JPH04110971A - Image forming device - Google Patents

Abstract

PURPOSE:To obtain stable printing quality by forming an electrostatic latent image on the surface of an uniformly electrified electrostatic latent image carrier with a light source, developing the electrostatic latent image in an application state where a bias voltage is changed, with electrified toner, and transferring the developed toner image to an image receiving material. CONSTITUTION:The surface 12A of the electrostatic latent image carrier is uniformly electrified by a first electrifying means 14, and the electrostatic latent image is formed on the surface 12A of the electrostatic latent image carrier with the light source. Then, development is carried out in a bias voltage applied state, with the electrified toner, on an electrostatic latent image developing means 18. At this time, the bias voltage of the developing means 18 is changed by a bias voltage changing means 19. After that, the toner image on the electrostatic latent image carrier 12 electrified to the same polarity by a second electrifying means 20 is transferred to the image receiving material. Thus, since the bias voltage of the developing means 18 is changed by the bias voltage changing means 19, the degradation of image quality caused by base fogging, etc., can be prevented on the development, and the stable printing quality is always obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image forming apparatus that forms monochrome or multicolor images.

[Prior Art] Electrophotographic image forming apparatuses apply black or other colors to an electrostatic latent image corresponding to an image signal formed on an electrostatic latent image carrier (photoreceptor) in order to obtain a visible image. This is a device that obtains a desired image by electrostatically depositing toner, transferring this to paper, etc., and fixing it using pressure, heat, etc. With the diversification of information in recent years, electrophotographic image forming apparatuses are being used as an alternative to conventional line printers.

The electrophotographic image forming apparatus has a 10th
It is divided into a regular development type shown in the figure and a reversal development type shown in FIG. Regular development is mainly used in ordinary PPC copying machines, and reversal development is mainly used in page printers and the like. Further, in a simple device for realizing two-color printing, such as that disclosed in Japanese Patent Laid-Open No. 60-117266, normal development and reversal development exist simultaneously, as shown in FIG.

The reason why reversal development is used here is that in a typical original document, the background portion occupies an overwhelmingly larger area than the character signal portion. That is, in the case of normal development, it is necessary to turn on the light source for a background portion that occupies an overwhelmingly large area, but in the case of reverse development, it is only necessary to turn on the light source for a small region of the character signal portion. For example, when considering LED printers, laser printers, etc., the latter is more advantageous in terms of head control, head life, and power consumption.

However, in order to realize reversal development, it is necessary to set the bias voltage as shown in FIGS. 11 and 12. Normally, in the case of monochrome printing, the uniform charging potential vO
A bias voltage of approximately the same level or slightly lower is applied. Furthermore, in the case of two-color printing disclosed in JP-A-60117266, approximately half of the uniform charging potential VD (V
O/2).

Setting the bias voltage in these reversal developments has a very important meaning in order to obtain a high quality image. Here, this point will be explained using FIG. 11 as an example.

Reversal development is a device for electrostatically adhering toner to a portion of an electrostatic latent image carrier (photoreceptor) at a completely opposite potential level compared to regular development. That is, originally, toner would not adhere to the part of the potential level "0" in Figure 11, but by setting the potential level of the part where toner exists to the bias level shown in the figure, the apparent The potential of the toner becomes VO, and conversely, the exposed portion is lower than the potential level of the toner by VO, so the positively charged toner electrostatically adheres to the exposed portion.

Here, as shown in FIG. 13, if the relationship between the uniform charging potential VO in the developing unit and the bias voltage VB of the developing unit is VO>VB, each potential in FIG. The relationship between levels will be disrupted. That is, a potential difference also occurs between the bias voltage and the background portion. As a result, the oppositely charged toner (negatively charged toner in FIG. 13) present in the developing device adheres to this portion, causing deterioration in image quality due to background fog.

Furthermore, if the above relationship becomes extreme and the potential difference between the bias voltage and the exposed area is smaller than the relationship shown in FIG. 11, the amount of toner adhering to the printed area will decrease and sufficient development will not be possible. As a result, image quality deteriorates, such as a decrease in density and white spots in solid black areas.

On the other hand, as shown in Figure 14, the completely opposite relationship is
A situation where VO<VB is possible. In this case, the relationship between the surface potential and bias voltage of the background area becomes the same as the relationship between the surface potential and bias voltage of the exposed area (print signal area), and toner should not adhere to the background area. Positively charged toner also adheres to the surface. Therefore, in this case as well, deterioration in image quality due to background fog occurs.

To avoid these problems, it is ideal that the bias voltage VB and the uniform charging potential VB be equal.

However, in reality, the uniform charging potential changes due to changes in charger characteristics due to environmental changes, implicit attenuation characteristics of the electrostatic latent image carrier, potential attenuation due to entry of external light into the device, and the like. Normally, a bias voltage that is slightly lower than the target uniform charging potential is applied in consideration of these surface potential attenuation amounts, but since the factors that change the surface potential of the electrostatic latent image carrier are complex, this The settings cannot necessarily be said to be appropriate bias voltage settings.

On the other hand, in the case of two-color printing as shown in FIG.
/2, reverse development is performed for positively charged toner, and regular development is performed for negatively charged toner.

At this time, the electrostatic force F acting on the toner particles in the direction of the electrostatic latent image carrier can be approximated by the following equation.

F-) Charge amount of toner × α・(Surface potential of electrostatic latent image carrier − bias potential) Here, α is a coefficient when calculating the electric field from the potential difference, and normally the electrostatic latent image carrier is connected to the developer sleeve. It is a coefficient that depends on the distance. If the charge amounts of positively charged toner and negatively charged toner are q(=) and q(-), the magnitude of the force acting on each toner is F(+)l = l q(+)xα
・VO/2F(-)l = l q(-)xα・・VC
It becomes l2. From this, it can be determined that if the charging characteristics of each toner are the same, the amount of adhesion is the same for positive and negative toners. However, as a practical matter, the charging characteristics of positively charged toner and negatively charged toner are different, and the 12th
The development shown in the figure has a problem in that differences in density occur due to differences in color.

[Problems to be Solved by the Invention Second Problems to be Solved by the Invention The present invention takes the above facts into consideration and aims to provide an image forming apparatus that can provide stable print quality.

Means for Solving the Problems: The present invention as set forth in claim (1) includes a first charging means for uniformly charging the surface of an electrostatic latent image carrier; a light source for forming an electrostatic latent image on the surface of an electrostatic latent image carrier; a developing means for developing the electrostatic latent image formed by the light source with charged toner under a bias voltage applied state; It is characterized in that it includes a bias voltage changing means for changing the bias voltage of the developing means, and a transfer means for transferring the toner image on the electrostatic latent image carrier developed by the developing means to an image receiving material.

Further, the present invention as set forth in claim (2) provides that the bias voltage changing means includes surface potential detecting means for detecting the potential on the surface of the electrostatic latent image carrier, and a bias voltage that increases the bias voltage as the surface potential becomes higher. It is characterized by consisting of a voltage change circuit.

The present invention as set forth in claim (3) further provides a first charging means for uniformly charging the surface of the electrostatic latent image carrier, and image information including two-color signals of a first color and a second color. a signal processing means for converting and outputting a signal distinguishable into a first color, a second color, and a background; and a surface of the electrostatic latent image carrier charged by the first charging means based on the output of the signal processing means. one light source for forming an electrostatic latent image with three potential levels corresponding to a first color, a second color, and a background; and applying a bias voltage to the electrostatic latent image formed by the light source. A developing means that attaches and develops two color toners charged with different polarities under different conditions, and changes a bias voltage so that the electrostatic force of the first toner and the electrostatic force of the second toner are equal. The present invention is characterized in that it includes a bias voltage changing means for changing the bias voltage, and a transfer means for transferring the two-color toner image on the electrostatic latent image carrier attached by the developing means to an image receiving material.

[Effect]

In the present invention as set forth in claim (1), the surface of the electrostatic latent image carrier is uniformly charged by the first charging means, and an electrostatic latent image is formed on the surface of the electrostatic latent image carrier by a light source. Next, this electrostatic latent image is developed with charged toner in a developing means under application of a bias voltage. In this case, the bias voltage of the developing means is changed by the bias voltage changing means. Thereafter, the toner image on the electrostatic latent image carrier, which has been charged to the same polarity by the second charging means, is transferred to the image-receiving material by the transfer means.

Therefore, by changing the bias voltage of the developing means using the bias voltage changing means, deterioration of image quality such as background fog during development can be prevented, and stable printing quality can always be obtained.

Further, in the present invention as set forth in claim (2), the surface potential detection means detects the potential on the surface of the electrostatic latent image carrier, and as the surface potential increases, the bias voltage changing circuit changes the bias voltage of the developing means. make it expensive Therefore, during development, deterioration in image quality such as background fog can be prevented, and stable printing quality can always be obtained.

Here, a method for realizing surface potential detection of the background portion will be described.

Since the print area includes both exposed and non-exposed areas, detecting the surface potential of this area will not accomplish the purpose. Therefore, by detecting the surface potential before exposure starts using the surface potential detection means and using this as the surface potential of the background part of the image that is currently being printed, it is possible to eliminate the effects of the environment, etc. .

Further, in the present invention as set forth in claim (3), the surface of the electrostatic latent image carrier is uniformly charged by the first charging means. on the other hand,
The signal processing means converts image information including a two-color signal of a first color and a second color into a signal that can be distinguished into a first color, a second color, and a background, and outputs the signal to one light source. One light source forms an electrostatic latent image on the surface of the electrostatic latent image carrier with three potential levels corresponding to a first color, a second color, and a background by varying the exposure amount of the light source. This amount of exposure can be changed by changing the exposure time or the intensity of light.

Next, this electrostatic latent image is developed in a developing means by applying two colored toners charged to very different polarities while applying a bias voltage.

In this case, the bias voltage changing means changes the bias voltage so that the electrostatic force of the first toner and the electrostatic force of the second toner become equal.

Thereafter, the two-color toner image on the electrostatic latent image carrier, which has been charged to the same polarity by the second charging means, is transferred to the image-receiving material by the transfer means.

As a result, deterioration in image quality such as background fog during development can be prevented, and a stable two-color image can always be obtained.

That is, in the case of two-color simultaneous development, as shown in FIG. 12, the relationship between the bias voltage and the background surface potential affects background fogging and color mixing of the two colors. Therefore, setting an appropriate bias voltage affects image quality more than in the case of monochrome. Furthermore, although the background potential is set to VO/2 in FIG. 12, the electrostatic force acting on the positive and negative toners and the electrostatic latent image carrier can be approximated by the following equation.

F=) Amount of charge on the toner It is a distance-dependent coefficient.

Therefore, when the background potential level and the bias potential level are both VO/2, in order for the positive and negative toners to receive the same force, the charge amounts q(+) and q(-) of the positive and negative toners must be equal. becomes. However, in reality, the charging characteristics of positively charged toner and negatively charged toner are different, and the first
There is a problem with the development shown in Figure 2 that differences in density occur due to differences in color. Therefore, in such cases, in addition to changes in the uniform charging potential, it is necessary to consider the amount of charge of the color toner. It is necessary to set the bias voltage.

For example, if the charge amount q(+) of the positively charged toner is twice the charge amount q(-) of the negatively charged toner, the 15th
As shown in the figure, the background potential level and bias voltage are V
By setting it to be VO/3 instead of O/2, F(+) l = l 2 q(+) xα・VO
/3=l 2/3Xq(+)Xα・VO F(-)l=I Q(-)Xα・Vo ・2/3=
l2/3xq(-)Xα·VO, and the electrostatic forces acting on the positive and negative toners are equal.

Setting the bias voltage in this way is also effective for obtaining a good two-color image.

[Embodiments of the invention]

An image forming apparatus according to a first embodiment of the present invention will be described below.

As shown in FIG. 1, the basic configuration of the image forming apparatus 10 of this embodiment is that the surface 12A of the electrostatic latent image carrier (photoreceptor) 12 is
A charger 14 as a first charging means for uniformly charging the
It is equipped with A surface potential sensor 15 is provided on the downstream side of the charger 14 in the direction of rotation of the electrostatic latent image carrier 12 (in the direction of arrow B in FIG. The potential of the surface 12A of the electrostatic latent image carrier after being charged in a -like manner is detected. A light source 16 is provided on the downstream side of the surface potential sensor 15 in the rotational direction of the electrostatic latent image carrier 12, and the background potential and the first image detection potential are applied to the surface 12A of the electrostatic latent image carrier after −-like charging. and a second image pattern.

As shown in FIG. 2, the light source 16 includes a plurality of LEDs (L, ~L
n).

As shown in FIG. 3, each LED (L, ~
Ln) are connected to the drive circuit 30, respectively. This drive circuit 30 is provided with a first AND circuit 34 and a second AND circuit 36, and on the output side of the first AND circuit 34 is a first timer circuit that lights up the input signal for a time T1. 38 is connected to the output side of the second AND circuit 36, and a second timer circuit 40 is connected to the output side of the second AND circuit 36 to turn on the human power signal for a period of T1+T2. Further, the output side of the first timer circuit 38 and the output side of the second timer circuit 40 are respectively connected to an OR circuit 42 so that the output of the OR circuit 42 is inputted to each LED of the light source 16. It has become.

The drive circuit 30 is connected to a print signal generator 32,
The inverted signal of the print signal A and the inverted signal of the print signal B are the first
The A N D circuit 34 is operated manually. In addition, the inverted signal of the print signal A and the print signal B are used as a second AND signal.
The circuit 36 is designed to be powered manually. The print signal generator 32 is connected to image information output means such as a scanner and a computer (not shown).

As shown in FIG. 1, a developing device 18 as a developing means is provided on the downstream side of the light source 16 in the rotational direction of the electrostatic latent image carrier 12. Reversal development is performed using different color toners charged with different polarities under the condition of application of a negative voltage. Further, a bias voltage changing circuit 19 forming part of a bias voltage changing means for changing the bias voltage is connected to the developing device 18. This bias voltage change circuit 19
is connected to the surface potential sensor 15, and the bias voltage is changed based on the surface potential of the surface 12A of the electrostatic latent image carrier after negative charge, which is manually inputted from the surface potential sensor 15.

On the downstream side of the developing device 18 in the rotational direction of the electrostatic latent image carrier 12,
A pre-transfer charger 20 is provided as a second charging means, and is configured to charge the different color toners on the electrostatic latent image carrier surface 12A attached by the developing device 18 to the same polarity. Electrostatic latent image carrier 1 of pre-transfer charger 20
A transfer charger 22 as a transfer means is provided on the downstream side in the rotational direction of the image forming apparatus 2, and the different color toner on the electrostatic latent image carrier surface 12A charged to the same polarity by the pre-transfer charger 20 is transferred to a sheet of paper as an image receiving material. 24.

A fixing device (not shown) is provided on the downstream side of the sheet 24 in the conveying direction (in the direction of the arrow in FIG.
It is set to become fixed at 4.

Further, a cleaning section 26 is provided on the downstream side of the pre-transfer charger 20 in the rotational direction of the electrostatic latent image carrier 12, and is configured to remove toner from the surface 12A of the electrostatic latent image carrier after the transfer is completed. There is. An exposure device (erase lamp) 28 is provided on the downstream side of the cleaning section 26 in the rotational direction of the electrostatic latent image carrier 12, and is designed to neutralize the residual charge on the electrostatic latent image carrier 12 after cleaning. ing.

Next, the operation of this embodiment will be explained.

The electrostatic latent image carrier 12 moves clockwise in FIG.
direction). The surface 12 of this electrostatic latent image carrier 12
A is uniformly charged positively or negatively by the charger 14. In this embodiment, the following description will be made assuming that the battery is positively charged.

The uniformly charged surface 12A of the electrostatic latent image carrier 12 is exposed to positive light and negative light by the light source 16 to form positive and negative electrostatic latent images.

That is, as shown in FIG. 7, the electrostatic latent image carrier surface 12A
is charged to a voltage corresponding to vO, and the photoelectric properties of this electrostatic latent image carrier 12 (a phenomenon in which it becomes photoconductive when irradiated with light of a specific wavelength and harmonizes the charges on the surface) are as shown in FIG. 4, the image signal A (for example, black signal) and the image signal B (for example, red signal) of the print generator cause each drive circuit 30 to
is activated, turning each LED (L, to Ln) of the light source 16 on and off.

That is, both the image signal A (for example, a black signal) and the image signal B (for example, a red signal) from the print signal generator 32 are L(
For the low) level (background case), the first AND
An H (high) level signal is output from the circuit 34, and the LED is turned on for a time T1 when the surface potential becomes VD/2.
The surface 12A of the electrostatic latent image carrier is positively exposed.

Further, when the image signal A (for example, a black signal) from the print signal generator 32 is at H level, an L level signal is output from both the first AND circuit 34 and the second AND circuit 36. Therefore, the LED is not turned on and the surface potential does not change from VO.

Further, when the image signal B (for example, a red signal) from the print signal generator 32 is at H level, the second AND circuit 36 outputs an H level signal and the surface potential becomes approximately 0 for a time T1. The LED is turned on for only T2, and the surface 12A of the electrostatic latent image carrier is exposed to negative light.

As a result, the surface potential of the image signal A portion remains VO, the surface potential of the background portion becomes vO/2, and the surface potential of the image signal B portion becomes approximately O.

In this way, by controlling the exposure time using one light source 1, three different potential levels shown in FIG. 7 can be realized.

These three surface potential levels are reversely developed in the developer 18 using positive and negative toners. In this case, as a bias voltage during development, a potential of VO/2, which corresponds to the potential level of the background portion, is applied among the three potential levels shown in FIG. Further, the negatively charged toner is reversely developed in the area of surface potential O. This allows toners of different colors to be developed simultaneously.

However, the positive and negative toners may be developed using separate developing devices 18, and the present invention does not particularly relate to simultaneous development.

Next, the toners of different polarity formed on the electrostatic latent image carrier surface 18 are made to have the same charge polarity by a pre-transfer charger 20 and are transferred to a sheet 24 by a transfer charger 22. The toner on the paper 24 is fixed to the paper 24 by a heat or pressure fixing device (not shown) to obtain a desired two-color image.

On the other hand, the electrostatic latent image carrier 12 is covered with residual toner.
After being removed in step 6, the residual charge is erased in erase lamp 28, and the next image forming process is started again.

Therefore, three different surface potentials can be realized with one light source 16, miniaturization of the device and simplification of the process are achieved, resulting in significant cost effects.

Further, in this embodiment, a bias voltage changing circuit 19 is connected to the developing device 18, and the bias voltage changing circuit 19 uses the bias voltage changing circuit 19 to detect the electrostatic latent image carrier after being charged in a negative manner. The bias voltage for development is controlled based on the surface potential of surface 12A. This makes it possible to control the developing bias voltage so that it is always equal to the surface potential of the background portion, and enables image formation without background fog even under different environmental changes and usage conditions.

Further, for example, if the charge amount q (+) of the positively charged toner is twice the charge amount q (=) of the negatively charged toner, the bias voltage change circuit 19 stores the charge amount q (+) in advance in the bias voltage change circuit 19 . q(+)/q(-)=2
Based on the relationship, as shown in FIG. 9, the potential level of the background portion and the bias voltage are set to VO/3 instead of VO/2.

Therefore, the electrostatic force F acting on the toner particles in the direction of the electrostatic latent image carrier is expressed by the following formula, and the electrostatic forces acting on the positive and negative toners are equal.

F(+) l = l 2 q (+)xα・VO
/3=l 2/3Xq(+)xα・V0 F(-) l=:q()xα・vo ・2/3-l
2/3

Setting the bias voltage in this way is also effective for obtaining a good two-color image.

Next, a second embodiment of the present invention will be described with reference to FIGS. 5 and 6.

As shown in FIG. 5, in this embodiment, each LED of the light source 16
The lighting of (L, ~Ln) is controlled by a control circuit 44 using a microcomputer, and an image information output means such as a scanner or a computer (not shown) is connected to the control circuit 44. .

Further, the memory in the control circuit 44 stores in advance a program for controlling the lighting of each LED (L, -Ln>) of the light source 16, based on the image information from the image information output means and the stored program. , each L of the light source 16
E D ('L + ~L n ) is lit.

Next, the lighting control of each LED (L, -Ln) of the light source 16, which is performed by an interrupt routine caused by the rise of a synchronization pulse, will be explained according to the flowchart in section 6.

First, increment the counter N (step 1
00). Next, when the image information from the image information output means is neither image signal A (for example, black signal) or image signal B (for example, red signal), that is, when it is the background, the surface potential is VO /2 of the light source 16 for the time T1
The second LED is turned on, and the electrostatic latent image carrier surface 12, A is turned on.
is positively exposed (steps 102 and 104).

Next, when the image information from the image information output means is not a background, and when the image information from the image information output means is an image signal B (for example, a red signal), the time T1+T2 when the surface potential becomes almost 0 Steps 106 and 108) turn on the Nth LED of the light source 16 and expose the electrostatic latent image carrier surface 12A to negative light.

Further, the image information from the image information output means is the image signal B
If not, that is, in the case of image signal A, the LED is not lit and the surface potential does not change from vO.

The above control is performed for N LEDs (step 110
．． 112) In synchronization with the rotation of the electrostatic latent image carrier 12, the exposure control for the next pixel line is started.

As a result, the surface potential of the image signal A portion of the electrostatic latent image carrier surface 12A remains at vO, the surface potential of the background portion becomes vO/2, and the surface potential of the image signal B portion becomes approximately O. .

〔Effect of the invention〕

Since the present invention has the above-described configuration, it has an excellent effect that stable printing quality can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing main parts of an image forming apparatus according to a first embodiment of the present invention, FIG. 2 is a schematic perspective view showing a light source of an image forming apparatus according to a first embodiment of the present invention, and FIG. FIG. 4 is a block diagram showing a drive circuit for a light source of an image forming apparatus according to a first embodiment of the present invention; FIG. 4 is an eight-power signal diagram showing lighting control of a light source of an image forming apparatus according to a first embodiment of the present invention; FIG. 5 is a block diagram showing a drive circuit for a light source of an image forming apparatus according to a second embodiment of the present invention, and FIG. 6 is a flowchart showing lighting control of a light source of an image forming apparatus according to a second embodiment of the present invention. Figure 7 is a graph showing the relationship between the print signal and potential, Figure 8 is a graph showing the relationship between the exposure time potential of the electrostatic latent image carrier, and Figure 9 is the print signal when the bias level is set to VO/3. Figure 10 is an explanatory diagram of the regular development process, Figure 11 is an explanatory diagram of the reversal development process, and Figure 12 is an explanatory diagram of the two-color printing process involving regular development and reversal development. , Fig. 13 is an explanatory diagram of development in the case of "-like charging potential>bias voltage", Fig. 14 is an explanatory diagram of development in the case of "-like charging potential > bias voltage", and Fig. 15 has different charge amounts. FIG. 3 is an explanatory diagram of two-color development using two-color toners. 10... Image forming device, 12... Electrostatic latent image carrier, 14... Charger, 15 ・ 16 ・ 18 ・ 19 ・ 20 ・ 22 ・ 24 ・・Surface potential sensor/light source, ・Developer,・Bias voltage change circuit, ・Pre-transfer charger, ・Transfer charger, ・Paper.

Claims (3)

[Claims] (1) A first charging means for uniformly charging the surface of the electrostatic latent image carrier, and a light source for forming an electrostatic latent image on the surface of the electrostatic latent image carrier charged by the first charging means. , a developing means for developing the electrostatic latent image formed by the light source with charged toner under a bias voltage applied state; a bias voltage changing means for changing the bias voltage of the developing means; An image forming apparatus comprising: a transfer means for transferring a developed toner image on an electrostatic latent image carrier to an image receiving material. (2) The bias voltage changing means includes a surface potential detecting means for detecting the potential on the surface of the electrostatic latent image carrier, and a bias voltage changing circuit that increases the bias voltage as the surface potential increases. Claims characterized (
1) The image forming apparatus described above. (3) a first charging means for uniformly charging the surface of the electrostatic latent image carrier; and a first charging means for uniformly charging the surface of the electrostatic latent image carrier; a signal processing means for converting and outputting a signal distinguishable from the background; and a first color and a second color on the surface of the electrostatic latent image carrier charged by the first charging means based on the output of the signal processing means. and one light source for forming an electrostatic latent image with three potential levels corresponding to the background, and the electrostatic latent image formed by this light source is charged to different polarities under a bias voltage application state. a developing means for attaching and developing toners of two colors; a bias voltage changing means for changing a bias voltage so that the electrostatic force of the first toner and the electrostatic force of the second toner are equal; and the developing means. an image forming apparatus comprising: a transfer means for transferring a two-color toner image deposited on an electrostatic latent image carrier onto an image-receiving material;