JPH087490B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Field of the Invention The present invention forms an electrostatic latent image on the surface of an image carrier by an electrophotographic method or an electrostatic recording method, and develops the electrostatic latent image with a developing device. The present invention relates to an image forming apparatus that forms a toner image.

2. Description of the Related Art Conventionally, various electrophotographic or electrostatic recording type image forming apparatuses have been proposed and are commercially available. However, a typical electrophotographic type image forming apparatus, that is, an electrophotographic copying apparatus shown in FIG. An example is shown.

The electrophotographic copying apparatus is provided with a drum-shaped photosensitive member, that is, a photosensitive drum 10, which is an image bearing member having a photosensitive material on its surface.

The photosensitive drum 10 rotates in the direction of arrow K, and the primary charging device 11
Are uniformly charged by. Photosensitive drum uniformly charged
The surface of 10 is irradiated with image exposure 12 to form an electrostatic latent image, and then the latent image is developed by a developing device 13 to form a visible toner image. The toner image is transferred onto the transfer material 16 by the transfer device 15, fixed by a fixing device (not shown), and then carried out of the apparatus.

On the other hand, the photosensitive drum 10 that has completed the transfer process is cleaned by a cleaning device 17 to remove foreign matter such as transfer residual toner and paper dust attached to the surface thereof, and is further subjected to pre-exposure 18 to remove residual charge on the surface, and then Prepare for the image formation cycle.

As shown in FIG. 3, the developing device 13 has a developer carrying member, in which a magnet roller 31 is arranged, a developing sleeve 30 in this example. The developing sleeve 30 is rotated at a predetermined distance from the photosensitive drum 10, carries the developer on the surface thereof, and conveys the developer toward the photosensitive drum 10. As a developer,
A one-component magnetic developer in which silica or the like is externally added to magnetic toner,
A non-magnetic toner, a carrier, and a two-component developer in which silica or the like is mixed are used.

A developing bias is applied between the developing sleeve 30 and the surface of the photosensitive drum 10. The developing bias includes a type in which a DC component is superimposed on an AC component, an AC component only, or a DC component only type.

In any of the developing methods, the toner has a charge of a predetermined polarity, and the electrostatic latent image formed on the surface of the photosensitive drum 10 is developed with light and shade according to the contrast of the potential of the latent image.

A characteristic curve showing such a relationship between the electrostatic latent image potential and the density of the toner image is called a γ curve, and as shown in FIG. 4, the horizontal axis indicates the development contrast (electrostatic latent image potential and The difference amount from the DC component of the developing bias), and the vertical axis is a graph showing the density of the developed toner image.

Generally, the DC component of the developing bias is set to a voltage between the latent image potential of the image bright area (original white background area) and the latent image potential of the image dark area (original black background area), and toner adheres to the image dark area. ,
The toner does not adhere to the bright image area.
For the reason described below, the development contrast of the dark area of the image (the potential difference between the DC component of the development bias and the dark area of the image) is set to a value lower than the toner density saturation point of the γ curve to form an image. It is common.

First, since the chargeability of the photosensitive drum (the easiness of uniformly charging the surface of the photosensitive drum) is low, the potential of the image dark portion cannot be made sufficiently high.

Secondly, when the development is performed with the development contrast in the dark portion of the image being equal to or higher than the toner density saturation point of the γ curve, the toner development becomes excessive, and the image quality such as the scattering of the image and the tailing is deteriorated in the transfer process and the fixing process. The phenomenon of lowering occurs.

Thirdly, in a copying machine in which the gradation of the intermediate density is emphasized, the development is intentionally performed in the region where the gradient of the γ curve is small.

SUMMARY OF THE INVENTION However, the conventional image forming apparatus represented by the electrophotographic copying apparatus having the above-described structure has the following drawbacks.

The toner contained in the developing device or carried on the developing sleeve has a distribution with a certain width in terms of particle size and specific charge of the toner, and the toner is carried in the developing process. Toner particles that are easily developed and toner particles that are difficult to develop are mixed on the body. Toner particles that are difficult to develop are toner particles having a large particle diameter and toner particles having a small specific charge. Toner that is difficult to develop is not developed on the image carrier unless a development contrast equal to or higher than the toner density saturation point of the γ curve is given.

Therefore, when image formation is performed at a development contrast below the toner concentration saturation point as in the prior art, toner particles that are difficult to develop are accumulated inside the developing device or on the developing sleeve as the durability of the developing device advances. The developing characteristics of the developing device change greatly.

FIG. 5 shows changes in the γ curve before and after the endurance in the conventional developing device. It is understood from FIG. 5 that the toner concentration developed in the dark area of the image is lowered as the durability is advanced. Hereinafter, such development will be referred to as "a phenomenon in which the toner concentration is reduced by selective development of toner (toner particles that are easily developed are preferentially developed and consumed)".

As a conventional technique aiming at solving such a problem, a configuration as described below has been proposed.

That is, an electrostatic latent image having the same potential as the image dark area in the image area is formed outside the image area, and this is developed in the image area under the same developing bias condition as that applied to the developing sleeve.
A band-shaped toner image is formed.

By performing solid black development in this way, to a certain extent,
Although it is possible to suppress the change in the γ curve, the effect is not perfect with such a conventional technique, and in order to bring out the effect to some extent, it is necessary to form a belt-shaped toner image with a width of 10 mm or more. It had a number of drawbacks, such as problems, toner scattering, and overload on the cleaning device.

As described above, in the conventional image forming apparatus, a sufficient development contrast can be obtained for reasons such as insufficient chargeability of the image carrier, prevention of toner image scattering, and improvement of gradation of intermediate density. Therefore, there is a drawback that when the durability of the developing device progresses, a "phenomenon of lowering toner concentration due to selective development of toner" occurs. Also, the prior art for eliminating this drawback has many drawbacks.

Therefore, an object of the present invention is to provide an image forming apparatus capable of preventing toner particles existing in a developing device or on a developer carrying member, which are difficult to develop, from being discharged onto the image carrying member to prevent a density reduction phenomenon due to selective development. Is to provide.

Another object of the present invention is to provide an image forming apparatus having a plurality of developing devices, and even when environmental changes are applied, a phenomenon such as a decrease in density does not occur due to progress of durability, and is always stable. An object of the present invention is to provide an image forming apparatus capable of obtaining an image of the desired quality.

Means for Solving the Problems The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus that performs development by forming a potential difference between an image carrier that carries an electrostatic image and a developer carrier that carries and moves a developer. An image forming apparatus is characterized in that development is performed in an image area with a development contrast having a higher development density than the development contrast of the image area.

Embodiment Next, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

In this embodiment, a case where the present invention is embodied in the electrophotographic copying machine described with reference to FIG. 1 will be described.
Further, FIG. 2 shows an operation sequence of each part for carrying out the present invention.

In this embodiment, the construction and operation of the electrophotographic copying apparatus are as described above, but according to the present invention, the primary charging device 11
This is different from the conventional device in that an erase exposure device 20 is provided between the developing device 13 and the developing device 13.

The erase exposure device 20 irradiates the surface of the photosensitive drum 10 with erase exposure at an appropriate timing to erase the electrostatic latent image outside the image area.

Further, the operation of the copying apparatus having the above-mentioned structure according to this embodiment will be described.

Referring also to FIG. 2, when the image forming operation start signal is input, the photosensitive drum 10 starts rotating in the direction of the arrow, and at the same time, the erase exposure 20 is turned on and the DC component of the developing bias is also output. To be done.

Before entering the image area, the primary charging device 11 operates to start primary charging, and uniformly charges the surface of the photosensitive drum 10 to + 400V. However, until the image area is entered, the above erase exposure is performed.
Since it is lit 20 times, the potential on the surface of the photosensitive drum 10 is removed to + 20V. Further, since the developing bias DC component is set to + 150V, the toner is not developed on the photosensitive drum 10 outside the image area.

In the image area, the erase exposure 20 is extinguished, and at the same time, the image exposure 12 is projected on the surface of the photosensitive drum 10, and the image dark area potential is +.
At 400V, the image bright area potential becomes + 50V. At the same time, the DC component of the developing bias is set to + 100V, the developing contrast in the dark area of the image becomes + 300V, and the latent image becomes a visible toner image according to the pattern of the electrostatic latent image on the photosensitive drum 10. Erase exposure is turned on again outside the image area, and the DC component of the developing bias becomes +150 V, preventing toner from adhering to the photosensitive drum 10.

According to the present invention, the erase exposure is performed in a part outside the image area.
Is turned off and the DC component of the developing bias is set to -50V to develop with a developing contrast of + 450V, for example, a band-shaped toner image with a width of 2 mm is formed.

As described above, according to the present invention, a belt-shaped toner image is formed on a portion outside the image area with a higher contrast than the development contrast within the image area, thereby developing sleeve.
31 Toner particles that are hard to develop and remain on the photosensitive drum 10
Since the toner particles are transferred to the upper side and discharged from the developing device, it is possible to prevent toner particles which are difficult to be developed from being accumulated in the developing device and the developing sleeve.

According to the present embodiment, it is only necessary to form a belt-shaped toner image having a width of 2 mm, which is not found in the conventional developing device without causing adverse effects such as toner consumption, toner scattering, and overload on the cleaning device. It was possible to completely prevent a decrease in concentration due to durability.

In the above embodiment, the width of the belt-shaped toner image formed outside the image area is 2 mm, the potential of the photosensitive drum 10 is +400 V, the developing bias DC component is −50 V, and the developing contrast is +450 V. The width of the toner image and the size of the development contrast can be variously changed by selecting appropriate conditions depending on the type and configuration of the developer and the developing device, and the same effect can be achieved. it can.

Next, a modified embodiment of the present invention capable of achieving the same effect as the above embodiment will be described below.

Modified Example 1 In the above-described example, by changing the DC component of the developing bias when forming the belt-shaped toner image outside the image area,
Although the development contrast is increased, when a photosensitive drum having a sufficient chargeability is used, the same effect can be obtained even if the development contrast is increased by changing the primary charge amount.

For example, when the primary charging device 11 is a scorotron corona charger at a place where a belt-shaped toner image is formed outside the image area, the potential applied to the discharge wire of the device is increased, or the charge amount control grid is changed. If the surface potential of the photosensitive drum 10 is set to + 550V by increasing the applied voltage of, the DC component of the development bias remains + 100V and the development contrast is 450V.
Can be changed to.

Modified Example 2 In the above-mentioned Example and Modified Example 1, the case where the present invention was carried out by the sequence of forming a belt-shaped toner image each time one image was formed was described. A belt-shaped toner image outside the area may be formed.

In other words, when a plurality of images are formed by one start switch operation, that is, when the same document is copied a plurality of times, one strip toner image is formed per one start switch operation, Alternatively, a belt-shaped toner image can be formed every several sheets. In the latter case, for example, if a belt-shaped toner image is formed every five sheets, when continuously copying 20 sheets, the belt-shaped toner image is formed after the first, sixth, eleventh, and sixteenth image formation. To do.

When using a developing device in which the selective development phenomenon is not remarkable, a belt-shaped toner image may be formed every time the start switch operation is performed several times in order to avoid unnecessary toner consumption.

Modified Example 3 In the above-described Example and Modified Examples 1 and 2, the present invention was carried out for an image forming apparatus having only one developing device 13, but a plurality of developing devices were provided. The present invention can also be applied to an image forming apparatus.

As described above, the degree of the selective development phenomenon depends on the configuration of the developing device and the type of developer. Therefore, in an image forming apparatus equipped with a plurality of developing devices, if a belt-shaped toner image outside the image area is formed with a developing contrast, width and sequence suitable for the characteristics of each developing device, each developing device can be optimized. Can be used.

Modified Example 4 The degree of progress of the density reduction phenomenon due to the selective development of the developing device differs depending on the difference in the environment in which the durability is performed. For example, in a high-temperature and high-humidity environment, the development characteristic is deteriorated, that is, the inclination of the γ curve is reduced, so that the phenomenon of selective development becomes more remarkable.

Therefore, the band-shaped toner image has a large development contrast and width, and the sequence is configured so that the band-shaped toner image is frequently formed, so that the toner particles that are difficult to develop are more positively discharged onto the photosensitive drum. It is better to take

Further, since development characteristics increase in a low temperature and low humidity environment, it is suitable that the development contrast, width and band-shaped toner image formation frequency of the band-shaped toner image are small.

Therefore, if the developing contrast, the width and the sequence of the belt-shaped toner image are changed according to the temperature and humidity of the environment, it is possible to always discharge the toner which is difficult to be developed in the developing device under the optimum condition regardless of the environmental change.

Further, by providing an environment temperature and humidity detecting means and automatically controlling the development contrast, width and sequence of the band-shaped toner image by the output thereof, the above-mentioned effect can be obtained without the need for operation of the changeover switch. . Of course, it goes without saying that a greater effect can be obtained by using a combination of the two.

Although the above-described embodiment and each modified embodiment have been described by taking the mode in which the present invention is embodied in the electrophotographic copying apparatus as an example, the present invention is also applicable to image forming apparatuses using various other image carriers and image forming processes. The invention may be similarly embodied.

As described above, the image forming apparatus according to the present invention is
Since the belt-shaped toner image is formed outside the image area with a development contrast larger than the development contrast in the image area, the toner particles that are difficult to develop existing in the developing device and on the developer carrying body are not developed on the image carrying body. It is possible to prevent the density decrease phenomenon due to selective development.

The present invention also provides a developing contrast of a belt-shaped toner image,
Even in an image forming apparatus equipped with a plurality of developing devices, by adapting the width, the sequence of formation timing, and the like to the characteristics of the developing device and changes in characteristics due to environmental changes,
Further, there is an advantage that it is possible to always obtain an image of stable image quality without causing a phenomenon such as a decrease in density due to progress of durability even when an environmental change is added.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an electrophotographic copying machine which is an embodiment of the image forming apparatus according to the present invention. FIG. 2 is an operation sequence diagram of each part in the apparatus of FIG. FIG. 3 is a schematic sectional view of the developing device. FIG. 4 is a graph showing a γ curve showing the developing characteristics of the developing device, particularly the developing contrast and the density of the toner image developed with the developing contrast. FIG. 5 is a graph showing changes in the γ curve before and after the durability of the developing device having the conventional structure. 10: image carrier 11: primary charging device 12: image exposure 13: developing device 20: erase exposure device 30: developer carrier (developing sleeve) 31: magnet roller

Claims (1)

[Claims] 1. An image forming apparatus for performing development by forming a potential difference between an image bearing member carrying an electrostatic image and a developer bearing member carrying a developer and moving, in a non-image area on the image bearing member. An image forming apparatus characterized in that development is performed with a development contrast having a higher development density than the development contrast of the image area.