JP2022049165A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that employs an internal power supply system and can stably eliminate static electricity on a recording material with a static eliminating needle even when a transfer voltage is changed.SOLUTION: When a transfer voltage (particularly, reference voltage) is not changed in association with execution of "ATVC control" (NO in S7), a "static elimination bias reference value" determined from a "reference value table" as a static elimination voltage to be applied to a static eliminating needle is applied as is (S6, S10). On the other hand, when the transfer voltage (particularly, reference voltage) is changed (YES in S7), a static elimination voltage is applied which is changed so that the voltage difference between the transfer voltage before the change and the static elimination voltage before the change and the voltage difference between the transfer voltage after the change and the static elimination voltage after the change become the same as each other (S8, S10). The voltage difference between the transfer voltage and the static elimination voltage is maintained constant even when the electrical resistances of an intermediate transfer belt, a secondary transfer inner roller, and a secondary transfer outer roller are changed due to deterioration associated with their use. A static elimination function with the static eliminating needle can therefore be properly maintained and the occurrence of failure in transfer to a recording material can be prevented.SELECTED DRAWING: Figure 4

Description

The present invention relates to an image forming apparatus using electrophotographic technology, such as a printer, a copying machine, a facsimile or a multifunction device.

Conventionally, an intermediate transfer type image forming apparatus has been used in which a plurality of photosensitive drums are arranged side by side along the moving direction of the intermediate transfer belt for each color such as yellow, magenta, cyan, and black. .. In the intermediate transfer type image forming apparatus, when a voltage is applied to the primary transfer roller provided in contact with the inner peripheral surface of the intermediate transfer belt so as to correspond to the photosensitive drum, the toner image formed on the photosensitive drum is formed. Primary transfer to the intermediate transfer belt. After that, the transfer current flows through the secondary transfer portion (transfer nip portion) formed by the secondary transfer inner roller and the secondary transfer outer roller arranged so as to sandwich the intermediate transfer belt, so that the toner on the intermediate transfer belt is transferred. The image is secondary transferred to a recording material that passes through the secondary transfer section. As a method of passing a transfer current through the secondary transfer unit, a device of a method of applying a transfer voltage to a secondary transfer inner roller that abuts on the inner peripheral surface of the intermediate transfer belt by a high voltage power supply (so-called internal power supply method) has been proposed. (Patent Document 1).

Then, on the downstream side of the secondary transfer section in the transport direction of the recording material, static elimination is performed to remove charges from the recording material in order to facilitate separation of the recording material that has passed through the secondary transfer section from the intermediate transfer belt. A needle is provided. In the apparatus described in Patent Document 1, the static elimination voltage applied to the static elimination needle is adjusted according to the charge amount of the recording material detected by the charge amount detection sensor, so that the static elimination of the recording material can be stably performed. ing.

Japanese Unexamined Patent Publication No. 2014-19706

By the way, recently, it is desired to reduce the cost of the image forming apparatus, and for that reason, the above-mentioned charge amount detection sensor may be omitted. However, in the above-mentioned internal power supply method, if the charge amount detection sensor is omitted, it becomes difficult to stably remove static electricity from the recording material. For example, when the electrical resistance of the intermediate transfer belt, the secondary transfer inner roller, or the secondary transfer outer roller changes due to deterioration due to use, the secondary transfer is performed in order to adjust the transfer current flowing through the secondary transfer section. When the transfer voltage applied to the inner roller was changed, static elimination may not be performed properly. Therefore, in the case of the internal power supply system configuration, it has been desired that static elimination can be appropriately performed by a static elimination needle without providing a charge amount detection sensor, but such a thing has not yet been proposed.

The present invention is an internal power feeding method in which a transfer voltage is applied to a secondary transfer inner roller that abuts on the inner peripheral surface of the intermediate transfer belt to transfer a toner image from the intermediate transfer belt to a recording material, even if the transfer voltage is changed. It is an object of the present invention to provide an image forming apparatus capable of stably removing static electricity from a recording material by a static elimination needle.

The image forming apparatus of the present invention includes an endless image-supporting belt that supports and rotates a toner image, a first rotating body that abuts on the inner peripheral surface of the image-supporting belt, and the first rotating body and the image-supporting body. A second rotating body that abuts on the outer peripheral surface of the image-supporting belt so as to sandwich the belt and forms a transfer nip portion that transfers a toner image from the image-supporting belt to the recording material while sandwiching and transporting the recording material. The first power supply that applies a voltage to the first rotation and the recording material that is arranged downstream of the transfer nip portion in the transport direction of the recording material and that has passed through the transfer nip portion by applying a static elimination voltage removes the charge of the recording material. The static elimination means, a second power source for applying a voltage to the static elimination means, and a control means for controlling the first power supply and the second power supply, respectively, are provided, and the control means is the first rotating body at the time of image formation. When the transfer voltage applied to is changed, the static elimination voltage applied to the second power source so as to maintain the voltage difference between the transfer voltage and the static elimination voltage before and after the transfer voltage is changed. It is characterized by changing.

According to the present invention, the transfer voltage is changed in the case of the configuration in which the transfer voltage is applied to the first rotating body that abuts on the inner peripheral surface of the image-supporting belt to transfer the toner image from the image-supporting belt to the recording material. However, it is possible to easily realize stable static elimination of the recording material by the static elimination member.

The schematic diagram which shows the structure of the image forming apparatus of this embodiment. The schematic which shows the image formation part. The figure for demonstrating the structure of the secondary transfer part and the static elimination structure in the internal power supply system of this embodiment. The flowchart which shows the static elimination voltage control processing of this embodiment. The figure for demonstrating the structure of the secondary transfer part and the static elimination structure in the external power supply system of the comparative example.

<Image forming device>
Hereinafter, the image forming apparatus of this embodiment will be described. First, the schematic configuration of the image forming apparatus of this embodiment will be described with reference to FIGS. 1 and 2. The image forming apparatus 100 shown in FIG. 1 is an intermediate transfer type full-color printer provided with yellow, magenta, cyan, and black image forming portions Pa, Pb, Pc, and Pd along the intermediate transfer belt 10. Although not shown, the image forming apparatus 100 is attached to the recording material P according to image information from an external device such as a document reading apparatus connected to the apparatus main body or a personal computer communicably connected to the apparatus main body. Form an image. Examples of the recording material P include various types of sheet materials such as plain paper, thin paper, rough paper, uneven paper, coated paper and the like, plastic films, cloth and the like.

In the image forming unit Pa, a yellow toner image is formed on the photosensitive drum 1a and is primarily transferred to the intermediate transfer belt 10. In the image forming unit Pb, a magenta toner image is formed on the photosensitive drum 1b and is superposed on the yellow toner image on the intermediate transfer belt 10 for primary transfer. In the image forming portions Pc and Pd, a cyan toner image and a black toner image are formed on the photosensitive drums 1c and 1d, respectively, and are sequentially superimposed on the intermediate transfer belt 10 and primaryly transferred. The toner image of each color primaryly transferred to the intermediate transfer belt 10 is supported on the intermediate transfer belt 10 and conveyed to the secondary transfer unit T2 to be secondarily transferred to the recording material P. The intermediate transfer belt 10 as an image-carrying belt is supported by being hung on a tension roller 11, a drive roller 12, and a secondary transfer inner roller 13, and is driven by the drive roller 12 to move in a predetermined movement direction (arrow R2 direction). It is provided as possible.

The recording material P is stored in a form of being loaded in the recording material cassette 18, and is sent out one by one from the recording material cassette 18 by the pickup roller 17 in accordance with the image formation timing. The recording material P sent out by the pickup roller 17 is conveyed to the registration roller 16 arranged in the middle of the conveying path. Then, the recording material P is subjected to skew correction and timing correction by the registration roller 16 and sent to the secondary transfer unit T2.

The secondary transfer unit T2 is a transfer nip unit formed by the secondary transfer inner roller 13 and the secondary transfer outer roller 14 arranged so as to face each other. The secondary transfer inner roller 13 as the first rotating body is in contact with the inner peripheral surface of the intermediate transfer belt 10. The secondary transfer outer roller 14 as the second rotating body has an intermediate transfer belt 10 toward the secondary transfer inner roller 13 in order to form a transfer nip portion (secondary transfer portion T2) of the toner image with respect to the recording material P. Is pressed from the outer peripheral side.

In the secondary transfer unit T2, when the recording material P is sandwiched and conveyed, a transfer current flows between the secondary transfer outer roller 14 and the secondary transfer inner roller 13, so that the toner image is transferred from the intermediate transfer belt 10. It is secondarily transferred to the recording material P. In the present embodiment, when a secondary transfer voltage is applied to the secondary transfer inner roller 13, a transfer current flows through the secondary transfer unit T2, and a toner image is secondarily transferred to the recording material P, so-called “internal power supply”. "Method" is adopted (see FIG. 3 described later).

The recording material P to which the toner image is transferred in this way is conveyed to the fixing device 15. The fixing device 15 heats and pressurizes the recording material P while transporting the recording material P to fix the toner image on the recording material P. The recording material P on which the toner image is fixed by the fixing device 15 is discharged to the outside of the machine body by a discharge roller pair (not shown).

<Image forming part>
The image forming portions Pa, Pb, Pc, and Pd will be described. The image forming units Pa, Pb, Pc, and Pd are configured in almost the same manner except that the toner colors used in the developing devices 4a, 4b, 4c, and 4d are different from those of yellow, magenta, cyan, and black, and they are configured in substantially the same manner, respectively, according to the same control. Can work. Therefore, in the following, the yellow image forming unit Pa will be described as a representative, and the other image forming units Pb, Pc, and Pd will be omitted.

As shown in FIG. 2, the image forming unit Pa is provided with a charging roller 2a, an exposure device 3a, a developing device 4a, a primary transfer roller 5a, and a cleaning device 6a surrounding the photosensitive drum 1a. The photosensitive drum 1a is, for example, a cylindrical organic photoconductor (OPC) drum in which a photosensitive layer of an organic substance and a surface protective layer are sequentially laminated on an outer peripheral surface of a conductive aluminum cylinder (base), for example, “240 mm”. It rotates in the rotation direction (arrow R1 direction) at a peripheral speed of "/ sec".

The charging roller 2a comes into contact with the rotating photosensitive drum 1a and rotates drivenly by the photosensitive drum 1a. When a voltage is applied, the charging roller 2a generates an electric discharge with the rotating photosensitive drum 1a to uniformly charge the surface of the photosensitive drum 1a. The exposure apparatus 3a generates a laser beam L obtained by ON-OFF modulation of scanning line image data obtained by developing a yellow separated color image, scanning this with a rotating mirror (not shown), and charging the photosensitive drum 1a. An electrostatic latent image is formed on the surface. The developing apparatus 4a develops an electrostatic latent image formed on the surface of the photosensitive drum 1a into a toner image using a developer in which a non-magnetic toner and a magnetic carrier are mixed.

The primary transfer roller 5a is arranged to face the photosensitive drum 1a with the intermediate transfer belt 10 interposed therebetween, and abuts on the inner peripheral surface of the intermediate transfer belt 10 to form a toner image between the photosensitive drum 1a and the intermediate transfer belt 10. The primary transfer nip portion T1 is formed. In the primary transfer roller 5a, the toner image on the photosensitive drum 1a is primarily transferred to the intermediate transfer belt 10 by applying a DC voltage (primary transfer voltage) having a polarity opposite to the charging polarity of the toner by the primary transfer power supply D1. .. The transfer residual toner remaining on the photosensitive drum 1a after the primary transfer is removed by the cleaning device 6a rubbing against the photosensitive drum 1a.

<Secondary transfer voltage>
Here, the secondary transfer voltage applied to the secondary transfer inner roller 13 for the secondary transfer of the toner image from the intermediate transfer belt 10 to the recording material P will be described. In recent years, the types of recording materials P have become abundant, and the thickness and electrical resistance of the recording materials P have also become widespread. Therefore, the intermediate transfer method shown in FIG. 1 is widely adopted. Then, in order to avoid changing the amount of charge supplied to the toner image due to differences in the image ratio of the image formed on the recording material P, the size of the recording material P, and the like, constant voltage control is performed in the secondary transfer to the recording material P. Has been adopted. However, the electrical resistance of the intermediate transfer belt 10, the secondary transfer inner roller 13, and the secondary transfer outer roller 14 may change depending on changes in the environment such as temperature and humidity and deterioration due to use. Therefore, in order to optimize the secondary transfer voltage (Vb) applied to the secondary transfer inner roller 13 when those electrical resistances change, the reference voltage (Va) for constant voltage control is performed prior to image formation. "ATVC control" to change is executed. "ATVC control" will be described later.

The reference voltage (Va) changed by "ATVC control" is a voltage (non-) that allows a target current to flow through the secondary transfer unit T2 when the recording material P does not pass through the secondary transfer unit T2. It is a constant voltage value applied to the roller 13 in the secondary transfer when passing through). On the other hand, the secondary transfer voltage applied to the secondary transfer inner roller 13 at the time of image formation causes a target current to flow through the secondary transfer unit T2 while the recording material P is passing through the secondary transfer unit T2. If the voltage is not as high as possible, transfer defects and the like may occur. Therefore, as the secondary transfer voltage, in addition to the electrical resistance of the intermediate transfer belt 10, the secondary transfer inner roller 13, the secondary transfer outer roller 14, and the like, a voltage considering the electrical resistance of the recording material P for forming an image is applied. There is a need.

Therefore, in the present embodiment, the secondary transfer voltage (Vb) applied to the secondary transfer inner roller 13 at the time of image formation is a shared voltage (Vp) in consideration of the above-mentioned reference voltage (Va) and the electric resistance of the recording material P. ) Is set (Vb = Va + Vp). The shared voltage (Vp) is assigned a different voltage value in advance depending on the type of recording material P (for example, the size of the basis weight) and the like. The shared voltage of the recording material P having a large electric resistance (for example, thick paper having a relatively large basis weight) is higher than the shared voltage of the recording material P having a small electric resistance (for example, thin paper having a relatively small basis weight). A value (absolute value) is assigned.

By the way, when thin paper having a small basis weight is used as the recording material P, thin paper (for example, having a basis weight of 79 g / m ² or less) has lower rigidity than plain paper, so that separation failure from the intermediate transfer belt 10 occurs. Easy to do. Therefore, in the present embodiment, a voltage is applied to the static elimination needle 91 to remove the charged charge of the recording material P when passing through the secondary transfer unit T2, whereby the separability of the recording material P with respect to the intermediate transfer belt 10 is achieved. Is improving.

<Secondary transfer unit>
FIG. 3 is an explanatory diagram of the configuration of the secondary transfer unit T2 and the static elimination needle 91 described above. As shown in FIG. 3, the secondary transfer outer roller 14 is urged by a spring member at both ends, and is pressed against the outer peripheral surface of the secondary transfer inner roller 13 via the intermediate transfer belt 10 to perform intermediate transfer. A secondary transfer portion T2 is formed between the belt 10 and the secondary transfer outer roller 14. In the case of this embodiment, the secondary transfer outer roller 14 is connected to the ground potential.

Since the internal power supply method is adopted in this embodiment, the transfer power supply 120 for applying a voltage is connected to the secondary transfer inner roller 13. The transfer power supply 120 as the first power source can apply a secondary transfer voltage (Vb) to the secondary transfer inner roller 13 at the time of image formation. By applying the secondary transfer voltage (Vb), the toner image supported on the intermediate transfer belt 10 is secondarily transferred to the recording material P passing through the secondary transfer unit T2. As the secondary transfer voltage, for example, a DC voltage having the same polarity as the charging polarity (for example, negative electrode property) of the toner is applied. Further, in order to perform the above-mentioned "ATVC control" on the secondary transfer inner roller 13, the secondary transfer inner roller 13 and the secondary transfer inner roller 13 respond to the application of a plurality of different voltages to the secondary transfer inner roller 13. An ammeter 121 for detecting the current flowing between the outer roller 14 and the outer roller 14 is connected.

The intermediate transfer belt 10 is endlessly formed by using, for example, a semi-conductive polyimide resin having a relative permittivity “ε = 3 to 5” and a volume resistivity “ρv = 1 × 10 ⁶ to 10 ¹¹ Ω ・ m”. It is formed. As the secondary transfer inner roller 13, for example, a semi-conductive roller having an outer diameter of 20 mm and a core metal diameter of 16 mm in which conductive carbon was dispersed in EPDM rubber was used. The electric resistance value of the secondary transfer inner roller 13 is, for example, about "1 x 10 ¹ to 105 Ω" when the applied voltage is "10 ^V " in an environment where the temperature is "23 degrees" and the humidity is "50%". be. On the other hand, as the secondary transfer outer roller 14, for example, an ion conductive sponge roller having an outer diameter of 24 mm and a core metal diameter of 12 mm, which was formed by mixing nitrile rubber and an ethylene-epichlorohydrin copolymer, was used. The resistance value of the secondary transfer outer roller 14 is, for example, about "1 x ¹⁰⁶ to ¹⁰⁸ Ω" when the applied voltage is "2 kV" in an environment where the temperature is "23 degrees" and the humidity is "50%". ..

<Static elimination unit>
As shown in FIG. 3, a static elimination needle 91 as a static elimination means for statically eliminating the recording material P is arranged on the downstream side of the secondary transfer outer roller 14 in the transport direction of the recording material P (see arrow R3 in FIG. 1). It is set up. The static elimination needle 91 is, for example, a thin plate material of SUS304 having a thickness of “0.2 mm” processed into a sawtooth shape, and the pitch of adjacent sawtooths is “1 mm”. The static elimination needle 91 is arranged at a height position where it does not come into contact with the recorded material P to be conveyed so that the tip of the saw blade faces the back surface of the recording material P.

The static elimination needle 91 is attached to the static elimination needle holder 92. The static elimination needle holder 92 is an insulating member mainly composed of an insulating PBT (polybutylene terephthalate), and is provided to suppress high voltage leakage between the secondary transfer outer roller 14 and the static elimination needle 91. Be done.

A static elimination power supply 130 that applies a variable static elimination voltage to the static elimination needle 91 is connected to the static elimination needle 91. The static elimination power supply 130 as the second power supply can apply a static elimination voltage to the static elimination needle 91 at a predetermined timing. The polarity of the static elimination voltage is set to the same polarity (for example, negative electrode property) as the secondary transfer voltage applied to the secondary transfer inner roller 13.

When the static elimination voltage is applied by the static elimination power supply 130, the static elimination needle 91 generates a corona discharge, and the charged particles accompanying the corona discharge are transferred to the toner on the back surface of the recording material P (the toner opposite to the surface facing the intermediate transfer belt 10). Irradiate the surface on the side where the image is not transferred). In this way, the static elimination needle 91 eliminates the charged charge of the recording material P on the downstream side in the recording material transport direction that has passed through the secondary transfer unit T2. As a result, the electrostatic adsorption force of the recording material P with respect to the intermediate transfer belt 10 is reduced, and thus the separability of the recording material P is improved.

<Control unit>
Further, as shown in FIG. 1, the image forming apparatus 100 includes a control unit 110 capable of generally controlling the operation of the image forming apparatus 100. The control unit 110 will be described here mainly with reference to a voltage control system for applying a voltage to the static elimination needle 91 and the secondary transfer inner roller 13. In addition to the parts shown in FIG. 3, the control unit 110 is connected to various parts constituting the image forming apparatus 100 and various devices such as a drive source (motor, power supply, etc.) for operating each part. However, since it is not the main purpose of the invention here, the illustration and description thereof will be omitted.

The control unit 110 as a control means controls various operations related to image formation, and includes, for example, a CPU 111 (Central Processing Unit) and a memory 112. The memory 112 is composed of, for example, a ROM (Read Only Memory), a RAM (Random Access Memory), or the like. The memory 112 is a later-described "reference value table" (see Table 1) used for setting various programs for controlling the image forming apparatus 100, a shared voltage (Vp) described later, and a static elimination voltage applied to the static elimination needle 91. ) And other data are stored. The memory 52 can also temporarily store arithmetic processing results associated with the execution of various programs, image information received from a document reading device or an external device, and the like.

The control unit 110 can execute various programs such as "image formation job processing" (not shown) for forming an image on the recording material P and "static elimination voltage control processing" described later, and executes these various programs to obtain an image. The operation of each part of the forming device 100 can be controlled. When the "static elimination voltage control process" is executed, the control unit 110 can control the voltage applied to the static elimination needle 91 by the static elimination power supply 130. The “static elimination voltage control process” of this embodiment will be described later (see FIG. 4).

Further, the control unit 110 may, for example, perform the above-mentioned "ATVC" at the time of front rotation before the start of the image forming job, or between papers after the cumulative number of image-formed recording materials P exceeds a predetermined number (for example, 1000). "Control" (change mode) can be performed. The control unit 110 controls the voltage applied to the secondary transfer inner roller 13 by the transfer power supply 120 when the “ATVC control” is executed. Then, the control unit 110 detects the current value flowing between the secondary transfer inner roller 13 and the secondary transfer outer roller 14 when a voltage is applied to the secondary transfer inner roller 13 by the transfer power supply 120. Can be obtained from the ammeter 121 as. That is, the control unit 110 controls the transfer power supply 120 to output a predetermined constant voltage, detects the output of the ammeter 121 at that time, and makes the current flowing through the secondary transfer unit T2 a predetermined value. It has a function of calculating a reference voltage (Va) and setting it in the transfer power supply 120.

The image forming job is a series of operations from the start of the image forming operation to the completion of the image forming operation based on the print signal for forming an image on the recording material P. That is, after starting the preliminary operation (so-called forward rotation) required for performing image formation, the preliminary operation required for completing image formation (so-called backward rotation) is completed through the image forming step. It is a series of operations up to. Specifically, it refers to the period from the front rotation (preparatory operation before image formation) after receiving the print signal (input of the image formation job) to the rear rotation (operation after image formation), and the image formation period. , Including between papers.

Further, a temperature / humidity sensor 140 as a humidity detecting means is connected to the control unit 110, and the control unit 110 calculates the amount of water contained in the main body (ambient atmosphere) based on the detection result of the temperature / humidity sensor 140. Can be done. The amount of water contained in the main body affects the separability of the recording material P with respect to the intermediate transfer belt 10 on the downstream side of the secondary transfer unit T2. That is, the larger the amount of water contained in the main body, the weaker the rigidity of the recording material P, and the lower the separability of the recording material P on the downstream side of the secondary transfer unit T2.

Next, the “static elimination voltage control process” in which the static elimination voltage applied to the static elimination needle 91 is set and applied by the static elimination power supply 130 will be described with reference to FIG. 3 with reference to FIG. FIG. 4 is a flowchart showing the “static elimination voltage control process” of the present embodiment. The "static elimination voltage control process" of the present embodiment is started by the control unit 110 at the execution timing of the above-mentioned "ATVC control".

As shown in FIG. 4, the control unit 110 performs "ATVC control" (S1). The ATVC (Auto Transfer Voltage Control) control is known, but will be briefly described. In the "ATVC control", the output voltage of the transfer power supply 120 is switched in multiple stages, and the output current value with respect to the output voltage in the plurality of stages is detected by the ammeter 121. For example, the first voltage V1 is applied for one round of the secondary transfer inner roller 13, and the current value at that time is detected by the ammeter 121 and averaged to obtain the current value I1 with respect to the first voltage V1. .. Then, using the same procedure, the current value I2 for the second voltage V2 and the current value I3 for the third voltage V3 are obtained (V3 <V2 <V1).

The control unit 110 uses the output voltages (V1, V2, V3) at the three measurement points and the current values (I1, I2, I3 (I3 <I2 <I1)) detected by the ammeter 121 to perform secondary transfer. The "voltage-current characteristic (VI characteristic)" of the part T2 is derived. Among the three measurement points, the measurement data of the three measurement points is derived by linear interpolation arithmetic processing. The "VI characteristic" thus obtained is stored in the memory 112.

Here, the memory 112 also stores the transfer current (predetermined value Ib) required for the secondary transfer unit T2. The control unit 110 stores in the memory 112 the reference voltage (Va) applied to the secondary transfer inner roller 13, which is necessary for passing the transfer current of the predetermined value Ib to the secondary transfer unit T2, in the “VI characteristic”. ”.

For example, in the case of "Ib <I2", the reference voltage (Va) is obtained from the following equation (1). Further, when "Ib ≧ I2", the reference voltage (Va) is obtained from the following equation (2).
Va = (Ib-I2) × (V3-V2) / (I3-I2) + V2 ... (1)
Va = (Ib-I1) × (V2-V1) / (I2-I1) + V1 ... (2)

Returning to the description of FIG. 4, the control unit 110 determines a pre-allocated shared voltage (Vp) according to the type of recording material P (for example, the size of the basis weight) and the like (S2). The shared voltage (Vp) is stored in the memory 112. Then, the control unit 110 applies the sum of the reference voltage (Va) obtained by "ATVC control" (see S1) and the determined shared voltage (Vp) to the secondary transfer inner roller 13 at the time of image formation. The secondary transfer voltage (Vb = Va + Vp) is set (S3).

After setting the secondary transfer voltage (Vb), the control unit 110 determines whether or not the recording material P is "thin paper" (S4). In the case of this embodiment, the recording material P having a basis weight of "79 g / m ² or less" is regarded as "thin paper". When the recording material P is not "thin paper" (NO in S4), the control unit 110 changes the static elimination voltage to the ground potential (0V) (S5). When the recording material P is "thick paper to plain paper", the static elimination needle 91 is grounded in order to sufficiently secure the separability of the recording material P from the intermediate transfer belt 10 on the downstream side of the secondary transfer unit T2. Connected to the potential. When the static elimination voltage is set to the ground potential, the control unit 110 jumps to the process of step S9. In this case, the control unit 110 controls the transfer power supply 120 to apply the secondary transfer voltage (Vb = Va + Vp) to the secondary transfer inner roller 13 (S9), and controls the static elimination power supply 130 to the static elimination needle 91. A ground potential (0V) is applied (S10). In this way, when image formation is performed on "thick paper to plain paper", the secondary transfer voltage (Vb = Va + Vp) is applied to the secondary transfer inner roller 13, and the ground potential (0V) is applied to the static elimination needle 91. ..

On the other hand, when the recording material P is "thin paper" (YES in S4), the control unit 110 sets the static elimination voltage to the "static elimination bias reference value" (S6). The "static elimination bias reference value" is stored in the memory 112 as the "reference value table" shown in Table 1 below. The control unit 110 is based on the type (basis weight) of the recording material P and the amount of water contained in the main body calculated based on the detection result of the temperature / humidity sensor 140, from the “reference value table” in Table 1 to the “static elimination bias reference”. Determine the value. In Table 1, the basis weight is rounded down to the nearest whole number.

In the example shown in Table 1, when the basis weight of the recording material P is the first basis weight (80 g / m ² or more), the first reference value (0 kV) is used as the "static elimination bias reference value", and the basis weight of the recording material P is used. When the amount is smaller than the first basis weight, the second reference value (-5 kV), which is larger in absolute value than the first reference value, is used. Further, when the moisture content of the environment based on the detection result of the temperature / humidity sensor 140 is the first moisture content (0.9 g / kg or less), the first reference value (0 kV) is used, and the moisture content of the environment is higher than the first moisture content. When the amount of second water content is large, the second reference value (-5 kV), which is larger in absolute value than the first reference value, is used.

When the recording material P is "thick paper to plain paper", the static elimination needle 91 is changed to the ground potential, but the present invention is not limited to this, and the "static elimination bias reference value" is determined from the "reference value table". You may do so. For example, as shown in Table 1, when the recording material P is "thick paper to plain paper" (when the basis weight is 80 g / m ² or more), the "static elimination bias reference value" is related to the amount of water contained in the main body. It may be set to the ground potential (0V).

After setting the static elimination voltage to the "static elimination bias reference value" (S6), the control unit 110 determines whether or not the reference voltage (Va) has been changed with the execution of the "ATVC control" (see S1) (S7). ). If the reference voltage (Va) has not been changed (NO in S7) with the execution of "ATVC control", the control unit 110 jumps to the process in step S9. In this case, the control unit 110 controls the transfer power supply 120 to apply a secondary transfer voltage (Vb = Va + Vp) to the secondary transfer inner roller 13 (S9), and controls the static elimination power supply 130 to the static elimination needle 91. A "static elimination bias reference value" is applied (S10).

On the other hand, when the reference voltage (Va) is changed due to the execution of "ATVC control" (YES in S7), the control unit 110 determines the secondary transfer voltage (Vb) before and after the change of the reference voltage (Va). The static elimination voltage is changed so as to maintain the voltage difference with (S8). Specifically, the voltage difference between the secondary transfer voltage (Vb) based on the reference voltage (Va) before the change and the static elimination voltage before the change, and the secondary transfer voltage (Vb + α) based on the reference voltage (Va + α) after the change. ) And the changed static elimination voltage are changed so that the voltage difference is the same. In the case of the present embodiment, the difference value between the secondary transfer voltage before the change and the secondary transfer voltage after the change, substantially the difference value (α) of the reference voltage (Va), is set with respect to the static elimination voltage before the change. The sum is the changed static elimination voltage.

Table 2 shows the relationship between the applied voltage (Vt) of the secondary transfer outer roller 14, the secondary transfer voltage (Vb), and the static elimination voltage (Vc) in the case of the present embodiment.

As can be understood from Table 2, when the recording material P is "plain paper", "-7 kV" is applied as the secondary transfer voltage (Vb) to the secondary transfer inner roller 13. Then, in the present embodiment, when the recording material P is "plain paper", "0 kV" is applied as the static elimination voltage to the static elimination needle 91 (see S5). On the other hand, when the recording material P is "thin paper", "-3 kV" is applied as the secondary transfer voltage (Vb) to the secondary transfer inner roller 13. As described above, the secondary transfer voltage applied to the secondary transfer inner roller 13 at the time of image formation is the sum of the reference voltage (Va) and the shared voltage (Vp) (Vb = Va + Vp). Further, the shared voltage (Vp1) of the "plain paper" having a relatively large basis weight is assigned a higher voltage value (absolute value) than the shared voltage (Vp2) of the "thin paper" having a relatively small basis weight. Therefore, here, for example, assuming that the reference voltage (Va) is "-3 kV", the shared voltage (Vp2) of "thin paper" is "0 kV", and the shared voltage (Vp1) of "plain paper" is higher than that. It will be a high "-4kV". In the case of "thin paper", the voltage difference between the applied voltage (here, the ground potential) of the secondary transfer outer roller 14 and the static elimination voltage (Vc) is "5 kV (0- (-5))". The voltage difference between the next transfer voltage (Vb) and the static elimination voltage (Vc) is "2kV (-3- (-5))".

When "ATVC control" is executed on "thin paper" (during durability), the reference voltage (Va) is the electrical resistance of the intermediate transfer belt 10, the secondary transfer inner roller 13, and the secondary transfer outer roller 14. The voltage value is changed according to the change of. As shown in Table 2, for example, the reference voltage (Va) is changed from "-3 kV" to "-5 kV". Here, since the shared voltage of "thin paper" is "0 kV", the secondary transfer voltage (Vb) is also changed from "-3 kV" to "-5 kV". That is, the difference value (α) between the reference voltage before the change and the reference voltage after the change is “-2 kV”, and the voltage value larger by the difference value is set as the secondary transfer voltage (Vb) at the time of durability. ..

As described above, when the reference voltage (Va) is changed (YES in S7), the control unit 110 sets the static elimination voltage to the secondary transfer voltage (Vb) before and after the change of the reference voltage (Va). The voltage value is changed so that the voltage difference is maintained (S8). Therefore, the static elimination voltage (Vc) is changed from "-5kV" to "5kV" so as to maintain the voltage difference "2kV" between the secondary transfer voltage (Vb) and the static elimination voltage (Vc) before the reference voltage (Va) is changed. It will be changed to -7kV (-5+ (-2)) ". In this way, even if the electrical resistance of the intermediate transfer belt 10, the secondary transfer inner roller 13, and the secondary transfer outer roller 14 changes due to durability (deterioration with use), the secondary transfer voltage (Vb) and static elimination. Maintain the voltage difference from the voltage (Vc). By doing so, the static elimination function of the static elimination needle 91 can be appropriately maintained, and transfer defects from the intermediate transfer belt 10 to the recording material P can be suppressed, so that a high-quality image can be output to the recording material P. ..

For comparison, the control of the static elimination voltage in the case of the external power supply system in which the secondary transfer inner roller 13 is grounded and the secondary transfer voltage is applied to the secondary transfer outer roller 14 will be described. FIG. 5 is an explanatory diagram of the configuration of the secondary transfer unit T2 and the static elimination needle 91 in the external power supply system. As shown in FIG. 5, in the case of the external power supply method, the secondary transfer inner roller 13 is connected to the ground potential, and the transfer power supply 125 for applying a voltage is connected to the secondary transfer outer roller 14. The transfer power supply 125 can apply a secondary transfer voltage (Vb) to the secondary transfer outer roller 14 at the time of image formation. By applying the secondary transfer voltage (Vb), the toner image supported on the intermediate transfer belt 10 is secondarily transferred to the recording material P passing through the secondary transfer unit T2. A static elimination power supply 130 for applying a voltage is connected to the static elimination needle 91. The transfer power supply 125 and the static elimination power supply 130 are controlled by the control unit 110.

Table 3 shows the applied voltage (secondary transfer voltage Vt) of the secondary transfer outer roller 14, the applied voltage (Vb) of the secondary transfer inner roller 13, and the static elimination voltage (Vc) in the case of the external power supply method. Show the relationship.

As shown in Table 3, when the recording material P is “thin paper” in the external power supply method, the secondary transfer voltage (Vt = Va + Vp) applied to the secondary transfer outer roller 14 is controlled by a constant voltage as an example. It is a positive DC voltage of "+3 kV". The static elimination voltage (Vc) is determined by the type of recording material P, and in the case of "thin paper", it is a negative DC voltage of "-5 kV" (see Table 1). As described above, the secondary transfer voltage (Vt) is applied to the secondary transfer unit T2 due to the increase in electrical resistance of the intermediate transfer belt 10, the secondary transfer inner roller 13, and the secondary transfer outer roller 14 due to durability. The voltage required to pass a constant current rises. As an example, in order to pass a current of "70 μA" to the secondary transfer unit T2 after forming an image on 600,000 sheets of "thin paper" under the same conditions as the same member (during durability), the secondary transfer voltage (Vt) is used. It is necessary to apply a positive DC voltage of "+ 5 kV (3 + 2)".

Even if the electrical resistance of the intermediate transfer belt 10, the secondary transfer inner roller 13, and the secondary transfer outer roller 14 changes due to durability, the applied voltage (Vb) and static elimination voltage (Vc) of the secondary transfer inner roller 13 are increased. The static elimination voltage (Vc) is set so as to maintain the voltage difference with. Here, the static elimination voltage (Vc) during durability is maintained at "-5 kV". By doing so, in the case of the external power supply method, the static elimination function by the static elimination needle 91 can be appropriately maintained, and transfer failure from the intermediate transfer belt 10 to the recording material P can be suppressed.

However, in the case of the external power supply method, leakage current is likely to occur when the recording material P, which has a lower surface resistivity on the back surface than the front surface, passes through the secondary transfer unit T2. That is, most of the current flowing through the secondary transfer unit T2 passes through the recording material P, and is arranged on the downstream side in the recording material transport direction from the secondary transfer unit T2 (specifically, the fixing roller and the pressurizing device 15). There was a leak to the roller) or the transport roller pair (not shown). For example, when the recording material P is an OHP sheet for roentgen, the surface resistivity on the front surface side facing the intermediate transfer belt 10 is “about 9 × 10 ^ 15Ω”, and the surface resistivity on the back surface side is “3 × 10”. It is about 7Ω, and the above-mentioned leakage current can occur. When such a leakage current occurs, a transfer defect occurs in which the toner image is not sufficiently transferred from the intermediate transfer belt 10 to the recording material P.

In the case of the internal power supply method shown in FIG. 3 as opposed to the external power supply method shown in FIG. 5, when the recording material P having a lower surface resistivity on the back surface than the front surface passes through the secondary transfer unit T2, It has been confirmed by the experiments of the inventors that leakage current is less likely to occur. Therefore, as described above, the internal power supply method is adopted in this embodiment. Even when the internal power supply method is adopted, the static elimination voltage is maintained so as to maintain the voltage difference between the applied voltage (secondary transfer voltage Vb) and the static elimination voltage (Vc) of the secondary transfer inner roller 13 as in the external power supply method. By controlling the setting of (Vc), a stable static elimination function can be realized.

As described above, in the present embodiment, when the static elimination voltage is applied to the static elimination needle 91 to form an image, the static elimination needle 91 is changed according to the change of the secondary transfer voltage applied to the secondary transfer inner roller 13. The static elimination voltage to be applied is changed. The static elimination voltage is changed so that the voltage difference between the secondary transfer voltage before the change and the static elimination voltage before the change is the same as the voltage difference between the secondary transfer voltage after the change and the static elimination voltage after the change. .. That is, the voltage difference between the secondary transfer voltage and the static elimination voltage is always maintained constant. In particular, even if the electrical resistance of the intermediate transfer belt 10, the secondary transfer inner roller 13, and the secondary transfer outer roller 14 changes due to durability, the voltage difference between the secondary transfer voltage and the static elimination voltage is maintained constant. By doing so, the static elimination function by the static elimination needle 91 can be appropriately maintained without providing the charge amount detection sensor, and it is possible to suppress the occurrence of transfer failure from the intermediate transfer belt 10 to the recording material P.

10 ... Image carrying belt (intermediate transfer belt), 13 ... First rotating body (secondary transfer inner roller), 14 ... Second rotating body (secondary transfer outer roller), 91 ... Static elimination means (static elimination needle), 110 ... Control means (control unit), 120 ... first power supply (transfer power supply), 121 ... current detection means (ammeter), 130 ... second power supply (static elimination power supply), 140 ... humidity detection means (temperature / humidity sensor), P ... Recording material, T2 ... Transfer nip part (secondary transfer part)

Claims (7)

An endless image-supporting belt that supports and rotates a toner image,
The first rotating body that abuts on the inner peripheral surface of the image-carrying belt and
A transfer nip portion that abuts on the outer peripheral surface of the image-supporting belt so as to sandwich the first rotating body and the image-supporting belt and transfers a toner image from the image-supporting belt to the recording material while sandwiching and transporting the recording material. The second rotating body to form and
The first power supply that applies voltage to the first rotation,
A static elimination means that is arranged on the downstream side of the transfer nip portion in the transport direction of the recording material and removes the charge of the recording material that has passed through the transfer nip portion by applying a static elimination voltage.
A second power supply that applies a voltage to the static elimination means,
A control means for controlling the first power supply and the second power supply, respectively, is provided.
When the transfer voltage applied to the first rotating body is changed at the time of image formation, the control means maintains a voltage difference between the transfer voltage and the static elimination voltage before and after the change of the transfer voltage. As described above, the static elimination voltage applied to the second power source is changed.
An image forming apparatus characterized in that. The control means changes the static elimination voltage by adding the difference between the transfer voltage after the change and the transfer voltage before the change to the reference value determined according to the type of recording material.
The image forming apparatus according to claim 1. A current detecting means for detecting the current flowing through the transfer nip portion is provided.
The control means obtains and obtains voltage-current characteristics by applying a plurality of steps of voltage to the first rotating body by the first power supply and measuring the current flowing through the transfer nip portion by the current detecting means. Based on the voltage-current characteristics, it is possible to execute a change mode that changes the transfer voltage applied to the first rotating body at the time of image formation.
The control means adds the difference between the transfer voltage after the change and the transfer voltage before the change changed by executing the change mode.
The image forming apparatus according to claim 2. The control means uses the first reference value when the basis weight of the recording material is the first basis weight, and from the first reference value when the basis weight of the recording material is the second basis weight smaller than the first basis weight. Also uses a second reference value that is large in absolute value,
The image forming apparatus according to claim 2 or 3. Equipped with a humidity detection means to detect humidity,
The control means uses the first reference value when the water content of the environment based on the detection result of the humidity detecting means is the first water content, and the water content of the environment is larger than the first water content of the second water content. At this time, the second reference value, which is larger in absolute value than the first reference value, is used.
The image forming apparatus according to claim 2 or 3. The first reference value is the same potential as ground.
The image forming apparatus according to claim 4 or 5. The second rotating body is grounded.
The image forming apparatus according to any one of claims 1 to 6.

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