JP2009020326A - Image forming method and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming method and an image forming apparatus by which stable image quality is obtained over a long term by suppressing lowering of image density and fogging even if image forming with little toner consumption is consecutively performed, and also developer is prevented from being needlessly discharged. <P>SOLUTION: The developer is carried on a developer carrier facing a photoreceptor to develop an electrostatic latent image formed by an electrophotographic system on the photoreceptor. The amount of the developer used for such development is cumulatively added as the number of dots in the formed image, and also an average printing rate is calculated according to an cumulative added value and the number of image forming sheets. When the calculated average printing rate is smaller than a predetermined reference printing rate, the developer of an amount equivalent to one round of the developer carrier is forcibly consumed, and a cycle time taken until next forcible consumption is regarded as a time corresponding to the average printing rate. Thereby, an image defect caused by deterioration in the developer is prevented. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming method and an image forming apparatus in a copying machine, a printer, a facsimile, and a composite machine using an electrophotographic method, and particularly, when image formation with a low printing rate is continuously performed. The present invention relates to an image forming method and an image forming apparatus capable of effectively preventing a reduction in image density and fogging that occur.

  In image forming apparatuses such as copiers, printers, facsimiles, and composite machines using electrophotography, a uniformly charged photoconductor is exposed with image data to form an electrostatic latent image on the photoconductor. Development is performed by supplying the developer to the electrostatic latent image from a developer carrying body (hereinafter referred to as a developing roller) carrying the developer facing the surface.

  However, at this time, it is known that various image defects such as a decrease in image density and fogging occur when the state in which the image portion of the image to be formed is small and the amount of developer to be used continues. For example, the amount of developer (number of dots) used for actual image formation compared to the amount of developer (number of dots) used when the entire surface of the image is black (solid for each color used in the case of color) If the ratio is defined as the printing rate, normally, when characters are printed on the entire paper, this printing rate is about 4 to 6%.

  However, when image formation continues with this printing rate being as small as 3% or less, the amount of developer used on the photosensitive member used for developing the electrostatic latent image on the developing roller is small. The change of the developer particles in the developer is reduced, and the charge control external additive is peeled off or embedded on the contrary due to excessive charging of the developer, charging becomes unstable, and various image defects such as image density reduction and fogging occur.

  Since these problems do not occur when an image with a high printing rate is output, for example, Patent Document 1 calculates the printing rate, and if the printing rate is smaller than a predetermined printing rate, the developing unit is controlled. Thus, an image forming apparatus for forcibly consuming the developer is shown. This forced developer consumption may always be performed at a constant amount regardless of the printing rate, and the sum of the developer amount consumed by development and the developer amount due to forced consumption becomes, for example, 5% in terms of printing rate. It is said that it may be forced to consume.

  Further, Patent Document 2 includes an image area ratio detection unit that detects an area ratio of a formed image and a development drive time detection unit that detects a development drive time in the developing device, and an image area ratio per unit drive time of development. An image forming apparatus in which the developer is forcibly consumed according to the above is shown.

JP-A-4-068370 JP 2003-76079 A

  However, particularly in the one-component development system, the developer held on the developing roller hardly detaches from the developing roller until it is used for developing the electrostatic latent image on the photoreceptor. For this reason, the same developer stays on the developing roller for a long time when outputting an image with a low printing rate, and only the developer on the developing roller may cause deterioration such as peeling or embedding of an external additive. .

  That is, as disclosed in Patent Documents 1 and 2, with the measure for forcibly consuming the amount of developer corresponding to the amount of developer consumed for development, the amount of developer forced consumption less than one rotation of the developing roller is In this case, since the next developing operation is performed while the deteriorated developer remains on the developing roller, there is little effect on solving the problems such as image density and fog. On the other hand, when the printing rate is very small, the developer for one turn or more of the developing roller may be forcibly consumed. In this case, the developer that has not deteriorated is discharged, which is uneconomical. It is.

  Therefore, in the present invention, even when image formation with low toner consumption is continuously performed, it is possible to suppress image density reduction and fogging, to obtain stable image quality over a long period of time, and to discharge excess developer. An object is to provide an image forming method and an image forming apparatus without any problems.

In order to solve the above problems, an image forming method according to the present invention includes:
An electrostatic latent image formed on a photoconductor by an electrophotographic method is developed with a developer carried on a developer carrier facing the photoconductor, and the amount of developer used for the development is determined by the number of dots in the formed image. In the image forming method in which the developer is forcibly consumed on the photosensitive member between sheets based on the average print rate calculated by the cumulative addition value and the number of image formed sheets.
The forced consumption of the developer is determined whether the calculated average printing rate is less than a predetermined reference printing rate. If the calculated average printing rate is equal to or higher than the reference printing rate, the forced consumption is not performed. The developer for one cycle of the developer carrying member is forcibly consumed, and the cycle time until the next forcible consumption is increased or decreased in accordance with the average printing rate.

An image forming apparatus that implements the image forming method includes:
A photoconductor that forms an electrostatic latent image by electrophotography, and a developing device that has a developer carrier that faces the photoconductor and develops the electrostatic latent image on the photoconductor with a carried developer; Stored in the accumulated dot number storage means for storing the cumulative addition amount of the developer used for the development as the number of dots in the formed image, the image formation number counting means for counting the number of image formations, and the accumulated dot number storage means Average printing rate calculating means for calculating a developer consumption amount per formed image as an average printing rate based on the number of accumulated dots and the number of formed images counted by the image forming number counting means. In the image forming apparatus having a control unit for forcibly consuming the developer onto the photoconductor by the average printing rate calculated by the rate calculating unit,
The control means forcibly consumes the developer for one rotation of the developer carrier in a state where the average printing ratio calculated by the average printing ratio calculation means is less than a predetermined reference printing ratio, and until the next forced consumption. The cycle time is configured to increase or decrease in accordance with the average printing rate.

  As described above, when the average printing rate is less than the predetermined reference printing rate, the developer forcibly consuming the developer for one round of the developer carrying member, and when the developer is forcibly consumed, on the developer carrying member. The development operation is not performed with the deteriorated developer remaining, and the forced consumption is less than the circumference of the developer carrier as in the prior art. There is almost no possibility of occurrence of problems such as fogging. As a result, it is possible to reduce the occurrence of uneven image density and fog in the developing roller cycle. In addition, the forced consumption of the developer increases or decreases the cycle time until the next forced consumption corresponding to the calculated average printing rate, so that the developer can be discharged according to the size of the average printing rate, and the developer It is possible to obtain an economical and stable image quality over a long period of time without discharging the developer for one rotation or more of the carrier and without discharging the developer that has not deteriorated. An image forming method and an image forming apparatus can be provided.

  The cycle time until the next forced consumption of the developer is determined based on the number of image forming sheets corresponding to the calculated average printing rate, and therefore, a plurality of average printing rates that require the forced consumption of the developer. It is a preferred embodiment of the present invention to have a table storing the number of image forming sheets for determining the cycle time until the next forced developer consumption corresponding to each of the above.

  In addition, the forced consumption of the developer may be performed without developing the photosensitive member, or when the photosensitive member is charged, the region corresponding to one round of the developing roller is exposed and developed, and the photosensitive member is charged. By performing the transfer bias for transferring the developer image formed by developing the electrostatic latent image on the body onto the recording paper as a reverse bias, the deteriorated developer can be easily forcibly consumed.

  As described above, according to the present invention, when the average printing rate is lower than the reference printing rate, the developer for one rotation of the developer carrier is forcibly consumed, and the cycle time of the forced consumption is set to the average printing rate. Since the number of image formations corresponding to each other is set, it is possible to effectively discharge the deteriorated developer on the developer carrier without discharging extra developer, and there is a problem such as a decrease in image density and fogging. It is possible to provide an image forming method and an image forming apparatus capable of obtaining a stable image quality over a long period of time without the possibility of occurrence.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, but are merely illustrative examples. Not too much.

  FIG. 1 is a schematic sectional view of a laser printer as an example of an image forming apparatus for carrying out the image forming method of the present invention, FIG. 2 is a block diagram of a control circuit for performing the image forming method of the present invention, and FIG. FIG. 4 is a graph showing the results of an experiment on the relationship between the number of output sheets and the image density when a large number of images are formed using a document with a low printing rate. .

  First, the outline of the present invention will be briefly described. In the present invention, similarly to Patent Document 1, the amount of developer used for development is cumulatively added as the number of dots, and the average print rate is calculated from the value and the number of images formed. If the calculated average printing rate is less than the standard printing rate, the developer is forcibly consumed between papers to prevent image defects due to developer deterioration. The developer for one round of the agent carrier (hereinafter referred to as a developing roller) is discharged.

  In other words, the developer is forcibly consumed when image formation with a low printing rate continues in the above-described prior art. For example, the forced developer consumption in these prior arts is an average print every 10 sheets of image formation. 0.1% when the rate is between 3% and 2%, 0.3% when the average print rate is between 2% and 1%, and 0.5% when the average print rate is between 1% and 0% In particular, the amount corresponding to the amount (small) of the developer consumed in the development is forcibly consumed. Therefore, when the forced consumption is small (when the average printing rate is large), the developing roller 1 If the amount is less than the circumference and the forced consumption is large (when the average printing rate is small), it may be more than one circumference of the developing roller. However, when the forced consumption amount is less than one rotation of the developing roller, the deteriorated developer remains on the developing roller, and when it exceeds one rotation of the developing roller, the undegraded developer is consumed.

  On the other hand, in the present invention, since the developer for one rotation of the developing roller is always discharged, the deteriorated developer does not remain on the developing roller, so that problems such as a decrease in image density and fogging occur. Since there is no possibility that the undegraded developer is consumed, there is no inconvenience of discharging the undegraded developer.

  However, when the developer for one rotation of the developing roller is always consumed at regular intervals as described above, it is a matter of course that the printing rate is relatively large although the forced printing of the developer is required but the printing rate is relatively large. The forced consumption of developer increases.

  Therefore, the forced consumption of the developer in the present invention is determined by determining the number of image forming sheets for which the forced developer consumption is performed in correspondence with the average printing rate calculated in advance, and increasing or decreasing the cycle time of the forced developer consumption. However, if the printing rate is relatively high, the forced consumption cycle time is lengthened, and the cycle time is shortened as the printing rate decreases, and the developer consumption corresponding to the average printing rate is large. By doing so, the deteriorated developer is effectively discharged without waste.

  The above is the outline of the present invention. Next, the outline of the image forming apparatus will be described using the schematic cross-sectional view of a laser printer as an example of the image forming apparatus for carrying out the image forming method of the present invention shown in FIG. To do. The image forming apparatus shown in FIG. 1 is an example of an image forming apparatus using a magnetic one-component developer (toner). In the following description, the image forming apparatus shown in FIG. 1 will be described as an example. It is obvious that the present invention can be applied to, for example, an image forming apparatus using a nonmagnetic developer.

  In the image forming apparatus shown in FIG. 1, a photosensitive drum 10 having a diameter of, for example, 30 mm and using a positively charged single-layer type organic photosensitive member is disposed at approximately the center thereof, and around the photosensitive drum 10. Toner 11 formed on the photosensitive drum 10 by development, a charging unit 11 that charges the peripheral surface of the toner to, for example, positive 475 V, a developing unit 14 that includes a developing roller 14 and a rotating spiral blade member 15 that stirs the developer, and the like. A transfer roller 17 for transferring the image onto the recording paper, a cleaning device 18 for cleaning the developer remaining without being transferred onto the photosensitive drum 10, and the like are disposed.

  A paper transport path 20 is provided at a substantially central portion in the vertical direction of the printer main body, an exposure device 12 having a laser scanning unit and the like is disposed at the upper portion thereof, and a paper feed cassette is provided at a position indicated by 21 at the lower portion. It is detachably arranged. The sheet conveying path 20 extends substantially horizontally in the tangential direction so that the sheet can pass between the photosensitive drum 10 and the transfer roller 17, and the upstream end region of the sheet conveying path 20 is inverted downward and is denoted by 21. The downstream end region of the sheet conveyance path 20 extends upward and is connected to the sheet discharge tray 28.

  A pickup roller 25 is disposed above the front end of the paper feed cassette at the position indicated by 21, and the upper surface of the paper front end (not shown) housed in the paper feed cassette is pressed against the pickup roller 25. ing. For this reason, when the pickup roller 25 rotates, the sheets are taken out one by one and passed through the sheet conveying path 20 by the sheet feeding roller 24 and the conveying roller 23 to the registration roller 22 provided on the upstream side of the photosensitive drum 10. . The registration roller 22 has a function of correcting the posture of the fed paper and feeding the paper in accordance with the timing when the toner image formed on the photosensitive drum 10 comes to the position of the transfer roller 17.

  On the other hand, a fixing device 26 for fixing the transferred toner image onto the paper is provided on the downstream side of the photosensitive drum 10 in the paper conveyance path 20, and a discharge roller 27 is further provided on the downstream side thereof. The sheet on which the image is fixed is discharged to the paper discharge tray 28. The remaining toner that is not transferred onto the sheet on the photosensitive drum 10 is cleaned by a cleaning device 18, and this cleaning device 18 includes a counter blade (not shown) and is provided on the circumferential surface layer of the photosensitive drum 10. It is configured to remove residual toner and the like.

  The developing unit 13 includes a developing housing including a developing chamber, and a so-called one-component developer made of magnetic toner is accommodated in the developing chamber. In the developing chamber, a developing roller 14 provided with a stationary permanent magnet (not shown) and a developer stirring / conveying mechanism including a plurality of rotating spiral blade members 15 are disposed. A magnetic brush of the developer conveyed by the blade member 15 is formed. Then, one end of the developing housing and the side facing the photosensitive drum 10 is opened, and a partial region of the peripheral surface of the developing roller 14 is exposed outward from the opening. In addition, a toner container 16 whose upper side can be opened is disposed on the upper portion of the developing housing, and the toner container 16 contains magnetic one-component toner.

  A developing bias is applied to the developing roller 14 in order to cause toner to fly to the electrostatic latent image formed on the photosensitive drum 10, and a transfer roller 17 is used to transfer the toner image from the photosensitive drum 10 to a sheet. A transfer bias is applied to. The development bias is composed of direct current and alternating current. For example, as the direct current bias, a direct current voltage of 340 V having the same polarity (plus) as the charging polarity of the photosensitive drum 10 is used, Vp-p 1.6 kV, frequency 2.6 kHz, duty ratio A rectangular wave AC bias with a value of 50 is superimposed.

  This developing bias causes a potential difference between the developing roller 14 and the photosensitive drum 10, and the toner between the developing roller 14 and the photosensitive drum 10 is forcibly moved from the developing roller 14 to the photosensitive drum 10. Then, it adheres and is developed.

  Next, a block diagram of a control circuit for performing the image forming method of the present invention shown in FIG. 2 will be described. In FIG. 2, 30 is a control circuit for controlling the entire image forming apparatus, 31 is a storage device for data of an image to be formed from the outside, 32 is an arithmetic circuit, 33 is a result of counting the number of formed images. The stored image formation number storage device 34 counts the amount of developer used for developing an image to be formed from the data stored in the image data storage device 31 by the control circuit 30 as the number of dots of the formed image. A cumulative dot number storage device 35 for cumulative storage, a correspondence table of the average print rate and the number of image formations, which associates a plurality of reference average print rates that require forced consumption of the developer and the number of image formations, 36 An exposure device control circuit for performing control to expose the photosensitive drum 10 charged by modulating the light source in the exposure device 12 according to data stored in the image data storage device 31; 7 is a developing device control circuit for controlling ON / OFF of a developing bias in the developing device (developing unit) 13 in accordance with an image forming instruction, 38 is for controlling a transfer bias, and for normal image forming, a positive transfer bias is used. This is a transfer device control circuit that applies a reverse transfer bias during forced consumption.

  In the correspondence table between the average printing rate and the number of formed images indicated by 35, for example, when the average printing rate A is 0 ≦ A <1, every 100 sheets of image formation, and when 1 ≦ A <2, the number is 200. For each sheet, similarly, when A <3, it is stored for every 500 sheets that the developer for one rotation of the developing roller is forcibly consumed as described above. This numerical value is merely an example, and it is obvious that the value for forced developer consumption is not limited to this numerical value.

  In the control circuit of the image forming apparatus configured as described above, at the time of normal image formation, first, an image formation instruction is transmitted to the control circuit 30, and image data to be formed is sent to the image data storage device 31. Then, the control circuit 30 drives the charger 11 in FIG. 1 to uniformly charge the surface of the photosensitive drum 10. Based on the image data stored in the image data storage device 31, the amount of developer to be used for developing the image to be formed is counted as the number of dots of the formed image, added to the cumulative dot number storage device 34, and stored. Data is sent to the exposure device control circuit 36, and the charged photosensitive drum 10 is exposed by modulating the light source in the exposure device 12 to form an electrostatic latent image.

  At the same time, the control circuit 30 drives the developing roller 14 and the rotating spiral blade member 15 in the developing unit 13 by the developing device control circuit 37 to carry the developer on the developing roller 14 and apply a developing bias. When the electrostatic latent image formed on the photosensitive drum 10 comes to the facing position of the developing roller 14, toner is attached to the electrostatic latent image to develop the electrostatic latent image as a toner image. At this time, the control circuit 30 instructs the image forming number storage device 33 to increment the image forming number by one.

  In addition, as the photosensitive drum 10 rotates and the formed toner image reaches the transfer position where the transfer roller 17 is located, the pickup roller is fed so that the paper taken out from the paper feed cassette at the position indicated by 21 is sent. 25, the paper feed roller 24, the transport roller 23, and the registration roller 22 are driven, and a transfer bias is applied to the transfer roller 17 to transfer the toner image on the photosensitive drum 10 onto the paper.

  The toner image transferred to the paper is fixed on the paper by the fixing device 26 and discharged to the paper discharge tray 28 by the discharge roller 27, and the image formation is completed. At this time, the residual toner that is not transferred onto the sheet and remains on the photosensitive drum 10 is cleaned by the cleaning device 18 and is prepared for the next image formation.

  In this image formation, since the calculated average printing rate is small, when the developer is forcibly consumed, the control is performed according to the flowchart shown in FIG. That is, as described above in step S10, when the image forming instruction is transmitted to the control circuit 30 and the image forming operation is started, in step S11, the image forming number P in the image forming number storage device 33 and the cumulative number of dots are displayed. The accumulated dot number B of the storage device 34 is set to 0, respectively.

  When the image data to be formed is sent to the image data storage device 31, the control circuit 30 charges the photosensitive drum 10, and in step S12, the arithmetic circuit 32 develops the image to be formed from now on. The amount of agent is counted as the number of dots B ′ of the formed image, B = B + B ′ is calculated and stored in the cumulative dot number storage device 34 in step S13, and the image formation number storage device P is set to P = P + 1. 33.

  Thereafter, the image data is sent to the exposure device control circuit 36 as described above, and the photosensitive drum 10 is exposed by the exposure device 12 to form an electrostatic latent image, and development and fixing are performed to form an image. The In the next step S14, after the image is formed in this way, the control circuit 30 determines whether or not the image is continuously formed, that is, whether or not the continuous printing is being performed. If the continuous printing is being performed, the process returns to step S12. The same is repeated.

  If continuous printing is not in progress or if continuous printing is completed, the process proceeds to step S15, where the cumulative number of dots B is divided by the number of formed images P, and an average printing rate A is calculated. Then, in the next step S16, a value that the image forming number P stored in the correspondence table 35 of the average printing rate and the image forming number is 500 is read, and it is determined whether or not P is 500 or more. If the process proceeds to step S21, if the result is No, the process proceeds to step S17. Similarly, in step S17, it is determined whether the image forming number P is 200 or more, and in step S18, whether the image forming number P is 100 or more. If the determination in step S17 is Yes, the process proceeds to step S20. If the determination is No, the process proceeds to step S18. If the determination in step S18 is Yes, the process proceeds to step S19. If the determination is No, the process returns to step S12. .

  For example, if the determinations in steps S16 and S17 are No and the determination in step S18 is Yes, the process proceeds to step S19, where the control circuit 30 stores the correspondence table 35 in the average printing rate and the number of formed images. The average printing rate A when the stored image forming number P is 100 is read out, that is, 0 ≦ A <1, and it is determined whether or not the average printing rate A is within this range. That is, when the number of formed images exceeds 100, it is checked whether the average printing rate is smaller than 1% as described above.

  If the average printing rate is smaller than 1%, the process proceeds to step S22, and the control circuit 30 turns off the charger 11, and only the time T required for the developing device control circuit 37 to make the developing roller 14 make one round. The developing bias is turned on so that the developer for one rotation of the developing roller 14 is developed on the photosensitive drum 10, and the transfer bias applied to the transfer roller 17 by the transfer device control circuit 38 is set as a reverse bias. The developer is prevented from moving from the photosensitive drum 10. Therefore, the developer developed on the photosensitive drum 10 moves to the cleaning device 18 as it is without adhering to the transfer roller 17 and is cleaned.

  When the diameter of the developing roller 14 is R and the rotation speed is V, the developing bias application time T corresponding to one rotation of the developing roller 14 can be expressed by T = πR / V (sec). If the diameter of the roller 14 is 20 mm and the rotation speed is 185 mm / sec, T is about 33 msec. In addition, the forced consumption of the developer is performed with the charger 11 turned off in the above description. However, the charger 11 is turned on as usual and corresponds to one rotation of the developing roller 14 on the photosensitive drum 10. The region may be exposed by the exposure device 12.

  When the developer is forcibly consumed in step S22 in this manner, the process returns to step S11, and the image forming number P in the image forming number storage device 33 and the cumulative dot number B in the cumulative dot number storage device 34 are each set to zero. Then, the processing described above is repeated.

  In step S18, it is determined that the number of formed images has exceeded 100. In step S19, it is determined that the average printing rate A is not in the range of 0 ≦ A <1, and the process is repeatedly returned to step S12. If it is determined in step S17 that the number of formed images has exceeded 200, the process proceeds to step S20, where it is determined whether the average printing rate A is A <2. If the average printing rate A is 2 or more, the process returns to step S12 and the same is repeated. However, if A <2, the process proceeds to step S22, and the forced developer consumption operation described above is performed. .

  In step S17, it is determined that the number of formed images has exceeded 200. In step S20, it is determined that the average printing rate A is not A <2, and the process returns to step S12. The same applies to the case where it is determined that the number of formed sheets exceeds 500, and the process proceeds to step S21 to determine whether or not the average printing rate A is A <3. If Yes, the forcible developer consumption operation described above is performed in step S22. If No, the process returns to step S11, and the image forming number P in the image forming number storage device 33 and the cumulative dot number in the cumulative dot number storage device 34 are returned. The operations described above are repeated with B set to 0.

  Thus, the forced consumption operation of the developer in the case of the low printing rate is performed, so that the cycle time until the next forced consumption is increased or decreased in correspondence with the size of the average printing rate, and one cycle of the developing roller. Since the developer is consumed, and thus the deteriorated developer is effectively discharged without waste, as compared with a technique in which forced consumption may be less than one round of the developer carrier as described above, There is almost no possibility of problems such as a decrease in image density and fogging. Further, since the developer is not discharged more than one developer carrier, there is no inconvenience of discharging even a developer that has not deteriorated, and stable image quality can be obtained over a long period of time. An image forming method and an image forming apparatus can be provided.

  In the above flow chart, when the number of image formations is between 100 and 200, when the average print rate A is A <2 (that is, when the average print rate A is 0 ≦ A <1), and Between 200 and 500 sheets, when the average printing rate A is A <3 (that is, including the case where the average printing rate A is A <2), the forced developer consumption is performed. If the average print ratio A is not in the range of 0 ≦ A <1 when the number of image formations exceeds 100, for example, the developer is not forcibly consumed until the next 200, and similarly the number of image formations is 200, for example. If the average printing rate A is not in the range of A <2 when the value exceeds the upper limit, the forced developer consumption may not be performed until the next 500 sheets.

  The above is the forced consumption operation of the developer at a low printing rate according to the present invention, and when a large number of images are formed while forcibly consuming the developer by the method of the present invention as described above, How the image density changes depending on the number of output sheets in each case where a large number of images are formed while forcibly consuming the amount corresponding to the amount of developer consumed in the development as in the prior art described above. FIG. 4 is a graph showing a result of an experiment on whether to do this.

  The developing unit 13 of the image forming apparatus used in this experiment uses a DC developing bias of 340 V, an AC developing bias of Vp-p 1.6 kV, a frequency of 2.6 kHz, a rectangular wave with a duty ratio of 50%, and charging. The potential was 475V. The rotational speed of the photosensitive drum 10 was 185 mm / sec, the developing roller 14 had a diameter of 20 mm, the speed ratio with the photosensitive drum 10 was 1: 1, and the image forming speed was 30 sheets / minute.

  In the graph of FIG. 4, the horizontal axis is the number of formed images (number of output sheets), the vertical axis is the image density, and the line shown as “prior art” in which x is plotted corresponds to the amount of developer consumed in development. This is a case where the formed amount is forcibly consumed to form an image. Also, the lines shown as “Example 1” in which “◯” is plotted and “Example 2” in which “Δ” is plotted are the cases where image formation is performed while performing the forced consumption operation of the developer according to the present invention described above. Among them, in “Prior Art” and “Example 1”, a large number of images are formed using only a document with a printing rate of 0.36%, and “Example 2” has a printing rate of 0.36%. A large number of images were formed by mixing 90% of the original, 8% of the original with a printing rate of 2.00%, and 2% of the original with a printing rate of 4.00%.

  It should be noted that the forcible consumption operation condition of the developer in the “prior art” is forcibly consumed every time between sheets after output of each image and before output of the next image. Further, when the forced consumption of developer in “Example 1” is converted every time output between sheets after output of each image and until output of the next image, as in “Prior art”, Since the forced consumption amount is about 0.20%, the total consumption amount of the developer is made the same. Therefore, the forced consumption amount per time in the “prior art” is equivalent to 0.20% as the printing rate. I did it.

  In this case, since the developing roller 14 has a diameter of 20 mm, the circumference of the developing roller 14 is about 20% of that of the A4 document. Therefore, the printing ratio in the “prior art” is equivalent to 0.20%. 1/100 of 20%, which corresponds to forcibly consuming about 3.6 degrees of developer per rotation of the developing roller every image formation, and 356. The developer for 4 degrees remains on the developing roller and the deterioration proceeds.

  As can be seen from the graph shown in FIG. 4, both the first and second embodiments in which the forced consumption of the developer is always equivalent to one rotation of the developing roller as in the present invention described above, both have an image density of 1. 2 or more, especially 90% for a document with a printing rate of 0.36%, 8% for a document with a printing rate of 2.00%, and 2% for a document with a printing rate of 4.00% for normal image formation In the second embodiment, which is close, a large reduction in image density has not occurred even after the formation of 50,000 images.

  On the other hand, in the “conventional technology” in which the total consumption is the same, the image density becomes 1.2 after image formation of about 35,000 sheets, and thereafter, the image density greatly decreases as the image formation proceeds. Therefore, it can be seen that the amount of forced consumption per time is small and the developer on the developing roller is not consumed, and the deterioration of the developer proceeds. By using the present invention, stable image quality can be obtained over a long period of time. Is possible.

  According to the present invention, an image forming apparatus can be provided because stable image quality can be obtained over a long period of time without causing image density reduction or fogging even when image formation with low toner consumption is continuously performed. .

1 is a schematic cross-sectional view of a laser printer as an example of an image forming apparatus that performs an image forming method of the present invention. It is a block diagram of a control circuit for performing the image forming method of the present invention. FIG. 6 is a flowchart of forced consumption of deteriorated toner in the image forming method of the present invention. 6 is a graph showing the results of experiments on the relationship between the number of output sheets and image density when a large number of images are formed using a document with a low printing rate.

Explanation of symbols

10 Photosensitive drum 11 Charger 12 Exposure device (laser scanning unit)
13 Developing unit 14 Developing roller 15 Rotating spiral blade member 16 Toner container 17 Transfer roller 18 Cleaning device 20 Paper transport path 21 Paper feed cassette 22 Registration roller 23 Transport roller 24 Paper feed roller 25 Pickup roller 26 Fixing device 27 Discharge roller 28 Discharge unit 30 control circuit 31 image data storage device 32 arithmetic circuit 33 image formation number storage device 34 cumulative dot number storage device 35 correspondence table 36 of average print rate and image formation number exposure device control circuit 37 developing device control circuit 38 transfer device control circuit

Claims (5)

An electrostatic latent image formed on a photoconductor by an electrophotographic method is developed with a developer carried on a developer carrier facing the photoconductor, and the amount of developer used for the development is determined by the number of dots in the formed image. In the image forming method in which the developer is forcibly consumed between the sheets of paper on the basis of the average print rate calculated by the cumulative addition value and the number of image formed sheets,
The forced consumption of the developer is determined whether the calculated average printing rate is less than a predetermined reference printing rate. If the calculated average printing rate is equal to or higher than the reference printing rate, the forced consumption is not performed. An image forming method comprising: forcibly consuming developer for one rotation of the developer carrying member; and increasing or decreasing a cycle time until the next forced consumption in accordance with the average printing rate.   The image forming method according to claim 1, wherein a cycle time until the next forced consumption of the developer is determined based on the number of formed images corresponding to the calculated average printing rate.   The forced consumption of the developer can be performed without developing the photosensitive member, or when the photosensitive member is charged, the region corresponding to one rotation of the developing roller is exposed and then developed. 3. The image forming method according to claim 1, wherein the transfer bias for transferring the developer image formed by developing the electrostatic latent image is a reverse bias. A photoconductor that forms an electrostatic latent image by electrophotography, and a developing device that has a developer carrier that faces the photoconductor and develops the electrostatic latent image on the photoconductor with a carried developer; Stored in the accumulated dot number storage means for storing the cumulative addition of the developer amount used for the development as the number of dots in the formed image, the image formation number counting means for counting the number of image formations, and the accumulated dot number storage means Average printing rate calculating means for calculating a developer consumption amount per formed image as an average printing rate based on the number of accumulated dots and the number of formed images counted by the image forming number counting means. In the image forming apparatus having a control unit for forcibly consuming the developer onto the photoconductor by the average printing rate calculated by the rate calculating unit,
The control means forcibly consumes the developer for one rotation of the developer carrier in a state where the average printing ratio calculated by the average printing ratio calculation means is less than a predetermined reference printing ratio, and until the next forced consumption. An image forming apparatus, wherein the cycle time is controlled to increase or decrease in accordance with the average printing rate.   5. A table storing a number of image forming sheets for determining a cycle time until the next forced developer consumption, corresponding to each of a plurality of average printing rates that require the forced consumption of the developer. Item 5. The image forming apparatus according to Item 4.

Images