Image forming apparatus

Abstract

The goal is to improve the performance of recovering carriers from the photosensitive drum to the recovery roller while simultaneously suppressing image distortion on the photosensitive drum. [Solution] An image forming apparatus capable of performing an image forming operation, comprising: a rotatable sleeve positioned facing the image carrier downstream of a development position where an electrostatic latent image formed on the image carrier is developed, with respect to the rotation direction of the image carrier, and upstream of a transfer position where a toner image carried on the image carrier is transferred to a transfer member; a non-rotating magnet positioned inside the sleeve; a carrier recovery device for recovering carriers on the image carrier; a voltage application unit for applying a voltage to the sleeve; and a control unit that controls the voltage application unit so that when performing an image forming operation, a carrier recovery bias is applied to the sleeve, which consists of a pulse section in which a DC voltage and an AC voltage are superimposed, and a blank section in which only a DC voltage is periodically repeated.

Description

Technical Field

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine having a plurality of functions thereof.

Background Art

[0002] As an image forming apparatus, a configuration in which a toner image is formed using a two-component developer including a non-magnetic toner and a magnetic carrier has been conventionally known. In this configuration, usually, in the developing process, the electrostatic latent image on the photosensitive drum is developed into a toner image by the toner, but the carrier may also adhere to the photosensitive drum at a certain rate (carrier adhesion). When carrier adhesion occurs, it affects the output image. For example, Patent Document 1 discloses a configuration provided with a carrier recovery device that recovers the carrier adhering to the photosensitive drum.

[0003] The carrier recovery device described in Patent Document 1 includes a recovery roller and a magnet roller provided inside the recovery roller, and further, a voltage obtained by superimposing a DC voltage and an AC voltage is applied to the recovery roller. Thereby, the carrier on the photosensitive drum is recovered by the recovery roller by the magnetic force of the magnet roller and the electrostatic force by the applied voltage.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In Patent Document 1, a voltage obtained by superimposing an AC voltage on a DC voltage is applied to the recovery roller. As conditions of the applied voltage, for example, the DC voltage is 0 to 800 V, the AC voltage is a rectangular wave with a difference Vpp between the maximum voltage and the minimum voltage of 800 to 2000 V, and the frequency is 500 to 2000 Hz.

[0006] Although carriers have a larger particle size and are less prone to vibration compared to toner, reducing the frequency component of the AC voltage applied to the recovery roller slows down the vibration speed, making it easier for the carriers to detach due to vibration. On the other hand, if a low-frequency AC voltage is applied to the recovery roller, the variation in toner redistribution in the carrier recovery area by the recovery roller increases, which may disrupt the image on the photoreceptor drum.

[0007] The present invention has been made in view of the above-mentioned problems. The object of the present invention is to provide an image forming apparatus that can achieve both improved performance in recovering carriers from the photoreceptor drum to the recovery roller and suppression of distortion of the image on the photoreceptor drum. [Means for solving the problem]

[0008] To achieve the above objective, an image forming apparatus according to one aspect of the present invention has the following configuration. That is, an image forming apparatus capable of performing an image forming operation, comprising: a rotatable image carrier on which an electrostatic latent image is formed; a developing container containing a developer including toner and a carrier; a developer carrier that carries the developer in order to develop the electrostatic latent image formed on the image carrier into a toner image; a transfer member on which the toner image carried on the image carrier is transferred; and a position downstream of the developing position where the electrostatic latent image formed on the image carrier is developed with respect to the rotation direction of the image carrier, and where the toner image carried on the image carrier is transferred to the transfer member. A carrier recovery device for recovering carriers on the image carrier, comprising: a rotatable sleeve positioned facing the image carrier upstream of the transfer position to which the image is transferred; a non-rotating magnet positioned inside the sleeve; a voltage application unit for applying a voltage to the sleeve; and a control unit that controls the voltage application unit so that when an image forming operation is performed, a carrier recovery bias is applied to the sleeve, which consists of a pulse section in which a DC voltage and an AC voltage are superimposed, and a blank section consisting only of the DC voltage, which are periodically repeated. [Effects of the Invention]

[0009] According to the present invention, it is possible to improve the performance of recovering carriers from the photoreceptor drum to the recovery roller and to suppress distortion of the image on the photoreceptor drum at the same time. [Brief explanation of the drawing]

[0010] [Figure 1] This is a cross-sectional view showing the configuration of the image forming apparatus according to this embodiment. [Figure 2] This is a cross-sectional view showing the configuration of the developing apparatus according to this embodiment. [Figure 3] This is a cross-sectional view showing the configuration of the carrier recovery device according to this embodiment. [Figure 4] This figure illustrates the carrier recovery bias according to this embodiment. [Figure 5] This figure illustrates the duty cycle of the carrier recovery bias according to this embodiment. [Figure 6] This diagram illustrates the carrier recovery bias waveform when affected by lead. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the present invention as defined in the claims, and not all combinations of features described in these embodiments are necessarily essential to the solutions of the present invention. The present invention can be implemented in various applications, such as photocopiers, printers, facsimile machines, and combinations having multiple functions thereof.

[0012] [Image forming apparatus] First, the general configuration of the image forming apparatus 100 of this embodiment will be explained using Figure 1.

[0013] The image forming apparatus 100 of this embodiment is a full-color image forming apparatus employing an electrophotographic method and comprises four image forming units Pa, Pb, Pc, and Pd. The configuration of each image forming unit is substantially the same except for the development color. Therefore, unless otherwise specified, the image forming unit Pa will be described as a representative unit, and for the other image forming units, the subscripts b, c, and d will be added to indicate that they are the respective image forming units, and detailed descriptions will be omitted.

[0014] The image forming unit Pa includes a photoreceptor drum 1a as an image carrier that carries an electrostatic latent image on its surface. The photoreceptor drum 1a is an example of a photoreceptor for electrophotography and is formed in a cylindrical shape. Such a photoreceptor drum 1a rotates in the direction of arrow R1 (counterclockwise) in Figure 1. Around the photoreceptor drum 1a are arranged a charger 2a as a charging unit, a laser beam scanner 3a as a latent image forming unit, a developing device 4a, a carrier recovery device 5a, a primary transfer roller 6a, a cleaning device 8a, and the like.

[0015] Next, the overall image forming sequence (image forming operation) of the image forming apparatus 100 with the above configuration will be described. First, the surface of the photoreceptor drum 1a is uniformly charged to a predetermined charging potential by the charger 2a. The photoreceptor drum 1a, which has been charged by the charger 2a, is then subjected to scanning exposure by a laser beam scanner 3a, which is an example of an exposure apparatus, using laser light modulated by an image signal.

[0016] The laser beam scanner 3a is controlled based on input image data, and the image data from which the laser light is emitted is input from an external terminal such as a document scanner or a personal computer (PC). The laser light from the laser beam scanner 3a changes the surface potential of the photoreceptor drum 1a, which is charged by the charger 2a, in the image area, and an electrostatic latent image is formed on the photoreceptor drum 1a.

[0017] The electrostatic latent image formed on the photoreceptor drum 1a as described above is reversely developed with toner by the developing device 4a to form a visible image, that is, a toner image. In the present embodiment, the developing device 4a uses a two-component developing method that uses a developer containing non-magnetic toner and a magnetic carrier as the developer. Each of the developing devices 4a, 4b, 4c, 4d has a developing container that houses a two-component developer containing toner of each color. Specifically, the developing device 4a houses yellow (Y) toner, the developing device 4b houses magenta (M) toner, the developing device 4c houses cyan (C) toner, and the developing device 4d houses black (K) toner. Therefore, by performing the above-described steps for each of the image forming units Pa, Pb, Pc, Pd, toner images of four colors, namely yellow, magenta, cyan, and black, are formed on the photoreceptor drums 1a, 1b, 1c, 1d, respectively.

[0018] Also, an intermediate transfer belt 60, which is an intermediate transfer member, is disposed below each of the image forming units Pa, Pb, Pc, Pd. The intermediate transfer belt 60 is suspended by rollers 61, 62, 63 and is movable in the direction of arrow R2. An outer secondary transfer roller 64 is disposed on the intermediate transfer belt 60 stretched over the roller 63. The secondary transfer roller 64 is configured such that a recording material can pass between it and the intermediate transfer belt 60. Note that the recording material is a sheet such as paper or a plastic sheet, for example.

[0019] The toner images on the photoreceptor drums 1a, 1b, 1c, 1d are sequentially primary transferred to the intermediate transfer belt 60 in primary transfer portions T1a, T1b, T1c, T1d as primary transfer members by primary transfer rollers 6a, 6b, 6c, 6d. As a result, toner images of four colors, namely yellow, magenta, cyan, and black, are superimposed on the intermediate transfer belt 60 to form a full-color image. Also, the toner remaining on the photoreceptor drum 1a without being transferred onto the intermediate transfer belt 60 is collected by the cleaning device 8a.

[0020] The full-color image on the intermediate transfer belt 60 is secondarily transferred onto the recording material fed from a feeding unit (not shown) by the action of the secondary transfer roller 64 in the secondary transfer unit T2 formed by the secondary transfer roller 64 and the intermediate transfer belt 60. The toner remaining on the surface of the intermediate transfer belt 60 without being transferred to the recording material is recovered by the intermediate transfer belt cleaning device 65. On the other hand, the recording material onto which the toner image has been transferred is sent to the fixing device 7, where the image is fixed and then discharged outside the machine.

[0021] The carrier recovery device 5a is disposed to face the photosensitive drum 1a downstream of the developing device 4a and upstream of the primary transfer unit T1a with respect to the rotation direction R1 of the photosensitive drum 1a. Similarly, the carrier recovery devices 5b, 5c, and 5d are disposed to face the photosensitive drums 1b, 1c, and 1d downstream of the developing devices 4b, 4c, and 4d and upstream of the primary transfer units T1b, T1c, and T1d with respect to the rotation directions R1 of the photosensitive drums 1b, 1c, and 1d.

[0022] The toner developed into an electrostatic latent image in the developing area and the carrier unintentionally transferred from the developing device 4a to the photosensitive drum 1a are conveyed to the carrier recovery device 5a along with the rotation of the photosensitive drum 1a after passing through the developing area. Similarly, the toner developed into an electrostatic latent image in the developing area and the carriers transferred to the photosensitive drums 1b, 1c, and 1d are conveyed to the carrier recovery devices 5b, 5c, and 5d along with the rotations of the photosensitive drums 1b, 1c, and 1d, respectively, after passing through the developing area.

[0023] The carrier recovery devices 5a, 5b, 5c, and 5d are provided for the purpose of recovering the carriers transferred to the photosensitive drums 1a, 1b, 1c, and 1d. Details of the carrier recovery devices 5a, 5b, 5c, and 5d will be described later with reference to FIG. 3.

[0024] (Configuration of the Developing Device) Next, the developing devices 4a, 4b, 4c, and 4d in this embodiment will be described with reference to Figure 1 and using Figure 2. Figure 2 is a cross-sectional view showing the configuration of developing device 4a. Since the configurations of developing devices 4a, 4b, 4c, and 4d are the same, developing device 4a will be described as a representative example below.

[0025] In this embodiment, the developer contained in the developing device 4a is a two-component developer in which a negatively charged non-magnetic toner and a magnetic carrier are mixed. The non-magnetic toner (hereinafter referred to as toner) is made by pulverizing or polymerization a resin such as polyester or styrene, which contains colorants, wax components, etc., and is made into a powder. The magnetic carrier (hereinafter referred to as carrier) is made by coating the surface of a core made of resin particles mixed with ferrite particles or magnetic powder with resin.

[0026] The developing apparatus 4a includes a developing container 40 for containing a two-component developer, a developing sleeve 41 as a developer carrier, and a magnet roller 42 consisting of a magnet fixedly positioned inside the developing sleeve 41 as a means for generating a magnetic field. Furthermore, a regulating blade 43 for forming a thin layer of developer on the surface of the developing sleeve 41, and screw members 44 and 45 for stirring and conveying the developer inside the developing container 40 are also provided.

[0027] The inside of the developing container 40 is divided into a developing chamber 40A and an agitation chamber 40B by a partition wall 46 that extends vertically. Transfer sections (not shown) are provided at both longitudinal ends of the partition wall 46 to allow the developer to pass between the developing chamber 40A and the agitation chamber 40B.

[0028] The developing chamber 40A has an opening at a position corresponding to the developing area opposite the photoreceptor drum 1, and the developing sleeve 41 is rotatably positioned in this opening of the developing container 40 so as to be partially exposed.

[0029] In this embodiment, the developing sleeve 41 is made of a non-magnetic material and rotates in the direction of the arrow in Figure 2 during the developing operation. Inside the developing sleeve 41, a magnet roller 42 having multiple magnetic poles along the circumferential direction is fixed as a means of generating a magnetic field.

[0030] In this embodiment, a magnet roller 42 with five magnetized poles is used. The N1 pole is an adsorption pole that supports the developer sent from the screw member 44 onto the developing sleeve 41. The S1 pole is a layer thickness regulating pole that restricts the amount of developer transported to the developing area. The N2 pole is a transport pole that transports the developer. The S2 pole is a developing pole that contributes to development. The N3 pole is a peeling pole that peels off the developer supported on the developing sleeve 41. With respect to the rotation direction R3 of the developing sleeve 41, a repulsive magnetic field is formed between the N3 pole and the N1 pole.

[0031] The developer in the developing chamber 40A is supplied to the developing sleeve 41 by the screw member 44. The developer supplied to the developing sleeve 41 is then carried onto the developing sleeve 41 in a predetermined amount by the magnetic pole N1 generated by the magnetic roller 42, forming a developer reservoir. As the developing sleeve 41 rotates, the two-component developer on the developing sleeve 41 is transported to the layer thickness regulating magnetic pole S1, where the layer thickness is regulated by the regulating blade 43.

[0032] The regulating blade 43 is composed of a non-magnetic material made of stainless steel or the like, extending along the rotation axis of the developing sleeve 41, and is positioned upstream of the position facing the photoreceptor drum 1 in the direction of rotation of the developing sleeve 41 (direction of arrow R3). Both the toner and carrier of the developer pass between the tip of the regulating blade 43 and the developing sleeve 41, and are transported by the transport magnetic pole N2 to the developing area facing the photoreceptor drum 1, supplying toner to the electrostatic latent image.

[0033] Subsequently, the developer on the developing sleeve 41 is peeled off from the surface of the developing sleeve 41 by the peeling magnetic pole S3.

[0034] In this embodiment, the diameter of the developing sleeve 41 is 20 mm, and the diameter of the photoreceptor drum 1 is 30 mm. Furthermore, by making the rotational linear speed of the developing sleeve 41 faster than the rotational linear speed of the photoreceptor drum 1, the amount of developer magnetic particles in contact per unit area of ​​the photoreceptor drum 1 is increased, thereby increasing the amount of toner that can be supplied. In this embodiment, the rotational linear speed of the photoreceptor drum 1 is 400 mm / s, and the rotational linear speed of the developing sleeve 41 is 600 mm / s, which is the peripheral speed ratio multiplied by 1.5.

[0035] Furthermore, in this embodiment, the closest proximity area between the developing sleeve 41 and the photoreceptor drum 1 is set to a distance of approximately 250 μm. This configuration allows development to be performed with the developer transported to the developing area in contact with the photoreceptor drum 1.

[0036] In this embodiment, the voltage applied to the charger 2 is a DC voltage of -530V, with a sine wave having a peak-to-peak voltage of 1200V and a frequency of 2000Hz. Note that the charging potential Vd of the photoreceptor drum 1 differs between immediately after charging and in the development region due to dark attenuation. The same applies to the exposure potential Vl, and the charging potential Vd and exposure potential Vl refer to the values ​​in the development region. In this embodiment, the charging potential Vd is set to -700V and the exposure potential Vl to -300V.

[0037] A development bias is applied to the development sleeve 41, which is a superposition of a DC voltage (development potential Vdc) and an AC voltage. In this embodiment, a DC voltage of -550V and a square wave with a peak-to-peak voltage Vpp of 1400V, a frequency of 12000Hz, and a duty cycle of 50% are used.

[0038] The electric field formed by the charging potential Vd, development potential Vdc, and exposure potential Vl of the developing sleeve 41 and the photoreceptor drum 1 causes the negatively polarized toner present in the developer to be developed to the exposure potential on the photoreceptor drum 1. After development, the potential of the exposed area changes due to the charge of the toner. The potential of the exposed area after development is defined as the toner potential. After passing through the developing region, the potential is generally approximately equivalent to the development potential Vdc.

[0039] On the other hand, positively polarized carriers are subjected to a Coulomb force that moves them from the developing sleeve 41 towards the photoreceptor drum 1 in the region of the charging potential Vd on the photoreceptor drum 1. Meanwhile, magnetic carriers are attracted to the developing sleeve 41 by the magnetic force formed by the developing electrode S2 with a magnetic flux density of 120 mT. The vast majority of these carriers do not migrate to the photoreceptor drum 1. However, a very small number of carriers with small particle sizes or low magnetization may migrate from the developing sleeve 41 to the photoreceptor drum 1 due to the Coulomb force because their magnetic force is weak. If carriers migrate from the developing sleeve 41 to the photoreceptor drum 1, the image defects described above may occur.

[0040] (Carrier retrieval device) Next, the carrier recovery devices 5a, 5b, 5c, and 5d in this embodiment will be described with reference to Figure 1 and using Figure 3. Figure 3 is a cross-sectional view showing the configuration of the carrier recovery device 5. Since the configurations of the carrier recovery devices 5a, 5b, 5c, and 5d are the same, the carrier recovery device 5a will be described as a representative example below.

[0041] As shown in Figure 1, the carrier recovery device 5a recovers carriers (carriers on the image carrier) attached to the photoreceptor drum 1a downstream of the developing unit D and upstream of the primary transfer unit T1a with respect to the rotation direction of the photoreceptor drum 1a (direction of arrow R1). The developing unit D is the area where the developing device 4a and the photoreceptor drum 1a are in close proximity. The primary transfer unit T1a is the area where the primary transfer roller 6a and the photoreceptor drum 1a are in close proximity.

[0042] The carrier recovery device 5a includes a recovery sleeve 51 (recovery roller) positioned opposite the photoreceptor drum 1a and rotating. Furthermore, as shown in Figure 3, the carrier recovery device 5a includes a magnet roller 52 (magnet) positioned non-rotating inside the recovery sleeve 51, which acts as a magnetic field generating unit to attract the carrier to the surface of the recovery sleeve 51 by magnetic force. Also, as shown in Figure 3, the carrier recovery device 5a includes a recovery chamber 53 and a transport screw 54 for transporting the carriers collected in the recovery chamber 53. These are all located within the recovery container 56.

[0043] In this embodiment, the recovery sleeve 51 has a diameter of 18 mm and is positioned with a gap of approximately 250 μm in the closest proximity area to the photoreceptor drum 1a. The recovered carrier is transported by rotating it in the direction of arrow R4 in Figure 3.

[0044] The magnetic roller 52, which serves as both a carrier transport and carrier detachment means on the recovery sleeve 51, has multiple (three in this embodiment) magnetic poles (magnet pieces). The S11 pole (carrier recovery pole) is positioned opposite the photoreceptor drum 1a via the recovery sleeve 51. The S11 pole is a magnetic pole for attracting carriers attached to the outer surface of the photoreceptor drum 1a. The S11 pole is positioned near the closest proximity point between the photoreceptor drum 1a and the recovery sleeve 51. The magnetic field created by the S11 pole and the electric field created by the carrier recovery bias (described later) attract the carriers attached to the photoreceptor drum 1a to the surface of the recovery sleeve 51.

[0045] The N11 pole (carrier transport pole) is positioned adjacent to the S11 pole on the downstream side of the S11 pole with respect to the rotation direction of the recovery sleeve 51 (arrow R4 direction), and is an opposite pole to the S11 pole. The magnetic force created by the S11 pole and the N11 pole causes the carriers attracted to the surface of the recovery sleeve 51 to be transported as the recovery sleeve 51 rotates.

[0046] The N12 pole (carrier detachment pole) is positioned adjacent to the N11 pole on the downstream side of the N11 pole with respect to the rotational direction of the recovery sleeve 51, and is the same pole as the N11 pole. The N12 pole is a magnetic pole for detaching the conveyed carrier and recovering it into the recovery chamber 53. As the recovery sleeve 51 rotates, the carrier is detached from the surface of the recovery sleeve 51 into the recovery chamber 53 by the repulsive magnetic field created by the N11 pole and the N12 pole.

[0047] The following is a detailed explanation. The recovery sleeve 51 is positioned opposite the photoreceptor drum 1a, downstream of the developing section D (developing position) and upstream of the primary transfer section T1a (transfer position), with respect to the rotational direction of the photoreceptor drum 1a. The recovery sleeve 51 rotates in the direction of arrow R4 in Figure 3, and the magnetic field created by the S11 poles positioned around the photoreceptor drum 1a attracts carriers attached to the photoreceptor drum 1a to the surface of the recovery sleeve 51. The attracted carriers are transported as the recovery sleeve 51 rotates and are peeled off into the recovery chamber 53 by the repulsive magnetic field created by the N11 and N12 poles.

[0048] The transport screw 54, which serves as a carrier transport member, has a rotating shaft made of non-magnetic metal and resin blades formed spirally around the rotating shaft. By rotating, the transport screw 54 transports the carrier that has fallen from the recovery sleeve 51 in the direction of the transport screw 54's rotation axis. In this embodiment, the rotation axis direction of the transport screw 54 and the rotation axis direction of the recovery sleeve 51 are approximately parallel.

[0049] In this embodiment, a magnetic roller 52 is used as the carrier transport means and carrier detachment means, but it is not limited to this. For example, as the carrier transport means, a "field curtain method" may be used in which charged particles are placed on a linear group of electrodes, and a time-varying voltage is sequentially applied to this group of electrodes to form an electric field that becomes a traveling wave, and the charged particles are transported by the electrostatic force at this time. Also, as the carrier detachment means, a scraper or cleaning blade that physically contacts and detaches the recovery sleeve 51 may be used.

[0050] Furthermore, a voltage consisting of a superimposed DC voltage and AC voltage is applied to the recovery sleeve 51 from the power supply 55 (recovery high-voltage substrate), which acts as a voltage application unit. The power supply 55 is controlled by the control unit 110 (see Figure 1) of the image forming apparatus 100, and can apply a superimposed DC voltage and AC voltage in accordance with the operation of the carrier recovery device 5a when performing an image forming operation.

[0051] The carrier recovery device 5a recovers carriers attached to the photoreceptor drum 1a by the force of the magnetic field formed by the magnet roller 52 and the force of the electric field formed between the recovery sleeve 51 and the photoreceptor drum 1 by the voltage applied to the recovery sleeve 51.

[0052] (Carrier recovery bias) Next, the carrier recovery bias, a feature of this embodiment, will be explained using Figure 4. In Figure 4, the vertical axis represents potential, and the horizontal axis represents time, with the vertical axis being displayed so that the upper part is negative.

[0053] A carrier retrieval bias is applied to the retrieval sleeve 51 by the power supply 55 when performing the image forming operation.

[0054] In conventional examples, the waveform of the carrier recovery bias was a rectangular bias consisting of repeated rectangular pulses of relatively low frequency, where an AC voltage was superimposed on a DC voltage.

[0055] On the other hand, the waveform of the carrier recovery bias in this embodiment is characterized by the periodic repetition of a pulse portion in which an AC voltage is superimposed on a DC voltage, and a blank portion consisting only of a DC voltage, as shown in Figure 4. In other words, the waveform of the carrier recovery bias in this embodiment is a blank pulse waveform in which a portion consisting only of a DC voltage is provided, by intermittently thinning out the AC voltage.

[0056] By providing a blank section, the performance of recovering carriers from the photoreceptor drum 1 to the recovery sleeve 51 can be improved. On the other hand, by increasing the frequency of the pulse section, the toner image developed on the photoreceptor drum 1 can be rearranged in the carrier recovery area of ​​the recovery sleeve 51 without disturbing it, thereby suppressing distortion of the image on the photoreceptor drum 1.

[0057] The carrier recovery bias shown in Figure 4 is a double blank pulse (WBP) waveform, consisting of a pulse portion (time: Tp) made up of a two-period rectangular AC voltage followed by a blank portion (time: Tb). The length of the blank portion is equivalent to two periods (wavelengths) of the rectangular wave in the pulse portion.

[0058] Here, Vc, Vcgo, Vcre, Vtgo, Vtre, Tcgo, and Tcre in the carrier recovery bias waveform shown in Figure 4 are defined as follows. Vc: This is the DC voltage of the carrier recovery bias, and the voltage of the blank section. Vcgo: In the pulsed portion of the AC voltage, the voltage on the side opposite to the normal charging polarity (positive polarity) of the carriers relative to a predetermined charging potential Vd. Vcre: Of the AC voltages in the pulsed portion, this is the voltage on the side with the same polarity as the normal charging polarity (positive polarity) of the carrier relative to a predetermined charging potential Vd. Vtgo: Among the AC voltages in the pulse section, this is the voltage on the opposite side of the toner's normal charging polarity (negative polarity) relative to a predetermined toner potential Vt. Vtre: Of the AC voltages in the pulse section, this is the voltage on the side with the same polarity as the normal charging polarity (negative polarity) of the toner, relative to a predetermined toner potential Vt. Tcgo (=Ttre): The duration for which Vcgo (Vtre) is applied during one period of the pulse. Tcre (=Ttgo): The duration for which Vcre (Vtgo) is applied during one period of the pulse.

[0059] Vc is the DC voltage of the carrier recovery bias, and in this embodiment, Vc = -900V, which has a potential difference of 200V with respect to the charging potential Vd = -700V.

[0060] Vcgo is the carrier recovery drive component with respect to the electropotential Vd. The carrier recovery drive component is the voltage at which normally charged carriers (positive polarity) exert a force from the photoreceptor drum 1 towards the recovery sleeve 51.

[0061] Vcre is the carrier return component with respect to the charging potential Vd. The carrier return component is the voltage at which normal charged carriers (positive polarity) exert a force from the recovery sleeve 51 in one direction toward the photoreceptor drum.

[0062] Vtgo is the toner recovery drive-side component, with the toner potential Vt as the reference. The toner recovery drive-side component is the voltage at which a force acts from the photoreceptor drum 1 towards the recovery sleeve 51 when the toner is normally charged (negative polarity).

[0063] Vtre is the toner return component, relative to the toner potential Vt. The toner return component is the voltage at which the normally charged toner (negative polarity) exerts a force from the recovery sleeve 51 towards the photoconductor drum in one direction.

[0064] In this embodiment, the normal charging polarity of the toner is set to negative polarity, and the normal charging polarity of the carrier is set to positive polarity. However, this is not the only possible configuration. A modified example in which the normal charging polarity of the toner is set to positive polarity and the normal charging polarity of the carrier is set to negative polarity is also possible.

[0065] Furthermore, when the normal charging polarity of the toner (carrier) is negative (positive), saying that the polarity is opposite to the normal charging polarity of the toner (carrier) does not mean that it is positive (negative). Rather, it means that the voltage is on the positive (negative) side with respect to the reference (Vt or Vd), and when a negative polarity developing bias is applied as in this embodiment, even if the polarity is opposite to the normal charging polarity of the toner (carrier), it is still negative. However, it may also be positive.

[0066] When transitioning from the pulsed section to the blank section, the blank section is positioned to occur immediately after the termination of the application of the carrier recovery drive component of the AC voltage, that is, the component with the opposite polarity (negative polarity) to the normal charging polarity of the carrier (positive polarity in this embodiment).

[0067] This is because the blank area is easily affected by the preceding state, and ensuring that the blank area follows immediately after the termination of voltage application for the carrier recovery drive component improves carrier recovery performance. Furthermore, ensuring that the blank area follows immediately after the termination of voltage application for the carrier recovery drive component has the advantage that developed toner is less likely to adhere to the recovery sleeve 51.

[0068] Here, we will explain the duty cycle waveform with a changed duty cycle for the carrier recovery bias using Figure 5. First, as mentioned earlier in Figure 4, let Vcgo be the applied voltage for the carrier recovery drive side component of the AC voltage pulse, and Vcre be the applied voltage for the carrier return side component. In this case, the ratio of the carrier recovery drive side component to the total carrier recovery bias (Vcgo + Vcre), Vcgo / (Vcgo + Vcre), is called the duty cycle (in %).

[0069] At this time, the application time Tcgo for the carrier recovery drive component and the application time Tcre for the carrier return component are changed to be roughly inversely proportional to the applied voltage (Tcgo:Tcre ≈ Vcre:Vcgo). Therefore, the integral values ​​of the carrier recovery drive component and the carrier return component within one cycle of the AC voltage are approximately constant. That is, the area of ​​the hatched portion above Vc in Figure 5, Vcgo × Tcgo, and the area of ​​the hatched portion below Vc in Figure 5, Vcre × Tcre, are approximately equal to Vcgo × Tcgo ≈ Vcre × Tcre. By setting it in this way, the effective voltage level can be made approximately constant in the pulse portion where AC and DC voltages are superimposed and in the blank portion consisting only of the DC component.

[0070] As mentioned above, the duty cycle was defined as Vcgo / (Vcgo+Vcre) based on the applied voltage, but it is not limited to this. The duty cycle may also be defined as Tcre / (Tcgo+Tcre) based on the applied time.

[0071] In each experiment in this embodiment, the carrier recovery bias waveform was measured using a Tektronix oscilloscope, model number DPO2014B. The carrier recovery bias waveform is susceptible to the effects of distortion caused by changes in capacitance, etc., between the recovery sleeve 51 and the photoreceptor drum 1 during output. Therefore, in each experiment in this embodiment, when calculating the duty cycle by measuring the carrier recovery bias waveform, the calculation is based on the application time rather than the applied voltage, as this is particularly susceptible to the effects of distortion.

[0072] Figure 6 shows an example of a carrier recovery bias waveform affected by lead distortion. The lead distortion in the carrier recovery bias waveform is a delay in the potential response caused by transient phenomena that occur when the potential is changed. Therefore, the potential state is easily affected by capacitance and other factors, but the timing of the potential change is less affected. Accordingly, in each experiment in this embodiment, the duty cycle is calculated based on the start timing of the potential change, which is circled in Figure 6.

[0073] Here, we will specifically explain the effect of the blank pulse waveform applied to the recovery sleeve 51.

[0074] The normal charge carriers (positive in this embodiment) attached to the surface of the photoreceptor drum 1, which forms the charge potential Vd, are mainly attached by Coulomb force.

[0075] In the carrier recovery region, the magnetic force from the carrier recovery electrode and the Coulomb force from the carrier recovery drive component Vcgo in the pulse portion of the carrier recovery bias apply a force that pulls the normally charged carriers attached to the surface of the photoreceptor drum 1 away from the photoreceptor drum 1. As a result, the normally charged carriers detach from the photoreceptor drum 1. On the other hand, the detached normally charged carriers may reattach towards the photoreceptor drum 1 due to the Coulomb force from the carrier return component Vcre.

[0076] In the case of a conventional rectangular bias (i.e., a rectangular bias in which a relatively low-frequency rectangular pulse section is repeated, with an AC voltage superimposed on a DC voltage), the components reach the recovery sleeve 51 while reciprocating due to the Coulomb force between Vcgo and Vcre.

[0077] Here, if the frequency of the pulse section is high, the carrier has difficulty following the carrier recovery bias and has difficulty reaching the recovery sleeve 51 within the carrier recovery region, thus reducing the carrier recovery performance.

[0078] In this case, if a blank section is provided after the voltage application of the carrier recovery drive component of the pulse section has ended, a force acts toward the recovery sleeve 51 for a certain period of time with the normally charged carriers detached from the photoreceptor drum 1. As a result, the distance the carriers travel from the photoreceptor drum 1 to the recovery sleeve 51 becomes longer, improving carrier recovery performance.

[0079] By improving carrier recovery performance, the pulse frequency can be increased compared to conventional methods, thereby reducing variations in toner adhesion to non-image areas during toner redistribution in the carrier recovery region. Furthermore, the low electric field strength of the toner recovery drive component in the blank area makes it more difficult for toner from the image area to be recovered by the recovery sleeve 51 during toner redistribution, thus suppressing a decrease in image quality.

[0080] Furthermore, by changing the duty cycle of the carrier recovery bias, carrier recovery performance can be further improved, and the degradation of image quality during toner repositioning can be reduced. Specifically, by setting the duty cycle to 50% or more and 90% or less, the carrier recovery drive side component of the pulse section becomes larger, making it easier for the carrier to detach from the photoreceptor drum 1, and thus improving carrier recovery performance. On the other hand, by setting the duty cycle to 50% or more and 90% or less, the toner recovery drive side component is also reduced, making it more difficult for the toner in the image section to be recovered into the recovery sleeve 51, similar to the blank section, and thus suppressing the degradation of image quality.

[0081] Furthermore, the duty cycle can be made variable according to the temperature and humidity in which the image forming apparatus 100 is installed. When the image forming apparatus 100 is installed in an environment with low moisture content, the charge on the carriers generally increases. When the charge on the carriers increases, the electrostatic adhesion force between the photoreceptor drum 1 and the carriers increases, making it difficult for the carriers to detach from the photoreceptor drum 1. Therefore, by increasing the duty cycle in environments with low moisture content, it is possible to maintain carrier recovery performance.

[0082] Furthermore, by increasing the frequency of the pulse portion of the carrier recovery bias or the duty cycle relative to the frequency of the pulse portion of the development bias, it is possible to improve the image quality after passing through the carrier recovery region compared to the image quality before passing through the carrier recovery region.

[0083] As mentioned above, determining the image quality on the photoreceptor drum 1 greatly contributes to the downstream processes of the image formation process. Therefore, the frequency of the pulse portion of the carrier recovery bias is made higher than that of the pulse portion of the development bias, or the duty cycle is increased to reduce the toner recovery drive component. This reduces toner adhesion variations to non-image areas due to toner redistribution in the carrier recovery region compared to after passing through the development region, thereby improving image quality.

[0084] [Examples] Next, we will describe the experiments conducted on carrier recovery performance and image quality when the carrier recovery bias waveform is changed. In these experiments, we mainly changed the length of the blank portion, the duty cycle, and the frequency of the pulse portion of the carrier recovery bias.

[0085] The carrier recovery performance was evaluated by outputting a monochromatic image using only Pb (magenta) from the image forming section Pa, Pb, Pc, and Pd of the image forming apparatus 100, with a reflectance density of around 0.6 and a line image resolution of 212 lines / inch, and evaluating the number of mottled images with varying shades. If carriers are present on the photoreceptor drum 1 of the downstream image forming section (Pc, Pd) of the output image, the retransfer of the output image is inhibited at the primary transfer rollers 6c and 6d, resulting in mottled images.

[0086] The reflectance was measured using a spectrophotometer X-Rite504 / 508 (manufactured by X-Rite Corporation). Previous sensory evaluations have already shown that the speckled images with varying shades are particularly noticeable to the human eye at reflectances of around 0.5 to 0.8.

[0087] The number of carriers on the photoreceptor drum 1 after passing through the development area but before passing through the recovery area was set to 1 carrier / cm² (1247 carriers per A3 size sheet of paper), and 10 A3 size magenta monochrome images were printed. Images with an average of 1 or fewer dark spotted image per A3 size sheet of paper were marked with ◎, those with an average of 1 to 2 or fewer were marked with ○, those with an average of 2 to 10 or fewer were marked with △, and those with an average of 10 or more were marked with ×.

[0088] Furthermore, image roughness was evaluated as an indicator of image quality. Roughness was evaluated using line images with a reflectance of around 0.4 and 212 lines / inch. Previous sensory evaluations have already shown that roughness is particularly noticeable to the human eye at reflectances of around 0.3 to 0.5. Images with no roughness and particularly smooth were marked with ◎, images with no roughness and smooth were marked with ○, images with little roughness and smooth were marked with △, and images with noticeable roughness were marked with ×.

[0089] In both the carrier retrieval performance evaluation and the roughness evaluation, a result of at least △ or higher is required.

[0090] Table 1 shows the results of the evaluation of carrier recovery performance and roughness for each carrier recovery bias configuration in this experiment.

[0091] In Table 1, "WBP" ​​in the waveform indicates that the development bias waveform is a double-blank pulse waveform, consisting of a pulse portion with a blank portion after a two-period square wave AC voltage. Also in Table 1, "Square" in the waveform indicates a development bias waveform without AC voltage decimation (no blank portion). Furthermore, in Table 1, Vpp in the waveform indicates the peak-to-peak voltage.

[0092] [Table 1]

[0093] Comparative Example 1 uses a rectangular bias, and the pulse frequency is high. In Comparative Example 1, the noise evaluation is good, but the carrier recovery performance evaluation is low, and it does not fully function as a carrier recovery device.

[0094] Comparative Example 2 uses a rectangular bias, and the pulse frequency is low. In Comparative Example 2, the carrier recovery performance evaluation is good, but the noise evaluation is poor, and it is not possible to achieve both carrier recovery performance and image quality.

[0095] As seen in Comparative Examples 1 and 2, simply changing the frequency makes it difficult to achieve both carrier recovery performance evaluation and noise level evaluation simultaneously.

[0096] Example 1-1 is a waveform with a blank portion equivalent to one wavelength of the pulse. When a blank portion is provided, good results are obtained for both carrier recovery performance evaluation and noise evaluation, and it is possible to achieve both carrier recovery performance and image quality.

[0097] In Example 1-2, the length of the blank portion was changed to two wavelengths of the pulse portion compared to Example 1-1. Similar to Example 1-1, Example 1-2 obtained good results in both carrier recovery performance evaluation and roughness evaluation.

[0098] In Examples 1-3, the length of the blank section is changed to four wavelengths of the pulse section compared to Example 1-1. In Examples 1-3, both the carrier recovery performance evaluation and the roughness evaluation were slightly lower compared to Example 1-1. This is thought to be because making the blank section too long reduces the number of pulse sections in the recovery area, decreasing the amount of carriers released from the photoreceptor drum 1, and thus slightly lowering the carrier recovery performance. Similarly, the number of toner rearrangements decreased, and the roughness evaluation also slightly decreased. Therefore, it is preferable that the length of the blank section be between 1 and 4 times the wavelength of the AC voltage in the pulse section (1 to 4 wavelengths), and more preferably between 1 and 2 times the wavelength of the AC voltage in the pulse section (1 to 2 wavelengths).

[0099] Example 2 has a larger duty cycle compared to Example 1-1. Example 2 obtained even better results than Example 1-1 in both carrier recovery performance evaluation and roughness evaluation. This is thought to be because increasing the duty cycle increases the carrier recovery drive component in the pulse section, improving carrier recovery performance evaluation, while reducing the toner recovery drive component also improved roughness evaluation.

[0100] In Example 3, the pulse frequency was made even higher than in Example 1-1, and was set to be higher than the pulse frequency of the development bias. Regarding the roughness evaluation, Example 3 was even better than in Example 1-1. This is thought to be because, by making the pulse frequency higher than the development bias, the variation in toner adhesion to non-image areas due to toner redistribution in the carrier recovery area was reduced compared to after passing through the development area. On the other hand, the increased frequency of the pulse reduced the carrier travel distance in a single pulse, resulting in a slight decrease in the carrier recovery performance evaluation.

[0101] As described above, in this embodiment, when performing the image forming operation, a carrier recovery bias is applied to the recovery sleeve 51 of the carrier recovery device 5, which periodically repeats a pulse portion in which a DC voltage and an AC voltage are superimposed, and a blank portion in which only a DC voltage is applied. This makes it possible to improve the performance of recovering carriers from the photoreceptor drum to the recovery roller and to suppress distortion of the image on the photoreceptor drum.

[0102] In this embodiment described above, a double blank pulse was used, but a single blank pulse waveform may also be used, in which a blank section is provided after a pulse section consisting of a single-period rectangular AC voltage.

[0103] Furthermore, the image forming apparatus 100 is not limited to a full-color printer; it may also be a monochrome or monocolor printer. Additionally, the image forming apparatus 100 may be a printer, various printing machines, copiers, fax machines, or multifunction devices having multiple of these functions.

[0104] Furthermore, the photoreceptor drum 1 may be a drum-shaped organic photoreceptor, or an inorganic photoreceptor such as an amorphous silicon photoreceptor may 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. The developing apparatus is also not limited to the configuration of the developing apparatus 4 described in Figure 2, as long as it employs a two-component developing method with a carrier. For example, the present invention can be applied to developing apparatuses in which the developing chamber and agitation chamber are arranged vertically, as has been used conventionally, or to other forms of developing apparatuses. [Explanation of symbols]

[0105] 1. Photoconductor drum 4. Developing device 5. Carrier recovery device 51 Recovery Sleeves 52 Magnetic Roller 55 Power supply 60 Intermediate transfer belt 100 Image forming apparatus 110 Control Unit

Claims

1. An image forming apparatus capable of performing image forming operations, A rotatable image carrier on which an electrostatic latent image is formed, A developing apparatus comprising: a developing container for containing a developer containing toner and a carrier; and a developer carrier for carrying the developer in order to develop the electrostatic latent image formed on the image carrier into a toner image; A transfer member onto which the toner image supported on the image carrier is transferred, A carrier recovery device for recovering carriers on the image carrier comprises: a rotatable sleeve positioned opposite the image carrier downstream of the development position where the electrostatic latent image formed on the image carrier is developed, and upstream of the transfer position where the toner image carried on the image carrier is transferred to the transfer member, with respect to the rotation direction of the image carrier; and a non-rotating magnet positioned inside the sleeve. A voltage application unit that applies voltage to the sleeve, A control unit controls the voltage application unit so that when performing an image forming operation, a carrier recovery bias is applied to the sleeve, which consists of a pulse section in which a DC voltage and an AC voltage are superimposed, and a blank section consisting only of the DC voltage, which are periodically repeated. An image forming apparatus characterized by comprising the following:

2. In the pulsed section, if Vcgo is the voltage on the side opposite to the normal charging polarity of the carrier relative to a predetermined charging potential Vd, The waveform of the carrier recovery bias is the waveform in which the blank portion exists after Vcgo is applied. The image forming apparatus according to feature 1.

3. The length of the blank portion is between 1 and 4 times the wavelength of the AC voltage in the pulse portion. The image forming apparatus according to feature 1.

4. The length of the blank portion is between one and two times the wavelength of the AC voltage in the pulse portion. The image forming apparatus according to feature 1.

5. The waveform of the carrier recovery bias is a waveform in which the blank portion is present after the pulse portion, which consists of AC voltages of double blank pulses with multiple periods. The image forming apparatus according to feature 1.

6. In the pulsed section, among the AC voltages, Vcgo is defined as the voltage on the side with the opposite polarity to the normal charging polarity of the carrier relative to a predetermined charging potential Vd. In the pulsed section, if Vcre is the voltage on the side of the normal charging polarity of the carrier with respect to a predetermined charging potential Vd, If the duty cycle of the carrier recovery bias is set to Vcgo / (Vcgo+Vcre), The duty cycle is greater than 50% and less than or equal to 90%. The image forming apparatus according to feature 1.

7. When the image forming operation is performed, a developing bias is applied to the developer carrier, which is a superposition of a DC voltage and an AC voltage. The frequency of the AC voltage in the pulse section is greater than the frequency of the AC voltage of the development bias. The image forming apparatus according to feature 1.

Images