JP6700803B2 - Image forming device - Google Patents

Description

  The present invention has a developing device (40) for developing an electrostatic latent image formed on an image bearing member using a two-component developer composed of a carrier and toner to visualize it as a toner image, a copying machine, a printer, a recorded image. The present invention relates to an image forming apparatus such as a display device and a facsimile.

  In general, a two-component developer containing toner particles and carrier particles as a main component is widely used in a developing device included in an electrophotographic or electrostatic recording type image forming apparatus. In particular, in a color image forming apparatus that forms a full-color or multi-color image by an electrophotographic method, many developing apparatuses use a two-component developing apparatus from the viewpoint of image tint and the like.

  Conventionally, in the two-component developing method, a developer carrying member (developing sleeve) and an image carrying member (photosensitive drum) are opposed to each other at a predetermined interval, and a developing bias is applied to the developer carrying member to develop the developer carrying member. Development is performed by attaching the above toner to the image carrier. As the developing bias voltage, a DC voltage superimposed with an AC voltage is generally used.

  In such a developing system, it is known that the electrostatic capacity at the facing portion (developing region) between the developer carrying member and the image carrying member changes depending on the state of the developing device. The electrostatic capacitance changes are caused by the distance (SD gap) between the developer bearing member and the image bearing member, the developer carrying amount on the developer bearing member, and the vicinity of the SD gap in the magnet composed of a plurality of magnetic poles. The position of the developing pole to be positioned, the toner concentration of the developer, and the like are included.

  If these factors change, the developability, that is, the ease of movement of the toner on the developing sleeve when developing on the photosensitive drum changes. Therefore, under the condition that the developability is improved, the density is high and the density unevenness is less likely to occur, but under the condition where the developability is poor, the density is decreased and the density unevenness is likely to occur.

  Therefore, conventionally, it has been proposed to detect the electrostatic capacitance that changes according to the state of the developing device and perform control so as to change the current condition so that the developability becomes appropriate (for example, See Patent Document 1).

JP, 2007-322727, A

  However, as described above, the SD gap, the amount of developer conveyed on the developer carrier, and the like are related to the developability (the ease with which toner moves from the developer carrier to the image carrier). There is. Therefore, in order to suppress in-plane density unevenness due to a decrease in developability and obtain a uniform image, it is desirable to set the developability as high as possible. For example, in the case of the SD gap, the smaller the value is, and the larger the value of the developer transport amount is, the higher the developability is, and the less the density unevenness is.

  On the other hand, under such a high developability condition, it becomes difficult for the developer to pass through the SD gap portion. Then, the developer that could not pass is accumulated in the SD gap portion, which may scrape the image or cause image defects such as fogging on a white background. This phenomenon is particularly remarkable in the case of the counter development method in which the moving direction of the developer carrying member and the moving direction of the image carrying member are opposite in the SD gap portion.

  Originally, it is necessary to make both the aforementioned high developability and the difficulty in retaining the developer compatible with each other. However, in order to reduce the cost of the main unit, it is necessary to set a large tolerance for individual parts and adjustment tolerances.Therefore, when those tolerances fluctuate between the upper and lower limits, factors such as environmental fluctuations and durability fluctuations should be taken into consideration. It becomes difficult to achieve both the developability and the developer retention.

  Therefore, as a means for solving this problem, it is conceivable to detect the electrostatic capacitance in the SD gap portion, determine whether or not retention occurs, and change the developing condition. Since the electrostatic capacitance changes depending on the SD gap and the developer transport amount, it is possible to judge to some extent whether or not the condition is such that retention easily occurs by looking at the electrostatic capacitance. However, in addition to these, the electrostatic capacitance changes depending on the pole position of the developing electrode, the toner concentration of the developer, and the like. Therefore, it is determined whether or not retention is likely to occur in the control that only looks at the absolute value of the electrostatic capacitance. Was difficult.

  Therefore, an object of the present invention is to accurately detect a state in which a stagnation of the developer is likely to occur, and by performing control to change the development condition so as to avoid the stagnation, while suppressing the occurrence of density unevenness, and An object of the present invention is to provide an image forming apparatus that can simultaneously suppress the occurrence of image defects due to the retention of developer.

In order to achieve the above object, an image forming apparatus according to one aspect of the first invention has the following configuration. That is, a rotatable image bearing member on which an electrostatic latent image is formed, a developing container containing a developer containing toner and carrier, and a position for developing the electrostatic latent image formed on the image bearing member. And a rotatable developer carrier that carries and conveys the developer, and a developing device for developing an electrostatic latent image formed on the image carrier using the developer, and the image carrier. A capacitance detecting section for detecting a capacitance between a body and the developer carrying body, a driving means for rotationally driving the developer carrying body, and a control section for controlling the driving means. The rotation speed of the developer carrier in the image region is based on the inclination of the change in the capacitance when the rotation speed of the developer carrier in the non-image region is changed. Is characterized in that a mode for determining is executable.
Further, in order to achieve the above object, an image forming apparatus according to one aspect of the second invention has the following configuration. That is, a rotatable image carrier on which an electrostatic latent image is formed, a developing container containing a developer containing toner and carrier, and a position for developing the electrostatic latent image formed on the image carrier. And a rotatable developer carrier that carries and conveys the developer, and a developing device for developing an electrostatic latent image formed on the image carrier using the developer, and the image carrier. Capacitance detecting means for detecting the electrostatic capacitance between the developer and the developer carrying body, and a developing bias applying means for applying a developing bias in which a DC voltage and an AC voltage are superposed to the developer carrying body. And a control unit for controlling the developing bias applying unit, wherein the control unit controls the surface potential of the non-image portion of the image carrier in the non-image region and the developing bias applying unit to apply the developer to the developer carrier. A mode for determining the potential difference in the image region can be executed based on the inclination of the change in the capacitance when the potential difference from the applied DC voltage of the developing bias is changed.

According to the invention described in claim 1 and the invention described in claim 6, it is possible to detect whether there is a sign of staying and to set the developing condition so as to avoid staying only when there is a sign. Therefore, it is possible to prevent image deterioration due to residual developer without wasteful deterioration of the developer while maintaining a uniform image without density unevenness, and stably obtain a high-quality image. It is possible to provide an image forming apparatus that can do this.

FIG. 1 is a schematic configuration explanatory diagram of an image forming apparatus according to the present invention. It is a schematic structure explanatory view of the developing device which concerns on this invention. It is an explanatory view of an AC waveform of a developing bias according to the present invention. It is a figure explaining the behavior of the developer in the SD gap part concerning the present invention. FIG. 3 is a block diagram of a developing high-voltage board and a control board according to the present invention. 7 is a graph of output voltage according to electrostatic capacitance when the state of the developing device is changed. It is a figure which shows the flow of control which concerns on Example 1 of this invention. It is a figure which shows the outline of Vback electric potential control which concerns on Example 1 of this invention. 5 is a graph showing the relationship between the Vback potential and the output voltage according to the capacitance according to the first embodiment of the present invention. It is a figure which shows the flow of control which concerns on Example 2 of this invention. 7 is a graph showing the relationship between the Vback potential and the output voltage according to the capacitance according to Example 2 of the present invention.

  Hereinafter, preferred embodiments of the present invention will be illustratively described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in the following embodiments should be appropriately changed depending on the configuration of the device to which the present invention is applied and various conditions. Therefore, unless otherwise specified, the scope of the present invention is not intended to be limited thereto.

[Example 1]
FIG. 1 is a schematic configuration diagram of an image forming apparatus. FIG. 1 shows an image forming apparatus that forms an electrophotographic full-color image, which is an embodiment of the image forming apparatus to which the present invention is applicable.

  As shown in FIG. 1, in the present embodiment, the image forming apparatus 100 includes four image forming units P (PY, PM, PC, PBk). The subscripts of Y, M, C, and Bk indicate the four colors of yellow Y, magenta M, cyan C, and black Bk, respectively, and represent the colors of the toner images formed in each image forming portion P. Since the structure of each image forming portion P is the same, the subscripts Y, M, C, and Bk will be omitted in the following description unless necessary.

  The image forming portion P includes a photosensitive drum 1 (1Y, 1M, 1C, 1Bk) as an image carrier. The photosensitive drum 1 is a drum-shaped electrophotographic photosensitive member that rotates in the arrow direction (counterclockwise).

  An image forming unit is provided around the photosensitive drum 1 for each color. Specifically, around the photosensitive drum 1, a charger 2 (2Y, 2M, 2C, 2Bk) as a charging unit, a developing device 40 (40Y, 40M, 40C, 40Bk), and a drum cleaner 9 as a cleaning unit. (9Y, 9M, 9C, 9Bk). A laser beam scanner 3 (3Y, 3M, 3C, 3Bk) as an exposure unit is provided above the photosensitive drum 1. Further, a primary transfer roller 6 (6Y, 6M, 6C, 6Bk) is arranged for each image forming portion P via an intermediate transfer belt 5 described later.

  Next, an image forming sequence of the entire image forming apparatus having the above configuration will be described.

  The photosensitive drum 1 rotates in the direction of the arrow in FIG. 1 (counterclockwise direction) at a process speed (peripheral speed) of 150 mm/sec. First, the photoconductor drum 1 is uniformly charged by the charger 2.

  The photosensitive drum 1 uniformly charged by the charger 2 is then subjected to scanning exposure by the laser beam scanner 3.

  The laser beam scanner 3 has a built-in semiconductor laser. The semiconductor laser is controlled in response to a document image information signal output by a document reading device having a photoelectric conversion element such as a CCD. Therefore, by controlling the semiconductor laser, the laser light modulated from the image signal is emitted from the semiconductor laser of the laser beam scanner 3. Then, the surface potential of the uniformly charged photosensitive drum 1 changes at the portion where an image is formed, and the changed portion becomes an electrostatic latent image. When the electrostatic latent image on the photosensitive drum 1 is developed (visualized) by the toner supply from the developing device 40, it becomes a visible image, that is, a toner image.

  In the present embodiment, the developing device 40 uses a two-component developing method that uses a developer in which toner and carrier are mixed as the developer.

  By performing the above process for each image forming unit P (PY, PM, PC, PBk), four colors of yellow, magenta, cyan, and black are respectively formed on the photoconductor drum 1 (1Y, 1M, 1C, 1Bk). Toner image is formed.

  In this embodiment, the intermediate transfer belt 5 (intermediate transfer member) is arranged below the four image forming portions P. The intermediate transfer belt 5 is suspended by a tension roller 51, a secondary transfer inner roller 52, and a drive roller 53, and is movable in the arrow direction (clockwise direction) in the drawing.

  The toner image on the photoconductor drum 1 (1Y, 1M, 1C, 1Bk) is first transferred to the intermediate transfer belt 5 by the primary transfer roller 6 (6Y, 6M, 6C, 6Bk) as a primary transfer portion. .. As a result, four color toner images of yellow, magenta, cyan, and black are superposed on the intermediate transfer belt 5 to form a full-color image. The toner remaining on the photosensitive drum 1 without being transferred is collected by the drum cleaner 9 (9Y, 9M, 9C, 9Bk) of each image forming portion P. The superimposed full-color image is conveyed to a secondary transfer portion that faces the inner secondary transfer roller 52.

  On the other hand, the transfer material such as paper placed on the feeding cassette 12 is taken out by the feeding roller 13. After that, the transfer material S is conveyed to the secondary transfer portion formed by the inner secondary transfer roller 52 and the secondary transfer roller 10 (secondary transfer member) via the feeding guide 11.

  At the secondary transfer portion, the full-color image on the intermediate transfer belt 5 is transferred to the transfer material S by the action of the secondary transfer roller 10. The toner remaining on the surface of the intermediate transfer belt 5 without being secondarily transferred is collected by the intermediate transfer belt cleaner 18.

  Then, the transfer material S is sent to the fixing device 16 (heat roller fixing device). In the fixing device 16, the transfer material S on which the toner image is transferred is heated and pressed to fix the image. The transfer material S on which the toner image has been fixed is discharged to the discharge tray 17.

  In this embodiment, the photoconductor drum 1 which is a commonly used drum-shaped organic photoconductor is used as the image carrier, but the image carrier is not limited to this. For example, an inorganic photoconductor such as an amorphous silicon photoconductor can be used. It is also possible to use a belt-shaped photoreceptor. The charging method, transfer method, cleaning method, and fixing method are not limited to the above methods.

  Next, the outline of the developing device 40 will be described with reference to FIG.

  The developing device 40 according to the present embodiment has a developing container 41. The developer container 41 contains a two-component developer containing a toner and a carrier as a developer. Further, the developing device 40 includes a developing sleeve 42 (developer carrying member) for carrying the developer in the developing container 41, and a regulating blade 43 for regulating the height of the spikes of the developer carried on the developing sleeve 42. Have.

  The interior of the developing container 41 is divided into a developing chamber 45 and a stirring chamber 46 by a partition wall 44 having a substantially central portion extending in a direction perpendicular to the plane of FIG. The developer is contained in the developing chamber 45 and the stirring chamber 46, and is circulated and conveyed by the following configuration.

  Each of the developing chamber 45 and the stirring chamber 46 is provided with a transport screw for stirring and transporting the developer. Specifically, the developing chamber 45 is provided with a first carrying screw 47 (first carrying member), and the stirring chamber 46 is provided with a second carrying screw 48 (second carrying member).

  By the rotation of the first transport screw 47 and the second transport screw 48, the developer is transported in mutually opposite directions along the axial direction, and the developer is communicated with the developing chamber 45 through the openings (communication portions) at both ends of the partition wall 44. It is circulated with the stirring chamber 46.

  The position corresponding to the developing area of the developing container 41 facing the photosensitive drum 1 is open. The developing sleeve 42 is exposed in the direction of the photosensitive drum 1 through this opening.

  In the present embodiment, the developing sleeve 42 has a diameter (outer diameter) of 20 mm, and the photosensitive drum 1 has a diameter (outer diameter) of 40 mm. Further, the closest area between the developing sleeve 42 and the photosensitive drum 1 is set to a distance of about 320 μm, and the rotation speed of the developing sleeve 42 at the time of image formation is 225 mm/sec (the peripheral speed ratio of the photosensitive drum=150%). Is set.

The developing sleeve 42 carries the two-component developer in the developing chamber 45 while rotating in the arrow direction (counterclockwise direction) in the drawing during development. A layer of developer (so-called magnetic brush) is formed on the developing sleeve 42 by the action of the magnet member 42m. The layer thickness of the magnetic brush is regulated by the regulation blade 43, and the developer whose layer thickness is regulated is carried on the developing sleeve 42. In this embodiment, the regulated developer layer thickness is set to 30 mg/cm ² . In this state, the developer carried on the developing sleeve 42 is conveyed to the developing area facing the photosensitive drum 1, and the toner is transferred onto the photosensitive drum 1 by the action of the developing bias applied to the developing sleeve 42. By doing so, the electrostatic latent image is visualized. In this embodiment, in the developing area, a counter developing method is used in which the moving direction of the developing sleeve 42 and the moving direction of the photosensitive drum 1 are opposite to each other.

  The magnet member 42m used in this embodiment is fixed inside the developing sleeve 42 and has a plurality of magnetic poles. Here, the magnet member 42m has five magnetic poles, and the N1 pole is arranged at the position closest to the closest position between the developing sleeve 42 and the photosensitive drum 1. The magnetic pole N1 has a magnetic flux density of 105 mT, and the position of the magnetic pole is located 10° upstream with respect to the closest position between the developing sleeve 42 and the photosensitive drum 1 in the rotation direction of the photosensitive drum 1. ..

  The developing bias applied to the developing sleeve 42 is a direct current with an alternating current superposed, and is applied from a high voltage applying means as a developing bias power source, which will be described later. As the waveform of the AC component of the developing bias, a blank pulse wave in which a rectangular pulse portion and a blank portion consisting of only the DC component are alternately repeated as shown in FIG. 3 is used. The frequency of the pulse portion is set to 14 kHz, Duty 50%, and amplitude 1700V.

  Next, the behavior of the developer in the SD gap portion will be described below with reference to FIG.

  FIG. 4 is an enlarged view of the closest portion (SD gap portion) between the photosensitive drum 1 and the developing sleeve 42, and the broken line portion labeled SD is the closest position. The carrier particles 4C of the developer in the SD gap portion are in a state of standing up along the magnetic lines of force extending from the N1 pole in the magnet member 42m. Since the carrier particles 4C are conveyed by the rotation of the developing sleeve 42 while being attracted to the developing sleeve 42 by the magnetic force of the N1 pole, they are conveyed in the direction of the arrow b shown in FIG. The carrier particles in contact with each other are pulled in the direction of arrow a by the rotation of the photosensitive drum 1. Therefore, the force of the arrow a acting on the carrier particles 4C becomes a resistance force when the developer passes through the SD gap portion. As described above, in the counter development method, since the carrier forces acting on the carrier particles in the vicinity of the developing sleeve and the carrier particles in the vicinity of the photosensitive drum are in opposite directions, retention of the developer easily occurs.

  Further, at the time of image formation, a potential difference (Vback potential) which is a difference between the potential of the white background portion on the photosensitive drum and the DC component potential of the developing bias is set so that the toner does not move to the photosensitive drum in the white background portion of the image. It For example, by setting the white background potential on the photoconductor drum to −750V and the DC component potential of the developing bias to −610V, the Vback potential becomes 140V.

  Since the carrier particles are charged in the opposite polarity to the toner (plus polarity in this embodiment), the carrier particles are attracted to the photosensitive drum 1 side when the Vback potential is applied. Therefore, the larger the Vback potential, the more the carrier particles are attracted to the photoconductor drum, and the force indicated by the arrow a in FIG. 4A acts more strongly, so that retention easily occurs.

  Further, FIG. 4B is a diagram showing a case where the SD gap is narrowed. In this case, since the developer is pushed into the narrow space, the force of the arrow a becomes stronger, so that Even under such conditions, retention easily occurs.

  Similarly, when the developer transport amount on the developing sleeve is large, the length of the spikes of the carrier particles becomes long even if the SD gap is the same. Therefore, the carrier particles are more likely to come into contact with the photosensitive drum, and the force indicated by the arrow a acts more strongly, so that retention is likely to occur even under such conditions.

  Further, FIG. 4C shows the case where the position of the N1 pole of the magnet member 42m is located at the closest position between the photosensitive drum 1 and the developing sleeve 42. In this case, the ears of the carrier particles in the closest part are in a state of being erected than in the case of FIG. 4A, and in this case also, the carrier particles are more likely to come into contact with the photosensitive drum and cause retention. There is a risk.

  In this embodiment, the electrostatic capacity in the SD gap portion can be detected in order to grasp the sign of the developer retention. The capacitance detection configuration will be described below with reference to FIG.

  FIG. 5 is a block diagram showing the configurations of the developing high-voltage board and the control board of the image forming apparatus in this embodiment.

  In FIG. 5, the image forming apparatus is equipped with a developing high-voltage board 300 and a control board 305. On the developing high voltage substrate 300, an AC high voltage driving circuit 301 (high voltage applying means), an AC transformer 302, a DC high voltage circuit 303 (high voltage applying means), an AC voltage amplitude (pp voltage) detection circuit 304 (capacitance detection). Means), a capacitor C1, a capacitor C2, and an output resistor. An A/D conversion circuit 306 and a CPU 308 (control means) are mounted on the control board 305.

  In the developing high voltage substrate 300, the AC high voltage driving circuit 301 generates a developing AC bias voltage and supplies it to the developing sleeve 42. The DC high voltage circuit 303 generates a developing DC bias voltage and supplies it to the developing sleeve 42. The pp voltage detection circuit 304 converts the peak-to-peak voltage (pp voltage) across the capacitor C2 into a DC voltage and extracts it as a capacitance detection signal. The extracted DC voltage is converted from an analog signal to a digital signal by the A/D conversion circuit 306 of the control board 305 and input to the CPU 308. With the above configuration, it is possible to detect the voltage value corresponding to the electrostatic capacitance in the SD gap portion. Further, the CPU 308 controls the developing bias applied to the developing sleeve by using the AC high voltage drive circuit 301 and the DC high voltage circuit 303. Further, the CPU 308 controls the rotation speed of the developing sleeve by using a driving unit (not shown) that drives the developing sleeve.

  In the configuration of this embodiment, the output of the voltage value corresponding to the electrostatic capacitance between the photosensitive drum 1 and the developing sleeve 42 when the SD gap is actually changed by using the above detection circuit is shown in FIG. Shown in A). This graph also shows the output when the toner concentration of the developer is also changed at the same time. It can be seen that the smaller the SD gap is, the larger the capacitance is, and accordingly the output value is increased. Further, when the toner concentration of the developer changes, the magnetic permeability of the developer changes and the electrostatic capacity also changes, so the output changes. As described above, the capacitance changes due to a plurality of factors, and the output is determined by the combination thereof.

  Further, FIG. 6B shows a graph in which the output of the voltage value corresponding to the electrostatic capacity is plotted when the developer transport amount on the developing sleeve is changed in the configuration of this embodiment. This graph shows the output when the toner concentration of the developer is 9%. Thus, it can be seen that the electrostatic capacitance also changes depending on the amount of developer conveyed on the developing sleeve. In addition to this, it is also known that the output value changes depending on the pole position of the N1 pole near the SD gap portion.

  In this way, the voltage value corresponding to the electrostatic capacity changes depending on the combination of multiple factors, so it is difficult to judge whether the retention of the developer is likely to occur by simply looking at the absolute value of the output value. Met. Therefore, in this embodiment, in the non-image area, by detecting the inclination of the output fluctuation corresponding to the change in the electrostatic capacity when the developing conditions are changed, it is determined whether there is a sign that the retention of the developer occurs. The control to judge is performed. The control method will be described in detail below with reference to FIGS. 3 and 7.

  When the user gives a print instruction from the main body operation unit or the PC, the paper type of the print paper is selected (S1). In the image forming apparatus according to the present exemplary embodiment, when the plain paper is selected as the transfer material, the rotation speed of the photosensitive drum 1 is the first rotation speed (here, 150 mm/sec) for performing the printing operation (constant speed mode). )It has become. on the other hand. When thick paper is selected as the transfer material, the rotation speed of the photosensitive drum 1 is the second rotation speed slower than the first rotation speed (here, 75 mm/sec, which is half the first rotation speed), and the printing operation is performed. The specifications (low speed mode) are set. Therefore, when the printing operation is started (S2) after the paper type is selected, the pre-rotation is started at a speed corresponding to the paper type (S3). When the pre-rotation is started, the temperature of the installation environment of the main body is detected by a temperature sensor (not shown) as an environment detection unit installed on the rear surface of the main body (S4).

  Here, when the speed in the low speed mode (75 mm/sec) is selected (S5) and when the detected environmental temperature is lower than a predetermined value (less than a predetermined value Ta) (S6), a symptom of developer retention is generated. The retention detection control for determining whether there is any is executed. This is because retention tends to occur when the rotation speed of the developing sleeve 42 is reduced to half the speed of plain paper printing in accordance with the rotation speed of the photosensitive drum 1, and when the environmental temperature is low. Is.

  Normally, the peripheral speed ratio of the developing sleeve 42 to the photosensitive drum 1 is controlled to be constant, but if the rotation speed of the developing sleeve 42 is slowed, the amount of developer conveyed on the developing sleeve is reduced. There is a tendency to increase. It is considered that this is because the developer is less likely to slip on the surface of the developing sleeve when the rotation speed of the developing sleeve 42 is low. Therefore, when the rotation speed of the developing sleeve 42 becomes slow, the amount of developer conveyed increases and the developer tends to stay.

  Further, when the rotation speed of the developing sleeve 42 is slow, the amount of the developer carried by the developing sleeve 42 per unit time decreases, and the amount of the developer that can pass through the SD gap portion also decreases. Retention tends to occur.

  Further, when the ambient temperature is low, each member constituting the developing device is affected by thermal contraction, and thus the SD gap may fluctuate in the direction of narrowing. When the SD gap becomes narrower, the developer naturally does not easily pass through the SD gap portion, so that the developer tends to stay. Therefore, in this embodiment, the predetermined value Ta is set to 12°C.

  Therefore, in the present embodiment, control is performed to detect whether there is a sign of retention during the pre-rotation in the low speed mode in which the developer retention is likely to occur and in the low temperature state. The operation of the stay detection control will be described below.

  When the low speed mode is selected and the environmental temperature is lower than the predetermined value Ta (12° C.), the AC high voltage drive circuit 301 of the developing high voltage substrate 300 and the DC high voltage circuit 303 direct the current (DC)+AC to the developing sleeve 42. (AC) voltage is applied. At that time, the outputs corresponding to the respective electrostatic capacities when the voltages are set to +40 V and −40 V with respect to the Vback value in the image area (S7) are detected by the pp voltage detection circuit 304 (S8). For example, when Vback in the image area is 140V (white background potential=−750V, development DC potential=−610V), the white background potential remains −750V, the development DC potential is first switched to −650V, and Vback is set to 100V. In this state, the output (C1) corresponding to the capacitance is detected. Then, the developing DC potential is switched to −570V this time, and the output (C2) corresponding to the electrostatic capacitance is detected in the state where Vback is set to 180V. An image of this potential switching is shown in FIG.

  These detection results are input to the CPU 308 after A/D conversion, and the output slope P is calculated by the following equation (1) (S9). That is, the slope P represents the slope of the output per Vback value of 1V.

    P=(C2-C1)/(180-100)...(1)

  When the slope P calculated by the above equation (1) is equal to or larger than the predetermined value (S10), it is determined that there is a sign of staying. In this case, 225 mm/sec (speed “1”, peripheral speed ratio to photosensitive drum=300%), which is the same as that for printing on plain paper, is selected as the developing sleeve speed during image formation (S11). Here, the threshold Pt is set to a predetermined value to be compared with the calculated slope P, and in this embodiment, the threshold Pt is set to 1.0 mV/V. On the other hand, when the slope P is less than the threshold Pt (S10), it is determined that there is no sign of retention. In this case, the developing sleeve speed at the time of image formation is selected to be 112.5 mm/sec (speed “2”, ratio of peripheral speed of photosensitive drum=150%) (S12). In this way, the developing condition (here, the rotation speed of the developing sleeve) in the image area is determined based on the calculated inclination P. After the developing sleeve speed is selected, the pre-rotation is ended (S13), and image formation at the selected developing sleeve speed is started (S14).

  When plain paper is selected as the paper to be used, the developing sleeve speed is originally rotated at 225 mm/sec, which is advantageous for staying, and thus the staying detection control as described above is not performed. Even when the low speed mode is selected, it is determined that the SD gap variation due to temperature is small when the environmental temperature is equal to or higher than Ta, and the residence detection control is not performed in this case as well.

  FIG. 9 shows an example of the output value when the Vback potential is actually changed. In the settings of A and B in this graph, output detection is performed with different settings for the SD gap, the developer layer thickness on the developing sleeve, and the N1 pole position. In the setting of graph A of FIG. 9, the slope P when Vback is swung is 0.50 mV/V, and in the setting of graph B, the slope P when Vback is swung is 2.13 mV/V, It can be seen that the slopes differ greatly. For comparison, the slope of the threshold Pt is shown by a dotted line, but it can be seen that the slope P of the graph B greatly exceeds this threshold Pt (=1.0 mV/V).

  As described above, the larger the Vback potential, the more the carrier particles of the developer are attracted to the photosensitive drum, so that the force indicated by the arrow a in FIG. 4A becomes stronger. The fact that the output largely changes when the Vback potential is small and when it is large means that the flow of the developer in the SD gap portion is largely changed. That is, when the Vback potential was small, the developer flowed smoothly, whereas when the Vback potential was large, the flow of the developer deteriorated and the flow became clogged, and the density of the developer in the SD gap portion increased. It is considered that the capacitance has changed significantly. On the other hand, even if the Vback potential is shaken, if the flow of the developer in the SD gap portion is not largely changed, the change in capacitance is also small, and the output slope is small.

  For the reasons described above, it is possible to determine whether there is a sign of retention by looking at the output slope (magnitude of output change) corresponding to the electrostatic capacitance when the Vback potential is swung. Further, when it is determined that there is a sign of stagnation, the stagnation can be prevented by rotating the developing sleeve speed during image formation at the same speed as in plain paper printing even in the low speed mode.

  When the peripheral speed ratio of the rotation speed of the developing sleeve with respect to the photosensitive drum is increased, the number of rotations of the developing sleeve per unit number of prints is increased, so that the deterioration of the developer is likely to proceed correspondingly. However, by carrying out the control of this embodiment, the peripheral speed ratio can be increased only when there is a sign of stagnation. Deterioration can be suppressed. Further, in order to surely avoid the retention, it is conceivable to set the SD gap to a wide range, but in this case, the developing property is lowered and the density unevenness is likely to occur as described above. Performing the example control can increase the level of image quality as a whole.

  In this embodiment, when the low speed mode is selected and when the environmental temperature is lower than the predetermined value, the stay detection control is performed. However, the condition for executing the stay detection control is that the low speed mode is selected. If it is performed, or if the environmental temperature is lower than a predetermined value, it may be set.

  Further, in this embodiment, when the environmental temperature becomes low, the SD gap becomes narrow and the developer tends to stay, but depending on the configuration of the developing device used and the temperature characteristics of the materials used for the respective members. The SD gap may become narrower as the environmental temperature becomes higher. Therefore, in that case, the retention detection control may be executed when the temperature is equal to or higher than a predetermined temperature.

  Further, the temperature threshold Ta for determining whether to execute the retention detection control, the value of the Vback potential changed in the retention detection control, the output slope threshold Pt for determining whether there is a sign of retention occurrence, etc. It is not limited to the numerical values in the example. Optimum values are set for these values depending on the image forming apparatus used.

  Further, in the present embodiment, as the staying avoiding means when the staying detection control determines that there is a sign of occurrence of staying, control for increasing the developing sleeve speed in the low speed mode is performed, but the present invention is not limited to this. is not. For example, as the stay avoiding means, other than this, the effect can be obtained by performing control so as to reduce the Vback potential during image formation or to reduce the amplitude of the AC voltage of the developing bias.

  Further, in the present embodiment, the output voltage corresponding to the capacitance when the Vback potential is shaken is detected as the means for detecting the sign of the retention during the retention detection control, but the invention is not limited to this. .. For example, as the above-mentioned means, it is also possible to perform control so as to detect an output voltage corresponding to the electrostatic capacitance when the rotation speed of the developing sleeve is shaken.

  By performing the control as described above, the developing condition can be determined (changed) so as to avoid the stay only when the sign of the occurrence of the stay is detected. Therefore, while maintaining a uniform image without density unevenness, it is possible to prevent wasteful deterioration of the developer and prevent the occurrence of image defects due to the retention of the developer, and to stably obtain a high-quality image. You can

  In addition, since it becomes possible to reliably detect the sign of the occurrence of retention, it is possible to reliably prevent the occurrence of image defects due to the retention of the developer.

  Further, as a result of the staying detection control, when it is determined that there is a sign of staying, it is possible to set the developing conditions that can reliably avoid the staying of the developer during image formation.

[Example 2]
In the present embodiment, the overall configuration of the image forming apparatus, the schematic configuration of the developing device, the developing bias to be used, the SD gap, the setting of the developer transport amount on the developing sleeve, and the like are the same as those in the first embodiment. Further, the rotation speed of the photosensitive drum 1 during plain paper printing (constant speed mode) is set to 150 mm/sec, and the rotation speed during thick paper printing (low speed mode) is set to 75 mm/sec, as in the first embodiment. Is. However, the present embodiment is characterized in that in the developer staying detection control described in the first embodiment, the Vback potential is finer than in the first embodiment.

  The control flow in this embodiment will be described with reference to FIG.

  Similar to the first embodiment, when the paper type selection and the pre-rotation after the start of printing are started, the information of the selected image forming speed and the environmental temperature is read into the CPU 308, the low speed mode is selected, and the environmental temperature is set to a predetermined value. When the value is less than Ta (12° C.), the retention detection control is performed.

  In this embodiment, first, the Vback potential (140 V) at the time of image formation and the Vback potential (160 V) that is +20 V higher than the Vback potential (160 V) are controlled (S7-1), and the output value corresponding to the electrostatic capacitance at this time is set. Each is detected (S8-1). The slope P is calculated from the output of these two points (S9-1). For example, an example is shown in FIG. 11, but in the state of the graph C in this figure, the calculated slope P is 2.5 mV/V. If the slope P is equal to or greater than the threshold value Pt (set to Pt=1.0 mV/V in this embodiment) indicated by the dotted line in the figure (S10-1), the Vback potential during image formation may be increased slightly. It is judged that there is a sign of stagnation, that is, the possibility of stagnation is high, and 225 mm/sec (speed “1”, peripheral speed ratio to photosensitive drum=300%) is selected as the developing sleeve speed during image formation. (S11-1). Therefore, in the case of the graph C in FIG. 11, the speed “1” is selected, at which point the retention detection control and the pre-rotation are ended (S12), and the image formation is started (S13).

  If the slope P calculated above is less than the threshold Pt (1.0 mV/V), the Vback potential is further increased to +40 V (180 V) during image formation (S7-2), and the electrostatic capacitance at that time is increased. The output value corresponding to is detected (S8-2). The slope P is calculated from two outputs, this point and the point when the previously detected Vback potential is +20V (S9-2). Next, this inclination P is compared with the threshold value Pt (S10-2). When the inclination P is equal to or more than the threshold value Pt, it is determined that the possibility of occurrence of retention is somewhat high, and the developing sleeve speed during image formation is 168.8 mm/sec (speed “2”), which is slightly slower than the previous speed. , The peripheral speed ratio of the photosensitive drum=225%) is selected (S11-2).

  If the slope P calculated here is less than 1.0 mV/V, the Vback potential is further increased to +60 V (200 V) during image formation (S7-3), and the output corresponding to the electrostatic capacity at that time is output. The value is detected (S8-3). The slope P is calculated from two outputs, this point and the point when the previously detected Vback potential is +40V (S9-3). Next, this inclination P is compared with the threshold value Pt (S10-3). When the inclination P is equal to or larger than the threshold value Pt, it is determined that the possibility of staying is somewhat high at this time, and the developing sleeve speed during image formation is 168.8 mm/sec (speed “2”, The peripheral speed ratio to the photosensitive drum=225%) is selected (S11-2). The graph D in FIG. 11 corresponds to this case, and it can be seen that the output slope is larger than the threshold Pt indicated by the dotted line from the setting of the Vback potential of +60 V (200 V).

  Even if the Vback potential is increased to +60V (200V) at the time of image formation, if the inclination P is less than the threshold value Pt, it is determined that there is no possibility of retention, and the developing sleeve speed at the time of image formation is 112.5 mm/ sec (speed “3”, ratio of peripheral speed of photosensitive drum=150%) is selected (S11-3).

  In the present embodiment, since the Vback potential is finely varied in the retention detection control, it is possible to determine how high the possibility of retention is. Therefore, it becomes possible to appropriately select the developing sleeve speed according to the degree of the possibility that the staying occurs, and it becomes unnecessary to rotate the developing sleeve fast unnecessarily. Thereby, the deterioration of the developer can be suppressed as much as possible.

  Further, when the Vback potential is finely shaken and a sign of staying is observed, the staying detection control is ended without increasing the Vback potential any more. As a result, it is possible to prevent retention from occurring during the retention detection control by suddenly increasing the Vback potential.

  In the present embodiment, the Vback swing is described in three stages, but the Vback may be swung more finely, or the stage of the developing sleeve speed may be divided into finer levels for control. Further, similar to the first embodiment, as the staying avoiding means when the staying detection control determines that there is a sign of occurrence of staying, control is performed to increase the developing sleeve speed in the low speed mode. The effect can be obtained by controlling the Vback potential at the time of formation to be small or the amplitude of the AC voltage of the developing bias to be small.

  Also in this embodiment, as a means for detecting a sign of staying during the staying detection control, not the Vback potential but the output voltage according to the electrostatic capacitance when the rotational speed of the developing sleeve is stepwise shaken is detected. It is also possible to control by.

  By performing the control as described above, it is possible to prevent more wasteful deterioration of the developer than in the first embodiment and to prevent retention from occurring during control, and at the same time, to prevent image defects due to retention of the developer during image formation. Can be prevented, and a higher-quality image can be stably obtained.

  Although the four image forming units (image forming units) are used in the above-described embodiment, the number of the image forming units used is not limited, and may be appropriately set as necessary.

  Further, in the above-described embodiment, the printer is exemplified as the image forming apparatus, but the present invention is not limited to this. For example, the image forming apparatus may be another image forming apparatus such as a copying machine or a facsimile machine, or another image forming apparatus such as a multi-function machine that combines these functions. Similar effects can be obtained by applying the present invention to these image forming apparatuses.

1... Photosensitive drum 4C... Carrier particles 40... Developing device 42... Developing sleeve 42m... Magnet member 43... Layer thickness regulating member 300... Development high voltage substrate 301... AC high voltage drive circuit 303... DC high voltage circuit 304... pp voltage detection Circuit 305... Control board 308... CPU

Claims (10)

A rotatable image carrier on which an electrostatic latent image is formed,
A developing container that contains a developer containing toner and a carrier; and a rotatable developer carrier that carries and conveys the developer toward a position where the electrostatic latent image formed on the image carrier is developed. And a developing device for developing the electrostatic latent image formed on the image carrier with the developer,
A capacitance detection unit for detecting the capacitance between the image carrier and the developer carrier,
Drive means for rotationally driving the developer carrier,
A control unit for controlling the driving means,
Equipped with
A mode in which the control unit determines the rotation speed of the developer carrier in the image area based on the inclination of the change in the capacitance when the rotation speed of the developer carrier in the non-image area is changed. An image forming apparatus characterized by being capable of executing. The control unit, in the mode,
When the inclination of the change in the capacitance when the rotation speed of the developer carrier in the non-image area is changed is a predetermined value or more , the rotation speed of the developer carrier in the non-image area. The drive means is controlled so that the rotation speed of the developer carrying member in the image region is faster than when the inclination of the change in the capacitance when the value is changed is less than a predetermined value. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. The image forming apparatus includes a first image forming mode in which the image carrier is operated at a first rotation speed and a second image formation mode in which the image carrier is operated at a second rotation speed slower than the first rotation speed. It is possible to select and execute one image forming mode from a plurality of image forming modes including two image forming modes,
The image forming apparatus according to claim 1 or 2, wherein the control unit is capable of executing the mode when the second image forming mode is selected. The image forming apparatus further includes an environmental temperature detection unit that detects an environmental temperature,
The control unit is capable of executing the mode when the environmental temperature detected by the environmental temperature detecting means is lower than a predetermined value. Image forming apparatus. The image forming apparatus according to any one of claims 1 to 4, wherein the developer carrying member moves at a position closest to the image carrying member in a direction opposite to a moving direction of the image carrying member. . A rotatable image carrier on which an electrostatic latent image is formed,
A developing container that contains a developer containing toner and a carrier; and a rotatable developer carrier that carries and conveys the developer toward a position where the electrostatic latent image formed on the image carrier is developed. And a developing device for developing the electrostatic latent image formed on the image carrier with the developer,
Capacitance sensing means for sensing the capacitance between the image carrier and the developer carrier,
Developing bias applying means for applying a developing bias in which a DC voltage and an AC voltage are superposed to the developer carrier,
A control unit for controlling the developing bias applying unit,
Equipped with
When the control unit changes the potential difference between the surface potential of the non-image portion of the image carrier in the non-image area and the DC voltage of the developing bias applied to the developer carrier by the developing bias applying unit. The image forming apparatus is capable of executing a mode for determining the potential difference in the image area based on the inclination of the change in the capacitance. The control unit, in the mode,
When the inclination of the change in the capacitance when the potential difference in the non-image area is changed is a predetermined value or more , the change in the capacitance when the potential difference in the non-image area is changed. The image forming apparatus according to claim 6, wherein the developing bias applying unit is controlled so that the absolute value of the potential difference in the image area is smaller than that when the inclination of is less than a predetermined value. The image forming apparatus includes a first image forming mode in which the image carrier is operated at a first rotation speed and a second image formation mode in which the image carrier is operated at a second rotation speed slower than the first rotation speed. It is possible to select and execute one image forming mode from a plurality of image forming modes including two image forming modes,
The image forming apparatus according to claim 6 or 7, wherein the control unit can execute the mode when the second image forming mode is selected. The image forming apparatus further includes an environmental temperature detecting unit that detects an environmental temperature,
9. The control unit is capable of executing the mode when the environmental temperature detected by the environmental temperature detecting means is lower than a predetermined value. 9. The control unit according to claim 6, wherein the mode is executable. Image forming apparatus. 10. The image forming apparatus according to claim 6, wherein the developer carrying member moves in a direction closest to the image carrying member in a direction opposite to a moving direction of the image carrying member. ..

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