JP2008033014A - Image forming device and charging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the wear amount of the surface of a photoreceptor drum while suppressing the shift of toner and carrier from a developing device to the photoreceptor drum. <P>SOLUTION: In the case of supplying an AC current from a primary power source 51 to a charging roll 32 to charge the photoreceptor drum 31, a power supply control part 62 that controls the AC current to be supplied to the charging roll 32 sets the AC current value as the AC current having a first frequency and a first AC current value during an image formation period to form an image on the photoreceptor drum 31, on the other hand, during a non-image formation period, the control part 62 sets the AC current as the AC current having a second frequency lower than the first frequency and a second AC current value lower than the first AC current value. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using an electrophotographic system, for example.

  In an image forming apparatus such as a digital copying machine or a printer using an electrophotographic method, generally, the surface of a photosensitive drum rotating at a constant speed is uniformly charged by a charging device and then controlled based on image information. Laser light is scanned and exposed to form an electrostatic latent image. The electrostatic latent image formed on the photosensitive drum is developed by a developing device, and the developed toner image is electrostatically transferred onto the recording paper. Then, the toner image is fixed on the recording paper by the fixing device, and a print image is formed.

  In recent years, as a charging device for uniformly charging the surface of a photosensitive drum, a contact charging device using a charging roll as a contact charging member, for example, is widely used in place of the conventional scorotron. A contact charging device using a charging roll has a configuration in which a charging roll in which a conductive elastic layer or a conductive surface layer is coated around a cored bar is disposed so as to be in contact with the surface of the photosensitive drum. . The surface of the photosensitive drum is uniformly charged by applying a DC voltage or a DC voltage on which an AC voltage is superimposed on the charging roll as a bias voltage. Such a contact charging device has the advantage that the voltage value to be applied can be set low, so that the generation of ozone is small, and further, the high-voltage power supply can be reduced in size and cost. Suitable for image forming apparatus.

By the way, in such a contact charging device, the uniformity of the charged potential on the surface of the photosensitive drum can be improved by using a DC voltage on which an AC voltage is superimposed as a bias voltage. However, since the bias voltage superimposed with the alternating voltage vibrates the photosensitive drum, there is a technical problem that the surface of the photosensitive drum is easily worn during cleaning by the cleaning member. Therefore, when an AC voltage is used as the bias voltage, inconveniences such as occurrence of image defects and shortening of the life of the photosensitive drum are likely to occur due to wear on the surface of the photosensitive drum caused by the bias voltage.
In order to suppress such wear on the surface of the photosensitive drum, it is effective to shorten the application time of the AC voltage as much as possible. That is, from the viewpoint of suppressing the abrasion of the surface of the photosensitive drum as much as possible, it is possible to apply a DC voltage on which an AC voltage is superimposed only in an image forming operation necessary for improving the image quality in the contact charging device. preferable. As a conventional example of such a technique, a DC voltage on which an AC voltage is superimposed is applied during image operation, and during operation other than the image operation, the application of the AC voltage is stopped and only the DC voltage is applied. Some control the bias voltage (see, for example, Patent Document 1).

Japanese Patent Laying-Open No. 2004-184876 (pages 8-9)

  Here, in general, when the bias voltage is controlled so as to shorten the AC voltage application time as much as possible, the transition from the image forming operation to the non-image forming operation or vice versa is performed. At the time of transition to, switching processing for turning on / off the AC voltage at any timing is required. However, when the AC voltage is switched from ON to OFF or from OFF to ON, for example, after the control signal for turning ON the AC voltage is input to the charging power source, the AC voltage actually supplied rises to a normal waveform. A time lag of about 100 to 200 msec occurs due to the influence of the processing algorithm and the control circuit. For this reason, even if the AC voltage is switched ON / OFF, the surface potential of the photosensitive drum is not switched immediately but changes over time. As a result, a state occurs in which the potential difference between the surface potential of the photosensitive drum and the developing bias applied to the developing device deviates from a predetermined value, and the toner and the carrier are easily transferred from the developing device to the photosensitive drum. As a result, wasteful consumption of toner and occurrence of image defects may be induced.

  Accordingly, the present invention has been made to solve the technical problems as described above, and an object of the present invention is to suppress the transfer of toner and carrier from the developing device to the photosensitive drum, while suppressing the transfer of the photosensitive drum. It is to reduce the amount of surface wear.

  For this purpose, the image forming apparatus of the present invention includes a photosensitive member on which an electrostatic latent image is formed, a charging member that charges the photosensitive member, a power source that supplies an alternating current to the charging member, and a charging member from the power source. A power control unit that controls an alternating current supplied to the power source, and the power control unit sets the alternating current to the first frequency and the first alternating current value during an image forming operation for forming an image on the photoconductor, In the non-image forming operation, the AC current is set to a second frequency lower than the first frequency and a second AC current value lower than the first AC current value.

Here, the power supply control unit sets the first DC voltage value during the image forming operation when controlling the DC voltage that generates the AC current supplied by the power supply and the AC voltage superimposed on the DC voltage. In the image forming operation, the second DC voltage value having an absolute value larger than the absolute value of the first DC voltage value may be set.
The power supply control unit further includes a storage unit that stores a combination of the alternating current frequency and the alternating current value set by the power supply control unit, and the power supply control unit stores the first combination stored in the storage unit during the image forming operation. It is possible to set and set a second combination consisting of a lower frequency and a lower alternating current value than the first combination stored in the storage unit during the non-image forming operation. In particular, the storage unit may store a combination of an alternating current frequency and an alternating current value set corresponding to each of a plurality of process speeds applied to the image forming apparatus.

Furthermore, the power supply control unit sets the second frequency and the alternating current of the second alternating current value between the time when the image forming apparatus is started up and the time when the image forming operation is started. Can be a feature.
In addition, the image forming apparatus further includes a developing unit that develops the electrostatic latent image formed on the photosensitive member with toner held on a rotatable toner holding member, and the developing unit has a second frequency during the non-image forming operation. The rotation of the toner holding member may be started or stopped in a state where the alternating current is supplied.

Further, the present invention is regarded as a charging device. The charging device of the present invention includes a charging member for charging a photosensitive member on which an electrostatic latent image is formed, a power source for supplying an alternating current to the charging member, and a power source to a charging member. A power control unit that controls an alternating current to be supplied, and the power control unit sets the alternating current to the first frequency and the first alternating current value during an image forming operation for forming an image on the photosensitive member. The image forming step is characterized in that the alternating current is set to a second frequency lower than the first frequency and a second alternating current value lower than the first alternating current value.
Here, the power supply control unit sets the first DC voltage value during the image forming operation when controlling the DC voltage that generates the AC current supplied by the power supply and the AC voltage superimposed on the DC voltage. In the image forming operation, the second DC voltage value having an absolute value larger than the absolute value of the first DC voltage value may be set.

According to claim 1 of the present invention, the amount of wear on the surface of the photosensitive drum can be reduced. Further, transfer of toner and carrier from the developing device to the photosensitive drum can be suppressed, and wasteful consumption of toner and occurrence of image defects can be suppressed.
According to the second aspect of the present invention, it is possible to suppress a decrease in the latent image potential in the non-latent image forming step and to suppress transfer of toner or carrier to the photosensitive drum.
According to the third aspect of the present invention, it is possible to easily set the frequency and the alternating current value in the latent image forming step and the non-latent image forming step.

According to claim 4 of the present invention, in the image forming apparatus that can be driven at a plurality of process speeds, the frequency and the alternating current value can be easily set in the latent image forming step and the non-latent image forming step. it can.
Moreover, according to Claim 5 of this invention, it can suppress that a user senses the discharge sound from a charging member unpleasantly.
According to the sixth aspect of the present invention, wasteful consumption of toner can be suppressed, and generation of friction noise and turning / breaking of the cleaning member can be suppressed in the drum cleaning device.

According to the seventh aspect of the present invention, the amount of wear on the surface of the photosensitive drum can be reduced. Further, transfer of toner and carrier from the developing device to the photosensitive drum can be suppressed, and wasteful consumption of toner and occurrence of image defects can be suppressed.
According to the eighth aspect of the present invention, it is possible to suppress a decrease in the latent image potential in the non-latent image forming step and to suppress transfer of toner or carrier to the photosensitive drum.

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
[Embodiment 1]
FIG. 1 is a diagram illustrating a configuration of a digital color printer 1 as an example of an image forming apparatus to which the exemplary embodiment is applied. A digital color printer 1 shown in FIG. 1 is configured in a so-called tandem system, and an image forming process unit 20 that forms an image corresponding to image data of each color, and a control unit 60 that controls the image forming process unit 20, for example, An image processing unit (IPS: Image Processing System) 22 is connected to a personal computer (PC) 3 and the image reading device 4 and performs predetermined image processing on image data received therefrom.

The image forming process unit 20 includes four image forming units 30Y, 30M, 30C, and 30K that are arranged in parallel at a predetermined interval. The image forming units 30Y, 30M, 30C, and 30K (hereinafter collectively referred to as “image forming unit 30”) form an electrostatic latent image while rotating in the direction of arrow A, and further form a toner image. Developing a photosensitive drum 31 as a carrier, a charging roll 32 as an example of a contact charging member that uniformly charges the surface of the photosensitive drum 31 at a predetermined potential, and an electrostatic latent image formed on the photosensitive drum 31 Developing units 33Y, 33M, 33C, and 33K (hereinafter also collectively referred to as “developing unit 33”) as developing units to be developed, and a drum cleaner 34 that cleans the surface of the photosensitive drum 31 after transfer. . Further, a laser exposure device 26 that exposes the respective photosensitive drums 31 of the image forming units 30Y, 30M, 30C, and 30K is provided.
Here, each image forming unit 30 is configured in substantially the same manner except for the toner stored in the developing devices 33Y, 33M, 33C, and 33K. In each of the image forming units 30, yellow (Y), magenta (M), cyan (C), and black (K) toner images are formed.

  The image forming process unit 20 also includes an intermediate transfer belt 41 to which the toner images of the respective colors formed on the photosensitive drums 31 of the image forming units 30 are transferred, and the image forming units 30Y, 30M, 30C, and 30K. A primary transfer roll 42 that sequentially transfers (primary transfer) each color toner image to the intermediate transfer belt 41 in the primary transfer portion T1, and a superimposed toner image transferred on the intermediate transfer belt 41 is recorded in the recording material (secondary transfer portion T2). A secondary transfer roll 40 that performs batch transfer (secondary transfer) onto a sheet P that is a recording sheet) and a fixing unit 80 that fixes the secondary transferred image onto the sheet P are provided.

  In the digital color printer 1 of the present embodiment, the image forming process unit 20 performs an image forming operation based on a control signal supplied from the control unit 60. At that time, the image data input from the PC 3 or the image reading device 4 is subjected to predetermined image processing by the IPS 22 and supplied to the laser exposure device 26. In the yellow (Y) image forming unit 30Y, for example, the surface of the photosensitive drum 31 uniformly charged at a predetermined potential by the charging roll 32 is applied by the laser exposure device 26 based on the image data obtained from the IPS 22. An electrostatic latent image is formed on the photosensitive drum 31 by scanning exposure. The formed electrostatic latent image is developed by the developing device 33Y, and a Y toner image is formed on the photosensitive drum 31. Similarly, M, C, and K color toner images are also formed in the image forming units 30M, 30C, and 30K.

Each color toner image formed by each image forming unit 30 is sequentially electrostatically transferred by the primary transfer roll 42 onto the intermediate transfer belt 41 that rotates in the direction of arrow B in FIG. A toner image is formed. As the intermediate transfer belt 41 moves, the superimposed toner image is conveyed toward the secondary transfer portion T2 where the secondary transfer roll 40 and the backup roll 49 are disposed. On the other hand, the paper P is taken out from the paper tray 71 by the pickup roll 72 and conveyed one by one to the position of the registration roll 74 by the conveyance roll 73.
When the superimposed toner image is conveyed to the secondary transfer portion T2, the paper P is supplied from the registration roll 74 to the secondary transfer portion T2 in accordance with the timing at which the toner image is conveyed to the secondary transfer portion T2. The superimposed toner images are collectively electrostatically transferred (secondary transfer) onto the paper P by the action of the transfer electric field formed between the secondary transfer roll 40 and the backup roll 49 in the secondary transfer portion T2. Is done.

Thereafter, the sheet P on which the superimposed toner image is electrostatically transferred is peeled from the intermediate transfer belt 41 and conveyed to the fixing device 80. The unfixed toner image on the paper P conveyed to the fixing device 80 is fixed on the paper P by being subjected to fixing processing by heat and pressure by the fixing device 80. Then, the paper P on which the fixed image is formed is conveyed to a paper discharge placement unit 91 provided in a discharge unit of the image forming apparatus. On the other hand, the toner (transfer residual toner) adhering to the intermediate transfer belt 41 after the secondary transfer is removed by the belt cleaner 45 in contact with the intermediate transfer belt 41 after the completion of the secondary transfer, and the next image forming cycle. Prepared for.
In this way, image formation is performed by the digital color printer 1.

Next, the contact charging device 50 provided in the digital color printer 1 of the present embodiment will be described.
FIG. 2 is a diagram illustrating a configuration of the contact charging device 50 according to the present embodiment. As shown in FIG. 2, the contact charging device 50 includes a charging roll 32 as an example of the contact charging member (charging member) described above, and an alternating current (AC) voltage is superimposed on a direct current (DC) voltage with respect to the charging roll 32. The power supply control unit 62 generates a control signal for controlling the charging bias voltage supplied to the charging roll 32 and outputs the control signal to the primary power supply 51.
In FIG. 2, the contact charging device 50 disposed in the image forming unit 30K is shown as an example, but the same contact charging device 50 is also disposed in the image forming units 30Y, 30M, and 30C.

  The charging roll 32 is a roll member formed by sequentially laminating a conductive elastic body layer and a conductive surface layer on a conductive core bar such as aluminum or stainless steel. The conductive elastic layer is composed of urethane rubber, epichlorohydrin rubber, polyether urethane rubber, polyester urethane rubber, chloroprene rubber, NBR rubber, SBR rubber, butyl rubber, etc., and conductive particles such as carbon black and metal oxide. Are distributed. The conductive surface layer is made of polyester, polyamide, polyurethane, melamine resin, acrylic resin such as PMMA or PMBA, polyvinyl butyral, polyvinyl acetal, polyarylate, polycarbonate, phenoxy resin, polyurea, polyvinyl acetate and the like as a binder. In addition, conductive particles such as carbon black and metal oxide are dispersed. The charging roll 32 is driven to rotate while being in contact with the photosensitive drum 31, and supplies a current to the charging roll 32.

  The primary power source 51 is a constant current control type power source that controls the output voltage value of the AC voltage to the charging roll 32 so that the alternating current (AC) current value Iac from the charging roll 32 to the photosensitive drum 31 becomes a predetermined value. It is. Since the primary power supply 51 of such a constant current control method can keep the amount of charge supplied to the photosensitive drum 31 constant, the charged potential of the photosensitive drum 31 can be stabilized.

  The power supply control unit 62 is a CPU (Central Processing Unit) that performs arithmetic processing, an I / O circuit that manages input / output with the primary power supply 51 and a memory (not shown), and a ROM (ROM) that stores processing programs. Read Only Memory) and RAM (Random Access Memory) as a primary storage unit. Then, based on the instruction signal from the control unit 60, the power supply control unit 62, for example, in each of image formation and non-image formation, the AC voltage (the AC component of the charging bias voltage: simply “AC component”) is used. A control signal for setting the frequency of (described below) to a predetermined value, a control signal for setting the AC current value Iac supplied to the charging roll 32 to a predetermined value, and a DC voltage applied to the charging roll 32 (DC of the charging bias voltage). A control signal or the like for setting the component (also simply referred to as “DC component”) to a predetermined value is output to the primary power source 51.

Next, switching control of the AC component frequency and the AC current value Iac performed by the contact charging device 50 of the present embodiment will be described.
First, FIG. 3 is an example of a timing chart showing operation timings of the image forming units 30Y, 30M, 30C, and 30K. In FIG. 3, as an example, when an image forming operation of two cycles is performed, that is, after an image forming operation of the first cycle (first sheet) is performed, a non-image forming operation in which the image forming operation is not performed is performed. The case where the image forming operation of the second cycle (second sheet) is executed and then the image forming operation is stopped is shown.
The digital color printer 1 of the present embodiment is configured in a tandem system, and each image forming unit 30 is driven by a common drive source. Therefore, when the operation of the digital color printer 1 starts (starts), all the image forming units 30 start driving. Then, in order to superimpose the toner image on the intermediate transfer belt 41 without positional deviation, the image forming operation in each image forming unit 30 is executed while being shifted by a predetermined timing.

Therefore, as shown in FIG. 3, each image forming unit 30 according to the present embodiment is provided with a (A) pre-waiting step between the start of driving and the start of the image forming operation. ing. In the digital color printer 1 having the configuration shown in FIG. 1, since image formation is started in the order of the image forming units 30Y, 30M, 30C, and 30K, (A) in the pre-standby step, the image forming unit 30Y on the most upstream side The shortest, the longer the image forming unit 30 installed on the downstream side, the longer the image forming unit 30K on the most downstream side.
Similarly, for example, in the image forming units 30Y, 30M, and 30C, after the last page image forming operation is finished, the last page image forming operation in the most downstream image forming unit 30K is finished. (E) A post-waiting step is provided. (E) In the post-standby step, the image forming unit 30Y on the most upstream side is the longest, the image forming unit 30 installed on the downstream side is shorter, and the image forming unit 30K on the most downstream side is the shortest.

Accordingly, in each image forming unit 30, (A) a pre-standby step, (B) an image forming step in the first cycle (first sheet), (C) a paper standby step corresponding to the conveyance interval of the paper P, (D) The second cycle (second sheet) image forming process, (E) standby process, (F) the entire surface of the photosensitive drum 31 is uniformly discharged, and the charging history remains on the photosensitive drum 31. Post-processing steps such as suppression processing are sequentially performed. Here, in any of the (A) pre-standby process and (C) paper standby process, which is a process in which an image forming operation is not performed (hereinafter referred to as “non-image forming process”), the toner density is adjusted. A step of forming a patch can also be assigned.
In the most downstream image forming unit 30K, the (E) post-waiting step is not necessary, and the (F) post-processing step is immediately executed.

Next, the operation of each part in each image forming unit 30 will be described.
Here, in the contact charging device 50 of the present embodiment, charging is performed in the (A) pre-waiting step, (C) paper waiting step, and (E) post-waiting step which are non-image forming steps (during non-image formation). The frequency of the AC component of the bias voltage and the AC current value Iac are set to a frequency lower than the frequency of the AC component applied in the image forming process (at the time of image formation) of (B) and (D), and ( The AC current value Iac is set lower than the AC current value Iac supplied in the image forming steps B) and (D). Correspondingly, the DC components of the charging bias voltage applied in (A) the pre-standby step, (C) the paper stand-by step, and (E) the post-standby step are used to form the image formation in (B) and (D). It is set so as to shift to the negative side with respect to the DC component applied in the process.

FIG. 4 is an example of a timing chart showing the application timing of the bias voltage and the setting value of the charging bias voltage in each part of each image forming unit 30.
As described above, the execution time of the (A) pre-standby step and the (E) post-standby step is different for each image forming unit 30, and the (E) post-standby step is not executed in the image forming unit 30K.
The timing chart shown in FIG. 4 will be described in detail below.
(A) Pre-standby process First, in the image forming unit 30, when image data is input to the IPS 22 from, for example, the PC 3 or the image reading device 4, the photosensitive drum 31 starts (starts), and the photosensitive drum 31 is rotated. For example, a predetermined pre-process such as a pre-exposure process is performed. Then, the primary power supply 51 is connected from the charging roll 32 under the control of the power supply control unit 62 in accordance with the timing when the region on the surface of the photosensitive drum 31 on which the predetermined pretreatment has been performed reaches the charging roll 32. Supply of the AC current value Iac = 1.0 mA by constant current control to the photosensitive drum 31 is started. The charging bias voltage at that time is set such that the DC component is -750 V, for example, and the frequency of the AC component is 800 Hz, for example. As a result, the photosensitive drum 31 is charged to a charging potential V _H = −680V.
The AC current value Iac = 1.0 mA and the frequency of the AC component: 800 Hz are set lower than those in the image forming steps (B) and (D). The DC component: −750 V is higher on the minus side than the image forming steps (B) and (D), that is, the absolute value of the DC component is set large. This reduces the amount of wear on the surface of the photosensitive drum 31 as will be described later.
Further, substantially simultaneously with the start of application of the charging bias voltage to the charging roll 32, a developing bias voltage of, for example, a DC voltage of −580V is applied to the developing device 33.

(B) First cycle (first sheet) image forming step And in synchronization with the start of scanning exposure by the laser exposure device 26 based on the image data, the power supply control unit 62 Then, the setting is changed so that the AC current value Iac = 2.5 mA by constant current control is supplied from the charging roll 32 to the photosensitive drum 31. The charging bias voltage at that time is set such that the DC component is -700 V, for example, and the frequency of the AC component is 1800 Hz, for example. As a result, the photosensitive drum 31 is charged to approximately the same charging potential V _H = −680 V as in the preceding standby step (A).
The power supply control unit 62 sets the AC current value Iac, the frequency of the AC component, and the DC component based on the instruction signal from the control unit 60.
Then, when scanning exposure by the laser exposure device 26 is started, an electrostatic latent image of, for example, an image portion potential V _L : −50 V and a background portion potential (= charge potential V _H ): −680 V is formed.
Further, a developing bias voltage of DC voltage: −580 V is applied to the developing device 33 in the same manner as in the preceding standby step (A). For this reason, the electrostatic latent image (image portion potential: −50 V, background portion potential: −680 V) formed on the photosensitive drum 31 has toner in the image portion due to a potential difference from the developing bias voltage (DC voltage: −580 V). Is developed.
Subsequently, in accordance with the timing at which the toner image formed on the photosensitive drum 31 reaches the primary transfer portion T1, the transfer bias voltage (for example, +1500 V) having a polarity opposite to the charging polarity of the toner is applied to the primary transfer roll 42. Is applied. As a result, the toner image held on the photosensitive drum 31 is transferred onto the paper P.

(C) Paper Waiting Step Next, after the image forming operation (image forming step) of the first cycle (first sheet) is completed and before the image forming operation of the second cycle (second sheet) is started. The sheet standby process corresponding to the sheet P conveyance interval is executed.
In this paper standby process, the developing bias voltage is maintained at a DC voltage of −580 V in the image forming process of (B), and the transfer bias voltage is also maintained at the set value (+1500 V) in the image forming process of (B). Is done.
In the contact charging device 50 according to the present embodiment, the charging bias voltage in the (C) paper standby process is set in the same manner as the set value in the (A) standby process. That is, the AC current value Iac = 1.0 mA, the DC component is set to −750 V, and the frequency of the AC component is set to 800 Hz. Thereby, the amount of wear on the surface of the photosensitive drum 31 is reduced.

(D) Second cycle (second sheet) image forming step After the passage of the paper standby step (C), the second cycle (second sheet) image forming step is the same as the (B) image forming step. Executed. Here, the same setting as that in the image forming step (B) is performed for each part. That is, the development bias voltage is maintained at DC voltage: −580V, and the transfer bias voltage is also maintained at the set value (+ 1500V).
Further, in the contact charging device 50, the AC current value Iac = 2.5 mA, the DC component is set to −700 V, and the frequency of the AC component is set to 1800 Hz.

(E) Post standby process After the image forming process of (D) is completed, a predetermined period of time until the image forming process of the second cycle in the image forming unit 30K arranged at the most downstream part is completed ( E) A post standby process is performed. Here, while the amount of wear on the surface of the photosensitive drum 31 is reduced, a predetermined potential state in the photosensitive drum 31 and the developing device 33 is maintained until the image forming process in the image forming unit 30K in the most downstream portion is completed. Thus, toner transfer from the developing device 33 to the photosensitive drum 31 is suppressed.
For this reason, in the (E) post-standby step, the same setting as in the (A) pre-standby step and (C) paper standby step is set. That is, the AC current value Iac = 1.0 mA, the DC component is set to −750 V, and the frequency of the AC component is set to 800 Hz.

(F) Post-processing step After the post-standby step (E) is completed, the entire surface of the photoconductive drum 31 is uniformly discharged, and a process for suppressing the charging history from remaining on the photoconductive drum 31 is performed. A post-processing step is performed. In this (F) post-processing step, the transfer bias voltage, the charging bias voltage, and the developing bias voltage are sequentially turned off, and then the driving of the digital color printer 1 is stopped (end).

As described above, in the digital color printer 1 of the present embodiment, the frequency of the AC component is lowered in the (A) pre-standby process, (C) paper standby process, and (E) post-standby process that are non-image forming processes. By setting the AC current value Iac to be low, the wear amount of the photosensitive layer (CTL: charge transport layer) of the photosensitive drum 31 is reduced.
In experiments by the present inventors, there is a correlation between the frequency of the AC component included in the charging bias voltage and the amount of wear of the photosensitive layer of the photosensitive drum 31 as the image forming cycle progresses. It has been found that the amount of wear of the photosensitive layer tends to increase as the value is set. As this mechanism, the AC component of the charging bias voltage applied to the charging roll 32 gives vibration to the photosensitive drum 31, which is a cleaning blade 34 a (see FIG. 2) installed on the drum cleaner 34 and the photosensitive drum 31. As a result of increasing the frictional force between the two, it is estimated that the surface of the photosensitive drum 31 is scraped.

Therefore, in the digital color printer 1 of the present embodiment, (A) a pre-waiting process, (C) a paper waiting process, and (E) which are non-image forming processes other than the image forming processes of (B) and (D). In the post-standby process, the frequency of the AC component is set low. Thereby, the frictional force between the cleaning member and the photosensitive drum 31 in the non-image forming process can be set low. Therefore, it is possible to reduce the amount of wear on the surface of the photosensitive drum 31 when the image forming cycles are repeated.
In addition, in the non-image forming steps (A), (C), and (E), the AC current value Iac is also set low in response to setting the frequency of the AC component low. Accordingly, the vibration energy applied from the charging roll 32 to the photosensitive drum 31 can be reduced at the same time, so that the effect of reducing the amount of wear on the surface of the photosensitive drum 31 is further enhanced.

Further, by setting the frequency of the AC component low in (A) the pre-standby step, (C) the paper stand-by step, and (E) the post-standby step, the following action also works.
As described above, the wear amount of the photosensitive layer increases as the frequency of the AC component included in the charging bias voltage is set higher. Therefore, if importance is attached to the effect of reducing the wear amount of the photosensitive drum 31, in principle, it is preferable to completely turn off the AC component during non-image formation.
However, if the AC component is completely turned off at the time of non-image formation, the transition from the image formation process to the non-image formation process, for example, (A) at the start of the pre-waiting process or (C) from the paper standby process to the next. The following inconvenience occurs at the time of shifting to the image forming step (D).

That is, when the AC component is completely turned off during the non-image forming process, for example, in the paper standby process of (C), only the DC component of −750 V is applied to the charging roll 32 (AC current value Iac≈0 mA). Therefore, the charging potential V _H of the photosensitive drum 31 is about 0V. That is, the charging potential V _H hardly occurs.
When the AC component is turned on again during the transition from the potential state of the photosensitive drum 31 in the paper standby process (C) to the image forming process (D), the AC current value Iac starts from 0 mA. At this time, in order to stabilize the AC current value Iac to a normal waveform, a time lag of about 100 to 200 msec occurs after the AC component is turned on. As a result, at the initial operation of the image forming step (D), the charging potential V _H of the photosensitive drum 31 increases from 0 V to −680 V with time, over a period of 100 to 200 msec.
On the other hand, in the image forming step (D), since the DC voltage: −580 V is applied as the developing bias voltage, the toner and the carrier are transferred to the photosensitive drum 31 in the process in which the charging potential V _H increases from 0 V to −680 V. An area is formed that transitions to. That is, since there is a region where the cleaning potential V _CLN (= development bias voltage−charging potential V _H ) that affects toner adhesion to the background portion and carrier adhesion is about −580 V−0 V = −580 V, the developing device 33 The amount of toner and the amount of carrier transferred from the toner to the photosensitive drum 31 increases. As a result, wasteful consumption of toner is caused, and the occurrence of image defects due to transferred carriers is caused.

  In response to this, the time lag of 100 to 200 msec generated in the AC component is considered in advance, and the start-up time of the AC component is set to be advanced by 100 to 200 msec from the time of transition to the image forming process of (D). It is also possible to do. However, in the contact charging device 50 of the present embodiment, for example, a process of shifting the DC component from −750 V in the paper standby process (C) to −700 V in the image forming process (D) is also performed. At that time, the DC component also has a time lag of about 50 to 100 msec in the voltage shift. Then, the time lag of each of the AC component and the DC component is measured, and based on the measured value, the AC current value Iac is stabilized to a normal waveform, and the DC component is completely shifted to −700V. Even if such a setting is made, there are many cases where they cannot always be matched due to the influence of the processing algorithm, control circuit, etc. in the power supply control unit 62.

In this case, if the shift of the AC component is delayed with respect to the DC component, the charging potential V _H does not rise sufficiently as described above, so that the absolute value of the charging potential V _H is the development bias voltage (DC voltage: − 600V), the amount of toner transferred to the photosensitive drum 31 increases.
Conversely, when the AC component shifts faster than the DC component, the AC current value Iac = 2.5 mA is supplied with the DC component remaining at a voltage close to the setting value: −750 V in the paper standby process of (C). Will be. In this case, the cleaning potential V _CLN (= development bias voltage−charging potential V _H ) is about −580 V − (− 750 V) = about 170 V, and the positively charged carrier is easily transferred to the photosensitive drum 31. Become. As a result, the occurrence of image defects may be induced.
The inconveniences as described above are the same at the time of shifting from the image forming process of (B) to the paper standby process of (C) and at the time of shifting from the image forming process of (D) to the post standby process of (E). Occurs.

Therefore, in the contact charging device 50 of the present embodiment, the frequency of the AC component applied in the non-image forming step (A) pre-standby step, (C) paper standby step, and (E) post-standby step is The frequency is set lower than the frequency of the AC component applied in the image forming step (B) or (D).
By setting in this way, the time lag in the frequency shift can be made extremely small as compared with ON / OFF of the AC component. Therefore, it is possible to substantially maintain the charge potential V _H at each step, it is possible to reduce the amount of toner and the amount of carriers transferred to the photosensitive drum 31.
Further, since there is no need to consider in advance the time lag caused by ON / OFF of the AC component, the transfer of the carrier to the photosensitive drum 31 can be suppressed.

The same applies to the AC current value Iac. That is, the time lag in the shift of the AC current value Iac can be made extremely small compared to the ON / OFF of the AC current value Iac. Therefore, even if the AC current value Iac is shifted (here, the setting value of the DC component is also shifted simultaneously), the charging potential V _H in each step can be substantially maintained and transferred to the photosensitive drum 31. The amount of toner and the amount of carrier to be kept can be kept low.

Here, the crest factor of the AC component of the charging bias voltage formed by the primary power supply 51 is preferably about 1.41 (= √2). The crest factor is defined as crest value / effective value (crest value = peak-to-peak value _Vp−p / 2 of AC component), and the crest factor is closer to 1.41 (= √2), The waveform is close to a sine wave. As the AC component waveform is closer to the sine wave, the charging efficiency can be increased, and a stable charging potential V _H can be formed even with a relatively small AC current value Iac. Therefore, by forming the crest factor of the AC component at approximately 1.41 (= √2), the charging potential V _H can be set more stably at the time of transition between the image forming process and the non-image forming process. It becomes possible.

Subsequently, (A) in the pre-standby step, the frequency of the AC component of the charging bias voltage is set low and the AC current value Iac is set low, so that the discharge sound from the charging roll 32 becomes annoying for the user. It also works as difficult.
As described above, in the pre-standby step (A), the photosensitive drum 31 starts to rotate, and after a predetermined pretreatment is performed on the photosensitive drum 31, application of the charging bias voltage is started. However, depending on the image forming apparatus, for example, in the case of performing a quick start, for example, a sheet transport system from the pick-up roll 72 to the fixing device 80 after a predetermined time has elapsed after the rotation of the photosensitive drum 31 is started. Etc. may be started.
In that case, since there is no driving sound of the paper conveyance system or the like when the application of the charging bias voltage is started, the discharging sound by the charging roll 32 is likely to enter the user's ear. However, even in that case, since the frequency of the AC component of the charging bias voltage is set low and the AC current value Iac is set low, the discharge sound from the charging roll 32 is unlikely to be annoying to the user.

In addition, it is preferable that the rotation start timing of the developing roll 33a (see FIG. 2) of the developing device 33 in (A) the pre-standby step is after the charging bias voltage is applied.
(A) In the pre-standby step, a charging bias voltage is applied, the charging potential V _H of the photosensitive drum 31 is set to −680 V, and the developing bias voltage of the developing device 33 is set to a DC voltage: −580 V. The amount of toner transferred to the photosensitive drum 31 is set by starting the developing roll 33a of the developing unit 33 when the cleaning potential V _CLN is set to about 100V. This is because the carrier amount can be kept low.
At this time, since the amount of toner supplied to the drum cleaner 34 is small, the frictional force between the cleaning member such as the cleaning blade 34 a and the photosensitive drum 31 tends to increase. However, (A) in the pre-standby step, as described above, the frictional force between the cleaning member and the photosensitive drum 31 is reduced by setting the frequency of the AC component low and setting the AC current value Iac low. Set low. Thereby, even when the amount of toner interposed between the cleaning member and the photoconductive drum 31 is small, the generation of frictional noise caused by the cleaning member and the photoconductive drum 31 is suppressed. The occurrence of damage can be suppressed.
For the same reason, the rotation stop timing of the developing roll 33a (see FIG. 2) of the developing device 33 in the post-processing step (F) is preferably before the charging bias voltage is turned off.

Next, in the contact charging device 50 of the present embodiment, the DC of the charging bias voltage applied in the (A) pre-standby step, (C) paper standby step, and (E) post-standby step that are non-image forming steps. The operation by setting the component so as to shift to the minus side from the DC component applied in the image forming steps (B) and (D) will be described.
As described above, in (A) the pre-standby step, (C) the paper stand-by step, and (E) the post-standby step, the charging bias voltage is set such that the frequency of the AC component is, for example, 800 Hz. The AC current value Iac is set to 1.0 mA. For this reason, if the DC component is kept at the same set value as the DC component applied in the image forming steps (B) and (D), the charging ability by the AC current value Iac is reduced. It can not be maintained at -680V to the charging potential _{V H.} In particular, there is a decrease tends to increase the charge potential V _H at a low temperature and low humidity environment.
Furthermore, when the frequency of the AC component is set low, the potential ripple generated in the charging potential V _H of the photosensitive drum 31 tends to increase. For example, when the frequency of the AC component is 1800 Hz, for example, the potential ripple is about 5V, but when the frequency is 800 Hz, it becomes about 15V.

Therefore, for example, in the (A) pre-standby step, (C) paper standby step, and (E) post-standby step, the DC component is applied in the image forming steps (B) and (D): −700 V In the case of setting to 1, the charging potential V _H of the photosensitive drum 31 is lowered to about −630V. In addition, since the potential ripple due to the charging potential _{V H} is 15V, the cleaning potential _{V CLN} become (= developing bias voltage - - charging potential _{V H} potential ripple / 2) of about 43~57V about, toner photosensitive It becomes easy to transfer to the body drum 31.
Therefore, in (A) the pre-standby step, (C) the paper stand-by step, and (E) the post-standby step, the set value −750 V is obtained by shifting the DC component of the charging bias voltage to the negative side. Thereby, even if the charging ability is reduced due to the AC current value Iac, the charging potential V _H of the photosensitive drum 31 can be set to −680V. Therefore, about 100 V or more at the cleaning potential V _CLN can be secured, and the transfer of the toner to the photosensitive drum 31 can be suppressed.

  As described above, in the digital color printer 1 of the present embodiment, the charging bias applied in the (A) pre-standby step, (C) paper standby step, and (E) post-standby step that are non-image forming steps. The frequency of the AC component of the voltage is set lower than the frequency of the AC component applied during the image formation of (B) and (D). At the same time, the AC current value Iac in the non-image forming step (A) pre-standby step, (C) paper standby step and (E) post-standby step is determined in the image forming steps (B) and (D). It is set lower than the supplied AC current value Iac. As a result, it is possible to reduce the wear amount of the photosensitive drum 31 when the image forming cycle is repeated while suppressing transfer of toner and carrier to the photosensitive drum 31.

[Embodiment 2]
In the first embodiment, in the digital color printer 1 operating at a single process speed, the frequency of the AC component of the charging bias voltage in the non-image forming process is set low and the AC current value Iac is set low. explained. However, in recent years, for example, in order to stabilize toner fixability according to the paper thickness of the paper P to be used, a configuration in which the process speed setting is changed corresponding to the paper P may be used. Therefore, in the present embodiment, in the digital color printer 1 in which a plurality of process speeds are set, the frequency of the AC component of the charging bias voltage in the non-image forming process is set low, and the AC current value Iac is set low. A configuration to be set will be described. In addition, the same code | symbol is used about the structure similar to Embodiment 1, and the detailed description is abbreviate | omitted here.

The digital color printer 1 of the present embodiment has a configuration similar to that shown in FIG. 1 and is configured to be able to operate at a plurality of process speeds. In the present embodiment, the control unit 60 determines the paper type of the paper P loaded in the paper tray 71 or designates the paper type by the user from the operation panel (not shown). Set the process speed corresponding to.
FIG. 5 is a diagram showing a configuration of the contact charging device 50 of the present embodiment. As shown in FIG. 5, in the contact charging device 50 of the present embodiment, the power supply control unit 62 includes the frequency of the AC component, the AC current value Iac, and the DC component supplied from the control unit 60 to the charging roll 32. In response to the instruction signal instructing the set value, the control signal according to the instruction signal from the control unit 60 is output to the primary power source 51.
Here, the storage unit 65 is connected to the control unit 60 of the present embodiment, and the storage unit 65 has the frequency of the AC component and the AC current value Iac set in the power supply control unit 62 according to the process speed. Are stored as a table. Then, the control unit 60 reads the AC component frequency and the AC current value Iac according to the process speed from the storage unit 65, and outputs them to the power supply control unit 62.

FIG. 6 is a diagram illustrating an example of the relationship between the frequency of the AC component set in the power supply control unit 62 from the control unit 60 and the process speed. The controller 60 sets the frequency of the AC component according to the process speed shown in FIG. 6 in order to form a stable charging potential V _H on the surface of the photosensitive drum 31.
In this case, FIG. 7 is a diagram showing an example of the relationship between the frequency of the AC component and the AC current value Iac in order to form a stable charging potential V _H on the surface of the photosensitive drum 31. Similarly, the control unit 60 sets an AC current value Iac corresponding to the frequency of the AC component shown in FIG. 7 to form a stable charging potential V _H on the surface of the photosensitive drum 31.
As described above, in the contact charging device 50 according to the present embodiment, based on the instruction signal from the control unit 60, a stable charging potential _VH is formed on the surface of the photosensitive drum 31 according to each process speed. A combination of the frequency of the AC component and the AC current value Iac is set.

In the digital color printer 1 of the present embodiment, it is set in the non-image forming process (A) pre-standby process, (C) paper standby process, and (E) post-standby process (see FIGS. 3 and 4). As the AC component frequency and the AC current value Iac, a combination of the AC component frequency and the AC current value Iac set corresponding to a process speed slower than the currently set process speed is selected.
For example, if the currently set process speed is PS1 in FIG. 6, (A) pre-waiting step, (C) paper waiting step, and (E) post-waiting step (see FIGS. 3 and 4). The combination of the frequency of the AC component and the AC current value Iac set in is read from the storage unit 65 and used corresponding to any one of PS2 to PS4 that is slower than PS1. The same applies to PS2 and PS3.
However, for example, in the slowest process speed PS4, since there is no slower process speed setting, (A) the pre-waiting step, (C) the paper waiting step, and (E) the post-waiting step (FIGS. 3 and 4). It is necessary to separately store the combination of the frequency of the AC component and the AC current value Iac set in (see) in the storage unit 65.
In the present embodiment as well, as in the case of the first embodiment, the crest factor of the AC component of the charging bias voltage formed by the primary power supply 51 is approximately 1.41 (= √2). preferable.

As described above, in the digital color printer 1 according to the present embodiment, the non-image forming operation is performed using the frequency of the AC component and the AC current value Iac for each process speed stored in the storage unit 65 (A). The frequency of the AC component and the AC current value Iac in the pre-standby process, (C) paper standby process, and (E) post-standby process are set.
Thus, it is not necessary to separately prepare the AC component frequency and AC current value Iac in (A) the pre-standby step, (C) the paper stand-by step, and (E) the post-standby step for each process speed. It is possible to easily set the charging bias voltage in the pre-standby process, (C) paper standby process, and (E) post-standby process.

1 is a diagram illustrating a configuration of a digital color printer of the present invention. It is the figure which showed the structure of the contact charging device. 3 is an example of a timing chart showing operation timing of each image forming unit. 6 is an example of a timing chart showing bias voltage application timing and charging bias voltage setting values in each part of each image forming unit. It is the figure which showed the structure of the contact charging device. It is the figure which showed an example of the relationship between the frequency of the AC component set to a power supply control part from a control part, and a process speed. It is the figure which showed an example of the relationship between the frequency of AC component, and AC current value Iac.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Digital color printer, 20 ... Image formation process part, 30Y, 30M, 30C, 30K ... Image formation unit, 31 ... Photosensitive drum, 32 ... Charging roll, 33Y, 33M, 33C, 33K ... Developer, 34 ... Drum Cleaner 40 ... secondary transfer roll 41 ... intermediate transfer belt 42 ... primary transfer roll 50 ... contact charging device 51 ... primary power source 60 ... control unit 62 ... power control unit 65 ... storage unit 80 ... Fixing device

Claims (8)

A photoreceptor on which an electrostatic latent image is formed;
A charging member for charging the photoreceptor;
A power source for supplying an alternating current to the charging member;
A power control unit for controlling an alternating current supplied from the power source to the charging member,
The power supply control unit sets the alternating current to a first frequency and a first alternating current value during an image forming operation for forming an image on the photoconductor, and the alternating current to the first alternating current value during a non-image forming operation. An image forming apparatus comprising: a second frequency lower than a first frequency and a second alternating current value lower than the first alternating current value.   The power supply control unit sets a first DC voltage value during the image forming operation when controlling a DC voltage that generates the AC current supplied by the power supply and an AC voltage superimposed on the DC voltage. 2. The image forming apparatus according to claim 1, wherein a second DC voltage value having an absolute value larger than an absolute value of the first DC voltage value is set during the non-image forming operation. A storage unit for storing a combination of the alternating current frequency and the alternating current value set by the power supply control unit;
The power supply control unit sets the first combination stored in the storage unit during the image forming operation, and is lower than the first combination stored in the storage unit during the non-image forming operation. The image forming apparatus according to claim 1, wherein a second combination including a frequency and a low alternating current value is set.   4. The storage unit according to claim 3, wherein a combination of the alternating current frequency and the alternating current value set corresponding to each of a plurality of process speeds applied to the image forming apparatus is stored. Image forming apparatus.   The power supply control unit sets the alternating current of the second frequency and the second alternating current value between the time when the image forming apparatus is started up and the time when the image forming operation is started. The image forming apparatus according to claim 1. A developing unit that develops the electrostatic latent image formed on the photoreceptor with toner held on a rotatable toner holder;
The developing unit starts or stops the rotation of the toner holding member in a state where the alternating current having the second frequency and the second alternating current value is supplied during the non-image forming operation. The image forming apparatus according to claim 1. A charging member for charging a photoreceptor on which an electrostatic latent image is formed;
A power source for supplying an alternating current to the charging member;
A power control unit for controlling an alternating current supplied from the power source to the charging member,
The power supply control unit sets the alternating current to a first frequency and a first alternating current value during an image forming operation for forming an image on the photoconductor, and the non-image forming step uses the alternating current as the first current. And a second AC current value lower than the first AC current value.   The power supply control unit sets a first DC voltage value during the image forming operation when controlling a DC voltage that generates the AC current supplied by the power supply and an AC voltage superimposed on the DC voltage. 8. The charging device according to claim 7, wherein a second DC voltage value having an absolute value larger than an absolute value of the first DC voltage value is set during the non-image forming operation.

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