JPH03267972A - Image forming device - Google Patents

Abstract

PURPOSE:To improve potential convergency by supplying a small electric current to a transferring means so that an electric field applied from an electrifying means to an image support at the time when the electrifying means passes through the region of the image support corresponding to the transferring means is made higher than an electric field applied in the other region. CONSTITUTION:While an image support 1 and a transferring means 2 are in direct contact with each other, a small electric current for obtaining the optimum transfer voltage is supplied to a transferring means 2 so that an electric field applied from an electrifying means 3 to the support 1 at the time when the means 3 passes through the region of the support 1 corresponding to the means 2 is made higher than an electric field applied in the other region. AC voltage superposed on DC is increased in the region at the time of electrification, potential convergency is improved and uniform electrification is performed. The occurrence of unevenness in density and sanding phenomenon due to defective electrification is prevented and noise generated by electrification can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Object of the Invention (Field of Industrial Application) The present invention relates to an image forming apparatus using an electrostatic transfer process, such as an electrostatic copying machine and a printer thereof.

(Prior art and problems to be solved) A toner image is formed on the surface of the image bearing member, which is uniformly charged by the charging means, and is equipped with a charging means and a transfer means that come into contact with a moving image bearing member, through an image exposure and development process. is formed, and when the toner image arrives at a transfer site formed as a contact portion between an image carrier and a transfer means such as a transfer roller that comes into contact with the image carrier, supplying a transfer material to this transfer site, and
2. Description of the Related Art Image forming apparatuses have already been proposed in which a toner image on an image carrier is transferred to a transfer material by applying a transfer bias to a transfer means.

FIG. 11 is a side view of essential parts schematically showing a typical example of such an image forming apparatus.

The surface of an image carrier (referred to as a photoreceptor) (1), which has an axis perpendicular to the plane of the paper and rotates in the direction of arrow a, is connected to a high-voltage power source 11.
After being uniformly charged by a charging roller 3 connected to a charging surface 4, this charging surface is irradiated with an image signal such as an image-modulated laser beam or slit exposure to form an electrostatic latent image.

As the photoreceptor 1 rotates, this electrostatic latent image reaches a development site where the photoreceptor 1 and the developing device 6 face each other, and is developed by the charged toner supplied from the developing device 6 to become a toner image.

When the photoconductor 1 further rotates and this toner image reaches the transfer site where the photoconductor 1 and the transfer roller 2 come into contact and are formed, the transfer material P is moved from the conveyance path 7 to the transfer site at the same timing. At the same time, a transfer bias having a polarity opposite to that of the toner is applied to the transfer roller 2 by the power supply 114, and due to the action of the electric field formed, the photoreceptor 1
The side toner image is transferred to the transfer material.

Thereafter, the transfer material is separated from the photoreceptor 1 and conveyed to a fixing site (not shown), and residual toner that has not been transferred to the transfer material at the transfer site is removed by a cleaner 8 and the photoreceptor 1 is removed.
is ready for the next process.

An image forming apparatus with such a configuration requires a lower power supply voltage than one using a well-known corona discharger.
Although it has various advantages such as strong support of the transfer material, less transfer misalignment, and less generation of ozone, it also has the following problems.

To briefly explain this, first, when using contact charging means such as a charging roller as the charging means, the charging means is charged by superimposing a constant current controlled alternating current bias on a constant voltage controlled direct current bias to increase the charging surface potential. ■ a given dark potential. A method to converge has been proposed.

In such a case, there is an inflection point in the peak value V, p (peak-to-peak voltage) of the AC bias as shown in FIG. It is known that local charging failures occur even at +200V below the inflection point.

This is because when charging is performed by discharge between the photoreceptor and contact charging means, partial charging failure occurs due to the surface roughness of the charging means, and minute uncharged areas are formed. This is noticeable in low temperature and low humidity environments.

Particularly in the case of reversal development, there is a problem in that uncharged portions as described above are developed and appear on the image (called sandy areas), resulting in deterioration of image quality.

For this reason, the voltage normally applied is set to an appropriate high potential in consideration of the above to avoid the occurrence of the above situation, and also to cope with changes in the characteristics of the charging means due to environmental changes. Furthermore, changes in V and p are compensated for by an alternating current bias that is controlled by a constant current.

Regarding contact type transfer means such as a transfer roller, it has been difficult to constantly apply an appropriate transfer bias due to variations in manufacturing characteristics and fluctuations due to environmental changes.

In response to this, the present applicant uniformly charges the surface of the photoreceptor, first controls the bias applied to the transfer means at a constant current, and maintains the generated voltage at the time of primary transfer. We proposed a method of constant voltage control (called ATVC).

By this method, a remarkable effect was observed from the viewpoint of always performing transfer with the optimum bias, regardless of variations in the resistance value of the transfer means, fluctuations due to the environment, size of the transfer material, etc.

However, as described above, when the ATVC method is applied to the surface of the photoreceptor that has been subjected to -order charging, the following problems occur.

In other words, when a constant current is passed through the photoreceptor during constant current control in order to obtain the transfer bias voltage, the surface potential of the photoreceptor decreases significantly, and if sufficient convergence is not achieved in the next charging step, charging failure will occur. This causes problems such as sandy areas appearing in the areas where the current flows, and the contrast between the dark potential and the bright potential being smaller than other areas (which appears as density unevenness in the image).

In order to avoid such a situation, it may be possible to set Vpp high in advance, but if Vpp becomes high (Vpp becomes high), the charging noise due to vibration between the charging means and the photoreceptor becomes louder and becomes harsher, and it becomes especially quiet. This method cannot be put to practical use in laser printers, etc., which require high performance.

The present invention has been made to cope with the above-mentioned situation, and in an apparatus that performs the ATVC method, the Vpp of the AC voltage superimposed on the DC voltage is set to be thicker than when charging other areas (and the potential is increased). In addition to improving convergence and ensuring uniform charging, it also prevents uneven density and sandy areas due to poor charging, and there is virtually no effect on charging noise, resulting in stable and good transfer performance regardless of the environment. The object of the present invention is to provide an image forming apparatus that can provide the following.

(2) Structure of the invention (technical means for solving the problem and its operation) In order to achieve the above object, the present invention includes an image carrier, a charging means that comes into contact with the image carrier and charges the surface uniformly, In an image forming apparatus including a transfer means disposed in contact with the image carrier for transferring the toner image formed on the surface of the image carrier onto a transfer material, the image carrier and the transfer means are in direct contact with each other. a means for passing a minute current through the transfer means in order to obtain an optimum transfer voltage; It is characterized by making the electric field stronger than that in other areas.

With this configuration, it is possible to prevent the occurrence of density unevenness and sandy area phenomenon due to poor charging, and to obtain an image forming apparatus in which charging noise is not bothersome.

(Description of Embodiments) FIG. 1 is a schematic side view of main parts showing the configuration of an image forming apparatus suitable for applying the present invention, which has an axis perpendicular to the plane of the paper and rotates in the direction of arrow a. The surface of a traveling cylindrical OPC photoreceptor 1 is uniformly negatively charged via a charging roller 3 by a power source 4 that is connected to a controller 9 and performs DC constant voltage and AC constant current control.

Thereafter, this charged surface is scanned with an image-modulated laser beam in the case shown in Figure 1 to form an electrostatic latent image, and the latent image is
When the photoreceptor 1 and the developing device 6 reach the opposing development area,
Toner is supplied from the developing device 6 to form a toner image.

On the downstream side of the development area when viewed in the traveling direction of the photoreceptor 11, there is a transfer area formed as a pressure nip between the photoreceptor 1 and the transfer roller 2 that is in pressure contact with the photoreceptor 1, and the toner image is transferred to the transfer area. When the transfer material P arrives, the transfer material P is supplied from the conveyance path 7 to the transfer site in synchronization with the toner image, and a transfer bias is applied to the transfer roller 2 by the power source 4 for performing ATVC, so that the transfer material P is transferred to the photoreceptor side. The toner image is transferred to the transfer material.

Thereafter, the transfer material P carrying the toner image is separated from the photoconductor l and conveyed to a fixing site (not shown), and residual toner that has not been transferred to the transfer material at the transfer site is removed by a cleaner 8, and the photoconductor l is removed by a cleaner 8. The state is now ready for the Hagi process.

The above device will be explained in more detail.

As the photoreceptor, a device that is negatively charged and performs reversal development was used.

The diameter of the photoreceptor is 30φ, the process speed is 30IllI
l/sec, and the diameter of the transfer roller is 20φ.

The transfer roller 2 has a volume resistance of 10a to xo12ΩcI1 by dispersing filler such as ZZnO35no in EPDM.
1, and the hardness is 20-40° (Asker-C hardness)
A controlled sample was used.

ATVC is performed during the pre-rotation and between the sheets, and print 1
A transfer bias is set for each sheet, so constant current control is performed between sheets during the pre-rotation, and the voltage generated at this time is maintained.

In the illustrated device, the resistance value of the transfer roller is 10' to 1010.
The constant current value of ATVC was set to about 5 μA.

When using a transfer roller with the above resistance value, if the constant current value is higher than this value, there is a possibility that positive memory will be formed on the photoconductor or that the image quality will deteriorate due to the transfer material being penetrated during transfer. If it is less than 3μ8, transfer defects will occur.

The charging roller 3 is configured to rotate following the photoreceptor 1, and has a 10φ core metal wound with carbon-dispersed EPDM, and a medium resistance layer of hydrin rubber having a volume resistivity of 108 to 2Ωc111 provided on the surface thereof. The outer diameter was formed to 16φ.

As mentioned above, using such a charging roller, the dark potential on the photoreceptor I is set by constant voltage control. ■ By setting and superimposing AC constant current control, the charging potential can be controlled regardless of the environment.

It is converging to.

As shown in FIG. 3, charging failure occurs when the alternating current value is less than 300 μA, so it was set to 300 μA.

The voltage generated at this time is approximately 1800 to 2200 V depending on the variation in the resistance value of the charging roller, but this value is within the range that performs good charging and does not leak the photoreceptor layer, and the charging frequency at this time f is 200Hz
It is.

By the way, as mentioned above, ATVC is applied to this −order charged region.
When constant current control is performed, a positive charge having a polarity opposite to that on the surface of the photoreceptor is applied, resulting in a decrease in surface potential. If there are variations in the resistance value in the circumferential direction and the longitudinal direction of the transfer roller 2, which is the transfer means, the electric charge will be concentrated in the part with the lower resistance value, and a positive memory will be formed on the photoreceptor, leading to the primary charging process. A situation occurs in which a predetermined dark potential V0 cannot be secured, and this appears as density unevenness, black spots, etc. on the image.

FIG. 4 shows the relationship between the constant current value of ATVC and the alternating current value of primary charging required to converge the potential to VD. As shown in the diagram, if a positive charge with the opposite polarity to the charging is not injected, If the ATVC current value is zero, the above 300μ
The generated voltage V pp when A is flowing is sufficient for the potential ■. converges to.

However, when a positive constant current is applied to perform ATVC, the potential convergence gradually deteriorates, and when the voltage is about 7 μA, it becomes impossible to recover with one charging even if a constant current of 400 μA is applied.

In the device shown in the figure, ATV
Since we are performing C, it takes about 3 to converge to the specified VD.
It turns out that 30 μA is required.

That is, by applying the voltage generated during the above-mentioned constant current control to the region where a constant current flows, it is possible to prevent charging defects, sand spots, etc. that tend to occur in the ATVC constant current region.

FIG. 5 shows the sequence in the above case.

As shown in the figure, in the region where a positive current is passed through the photoreceptor in ATVC, the alternating current component of the -order charging is increased to improve potential convergence.

By the way, in a device that scans a charged surface with an image-modulated laser beam as an electrostatic latent image forming means, the laser beam is irradiated during pre-rotation or between sheets in order to correct the intensity of the laser beam. A system that corrects the light intensity for each print (referred to as APC) has been proposed.

6A and 6B show the sequence in which the present invention is applied in such a case, in which FIG.
When performing TVC, Figure 6B shows APC followed by ATVC.
This is the sequence when performing.

As shown in the figure, in both cases ATVC is executed at a time when APC is not performed;
This is because the surface of the photoreceptor is irradiated with laser light by C and the potential of that portion is lowered, so if the positive current of the ATVC flows into that part, the low potential easily causes positive memory.

As explained above, in an apparatus that uses a charging roller as a contact charging means and a transfer roller as a contact transfer means, by increasing the voltage V□ of the AC component of primary charging only in the constant current region of ATVC, It is possible to prevent the occurrence of transfer defects, sandy areas, etc., and also to suppress charging noise to a level that is virtually unnoticeable.

Next, a description will be given of an apparatus having the same configuration as that shown in FIG. 1 and having a process speed of 60 mm/see.

As the process speed increases, the constant current value of the ATVC and the charging frequency f applied to the charging roller also change, and the amount of charge per unit area of the charged photoreceptor surface is approximately twice that of the first embodiment. There must be.

For this reason, in this embodiment, a constant current of 10 μA
TVC was carried out, and as a result, a generated voltage almost the same as that at 5 μA in FIG. 2 of the above embodiment was obtained. The resistance value of the transfer roller at this time is the same as that of the first embodiment.

The value of the charging frequency f largely depends on the process speed,
The potential is V when it is below a certain value. Since it does not converge and uneven charging occurs, it is necessary to increase this value.The relationship between the two shows an almost linear change, so in this example, f is set to 400H.
It was set to z.

A: As the process speed increases, the paper gap becomes shorter.
It is difficult to find time to conduct TVC and APC (because it becomes
It is preferable to perform ATVC only during the previous rotation of the PC between each paper during the previous rotation, and in the case of continuous printing, to continuously apply the voltage detected during the previous rotation.

In the case of this embodiment, constant current control of ATVC is performed for one rotation of the transfer roller, and fluctuations in the generated voltage due to resistance unevenness in the circumferential direction of the roller can be suppressed as much as possible.

FIG. 7 shows the sequence of the above device.

As the charging frequency increases, the impedance decreases (because of the relationship Z = 1/2πfC, so the AC constant current value also changes.
By increasing the current to 660 μA in some cases, the charging sound did not become harsh, it was possible to prevent transfer defects and sandy areas, and it was possible to obtain high-quality images.

In the illustrated sequence, ATV (:) is performed after APC, but it goes without saying that ATVC may be performed either before or after APc.

Above, an apparatus using a charging roller as a charging means and a transfer roller as a transfer means has been described.
It goes without saying that the transfer means can also be applied to belts or other contact type means.

FIG. 8 shows still another embodiment of the present invention, which is a system for reversal development using a negatively charged photoreceptor l, in which a blade-shaped charging blade is used as the charging means and a transfer belt is used as the transfer means. I am using it.

The surface of the photoreceptor 1 is uniformly negatively charged by a charging blade 83 connected to a power source 4 which is controlled by a controller 9 to control DC constant voltage and AC constant current, and is then irradiated with an image signal 5 by a laser beam to become static. A latent image is formed.

Negatively charged toner is supplied from the developing device 6 to this latent image to form a toner image, and when this reaches the transfer site where the photoreceptor 1 and the transfer belt 81 suspended between rollers 84 and 85 come into contact, the timing The transfer material P is conveyed to the transfer site together with the photoreceptor l and the transfer belt 81.
A transfer bias is applied by the power source 4 controlled by the controller 9 to the electrode roller 82 facing the electrode roller 82 via the transfer bias, and the toner image on the photoreceptor side is transferred to the transfer material.

After the transfer, the transfer material is conveyed by a belt 81 to an unspecified fixing site, residual toner remaining on the photoreceptor is removed by a cleaner 8, and the next image forming process begins.

Next, the above device will be explained in more detail.

The charging blade 83 has a 2 mm thick conductive layer made of urethane rubber with a volume resistance of 10"ΩCff1, and a 30 μm thick conductive layer made of N-methylmethoxylated nylon with a volume resistance of 109 Ωcm as a medium resistance layer for uniform charging. The photoreceptor 1 is in contact with the photoreceptor 1 in the counter direction.

The transfer belt 81 is made of a difluorinated fluororesin with polar groups such as secondary amino groups and hydroxyl groups, and has a thickness of 150 μm and is made of a resin with conductivity and a volume resistivity of 10 12 to 10 14 ΩcIffl. .

The electrode roller 82 disperses carbon and has a volume resistance of 10
Urethane rubber adjusted to 10' to 10' ΩCIn was wound around a 6φ core metal to a thickness of 2ffiI11 to give a diameter of 10ψ and a hardness of 40° (Asker-C).

In this example device, the process speed is 901tlII.
l/sec, and the charging frequency f was set to 600 Hz.

In addition, in order to obtain good transfer performance in ATVC, the electrode roller 8
It was found that the current flowing through the ATV 2 is about 15-20 μA, so with this device, a constant current of 15 μA is required for the ATV.
When C was carried out, a voltage almost the same as that obtained in the case of a constant current of 5 μA with the apparatus of the first embodiment was obtained, and good transferability could be obtained.

Furthermore, it has been found that there is not much difference between the charging roller and the charging blade in the alternating current value that influences the -order charging alternating current component.

This is because experiments have already revealed that the charging on the surface of the photoreceptor is mainly due to Paschen discharge1.
In the case of electrical discharge between rotating curved surfaces, it has been confirmed that when the gap between the two is in the direction of widening, in other words, the electrical discharge in the latter half of the pressure nip between the two greatly contributes to charging. Therefore, it is thought that this is because there is no significant difference in the portion that substantially contributes to charging whether the charging member is a roller or a blade.

In the device shown, typically 1 mA, ATV
Constant current control was performed at 1.1 mA in the C4l region. Also,
Since the process speed is fast, ATVC was performed during the pre-rotation, and only APC was used between sheets.

The sequence is the same as that shown in FIG. 7, so it has been omitted.

Using the above-mentioned device, there is no disturbing charging noise, no density unevenness due to poor transfer, no sandy areas, etc.
Good transferability was obtained in each environment.

FIG. 9 is a schematic side view of an image forming apparatus showing still another embodiment of the present invention.

This device also uses a system in which the photoreceptor is negatively charged and reverse development is performed using negative toner, and the process speed is 60ffI.
III/seC.

This embodiment of the apparatus is equipped with a variable duty ratio oscillator 10 for changing the duty ratio of the AC voltage applied to the charging roller 3 as a means for strengthening the electric field in the ATVC constant current region on the photoreceptor. Other than that, this device has the same structure and function as the device shown in the first figure, and a description of the common parts will be omitted.

Furthermore, since the process speed was 60 mm/sec, the ATVC constant current value and charging frequency were set to 1oILA and 400Hz, respectively, as in the case of the apparatus shown in Figure 2.

When changing the duty ratio without changing the frequency of the AC voltage applied to the charging roller, the convergence of the potential changes depending on the effective value of the applied bias.

In Figure 10, dark potential ■. = -600V charged surface, 10
This figure shows the relationship between potential convergence and changes in the duty ratio (a:b in the figure) of the AC voltage applied to the charging roller when ATVC is performed at μA.

As shown in the figure, when the effective value is increased in the direction of increasing the potential, the convergence improves, and in the case shown, a / b =
It can be seen that it generally converges when it is 2 or more.

Therefore, when charging in the ATVC constant current region, the duty ratio (a/b) of the AC constant current applied to the charging roller 3 is
) was set as 4.

As a result, we tested the effects using the sequences shown in Figures 6A, 6B, and 7 above, and found that sufficient potential convergence was obtained even when the AC constant current was 300 μA.
It was confirmed that good images could be obtained without an increase in charging noise, poor transfer, or occurrence of sandy areas.

It goes without saying that the device of this embodiment is also effective for other contact type charging and transfer means such as belts and blades.

(3) As described in detail, according to the present invention, in an image forming apparatus equipped with a charging means and a transfer means configured to come into contact with a photoreceptor such as a roller or a blade, there is a difference in the environment for the transfer means. In order to obtain the optimum transfer voltage, a minute current is passed through the transfer means, and when the charging means passes through a region of the image bearing member corresponding to this, the electric field extending from the charging means to the image bearing member is made smaller than other regions. By increasing the strength of the voltage, the potential convergence can be improved, thereby preventing the occurrence of sandy ground phenomenon without increasing the charging noise to the point where it becomes annoying, and preventing charging failures and the resulting separation between the ATVC area and other areas. This effectively eliminates the unevenness in image density that occurs between images, and has a remarkable effect on obtaining high-quality images.

[Brief explanation of drawings]

FIG. 1 is a schematic side view of an image forming apparatus showing a first embodiment of the present invention, FIG. 2 is a graph showing the relationship between the ATVC constant current value of the transfer means and the generated voltage, and FIG. 3 is an AC of the charging means. A graph showing the relationship between the constant current value and the generated Vpp, FIG. 4 is a graph showing the relationship between the ATVC constant current value and the primary charging AC constant current value that allows potential convergence, and FIG. The operation sequence of the example device when APC is not performed. Figure 6A is the sequence of the same device as above when APC is performed and before that ATVC is performed. Figure 6B is the sequence of the same device as above when APC is performed and then FIG. 7 shows the sequence when performing ATVC in the device of the first embodiment,
The sequence of the second embodiment with different process speeds,
FIG. 8 is a schematic side view of a device according to a third embodiment of the present invention, FIG. 9 is a schematic side view of a device according to a fourth embodiment of the present invention, and FIG. A graph showing the relationship between electron convergence. FIG. 11 is a schematic side view showing the configuration of a known image forming apparatus. FIG. 12 is a graph showing the relationship between primary charging AC voltages V and p and potential convergence. This is a graph to explain. 1... Image carrier (photoreceptor), 2... Transfer roller, 3
... Charging roller, 4... Power supply for charging and transfer, 5...
・Image exposure, 6...Developer, 7...Transport path, 8...
Cleaner, lO... variable duty ratio oscillator, 81.
...Transfer belt, 82...Back electrode roller, 83...
・Charged blade. i ÷ Figure 1 Act tAc 8 ωA] 2 Figure 6A Figure (V) 1 Figure 6B, -7 τ type 79 diagram. 0. Hito also has an audience 81Σ Soldier 11 figure Tsuta 12 figure

Claims (3)

[Claims] (1) An image carrier, a charging means that comes into contact with the image carrier and charges the surface uniformly, and a charging device that is arranged in contact with the image carrier to transfer the toner image formed on the surface of the image carrier to a transfer material. an image forming apparatus including a transfer means, comprising: means for passing a minute current through the transfer means to obtain an optimum transfer voltage while the image bearing member and the transfer means are in direct contact with each other; An image forming apparatus characterized in that when the charging means passes through a region on the image bearing member that has been charged, an electric field from the charging means to the image bearing member is made stronger than that in other regions. (2) The image forming apparatus according to claim 1, wherein when charging the area on the image carrier through which a minute current is passed, the peak value of the AC voltage applied to the charging means is larger than that of the other area. . (3) The image forming apparatus according to claim 1, wherein the duty ratio of the AC voltage applied to the charging means is changed when charging the area on the image carrier through which a minute current is passed.