JP6261204B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer having a function of forming an image on a recording material such as a sheet.

In a conventional image forming apparatus such as a copying machine, a printer, and a facsimile using an electrophotographic system, an image is formed through a charging process, an exposure process, a development process, a transfer process, and a fixing process. Among these, the transfer process for transferring the toner image on the photosensitive drum formed in the developing process to the recording material is an important process for determining the quality of the image formed on the recording material, and performs an optimal image formation. For this purpose, it is necessary to appropriately convey the recording material to the transfer position on the surface of the photosensitive drum. Therefore, in an image forming apparatus using an electrophotographic system, in order to guide the recording material conveyed from the registration roller pair to the transfer position to a better position, a pair of guides that guide both sides of the recording material upstream of the transfer position. By arranging the guide, high transfer performance is secured.
However, especially when printing a large amount of high-printed images continuously, the upper guide (guide member) that guides the image forming surface side of the recording material out of the pair of guides rides on the airflow generated by the rotation of the photosensitive drum. As a result, toner on the photosensitive drum may accumulate. For this reason, the toner deposited on the upper guide may fall on the recording material to be conveyed due to an impact such as conveyance of the recording material, resulting in image defects due to toner dropping or toner scattering. Therefore, in Patent Document 1, the toner is electrostatically prevented from being transferred to the upper guide by applying a negative bias having the same polarity as the normal charging polarity of the toner to the upper guide.

JP 2007-264342 A

However, if a negative bias is applied to the upper guide, negative toner for image printing that has a negative polarity will not easily accumulate on the upper guide, but a positive toner having a positive polarity will be developed by reversing the polarity like fog toner. As a result, it tends to be electrostatically deposited on the guide. The reversal positive toner is easily developed when the relationship of drum potential <development potential is satisfied at the development position such that the drum potential is −1100 V and the development potential is −350 V. For example, this is conspicuous in a low printing area, a non-sheet passing area at the time of small size printing, and a non-printing timing such as a pre-rotation, a post-rotation, and a paper interval.
Therefore, in the configuration in which a negative bias is applied to the upper guide, the following phenomenon occurs when a large-scale print of a small size, a large-scale print of a low-print image, a pre-rotation or post-rotation, or a paper passing job with a long interval between papers is repeated. There is concern. That is, there is a concern that even if guide smear due to negative toner for image printing can be suppressed, guide smear due to reversal positive toner such as fogging will occur, and image defects due to this will easily occur. This phenomenon is likely to occur in a jumping development system that easily develops fog toner.

  The present invention has been made in view of the above-described circumstances, and suppresses adhesion of the printing toner and the toner having a charging polarity different from the charging polarity of the toner to the guide member. For the purpose.

In order to achieve the above object, the present invention provides:
A photoreceptor on which an electrostatic latent image is formed;
A developing member for developing the electrostatic latent image;
A transfer member that forms a transfer nip portion with the photoconductor, and transfers a toner image on the photoconductor to a recording material at the transfer nip portion;
A guide member that is disposed upstream of the transfer nip portion in the conveying direction and guides the recording material, and is disposed between the surface of the recording material on which the toner image is transferred and the photoreceptor . A guide member;
A power supply for applying a voltage to the guide member in order to suppress toner from adhering to the guide member;
In an image forming apparatus for forming an image on a recording material,
The polarity of the voltage applied to the guide member by the power supply unit is the same as the normal charging polarity of the toner when a region of the photoconductor having a surface potential higher than the developing potential of the developing member faces the guide member. Is applied to the guide member, and the region of the photoconductor having a surface potential smaller than the developing potential of the developing member is opposite to the guide member, the polarity is opposite to that of the first voltage. Control means capable of controlling the power supply unit so as to apply a second voltage to the guide member is provided.
In order to achieve the above object, in the present invention,
A photoreceptor on which an electrostatic latent image is formed;
A developing member for developing the electrostatic latent image;
A transfer member that forms a transfer nip portion with the photoconductor, and transfers a toner image on the photoconductor to a recording material at the transfer nip portion;
A guide member arranged on the upstream side of the transfer nip portion in the conveying direction for guiding the recording material, and disposed between the surface of the recording material on which the toner image is transferred and the photosensitive member; A guide member provided with a plurality of conductive portions with respect to the rotation axis direction of the photoreceptor;
In order to suppress the toner from adhering to the guide member, a voltage is applied to the guide member, and a power supply unit capable of independently applying a voltage to each conductive portion;
The polarity of the voltage applied to the guide member by the power supply unit is the same as the normal charging polarity of the toner when a region of the photoconductor having a surface potential higher than the developing potential of the developing member faces the guide member. Is applied to the guide member, and the region of the photoconductor having a surface potential smaller than the developing potential of the developing member is opposite to the guide member, the polarity is opposite to that of the first voltage. Control means capable of controlling the power supply unit so as to apply a second voltage to the guide member;
A printing rate deriving unit for deriving a printing rate of a toner image formed on the photoreceptor;
In an image forming apparatus for forming an image on a recording material,
The control means derives the printing rate of each region facing each conductive portion in the rotation axis direction on the photosensitive member by the printing rate deriving unit, and based on the derived printing rate of each region, The voltage application operation to the conductive portion is controlled.

  According to the present invention, it is possible to suppress adhesion of the printing toner and the toner having a charging polarity different from the charging polarity of the toner to the guide member.

FIG. 2 is a schematic cross-sectional view showing the main part of the image forming apparatus of Embodiment 1. Sectional drawing which shows schematic structure of the image forming apparatus of Example 1. FIG. The figure which shows the drum potential and developing potential in the developing position of Example 1. The figure explaining the switching timing of the bias applied to the pre-transfer upper guide The figure which shows the relationship between the bias applied to the pre-transfer upper guide and the amount of adhered toner The figure for demonstrating the printing rate calculation means in Example 2. FIG. Diagram showing how bias applied to the pre-transfer upper guide changes in time series The figure which shows the image each printed on four recording materials shown in FIG. The figure which shows schematic structure of the pre-transfer upper guide in Example 3.

  DETAILED DESCRIPTION Exemplary embodiments for carrying out the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. It is not intended to limit the scope to the following embodiments.

[Example 1]
Example 1 will be described below. FIG. 2 is a cross-sectional view illustrating a schematic configuration of the image forming apparatus according to the present exemplary embodiment.
In FIG. 2, a photosensitive drum 1 as an image carrier is uniformly charged by a charging means 2 while rotating in the direction of the arrow shown. Next, the photosensitive drum 1 is irradiated with a laser beam 3 modulated in accordance with an image signal, and an electrostatic latent image (latent image) is formed on the surface thereof. Subsequently, the electrostatic latent image formed on the photosensitive drum 1 (on the image carrier) is developed with negative toner in the developing device 4 and is visualized as a toner image T. A predetermined gap is provided between the developing roller 4 a provided in the developing device 4 and the photosensitive drum 1 by disposing spacers (not shown) at both ends of the photosensitive drum 1 in the rotation axis direction. Yes. In the image forming apparatus of the present embodiment, a so-called jumping development system is employed in which the toner on the developing roller 4a is developed on the photosensitive drum 1 by using the electric field force generated in the gap. Here, in this embodiment, the rotation axis direction (hereinafter referred to as the longitudinal direction) of the photosensitive drum 1 and the width direction of the recording material to be conveyed are the same direction. Further, the developing roller 4 a supplies toner to the photosensitive drum 1 at a developing position (position (area) indicated by A in FIG. 1, hereinafter referred to as developing position A) between the developing roller 4 a and the photosensitive drum 1. This corresponds to a developing member for forming a toner image.

On the other hand, the recording material P passes between the pre-transfer upper guide 6 and the pre-transfer lower guide 7 while the timing is adjusted by the registration roller pair 5 in the direction of the arrow shown in the drawing, and then the photosensitive drum 1 and the transfer roller 8 Are guided to a transfer nip portion (transfer position) N1 formed between the two. Here, the transfer roller 8 corresponds to a transfer member. Further, the pre-transfer upper guide 6 is a guide member that guides the recording material P to be conveyed toward the transfer nip portion N1, and is a surface on which the toner image is transferred (printing surface, This corresponds to a guide member disposed to face the image forming surface.
When the recording material P is conveyed to the transfer nip N1, a positive transfer bias is applied to the transfer roller 8 from the back side of the recording material by the power source, and the toner image T on the photosensitive drum 1 is transferred onto the recording material P. Is done. The transfer roller 8 is a sponge type made of NBR and hydrin, and uses a roller having a resistance value of 1.0 × 10 ⁸ Ω in a normal temperature environment.

The recording material P carrying the toner image is conveyed to the fixing device 10 through the guide 9. Thereafter, the unfixed toner image on the recording material P is fixed on the recording material P by being heated and pressed by the fixing device 10, and the recording material P on which the toner image is fixed is taken out of the machine as a hard copy. Discharged.
Untransferred toner (residual toner) remaining on the photosensitive drum 1 is cleaned and removed by the cleaning device 11, and the photosensitive drum 1 is subjected to the next image forming process (image forming operation) starting from the charging step.

Next, the pre-transfer upper guide 6 and the pre-transfer lower guide 7 provided in the image forming apparatus of this embodiment will be described with reference to FIG. FIG. 1 is a schematic cross-sectional view showing the main part of the image forming apparatus of this embodiment.
The pre-transfer upper guide 6 and the pre-transfer lower guide 7 are transport guides for guiding the recording material P transported to the transfer nip portion N1 to a better position in the transfer nip portion N1. The pre-transfer upper guide 6 and the pre-transfer lower guide 7 are rectangular plate-like members having substantially the same width as the width of the photosensitive drum 1 so that the long side direction (longitudinal direction) is orthogonal to the recording material conveyance direction. Has been placed. In order to guide the recording material P to a better transfer nip portion N1, the tip of the pre-transfer upper guide 6 is bent or bent in the recording material conveyance direction so as not to interfere with the photosensitive drum 1. It is preferable to approach to the nip portion N1.

  The pre-transfer upper guide 6 is formed of a conductive material such as SUS, and the pre-transfer upper guide 6 is connected to the pre-transfer upper guide 6 through a bias applying unit 12 in order to prevent adhesion of toner to the pre-transfer upper guide 6. (Voltage) Vtg is applied. The bias applying unit 12 includes a power supply 13 that generates a positive bias Vtg1 and a power supply 14 that generates a negative bias Vtg2. The power supplies 13 and 14 to be used are switched by a switching element 15 according to a selection signal from the CPU 20. Here, the bias applying unit 12 and the power sources 13 and 14 constitute a power source unit. The CPU 20 corresponds to a control unit capable of performing control to change the magnitude of the voltage applied to the pre-transfer upper guide 6 by the power supply unit at a predetermined timing during image formation.

Here, the value of the potential of the photosensitive drum 1 during image formation is a drum potential, and the value of the potential of the developing roller 4a is a developing potential. At this time, in this embodiment, at the development position A on the photosensitive drum 1 shown in FIG. 1, the relationship of development potential <drum potential is established, and the negative toner T2 whose charging polarity is normal negative is easily developed. The negative bias Vtg2 is applied to the pre-transfer upper guide 6 at the following timing. The timing (predetermined timing) is the timing at which the toner developed at the development position A reaches the region (closest region) B closest to the pre-transfer upper guide 6.
On the other hand, at the development position A on the photosensitive drum 1, the relationship of development potential> drum potential is established, and the positive toner T1 is easily developed preferentially at the timing when the developed toner reaches the region B. A positive bias Vtg 1 is applied to the pre-transfer upper guide 6. The power supplies 13 and 14 that output a high voltage may be shared with power supplies used for charging, development, transfer, fixing, and the like. However, in order to stabilize the output, before the transfer, as in this embodiment, A power supply dedicated to the guide 6 is preferable.

FIG. 3 is a diagram showing the toner on the photosensitive drum 1 at the development position A and changes in the drum potential and the development potential in time series when two high-print images are continuously printed.
Time X0 in the figure is the timing at which the charged surface of the charged photosensitive drum 1 reaches the developing position A immediately after the start of the printing operation. Next, times X1, X2, X3, and X4 are the surfaces on the photosensitive drum 1 corresponding to (corresponding to) the leading edge, the trailing edge, the leading edge, and the trailing edge of the first sheet of recording material (hereinafter referred to as the drum surface). ) Is the timing to reach the development position A.
Accordingly, at time X1 to X2 and time X3 to X4, since the negative toner T2 printed on the recording material is developed, the development potential Vdc is lowered to −350V, and the drum potential is lowered to Vl = −150V due to the exposure action. There is a relationship of potential <drum potential. Here, in the time X1 to X2 and the time X3 to X4, the region on the photosensitive drum 1 located at the development position A can be printed on which a toner image for forming an image on one recording material P can be formed. An area (corresponding to an image forming area, hereinafter referred to as a print area). Here, the print area is a photosensitive region located at the development position A when the value of the potential of the photosensitive drum 1 is a value on the opposite side of the normal charging polarity of the toner from the value of the potential of the developing roller 4a. This is an area on the drum 1.

  On the other hand, at other timings, the time X0 to X1 corresponds to the pre-rotation, the time X2 to X3 corresponds to the paper interval, and the time X4 and subsequent times correspond to the post-rotation, but since it is not a printing area, the development potential Vdc is 0V or −350V. Since the drum potential is not exposed, Vd = −600 V as it is after charging. Therefore, the relationship of development potential> drum potential is established, and the positive toner T1 that is inverted on the photosensitive drum 1 is easily developed as fog toner.

FIG. 4 is a diagram for explaining the switching timing of the biases Vtg1 and Vtg2 applied to the pre-transfer upper guide 6. In FIG.
The time Yn (n = 0, 1, 2, 3, 4) in the figure is located at the developing position A at the time Xn (n = 0, 1, 2, 3, 4) shown in FIG. This is the timing (arrival timing) at which the drum surface of the photosensitive drum 1 rotates and moves to the region B and is closest to the pre-transfer upper guide 6. That is, the negative toner T2 developed at the developing position A at the timings of the times X1 to X2 and X3 to X4 is on the drum surface that reaches the region B at the timings of the times Y1 to Y2 and Y3 to Y4. .
Therefore, at the timing of times Y1 to Y2 and Y3 to Y4, the negative bias Vtg2 (first voltage) having the same polarity as the negative toner T2 is applied to the pre-transfer upper guide 6 so that the negative toner T2 is transferred from the drum surface to the upper before transfer. Flying and adhering to the guide 6 can be suppressed (reduced).

On the other hand, on the drum surface that reaches the region B at the timing of time Y0 to Y1, Y2 to Y3, Y4 to Y5, the timing after time X0 to X1, X2 to X3, X4, which is the forward rotation, the sheet interval, and the backward rotation. The reversal positive toner T1 developed at the development position A is on.
Therefore, at the timing after time Y0 to Y1, Y2 to Y3, Y4, the positive bias Vtg1 having the same polarity as the positive toner T1 is applied to the pre-transfer upper guide 6 so that the positive toner T1 is transferred from the drum surface to the pre-transfer upper guide 6. It is possible to suppress flying and adhering to the surface. Here, the timings of times Y0 to Y1, Y2 to Y3, and Y4 to Y5 are arrival timings during image formation (timing of times Y1 to Y2 and Y3 to Y4 for the print area on photosensitive drum 1 to reach area B). Corresponds to other times. Further, the positive bias Vtg1 corresponds to a second voltage having a value opposite to the normal charging polarity of the toner with respect to the negative bias Vtg2 (first voltage).

Here, in this embodiment, the positive bias Vtg1 is set to + 750V and the negative bias Vtg2 is set to -750V, for the following reason.
If the output of the bias Vtg applied to the pre-transfer upper guide 6 is too high, discharge may occur between the adjacent drum surface or transfer roller and the pre-transfer upper guide 6 and an abnormal image may be generated. Is done. Further, there is a concern that a transfer current leaks to the pre-transfer upper guide 6 through the moisture-absorbing paper, thereby causing a transfer failure. Therefore, the applied bias is preferably in the range of −1000V to + 1000V. Further, in order to shorten the response time when switching the output polarity, it is preferable that the output difference at the time of switching is as small as possible.

FIG. 5 shows the bias applied to the pre-transfer upper guide 6 and the amount of toner adhering to the pre-transfer upper guide 6 when 500 high-print images having a printing rate of 100% are printed continuously in an accelerated test. It is a figure which shows the measurement result.
If the amount of toner deposited in this evaluation can be suppressed to 150 mg or less, even if 200,000 low-printed images (printing rate 4%) are printed, images such as toner dropping from the upper guide 6 before transfer or toner scattering Defects can be suppressed.
Here, in this evaluation, by setting a high print image (print rate 100%), the toner is transferred at a speed about 5 to 10 times faster than a low print image (print rate 4%) assumed in the market. Deposited on the upper guide 6. In this evaluation, a constant bias is always applied to the pre-transfer upper guide 6 from X0 to X4 during the printing operation described above. That is, it is different from the control of this embodiment. Further, the toner amount adhering to the pre-transfer upper guide 6 is the total amount (T1 + T2) of the adhering positive toner T1 and negative toner T2. The positive toner T1 increases as the bias Vtg applied to the pre-transfer upper guide 6 is increased in the negative direction. On the other hand, the negative toner T2 increases as the bias applied to the pre-transfer upper guide 6 is increased in the positive direction. As a result, the peak in which the total amount of toner (T1 + T2) adhering to the pre-transfer upper guide 6 was the smallest was when -250 V was continuously applied to the pre-transfer upper guide 6. However, the value of -250V is a value that depends on the printing conditions. When a low-print image is continuously printed, the negative toner T2 to be printed is reduced, so that the influence of the positive toner T1 that increases the ratio is suppressed. It is considered that the peak shifts in the positive direction from −250V.

Hereinafter, the result of verifying the effect in the present embodiment will be described.
Table 1 shows the results of comparing conditions 1 to 6 shown in FIG. 5 and the amount of toner adhering to the pre-transfer upper guide 6 in this embodiment when 500 high-print images are continuously printed.
The bias Vtg applied to the upper guide 6 before transfer is always -750 V in condition 1, always -500 V in condition 2, always -250 V in condition 3, always 0 V in condition 4, always +500 V in condition 5, and always +750 V in condition 6. Was applied. Further, in this embodiment, as described above, +750 V is applied to the pre-transfer upper guide 6 when the developing potential> drum potential, and −750 V is applied when the developing potential <drum potential.

From Table 1, the present embodiment has a smaller amount of toner adhering to the pre-transfer upper guide 6 than the conventional case of -250 V in condition 3 where the amount of adhering toner is the smallest in the system to which a constant bias is applied. . As described above, a system that applies a positive or negative constant bias during printing cannot suppress adhesion of the positive toner and the negative toner to the pre-transfer upper guide 6.
On the other hand, in this embodiment, the polarity of the bias applied to the pre-transfer upper guide 6 is controlled by targeting a toner that is more easily developed in accordance with the timing at which the magnitude relationship between the developing potential during printing and the drum potential changes. doing. For this reason, it is possible to suppress the adhesion to the pre-transfer upper guide 6 more efficiently with respect to both the positive toner and the negative toner.

That is, in this embodiment, the drum surface located at the development position A is a region in the case where the developing potential is less than the drum potential during printing, the reversal positive toner is easily developed, the front rotation, the sheet interval, the rear rotation, and the like. A positive bias is applied to the pre-transfer upper guide 6 at the timing of reaching B. This can suppress the reversal positive toner from adhering to the pre-transfer upper guide 6.
On the other hand, when the negative potential can be developed, where the development potential is greater than the drum potential during printing, a negative bias is applied to the pre-transfer upper guide 6 at the timing when the print area located at the development position A reaches the area B. Applied. This can suppress the negative toner from adhering to the pre-transfer upper guide 6.
As described above, in this embodiment, it is possible to suppress flying and adhesion to the pre-transfer upper guide 6 with respect to both the printing negative toner and the positive toner having the reversed polarity. As a result, the total amount of toner deposited on the pre-transfer upper guide 6 can be suppressed (reduced) compared to a configuration in which only a constant negative bias is applied to the pre-transfer upper guide 6 as in the conventional example. It is possible to suppress image defects such as toner dropping and toner scattering caused by the toner.

Note that the amount of toner adhering to the above-mentioned upper guide 6 before transfer varies somewhat depending on the usage environment of the image forming apparatus in which the ratio of the positive toner and the negative toner changes, the usage history of the developing device 4, and the like. However, the tendency of the toner adhesion amount with respect to the bias applied to the pre-transfer upper guide 6 does not change.
In the case where the positive bias Vtg1 cannot be applied to the upper guide 6 before transfer (the polarity cannot be changed), the negative bias is weakened or the bias is turned off at the timing of applying the positive bias Vtg1 described above. You may comprise so that a voltage may not be applied to the guide 6. FIG. Decreasing the negative bias means changing the magnitude of the negative bias to a value on the positive side of the negative bias Vtg2.

In the present embodiment, the case where the regular charging polarity of the toner is negative has been described. However, the present invention is not limited to this, and the present invention is preferably applied to the case where the regular charging polarity of the toner is positive. Can do. In this embodiment, the surface corresponding to the image information on the surface of the photosensitive drum 1 is exposed, and development is performed by attaching toner charged to the same polarity as the charged polarity of the photosensitive drum 1 to the exposed portion. Although the image forming apparatus employing the reversal development method has been described, the present invention is not limited to this. The present invention can be suitably applied even to an image forming apparatus that employs a regular development method. Here, in the normal development method, the surface corresponding to the background portion of the image information on the surface of the photosensitive drum 1 is exposed, and the portion other than the exposed portion is charged with a polarity opposite to the charged polarity of the photosensitive drum 1. In this method, development is performed by attaching toner.
That is, at the time when the printing area on the photosensitive drum 1 reaches the area B during image formation, the first voltage having the same polarity as the normal charging polarity of the toner is applied to the pre-transfer upper guide 6. Any configuration may be used. Further, when the image formation is not at the arrival timing, a second voltage having a value opposite to the normal charging polarity of the toner with respect to the first voltage is applied to the pre-transfer upper guide 6. I just need it.
As described above, the charging polarity of the toner, the charging polarity of the photosensitive drum 1, the developing method, and the like may be appropriately set according to the specifications of the image forming apparatus.

[Example 2]
Example 2 will be described below.
In this embodiment, even if the area on the photosensitive drum 1 is a printing area, the area where the printing rate calculated by the printing rate calculating means (printing rate deriving means) is 0% reaches the area B. A positive bias is applied at the timing. This suppresses the reversal positive toner from adhering to the pre-transfer upper guide 6. Here, as described above, the printing area is an area on the photosensitive drum 1 corresponding to an area where printing can be performed between the leading edge and the trailing edge of one recording material. Explanation of the same configuration as that of the first embodiment is omitted.

  First, the printing rate calculation means in the present embodiment will be described. FIG. 6 is a diagram for explaining the printing rate calculation means in the present embodiment. In the present embodiment, a laser lighting video signal is sampled at predetermined time intervals within a predetermined printing area, and the laser lighting ratio, that is, the printing rate is calculated from the ratio of the number of video signals to the total number of samples. Is calculated (derived). In FIG. 6, reference numeral 21 denotes an area (hereinafter referred to as an image area) in which printing is possible within the recording material P (on the recording material).

The image area 21 is divided into n numbers (n: natural number). In FIG. 6, the n divided areas are numbered from 1 to n . The hatched portion in FIG. 6 is a point that is randomly selected at one place for each of the n divided areas.
At this point, it is determined whether the video signal is on or off, and the number of video signals that are on is counted. The printing rate can be calculated by dividing the number by the number of divided image areas (in this case, n). Further, if the calculation is limited to a certain section in the image area 21, it is also possible to calculate the printing rate in a partial area such as the leading edge or the trailing edge of the recording material. However, the value calculated as described above does not necessarily coincide with the printing rate. However, if the number of samples n is sufficiently large, the calculated value and the actual printing rate are almost equal, although it takes time to determine on / off.

FIG. 7 shows how the bias applied to the toner on the photosensitive drum 1 and the pre-transfer upper guide 6 changes in time series in the region B when four recording materials having different printing rates are continuously printed. FIG. FIG. 8 is a diagram showing images printed on the four recording materials shown in FIG. Here, the region B is a position where the drum surface of the photosensitive drum 1 is closest to the pre-transfer upper guide 6 as described above.
FIG. 8A shows an image of the first recording material and an overall printing rate of 100%, and FIG. 8B shows an image of the second recording material and the entire printing rate. An image with a printing rate of 0% is shown. FIG. 8C shows an image of the third recording material and an overall printing rate of 50%, and FIG. 8D shows an image of the fourth recording material. An image with an overall printing rate of 50% is shown.

In FIG. 7, time Y <b> 10 is the timing at which the charged surface of the charged photosensitive drum 1 reaches the region B immediately after the start of the printing operation. Times Y11, Y12, Y13, Y14, Y15, Y18, Y19, and Y23 are timings at which the drum surface corresponding to the leading and trailing edges of the first to fourth recording materials reaches the region B, respectively. As in the first embodiment, a positive bias Vtg1 is applied to the pre-transfer upper guide 6 after Y10 to Y11, Y12 to Y13, Y14 to Y15, Y18 to Y19, and Y23, which correspond to the pre-rotation, the sheet interval, and the post-rotation. ing. As a result, it is possible to prevent the positive toner T <b> 1 that has been inverted such as fogging from adhering to the pre-transfer upper guide 6.
On the other hand, at the timing of Y11 to Y12 corresponding to the first recording material, the overall printing rate is 100%, and the negative bias Vtg2 is applied to the pre-transfer upper guide 6. Thereby, it is possible to suppress the printed negative toner T2 from adhering to the pre-transfer upper guide 6.

Further, at the timing of Y13 to Y14 corresponding to the second recording material, since the overall printing rate is 0%, the positive bias Vtg1 is applied to the pre-transfer upper guide 6. As a result, it is possible to suppress the positive toner T1 that has been inverted such as fogging from adhering to the guide.
Furthermore, at the timings Y15 to Y18 and Y19 to Y23 corresponding to the third and fourth recording materials, the overall printing rate is 50%, so a negative bias Vtg2 is applied to the pre-transfer upper guide 6. Yes. Thereby, it is possible to suppress the printed negative toner T2 from adhering to the guide.

However, even if the overall printing rate is 50%, the printing rate area of 0% is the entire printing area in the longitudinal direction and over the wide range of 200 msec or more in the recording material conveyance direction, as in the timing of Y16 to Y17. If it is in a part, the positive bias Vtg1 is applied to the region. As a result, it is possible to prevent the positive toner T <b> 1 that has been inverted such as fogging from adhering to the pre-transfer upper guide 6.

Here, a case will be described in which there is a printing rate 0% region in a part of the recording material in the longitudinal direction, such as the timing of Y20 to Y21 of the fourth recording material. In such a case, it is impossible to apply a positive bias only to that region in the present embodiment because of the guide structure. Therefore, in order to prevent the printed negative toner T3 from adhering to the pre-transfer upper guide 6, a negative bias is applied. Vtg2 is applied.
Further, a case will be described in which there is an area with a printing rate of 0% in a short range of less than 200 msec in the recording material conveyance direction, such as the timing of Y21 to Y22 of the fourth recording material. In such a case, since the interval in the recording material conveyance direction is too short and the bias applied to the pre-transfer upper guide 6 cannot reach the desired bias value, the polarity is not switched and the negative bias Vtg2 is set. Continue to apply. In this embodiment, at least 200 msec is required until the desired bias value is reached after the bias output polarity is switched.

Hereinafter, the result of verifying the effect in the present embodiment will be described.
Table 2 shows the amount of toner adhering to the pre-transfer upper guide 6 after printing 500 sheets of the three types of images shown in FIGS. 8A to 8C in the case of Example 1 and this example. The comparison result is shown.
In Example 1, a negative bias of −750 V was always applied to the pre-transfer upper guide 6 in the print area. On the other hand, in this embodiment, even in the print image area, if there is an area with a printing rate of 0% in a wide range in the recording material conveyance direction, a positive bias +750 V is applied to the upper guide 6 before transfer for that area. It is characterized by applying. Further, a positive bias +750 V is applied to the timings of the pre-rotation, paper interval, post-rotation, etc. excluding the print image area.

  In this example, the amount of toner adhering to the pre-transfer upper guide 6 was smaller than that of Example 1 (Comparative Example in Table 2). In addition, the lower the printing rate, the greater the improvement effect. The improvement effect of the present embodiment over the first embodiment is that the reversal toner in the printing rate 0% region is prevented from adhering to the pre-transfer upper guide 6. In other words, an image having a larger area with a printing rate of 0% is more effective. Also, the area with a printing rate of 0% tends to increase as the printed image becomes lower.

  Therefore, according to the present embodiment, the pre-transfer upper guide 6 is prevented from being soiled particularly when a large amount of character images, images with a large margin, etc. are printed. Image defects can be suppressed.

Here, in order to optimize the effect of the present embodiment, the threshold of the printing rate may be finely adjusted from 0% to another value, or the bias setting value may be changed stepwise according to the printing rate. .
In this embodiment, the printing rate calculated by the printing rate calculation means is 0 even in the printing area.
A positive bias is applied at the timing when the% region reaches the region B, but the present invention is not limited to this. That is, even if it is a printing area, it suffices if a positive bias is applied to the area where the printing rate is equal to or less than a predetermined value set in advance at the timing of reaching the area B. May be set as appropriate according to the specifications of the image forming apparatus.

[Example 3]
Example 3 will be described below.
In this embodiment, the conductive portion of the above-mentioned upper guide before transfer is divided in the longitudinal direction and a plurality of conductive portions are arranged. In the outer non-sheet passing area, different biases can be applied independently. With this configuration, in this embodiment, it is possible to suppress contamination of the pre-transfer upper guide in the non-sheet passing area when narrow paper is printed. Here, the sheet passing area is an area (opposite area) facing the conveyed (passing) recording material in the upper guide before transfer, and the recording is performed in a direction perpendicular to the printing surface of the recording material. This is an area projected onto the pre-transfer upper guide when a material is projected. The non-sheet passing area is an area other than the opposing area in the longitudinal direction of the upper guide before transfer.

FIG. 9 is a diagram showing a schematic configuration of the pre-transfer upper guide 30 in the present embodiment.
A recording material P indicated by a dotted line in FIG. 9 is an A4 size recording material and is conveyed in the direction of the arrow.
The pre-transfer upper guide 30 has two conductive portions, that is, a guide 32 disposed in a region (sheet passing region) facing the recording material P having A4 width, and an A4 width in the longitudinal direction via an insulating member 31. The guide 33 is arranged outside (non-sheet passing area) outside the area facing the recording material P. The pre-transfer upper guide 30 is made of a conductive material such as SUS.
The guides 32 and 33 are connected to a power source 13 that generates a positive bias Vtg1 and a power source 14 that generates a negative bias Vtg2 through different bias applying units 34 and 35, respectively. Here, in the present embodiment, the bias application units 34 and 35 and the power sources 13 and 14 constitute a power source unit.
Therefore, different biases can be independently applied to the guides 32 and 33 according to a selection signal from the CPU 20.

Here, a bias application operation when an A4 size recording material is printed by vertical feeding will be described.
For the guide 32 inside the A4 width, as in the first and second embodiments, the polarity and value of the bias applied during printing are set according to the magnitude relationship between the development potential and the drum potential and the result of the printing rate. Let's switch. Thereby, it is possible to suppress both the reverse positive toner and the negative toner from adhering to the guide 32.
On the other hand, since the guide 33 outside the A4 width is a non-sheet passing area, the inverted positive toner is more easily developed than the negative toner. Therefore, a positive bias is always applied to the guide 33 during printing. Thereby, it is possible to suppress the reversal toner developed in the non-sheet passing area from adhering to the guide 33.

  Accordingly, when a large amount of A4 size recording material is printed, the guide stains in the sheet passing area are maintained at the same level as in the first and second embodiments, while the guide stains in the non-sheet passing area are the same as in the first and second embodiments. Can also be suppressed.

In the configuration of this embodiment, even when printing on a small-sized recording material that is narrower than the A4 portrait, the bias control is the same as the A4 portrait feed, but it is part of the non-sheet passing portion. The amount of reversal toner adhering to the guide can be suppressed. Further, when a large-sized recording material wider than A4 length is printed, the same effect as in the first and second embodiments can be obtained by applying the same bias to both the guides 32 and 33.

Here, in the present embodiment, the pre-transfer upper guide 30 has a plurality of conductive portions arranged in the longitudinal direction according to the size in the width direction of the recording material (hereinafter referred to as width size). It is not limited. The pre-transfer upper guide 30 may be provided with a plurality of conductive portions so as to correspond to the size of the print area of the photosensitive drum 1 in the longitudinal direction. In this case, the guide 33 is arranged in a region of the pre-transfer upper guide 30 that opposes the region other than the print region (other than the image formation region) of the photosensitive drum 1 in the longitudinal direction.
Further, the pre-transfer upper guide 30 may be composed of a plurality of conductive portions (guides) according to the size in the width direction of a plurality of types of recording materials, and the plurality of conductive portions are arranged as follows. It is good that it is done. That is, the printing area in which the longitudinal area is changed corresponding to the size of the recording material to be printed (used), and the pre-transfer upper guide respectively facing the areas of the photosensitive drum 1 other than the printing area in the longitudinal direction. It may be arranged so that a voltage can be independently applied to 30 regions.
In addition, it has a detecting means for detecting the width size of the recording material, and the bias applied to each conductive portion is controlled to be switchable based on the detection result (the voltage application operation to each conductive portion is controlled). Be good). Further, if the configuration is such that the user sets the size of the recording material (for example, selected on the operation panel unit (display unit)) or sets the print mode according to the size of the recording material, the setting operation The bias applied to each conductive part may be configured to be switchable.
Further, by utilizing the printing rate information of the second embodiment, the printing rate is calculated for each area on the photosensitive drum 1 corresponding to (facing) a plurality of conductive portions arranged in the longitudinal direction, and printing of each calculated area is performed. A bias may be applied independently for each conductive portion based on the rate.

  DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum, 6 ... Upper guide before transfer, 8 ... Transfer roller, 12 ... Bias application part, 13, 14 ... Power supply, 20 ... CPU, N1 ... Transfer nip part, P ... Recording material

Claims (10)

A photoreceptor on which an electrostatic latent image is formed;
A developing member for developing the electrostatic latent image;
A transfer member that forms a transfer nip portion with the photoconductor, and transfers a toner image on the photoconductor to a recording material at the transfer nip portion;
A guide member that is disposed upstream of the transfer nip portion in the conveying direction and guides the recording material, and is disposed between the surface of the recording material on which the toner image is transferred and the photoreceptor . A guide member;
A power supply for applying a voltage to the guide member in order to suppress toner from adhering to the guide member;
In an image forming apparatus for forming an image on a recording material,
The polarity of the voltage applied to the guide member by the power supply unit is the same as the normal charging polarity of the toner when a region of the photoconductor having a surface potential higher than the developing potential of the developing member faces the guide member. Is applied to the guide member, and the region of the photoconductor having a surface potential smaller than the developing potential of the developing member is opposite to the guide member, the polarity is opposite to that of the first voltage. An image forming apparatus comprising: a control unit capable of controlling the power supply unit so as to apply a second voltage to the guide member . The control means includes
If the regular charging polarity of the toner is negative,
The value of the photosensitive member potential is, the area on the photosensitive member, wherein was located at the developing position when larger than the value of the potential of the developing member, at the timing when reaching the nearest region, polarity negative Is applied to the guide member,
The value of the photosensitive member potential is, areas on the photosensitive member, wherein was located at the developing position when less than the value of the potential of the developing member, at the timing when reaching the nearest region, the polarity is positive The image forming apparatus according to claim 1 , wherein the voltage is applied to the guide member. A printing rate deriving unit for deriving a printing rate of a toner image formed on the photoreceptor;
The control means includes
Among the areas on the photoconductor, an image forming area where a toner image for forming an image on one sheet of recording material can reach a closest area closest to the guide member, and When the printing rate derived by the printing rate deriving unit for the image forming area is larger than a predetermined value,
Applying the first voltage to the guide member,
The image forming area reaches the closest area during image formation at a time other than the arrival timing, or when the printing ratio derived by the printing ratio deriving unit is equal to or less than the predetermined value with respect to the image forming area. At the timing to
I you apply a pre-Symbol second voltage to the guide member urchin,
The image forming apparatus according to claim 1, wherein the power supply unit is controlled. The control means is arranged in the direction of the rotation axis of the photoconductor in the image forming area even when the printing ratio derived by the printing ratio deriving means with respect to the image forming area is larger than the predetermined value. When the printing rate derived by the printing rate deriving means for a part of the area is equal to or less than the predetermined value, the second voltage is set at the timing when the region reaches the closest region. you applied to the guide member urchin, the image forming apparatus according to claim 3, wherein the controller controls the power supply unit. The guide member is provided with a plurality of conductive portions with respect to the rotation axis direction of the photoconductor ,
The power supply unit, an image forming apparatus according to any one of claims 1 to 4, characterized in that it is possible to apply the independently voltage to each conductive portion. The plurality of conductive portions are arranged so that a voltage can be independently applied to an opposing area of the guide member facing the recording material to be conveyed and an area of the guide member other than the opposing area in the rotation axis direction. The image forming apparatus according to claim 5 , wherein the image forming apparatus is an image forming apparatus. Comprising a detecting means for detecting the size of the recording material in the width direction;
The image forming apparatus according to claim 6 , wherein the control unit controls a voltage application operation to each conductive portion based on a size in a width direction of the recording material detected by the detection unit. The second voltage or no voltage is applied to the conductive portion arranged in the region of the guide member other than the facing region in the rotation axis direction during image formation. The image forming apparatus according to claim 6 or 7 . Said control means, based on the printing rate of each region facing the respective conductive portions in the rotation axis direction of the the photosensitive member, according to claim 5, characterized in that for controlling the voltage application operation to the respective conductive portions 9. The image forming apparatus according to any one of items 1 to 8 . A photoreceptor on which an electrostatic latent image is formed;
A developing member for developing the electrostatic latent image;
A transfer member that forms a transfer nip portion with the photoconductor, and transfers a toner image on the photoconductor to a recording material at the transfer nip portion;
A guide member arranged on the upstream side of the transfer nip portion in the conveying direction for guiding the recording material, and disposed between the surface of the recording material on which the toner image is transferred and the photosensitive member; A guide member provided with a plurality of conductive portions with respect to the rotation axis direction of the photoreceptor;
In order to suppress the toner from adhering to the guide member, a voltage is applied to the guide member, and a power supply unit capable of independently applying a voltage to each conductive portion;
The polarity of the voltage applied to the guide member by the power supply unit is the same as the normal charging polarity of the toner when a region of the photoconductor having a surface potential higher than the developing potential of the developing member faces the guide member. The first voltage is applied to the guide member, and the region of the photoconductor having a surface potential lower than the developing potential of the developing member faces the guide member.
Control means capable of controlling the power supply unit so as to apply a second voltage having a polarity opposite to that of the voltage to the guide member;
A printing rate deriving unit for deriving a printing rate of a toner image formed on the photoreceptor;
In an image forming apparatus for forming an image on a recording material,
The control means derives the printing rate of each region facing each conductive portion in the rotation axis direction on the photosensitive member by the printing rate deriving unit, and based on the derived printing rate of each region, An image forming apparatus that controls an operation of applying a voltage to a conductive portion.

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