JP2004163929A - Liquid development apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To easily collect toner from a thin layer of liquid developer on a development roll, in a liquid development apparatus that develops using liquid developer comprising liquid carrier and toner. <P>SOLUTION: A supply roll DC bias voltage is applied to a supply roll 21 which holds liquid developer. The development roll 22 forms a thin layer of the liquid developer by receiving the liquid developer from the supply roll and forms a toner image by developing an electrostatic latent image formed on a photoreceptor drum 3. A development bias in which an AC bias voltage overlaps a DC bias voltage is applied to the development roll. A first DC bias voltage is set higher than a development roll DC bias voltage whereas a second DC bias voltage is set lower than the development DC bias voltage. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to a developing device used in an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction machine using an electrophotographic method, and in particular, develops an electrostatic latent image using a liquid developer. The present invention relates to a liquid developing device that obtains an image by using a liquid developing device.

  Generally, in a developing device (hereinafter, referred to as a liquid developing device) that performs development using a liquid developer containing a toner in a liquid carrier (a so-called liquid toner), if fine toner is used, a high level of offset printing can be obtained. Not only can image quality be realized, but even if the amount of toner is small, sufficient image density can be obtained, which is economical.

  By the way, when developing an electrostatic latent image formed on an image carrier such as a photosensitive drum, if the viscosity of the liquid developer is low, the liquid developer is caused to flow or circulate on the developing roll, The electrostatic latent image formed on the photoconductor drum is developed by the liquid developer above, and the liquid developer is directly sprayed on the photoconductor drum without using a developing roll. Is being done.

  If a solvent having high volatility at room temperature is used as the liquid carrier, for example, the surplus carrier volatilizes into the air during heating and melting, and the residual carrier also volatilizes. However, volatilized solvents are generally toxic and ignitable, so not only is a carrier recovery device required to liquefy and recover the vaporized carriers, but also air dedicated to air containing solvents that cannot be recovered is used exclusively. It is necessary to exhaust the air outdoors through the ventilation duct.

  For this reason, when a high-concentration toner is dispersed in silicon oil or the like, and a high-viscosity, high-concentration developer is used, a problem of harm to the human body does not occur. There is an advantage that a liquid developer need not be used. However, when using a non-volatile, high-viscosity, high-concentration liquid developer, the high-viscosity toner should be prevented from adhering to the non-exposed portions of the electrostatic latent image formed on the photosensitive drum. Before applying the liquid developer to the photosensitive drum, a pre-wet process for applying a pre-wet liquid to the photosensitive drum is required.

  However, when the development is performed as described above, non-volatile oil is inevitably present in the toner image, and when the toner image transferred to the recording medium is fixed, the non-volatile oil may cause the non-volatile oil. In addition to fixing failure, the toner image is insufficiently aggregated, so that fine streak unevenness may occur in the image due to destruction and separation of the toner layer during development or transfer.

  In order to prevent such problems, when using a non-volatile, high-viscosity, high-concentration liquid toner as the liquid developer, the oil removal mechanism is brought into contact with the photosensitive drum at a position on the photosensitive drum after development. The oil removing mechanism removes excess pre-wet liquid and carrier liquid on the photoreceptor drum after development, and re-aggregates the toner to apply a voltage exceeding the developing bias to the oil removing roller. (See Patent Document 1).

  On the other hand, when a high-concentration and high-viscosity liquid developer is used, the stress caused by the viscosity of the liquid carrier (toner liquid) is extremely large compared to the electrostatic force acting on the toner particles. In the developing section, the turbulence of the liquid developer layer generated at the time of destructive separation increases, and it is difficult to form the liquid developer layer uniformly. Therefore, an image cannot be formed uniformly, and image fogging and the like are likely to occur.

  In order to solve such a problem, a plurality of applicator rolls for supplying the liquid developer to the developing roll are provided, and the liquid developer supplied to the nip between the applicator roll and the other applicator roll is provided by one of the applicator rolls. 2. Description of the Related Art It has been practiced to spread a thin and wide layer to form a uniform and thin layer and apply it to the surface of a developing roll. Further, a conductive blade is arranged in contact with the liquid developer layer on the developing roll, a predetermined amount of the liquid developer is passed between the conductive blade and the developing roll, and a voltage is applied to the conductive blade. In some cases, development is performed by forming a state in which toner particles contained in a liquid developer are sparse on a surface on a developing roll (see Patent Document 2).

  Further, as a developing device, a toner transport roller is disposed adjacent to the photosensitive drum, and a first size for attracting toner from the toner supply roller to the toner transport roller and a toner for attracting toner from the toner transport roller to the toner supply roller are provided. There is one in which a voltage difference between a toner supply roll and a toner conveyance roller is generated by a voltage difference having a second magnitude different from the first magnitude (see Patent Document 3).

JP-A-2000-250319 (paragraphs (0015) to (0031), FIG. 1) JP 2001-324876 A (paragraphs (0014) to (0022), FIG. 1) JP-A-7-72733 (paragraphs (0022) and (0023), FIG. 2)

  However, the developing device described in Patent Literature 1 requires not only a mechanism for performing a pre-wet process for applying a pre-wet liquid to the photosensitive drum (pre-wet processing mechanism), but also an oil removing mechanism. Is required, and as a result, there is a problem that the entire apparatus becomes complicated.

  On the other hand, in the developing device described in Patent Document 2, although a uniform thin layer (liquid developer layer) is formed on the developing roll using an applicator roll and a conductive blade, a uniform and stable liquid developing is performed. In order to form the agent layer, a plurality of applicator rolls are required, and there is a problem that the developing device itself becomes complicated and large. Further, when the conductive blade is in contact with the liquid developer layer on the developing roll, if dust or the like adheres, a vertical streak or the like may be formed and a good image may not be formed.

  In addition, in order to perform stable development over a long period of time in a liquid developing device, it is important not only to make the liquid developer layer a uniform thin layer, but also to remove the residual liquid developer remaining on the developing roll. It is also important to collect the toner. However, both the developing devices described in Patent Documents 1 and 2 collect the residual liquid developer itself, and require a complicated mechanism.

  Further, in Patent Document 3, when only a DC voltage is applied, only toner is supplied to a toner roller (toner transport roller) during image formation, and no image is collected at all, and image formation is continuously performed. Then, the amount of toner that is not suitable for forming an image with high charge on the toner roller increases, and a phenomenon such as a decrease in image density occurs.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a liquid developing apparatus capable of forming a uniform liquid developer layer on a developing roll with a simple configuration in view of the problems of the related art.

  Another object of the present invention is to provide a liquid developing apparatus which facilitates recovery of toner in a liquid developer remaining on a developing roll and can form a good image.

  Accordingly, in order to solve the problem, the present invention forms a supply roll that holds a liquid developer including at least a liquid carrier and a toner, and forms the liquid developer thin layer by receiving the liquid developer from the supply roll. A developing roller that develops the electrostatic latent image formed on the image carrier with the liquid developer thin layer to form a toner image, a DC bias voltage with respect to the supply roll, DC bias applying means for applying as a supply roll DC bias voltage; and developing bias applying means for applying a developing bias in which an AC bias voltage is superimposed on a developing roll DC bias voltage with respect to the developing roll. The applying means applies a first DC bias voltage as the supply roll DC bias voltage in the first step of performing development. In the second step of reducing the toner concentration of the liquid developer thin layer carried on the developing roll after the development of the electrostatic latent image formed on the image carrier, the supply roll DC bias voltage may be reduced to a second value. 2, the first DC bias voltage is higher than the developing roll DC bias voltage, and the second DC bias voltage is lower than the developing roll DC bias voltage. Features. In this way, by changing the DC bias voltage applied to the supply roll, the toner concentration of the liquid developer thin layer can be adjusted to a state suitable for development at the time of development. The toner in a certain liquid developer can be easily collected.

  In the present invention, the developing bias may be used to adjust the toner concentration in the liquid developer thin layer on the developing roll facing the image carrier to the liquid developer thin layer on the developing roll facing the supply roll. By making the toner concentration lower than the toner concentration in the layer, it is possible to effectively prevent the toner from adhering to the non-electrostatic latent image portion on the image carrier.

The developing roll has a volume resistivity of 1 × 10 ³ Ω · cm or more and 1 × 10 ⁷ Ω · cm or less, and the supply roll has a volume resistivity of 1 × 10 ¹ Ω · cm or more and 1 × 10 ⁷ Ω · cm or more. It is desirable to set it to 10 ⁵ Ω · cm or less. Further, it is preferable that the AC bias voltage has a duty ratio of 30% or more and 50% or less, and a bias difference between the first DC bias voltage and the developing roll DC bias voltage ranges from 100 V to 350 V. In the second step of reducing the toner concentration of the liquid developer thin layer carried on the developing roll after developing the electrostatic latent image formed on the image carrier, the developing roll facing the image carrier side is used in the second step. It is desirable that the toner concentration in the above liquid developer thin layer is 7% or less. When the volume resistivity and the bias difference are defined in this manner, a thin developer layer can be formed satisfactorily, the image quality can be improved, and when the duty ratio is defined, the liquid on the developing roll can be improved. The replacement of the developer can be easily performed.

  As described above, in the present invention, in the first step of forming the liquid developer thin layer on the developing roll, the first DC bias voltage is applied as the supply roll DC bias voltage to form the liquid developer thin layer on the developing roll. In the second step of moving the liquid developer thin layer to the supply roll side, a second DC bias voltage is applied as a supply roll DC bias voltage, and the first DC bias voltage is changed to a development roll DC bias voltage. And the second DC bias voltage is made lower than the DC bias voltage of the developing roll, so that not only can the toner concentration of the liquid developer thin layer be suitable for development during development, but also There is an effect that the toner in the residual liquid developer on the roll can be easily collected, and as a result, image defects such as fog do not occur.

  Further, in the present invention, the toner density in the liquid developer thin layer on the developing roll facing the image carrier is adjusted by the developing bias from the toner density in the liquid developer thin layer on the developing roll facing the supply roll. Since it is lowered, there is an effect that the amount of toner attached to the non-electrostatic latent image portion on the image carrier is extremely small.

  Further, in the present invention, as described above, the volume specific resistance value of each of the developing roll and the supply roll is defined, and the bias difference is defined, so that the toner concentration in the liquid developer thin layer carried on the developing roll is reduced. It can be set to a value suitable for development, and good development can be performed. Further, when the duty ratio of the AC bias voltage is defined as described above, the toner in the liquid developer can be easily collected, and the supply and the collection of the toner are repeated according to the AC cycle. There is an effect that the image density does not decrease.

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

  FIG. 1 is a view showing an image forming apparatus using an example of a liquid developing apparatus according to the present invention. Referring to FIG. 1, a liquid developing apparatus (hereinafter simply referred to as a developing apparatus) 2 is provided in an image forming apparatus main body 1. , Latent image carrier (photosensitive drum) 3, exposure unit 4, intermediate transfer belt 5, liquid developer container (hereinafter simply referred to as developer container) 6, cleaning unit 7, charger (charging roller) 8, transfer device A (secondary transfer roller) 9 and a fixing roller pair 10 are disposed, and a recording medium (recording paper) is stored in a paper feed cassette (not shown). Then, after the photosensitive drum 3 is uniformly charged by the charger 8, the photosensitive drum 3 is exposed by the exposure unit 4 in accordance with the image data, and an electrostatic latent image is formed on the photosensitive drum 3.

  The developer container 6 contains a liquid developer composed of at least a liquid carrier and a toner. The electrostatic latent image on the photosensitive drum 3 is developed by the developing device 2 as described later. It becomes an image. The toner image on the photosensitive drum 3 is sequentially transferred to the intermediate transfer belt 5 by the primary transfer roller 3a, and a color transfer image is formed on the intermediate transfer belt 5. The recording medium supplied from the paper supply cassette is conveyed by conveyance rollers (not shown), and a color transfer image is transferred from the intermediate transfer belt 5 to the recording medium at a nip portion between the intermediate transfer belt 5 and the secondary transfer roller 9. The secondary transfer is performed. After that, the recording medium is sent to the fixing roller pair 10, where the color transfer image is fixed on the recording medium.

  When a positively charged material (positive OPC) is used as the photosensitive material used for the photosensitive drum 3, the generation of ozone and the like is small and the charging is stable. In particular, when the single-layer positive OPC is used, the photosensitive characteristics are not changed and the image quality is stable even if the film thickness changes due to long-term use. Further, an a-Si photoreceptor may be used. The a-Si photoreceptor has excellent cleaning properties, and thus is advantageous in extending the life.

  When positive OPC is used as a photosensitive material, its film thickness is set to about 20 μm to 40 μm. When the film thickness is less than 20 μm, the film thickness is reduced by long-term use, and when it reaches 10 μm, the occurrence of black spots becomes noticeable due to dielectric breakdown. On the other hand, if the film thickness exceeds 40 μm, the sensitivity is reduced, which causes a decrease in image quality.

  As the exposure unit 4, for example, a semiconductor laser or an LED is used. It is effective to use light having a wavelength around 770 nm for positive OPC, and light having a wavelength around 685 nm is effective for an a-Si photosensitive member.

  As the aforementioned liquid carrier, for example, a non-volatile oil such as silicone oil or liquid paraffin is used, and the toner contains a coloring material such as a resin or a pigment having at least a fixing function on toner particles, a charge controlling agent and a charge inducing material. A material having an average particle size in the range of 0.5 μm to 5 μm is used. Then, such a toner was mixed in a range of 5 to 20% with respect to the liquid carrier to obtain a liquid developer. The viscosity of the liquid developer may be any as long as it has fluidity in the range of 0.5 to 10000 centistokes at room temperature. For example, when producing a liquid developer, 10% of KetBlue 105 (manufactured by Dainippon Ink and Chemicals) as a coloring agent and 10% of FCA222P (manufactured by Fujikura Kasei) are added as a colorant to a styrene-acryl resin as a toner. After mixing by a dry mixing device, the mixture was pulverized and classified to an average particle size of 100 μm by a pulverizer. Thereafter, dimethyl silicone oil (KF-96, manufactured by Shin-Etsu Chemical Co., Ltd.) was used as a liquid carrier liquid, and 20 parts by weight of the toner described above and Solspers 31845 (manufactured by Avicia) as a dispersion stabilizer were added to 100 parts of the liquid carrier. % And mixed with a Dyno mill (manufactured by Shinmaru Enterprise Co., Ltd.) to give a viscosity at room temperature of 50 mPa.s at 50 rpm. s liquid developer. The viscosity of the liquid developer was measured with a viscosity measuring device (product number: SR-100) manufactured by Haake.

  In the following description, a case in which normal OPC is used as a photosensitive material will be described. First, the photosensitive drum 3 is charged to 600 V by the charger 8. Thereafter, when the exposure unit 4 using LEDs exposes the photosensitive drum 3 with light having a wavelength of 770 nm, the potential after exposure becomes 70 volts (set).

  Here, referring also to FIG. 2, the developing device 2 has a developing roll 22 and a supply roll 21. The supply roll 21 is supplied from the developer container 6 (FIG. 1) to the liquid developer ( This is a liquid developer carrier that supplies a developer (liquid developer mixed with toner) 23 to the developing roll 22, and the thickness of the liquid developer layer on the supply roll 21 is regulated by a regulating blade 29. A thin layer of liquid developer (liquid developer thin layer) 26 is formed on the developing roll 22 as described later, and the electrostatic latent image formed on the photosensitive drum 3 by the liquid developer thin layer is developed. I do.

The photoconductor drum 3 is in contact with the developing roll 22, and the developing roll 22 is a rotating body, and the surface thereof is formed of, for example, a conductive elastic body having a volume resistivity of 10 ³ to 10 ⁷ Ωcm. For example, the material of the sleeve of the developing roll 22 is not particularly limited, but the surface thereof may be a uniform conductor, and SUS or a conductive resin material coated on the surface with a conductive material having the above-described volume specific resistance value may be used. Used. In the illustrated example, a conductive rubber made of silicon rubber of 3 × 10 ⁴ Ω · cm was used.

  A developing bias 27 is applied to the developing roll 22. The developing bias 27 has a DC bias (Vdc1) 27a and an AC bias (AC) 27b, and the developing bias is transmitted to the conductive sleeve via the shaft (not shown) of the developing roll 22. The DC bias (Vdc1) 27a is, for example, 200 V, and the AC bias 27b has Vpp of 1.5 kV, a frequency of 3.0 kHz, and a duty ratio of 30% (the waveform of the AC bias is a rectangular wave). Is desirable).

The developing bias 27 in which the DC bias 27a and the AC bias 27b are superimposed is applied to the conductive sleeve, and the outermost surface of the developing roll 22 is electrically conductive with a volume resistivity of 10 ³ to 10 ⁷ Ωcm. As described later, favorable development is performed on the electrostatic latent image on the photosensitive drum 3 as described later.

The supply roll 21 is a conductive elastic body having a surface having a specific volume resistance value of 10 ¹ to 10 ⁵ Ω · cm. A DC bias (Vdc2) 28 is applied to the supply roll 21 to hold the liquid developer 23 as a developer carrier. The liquid developer thin layer 26 is formed on the developing roll 22 while regulating the thickness of the liquid developer 23 by the regulating blade 29. The gap between the regulating blade 29 and the supply roll 21 is preferably 0.3 to 1.5 mm.

  Specifically, the liquid developer 23 on the supply roll 21 comes into contact with the development roll 22 and the liquid developer 23 partially moves to the development roll 22 side. Is moved from the developing roll side 22 side to the supply roll 21 side by the potential difference and the AC bias, and the toner concentration in the liquid developer thin layer 26 carried by the developing roll 22 in the developing nip portion facing the photosensitive drum 3 is reduced. Let it. The toner concentration of the liquid developer thin layer 26 carried on the developing roll 22 in the developing nip portion changes in accordance with the above-mentioned potential difference and the AC bias, and thereby the toner concentration in the liquid developer thin layer 26 of the developing nip portion is changed. To adjust.

  When collecting the toner in the liquid developer thin layer 26 on the developing roll 22 at the end of the development, the DC bias (Vdc2) 28 is changed while the AC bias 27b is applied, and the liquid developing on the developing roll 22 is changed. The toner in the agent thin layer 26 is collected on the supply roll 21.

  The developing bias also has a function of assisting the formation of the liquid developer thin layer 26 on the developing roll 22. The amount of the liquid developer thin layer 26 on the developing roll 22 is determined by a DC bias (Vdc1) 27a and a DC bias (Vdc2). ) 28. Assuming that the DC bias (Vdc1) 27a is 200 V and the DC bias (Vdc2) 28 is 450 V, the liquid developer layer 26 of about 10 μm is formed on the developing roll 22 when the developing roll 22 makes two turns.

  As described above, when the liquid developer thin layer 26 is formed, the liquid developer is calibrated to an optimum value and its thickness is adjusted by the potential difference between Vdc2 and Vdc1, but the viscosity of the liquid developer and the toner Also varies depending on the amount of charge of If the adjustment of the liquid developer thin layer 26 is performed according to image data, for example, a uniform image can be obtained. Then, when performing continuous printing, it is desirable to set the potential difference between Vdc2 and Vdc1 higher.

  As described above, the liquid developer thin layer 26 on the developing roll 22 is collected by the supply roll 21, and new liquid developer is conveyed to the developing roll 22 while being regulated by the regulating blade 29. The liquid developer supplied from the supply roll 21 to the developing roll 22 has a uniform charge due to the bias effect, and the liquid carrier concentration on the non-latent image portion of the photosensitive drum 3 Since a high liquid developer is in contact with the liquid developer (that is, a liquid developer having a low toner concentration is in contact with the liquid developer), a fogging phenomenon due to physical adhesion can be suppressed.

  On the other hand, as described above, unless the toner in the residual liquid developing thin layer 26 remaining on the developing roll 22 is sufficiently collected by the supply roll 21, that is, if the function of refreshing the developing roll 22 is not sufficient, image unevenness ( Ghost) is likely to occur. For example, as shown in FIG. 3A, when a solid image 35 composed of a rectangular black solid is formed and then a halftone image 36 following the solid image 35 is formed in the subsequent image formation (halftone image 36 is wider than the solid image 35), if the recovery of the toner in the residual liquid developed thin layer 26 is sufficient, no afterimage occurs, but if the recovery of the residual liquid developed thin layer 26 is insufficient, As shown in FIG. 3B, when a half image is formed at the position of the photosensitive drum 3 where the solid image at the time of the previous image formation is formed, a halftone image 37 is formed around the halftone image 37. An afterimage portion (ghost) 38 having a higher image density occurs.

  Here, with reference to FIG. 4, an operation when the liquid developer thin layer 26 is formed on the developing roll 22 and the liquid developer 23 is collected on the supply roll 21 will be described. In FIG. 4, Vslv-dc represents the DC voltage level applied to the developing roll 22, Vmag-dc represents the DC voltage level applied to the supply roll 21, and Vmag-dc is 450 V during development, and When the toner in the liquid developer 23 is collected, the voltage is set to 100 V, and Vslv-dc is set to 200 V as described above. The alternating current (AC) bias 27b is a rectangular wave having a Vpp of 1.5 kV, a frequency of 3.0 kHz, and a duty (Duty) ratio of 30%.

During development, first, at time _{t 1,} the DC voltage Vmag-dc applied to the supply roll 21 is set to 450V. Thus, the DC voltage Vmag-dc applied to the AC (AC) bias 27b and supply roll 21 to be applied to the development roller 22, at time _{t 1,} the liquid developer thin layer to the supply roll 21 from the developing roll 22 The toner in the liquid developer thin layer 26 on the developing roll 22 is collected by the potential difference _Vb1 at which the toner in the 26 moves. Then, at time t _2, in turn, by the potential difference V _a1 which the toner in the developing roller 22 the liquid developer thin layer 26 to the supply roll 21 moves, the toner to a liquid developer thin layer 26 on the developing roll 22 Moving. That is, the toner amount of the liquid developer thin layer 26 on the developing roll 22 is adjusted to an amount suitable for development by the rectangular wave having a duty ratio of 30% and the potential difference of Vmag-dc applied to the supply roll 21.

Completed development from the above state, when collecting toner from the liquid developer thin layer 26 on the developing roll 22, at time t _3, the DC voltage Vmag-dc applied to the supply roll 21 is set to 100V Is done. Thus, the potential difference _{V b1} by a DC voltage Vmag-dc applied duty ratio of 30% of the square wave to the supply roll 21 is larger 350V than at time _{t 1,} conversely, the potential difference _{V a1} is, 350V smaller . As a result, the toner is collected from the thin liquid developer layer 26 on the developing roll 22 to the supply roll 21. In the continuous printing, the collection of the toner from the liquid developer thin layer 26 from the developing roll 22 is performed, for example, between recording media (that is, between papers). Further, it may be performed after printing is completed.

  In the image forming apparatus of the type including the above-described liquid developing device, liquid developers corresponding to respective colors such as yellow, cyan, magenta, and black are supplied from the developer container 6 to the developing device 2 and are shown in FIG. The liquid developer 23 is regulated on the supply roll 21 by the regulating blade 29 to form a layer. The toner in the liquid developer 23 is charged by the stirring mixer 24. The potential difference between the DC bias (Vdc2) 28 applied to the supply roll 21 and the DC bias (Vdc1) 27a applied to the developing roll 22, and the developer thin layer carried on the developing roll 22 by the AC bias 27b. 26 is adjusted.

  Then, when a print start signal is received from a control circuit (not shown), the photosensitive drum 3 is first charged to, for example, 600 V by the charger 8, and then the exposure unit 4 causes the photosensitive drum 3 to have a wavelength of, for example, 770 nm. By the exposure, the potential of the photosensitive drum 3 after exposure becomes about 70 V, and an electrostatic latent image is formed. The electrostatic latent image is developed with toner moving from the thin liquid developer layer 26 on the developing roll 22 to the photosensitive drum 3 by a DC bias (Vdc1) 27a and an AC bias 27b applied to the developing roll 22, A toner image is formed.

  After the toner image is formed on the photosensitive drum 3 in this manner, the toner image is sequentially transferred to the intermediate transfer belt 5, and the color transfer image on the intermediate transfer belt 5 is recorded at the timing of reaching the secondary transfer position. The medium is taken out of the paper feed cassette, conveyed by a conveying roller, and the color transfer image is secondarily transferred onto the recording medium at a nip portion between the intermediate transfer belt 5 and the transfer roller 9. Thereafter, the color toner image transferred to the recording medium is fixed by the fixing roller pair 10 and discharged.

Here, the image density and the image fogging after the image formation were examined by variously changing the volume resistivity of the surface (conductive elastic body) of the developing roll 22. The result is shown in FIG. In this case, the volume resistivity of the surface of the supply roll 21 is 6 × 10 ³ Ω · cm, the DC voltage 27a applied to the developing roll 22 is 200 V, the AC voltage 27b is Vpp 1.5 kV, and the frequency is The frequency was 3.0 kHz, the duty ratio was 30%, and the DC voltage 28 applied to the supply roll 21 was 400 V. As shown in FIG. 5, although the image density is highest when the volume resistivity of the developing roll 22 is 4 × 10 ² Ω · cm, when the volume resistivity is 4 × 10 ² Ω · cm, Image fog occurs.

It is assumed that the image density is good when the image density is 1.35 or more, and from FIG. 5, it is good when the volume specific resistance value of the developing roll 22 is 8 × 10 ³ Ω · cm to 4 × 10 ⁶ Ω · cm. An image density can be obtained, and no image fogging occurs. It is desirable that the volume specific resistance value of the developing roll 22 is selected to be 8 × 10 ³ Ω · cm to 4 × 10 ⁶ Ω · cm.

Next, the image density and image fogging after image formation were examined by variously changing the volume specific resistance value of the surface of the supply roll 21. FIG. 6 shows the result. In this case, the volume resistivity of the surface of the developing roll 22 is 3 × 10 ⁴ Ω · cm, the DC voltage 27a applied to the developing roll 22 is 200 V, the AC voltage 27b is Vpp of 1.5 kV, and the frequency is The frequency was 3.0 kHz, the duty ratio was 30%, and the DC voltage 28 applied to the supply roll 21 was 400 V. As shown in FIG. 6, although the image density is highest when the supply roll 21 has a volume resistivity of 4 Ω · cm, image fog occurs when the volume resistivity is 4 Ω · cm. Here, it is assumed that the image density is good if the image density is 1.35 or more. From FIG. 6, it is assumed that the volume resistivity of the supply roll 21 is 8 × 10 ¹ Ω · cm to ⁴ × 10 ⁴ Ω · cm. Good image density can be obtained, and no image fogging occurs. It is desirable to select the volume specific resistance value of the supply roll 21 to be 8 × 10 ¹ Ω · cm to ⁴ × 10 ⁴ Ω · cm.

According to the experiment of the inventor, the volume resistivity of the supply roll 21 is set to 1 × 10 ¹ Ω · cm or more and 1 × 10 ⁵ Ω · cm or less, and the volume resistivity of the developing roll 22 is set to 1 × 10 ³ Ω · cm. It was found that a good image density was obtained and the image fogging did not occur when the value was Ω · cm or more and 1 × 10 ⁷ Ω · cm or less. When measuring the volume specific resistance, the shaft of the developing roll 22 or the supply roll 21 and the metal drum are rotated while the developing roll 22 or the supply roll is in contact with the cylindrical metal drum and rotated. A voltage of 500 V DC or 1 kV is applied in between, and a voltage applied to a resistor connected in series with the metal drum is measured to obtain a resistance value. The contact width with the metal drum measured in advance and the layer of the conductive elastic body are measured. Using the contact area determined from the thickness, the above resistance value was converted to a volume specific resistance value.

Further, at the time of development, the level of the DC voltage 28 (Vmag-dc shown in FIG. 4) was changed to examine the image density and image fog after image formation. FIG. 7 shows the result. In this case, the level of the DC voltage 27a (Vslv-dc shown in FIG. 4) is 200 V, the volume resistivity of the surface of the developing roll 22 is 3 × 10 ⁴ Ω · cm, and the surface of the supply roll 21 is The volume specific resistance was 6 × 10 ³ Ω · cm, and the alternating current (AC) voltage 27b was a rectangular wave having a Vpp of 1.5 kV, a frequency of 3.0 kHz, and a duty (Duty) ratio of 30%.

  As shown in FIG. 7, when Vmag-dc is 270 V (DC bias difference 70 V), image fog does not occur, but the image density decreases (image density 1.27). On the other hand, when Vmag-dc became 650 V (DC bias difference of 450 V), it was found that image fogging and the like occurred. According to an experiment conducted by the inventor, when developing, when Vmag-dc-Vslv-dc (DC bias difference) is less than 100 V, the image density is reduced. When the DC bias difference exceeds 300 V, image fog is reduced. And the like, and it was found that it is desirable to set the DC bias difference between 100 V and 350 V.

Further, FIG. 8 shows the result of forming an image by forming a liquid developer thin layer on the developing roll 22 by changing the duty ratio of the AC voltage 27b variously. In this case, the level of the DC voltage 27a was 200 V, the level of the DC voltage 28 was 400 V, and the AC voltage 27b was a rectangular wave having a Vpp of 1.5 kV and a frequency of 3.0 kHz. The volume resistivity of the surface of the developing roll 22 was 3 × 10 ⁴ Ω · cm, and the volume resistivity of the surface of the supply roll 21 was 6 × 10 ³ Ω · cm. As shown in FIG. 8, when the duty ratio exceeds 50% and becomes 55%, the image density at the time of endurance when used for a long time decreases (for example, the image density at the time of 1000 sheets of continuous printing is reduced from 1.48 to 1.48). 1.33).

  On the other hand, when the duty ratio becomes 15%, the image density becomes low (the image density becomes 1.30). According to the experiment of the inventor, when forming a thin developer layer on the developing roll 22, if the duty ratio of the AC voltage 27b exceeds 50%, a good image cannot be obtained when printing is performed continuously. It was found that good image density could not be obtained even when the duty ratio of the voltage 27b was less than 30%. Therefore, it is desirable to set the duty ratio of the AC voltage 27b to 30% to 50%.

In addition, the toner density in the liquid developer thin layer 26 carried on the surface of the developing roll 22 on which the image has been formed is examined after the image formation, and the occurrence of a ghost image when the image is formed thereafter FIG. 9 shows the result of an investigation on the relationship between the presence or absence of toner and the toner density. At this time, the level of the DC voltage 27a was 200 V, the level of the DC voltage 28 was 400 V, and the AC voltage 27b was a rectangular wave having Vpp of 1.5 kV, a frequency of 3.0 kHz, and a duty ratio of 30%. The volume resistivity of the surface of the developing roll 22 was 3 × 10 ⁴ Ω · cm, and the volume resistivity of the surface of the supply roll 21 was 6 × 10 ³ Ω · cm. As shown in FIG. 9, when the toner concentration in the developer thin layer was 10%, a ghost image was generated, and when the toner concentration was 20%, the ghost image became severe.

  The presence or absence of a ghost image is determined by forming a solid image of 5 cm on each side during image formation before toner density adjustment, and forming a ghost image by forming an entire half image (1 dot 25%) during the next image formation. I investigated whether to do it. “×” in FIG. 9 indicates the occurrence of a ghost image, and “xx” indicates the occurrence of a severe ghost image. It is a condition that a ghost image does not occur in color image formation because it deteriorates color. Therefore, the toner concentration after the completion of the development and the collection of the toner in the liquid developer thin layer 26 carried on the surface of the developing roll 22 is set to 3% to 7%.

As described above, the DC bias (Vdc1) 27a and the AC bias 27b are applied to the developing roll 22, and the outermost surface of the developing roll 22 is 1 × 10 ³ Ω · cm or more and 1 × 10 ⁷ Ω · cm or less. Since it is conductive, the peak of the AC component when the toner in the liquid developer thin layer 26 on the developing roll 22 is pulled back to the supply roll 21 becomes sharp, and the toner in the liquid developer thin layer 26 pulls back. Done effectively. Therefore, the effect of pulling back the toner in the liquid developer from the developing roll 22 to the supply roll 21 as the developer conveying body is increased, and the occurrence of a ghost phenomenon is prevented, and a long-term stable image quality is obtained.

  Further, as described above, since the AC bias voltage is superimposed on the DC bias voltage, the toner in the liquid developer moves quickly due to the bias effect, and the toner moves quickly in the bias direction. As a result, the toner density becomes higher on the lower potential side. For this reason, the toner concentration on the liquid developer surface in contact with the photosensitive drum 3 becomes lower (sparser), and toner adhesion to the non-electrostatic latent image portion on the photosensitive drum 3 can be effectively prevented.

  Moreover, such a phenomenon prevents the uneven distribution of the density of the toner in the liquid developer on the surface of the electrostatic latent image, so that the transfer can be performed satisfactorily. That is, at the start of development, the liquid developer is separated between the developing roll 22 and the photosensitive drum 3, and the liquid developer containing a relatively high-density toner is applied to the electrostatic latent image portion of the photosensitive drum 3. Will be held.

  In the above description, if the duty ratio is 50% or less (preferably 30% or more and 50% or less), the toner in the liquid developer thin layer on the developing roll can be easily replaced after the development is completed. I understood. Moreover, in the above-described example, there is no need to press the blade or the like against the developing roll 22 to scrape off the residual liquid developer, so that the developing roll does not deteriorate and a special mechanism for collecting the liquid developer is used. There is no need.

  By the way, after the development is completed, when the liquid developer thin layer is left on the developing roll in contact with the photosensitive drum, if the collection of the toner in the liquid developer thin layer is insufficient, the photosensitive drum is Indentations may occur, and if the liquid developer thin layer is left on the developing roll for a long period of time, this may also deteriorate the photosensitive drum. That is, the toner easily stays at both ends of the nip portion between the developing roll and the photosensitive drum, and the toner stay may cause an impression on the photosensitive drum.

  On the other hand, in the above-described example, after the development is completed, the DC bias voltage is changed to collect the toner in the liquid developer thin layer on the developing roll to the supply roll, so that the liquid carrier concentration increases ( (The toner concentration is reduced), and the above-described problem does not occur. When the toner in the liquid developer thin layer is collected, the toner in the liquid developer thin layer can be collected more effectively by reversing the supply roll in the rotation direction of the developing roll.

  In this manner, in the present invention, the toner concentration of the liquid developer thin layer supported on the developing roll can be made suitable for development. A good image can be obtained by preventing toner adhesion. Further, since the toner concentration of the liquid developer thin layer carried on the developing roll can be adjusted by a simple mechanism, the liquid developing device itself can be made compact and the cost can be reduced.

  A supply roll holds a liquid developer composed of a liquid carrier and a toner, and the development roll receives the liquid developer from the supply roll to form a liquid developer thin layer and form an electrostatic latent image formed on the image carrier. When developing with a thin liquid developer layer to form a toner image, supply and recovery of the toner in the liquid developer is performed by adjusting the DC bias voltage, so that a uniform liquid developer layer is formed on the developing roll. Not only is it possible to obtain a liquid developer thin layer having a toner concentration suitable for development on the developing roll, but also it is easy to collect the toner in the liquid developer thin layer remaining on the developing roll. Since such an image can be formed, it can be applied to an image forming apparatus such as a copying machine, a printer, or a facsimile machine.

1 is a configuration diagram illustrating a tandem-type image forming apparatus as an example of an image forming apparatus using a liquid developing device according to the present invention. FIG. 4 is a configuration diagram for explaining an operation of an example of the liquid developing device according to the present invention. FIG. 3 is a diagram for explaining occurrence of an afterimage portion (ghost). FIG. 7 is a diagram for explaining bias application when forming and collecting a liquid developer thin layer on a developing roll. FIG. 3 is a diagram illustrating a relationship between a volume specific resistance value (Ω · cm) of a developing roll 22 and image density and image fog. FIG. 3 is a diagram illustrating a relationship between a volume specific resistance value of a supply roll 21, (Ω · cm), image density, and image fog. FIG. 4 is a diagram illustrating a relationship between a difference between a DC bias applied to a developing roll 22 and a DC bias applied to a supply roll, and image density and image fog. FIG. 3 is a diagram illustrating a relationship between a duty ratio of an AC bias applied to a developing roller 22 and image density and image fog. FIG. 4 is a diagram illustrating a relationship between a toner concentration in a thin liquid developer layer on a developing roll and the presence or absence of a ghost image.

Explanation of reference numerals

2 Developing device 3 Photoreceptor drum 4 Exposure unit 21 Supply roll 22 Developing roll 23 Liquid developer 24 Stirring mixer 26 Liquid developer thin layer 27 Developing bias 28 Direct current (DC) bias 29 Regulator blade

Claims (6)

A supply roll for holding a liquid developer including at least a liquid carrier and a toner, and an electrostatic latent image formed on an image carrier by receiving the liquid developer from the supply roll to form a liquid developer thin layer A developing roll that develops the liquid developer thin layer to form a toner image,
DC bias applying means for applying a DC bias voltage to the supply roll as a supply roll DC bias voltage,
Developing bias applying means for applying a developing bias in which an AC bias voltage is superimposed on a developing roll DC bias voltage with respect to the developing roll,
In the first step of performing development, the DC bias application unit applies a first DC bias voltage as the supply roll DC bias voltage, and after developing the electrostatic latent image formed on the image carrier, In the second step of reducing the toner concentration of the liquid developer thin layer carried on the developing roll, a second DC bias voltage is applied as the supply roll DC bias voltage,
The first DC bias voltage is higher than the developing roll DC bias voltage,
The liquid developing device according to claim 1, wherein the second DC bias voltage is lower than the developing roll DC bias voltage.   By the developing bias, the toner concentration in the liquid developer thin layer on the developing roll facing the image carrier side is made smaller than the toner concentration in the liquid developer thin layer on the developing roll facing the supply roll. 2. The liquid developing device according to claim 1, wherein the liquid developing device is configured to be lowered. The development roll has a volume resistivity of 1 × 10 ³ Ω · cm or more and 1 × 10 ⁷ Ω · cm or less, and the supply roll has a volume resistivity of 1 × 10 ¹ Ω · cm or more. 2. The liquid developing device according to claim 1, wherein the pressure is equal to or less than × 10 ⁵ Ω · cm.   2. The liquid developing apparatus according to claim 1, wherein the AC bias voltage has a duty ratio of 30% or more and 50% or less.   The liquid developing apparatus according to claim 1, wherein a bias difference between the first DC bias voltage and the developing roll DC bias voltage is in a range of 100V to 350V.   In the second step of reducing the toner concentration of the liquid developer thin layer carried on the developing roll after developing the electrostatic latent image formed on the image carrier, the developing step facing the image carrier is performed. 2. The liquid developing apparatus according to claim 1, wherein the toner concentration in the liquid developer thin layer on the roll is 7% or less.

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