JPH035585B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a developing device using one-component magnetic toner. Various developing devices using one-component magnetic toner are known, but they can be classified into two types: one uses low-resistance magnetic toner, which is developed using electrostatic induction, and is transferred to processed paper, and the other uses insulating magnetic toner, which uses triboelectric charging. There are two main types of methods: applying a charge and transferring it to plain paper. However, the former has the disadvantage of using treated paper, and the latter has the disadvantage of deterioration over time due to frictional electrification.
Therefore, attempts have been made to use magnetic toner with relatively high resistance, apply electric charge to the toner by electrostatic induction, polarization, injection, etc., and perform electrostatic transfer onto plain paper. However, with this method, only the image quality, especially the peripheral part of the image, is disturbed, so-called poor sharpness, and the reproducibility of low densities and fine lines is poor. In order to overcome this drawback, Japanese Patent Application Laid-open No. 55-73058,
As disclosed in JP-A-55-138766 and JP-A-56-14268, using two developing rolls,
Attempts have been made to improve image sharpness and low density reproducibility by once overdeveloping and then removing toner adhering to non-image areas. The inventors of the present invention have conducted extensive studies on a developing device using the two developing rolls described above (hereinafter referred to as a two-roll developing device), and have found that the settings of each developing roll have a large effect on image quality. . In other words, with the conventional configuration in which two developing roll devices are arranged, each controlling the developing bias or the toner layer potential on the developing roll using an injection electrode, etc., the fog density is reduced, but the reproducibility of low density is poor. It was confirmed that it was not possible to obtain sufficient performance in terms of sharpness, etc. In view of the drawbacks of the two-stage roll developing device described above, the object of the present invention is to provide a two-stage roll developing device that is capable of reducing fog density, as well as providing image quality with excellent low density reproducibility, fine line reproducibility, and excellent sharpness. A roll developing device is provided. An object of the present invention is to provide a developing device that develops an electrostatic latent image formed on a recording medium using first and second magnetic brush rolls using magnetic toner, in which the moving speed of the recording medium is set to Vp, When the speed of the second magnetic brush roll is Vu and the speed of the second magnetic brush roll is Vl, Vu and Vl are
are 2 to 7 times higher, respectively, and Vl/Vu is 0.5 to 1.0.
and the packing density (dT/dp; dT is the trimming gap, dp is the distance between the photoreceptor drum and the developing roll) of the toner on the second magnetic brush roll is within the range of 0.25 to 0.50. This can be achieved by a magnetic one-component developing device characterized in that the packing density of the first developing roll is set to be higher than the packing density of the second developing roll so as to perform excessive development. The present invention will be described in detail below with reference to the drawings. FIG. 1 shows a conventional developing device using one developing roll, FIG. 2 shows the relationship between surface potential and bias potential, and FIG. 3 shows developing characteristics when using one developing roll. In FIG. 1, 1 indicates a photoreceptor. 2 is a developing device, which consists of a magnet roll 4 fixed in a rotating cylindrical non-magnetic sleeve 3, and an injection electrode 5 which also serves as a toner layer thickness adjusting member, and which can be set to any voltage by a bias power supply shown in 6. is applied. When using the apparatus shown in Fig. 1 to develop the original image with a low contrast potential shown in Fig _. 2 (the figure shows a latent image potential of 300 V for an original density of 0.2), The bias voltage shown indicates the development bias level (determined by the toner development start voltage) necessary to suppress the fog density of the non-image area (BKGD) to 0.01 or less. On the other hand, the development bias level required to reproduce a low original image with sufficient density is indicated by _V2 (-200V in the figure).
Therefore, since there is no overlapping range of the developing bias potential required to satisfy fog density and low density reproduction,
After all, if the bias voltage is set while suppressing the fog density, only developing characteristics with poor density reproduction on the low density side can be obtained, as shown in FIG. Therefore, a two-stage roll development method has been proposed with the aim of satisfying both fog density and low density reproducibility. The basic concept of the two-stage roll development method is considered to be a two-stage development process of over-development and pick-up, but as a result of investigation, it was found that the upper stage, the improvement of image sharpness, the improvement of low-density reproducibility, and the suppression of fog density are compatible with each other. It was confirmed that it greatly depends on the condition of the lower toner brush and the roll speed. In particular, it has been found that the setting conditions of the lower developing roll have a large influence on the above-mentioned image characteristic values in terms of forming the final image. FIG. 4 shows a conventional two-roll developing device. Two independent developing devices, an upper developing device 2 and a lower developing device 7, are provided. Reference numerals 6 and 11 indicate bias power supplies applied to the upper and lower developing rolls, respectively. Regarding the lower developing device 7, if you change the settings for the trimming gap _dT and the distance _dP between the photosensitive drum 1 and the developing roll (non-magnetic cylindrical sleeve 8), what will happen to the image quality? We checked to see if it affected. Figure 5 shows the trimming gap d _T
(corresponding to the thickness of the toner brush) and the distance _dP between the photoreceptor drum and the developing roll. In the area indicated by diagonal lines A (d _P 0.4, d _T 0.2), toner stagnation occurs in the developing nip, and most of the developed image visualized in the upper developing device is scraped off. . In addition, a developing method (touch-down) in which the distance between the photoreceptor drum and the developing roll is in contact has a limit in terms of maintainability. On the other hand, in the area indicated by the diagonal line B, the developed image overdeveloped by the upper developing device adheres not only to the non-image area (background area) but also to the low-density image where the adhesion force with the latent image is weak. The fog density was low because the toner in the image was picked up, but the image quality was poor in low density reproducibility. On the other hand, in the area indicated by the diagonal line C, the toner brush is weakly rubbed, so the toner that was forcibly attached to the background area is pulled back to the roll side by controlling the developing bias in the upper developing roll. As a result, only images with high fog density were obtained. In other words, maintaining the reproducibility of low-density images and suppressing fog density are
In addition to the polarity of the developing bias voltage (or control of the toner layer potential) and the selection of the applied voltage, the packing density of toner in the developing nip (indicated by d _T /d _P ) is an important factor. It turned out. In order to achieve both of the above image characteristics, it is necessary to maintain the trimming gap and the distance between the photosensitive drum and the developing roll within the range shown by straight lines A and B in FIG. It is not necessary experimentally and the d _T /d _P value is
It was confirmed that it was 0.25-0.5. Furthermore, what is important here, as mentioned above, is the packing density of the toner in the developing nip, in other words, the toner brush is very soft, and almost the most advanced toner lightly rubs the developed image developed on the upper roll. By doing so, it is possible to prevent disturbances at the edge of the image that are characteristic of one-component development. FIG. 6 is a schematic diagram showing an example of a developing device used to carry out the magnetic brush developing method of the present invention. In the figure, 1 is a photoreceptor, 2 is a first developing roll having a non-magnetic sleeve 3 and a magnet roll therein. Reference numeral 5 represents an electrode member that also serves as a trimming member, and 6 represents a bias power source. The second developing roll 7 has almost the same structure as the first developing roll, and is composed of a non-magnetic sleeve and a magnetic roll 9, and is also provided with a trimming electrode member 10 and a bias power source. The sleeves of the two developer rolls are each rotating counterclockwise, in a direction opposite to the photoreceptor. The electrostatic latent image formed on the photoreceptor 1 is first developed by the first developing roll 2. Magnetic toner is supplied from a toner container and moves on the non-magnetic sleeve by interaction between the non-magnetic sleeve 3 and the magnetic roll 4. The thickness of the magnetic toner layer on the non-magnetic sleeve 3 is regulated by the trimmer-cum-electrode member 5. This toner layer then advances to the development area. At this time, a voltage is applied to the trimmer-cum-electrode member 5 to apply the charge necessary for development and to regulate the toner layer thickness. A bias voltage having a polarity opposite to that of the latent image is applied to the magnetic sleeve 3 by a bias power supply 6, and the magnetic brush of the magnetic toner that has advanced to the development area develops the electrostatic latent image on the photoreceptor. This development is carried out as so-called overdevelopment, in which the toner is in almost a single layer and adheres even to the background portion of the image. After receiving the developing action by the first developing roll 2,
The toner image on the photoreceptor comes under the action of a second developer roll 7. The second developing roll adsorbs and scavenges the toner that has adhered to the background area by the first developing roll, so that a high quality image with high sharpness and no fog in the background area of the image is obtained. The bias voltage applied to the second developing roll 7 has a polarity opposite to that of the first developing roll 2, that is, the same polarity as the latent image. The magnetic toner whose layer thickness is regulated by the action of the trimming electrode member 10 advances to the second development area by the mutual rotation action of the non-magnetic sleeve 8 and the magnetic roll 9. At this time, the magnetic toner on the non-magnetic sleeve to which a bias voltage of the same polarity as the latent image on the photoconductor 1 is applied does not transfer to the photoconductor side, and the toner on the photoconductor 1 loosely adheres to the photoconductor. Transfer the toner to photoconductor 1.
It acts to transfer the magnetic material from the magnetic layer to the non-magnetic sleeve 8 side. This toner transfer is a transfer of toner attached to the image background part or the periphery of the image part of the photoconductor, and the toner image overdeveloped by the first developing roll is transferred to the image background part and the image periphery by the action of the second developing roll. The toner is adsorbed in the area of the image, giving a high quality image with no fog and high sharpness. The toner is returned from the lower developing roll 7 to the upper developing roll 2 by magnetically picking it up in the region of the trimmer-cum-electrode 10. Each developing roll has the same construction as that shown in FIGS. 1 and 4, and is arranged so that its rotational speed can be controlled independently. V _P indicates the rotation speed of the photoreceptor 1, Vu indicates the rotation speed of the upper developing roll 2, Vl indicates the rotation speed of the lower developing roll 7, and the rotation directions are indicated by arrows. Furthermore, the packing density of toner in the developing nip (d _T /
_dP ) is set to 0.5 for the upper developing roll and 0.3 for the lower developing rolls. The relationship between the rotational speed V _P of the photosensitive drum and the rotational speeds Vu and Vl of the upper and lower developing rolls was tested in terms of low density reproducibility, image edge disturbance, and density, and the results were as shown in Figure 7. Furthermore, it has been found that the speed and speed ratio of the upper and lower developing rolls greatly affect image quality. That is, the diagonal line A in Figure 7
In the area shown in , the image density was low and the edges of the image were disturbed when the speed of the upper developing roll was slow (Vu2V _P ). It is assumed that this is because the sliding speed of the toner brush is slow, so the developed image is poorly filled and the development efficiency is low. The area indicated by diagonal line B is when the lower developing roll speed is slow (Vl×
2V _p ), only images with disturbances at the edges of the image and high fog density can be obtained. On the other hand, regarding the speed ratio (Vl/Vu) of the upper and lower developing rolls,
In the area indicated by the diagonal line C, that is, when the roll speed Vl of the lower stage is faster than the roll speed Vu of the upper stage, fog density and disturbance of the image edges can be suppressed, but only images with poor low density reproducibility can be obtained. That is, it is presumed that if the rotational speed of the lower developing roll is high, the scavenge effect will be emphasized. In addition, the area indicated by the diagonal line D, that is, the speed Vu of the upper roll
On the other hand, when the lower developing roll speed Vl is slow, although low density reproducibility is satisfied, only very low quality images with high fog density and disturbed image edges are obtained. did. It was also confirmed that when the developing roll speeds Vu and Vl were 7 times or more the photoreceptor speed _VP , the toner on the sleeve was poorly conveyed and the toner was missing in stripes on the developing roll. As a result of further investigation, it was found that only within the range indicated by straight lines A and B, the fog density was low, the low density reproducibility was good, and an image with sharp edges could be obtained. In other words, the speed of the photoreceptor drum V _P
On the other hand, it was found that the speeds of the upper and lower developing rolls were in the range of 2 to 7 times, and the speed ratio Vl/Vu of the upper and lower developing rolls was in the range of 0.5 to 1.0. Regarding the speed and speed ratio of the upper and lower developing rolls, it has been confirmed that each has an optimal range as described above, but when observing the above effects on the shape of the developed image using a microscope etc., it is found that the upper roll has a Disturbances in the filling surface (density) and image periphery,
In particular, the speed of the toner brush has an effect on the protrusion at the rear end of the image, and for the lower roll, the developing force is suppressed by the developing bias, and its functions are to remove toner attached to the background area and to remove the toner from the rear end of the image. The problem lies in the correction (pickup) of the toner image that has been pushed out, and it is therefore presumed that the contact pressure and sliding speed of the magnetic brush are the keys. That is, the pick-up force including electrostatic, magnetic, and mechanical forces is proportional to the brush speed and contact pressure (substituted by d _T /d _P ). On the other hand, since the developed image, which is almost uniformly overdeveloped on the photoconductor, has different adhesion forces with the latent image depending on the background, low density, and high density areas, the toner adhering to the background is completely collected by the brush and the The toner that adheres to the density area is not re-adhered (recovered) to the toner brush, and only the peripheral area needs to be corrected, so it is necessary to control the speed of the lower roll, and the speed and speed ratio of the upper and lower developing rolls. was found to be an important parameter. As described in detail above, in the two-roll development method that performs two stages of development: overdevelopment and pick-up, in order to fully demonstrate the functions of the upper and lower development rolls, the bias voltage of each development roll must be and control of toner packing density in the development nip;
It was confirmed that the speed of each developing roll and the speed ratio are very important. According to the present invention, in a developing device in which two developing rolls are integrated, the speed of the upper and lower developing rolls is set to a speed that is 2 to 7 times (preferably 3 to 5 times) the speed of the photoreceptor drum. ,
In addition, the speed ratio Vl/Vu of the upper and lower developing rolls is
It has been confirmed that by setting the value between 0.5 and 1.0, it is possible to reduce the density of dust and achieve high-quality image reproduction with good density reproducibility and further suppressed image roughness. Examples of the present invention will be described below in comparison with comparative examples. Example Development was carried out under the following conditions. Photoconductor drum: Se-based photoconductor V _DDP /V _BKG =900/
100 Magnetic toner: 10 ¹⁵ Ω (8×10 ⁴ v/cm) average particle size 14μ
m Photoreceptor drum speed: 120mm/sec (20rpm) Development conditions

[Table] Comparative Example Among the developing conditions of the example, only the following conditions were different.

[Table] The image quality obtained in Examples and Comparative Examples is shown below.

[Table] From the above results, it is clear that copies with excellent image quality can be obtained by the present invention.

[Brief explanation of the drawing]

Fig. 1 is a schematic cross-sectional view of a developing device using one developing roll, Fig. 2 shows the relationship between document image density, surface potential, and bias potential using the device shown in Fig. 1 shows a developing curve using the apparatus shown in FIG. 4, FIG. 4 is a schematic cross-sectional view of a conventional developing apparatus using two developing rolls, and FIG. Distance between and developing roll d _P and trimming gap P _T
FIG. 6 is a schematic sectional view of an apparatus for explaining the method of the present invention, and FIG. 7 is a diagram showing the relationship between the speed Vu of the upper developing roll and the speed Vl of the lower developing roll using the apparatus shown in FIG. It is a relationship diagram. Codes in the figure: 1... Photosensitive drum; 2, 7... Developing device; 3, 8... Non-magnetic cylindrical sleeve: 4, 9
... Magnet roll; 5, 10 ... Trimming electrode; 6, 11 ... Bias power supply; d _T ... Distance between electrode rolls; d _P ... Distance between photoreceptor rolls; Vp...
Photosensitive drum rotation speed; Vu... Upper developing roll rotation speed; Vl... Lower developing roll rotation speed.

Claims (1)

[Claims] 1. In a developing device that develops an electrostatic latent image formed on a recording body using first and second magnetic brush rolls using magnetic toner, the moving speed of the recording body is set to Vp, and the speed of the first magnetic brush roll is set to Vp. speed
When Vu and the speed of the second magnetic brush roll are Vl, Vu and Vl are respectively 2 to 7 with respect to Vp.
times, Vl/Vu is 0.5 to 1.0, and the second
The packing density of toner on the magnetic brush roll (dT/dp; dT is the trimming gap, and dp is the distance between the photoreceptor drum and the developer roll) is
The packing density of the first developing roll is within the range of 0.25 to 0.50, and the packing density of the first developing roll is such that the packing density of the second developing roll is within the range of 0.25 to 0.50.
A magnetic one-component developing device characterized in that the packing density is set to be higher than the packing density of a developing roll.