JPH09204109A - Wet image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform uniform and stable discharge with a small current without causing rapid increase of an electric current between a photoreceptor and an electric field roller, to improve operability by relaxing the gap control between the photoreceptor and the roller, to improve the durability of a discharging surface and to prolong the service life. SOLUTION: An electric field roller 6 facing adjacently a photoreceptor drum 4 consists of a conductive core and a plasma thermal spray film 2 formed on the surface of the core. The plasma thermal spray film 12 consists of, for example, two components of plasma thermal spraying material such as titanium oxide (TiO2 ) and alumina (Al2 O3 ) whose component ratio is reqrlated. By increasing the film thickness of the plasma thermal spray film 12, the thermal spay film 12 has an enough resistance as a resistor during discharge so as to suppress the increase of electric current by this resistance. On the other hand, the increase of insulating property caused by that the film made thick in thickness can be suppressed by incorporating titanium oxide (TiO2 ).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wet type image forming apparatus such as an electrostatic printer using liquid toner as a developer.

[0002]

2. Description of the Related Art In a wet image forming apparatus using a liquid toner as a developer, in order to transfer a liquid toner image without crushing it,
It has been proposed that a minute gap is provided between the toner image carrier (photoreceptor) and an electric field roller is provided, and a discharge current is passed through the minute gap to obtain a sharp image. The flow of the discharge current strengthens the bond (cohesive force) between the surface of the photoconductor and the toner particles and between the toner particles, and as a result, image crushing can be avoided.

If the discharge current is too large, the transfer conditions will be disadvantageous (high transfer voltage is required), and if it is too small, a locally crushed toner image will be formed (the reason is that the glow-like discharge light causes a local dark portion to occur). It is considered that there is a portion that does not discharge locally because it is visible). Therefore, in order to obtain a sharp image, it is necessary to generate a stable and uniform glow-like discharge with a small current in the minute gap. Ultimately, an ideal discharge mode is one in which the image is discharged only in the image area and not in the background area with the minimum necessary current. As is well known, a technique for obtaining a stable discharge in a minute gap has been established in a decharging / charging process in a conventional image forming apparatus, and a number of methods in which a discharge electrode is composed of a resistor are proposed (Japanese Patent Laid-Open No. 62-296
174, JP-A-2-256077, JP-A-5-107866, etc., but all of these purposes are to uniformly charge an object to be charged (photoreceptor) to a predetermined potential, and Since a large current is required in terms of characteristics, it is difficult to apply it as a stable discharge technique for obtaining a sharp image in a wet image forming apparatus.

In view of such a situation, the conventional wet image forming apparatus is constructed such that an electric field roller having a hard alumite film as a resistor is used and a voltage is applied through a protective resistor.

[0005]

However, in the prior art as described above, the gap distance cannot be increased unless a voltage is applied via a protective resistance of several MΩ or more,
Further, if the gap distance is inclined even slightly (non-parallel), one side becomes a dark part and discharge unevenness occurs, so that there is a problem that the degree of freedom in manufacturing accuracy is narrow and a high skill is required for assembly. Also, when the electric field in the gap becomes large,
There was also a problem that the discharge current suddenly increased.

According to the present invention, the control of the gap can be eased and the handling is easy, and the stable and uniform glow discharge can be maintained with a small current, and the discharge current sharply increases even if the electric field in the gap becomes large. It is an object of the present invention to provide a wet-type image forming apparatus that has a discharge surface that is excellent in durability and longevity.

[0007]

DISCLOSURE OF THE INVENTION In order to find a clue for solving the problems of the prior art, the inventors of the present invention have tried to find a clue for solving the problems, and a discharge voltage when an electric field roller having a hard alumite coating is used.・ The experiment of current characteristics was conducted. As shown in FIG. 6, the stainless metal 20 is used instead of being mounted on an actual device in which the electric field roller is opposed to the photoconductor and discharged.
A circuit for measuring the discharge voltage V _d and the current I _d, which is used as the electrode surface and faces the electric field roller 22, is used. The characteristics of the electric field roller having a hard alumite coating (strictly speaking, the discharge voltage / current characteristics with the protective resistance of the hard alumite coating interposed) were measured using the gap as a parameter, and the results are shown in FIGS. Although the voltage at which the current starts to flow (discharge start) differs depending on the discharge gap, it becomes almost linear as the voltage increases. At this time, the uniform glow discharge light in the gap was confirmed and measured. Since the discharge current may contain many harmonics, the average value current used for measuring the direct current is displayed. Therefore, when recording is performed by detecting the current, a capacitor of approximately 1 μF is connected in parallel with the detection resistor. Therefore, the analog average value ammeter and the recorded data showed the same value.

From the graphs of FIGS. 7 and 8 in which the voltage is applied through the protective resistors R of 12 MΩ and 100 MΩ, respectively, the ratio (ΔV _d / ΔI _d of the increments ΔV _d and ΔI _d of the straight line is obtained.
= R _i , referred to as incremental resistance) indicates a slope, and this slope has the same value as the protection resistance R. In addition,
The sampling position of the incremental resistance value R _i is as shown in FIG.
20μA site at 5μm, Figure 8 shows 5 at gap = 75μm
μA site. From this fact, it is considered that the gap and the resistance value of the hard anodized film behave like a conductor during discharge. The gap behaves as an insulator up to the discharge start voltage, and when the applied voltage is higher than that, the increase in the voltage is consumed for carrier transport like a Zener element. The thickness of the hard alumite coating used here is 34 μm,
The results of measuring the resistance of the coating with the conductive adhesive are shown in the graph of FIG. As is apparent from the graph, the resistance value depends on the polarity and also on the applied voltage.
Thus, the volume resistance value of hard anodize is about 10 ^10.
Despite the high resistance value of Ω-cm or more, it is unknown why it becomes a conductor during discharge, but due to this phenomenon,
It has been conventionally considered that the discharge characteristic cannot be regulated by the volume resistivity value of the coating material described in patent publications and the like. Therefore, the protective resistance is interposed. Even if the hard alumite coating is made thicker to eliminate the protective resistance, the resistance of the coating increases, but stable discharge is performed with a large current similar to that used in the decharging / charging process, Unstable under current (especially in high humidity environment).

FIG. 10 shows a measurement method in which a conductive adhesive is applied as in FIG. 9, and the composition ratio of titanium oxide (TiO ₂ ) is 10%.
% Of three types of plasma sprayed film (175 μm,
It is the graph which displayed each volume specific resistance voltage dependence of 220micrometer, 320micrometer. From the graph, it can be seen that the volume specific resistance value changes by three digits depending on the film thickness and the applied voltage.
This is natural because we are dealing with thin films.

In view of the above experimental facts, the present invention suppresses a sudden increase in current by making the resistance value of the coating exist during the operation function, that is, at the time of discharging, and improves the durability and prolongs the service life. The aim is to achieve both at the same time. Specifically, in the invention according to claim 1, the resistance value of the coating at the time of discharge is made uniform by increasing the thickness of the coating, and the insulation due to the increasing thickness is adjusted by the inclusion of titanium oxide (TiO ₂ ), This suppresses a sharp increase in current, and improves the durability and extends the life (which can also be obtained by reducing the current) due to the high hardness peculiar to the plasma sprayed coating containing titanium oxide. According to a second aspect of the present invention, in the configuration of the first aspect, the plasma sprayed film includes at least two plasma sprayed materials of titanium oxide (TiO ₂ ) and alumina (Al ₂ O ₃ ) and the composition of each component. It is formed by adjusting the ratio, and the incremental resistance value R _i = ΔV _{d at} the discharge current I _d above the discharge start point and the applied voltage V _d .
/ A [Delta] I _d and a control means for operating within a 200Keiomega～600keiomega, adopts a configuration that.

According to a third aspect of the present invention, in the structure of the first aspect, the plasma sprayed film having a relatively low resistance of 10 MΩ or less is obtained under the condition that the diameter of the electric field roller is 20 mm and the axial length of the discharge surface is 50 mm. The electric field roller having the configuration adopts a configuration in which a discharge current flows through a fixed resistor when a voltage is applied. According to a fourth aspect of the invention, in the configuration of the first aspect, when the incremental resistance value is low, a fixed resistor of at least 5 MΩ or more is connected and operated. According to a fifth aspect of the invention, in the configuration of the first aspect, the plasma sprayed film is titanium oxide (TiO ₂ ) and alumina (Al ₂ O ₃ ).
Titanium oxide (TiO 2)
₂ ) the composition ratio is set to 2% or more and 20% or less, and the thickness of the plasma sprayed film is set to 50 to 450 μm,
The configuration is adopted.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. In the wet image forming apparatus 2 shown in this embodiment, as shown in FIG. 1, an electric field roller closely facing each other with a minute gap g between the photosensitive drum 4 and the surface of the photosensitive drum 4. 6 and a power source 8 controlled so as to apply a voltage to the electric field roller 6 to generate a discharge in the minute gap before transferring the developed toner image to the surface of the photosensitive drum 4. Note that other image forming elements are omitted.

As shown in FIG. 2, the electric field roller 6 comprises a core portion 10 made of a conductive material and a plasma sprayed film 12 formed on the surface of the core portion 10. In FIG. 2, the thickness of the plasma sprayed film 12 is exaggerated with respect to the size of the core 10. The plasma sprayed film 12 is
It contains titanium oxide (TiO ₂ ), or contains two or more components of plasma spray material of titanium oxide (TiO ₂ ) and alumina (Al ₂ O ₃ ) and the composition ratio thereof is adjusted. Desirably, titanium oxide is used. The composition ratio of (TiO ₂ ) is 2
% And 20% or less, and the film thickness is appropriately selected within the range of 50 to 450 μm (claims 1, 2 and 5).

3 and 4 are experimental graphs showing the discharge characteristics of the plasma sprayed film 12 at a specific titanium oxide composition ratio and film thickness, which are measured using the minute gap g as a parameter. Further, the voltage from the power source 8 is directly applied without passing through the protection resistor R. Figure 3 shows titanium oxide (TiO ₂ )
4 has a composition ratio of 10% and a film thickness of 175 μm, and FIG. 4 shows a composition ratio of titanium oxide (TiO ₂ ) of 10% and a film thickness of 220 μm. In both figures, the sampling position of the film resistance (incremental resistance) R _i is very small. It is a 10 μA site with a gap g = 75 μm.
In FIG. 3, the incremental resistance value R _i = ΔV _d / ΔI _d = 200 _v /
10 μA = 20 MΩ, and in FIG. 4, the incremental resistance value R _i = ΔV _d / ΔI _d = 400 _v / 10 μA = 40 MΩ
It has become. These incremental resistance values R _i are estimated by analogy from the characteristic diagram of the hard anodized coating, and the plasma sprayed coating 12
It is considered to be the resistance value of. In the case of FIG. 3, that is, when the film thickness is small, it can be seen that the incremental resistance value R _i becomes smaller as the applied voltage becomes higher, and the voltage dependence becomes remarkable. The electric field roller 6 used in this embodiment has a diameter of 20 mm and the discharge surface has an axial length of 50 mm.

As described above, when an electric field roller having a hard alumite coating is used, unless a voltage is applied through a protective resistance of several MΩ or more, bright spot-like non-uniform discharge occurs, so that a minute gap g is formed. Although it could not be made large (100 μm or more), in the case of the present embodiment, without a protective resistor,
It is possible to obtain a uniform glow discharge in a gap region of 100 μm or more. Further, in the case of a hard alumite coating, if the minute gap is intentionally tilted, discharge unevenness in which one side becomes a dark part occurs, but in the case of this example without a protective resistance, a uniform discharge that cannot be visually judged. I was able to get the status. As a result, the management of minute gaps when mounted on an actual machine is relaxed, and manufacturing can be facilitated and quality can be made uniform (reliability).

In the case of a hard alumite coating, FIG.
And as is clear from FIG. 8, the minute gap g is 150 μm.
The above characteristics near the discharge start voltage are steep gradients, the incremental resistance is smaller than the protective resistance, and unstable non-uniform discharge (visual confirmation is possible). However, in the case of FIGS. 3 and 4 according to the present invention, A uniform discharge light can be confirmed even at a current of 1 μA level with a gentle gradient. It is considered that this is because the difference in film thickness causes a difference in phenomenon even if the volume resistivity is almost the same order as that of alumite. 1μ near the discharge start point of the plasma sprayed film 12
The reason why the minute current of A level is not greatly affected by the applied voltage is not clear, but it seems that it depends on the structure and surface property of the film body.

As a result of the implementation test, titanium oxide (TiO 2
₂ ) and alumina (Al ₂ O ₃ ), the plasma sprayed film 12 has a smaller resistance value as the amount of titanium oxide increases.
At 0% or more, it was close to a conductor. The composition ratio of titanium oxide is 5
If it is less than or equal to%, it becomes like an insulator, but if the film thickness is thinly adjusted by polishing, discharge is possible, and the limit is 2%. When the plasma sprayed film 12 is used, the discharge is stable with a small current.
The value of A was lowered to 5 to 30 μA, and a uniform sharp image was obtained. In the forced running test, it was confirmed that the life was 10 times or more due to the reduced current and the high hardness peculiar to the plasma sprayed film. The life is determined when the glow discharge light changes to a bright spot discharge light,
The life at the time of peeling the film was set as the limit.

Further, as shown in FIG. 1, a control means 14 for controlling the voltage is provided, and the wet image forming apparatus 2 is controlled by the control means 14 at the discharge current I _d and the applied voltage V _d above the discharge start point. It is also possible to maintain the incremental resistance value R _i = ΔV _d / ΔI _d within the range of 200 KΩ to 600 KΩ (Claim 2).

FIG. 5 shows an embodiment corresponding to claims 3 and 4. The difference from the above embodiment is that a fixed resistor 16 (5 MΩ or more) is provided. This is a configuration for dealing with the case where the thickness of the plasma sprayed film 12 is small and the weight of the conductor behavior during discharge is high. Even in this case, the durability and the extension of the life of the electric field roller 6 are not impaired by the inclusion of titanium oxide (TiO ₂ ).

[0020]

As described above, according to the present invention, the resistance value of the coating film at the time of discharge is made to exist by thickening the plasma sprayed film, and the insulation due to the thickening (of the resistor) is made of titanium oxide. Since the composition is adjusted by the content of (TiO ₂ ), it is possible to suppress the rapid increase of the current, relax the gap management by the uniform discharge of the minute current, and further, the plasma sprayed film containing titanium oxide. Due to the unique high hardness, it is possible to improve durability and extend the service life.

[Brief description of drawings]

FIG. 1 is a schematic view of a wet image forming apparatus according to an embodiment of the present invention with a part thereof omitted.

FIG. 2 is a sectional view of an electric field roller.

FIG. 3 is a graph showing discharge characteristics of a plasma sprayed film.

FIG. 4 is a graph showing discharge characteristics of a plasma sprayed film.

FIG. 5 is a schematic view of a wet image forming apparatus according to another embodiment with a part thereof omitted.

FIG. 6 is a schematic diagram of a discharge characteristic measuring circuit.

FIG. 7 is a graph showing discharge characteristics of a hard alumite coating.

FIG. 8 is a graph showing discharge characteristics of a hard alumite coating.

FIG. 9 is a graph showing the volume resistivity voltage dependence of a hard alumite coating.

FIG. 10 is a graph showing the volume resistivity voltage dependence of a plasma sprayed coating containing titanium oxide.

[Explanation of symbols]

 4 Photoreceptor 6 Electric Field Roller 10 Core 12 Plasma Sprayed Film g Small Gap

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Goo Ikeda 1-3-6 Nakamagome, Ota-ku, Tokyo ・ Inside Ricoh Co., Ltd.

Claims (5)

[Claims] 1. A toner image developed on the photoconductor, comprising a photoconductor on which a toner image is developed, and an electric field roller closely facing the photoconductor surface with a minute gap therebetween. In order to strengthen the bond between the photoreceptor and the toner, the wet type image forming apparatus applies a voltage to the electric field roller to generate discharge in the minute gap before transfer, and the electric field roller is a core which is a conductor. And a plasma sprayed film containing titanium oxide (TiO ₂ ) formed on the surface of the core portion, a wet image forming apparatus. 2. The plasma sprayed film is formed by containing at least two plasma sprayed materials of titanium oxide (TiO ₂ ) and alumina (Al ₂ O ₃ ) and adjusting the composition ratio of each component. Discharge current I _d above the discharge start point
And the incremental resistance value R _i = ΔV _d / ΔI at the applied voltage V _d
The wet image forming apparatus according to claim 1, further comprising control means for operating _d within a range of 200 KΩ to 600 KΩ. 3. The diameter of the electric field roller is 20 mm, and the axial length of the discharge surface is 5
2. An electric field roller having a plasma sprayed film having a relatively low resistance with an incremental resistance value of 10 MΩ or less under a condition of 0 mm, discharge current is caused to flow through a fixed resistor when voltage is applied. Wet image forming apparatus. 4. At least 5 if the incremental resistance is low.
The wet image forming apparatus according to claim 1, wherein a fixed resistor having a resistance of MΩ or more is connected to operate. 5. The plasma sprayed film is made of titanium oxide (Ti).
O ₂ ) and alumina (Al ₂ O ₃ ) plasma spray material, the composition ratio of titanium oxide (TiO ₂ ) is set to 2% or more and 20% or less, and the film thickness of the plasma spray film is 50%.
The wet image forming apparatus according to claim 1, wherein the wet image forming apparatus has a thickness of about 450 μm.