JP5075558B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus that develops an electrostatic latent image by supplying toner to the electrostatic latent image.

  In the electrophotographic image forming apparatus, the electrostatic latent image is developed by attaching the toner contained in the one-component developer or the two-component developer to a part of the electrostatic latent image carrier with electrostatic force. .

  For example, when a two-component developer composed of a carrier and a toner is used, a magnetic brush made of a developer is formed on the developer carrier, and a developing bias voltage is applied to the developer carrier so that the developer carrier and A bias electric field is generated between the electrostatic latent image carrier and the electrostatic latent image is developed.

In such an image forming apparatus, when the charge amount of toner changes, the amount of toner adhering to the electrostatic latent image carrier changes, so that the image density changes. In order to adjust the image density, the conventional image forming apparatus adjusts the DC voltage component of the developing bias voltage applied to the developer carrier (see, for example, Patent Document 1).
JP 2005-165091 A

  However, if only the DC voltage component is changed, even if the image density can be kept constant, the reproducibility of the image is lowered. For example, when a DC bias voltage component is lowered due to a decrease in toner charge amount compared to when a standard level development bias voltage is applied, dots or fine lines are chipped.

  An object of the present invention is to provide an image forming apparatus capable of adjusting an image density while suppressing a decrease in image reproducibility.

  An image forming apparatus of the present invention is an electrophotographic image forming apparatus that develops an electrostatic latent image by using a developer containing toner and supplying the toner to the electrostatic latent image. A carrier, a developer carrier, a voltage application unit, and a control unit are provided. The electrostatic latent image carrier carries an electrostatic latent image on its surface. The developer bearing member faces the electrostatic latent image bearing member and carries toner for developing the electrostatic latent image. The voltage application unit is a development bias voltage in which an AC voltage component and a DC voltage component are superimposed, and a development-side potential and an electrostatic latent image carrier that apply a force from the developer carrier to the electrostatic latent image carrier. A developing bias voltage is applied to the developer carrying member, with each of the reverse development side potentials giving the toner a force directed from the body to the developer carrying member as a peak potential. The control unit controls the developing bias voltage applied to the developer carrying member by the voltage applying unit while maintaining the reverse developing side potential at a predetermined value when adjusting the image density.

  When the potential of the DC voltage component of the development bias voltage is changed when adjusting the image density, the developer adheres to the reverse development side potential and the electrostatic latent image carried on the surface of the electrostatic latent image carrier. The potential difference between the image portion and the potential of the image portion is changed. Image reproducibility tends to be reduced by an electric field in which the toner acts in a direction from the electrostatic latent image carrier to the developer carrier.

  In this configuration, when adjusting the image density, elements other than the reverse development side potential of the development bias voltage are adjusted. When adjusting the image density, the potential of the image portion is not changed. For this reason, at the time of adjusting the image density, the potential difference between the reverse development side potential and the potential of the image portion is not changed, and a decrease in image reproducibility is suppressed.

  In the above configuration, the potential difference obtained by subtracting the potential of the image portion of the electrostatic latent image carrier from the reverse development side potential when the toner charging polarity is negative, or the potential of the image portion when the toner charging polarity is positive. The potential difference obtained by subtracting the reverse development side potential from is desirably less than 100 volts. When the toner charging polarity is negative, the reverse development side potential is too large and out of the above range, and when the toner charging polarity is positive, the reverse development side potential is too small and out of the above range. In this case, the electric field acting on the toner in the direction from the electrostatic latent image carrier to the developer carrier becomes strong. For this reason, a sufficient amount of toner does not move to the image portion, or the toner once moved to the image portion is pulled back from the image portion, so that the reproducibility of the image is lowered. On the other hand, if the potential difference is within the above range, the image density is adjusted while suppressing a decrease in the reproducibility of the image.

  In addition, the control unit may be configured to adjust the development-side potential when adjusting the image density. By adjusting the development side potential without changing the reverse development side potential, the image density can be adjusted without changing the electric field acting in the direction from the electrostatic latent image carrier to the developer carrier. For this reason, it is possible to adjust the image density while suppressing a decrease in image reproducibility.

  Furthermore, the control unit may change the amplitude of the AC voltage component by the potential ΔV and change the DC voltage component by the potential ΔV when the developing side potential is changed by the potential 2ΔV when adjusting the image density. For example, when the polarity of the developing side potential is negative and the developing side potential is decreased by 2ΔV, the amplitude of the AC voltage component is increased by the potential ΔV, and the DC voltage component is decreased by the potential ΔV. On the other hand, when the development-side potential is increased by 2ΔV, the amplitude of the AC voltage component is decreased by the potential ΔV, and the DC voltage component is increased by the potential ΔV. As a result, the development side potential can be changed while the reverse development side potential is maintained at a predetermined value. Therefore, it is possible to adjust the image density while suppressing a decrease in image reproducibility.

  The control unit may adjust the application time of at least one of the development side potential and the reverse development side potential when adjusting the image density. When adjusting the image density, the reverse development side potential is changed by adjusting either the development side potential application time or the reverse development side potential application time without changing the development side potential and the reverse development side potential. The same effect as changing the DC voltage component without changing the image density can be obtained. Therefore, the image density is adjusted while suppressing a decrease in image reproducibility.

  The configuration further includes a humidity measuring unit that measures the humidity in the vicinity of the developer carrier, and the control unit adjusts the toner concentration in the developer according to the humidity acquired by the humidity measuring unit when adjusting the image density. May be. When the humidity is low, the charge amount of the toner becomes high and the image density becomes low. Therefore, by increasing the toner density in the developer, the toner is easily supplied to the image portion so that the image density becomes constant. Adjust to. When the charge amount of the toner is high, even if the toner concentration is high, the fog in which the toner is supplied in a thin film shape to the non-image portion where the image should not be formed is suppressed. On the other hand, when the humidity is high, the charge amount of the toner becomes low and the image density becomes high. By reducing the toner density, it is difficult to supply the toner to the image portion, and the image density is made constant. adjust. When the charge amount of the toner is low, the fog is generally increased, but the fog is suppressed by lowering the toner concentration. In any case, since the reverse development side potential is not changed, the image density is adjusted while suppressing a decrease in image reproducibility.

  The configuration further includes a humidity measuring unit that measures the humidity in the vicinity of the developer carrier, and the control unit adjusts the frequency of the developing bias voltage according to the humidity acquired by the humidity measuring unit when adjusting the image density. Also good. When the humidity is high, the charge amount of the toner decreases and the image density increases. Therefore, by adjusting the frequency of the developing bias voltage, the image density is adjusted to be constant. When the charge amount of the toner is low, the fog is generally increased, but the fog is suppressed by increasing the frequency of the developing bias voltage. When the humidity is low, the charge amount of the toner becomes high and the image density becomes low. Therefore, by adjusting the frequency of the developing bias voltage, the image density is adjusted to be constant. Fog is suppressed when the charge amount of the toner is high. In any case, since the reverse development side potential is not changed, the image density is adjusted while suppressing a decrease in image reproducibility.

  When the development side potential application time is T1 and the reverse development side potential application time is T2, the control unit applies within the range in which 0.3 <T1 / (T1 + T2) <0.45 holds when adjusting the image density. The structure which adjusts at least any one of time T1 or T2 may be sufficient. When the duty ratio T1 / (T1 + T2) is 0.45 or more, the carrier rising rate at which the carrier contained in the two-component developer adheres to the electrostatic latent image carrier is greatly increased. When the duty ratio is 0.3 or less, the fog is greatly increased, the amount of toner attached to the electrostatic latent image carrier is decreased, and the image density is extremely lowered as it deviates from a predetermined allowable range. To do. If the duty ratio is in the range of more than 0.3 and less than 0.45, it is possible to reduce the carrier rise, suppress the increase in fogging, and increase the image density to a predetermined value or more. Therefore, it is desirable that the duty ratio is in the range of more than 0.3 and less than 0.45.

  According to the present invention, when adjusting the image density, by controlling the developing bias voltage while maintaining the reverse developing side potential at a predetermined value, the electric field acting in the direction in which the reproducibility of the image tends to be reduced is changed. You can avoid it. For this reason, it is possible to adjust the image density while suppressing a decrease in image reproducibility.

  Hereinafter, the best mode for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a schematic front sectional view of an image forming apparatus 100 according to an embodiment of the present invention.

  The image forming apparatus 100 is a tandem color image forming apparatus that forms an image using four color developers of yellow, magenta, cyan, and black. In this embodiment, a two-component developer containing toner and carrier is used as the developer. The image forming apparatus 100 is based on image data read by a document reading device such as a scanner, or image data transmitted from a terminal device such as a PC (Personal Computer) that is communicably connected via a network (not shown). Thus, a color image or a monochrome image is formed on the paper P that is a recording medium.

  The image forming apparatus 100 includes a paper feed tray 110, a transport device 120, an image forming unit 130, and a fixing device 140.

  The paper feed tray 110 accommodates a large number of sheets on which images are to be formed.

  The conveyance device 120 includes a driving roller 121, a driven roller 122, and a conveyance belt 123. The conveyor belt 123 is an endless belt and is stretched between the driving roller 121 and the driven roller 122. The transport belt 123 rotates to transport the paper P supplied from the paper feed tray 110 toward the fixing device 140.

  The image forming unit 130 includes photosensitive drums 31A to 31D, charging devices 32A to 32D, exposure units 33A to 33D, developing devices 34A to 34D, transfer rollers 35A to 35D, and cleaning units 36A to 36D. The photosensitive drums 31A to 31D correspond to the electrostatic latent image carrier of the present invention. The developing devices 34A to 34D have developing rollers 41A to 41D corresponding to the developer carrier of the present invention.

  The image forming unit 130 forms an image using image data corresponding to four hues of black and cyan, magenta, and yellow, which are three subtractive primary colors obtained by color separation of a color image. It comprises four image forming units 30A, 30B, 30C, 30D.

  The image forming unit 30A for black, the image forming unit 30B for cyan, the image forming unit 30C for magenta, and the image forming unit 30D for yellow are arranged in this order from the downstream side in the paper transport direction along the transport belt 123. Are arranged in a row.

  Hereinafter, the image forming unit 30A for black will be mainly described. The other hue image forming units 30B to 30D are configured in the same manner as the image forming unit 30A.

  The image forming unit 30A includes a photosensitive drum 31A that rotates clockwise in FIG. Around the photosensitive drum 31A, a charging device 32A, an exposure unit 33A, a developing device 34A, a transfer roller 35A, and a cleaning unit 36A are arranged in this order along the rotation direction of the photosensitive drum 31A. The transfer roller 35A is disposed at a position facing the photosensitive drum 31A with the conveyance belt 123 and the paper P interposed therebetween.

  The charging device 32A uniformly charges the surface of the photosensitive drum 31A to a predetermined potential. In this embodiment, a contact-type roller charging device is used, but a charger-type or brush-type charging device may be used.

  The exposure unit 33A includes a semiconductor laser, a polygon mirror, a first fθ lens, and a second fθ lens (not shown). The exposure unit 33A irradiates the photosensitive drum 31A with a laser beam modulated by black hue image data. On each of the photosensitive drums 31A to 31D, an electrostatic latent image is formed based on image data of each hue of black, cyan, magenta, and yellow. As the exposure unit 33A, a laser scanning unit (LSU) or a writing device in which light emitting elements such as EL and LED are arranged in an array can be used.

  The developing device 34A contains black toner and has a developing roller 41A. The developing roller 41A conveys the developer to a developing area where the toner can move to the surface of the photosensitive drum 31A. The developing device 34A supplies toner to the electrostatic latent image formed on the surface of the photosensitive drum 31A, thereby converting the electrostatic latent image into a toner image.

  Each of the developing devices 34B to 34D contains toners of cyan, magenta, and yellow hues, and the electrostatic latent images of the respective hues formed on the photoreceptor drums 31B to 31D are cyan, magenta, and yellow, respectively. The toner images of each hue are visualized.

  In this embodiment, the toner is charged with the same polarity as the surface potential of the photosensitive drum 31A. Both the polarity of the surface potential of the photosensitive drum 31A and the charging polarity of the toner are negative.

  The cleaning unit 36A collects toner remaining on the surface of the photosensitive drum 31A after development and image transfer.

  In order to transfer the toner image carried on the surface of the photosensitive drum 31A onto the paper P, a transfer bias voltage having a polarity opposite to the toner charging polarity (plus in this embodiment) is applied to the transfer roller 35A. . As a result, the black hue toner image formed on the photosensitive drum 31A is transferred onto the paper P so as to overlap the other hue toner image on the paper P. In each of the image forming units 30 </ b> A to 30 </ b> D, a toner image of each hue is transferred onto the paper P so that a full-color toner image is formed on the paper P.

  However, when image data of only a part of the hues of yellow, magenta, cyan, and black is input, only a part corresponding to the hue of the input image data among the four photosensitive drums 30A to 30D. In step S1, an electrostatic latent image and a toner image are formed. For example, in the monochrome printing mode, the electrostatic latent image and the toner image are formed only on the photosensitive drum 31A corresponding to the black hue, and only the black toner image is transferred to the paper P.

  The fixing device 140 includes a heating roller 141 and a pressure roller 142. The sheet P on which the toner image has been transferred is guided to the fixing device 140 and is heated and pressed by passing between the heating roller 141 and the pressure roller 142. As a result, the toner image is firmly fixed on the surface of the paper P. The paper on which the toner image is fixed is discharged onto a paper discharge tray (not shown).

  FIG. 2 is a front sectional view of a part of the developing device 34 </ b> A provided in the image forming apparatus 100. The developing devices 34B to 34D are configured similarly to the developing device 34A.

  In addition to the developing roller 41A, the developing device 34A includes a regulating blade 42A, two agitating and conveying screws 43A and 44A, and a developing tank 45A.

  A two-component developer containing toner and carrier is contained in the developing tank 45A. The agitating / conveying screws 43A and 44A are disposed in the developing tank 45A. A partition wall 46A is disposed between the stirring and conveying screw 43A and the stirring and conveying screw 44A. The partition 46A divides the region in the vicinity of the stirring and conveying screw 43A and the region in the vicinity of the stirring and conveying screw 44A except for both end portions in the rotation axis direction of the stirring and conveying screws 43A and 44A.

  The toner in the developer accommodated in the developing tank 45A is triboelectrically charged by being stirred together with the carrier by the stirring operation of the stirring and conveying screws 43A and 44A.

  The developing tank 45A has an opening 47A at a portion facing the photosensitive drum 31A. The developing roller 41A is partially exposed from the opening 47A of the developing tank 45A, and a developing gap of about 0.3 mm to 1.0 mm is provided between the surface of the photosensitive drum 31A. Are disposed in the developing tank 45A.

  The developing roller 41A includes a magnet roller 48A and a nonmagnetic developing sleeve 49A. The magnet roller 48A contains a plurality of magnetic pole members arranged along the circumferential direction. The magnet roller 48A is fixedly disposed in the developing tank 45A. The developing sleeve 49A is formed of a substantially cylindrical aluminum alloy, brass or the like, and is externally fitted to the magnet roller 48A so as to be rotatable in a predetermined direction (the arrow direction in FIG. 2). The developing sleeve 49A is rotated in a predetermined direction by a driving source (not shown).

  The carrier contained in the developer is made of a magnetic material. The toner adheres to the surface of the carrier by the Coulomb force generated by frictional charging. The developer is attracted to the surface of the developing sleeve 49A by the magnetic force of the magnet to form a magnetic brush, and the developing sleeve 49A is rotated, so that the developer is transported to the developing region that is opposed to the photosensitive drum 31A.

  The regulating blade 42A is disposed on the upstream side of the opening 47A in the rotation direction of the developing sleeve 49A in the developing tank 45A so that the tip thereof faces the developing sleeve 49A. The regulating blade 42A regulates the layer thickness of the developer formed on the surface of the developing roller 41A.

  The developing device 34A supplies a predetermined amount of developer to the developing area. The toner contained in the developer supplied to the development area is attracted to the electrostatic latent image carried on the surface of the photoreceptor drum 31A by electrostatic force. As a result, the electrostatic latent image is developed into a toner image.

  The developing device 34A returns the toner that has not been used for the carrier and the development among the developer supplied to the development area to the developing tank 45A again by the rotation of the developing sleeve 49A.

  FIG. 3 is a block diagram illustrating a configuration of the control unit 50 of the image forming apparatus 100.

  The image forming apparatus 100 includes a control unit 50 that comprehensively controls the entire image forming apparatus 100. The control unit 50 is electrically connected to an image processing unit 61, an input unit 62, a developing bias voltage applying unit 71A, drive motors 72A and 73A, and a charging bias voltage applying unit 74A.

  The image processing unit 61 receives image data read by the document reading device 200 or image data transmitted from a terminal device that is communicably connected via a network (not shown), and the image data is received as a predetermined image. It is converted into a formation signal and output to the control unit 50.

  The input unit 62 accepts an input of setting by the user of the image density at the time of image formation. The control unit 50 stores the image density input from the input unit 62 as an input setting value of image density. When there is no input setting value of image density by the user, the control unit 50 stores a predetermined standard density determined in advance as the input setting value. The controller 50 adjusts the developing bias voltage V10 so that an actual image is formed with the image density stored as the input set value.

  The control unit 50 includes a CPU 51, and a ROM 52 storing a program executed by the CPU 51 and various data and a RAM 53 serving as a working memory are connected to the CPU 51. The CPU 51 controls the operation of each unit of the image forming apparatus 100 according to the input image forming signal.

  The CPU 51 transmits an image forming operation by transmitting a control signal corresponding to the input image forming signal to each of the developing bias voltage applying unit 71A, the driving motor 72A, the driving motor 73A, and the charging bias voltage applying unit 74A. Let

  The development bias voltage application unit 71A applies a development bias voltage to the development sleeve 49A of the development roller 41A. The drive motor 72A rotates the developing sleeve 49A. The drive motor 73A rotates the photosensitive drum 31A. The charging bias voltage application unit 74A applies a charging bias voltage to the charging device 32A.

  A single drive motor can be shared as the drive motor 72A for rotating the developing sleeve 49A and the drive motor 73A for rotating the photosensitive drum 31A.

  FIG. 4 is a diagram showing the developing bias voltage V10 before the image density adjustment. In FIG. 4, the negative value increases toward the upper side.

  The development bias voltage V10 is a vibration bias voltage in which an AC voltage component and a DC voltage component V11 are superimposed. The development bias voltage V10 has a peak potential at each of the development side potential V1 and the reverse development side potential V2. The developing-side potential V1 gives the toner a force from the developing roller 41A toward the photosensitive drum 31A. The reverse development side potential V2 gives the toner a force from the photosensitive drum 31A toward the developing roller 41A.

  When the application time of the development side potential V1 is T1 and the application time of the reverse development side potential V2 is T2, the duty ratio is expressed by T1 / (T1 + T2). In the image forming apparatus 100 according to the first embodiment, the developing bias voltage V10 that is a rectangular wave having a duty ratio of 0.5 is used.

  The developing bias voltage V10 is applied to the developing sleeve 41A and generates a bias electric field between the developing roller 41A and the photosensitive drum 31A.

  The image density, the fog in which the toner is supplied to the non-image area where the image should not be formed, the carrier rising in which the carrier in the developer adheres to the photosensitive drum 31A, and the image reproducibility are the development bias voltage. V10 and the image portion potential V21 that is the potential of the image portion of the surface of the photosensitive drum 31A or the non-image portion potential V20 that is the potential of the non-image portion of the surface of the photosensitive drum 31A. To do.

  In the electrophotographic image forming process, the toner charge amount greatly affects the image density. Even if an appropriate image density is obtained with the developing bias voltage V10 as shown in FIG. 4 in a certain environment, if the charge amount of the toner decreases due to an environmental change such as an increase in humidity around the developing roller 41A, the photosensitive drum 31A. As the amount of toner adhering to the toner increases, the image density becomes too high under the same developing bias voltage V10.

  FIG. 5 is a diagram illustrating the developing bias voltage after adjusting the image density in the comparative example. In FIG. 5, the negative value increases toward the upper side.

  As a comparative example for adjusting the image density, when the image density is lowered, the peak-to-peak voltage Vpp of the developing bias voltage V10 is constant and the DC voltage component V11 is raised as shown in FIG. As a result, the development-side potential V1 increases. In this case, if the increment of the DC voltage component V11 is ΔV, the charging potential of the photosensitive drum 31A is also changed, and the non-image portion potential V20 is increased by ΔV. This is because if the non-image portion potential V20 is kept constant, the difference between the direct-current voltage component V11 and the non-image portion potential V20 increases, and the carrier rise increases.

  In contrast, in the comparative example, when the DC voltage component V11 is lowered to adjust the image density, the difference between the DC voltage component V11 and the non-image portion potential V20 is small unless the charging potential of the photosensitive drum 31A is changed. And the fog increases.

  Therefore, in either case, the DC voltage component V11 and the non-image portion potential V20 are changed in conjunction so that the potential difference hardly changes.

  In the comparative example, when the charging potential of the photosensitive drum 31A is increased, the image portion potential V21 also changes somewhat. However, since the change in the image portion potential V21 is relatively small, FIG. 5 shows that the image portion potential V21 is not changed.

  FIG. 6 is a diagram illustrating the developing bias voltage after the adjustment of the image density in the first embodiment.

  In this embodiment, when adjusting the image density, the reverse development side potential V2, which is the peak potential on the higher side of the development bias voltage V10, is maintained at a predetermined value without being changed, and the peak potential on the lower side. The development side potential V1 is changed. Specifically, for example, as shown in FIG. 6, when the development-side potential V1 is to be increased by 2 × ΔV, the amplitude (peak-to-peak voltage Vpp) of the AC voltage component is decreased by 2 × ΔV, and the DC voltage component V11 is decreased by ΔV. increase. When the development-side potential V1 is to be lowered by 2 × ΔV, the amplitude of the AC voltage component (peak-to-peak voltage Vpp) is increased by 2 × ΔV, and the DC voltage component V11 is lowered by ΔV. As a result, the development side potential V1 can be changed while the reverse development side potential V2 is maintained at a predetermined value.

  Next, a method for determining ΔV will be described. A pixel having a predetermined size serving as a reference is exposed and developed to form a density patch. The density of the formed density patch is optically detected. In an image forming apparatus configured to transfer a toner image onto a sheet via a transfer belt, the detection of the density patch density is not limited to being performed on the photosensitive drum 31A, but can be performed on the transfer belt. The optical sensor for detecting the density includes a light emitting element such as an LED (Light Emitting Diode) and a light receiving element such as a PD (Photo Diode), and detects specularly reflected light or diffused light according to the hue or material of the toner. .

  A density patch based on the previously set development bias voltage V10 and a density patch when shifted by ± 50 V from the development bias voltage V10 are formed, and the density of these three density patches is detected by the method described above. The CPU 51 determines the development bias voltage V10 corresponding to an appropriate density from the three density values, determines the value of ΔV, and applies the development bias voltage V10 after adjusting the image density from the development bias voltage application unit 71A to the development sleeve 41A. Apply.

  In order to verify the effect of adjusting the image density while keeping the reverse development side voltage V2 constant, a comparative experiment was performed in the case of adjusting the density only with the DC voltage component V11. Before adjusting the image density, the waveform of the developing bias voltage V10 in FIG. 4 was set. At this time, the non-image portion potential V20 is −500 V, the image portion potential V21 is −100 V, the DC voltage component V11 is −350 V, and the peak-to-peak voltage Vpp is 800 V (development side potential V1 = −750 V, reverse development side potential V2 = + 50 V). ).

  Next, the toner charge amount and the developing bias voltage V10 were changed. In the experiment, a toner deteriorated by being left for a long period of time was used as a toner having a decreased charge amount, and an experiment on a toner having an increased charge amount was performed in an environment having a reduced humidity.

  FIG. 7A is a diagram showing a photograph of an isolated dot formed in an initial stage before the change in the toner charge amount, and FIG. 7B is an image after adjusting the image density with only the DC voltage component V11. It is a figure showing the photograph of the formed isolated dot.

  Here, an experiment was conducted in the case where the toner charge amount decreased. When the image density was adjusted only with the DC voltage component V11, the shift amount ΔV of the developing bias voltage V10 was 150V. As shown in FIG. 7B, it can be seen that the isolated dots after adjusting the image density are very thin and small. That is, when the image density is adjusted only with the DC voltage component V11, the reproducibility of isolated dots greatly changes even if the density of the solid image can be kept constant.

  On the other hand, when the image density is adjusted by the control method of the first embodiment, the shift amount ΔV of the developing bias voltage V10 is 80V. The DC voltage component V11 increased by 80V, and the peak-to-peak potential Vpp decreased by 160V. Since there are both an effect of lowering the image density by increasing the DC voltage component V11 and an effect of lowering the image density by reducing the peak-to-peak potential Vpp, the density adjustment is performed only by the DC voltage component V11. In addition, the amount of change in each of the DC voltage component V11 and the peak-to-peak potential Vpp can be smaller than when the concentration is adjusted only by the peak-to-peak potential Vpp. For this reason, it is possible to adjust the image density while suppressing a decrease in image reproducibility.

  FIG. 8 is a diagram illustrating a photograph of an isolated dot on which an image is formed after adjusting the image density in the first embodiment. It can be seen that the isolated dots shown in FIG. 8 are almost the same as the initial isolated dots shown in FIG.

  The difference between the isolated dots in FIG. 7B and the isolated dots in FIG. 8 is considered to largely depend on the relationship between the reverse development side potential V2 and the image portion potential V21. In the control method in which only the DC voltage component V11 is shifted, the positional relationship between the reverse development side potential V2 and the image portion potential V21 is changed by the shift of the development bias voltage V10 as the development bias voltage V10 is shifted. The reproducibility of isolated dots tends to decrease due to the electric field acting in the direction in which the toner returns from the photosensitive drum 31A toward the developing sleeve 41A. For this reason, when the potential difference between the reverse development side potential V2 and the image portion potential V21 is changed, the reproducibility of the isolated dots greatly changes.

  In the present invention, the reverse development side potential V2 is not changed, and the image portion potential V21 is hardly changed, so that isolated dots are reproduced stably and satisfactorily.

  FIG. 9 is a diagram illustrating experimental results regarding the relationship between the voltage shift amount ΔV and the line width. FIG. 10A shows the line width of an image formed in the initial stage before the change in the toner charge amount, and FIG. 10B shows the image after adjusting the image density according to the change in the toner charge amount. The line width of the formed image is shown.

  As shown in FIGS. 10A and 10B, the initial line width X0 and the line width X1 after adjusting the image density with the change in the charge amount were measured. In FIG. 9, the horizontal axis represents the voltage shift amount ΔV, and the vertical axis represents the ratio (X1 / X0) to the initial (ΔV = 0) line width. When the image density is adjusted by shifting only the DC voltage component V11 of the comparative example, the line width is likely to change, whereas in the present invention, the line width is formed stably.

  As described above, at the developing bias voltage V10, the peak potential that gives the toner a force from the developing roller 41A toward the photosensitive drum 31A is the developing-side potential V1, whereas the photosensitive drum 31A moves toward the developing roller 41A. The peak potential that applies force to the toner is the reverse development side potential V2. At this time, the dot reproducibility and fine line reproducibility of the toner image transferred to the paper or the like vary greatly depending on the relationship between the reverse development side potential V2 and the image portion potential V21.

  FIG. 11 shows the experimental results regarding the relationship between the potential difference between the reverse development side potential V2 and the image portion potential V21 and the dot reproducibility when the image portion potential V21 is kept constant at −100V.

  As shown in FIG. 11, when V2−V21 <+100, that is, V2 <V21 + 100, the dot reproducibility of the transferred image onto the paper or the like is good. On the other hand, when V2 ≧ V21 + 100, the dot reproducibility decreases. This is considered to be due to the fact that the electric field that gives the toner a force from the image portion toward the developing roller 41A is strengthened, and the toner that has once reached the image portion is pulled back and the dots are disturbed.

  From the above examination results, it is understood that it is desirable to control the developing bias voltage applying unit 71A by the control unit 50 so that the potential difference of the reverse developing side potential V2 with respect to the image portion potential V21 of the photosensitive drum 31A does not exceed 100V. .

  In this embodiment, the case where the charging polarity of the toner is negative has been described, but the same experiment was performed when the charging polarity of the toner was positive, and the result shown in FIG. 12 was obtained. From this experimental result, it can be seen that dot reproducibility is good when V21−V2 <+100, that is, V2> V21-100.

  Therefore, the potential difference obtained by subtracting the image portion potential V21 from the reverse development side potential V2 when the toner charging polarity is negative, or the reverse development side potential V2 is subtracted from the image portion potential V21 when the toner charging polarity is positive. The potential difference is desirably less than 100 volts.

  As a result, the image density can be adjusted while further suppressing the decrease in dot reproducibility and fine line reproducibility.

  FIG. 13 is a diagram illustrating the developing bias voltage V10 after the adjustment of the image density in the second embodiment.

  In this embodiment, when adjusting the image density, the development side potential V1 and the reverse development side potential V2 of the development bias voltage V10 are not changed, and the development time V1 application time T1 or the reverse development side potential V2 application time T2 is maintained. Change at least one of them.

  Here, a case where the application time T1 is changed will be described.

  Assuming T1 = T2 as the standard state, the shift amount ΔV of the DC voltage component V11 of the developing bias voltage V10 when the application time T1 is changed by ΔT is expressed by the following equation.

ΔV = Vpp × ΔT / 2 (2 × T1 + ΔT)
FIG. 14 shows the experimental results regarding the relationship between the application time T1 and the image density. Here, the non-image portion potential V20 of the photosensitive drum 31A is -600V, the image portion potential V21 is -100V, the peak-to-peak voltage Vpp is 800V, the development side potential V1 is -850V, and the reverse development side potential V2 is -50V. The experiment was conducted at time T2 = 0.25 msec. From this experimental result, it can be seen that the image density can be adjusted over a wide range by changing the application time T1.

  Also in this case, since the reverse development side potential V2 is not changed, the electric field acting in the direction in which the toner returns from the photosensitive drum 31A toward the developing sleeve 41A is constant, and the toner moves from the developing sleeve 41A toward the photosensitive drum 31A. Even if the application time T1 of the developing-side voltage V1 acting in the direction is increased, there is almost no decrease in reproducibility of isolated dots or line width.

  The adjustment range of the application time T1 varies depending on the peak-to-peak voltage Vpp. The width of the range to be adjusted differs depending on how much the toner charge amount changes. When the adjustment range needs to be wide, the peak-to-peak voltage Vpp value is set higher (for example, 1600 V).

  Next, an image forming apparatus according to a third embodiment will be described. The image forming apparatus according to the third embodiment is configured in the same manner as the image forming apparatus 100 according to the first embodiment, except that the toner density is adjusted according to the humidity near the developing roller 41A. .

  As described above, in the electrophotographic image forming process, the charge amount of the toner greatly affects the toner amount for developing the electrostatic latent image carried on the photoreceptor drum 31A, and the main change in the image density. It becomes a factor.

  Therefore, the image forming apparatus according to the third embodiment further includes a humidity measuring unit 63 that measures humidity near the developing roller 41A and outputs measurement data to the CPU 51, and is based on the humidity measured by the humidity measuring unit 63. Thus, when the toner charge amount is high at low humidity, the toner concentration in the developer is set high, and when the toner charge amount is low at high humidity, the toner concentration in the developer is set low.

  When the charge amount of the toner is high, the image density becomes low. Therefore, the image density can be supplemented by setting the toner density high. Further, when the toner charge amount is high, fogging can be suppressed even if the toner concentration is high.

  On the other hand, when the charge amount of the toner is low, the image density tends to be high. Therefore, the increase in the image density can be suppressed by setting the toner density low. Further, when the charge amount of the toner is low, the fog generally increases. However, the fog can be suppressed by lowering the toner density, and the toner charge amount is increased by lowering the image density. There is also an effect of alleviating the decrease in charge amount.

  Generally, the toner concentration in the developing tank 45A is kept constant using a magnetic permeability sensor or the like. When the toner concentration in the developer is changed according to the humidity, it is realized by changing the set value of the toner concentration in the developing tank 45A according to the humidity. At this time, the set value of the toner density may be continuously changed according to the humidity, or the toner density may be set to the set value according to the humidity for each predetermined range using a conversion chart of several stages. Good.

  The image density control method according to the third embodiment is desirably used in combination with the image density control method according to the first embodiment or the image density control method according to the second embodiment.

  According to the first embodiment or the second embodiment, it is possible to adjust the image density while suppressing a decrease in image reproducibility. However, in the first embodiment and the second embodiment, the adjustment amount of the developing bias voltage V10 increases when the change in the toner charge amount is large. Therefore, in order to further suppress the decrease in image reproducibility, By using the image density control method in the third embodiment in combination, the adjustment amount of the developing bias voltage V10 can be reduced. As a result, it is possible to adjust the image density while further suppressing changes in the dot size and line width and further suppressing deterioration in image reproducibility.

  Next, an image forming apparatus according to a fourth embodiment will be described.

  The image forming apparatus according to the fourth embodiment includes a humidity measuring unit 63, and the image forming apparatus according to the first embodiment except that the frequency of the developing bias voltage is adjusted according to the humidity near the developing roller 41A. The configuration is the same as that of the forming apparatus 100.

  FIG. 15 shows the absolute value of the potential difference between the average voltage V12 of the developing bias voltage V10 (here, equivalent to the potential of the DC voltage component V11) and the non-image portion potential V20 for a plurality of different frequencies of the developing bias voltage V10. The experimental result about the relationship with fog is shown. Here, a value obtained by measuring the optical density of the toner adhering to the non-image portion on the photosensitive drum 31A is shown as the fog value.

  Referring to FIG. 15, it can be seen that the fog decreases as the frequency of the developing bias voltage V10 increases.

  Therefore, when the humidity is high, the charge amount of the toner decreases and the image density increases. Therefore, the image density is adjusted to be constant by increasing the frequency of the development bias voltage V10. When the toner charge amount is low, fogging generally increases, but as shown in FIG. 15, the fogging can be reduced by increasing the frequency of the developing bias voltage V10.

  Further, when the humidity is low, the charge amount of the toner becomes high and the image density becomes low. Therefore, by adjusting the frequency of the developing bias voltage V10, the image density is adjusted to be constant. When the charge amount of the toner is high, the fog is reduced.

  In either case, since the reverse development side potential V2 is not changed, the image density can be adjusted while suppressing a decrease in image reproducibility.

  The image density control method in the fourth embodiment is desirably used in combination with the image density control method in the first embodiment or the image density control method in the second embodiment.

  By using the image density control method in the first embodiment or the second embodiment together with the image density control method in the fourth embodiment, the adjustment amount of the developing bias voltage V10 is reduced. Therefore, it is possible to adjust the image density while further suppressing changes in the dot size and line width, and further suppressing deterioration in image reproducibility.

  Next, the duty ratio of the developing bias voltage V10 will be described.

  As described above, when the application time of the development side potential V1 is T1, and the application time of the reverse development side potential V2 is T2, the duty ratio is represented by T1 / (T1 + T2). The duty ratio has a very large correlation with the carrier up.

  FIG. 16 shows experimental results regarding the relationship between the absolute value of the potential difference between the average voltage V12 and the non-image portion potential V20 and the carrier rise at a plurality of different duty ratios. Here, as the value of the carrier rising, a value obtained by capturing the carrier adhering to a predetermined area (300 mm × 15 mm) in the non-image portion on the photosensitive drum 31A with an adhesive tape and visually counting the number of carriers is shown. . In this experiment, the average voltage V12 is constant.

  In FIG. 16, the vertical axis indicates the amount of carrier rise (unit: au), and the horizontal axis indicates the absolute value of the potential difference between the average voltage V12 of the developing bias voltage V10 and the non-image portion potential V20 of the photosensitive drum 31A. Indicates. In this experiment, a carrier having an average particle diameter of 35 μm was used as a carrier in the developer. From this experiment, the correlation between the duty ratio and the carrier uplink is known.

  Referring to FIG. 16, it can be seen that the carrier uplink decreases as the duty ratio decreases to 0.5, 0.38, and 0.25. In particular, by reducing the duty ratio of the developing bias voltage V10, even when the potential difference between the average voltage V12 of the developing bias voltage V10 and the image portion potential V21 of the photosensitive drum 31A is increased, the carrier rise is reduced. You can see that

  Thus, by increasing the duty ratio of the developing bias voltage V10, it is possible to reduce the carrier ascent. However, in general, if the duty ratio is too small, the fog increases. For this reason, in order to suppress fogging, the duty ratio of the developing bias voltage V10 applied to the developing roller 41A is limited.

  Next, experimental results for verifying a suitable duty ratio that can achieve both a reduction in fog and a decrease in carrier uplink are shown.

  FIG. 17 shows the relationship between the absolute value of the potential difference between the non-image portion potential V20 of the photosensitive drum 31A and the average voltage V12 of the developing bias voltage V10 and the respective amounts of fogging and carrier rising at different duty ratios. It is a figure which shows the experimental result about. In this experiment, while maintaining the bias waveforms of the non-image portion potential V20 and the developing bias voltage V10, the potential difference is changed by shifting the average voltage V12, and the duty ratios are 0.25, 0.38, and 0.5. For each case, the respective amounts of fog and carrier up were measured.

  As the range of absolute values of the potential difference between the non-image portion potential V20 of the photosensitive drum 31A and the average voltage V12 of the developing bias voltage V10 that can achieve both reduction in fog and reduction in carrier rising, | V20−V12 | = 100V The range of ~ 200V is mainly compared. Then, as shown in FIG. 17, at the duty ratio of 0.5, although the fogging is reduced, the carrier ascending rate is greatly increased at the potential difference of 200V. On the other hand, when the duty ratio is 0.25, although the carrier rise decreases to about 250V, the fog at the potential difference of 100V increases. Further, when the duty ratio is 0.38, both the fog and the carrier up are reduced.

  Similar experiments were also performed for duty ratios of 0.43 and 0.32, and the results of these experiments are collectively shown in FIG. As shown in FIG. 18, when the duty ratio T1 / (T1 + T2) is in the range of 0.3 to 0.45, both the fog and the carrier uplink decrease.

  Further, when the duty ratio is in the range of 0.3 to 0.45, the toner density can be kept high. FIG. 19 shows experimental results regarding the relationship between the duty ratio and the amount of toner adhering to the photosensitive drum 31A. Here, a value obtained by measuring the weight per unit area of the toner attached to the photosensitive drum 31A is shown as the value of the amount of toner attached.

  In a range where the duty ratio exceeds 0.3, it is possible to maintain an image density substantially equal to or higher than that when the duty ratio is 0.5. On the other hand, when the duty ratio is 0.3 or less, the amount of toner adhering to the photosensitive drum 31A decreases, and the image density of the toner image transferred to the paper is significantly reduced.

  In short, when the duty ratio T1 / (T1 + T2) is 0.45 or more, the carrier ascending rate is greatly increased, and when the duty ratio is 0.3 or less, the fog is greatly increased and the photosensitive drum of toner is used. The amount of adhesion to 31A decreases, and the image density decreases so much that it deviates from a predetermined allowable range. If the duty ratio is in the range of more than 0.3 and less than 0.45, it is possible to reduce the carrier up and suppress the increase of fogging and maintain the image density within a predetermined allowable range. Therefore, it is desirable that the duty ratio is in the range of more than 0.3 and less than 0.45.

  Therefore, it is desirable that the control unit 50 adjusts at least one of the application time T1 and the application time T2 within the range where the following expression is satisfied when adjusting the image density.

0.3 <T1 / (T1 + T2) <0.45
In the above-described embodiment, the case of using a two-component developer has been described. However, the present invention can also be applied to the case of using a one-component developer.

  Further, when developing an electrostatic latent image, regardless of one-component or two-component developer, development is performed using toner charged to the same polarity as the surface potential charged by the photosensitive drum 31A. In some cases, development is performed using toner charged to a polarity opposite to the surface potential charged by the photosensitive drum 31A. In any case, the present invention can be applied.

  Finally, the description of the above-described embodiment is to be considered in all respects as illustrative and not restrictive. The scope of the present invention is shown not by the above embodiments but by the claims. Furthermore, the scope of the present invention is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

1 is a schematic front sectional view of an image forming apparatus according to an embodiment of the present invention. 2 is a front sectional view of a part of the image forming apparatus. FIG. 2 is a block diagram illustrating a configuration of a control unit of the image forming apparatus. FIG. It is a figure which shows the developing bias voltage before adjustment of image density. It is a figure which shows the developing bias voltage after adjustment of the image density in a comparative example. It is a figure which shows the developing bias voltage after adjustment of the image density in 1st Embodiment. (A) is a view showing a photograph of an isolated dot formed in an initial stage before changing the charge amount of the toner, and (B) is an isolated dot formed after adjusting the image density with only a DC voltage component. It is a figure showing this photograph. It is a figure showing the photograph of the isolated dot image-formed after adjustment of the image density in 1st Embodiment. It is a figure which shows the experimental result about the relationship between voltage shift amount (DELTA) V and line width. (A) is a diagram showing the line width of an image formed at an early stage before the change of the toner charge amount, and (B) is an image formation after adjustment of the image density according to the change of the toner charge amount. It is a figure which shows the line width of the done image. It is a figure which shows the experimental result about the relationship between the electric potential difference of the reverse development side electric potential when an image part electric potential is made constant, and an image part electric potential, and dot reproducibility. FIG. 10 is a diagram illustrating an experimental result regarding a relationship between a potential difference between a reverse development side potential and an image portion potential and dot reproducibility when the toner charging polarity is positive. It is a figure which shows the developing bias voltage after adjustment of the image density in 2nd Embodiment. It is a figure which shows the experimental result about the relationship between application time T1 and image density. It is a figure which shows the experimental result about the relationship between the absolute value of the potential difference of the average voltage of a developing bias voltage, and the non-image part electric potential, and a fog about several frequency from which a developing bias voltage differs. The experimental result about the relationship between the absolute value of the electric potential difference of an average voltage and a non-image part electric potential in a several different duty ratio, and a carrier going up is shown. FIG. 10 is a diagram showing experimental results on the relationship between the absolute value of the potential difference between the non-image portion potential of the photosensitive drum and the average voltage of the developing bias voltage and the respective amounts of fogging and carrier rising at different duty ratios. is there. It is a figure which shows the experimental result about the relationship between a duty ratio, fogging, and carrier up. It is a figure which shows the experimental result about the relationship between a duty ratio and the adhesion amount of the toner to a photoreceptor drum.

Explanation of symbols

30A-30D Image forming unit 31A-31D Photosensitive drum (electrostatic latent image carrier)
34A to 34D Developing device 41A to 41D Developing roller (developer carrier)
50 control unit 51 CPU
63 Humidity measurement unit 71A to 71D Development bias voltage application unit 100 Image forming apparatus 130 Image forming unit V1 Development side potential V2 Reverse development side potential V10 Development bias voltage V11 DC voltage component V12 Average voltage V20 Non-image part potential V21 Image part potential Vpp Peak-to-peak voltage

Claims (5)

An electrophotographic image forming apparatus that develops the electrostatic latent image by supplying toner to the electrostatic latent image using a developer containing toner,
An electrostatic latent image carrier carrying an electrostatic latent image on the surface;
A developer carrying body facing the electrostatic latent image carrying body and carrying toner for developing the electrostatic latent image;
A developing bias voltage in which an AC voltage component and a DC voltage component are superimposed, and a developing-side potential that applies a force from the developer carrying member to the electrostatic latent image carrying member to the toner and from the electrostatic latent image carrying member to the above-mentioned A voltage applying unit that applies a developing bias voltage to each developer carrying member with a peak potential at each of the reverse development side potentials that gives the toner a force toward the developer carrying member;
A control unit that controls the developing bias voltage applied to the developer carrying member by the voltage applying unit while maintaining the reverse developing side potential at a predetermined value when adjusting the image density. ,
The control unit adjusts the developing-side potential when adjusting the image density, and when changing the developing-side potential to a potential 2ΔV, changes the amplitude of the AC voltage component to a potential 2ΔV, and changes the DC voltage component to a potential ΔV. The image forming apparatus to be changed .   The potential difference obtained by subtracting the potential of the image portion of the electrostatic latent image carrier from the reverse development side potential when the toner charging polarity is negative, or the potential of the image portion when the toner charging polarity is positive. The image forming apparatus according to claim 1, wherein the potential difference obtained by subtracting the reverse development side potential is less than 100 volts. 3. The image formation according to claim 1, wherein the control unit changes a non-image portion potential of the electrostatic latent image carrier so that a potential difference between the DC voltage component and the non-image portion potential becomes constant. apparatus. A humidity measuring unit for measuring the humidity in the vicinity of the developer carrying member;
Wherein, when adjusting the image density, the image forming apparatus according to any one of claims 1-3 for adjusting the toner concentration in the developer according to the humidity acquired by the humidity measurement unit. A humidity measuring unit for measuring the humidity in the vicinity of the developer carrying member;
Wherein, when adjusting the image density, the image forming apparatus according to any one of 4 claims 1 to adjust the frequency of the developing bias voltage in accordance with the humidity acquired by the humidity measurement unit.

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