JP3138085B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine or a printer of an electrophotographic system or an electrostatic recording system.
More specifically, a contact charging type transfer member is used as a transfer unit for transferring a visible image formed on an image carrier such as a photoreceptor or a dielectric to a recording material to contact the back surface of the recording material and apply a charge. Related to an image forming apparatus.

[0002]

2. Description of the Related Art Conventionally, for example, in an electrophotographic image forming apparatus or an electrostatic recording type image forming apparatus, a visible image (toner image) formed on an image carrier such as a photoconductor or a dielectric is printed on paper. As a transfer means for transferring onto a recording material such as, for example, a contact type transfer member such as a transfer roller or a transfer belt is brought into contact with the image carrier, and the recording material is inserted together with the toner image into a transfer area configured as the corresponding contact portion. A configuration has been proposed in which a transfer bias is applied to a transfer member to transfer a toner image on an image carrier to a recording material.

In this type of apparatus, in order to perform appropriate transfer, a transfer current flowing per unit area of the recording material at the time of transfer is set within a certain range irrespective of the resistance, width, etc. of the recording material. Must be applied to the recording material.

Generally, an image forming apparatus such as a copying machine is configured to be able to use a plurality of types of recording materials. Therefore, depending on the size of the recording material, an image carrier or a transfer member (transfer roller)
However, the area in which the sheet contacts the recording material also differs. In such a case, if the constant current control is performed while applying the transfer bias as described above, since the size of the recording material is different, the area where the transfer member directly contacts the image carrier is different.
It has been difficult to achieve the intended purpose of flowing a substantially constant current only to the recording material portion regardless of the type of the recording material. This problem is caused not only by the size of the recording material but also by the thickness and the material of the recording material.

In the above case, for example, when the size of the fed recording material is small and the contact area between the transfer roller and the image carrier is too large, a constant current is applied to the transfer means by constant current control. Even if the control is performed, the amount of charge applied to the recording material may be insufficient, which may cause a transfer failure, or may weaken the toner holding power of the recording material, causing image disturbance such as toner scattering. Conversely, if the size of the fed recording material is too large and the contact area between the transfer roller and the image carrier is too small, a constant current control may be applied to apply a constant current to the recording material. In some cases, the amount of charge applied to the recording material may be excessive, and the toner may be charged to a polarity opposite to the original charge polarity, resulting in image transfer loss.

In order to avoid such drawbacks of the constant current control, a method of controlling the transfer bias at a constant voltage can be considered immediately. However, the transfer roller, the roller used for the transfer belt, the belt, and the like are different depending on the material. However, its resistance value changes greatly depending on the environment, especially humidity,
Paper, which is most frequently used as a recording material, also has a large environmental dependency of its resistance, so that it is also difficult to perform stable transfer regardless of the recording material size, material, environment, and the like.

In order to eliminate these drawbacks, the constant current control is performed when the recording material is present in the transfer area so that the resistance of the transfer roller and the belt and the resistance of the recording material are changed at the same time depending on the environment. Means for performing constant voltage control with a voltage that is a constant multiple of the voltage that is sometimes held has been proposed in, for example, Japanese Patent Application Laid-Open No. 2-264278.

[0008] However, the following problems also arise in the case where the transfer bias is controlled by using both the constant current control and the constant voltage control.

For example, if the resistance value of the transfer roller is small,
The voltage value obtained (held) by the constant current control is small because it depends on the resistance value of the transfer roller. At this time, if the resistance value of the recording material is large, the bias voltage required for the constant voltage control is the sum of the partial pressure of the image carrier (for example, the photosensitive drum), the partial pressure of the transfer roller, and the partial pressure of the recording material. Therefore, the bias voltage naturally becomes large, but the bias voltage obtained at the time of the constant current control becomes small, and the required bias voltage cannot be obtained. As a result, the amount of electric charge necessary for performing good transfer is not applied to the recording material, resulting in poor transfer and image disturbance.

As is clear from the above description, conventionally, a change in the resistance of the roller and the image carrier due to the environment or the like is detected, and the voltage is determined on the assumption that the recording material adapted to the same environment is fed. Due to the adoption of the system, an appropriate voltage is applied to recording materials that are unfamiliar with the environment, high-resistance recording materials such as OHP sheets, and dry recording materials that have just been fixed once with a duplex copying machine. It was difficult to apply.

On the other hand, as disclosed in JP-A-3-213888, an image area is detected by detecting a voltage at a constant current and adding a constant voltage to the voltage or a value obtained by multiplying the voltage by a constant. There is proposed a method of performing constant voltage control by using. A control method for setting a current by a current limiter during the constant voltage control of the transfer roller is disclosed in
87380.

[0012]

However, the above-mentioned prior art has the following disadvantages.

In order to provide a certain range of charge density to the recording material even if the resistance value of the recording material changes due to the environment, the resistance value of the transfer roller needs to be higher than a certain value. From the upper limit to the lower limit of the resistance value (for example, 10 ⁹ to 10
^It must be within a narrow range such as ¹⁰ Ω · cm)
There is. It is very difficult to control the transfer roller so that its resistance value falls within a certain resistance range, particularly within a range of about 10 ⁸ to 10 ¹¹ Ω · cm, and the yield is extremely deteriorated. On the other hand, in order to increase the yield, it is necessary to make full use of a transfer roller having a lower resistance value.

When a transfer roller having a low resistance value is used, it is conceivable to set the current by using a current limiter as in the conventional example. However, the optimum current limiter value is set for the following reason. There was a disadvantage that it could not be set.

For example, in a conventional electrophotographic image forming apparatus, as shown in FIG. 18, the photosensitive drum 1 and the transfer roller 2 are arranged such that the paper width of the recording material 5 is reduced during the passage of the small-sized recording material 5. The photosensitive drum 1 and the transfer roller 2 are in contact on both outer sides. These contacts are shown in (b). As described above, since the photosensitive drum 1 and the transfer roller 2 are in contact with each other in the portion (b), a current also flows in these portions (b), and the current during constant voltage control is set by the current limiter. The optimal current value cannot be set.

That is, in the case of moisture-absorbing recording paper, its resistance value is considerably low. Therefore, as shown in FIG. 18, when the photosensitive drum 1 and the transfer roller 2 are in contact on both outer sides of the paper width,
Even when the recording paper size is different, the current amount is almost the same regardless of the recording paper size, so that the current amount per unit area changes by up to five times, and the effect of installing the current limiter Was gone.

Accordingly, an object of the present invention is to enable the transfer of a visible image having a high transfer efficiency to any recording material even if the resistance value of the transfer means slightly fluctuates, so that a sharp high-quality image can be constantly transferred. An object of the present invention is to provide an image forming apparatus which can be obtained.

[0018]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image carrier, developing means for developing an electrostatic latent image formed on the image carrier to form a visible image, and feeding the visible image to a transfer unit. A transfer member that contacts the back surface of the recording material to apply a charge to the recording material to perform transfer, and controls the transfer member at a constant current when there is no recording material between the image carrier and the transfer member. In an image forming apparatus that performs transfer by performing constant voltage control on a transfer member during transfer with a voltage obtained based on the voltage generated at this time, the transfer member is brought into contact with an image carrier, and a recording material is supplied to a transfer unit. The image forming apparatus is characterized in that the transfer member and the image carrier are not in contact with each other outside the recording material in the width direction when the paper is fed.

[0019]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram showing one embodiment of an image forming apparatus according to the present invention. The image forming apparatus includes a charging device for uniformly charging an image carrier, and a visible image (toner) formed on the image carrier. The present invention is applied to an electrophotographic image forming apparatus using a contact charging type charging device and a transfer device as transfer devices for transferring the image) onto a fed recording material.

As shown in the figure, the image forming apparatus has a photosensitive drum 1 as an image carrier which has an axis in a direction perpendicular to the plane of the drawing and is driven to rotate in a direction indicated by an arrow. , A roller type contact charging type primary charging device 8 for uniformly charging the photosensitive drum 1,
Image exposure means (only the light source 7 is shown) for forming an electrostatic latent image according to image information on the photosensitive drum 1 and develops the electrostatic latent image formed on the photosensitive drum 1 into a visible image Developing device 10 and a transfer member as a roller-type contact-charging type transfer device for transferring a visible image on photosensitive drum 1 onto recording material 5 fed from guide device 3 through a guide device (not shown). 2 (hereinafter abbreviated as a transfer roller), and a cleaner 11 for removing the developer remaining on the photosensitive drum 1 after the transfer of the visible image, and the like are sequentially arranged.

In this embodiment, the photosensitive drum 1 is a drum-type electrophotographic photosensitive member having a basic structure in which a negatively charged OPC photosensitive member is adhered to the outer peripheral surface of a conductive base layer made of aluminum, iron or the like. And a visible image (toner image) formed by the positively charged toner formed on the photosensitive drum 1.
Before the transfer reaches a transfer portion (area), which is a contact portion (nip portion) between the photosensitive drum 1 and the transfer roller 2, the transfer roller 2 is controlled by a constant current by a power supply 6 as a bias applying unit. At the same time when the recording material 5 arrives at the transfer site, a predetermined voltage of negative polarity is applied to the transfer roller 2 by the power source 6 (constant voltage control), and the electric field formed between the photosensitive drum 1 and the recording material 5 The toner image on the drum 1 is transferred onto the recording material 5.

Thereafter, the recording material 5 is separated from the photosensitive drum 1, and is advanced to a fixing portion (not shown) by a conveying means (belt) 4. On the other hand, the attached matter such as the remaining toner is removed from the photosensitive drum 1 by the cleaner 11 and is ready for the next image forming operation.

The photosensitive drum 1 has a diameter of 30 mm, the thickness of the OPC photosensitive layer is 25 μm, the relative dielectric constant is 3.0, and the photosensitive member surface potential is dark (dark portion) potential (solid black) V _D = −700.
V, potential V _CON at constant current V _CON = −100 V, and write (bright portion) potential V _L = −100 V.

The toner is a one-component magnetic positively charged toner, and has a tribo Q / M of up to +8 μc / at room temperature and normal humidity.
g, the amount of toner on the photosensitive drum after the solid black development is ~ 1.
0 mg / cm ² .

The maximum image width that can be copied by the apparatus is 2
The transfer roller 2 has a diameter of 20.0 mm, a width of 220 mm, and a process speed of 50 mm / sec.

The transfer roller 2 has a configuration in which foamed EPDM is integrally molded on a SUS core having a diameter of 8 mm, and the overall diameter is 20.0 mm as described above. Its resistance value is about 10 ⁹ to 10 ¹² Ω · cm in terms of volume resistance value. The hardness is ASCER-C (300g weight)
Approximately 30 to 40, and in the apparatus, a total pressure of 30 g is applied in the direction of the photosensitive drum 1 via coil springs of bearings at both ends. Further, the appropriate current at the time of solid black is I _MIN = −1
μA, I _MAX = −2.5 μA.

Due to the contact pressure of the transfer roller 2, when a small-sized recording material is passed, the photosensitive drum 1 and the transfer roller 2 on both outer sides in the width direction of the recording material have a gap substantially corresponding to the thickness of the recording material. In a non-contact state.

FIG. 2 is an enlarged sectional view showing the positional relationship between the photosensitive drum 1, the transfer roller 2 and the recording material 5 at the transfer portion when the small-size recording material 5 is passed through the transfer portion. Indicates that the gap G corresponding to the thickness of the recording material 5 is formed on both outer sides in the width direction of the recording material 5. As will be described later, the current amount in the region shown in FIG. 18B where the gap G is formed can be suppressed considerably lower than that in the region shown in FIG.

FIG. 3A is a schematic side view showing a state near the transfer site when the recording material 5 exists at the transfer site.
FIG. 2B is an equivalent circuit diagram of FIG. FIG.
As shown in (b), the voltage V applied from the power source 6 to the transfer roller 2 is divided into the partial pressures V _ROL (I) and V _PAP (I) of the transfer roller 2, the recording material 5, and the photosensitive drum 1, respectively. ),
And V _DRM (I). That is, V = V _ROL (I) + V _PAP (I) + V _DRM (I) holds.

FIG. 4 is a characteristic diagram showing the relationship between the voltage V applied from the power supply 6 and the current I to the transfer roller 2 in the state shown in FIG. 3A, and FIGS. 5, 6, and 7 respectively. _Partial pressure V _ROL of transfer roller 2 (I), partial pressure V _{PAP of} recording material 5
FIG. 2I is a characteristic diagram illustrating a relationship between a partial pressure V _DRM of the photosensitive drum 1 and a current I.

FIG. 8 is a characteristic diagram showing the relationship between the partial pressure V _PAP (I) of the recording material 5 and the current I when the type of the recording material and the leaving conditions are changed, and the characteristic curve A in FIG. The characteristic curve B shows the case where plain paper left under wet conditions is used, and the characteristic curve B shows the case where thick paper is left under low humidity conditions. These sheets show examples of the recording material exhibiting the lowest impedance and the recording material exhibiting the highest impedance among the recording materials used in the image forming apparatus of the present embodiment.

In the transfer step, the toner image on the photosensitive drum 1 is transferred to the recording material 5 by an electric field generated by the charge applied to the recording material 5, so that the transfer efficiency is determined by the charge density on the recording material. That is, it is necessary to apply a constant charge density to the area of the recording material corresponding to the area (image area) on the photosensitive drum where the toner exists. When the voltage applied to the transfer roller 2 is constant, regardless of the image ratio (the ratio of the toner image area to the image area), the surface potential of the transfer roller 2 and the image area (toner image area) potential on the photosensitive drum 1 (toner image area) Since the relationship between the solid black potential and the dark potential is constant, if the voltage applied to the transfer roller 2 is constant, the charge density applied to the recording material 5 corresponding to the image area is always constant.

Therefore, the charge density of the area on the recording material corresponding to the image portion and the current value when the front surface is solid black can be in one-to-one correspondence, and the correspondence with the voltage applied to the transfer roller at this time can also be taken.

FIG. 9 shows the relationship between the solid black current, that is, the current applied to the recording material 5 and the transfer efficiency. The transfer efficiency is lowered when the solid black current is too small or too large.
It can be seen that it is largest within a certain range. It is considered that the reason for this is that the toner is reversely charged by the excessive charge, and the transfer efficiency is reduced.

On the other hand, if the charge density given to the recording material corresponding to the image area other than the transfer efficiency is too small or too large,
It is known that inconvenience on an image appears.

First, when the charge density is too low, a sufficient charge for transferring the toner to the recording material is not given to the recording material or the toner image portion, and transfer failure occurs or the toner is lost in the dry recording material. A phenomenon of large scattering occurs. If the charge density applied to the recording material is too high, the toner is transferred to the recording material before the recording material comes into close contact with the photosensitive drum, and a phenomenon in which the toner scatters such as blurred characters appears.

That is, there is an optimum current range that can improve the transfer efficiency and prevent the scattering and bleeding. This range is the toner tribo Q / M
And the curvature of a transfer member such as a transfer roller and a transfer roller, the curvature of a photosensitive drum, and the like. FIG. 9 shows the range of the current optimal for this transfer as a region between the minimum value I _MIN and the maximum value I _MAX .

FIG. 10 shows the characteristics in the case where the surface potential of the photosensitive drum is varied in the relationship between the partial pressure V _DRM of the photosensitive drum 1 and the current I shown in FIG. Such a relationship is explained from the principle that when an electric field of a certain value or more is applied in the gap between the photosensitive drum 1 and the transfer roller 2, a discharge is generated and a current starts to flow rapidly. Characteristic curve A in the figure,
B and C show characteristics when the surface potential (before transfer) of the photosensitive drum 1 is 0 V, V _CON , and V _D , respectively. As described above, V _CON indicates the surface potential of the photosensitive drum during constant current control, and V _D indicates the surface potential when solid black.

Here, when the current is I ₀ , the divided voltage of the characteristic curve B is V ′ _DRM (I ₀ ), and the _divided voltage of the characteristic curve C is V _DRM.
Expressed in _{(I 0), V DRM (} I 0) -V 'DRM (I 0) = a V _D -V _CON.

FIG. 11 shows a state in which there is no recording material between the photosensitive drum and the transfer roller using a transfer roller having the same resistance and an OPC photosensitive drum having the same resistance and the same potential (solid black) (during constant current control). ), A state in which plain paper left in a high-humidity environment exists between the photosensitive drum and the transfer roller (at the time of constant voltage control), and a state in which thick paper left in a low-humidity environment exists between the photosensitive drum and the transfer roller 7 shows voltage-current characteristics measured during (at the time of constant voltage control). The characteristic curve in the figure is the VI characteristic in a state where there is no recording material, the characteristic curve is a state where a high humidity plain paper is present, and the characteristic curve is a state where a low humidity thick paper is present.

That is, the characteristic curve shows the lowest resistance value among the recording materials used in the image forming apparatus of this embodiment.
The characteristic curve is V- when the one with the highest resistance value is used.
It is an I characteristic. Here, the partial pressure of the recording material of the characteristic curve at the time of _IMAX , which is the maximum value of the current value at which the optimum transfer can be performed, is C ₂ , and the minimum value of the current value at which the optimum transfer can be performed, I2
The partial pressure of the recording material characteristic curve when the _MIN and C _1, I
Voltage V _MAX characteristic curve when the _MAX, the voltage characteristic curve when the I _MIN and V _MIN. The voltage of the characteristic curve at the time of I _MAX is V _{D + R} (I _MAX ), and the voltage of the characteristic curve at the time of I _MIN is V _{D + R} (I _MIN ).

FIG. 12 shows a state between the photosensitive drum and the transfer roller.
The voltage-current characteristics without the recording material are shown.
The characteristic curve shows the VI characteristic under the same conditions as the characteristic curve of FIG.
The characteristic curve shows that the surface potential of the photosensitive drum is constant.
Surface potential V during flow control_CON VI characteristics when the same as
Sex. In addition, the highest current value that allows optimal transfer
I_MAX The voltage of the characteristic curve at the time of_{D + R} (I
_MAX ), I which is the minimum value of the current value at which the optimum transfer can be performed.
_MIN The voltage of the characteristic curve at the time of_{D + R} (I _MIN )When
I do.

Here, the characteristic curve is obtained by adding the partial pressure of the OPC photosensitive drum and the partial pressure of the transfer roller. Therefore, VD _{+ R} ( _IMAX ) -V'D _{+ R} ( _IMAX ) = V _DRM (I
_MAX ) −V ′ _DRM (I _MAX ) V _{D + R} (I _MIN ) −V ′ _{D + R} (I _MIN ) = V _DRM (I
_MIN ) −V ′ _DRM (I _MIN ) V _DRM (I _MAX ): partial pressure of the photosensitive drum with the solid black current I _MAX V _DRM (I _MIN ): partial pressure of the photosensitive drum with the solid black current I _MIN V ′ _DRM (I _MAX ): The potential of the photosensitive drum at a constant current (current I _MAX ) V ′ _DRM (I _MIN ): The potential of the photosensitive drum at a constant current (current I _MIN ) holds.

Assuming that both I _MIN and I _MAX are currents in the linear portion of the characteristic curve shown in FIG. 10, V _DRM (I _MAX ) −V ′ _DRM (I _MAX ) = V _DRM (I _MIN ) −V ′ _DRM (I _MIN ) = V _D −V _CON V _D : Surface potential (dark potential) of the photosensitive drum when solid black V _CON : Surface potential of the photosensitive drum when constant current V _D and V _CON are both durable and fluctuate Motomari advance in the absence, "V _D -V _CON" can be used as a known value.

From the above description, an example of a method of determining a constant voltage value when a recording material is passed through a transfer portion will be described below.

First, when a recording material is not passed, a constant current of I _MIN is supplied, and a voltage V ′ _{D + R} (I _MIN ) at that time is obtained. Next, the obtained voltage V ′ _{D + R} (I _MIN )
A value V _MIN obtained by adding “V _D −V _CON ” and “C ₁ ” is set as a constant voltage value. Note that “V _D −V _CON ” and “C ₁ ” are known numbers obtained in advance as described above.

That is, the constant voltage value when the recording material is passed through the transfer portion is a voltage obtained by adding a constant voltage to a voltage at a constant current when paper is not passed.

FIG. 11, FIG. 13 and FIG.
13 shows voltage-current characteristics of three types of transfer rollers having sequentially lower resistance values. The resistance value shown in FIG. 13 is the lower limit that can be used in the conventional transfer method, and is shown in FIG. It can be seen that the transfer roller having the resistance value cannot be used in the conventional method.

[0050] That is, in FIG. 14, although the constant voltage value is a value V _MIN when the I _MIN characteristic curve of humidity cardboard, when controlled by the voltage I _MIN, high humidity plain paper recording medium (curve ), The current value exceeds _IMAX , which causes a decrease in transfer efficiency and blurring of characters. FIG.
Even when using a transfer roller of low resistance as shown in, setting I _MAX as a current limit at constant voltage, it can be seen that the image problem in does not occur.

FIG. 15 shows a case where the same transfer roller as that shown in FIG. 14 is used.
Are the same as in FIG. The characteristic curve is a voltage-current characteristic when a low-humidity thick paper having the same thickness, dielectric constant, and moisture absorption as the characteristic curve and a small-size paper having a width in the longitudinal direction of 100 mm is used, and the photosensitive drum potential is almost the entire write potential. . The characteristic curve is a high-humidity plain paper having the same thickness, dielectric constant, and moisture absorption as the characteristic curve, and a small-sized paper having a width in the longitudinal direction of 100 mm. It is a current characteristic.

The following Table 1 summarizes the characteristic curves (1) to (4).

[0053]

[Table 1]

The characteristic curve shows that the recording material having the highest impedance among the recording materials is in a state where the largest current flows at a constant voltage, that is, the width in the longitudinal direction is short and the photosensitive drum potential is almost the entire surface write potential. Is shown.

When I _{MAX is set} to the upper limit at the time of constant voltage control at V _MIN , the voltage of the transfer roller is selected so that the voltage is reduced by the current limitation and the amount of charge per unit area of the recording material does not fall below the minimum. In FIG. 15, V
_Since V _MIN does not decrease even when the recording material having the characteristic curve at the constant voltage of _MIN arrives, it can be seen that the resistance value of the transfer roller shown in FIG. 15 can be used.

The characteristic curve of FIG. 15 shows that the recording material having the lowest impedance among the recording materials has the largest current flow at the time of constant voltage, that is, the width in the longitudinal direction is short, and the potential of the photosensitive drum is almost the entire surface. The case of a write potential is shown. At this time, the current limit is activated at the time of the constant voltage of V _MIN in FIG. 15, and the voltage drops to V _LIM .
Since the charge amount per unit area of the recording material at this time is the intersection E between the characteristic curve and V _LIM , it can be seen that the charge amount is higher than I _MIN .

On the other hand, the characteristic curve and the characteristic curve in the conventional example show the case where the current is limited while the transfer roller is kept in contact with the photosensitive drum on both outer sides in the width direction of the recording material. It is a low-moisture thick paper of a size, and since the current flowing outside the paper width is larger than the characteristic curve when the same recording material of the present invention is used, the voltage at the time of the current limitation is lower than the characteristic curve (point F). Therefore, the charge amount per unit area is lower than the minimum, and toner scattering and transfer failure occur.

The characteristic curve is a small-sized high-humidity plain paper, and the current flowing outside the paper width is larger than the characteristic curve when the same recording material of the present invention is used. Is lower than the characteristic curve (point G). Therefore, the charge amount per unit area is less than the minimum in the characteristic curve, and transfer failure occurs.

From the above results, it was found that in the present invention, the effect of limiting the current was significantly improved by reducing the contact pressure of the transfer roller. Further, it was shown that a transfer roller having a lower resistance value can be used as compared with the conventional example.

Further, the limiting current value at the time of constant voltage is -2.5
When the recording material is not passed, first, the constant current control means 14 controls the current -1 μm of the minimum value I _MIN of the current range optimal for transfer by the constant current control means 14.
A controls the voltage generator 12 at a constant current, and applies the voltage generated from the voltage generator 12 to the transfer roller 2.
At this time, the voltage generated by the voltage generator 12 is
6 and supplies the detected voltage value to the calculating means 15, and controls the voltage generating means 12 through the constant voltage control means 13.

Next, a current is detected by the current detecting means 17 while the recording material is passed and a constant voltage is applied to the transfer roller 2 from the voltage generating means 12 by the constant voltage control. The detected current value is supplied to the calculating means 15 and this is -2.5
It is determined whether it is smaller than μA, and if it is smaller than −2.5 μA, the constant voltage control is continued. If it is larger than −2.5 μA, an appropriate voltage value is calculated and supplied to the constant voltage control means 13. Then, the voltage value generated from the voltage generating means 12 is reduced, and the constant voltage control is continued. FIG. 17 is a flowchart showing the sequence.

In the above-described embodiment, the case where the voltage applied to the transfer roller is controlled with the upper limit of the current value kept constant without particularly detecting the width of the recording material in the longitudinal direction has been described. If the control is performed in combination with the recording material size detecting means for detecting the size of the cassette to be used, the resistance value of the transfer roller can be used in a wider range. That is, it is possible to use a transfer roller having a wider resistance value range than before.

By providing means for detecting the thickness, dielectric constant, and resistance of the recording material, and changing the upper limit of the current value according to the detected values, the resistance of the transfer roller can be further widened. Can be used with

Further, it is also possible to provide a current limiter only on the hardware circuit without detecting and feeding back the current value. Further, the upper limit of the current value may be changed according to the detection of the resistance value of the transfer roller or the resistance value that has been stratified in advance.

In the above embodiment, the case where the present invention is applied to an electrophotographic image forming apparatus has been described.
The present invention is not limited to the image forming apparatuses shown in the above embodiments, and the present invention can be applied to image forming apparatuses of various other configurations using a contact type transfer member such as a transfer roller and a belt. Needless to say, an effect can be obtained. Further, the configuration of the transfer roller is not limited to that shown in the above-described embodiment, and it is needless to say that various modifications and changes can be made as needed. It goes without saying that it is good.

[0066]

As described above, the present invention provides an image carrier, developing means for developing an electrostatic latent image formed on the image carrier to form a visible image, and transferring the visible image. A transfer member that applies a charge to the recording material and transfers the recording material by contacting the back surface of the recording material fed by the unit; and a transfer member when there is no recording material between the image carrier and the transfer member. In an image forming apparatus that performs constant voltage control and constant voltage control of a transfer member at the time of transfer with a voltage obtained based on a voltage generated at this time to perform transfer, the transfer member is brought into contact with an image carrier, and When the recording material is fed, the transfer member and the image carrier are not in contact with each other outside the width direction of the recording material. Therefore, when the recording material is not present, the transfer member and the image carrier are in contact with each other. The voltage detection during current control is performed reliably, and the non-sheet passing area outside the recording material in the width direction during transfer. No excessive current flows that in order not in contact, therefore, can be properly transferred, there is an effect that it is possible to obtain an image of the sharp image quality at all times.

[0067]

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an embodiment in which the present invention is applied to an electrophotographic image forming apparatus having a contact charging type transfer device.

FIG. 2 is a schematic cross-sectional view for explaining a state when a small-sized recording material is passed through a transfer portion in the image forming apparatus according to the present invention in FIG.

FIG. 3 is a schematic side view and an equivalent circuit diagram showing a state near a transfer portion when a recording material is present at the transfer portion in the image forming apparatus according to the present invention in FIG. 1;

FIG. 4 is a diagram illustrating voltage-current characteristics of a transfer roller during transfer.

FIG. 5 is a diagram illustrating voltage-current characteristics of a transfer roller alone during transfer.

FIG. 6 is a diagram illustrating voltage-current characteristics of a recording material alone during transfer.

FIG. 7 is a diagram illustrating voltage-current characteristics of a single photosensitive drum during transfer.

FIG. 8 is a diagram illustrating voltage-current characteristics of a single recording material during transfer when the type of recording material and the moisture absorption are changed.

FIG. 9 is a characteristic diagram showing a relationship between a current at the time of solid black and transfer efficiency.

FIG. 10 is a diagram illustrating voltage-current characteristics of a single photosensitive drum at the time of transfer when the photosensitive drum surface potential is different.

FIG. 11 is a diagram illustrating voltage-current characteristics at the time of constant current control and at the time of transfer when the type of recording material and moisture absorption are changed.

FIG. 12 shows a voltage at the time of constant current control and a transfer at the time of solid black.
It is a figure showing a current characteristic.

FIG. 13 is a diagram illustrating voltage-current characteristics of a transfer roller having a lower resistance value that can be used in a conventional transfer method during constant current control and when the type of recording material and moisture absorption are changed.

14 is a diagram illustrating voltage-current characteristics of a transfer roller having a lower resistance value than the transfer roller of FIG. 13 at the time of constant current control and when the type of recording material and the moisture absorption are changed.

FIG. 15 is a diagram illustrating voltage-current characteristics of the transfer roller having the same resistance value as in FIG. 14 during constant current control and when the type, size, and moisture absorption of the recording material are changed.

FIG. 16 is a block diagram illustrating an example of a control circuit that can be used in the image forming apparatus according to the present invention.

FIG. 17 is a flowchart for explaining an operation sequence of the control circuit in FIG. 16;

FIG. 18 is a schematic cross-sectional view for explaining a state when a small-sized recording material is passed through a transfer portion in a conventional image forming apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Transfer roller 5 Recording material 6 Power supply 7 Light source 8 Primary charging device 10 Developing device 11 Cleaner 12 Voltage generating means 13 Constant voltage control means 14 Constant current control means 15 Calculation means 16 Voltage detection means 17 Current detection means

Claims (1)

(57) [Claims] An image carrier, a developing unit for developing a latent image formed on the image carrier to form a visible image, and a recording material fed with the visible image by a transfer unit A transfer member that applies a charge to the recording material and transfers the recording material by contacting the back surface of the recording material. When the recording material is not present between the image carrier and the transfer member, the transfer member is controlled by a constant current. The transfer member is brought into contact with an image carrier and a recording material is fed to a transfer unit in an image forming apparatus that performs transfer by performing constant voltage control of a transfer member at the time of transfer with a voltage obtained based on a voltage to be transferred. An image forming apparatus, wherein a transfer member and an image carrier are not in contact with each other outside the recording material in the width direction.