JPH1172977A - Image forming device - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To obtain a high-quality image with no reduction in image density or uneven density in a solid black image portion regardless of an image forming mode. SOLUTION: A control means 19 switches a developing bias value by a developing bias applying means 11 according to an image forming mode, and also switches a transfer bias value by a transferring bias applying means 12 in conjunction with switching of the developing bias value. Accordingly, the transfer bias value is also corrected to an appropriate value in accordance with the toner charge amount Q / M that changes by switching the developing bias in accordance with the image forming mode, so that a high-quality image is always obtained regardless of the image forming mode. Can be For example, in the photographic image forming mode, a developing bias in which an AC bias is superimposed on a DC bias is used, and the transfer bias side is also switched to a larger bias value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a facsimile, and a printer for forming an image using electrophotographic technology.

[0002]

2. Description of the Related Art Conventionally, there is a two-component magnetic brush developing method as one of the developing methods in this type of image forming apparatus. In this development method, a developer is transported while being held on a rotatable developing sleeve surface having magnetic poles, thereby bringing the developer into contact with the photoreceptor surface to visualize an electrostatic latent image on the photoreceptor surface. Is the law. In this case, the charge amount Q / M [μC / g] per unit mass of the toner in the developing device has a certain distribution due to variations in the surface characteristics and mass of the toner and the carrier. For this reason, when the charge amount of the toner is high, there is a problem that the image density cannot be obtained due to a large adhesive force with the carrier. This causes a problem that the toner adheres to the background portion of the part and causes background contamination.

As a countermeasure against such image quality deterioration, there is known a method of superimposing an AC bias on a DC bias as a developing bias. For example, Japanese Patent Application Laid-Open No. 4-1
According to Japanese Patent No. 57486, in a photographic image forming mode, by applying an AC bias in which an AC component is superimposed on a DC bias component as a developing bias, an end of the image is blurred, and an image having a uniform image density is obtained. In the formation mode, an edge portion and a line image of an image are emphasized by applying a DC bias including no AC component as a developing bias.

[0004]

However, Japanese Unexamined Patent Publication No.
When image formation was performed while controlling the developing bias to be switched in accordance with the image forming mode according to the 157486 system, no image was observed in the photographic image forming mode (when the AC bias was superimposed) and in the character image forming mode. This is due to image quality deterioration such as a decrease in image density and unevenness of a solid black image portion.

Therefore, the present invention provides an image forming apparatus capable of forming a high-quality image without lowering the image density or uneven density of a solid black image portion, irrespective of the image forming mode, particularly in the photographic image forming mode. The purpose is to provide.

[0006]

According to the first aspect of the present invention,
An image carrier having a photoconductive layer and driven to rotate in one direction,
A charging device that charges the surface of the image carrier, a latent image forming unit that forms an electrostatic latent image on the image carrier by light irradiation,
A two-component developer consisting of a toner and a carrier is housed, and is transported while holding the developer by a developing sleeve having a magnet inside to develop an electrostatic latent image on the image carrier to form a toner image. Developing device, and a developing bias applying means for applying a developing bias to the developing sleeve,
Transfer means for electrostatically transferring the toner image on the image carrier to the transfer material; transfer bias application means for applying a transfer bias to the transfer means; and various components according to a designated image forming mode Control means for controlling the developing bias value by the developing bias applying means in accordance with the image forming mode and applying the transfer bias in accordance with the switching of the developing bias value. The transfer bias value by means is switched.

The charge amount Q / M [μC / g] per unit mass of the toner developed on the image carrier was measured. As shown in FIG. 3, there was a difference between when the AC bias was applied and when the DC bias was applied. (In the figure,
The horizontal axis is the development potential ΔV [V], and the vertical axis is the absolute value Q of the charge amount per unit mass of the toner developed on the image carrier.
/ M [μC / g]). According to the measurement results shown in FIG. 3, the charge amount Q / M of the toner when the AC bias is applied is higher than the charge amount Q / M of the toner when the DC bias is applied. On the other hand, when the adhesion amount M / A [mg / cm ² ] of the toner developed on the image carrier was measured when the AC bias was applied and when the DC bias was applied, the difference shown in FIG. 4 was obtained. (In the figure, the horizontal axis represents the development potential ΔV [V], and the vertical axis represents the toner adhesion amount M / A per unit area adhered to the image carrier.
[Mg / cm ² ]). According to the measurement results shown in FIG. 4, it can be seen that the toner adhesion amount when the AC bias is applied is larger than the toner adhesion amount when the DC bias is applied.

Considering the measurement results shown in FIGS. 3 and 4, it is assumed that the charge amount Q / M of the toner before development has a distribution as shown in FIG. It represents the average value of the charge amount Q / M of the toner. Here, the toner that is actually developed when a DC bias is applied as a developing bias is a portion A indicated by hatching in FIG. 5A, and is actually developed when an AC bias is applied as a developing bias. Assuming that the toner to be applied is a portion B indicated by hatching in FIG. 5B, the average value of the charge amount Q / M of the developed toner is higher when the AC bias is applied than when the DC bias is applied. It can be seen that the toner adhesion amount also increases.

That is, it is found that when an AC bias is applied as a developing bias, toner having a higher charge amount Q / M is developed than when a DC bias is applied. Further, observation of the toner image adhered on the image carrier confirmed that there was no reduction in image density and no density unevenness in a solid black portion when an AC bias was applied. On the other hand, the fact that the charge amount of the toner affects the transferability is as follows.
This is a commonly known fact. In other words, if the charge amount of the toner is large, the toner obtains a large adhesive force to the image carrier by a mirror image force, and if the adhesive force is larger than the electrostatic force due to the transfer bias, the toner is not transferred to the transfer material. In this case, toner remaining on the image carrier is likely to be generated.

Taking these points into consideration, it is expected that the causes of the problems of lowering the image density and uneven density in the solid black portion are not the development process but the transfer process. Therefore, in the present invention, when the developing bias value by the developing bias applying unit is switched according to the image forming mode, the transfer bias value by the transferring bias applying unit is switched according to the switching of the developing bias value.
The transfer bias value is also corrected to an appropriate value in accordance with the toner charge amount Q / M that changes with the switching of the developing bias,
A good quality image is always obtained regardless of the image forming mode.

According to Japanese Patent Application Laid-Open No. 8-160783, it is assumed that the developing current between the developing bias power supply and the developing sleeve is proportional to the charging amount Q / M per unit weight of toner. And a control unit that controls the transfer condition when the charge amount Q / M is out of the proper range. However, the charge amount Q / M falls within the proper range. There is no idea that the transfer conditions are controlled in conjunction with them under certain conditions.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the developing bias applying unit has an AC bias source and a DC bias source, and the transfer bias applying units differ in absolute value. A transfer bias source, wherein the control means causes the developing bias applying means to apply, as a developing bias, an AC bias obtained by superimposing an AC voltage on the DC bias in the first image forming mode, The transfer bias having a larger absolute value is applied by the developing bias applying means in the second image forming mode, and a DC bias including no AC component is applied as a developing bias. To be applied. Therefore, in the first image forming mode in which the AC bias is used as the developing bias, the transfer bias is switched to a higher value, so that a high-quality image can be obtained without lowering the image density or uneven density in the solid black image portion.

According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the transfer means is a corona charger for applying corona ions to the back side of the material to be transferred. Therefore, a transfer electric field can be formed in accordance with the charge amount Q / M of the toner developed in the corona transfer method, and appropriate transfer can be performed, thereby lowering the image density and reducing the density unevenness of the black solid image portion. No good quality images can be obtained.

According to a fourth aspect of the present invention, in the image forming apparatus according to the first or second aspect, the transfer means is a roller-shaped rotating body which forms a transfer electric field by a voltage while being in contact with the material to be transferred. Therefore, a transfer electric field can be formed in accordance with the charge amount Q / M of the toner developed in the roller transfer method, and appropriate transfer can be performed, thereby lowering the image density and reducing the density unevenness of the solid black image area. No good quality images can be obtained.

According to a fifth aspect of the present invention, in the image forming apparatus of the third or fourth aspect, the transfer bias applying means includes:
A voltage controlled at a constant voltage according to the designated image forming mode is supplied as a transfer bias. Therefore, a stable electric field can be obtained in the transfer means by using a constant voltage, and appropriate transfer can be performed, so that a high-quality image can be obtained without lowering the image density or uneven density in the solid black image portion.

According to a sixth aspect of the present invention, in the image forming apparatus of the third or fourth aspect, the transfer bias applying means comprises:
A voltage controlled at a constant current according to the designated image forming mode is supplied as a transfer bias. Therefore, by using a constant current, a constant charge can always be supplied to the transfer unit, and appropriate transfer is performed.
A good quality image can be obtained without a decrease in image density or density unevenness in a solid black image portion.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration of an image forming apparatus using an electrophotographic technique to which the present invention is applied. First, a drum-shaped photoconductor 1, which is an image carrier that is driven to rotate in one direction, is provided. I have. The photoreceptor 1 is an organic photoreceptor (OPC) having a photoconductive layer on its surface, and a charging roller 2 is provided around the photoreceptor 1 according to an electrophotographic process.
(Charging device), writing laser optical system 3 (latent image forming means), developing device 4, transfer device 5, cleaning device 6,
Exposure devices 7 for static elimination and the like are arranged in order.

The charging roller 2 uniformly charges the surface of the photosensitive member 1. The writing laser optical system 3 is a photoconductor 1 charged with a laser beam modulated in accordance with image information.
A latent image is formed by irradiating the surface. The developing device 4 forms a toner image by developing an electrostatic latent image on the photoconductor 1 by a two-component magnetic brush development method using a two-component developer including a toner and a carrier. For this reason,
A developing sleeve 8 having a magnet disposed therein;
The two-component developer is conveyed with the rotation while forming a magnetic brush while being held on the surface of the developing sleeve 8, and the magnetic brush is disposed so as to be in contact with the surface of the photoconductor 1. The transfer device 5 employs a roller transfer method of transferring a toner image on the photoreceptor 1 onto a transfer paper 10 (a material to be transferred) using a transfer roller 9 which is a roller-shaped rotating member. 9a and a surrounding conductive rubber roller 9b. The cleaning device 6 removes the toner remaining on the photoconductor 1 after the transfer, and the charge removing exposure device 7 removes the residual charge.

The developing sleeve 8 in the developing device 4
, A developing bias applying device 11 (developing bias applying means) is connected to the shaft roller 9 a of the transfer roller 9.
Is connected to a transfer bias applying device 12 (transfer bias applying means). Here, the developing bias applying device 11 applies a developing bias to the developing sleeve 8, and includes a DC bias source 13 and an AC bias source 14. This developing bias applying device 1
1 and the developing sleeve 8, an AC bias terminal 15 a for superimposing an AC bias from the AC bias source 14 on a DC bias from the DC bias source 13 as a developing bias, and a DC bias source 13 so that the AC bias is not applied. And a switching device 15 having a DC bias terminal 15b connected thereto and switching the bias state. Specifically, the DC bias from the DC bias source 13 is set to -600 [V], and the AC bias from the AC bias source 14 is
Peak voltage Vpp = 2.0 [kV], frequency f = 5 [kHz]
z] and a duty of 50 [%] (note that
The duty is defined as t1 / (t1 + t2) × 100 [%] (see FIG. 2). The transfer bias applying device 1
Numeral 2 applies a transfer bias to the transfer paper 10, and includes a transfer bias source 16 having a larger transfer bias value and a transfer bias source 17 having a smaller transfer bias value. These transfer bias sources 16 and 17 are configured to be supplied from, for example, a constant current source. As an example, the output current on the transfer bias source 16 side is 25 [μA], and the output current on the transfer bias source 17 side is 15 [μA]. A switch 18 having a large bias terminal 18a on the transfer bias source 16 side and a small bias terminal 18b on the transfer bias source 17 side to switch the bias state is provided between the transfer bias applying device 12 and the shaft roller 9a. Intervened.

Further, there is provided a control unit 19 (control means) for controlling the switching of the switches 15 and 18 and other controls. The control unit 19 is connected to an operation panel 20 for performing an image forming mode, other setting operations, and the like. Here, in the present embodiment, as an image forming mode arbitrarily set through the operation panel 20, a photographic image forming mode is prepared as a first image forming mode, and a character image forming mode is set as a second image forming mode. It is prepared. The control section 19 switches the switches 15 and 18 to the terminals 15a and 18a in conjunction with each other when the photographic image forming mode is designated, and switches the switches 15 and 18 to the terminals when the character image forming mode is designated. It is set so as to be interlocked and switched to the 15b and 18b sides.

In such a configuration, the operation panel 20
When the photographic image forming mode is selected and designated through the control unit 19, the control unit 19 switches the switch 15 to the AC bias terminal 15a side,
The switches 18 are each switched to the large bias terminal 19a side. As a result, a developing bias −600 [V] ± 1 [kV] in which an AC bias is superimposed on a DC bias as shown in FIG. 2 is applied to the developing sleeve 8, and the developing roller 8 is applied to the transfer roller 9. A transfer bias current of 25 [μA] having a larger bias value is applied. On the other hand, when the character image forming mode is selected and designated through the operation panel 20, the control unit 19 switches the switch 15 to the DC bias terminal 15b and the switch 18 to the small bias terminal 19b. Thus, a DC bias of -600 [V] is applied to the developing sleeve 8 as a developing bias, and a transfer bias current 15 [μA] having a smaller bias value is applied to the transfer roller 9. .

Therefore, according to the present embodiment, when the charge amount Q / M of the toner to be developed changes as the developing bias is switched between the AC bias and the DC bias in accordance with the switching of the image forming mode. In addition, since the transfer bias value also switches the bias value in accordance with the change, it is possible to eliminate the reduction in image density due to the image forming mode and the density unevenness of the solid black image portion, and to always obtain a good quality image. it can.

In the transfer device 5 of this embodiment, the roller transfer method using the transfer roller 9 is used. However, as shown in FIG. 6, a corona charger 21 for applying corona ions from the back side of the transfer paper 10 is used. Or a corona transfer method using In this case, the output from the transfer bias applying device 12 connected to the corona wire 21a is such that the total discharge current is 400 [μA] in the photographic image formation mode and 300 [μA] in the character image formation mode. A constant current source through which a current flows is used as the transfer bias sources 16 and 17. However, such a current value actually depends on the thickness of the transfer paper 10, the distance between the corona wire 21a and the photoreceptor 1, and the like, so that an appropriate value is used according to the apparatus and conditions used. Of course.

Further, although a constant current source is used for the bias sources 16 and 17 as the transfer bias applying device 12 for the transfer device 5, a constant voltage source may be used. In this case as well, the appropriate voltage value varies depending on the apparatus and conditions used. For example, when the transfer roller 9 is used, 1000 [V] is applied to the bias source 16 side, and 500 V is applied to the bias source 17 side.
For example, when the corona charger 21 is used as shown in FIG. 6, 7 [kV] is applied to the bias source 16 and 5 [k] is applied to the bias source 17 as shown in FIG.
V] may be used.

In the developing bias applying device 11, as shown in FIG. 7, a DC bias source 22 connected in series to an AC bias source 21 is used as an AC bias, and a DC bias source 23 is used as a DC bias. By arranging them, the center value voltage of the AC bias may be changed. For example, as an AC bias waveform, Vpp = 2.0 kV, f = 5 k
Hz], duty 20 [%], center value voltage -1200
[V], a good image was obtained in the photographic image forming mode.

[0026]

According to the first aspect of the present invention, the control means switches the developing bias value by the developing bias applying means according to the image forming mode and the transfer bias applying means according to the switching of the developing bias value. Since the transfer bias value is switched, the transfer bias value is also corrected to an appropriate value in accordance with the toner charge amount Q / M that changes when the developing bias is switched according to the image forming mode. Quality images can always be obtained.

According to the second aspect of the present invention, in the image forming apparatus according to the first aspect, the developing bias applying means includes:
A transfer bias source having an AC bias source and a dc bias source; a transfer bias source having a transfer bias source having an absolute value different in absolute value; An AC bias on which a voltage is superimposed is applied as a developing bias, and a transfer bias having a larger absolute value is applied by a transfer bias applying unit. In the second image forming mode, a DC bias containing no AC component is applied by the developing bias applying unit. In the first image forming mode in which an AC bias is used as the developing bias, the transfer bias is increased. Will be switched to Without density unevenness reduction or black solid image portion of the image density can be obtained a high-quality image.

According to the third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the transfer means is a corona charger for applying corona ions to the back side of the material to be transferred. A transfer electric field corresponding to the charge amount Q / M of the toner developed by the method can be formed, and a proper transfer can be performed. Images can be obtained.

According to a fourth aspect of the present invention, in the image forming apparatus according to the first or second aspect, the transfer means is a roller-shaped rotating body that forms a transfer electric field by a voltage while being in contact with the material to be transferred. It is possible to form a transfer electric field according to the charge amount Q / M of the toner developed in the roller transfer method, to perform proper transfer, to reduce the image density, and to reduce the density unevenness of the black solid image portion. You can get good quality images.

According to a fifth aspect of the present invention, in the image forming apparatus according to the third or fourth aspect, the transfer bias applying unit supplies a voltage controlled at a constant voltage in accordance with a designated image forming mode as a transfer bias. Because
By using a constant voltage, a stable electric field can be obtained in the transfer unit, so that proper transfer can be performed, and a high-quality image can be obtained without a decrease in image density or density unevenness in a solid black image portion.

According to a sixth aspect of the present invention, in the image forming apparatus according to the third or fourth aspect, the transfer bias applying means supplies a constant current-controlled voltage as a transfer bias in accordance with a designated image forming mode. Because
By using a constant current, a constant charge can always be supplied to the transfer means, and appropriate transfer can be performed, and a high-quality image can be obtained without a decrease in image density or density unevenness in a solid black image portion. it can.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a waveform diagram showing a developing bias on which an AC bias is superimposed.

FIG. 3 is a graph showing a measurement result of a charge amount for explaining a premise of the present invention.

FIG. 4 is a graph showing a measurement result of a toner adhesion amount for explaining a premise of the present invention.

FIG. 5 is a graph showing the result of consideration of the number of developed toners for explaining the premise of the present invention.

FIG. 6 is a schematic configuration diagram illustrating a modification of the transfer device.

FIG. 7 is a schematic configuration diagram illustrating a modified example of the developing bias application device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image carrier 2 Charging device 3 Latent image forming means 4 Developing device 5 Transfer device 8 Developing sleeve 9 Rotating member 10 Transfer material 11 Developing bias applying means 12 Transfer bias applying means 13 DC bias source 14 AC bias source 16, 17 Transfer Bias source 19 Control means

Claims (6)

[Claims] An image carrier having a photoconductive layer and driven to rotate in one direction, a charging device for charging the surface of the image carrier, and an electrostatic latent image formed on the image carrier by light irradiation. A latent image forming means for forming, and a two-component developer comprising a toner and a carrier, containing the developer and holding the developer by a developing sleeve having a magnet therein to convey the electrostatic latent image on the image carrier; Developing device that develops a toner image to form a toner image, a developing bias applying unit that applies a developing bias to the developing sleeve, and a transfer device that electrostatically transfers the toner image on the image carrier to a material to be transferred. Means, a transfer bias applying unit for applying a transfer bias to the transfer unit, and a control unit for controlling each unit according to a designated image forming mode. Depending on the forming mode Image forming apparatus characterized by switching the transfer bias value by the transfer bias applying means in accordance with the switching of the developing bias value switches the development bias value by the developing bias applying means. 2. The developing bias applying unit has an AC bias source and a DC bias source, the transfer bias applying unit has a transfer bias source whose absolute value differs in magnitude, and the control unit has a first image forming unit. In the mode, the developing bias applying unit applies an AC bias obtained by superimposing an AC voltage on a DC bias as a developing bias, and the transfer bias applying unit applies a transfer bias having a larger absolute value. 2. The method according to claim 1, wherein said developing bias applying means applies a direct current bias containing no AC component as a developing bias, and said transferring bias applying means applies a transfer bias having a smaller absolute value. Image forming apparatus. 3. The image forming apparatus according to claim 1, wherein the transfer unit is a corona charger that applies corona ions to the back side of the material to be transferred. 4. The image forming apparatus according to claim 1, wherein the transfer unit is a roller-shaped rotating body that forms a transfer electric field by a voltage while being in contact with the material to be transferred. 5. The image forming apparatus according to claim 3, wherein the transfer bias applying unit supplies a voltage controlled at a constant voltage according to a designated image forming mode as a transfer bias. 6. The image forming apparatus according to claim 3, wherein the transfer bias applying unit supplies a voltage controlled at a constant current as a transfer bias according to a designated image forming mode.