the axial direction of the photosensitive drum.

2. 2. The image forming apparatus according to claim 1, wherein the aperture ratio of the non-paper passing region of the screen grid electrode is set higher on the upstream side in the rotation direction of the photosensitive drum than on the downstream side.

3. 3. The image forming apparatus according to claim 2, wherein the aperture ratio of the non-paper passing area of ​​the screen grid electrode gradually decreases from the upstream side to the downstream side in the rotation direction of the photosensitive drum.

4. 4. The image forming apparatus according to claim 1, wherein an opening ratio of the non-paper passing area of ​​said screen grid electrode is varied by changing a grid mesh width constituting said screen grid electrode.

5. The aperture ratio of the screen grid electrode is set to the value of the grid 5. The image forming apparatus according to claim 1, wherein the width of the mesh is variable.

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