JP4393220B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using an electrophotographic system, and more particularly to an image forming apparatus provided with a contact charging device that charges a photosensitive member by contacting a charging roller.

  Conventionally, in an image forming apparatus such as a copying machine or a printer using such an electrophotographic system, a non-contact corona discharge is used as a charging device for uniformly charging the entire surface of the photosensitive member. The device is widely used. However, the corona discharge device is effective as a means for uniformly charging the surface of the photoconductor with a certain potential, but since a high voltage power source is required, ozone is generated, and if this increases, the human body is adversely affected. There was a problem that.

  In view of this, recently, a contact-type charging device that charges a photosensitive member by contacting a charging roller has been proposed and commercialized. Such a contact-type charging device has the advantage that the amount of ozone generated is extremely small because the required voltage is low. In addition, the contact charging device is advantageous in that the cost is lower than that of the corona discharge device and the space can be saved.

However, in such a contact-type charging device, since the charging roller is always pressed against the photosensitive member, if the photosensitive member and the charging roller are left in a stopped state,
(1) Low molecular substances contained in the charging roller ooze out from the surface of the charging roller at the nip portion and adhere to the photosensitive member.
(2) When pressed for a long time, permanent distortion remains on the charging roller, resulting in uneven charging.
(3) When an amorphous silicon photoconductor is used as the photoconductor, a slight corona discharge occurs even in contact charging using a charging roller, resulting in a charged product, and the charged product remaining in the vicinity of the nip portion is the photoconductor. It adheres to the surface and generates latent image blur (image flow) under high humidity.
There was a problem.

  Therefore, Patent Literature 1 discloses a solution to such a problem. This is a pressure release means for holding the charging member (charging roller) and the surface to be charged (photosensitive member) in a non-pressure contact state when the device is not used for a long period of time, such as before the device is shipped. By preventing the contamination, the contamination of the surface of the member to be charged and the deformation or deterioration of the charging member due to the pressure contact are prevented.

In addition, an “image forming apparatus” is disclosed that reduces the contact pressure between a charging unit (charging roller) and an image carrier (photoconductor) during use of the apparatus (see, for example, Patent Document 2). This means that the charging means is brought into contact with the image carrier during charging, and the charging means and the image carrier are brought into non-contact state or are in contact with each other at a pressure smaller than that during charging except during charging. Thus, deformation and deterioration of the charging means are prevented even during use of the apparatus.
JP-A-2-39169 Japanese Patent Laid-Open No. 3-48870

However, in the configuration described in each of the above patent documents,
(1) A system in which the charging roller is separated from the image carrier or the contact pressure is variable is structurally complicated and expensive.
(2) Even if the charging roller is separated or the contact pressure is variable, the charged product is present in the vicinity of the nip portion of the charging roller, so that the image flow of the charged product portion cannot be prevented.
Such problems remain unresolved.

  In particular, in recent years, in the market of copying machines, printers, and the like using electrophotography and electrostatic printing, printing speed and machine durability have been remarkably advanced. Along with the increase in durability of machines, many products using amorphous silicon as a photoreceptor have been commercialized. However, when an amorphous silicon photoconductor is used as the photoconductor, there is an image flow as a problem to be overcome. Image flow occurs when the charged product generated in the charging unit adheres to the surface of the photoconductor, which bonds with water molecules in the air, and the latent image is blurred due to a decrease in resistance in the direction of the photoconductor surface. It is.

  Such image flow has been thought to be reduced by changing the charging system from a corona discharge system to a contact roller system, which reduces charged products (such as nitrogen oxides). However, for example, the problem that image flow occurs when printing under high humidity after being left for a long time from the previous print has not been eliminated. In general, an amorphous silicon photoconductor is provided with a heater as a countermeasure against image flow. However, in recent years, energy saving is progressing, and a heaterless system is becoming mainstream. In the heaterless system, the image flow immediately below the charging roller becomes more prominent.

  In view of the above-described problems, the present invention has a simple configuration, avoids image flow, recovers permanent distortion of the charging roller, and further absorbs low molecular weight in the charging roller material that has oozed out on the surface of the photoreceptor. An object of the present invention is to provide an image forming apparatus capable of removing a substance and avoiding the occurrence of an image defect due to contamination on the surface of a photoreceptor.

In order to achieve the above object, according to the present invention, in an image forming apparatus having a charging roller that contacts a surface of a photosensitive member and charges the surface of the photosensitive member, the charging roller is moved to the photosensitive member with the photosensitive member stopped. Provide a function to rotate while being in contact with the body, measure the relative humidity Y% RH around the device, and set the time for rotating the charging roller to Y / 2 seconds or more based on the relative humidity Features.

Further, when the relative humidity is 60% RH or more, the time for rotating the charging roller is set to (Y / 2 + 5) seconds or more . Further, the photoconductor is an amorphous silicon photoconductor. Further, a voltage having the same polarity as the toner used is applied to the charging roller while the charging roller is rotating.

According to the present invention, in an image forming apparatus of a type in which the charging roller is in contact with the photosensitive member, the charging roller is moved for Y / 2 seconds or more based on the relative humidity Y% RH around the device with the photosensitive member stopped. The charged product can be removed with a simple configuration of rotating while in contact with the photoreceptor, thereby avoiding image flow, restoring permanent distortion of the charging roller, and further oozing out on the surface of the photoreceptor. The low-molecular substance in the charged roller material can be removed to prevent the charging roller from sticking to the photoreceptor.

Moreover, by determining the time to rotate the charging roller based on the humidity around the device, it is possible to sufficiently eliminate image blurring.

  Further, by using amorphous silicon for the photoreceptor, it is possible to take advantage of removing the charged product while maintaining durability.

  Further, by applying a voltage having the same polarity as the toner used to the charging roller while the charging roller is rotating, the toner on the charging roller can be returned to the surface of the photoreceptor.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram schematically showing a schematic structure of an image forming apparatus to which the present invention is applied. In the figure, reference numeral 1 denotes a photosensitive drum using amorphous silicon as a photosensitive member, for example. Around the photosensitive drum 1, along the rotation direction, a charging roller 2, an exposure unit 3, a developing unit 4, a developing unit 4, a transfer roller 5, a rubbing roller 7, A cleaning blade 6 and a static eliminator 8 are provided.

  FIG. 2 shows the structure of the charging roller. The charging roller 2 uniformly charges the peripheral surface of the photosensitive drum 1 to a predetermined polarity and potential while contacting the outer periphery of the photosensitive drum 1. The charging roller 2 includes a center core 2d, a rubber layer 2c formed on the outer periphery of the center core 2d, and two resistance layers 2b and 2a sequentially formed on the outer periphery of the rubber layer 2c. Yes. In this case, the resistance layer 2a is formed thinner than the resistance layer 2b.

  Both end portions of the center metal core 2d protrude from the rubber layer 2c, and a pressing mechanism (not shown) is provided at both end portions, and the charging roller 2 is applied to the photosensitive drum 1 by the pressing mechanism during image formation. It is designed to rotate while being pressed by the pressing force.

  In the present invention, the photosensitive drum 1 made of amorphous silicon has a structure in which an amorphous silicon photosensitive layer 20 is provided on a conductive substrate 12 as shown in FIG. The blocking layer 11, the amorphous silicon photoconductive layer 10, and the surface protective layer 9 are preferably used.

  The material for forming the photoconductive layer 10 in the photosensitive layer 20 is not particularly limited as long as it is amorphous silicon (a-Si). Preferred materials include a-Si, a-SiC, and a-SiO. , A-SiON and the like. Among these materials, a-SiC (amorphous silicon carbide) is particularly suitable as a photosensitive layer material because it has a particularly high resistance and can provide better charging ability, abrasion resistance, and environmental resistance.

  The above-described amorphous silicon photosensitive drum 1 is uniformly charged positively by the charging roller 2. After charging, the exposure device 3 irradiates light such as laser light based on predetermined image information to perform image exposure. By this light irradiation, the potential of the portion irradiated with light is lowered, and an electrostatic latent image is formed. The electrostatic latent image formed on the surface of the photosensitive drum 1 is developed by the developing device 4 to form a toner image on the surface of the photosensitive drum 1. Development by the developing device 4 may be performed by a regular development method or a reversal development method. When developing by the reversal development method, the light irradiation part by the image exposure becomes an image part, and the part not irradiated with light becomes a background part of the image. In normal development, the reverse pattern is obtained.

  As a developer used for development, a non-magnetic toner or a one-component developer composed of a magnetic toner in which magnetic powder is dispersed in a resin, a non-magnetic or magnetic toner and a magnetic carrier (for example, iron powder or ferrite) The developer is supplied to the surface of the photosensitive drum 1 by a known developing device 4 so that a positively charged toner is obtained in the reverse development, and a negatively charged toner is obtained in the normal development. Then, development is performed by adhering to the electrostatic latent image forming portion, and a toner image is formed.

  The toner image formed as described above is transferred onto a sheet passing between the photosensitive drum 1 and the transfer roller 5 by the transfer roller 5 to which a transfer bias is applied. That is, the toner image is transferred from the surface of the photosensitive drum 1 to the surface of the sheet by an electric field formed between the photosensitive drum 1 and the transfer roller 5 by the transfer bias. Therefore, when the charging polarity of the toner image is positive (in the case of reversal development), a negative transfer bias is applied to the transfer roller 5, and when the charging polarity of the toner image is negative (in the case of regular development). A positive transfer bias is applied to the transfer roller 5.

  After the transfer of the toner image onto the paper by the transfer roller 5, the paper having the transferred toner image is introduced into a fixing device (not shown), and the toner image is fixed on the paper surface by heat and pressure. After that, it is discharged out of the device. On the other hand, after the transfer by the transfer roller 5 is completed, the photosensitive drum 1 is in frictional contact with the rubbing roller 7. The rubbing roller 7 can be rubbed and rotated with respect to the photosensitive drum 1 to remove charged products and the like attached to the surface of the photosensitive drum 1 due to the frictional contact. Therefore, as such a rubbing roller 7, it is preferable to use a rubber roller such as EPDM (ethylene / propylene / diene / monomer) in order to ensure a sufficient nip width (contact width) with the photosensitive drum 1. .

  After the photosensitive drum 1 comes into contact with the rubbing roller 7, the toner remaining on the surface of the photosensitive drum 1 is removed by the cleaning blade 6, and the residual charge is removed by the static eliminator 8. As the cleaning blade 6, a rubber blade made of polyurethane or the like is usually used in pressure contact with the surface of the photosensitive drum 1, but a fur brush or the like may be used as necessary. Further, as the static eliminator 8, an LED or the like is used, and the static elimination is performed by light irradiation. However, a charging roller, a corona charger, or the like may be used.

  The rubbing roller 7 is composed of a metal shaft and the elastic member as described above, and is provided in pressure contact with the photosensitive drum 1 by providing a pressing means such as a spring at both ends of the metal shaft. The rubbing roller 7 is made of a conductive material, and is configured to hold a certain amount of transfer residual toner by applying a voltage and to always efficiently and uniformly polish the surface of the photosensitive drum 1. In order to perform this polishing satisfactorily, it is preferable to add about 1 to 3% of titanium oxide or the like having a polishing effect to the toner used for development.

  In this image forming apparatus, in this embodiment, when the power supply is turned on under high humidity, based on the humidity information of the surrounding environment, only the charging roller is rotated for a predetermined time while the photosensitive drum is stopped. It is configured to perform roller refreshing (removing toner attached to the charging roller, recovering permanent distortion of the charging roller, and removing charged product attached to the photosensitive drum).

  FIG. 7 is a block diagram showing the configuration of the control system of the image forming apparatus according to Embodiment 1 of the present invention. In the figure, 51 is a control unit constituted by, for example, a microcomputer, 52 is a fixing temperature sensor provided inside a fixing device (not shown) for measuring a fixing temperature, and 53 is a humidity sensor for measuring the humidity around the apparatus. Reference numeral 54 denotes a motor that rotates the charging roller 2, 55 denotes a voltage supply circuit that applies a voltage to the charging roller 2, and 56 denotes a transfer bias supply circuit that applies a transfer bias to the transfer roller 5.

  In this control system, the control unit 51 drives the motor 54 to rotate the charging roller 2 based on the measured values by the fixing temperature sensor 52 and the humidity sensor 53, and applies a voltage to the charging roller 2 by the voltage supply circuit 55. The transfer bias supply circuit 56 controls the transfer roller 5 to apply a transfer bias. Specific control contents by this control system will be described below.

  FIG. 4 is a flowchart when the power is turned on in the image forming apparatus according to Embodiment 1 of the present invention. As shown in the figure, first, after the power is turned on, in step S5, the control unit 51 determines from the measurement value of the fixing temperature sensor 52 whether or not the fixing temperature is 80 ° C. or higher. If the fixing temperature is not 80 ° C. or higher, the process proceeds to step S10. This is because only when the fixing temperature is low, the apparatus is regarded as being turned on after being left for a long time (for example, 3 hours or more), and roller refreshing is performed.

  Next, in step S <b> 10, the control unit 51 determines whether the humidity is 65% or more from the measurement value of the humidity sensor 53. And if humidity is 65% or more, it will transfer to Step S15. In this case, the higher the humidity is, the worse the image flow becomes, so the refresh time is determined based on the measured humidity value. Subsequently, roller refresh is started in step S15. Finally, the process proceeds to step S20, and an image can be formed. If the fixing temperature is 80 ° C. or higher in step S5, and if the humidity is not 65% or higher in step S10, the process proceeds to step S20, and an image can be formed.

  FIG. 5 is a timing chart at the time of roller refresh. As shown in the figure, the charging roller 2 is rotated in a state where the photosensitive drum 1 is not rotationally driven (stopped state) and a state where no developing bias is applied. In this case, the charging roller 2 is rotated by the motor 54 while being in contact with the stopped photosensitive drum 1. At this time, the voltage supply unit 55 may rotate the charging roller 2 while applying a voltage thereto, or may rotate the charging roller 2 without applying a voltage.

  In this embodiment, a voltage having the same polarity as that of the toner used is applied to the charging roller 2 (+800 V in this embodiment). This is because the toner or the like adhering to the surface of the charging roller 2 is transferred to the surface of the photosensitive drum 1 to clean the surface of the charging roller. At this time, the transfer bias is applied to the transfer roller 5 by the transfer bias supply circuit 56, and this transfer bias has the same polarity as the toner used (+ in this embodiment). This is to prevent contamination of the transfer roller 5 by preventing toner and the like from moving from the photosensitive drum 1 to the transfer roller 5.

  The roller refresh time (refresh time) is set based on Y / 2 seconds when the humidity is Y% RH (relative humidity) in this embodiment. If it is too short for Y / 2 seconds, the image flow is not sufficiently eliminated, and if it is too long for Y / 2 seconds, unnecessary rotation is performed, and the startup time of the apparatus is wasted. Because. FIG. 6 shows the relationship between an example of the refresh time at each humidity and the image flow.

  According to this, it is understood that the presence / absence of image flow is determined by the refresh time with Y / 2 seconds as a reference. Specifically, for example, when the humidity is 60%, the reference refresh time is 30 seconds, and image flow occurs in 25 seconds shorter than this, and image flow does not occur in longer 35 seconds. The same applies to the case where the humidity is 70% or more. However, when the humidity is 50%, no image flow occurs regardless of the refresh time.

  In addition, in order to avoid the possibility that the charged product or the like adheres to the surface of the photosensitive drum 1 before and after the nip portion of the charging roller 2 and remains adversely, the charging roller 2 is also provided before and after the nip portion. It may be configured to perform roller refresh by moving and rotating. Specifically, after the charging roller 2 is rotated at the nip portion, the photosensitive drum 1 is rotated by ± α in the forward and backward directions within a narrow range before and after the nip portion, and the charging roller 2 at each position. It is good also as a structure which rotates.

  Alternatively, a configuration may be adopted in which the charging roller 2 is rotated at each position by moving the photosensitive drum 1 three times such as one rotation-α, one rotation, and one rotation + α so as not to reversely rotate. . However, in any method, the region where the charging roller 2 rotates and rubs against the photosensitive drum 1 before and after the nip portion needs to be connected to the nip portion.

  As described above, according to the image forming apparatus of the present invention, after the humidity of the surrounding environment is detected by the humidity sensor, the charging roller is rotated in a state where the photosensitive drum is stopped, so that it adheres to the vicinity of the nip portion. The charged product can be removed by polishing, and as a result, the image flow generated in the nip portion can be effectively avoided in a short time. At this time, even if the charged product removed by polishing adheres to the photosensitive drum again, there is no problem because it is removed by the above-described cleaning blade (cleaning blade 6, see FIG. 1).

  Even if the device is left for a long time, even if the low-molecular substance in the charging roller material oozes out and adheres to the photosensitive drum, it can be effectively removed by roller refreshing. Can be avoided.

  Furthermore, even if permanent charging occurs on the charging roller by leaving the device for a long time, the charging roller recovers to its original shape while the charging roller rotates due to roller refresh. Unevenness can be avoided.

  In the above embodiment, the image forming apparatus of the type in which the charging roller 2 is always in contact with the surface of the photosensitive drum 1 has been described. However, as in the above-mentioned Patent Document 1, the present invention is charged when the apparatus is not used. The present invention can also be applied to an image forming apparatus that separates rollers. In this case, the charging roller may be brought into contact with the surface of the photosensitive drum prior to image formation, and a time for rotating the charging roller while the photosensitive drum is stopped in that state may be provided.

  Note that since the image flow phenomenon may occur even in cases other than the amorphous silicon photoconductor, the present invention can be suitably applied to such a case. Needless to say, the present invention can be applied not only to the drum-shaped photosensitive drum as shown in this embodiment but also to a belt-shaped photosensitive drum.

1 is a diagram schematically showing a schematic structure of an image forming apparatus to which the present invention is applied. The figure which shows the structure of a charging roller. FIG. 3 is a diagram schematically illustrating a layer cross-sectional structure of a photosensitive drum. The flowchart at the time of power-on. Timing chart during roller refresh. The figure which shows the relationship between the refresh time and image flow in each humidity. The block diagram which shows the structure of a control system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging roller 3 Exposure device 4 Developing device 5 Transfer roller 6 Cleaning blade 7 Rub roller 8 Static eliminator

Claims (4)

In an image forming apparatus having a charging roller that contacts the surface of the photoreceptor and charges the surface of the photoreceptor,
A function of rotating the charging roller while being in contact with the photoconductor while the photoconductor is stopped ,
An image forming apparatus , wherein the relative humidity Y% RH around the apparatus is measured, and the time for rotating the charging roller is set to Y / 2 seconds or more based on the relative humidity . 2. The image forming apparatus according to claim 1 , wherein when the relative humidity is 60% RH or more, the time for rotating the charging roller is set to (Y / 2 + 5) seconds or more .   The image forming apparatus according to claim 1, wherein the photoconductor is an amorphous silicon photoconductor.   4. The image forming apparatus according to claim 1, wherein a voltage having the same polarity as the toner to be used is applied to the charging roller during rotation of the charging roller. 5.

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