Image forming apparatus capable of smooth transmission of recording medium

Disclosed is an image forming apparatus including a first member having a surface endlessly moving in a specific direction a second member having a surface endlessly moving in the specific direction at a region facing the surface of the first member, and a supporting member holding the second member and to move together with the second member. The supporting member includes a release mechanism to release an engagement of the supporting member from the second member. A pressing member is included to press the second member via the supporting member to cause the second member to contact the first member under pressure to form a nip between the first and second members. The release mechanism releases the engagement of the supporting member from the second member when the second member is pushed away by a leading edge of a recording medium having a thickness greater than a reference thickness value.

PRIORITY STATEMENT

This patent specification is based on Japanese patent application, No. 2005-189547 filed on Jun. 29, 2005 in the Japan Patent Office, the entire contents of which are incorporated by reference herein.

BACKGROUND

This patent specification generally describes an image forming apparatus. For example, it generally describes one capable of smooth transmission of recording medium.

2. Discussion of the Background

Background image forming apparatuses, such as printers, facsimiles, copiers, and multifunction apparatuses which print, fax, copy, and so on generally use an electrophotographic process for image forming. The electrophotographic process includes a charging process of an image carrier, a latent image forming process to form the image on the image carrier, a developing process in which toner is adhered, a transferring process which transfers the toner image to an intermediate transfer belt and transfers the toner image to the recording medium at a transfer device and a fixing process to fix the toner image at a fixing apparatus.

The transfer device forms a transfer nip by a pressuring force of a spring mechanism so as to contact a transfer roller with a pressure to the intermediate transfer belt which is the image carrier moving endlessly. The transfer roller is pressed in a direction of a thickness of the recording medium so as to give a pressure to any recording medium even if the recording medium is thick.

When the recording medium is conveyed to the transfer nip, a necessary space equal to a thickness of the recording medium is formed by a pushing force of a leading edge of the recording medium so that the recording medium pass through the transfer nip. If the recording medium is thin, the necessary space for the thickness of the recording medium is easily formed by a reform of a rubber layer of the transfer roller. The thin recording medium can pass through the transfer nip without stacking at the transfer nip.

If the recording medium is thick, it is not possible to obtain a necessary space for the thickness of the recording medium simply by the reform of the rubber layer of the transfer roller. It is needed that the transfer roller is retracted by the leading edge of the recording medium to form the necessary space. Otherwise, the recording medium can not enter the transfer nip and stack at a position immediately before the transfer nip until the necessary space for the thickness of the recording medium is formed.

If the stacking of the recording medium occurred for a relatively long time, a load to the intermediate transfer belt is increased due to a friction between the recording medium and the intermediate transfer belt. Due to the increase of the load of the intermediate transfer belt, a speed of the intermediate transfer belt may change. As a result, an uniformity of the color density which is called a shock jitter is occurred.

SUMMARY

This patent specification describes at least one embodiment of a novel image forming apparatus which includes a first member having a surface endlessly moving in a specific direction a second member having a surface endlessly moving in the specific direction at a region facing the surface of the first member, a supporting member to hold the second member and to move together with the second member, the supporting member including a release mechanism to release an engagement of the supporting member from the second member and a pressing member to press the second member via the supporting member to cause the second member to contact the first member under pressure to form a nip between the first and second members. The release mechanism releases the engagement of the supporting member from the second member when the second member is pushed away by a leading edge of a recording medium having a thickness greater than a reference thickness value.

This patent specification further describes at least one embodiment of a novel image forming apparatus which includes a release mechanism having a detector to detect an event that the second member is pushed away by a recording medium having a thickness greater than the reference thickness value and an actuator to be driven to rapidly decrease the spring constant of the spring mechanism in response to a detection of the event by the detector.

Further, the patent specification describes at least one embodiment wherein the detector detects an event that a trailing edge of the recording medium having a thickness greater than the reference thickness value exits from the nip and the actuator is driven to rapidly increase the spring constant of the spring mechanism in response to a detection of the event by the detector.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly toFIG. 2, a transfer device for image forming apparatus according to an embodiment of the present invention is described.

FIG. 1illustrates an image forming apparatus100according to a first example embodiment. The image forming apparatus100is a multifunction peripheral (MFP) apparatus which is capable of copying, printing, faxing, scanning, storing, and so on. The MFP is called a digital color copying apparatus and copies a document by being scanned, read, digitized and printed to a recording medium P. The MFP may send and receive an image of the document communicating with a facsimile placed at a distant place. The MFP may print image information which is processed with a computer.

The image forming apparatus100includes an image forming unit1, a paper supply unit2, a document read unit3and an paper-output stock unit4. The paper supply unit2has multistage paper trays21which stock the recording mediums, for example, plain papers, OHP (over head projector) sheets, tracing papers and so on. Each paper tray21is configured to be released from the image forming apparatus100. A sensor is arranged at the image forming apparatus100to sense that the paper tray21is released.

An optional paper supply unit22can be installed additionally if necessary. A manual tray140configured to be openable and closable is arranged on a right side of the image forming unit1. When the manual tray140is opened by being separated from the main body of the image forming apparatus100with an upper portion of the manual tray140as shown inFIG. 1, a bunch of the recording mediums can be stocked onto the manual tray140. A manual-tray sensor may be arranged to detect whether paper is stocked on the manual tray140. The document read unit3is arranged over the image forming unit1to read a document. The paper-output stock unit4is arranged on a left side of the image forming unit1to stock the recording medium having the printed image.

The image forming unit1is arranged at a middle of the image forming apparatus100and includes an intermediate transfer belt5, image forming devices6, an exposure unit7and a fixing apparatus8. The four image forming devices6which form a four color image are arranged in parallel to face the intermediate transfer belt5formed endlessly.

Each image forming devices6includes a photosensitive drum61. Around photosensitive drum61, a charging device62is arranged to charge a surface of the photosensitive drum61. The four image forming devices6further includes a development device63and a cleaning device64. The development device63visualizes an electrostatic latent image which is formed on the photosensitive drum61by exposing a laser light. The cleaning device64removes and collects residual toner on the photosensitive drum61.

The document read unit3includes reader running units32and33. The reader running units32and33includes a document-illumination source and mirrors and are configured to move back and forth to scan to read a document placed on a contact glass31. During the scan, a document image is read and detected as an image signal by a CCD (charge coupled device)35arranged posterior to a lens34. The image signal is digitized so that image forming processing is processed.

Based on the executed signal of the image forming process, a laser diode (LD) emits a laser light. The laser light is exposed onto the surface of the photosensitive drum61so as to form an electrostatic latent image on the photosensitive drum61. The laser light arrives at the photosensitive drum61through polygon mirror and lens. An auto document feeder36is attached over the document read unit3to feed document automatically.

Around the intermediate transfer belt5, a transfer device51and an intermediate transfer cleaning device52are arranged. The transfer device51forms nip to transfer the full color image formed on the intermediate transfer belt5to the recording medium. The intermediate transfer cleaning device52removes and collects residual toner on the surface of the intermediate transfer belt5.

An image forming process on this image forming apparatus will be described. In the image forming device6ofFIG. 1, four color toner image is formed by a common electrophotographic process with a timing in accordance with a rotation of the intermediate transfer belt5.

At a position of the yellow color image forming device, a yellow toner color image formed on the leftmost photosensitive drum is transferred to the intermediate transfer belt5. At a position of the magenta color image forming device, a magenta toner color image formed on the photosensitive drum next to the photosensitive drum for the yellow color image is transferred to the intermediate transfer belt5by superimposing on the yellow toner color image. Similarly, at a position of the cyan color image forming device, a cyan toner color image formed on the photosensitive drum next to the magenta photosensitive drum for the magenta color is transferred to the intermediate transfer belt5.

Further, at a position of the black color image forming device, a black toner color image formed on the photosensitive drum next to the photosensitive drum for the cyan color image is transferred to the intermediate transfer belt5. Thus, as a result of the superimposition of four color toner images, a full color toner image is formed on the intermediate transfer belt5by transferring each color toner image formed on each photosensitive drum.

In parallel with the image forming processed on the intermediate transfer belt5, a recording medium is fed one by one by being separated from a specified paper tray21of the paper supply unit2. A bunch of recording mediums are piled on a baseplate24movably supported by the paper tray21. By the movement of the baseplate24, the bunch of recording mediums lifted up to a position at which a top recording medium of the bunch of the recording mediums contacts a pickup roller25. The top recording medium is fed in accordance with a rotation of the pickup roller25. The top recording medium is separated from a following recording medium by a reverse roller27. The top recording medium separated is fed from the paper tray21by a rotation of a paper-feed roller26and is sent to a resist roller23arranged at a downstream of a convey path.

When the recording medium is conveyed to the resist roller23, the recording medium is stopped to convey and is held at a nip formed by the resist roller23. The resist roller23is controlled to start to rotate at a timing so that a position of the full color image formed on the intermediate transfer belt5matches with a position of a leading edge of the recording medium with a designated positional relationship. By the rotation of the resist roller23, the recording medium held is to be fed again. The full color image formed on the intermediate transfer belt5is transferred onto the recording medium at a designated position by the transfer device51.

The recording medium having the full color image transferred is conveyed to the fixing apparatus8. The fixing apparatus8fixes the full color image transferred by the transfer device51onto the recording medium. The recording medium is output to the paper-output stock unit4by paper-output rollers41.

When a double-side image forming is performed, the recording medium is separated at a separation device (not shown) and is turned over by passing through a double-side unit (not shown). Then, the recording medium is sent to the nip of the resist roller23. A backside image is formed at a backside of the recording medium after an adjustment of a skew caused while conveying.

FIG. 2illustrates a schematic of the transfer device51ofFIG. 1. The transfer device51includes a transfer roller110, a support mechanism120and a coil spring150and forms a transfer nip. The support mechanism120supports the transfer roller110. The coil spring150gives pressure to the transfer roller110.

The transfer roller110is a roller with a light weight and a low inertial force. The transfer roller110has a pipe shape having a hollow cylinder and is formed with materials having a specific gravity of 2.8, for example, aluminum, alloy aluminum high-strength resin and so on. An elastic rubber is formed on the surface of the transfer roller110. The support mechanism120includes a support release member130and an arm121. The support release member130is attached to a roller bearing110bengaged with a shaft110awhich is extending from the transfer roller110. The arm121supports the transfer roller110via the support release member130and is rotatably attached to the main body of the image forming apparatus100by an attachment pin122.

The support mechanism120is pushed upwards by the coil spring150. The support mechanism120and the transfer roller110are configured to move up and down as one piece because of a following reason. The transfer roller110is a light weight roller with a low inertial force as described. If the transfer roller110moves up and down by being affected by an irregularity of a thickness of the recording medium P, the transfer roller110may not give a stable pressure to the intermediate transfer belt5to transfer the image.

In this example embodiment, the transfer roller110can move smoothly because the support mechanism120and the transfer roller110move up and down as one piece so that the support mechanism120works like a flywheel. Further, a weight of the support mechanism120is determined to be heavier than the weight of the transfer roller110so as to work well as a flywheel.

FIG. 3illustrates a schematic of the transfer nip of the transfer device51to explain a situation in which a leading edge of the recording medium P goes into the transfer nip. A necessary space for a thickness of the recording medium P is required to be formed so that the recording medium P passes through the transfer nip. The recording medium P can not goes into the transfer nip until the necessary space for the thickness of the recording medium P is formed. The recording medium P may stack at a position immediately before the transfer nip.

If the stacking of the recording medium occurred for a relatively long time, a load to the intermediate transfer belt5is increased due to a friction between the recording medium P and the intermediate transfer belt5. Due to the increase of the load of the intermediate transfer belt5, a speed of the intermediate transfer belt5may change. As a result, an uniformity of the color density which is called a shock jitter is occurred.

If the recording medium P is thin, a necessary space for the thickness of the recording medium P may be formed by a reform of the rubber layer of the transfer roller110. The thin recording medium P can easily pass the transfer nip because the necessary space for the thin recording medium P is small and can be formed in a short time. Thus, when the recording medium P is thin, a load of the intermediate transfer belt5due to the friction between the recording medium P and the intermediate transfer belt5is not increased because the stacking time is short. As a result, the speed of the intermediate transfer belt5is kept constant so as to avoid a shock jitter.

If the recording medium P is thick and has a basic weight of at least 60 [g/m2], it is not possible to obtain a necessary space for the thickness of the recording medium P only by the reform of the rubber layer of the transfer roller110. It is needed that the transfer roller110is retracted by the leading edge of the recording medium P to form the necessary space. The recording medium P can not go into the transfer nip and stacks at a position immediately before the transfer nip until the necessary space for the thickness of the recording medium P is formed.

During the stack of the recording medium P, a load to the intermediate transfer belt5is increased due to a friction between the recording medium P and the intermediate transfer belt5. Due to the increase of the load of the intermediate transfer belt5, the speed of the intermediate transfer belt5may change. As a result, an uniformity of the color density which is called a shock jitter is occurred during transferring the image to the intermediate transfer belt5.

The transfer roller110is needed to be retracted to form a space in a short time such that the recording medium P can pass the transfer nip without a slowing down i.e., without stopping at the position immediately before the transfer nip. If the conveying speed of the recording medium P is increased, the transfer roller110is needed to be retracted in much shorter time to form the necessary space. A necessary force to move the transfer roller110by a thickness of the recording medium P depends on an inertial force of the transfer roller110and the support mechanism120and a drag force of the coil spring150. The inertial force system will be focused and described.

If the conveying speed of the recording medium P is slow, a slower retraction of the transfer roller110is allowed to form a necessary space for the recording medium P passing through the transfer nip without stack of the recording medium P at the position immediately before the transfer nip. The inertial force of the transfer roller110and the support mechanism120can be smaller values. The necessary force to move the transfer roller110depends mostly on the drag force of the coil spring150which pushes the transfer roller110and the arm121to the intermediate transfer belt5. The recording medium P can be conveyed without stacking at the position immediately before the transfer nip by adjusting a spring constant of the coil spring150.

If the conveying speed of the recording medium P is faster, the transfer roller110is needed to be retracted in a shorter time. A quicker retraction of the transfer roller110is requested to avoid the stacking of the recording medium P at the position immediately before the transfer nip. In this case, a rapid acceleration of the transfer roller110and the support mechanism120is needed. The inertial force of the transfer roller110and the support mechanism120becomes large due to the large accelerated velocity. The inertial force is so large enough to neglect the drag force of the coil spring150.

The inertial force is generally proportional to square of the a speed and is depending on a mass. If the masses of the transfer roller110and the support mechanism120are large, the inertial force is larger. (When the support mechanism120is rotatably supported by one supporting point as shown inFIG. 2, an equivalent value which is calculated based on the transfer roller110and the support mechanism120will be used.)

To make the necessary force smaller, a transfer roller110having a smaller mass and a lower inertial force may be proposed to use. If the transfer roller110having a smaller mass is used, however, the transfer roller110is affected by small irregularities of the recording medium P. As a result, the transfer roller110can not give a designated pressure to the intermediate transfer belt5.

In this example embodiment, the transfer roller110is supported by the support mechanism120at a position between the transfer roller110and the support mechanism120during a normal operation. During the normal operation, the inertial force is large because the transfer roller110and the support mechanism120move as one unit. As a result, the transfer roller110is not affected by the small irregularities of the recording medium P.

When a leading edge of the recording medium P having a thickness greater than a reference thickness value is going into the transfer nip, the support release member130releases the transfer roller110from the support mechanism120so that the transfer roller110and the support mechanism120do not move as one unit. As a result, a value of the drag force to the recording medium P which is located in a side of the transfer roller110is equal to a value of the inertial force of the transfer roller110.

The leading edge of the recording medium P can easily push the transfer roller110away. The recording medium P can pass the transfer nip in a relatively short time without stacking at the position immediately before the transfer nip. The load of the intermediate transfer belt5is not increased so that a shock jitter is avoided. The release mechanism130will be described in detail referring to a few example embodiments.

FIGS. 4A and 4Billustrate schematics of the release mechanism130.FIG. 4Ais an illustration of the release mechanism130when a thick recording medium P is not passing through the transfer nip.FIG. 4Bis an illustration of the release mechanism130when the thick recording medium P is passing through the transfer nip.

The release mechanism130includes a plate-shaped spring131, a moving member132, a coil spring133and a base member134. The release mechanism130is configured to change the spring constant of the spring rapidly in a nonlinear way when the moving member132is pressed down. The moving member132is attached on the base member134via the coil spring133. The moving member132is configured to move up and down by a guide member (not shown).

On the upper surface of the moving member132, a bearing is arranged to engage with the shaft110aof the transfer roller110. The plate-shaped spring131is attached to a side wall of the base member134with an end of the plate-shaped spring131. The plate-shaped spring131has a dogleg shape at another end to support an under portion of the moving member132.

A spring constant of the plate-shaped spring131is determined to be a larger number than the spring constant of the coil spring150which presses the transfer roller110and the support mechanism120to the intermediate transfer belt5. A spring constant of the coil spring133is determined to be a smaller number than the spring constant of the coil spring150. The arm121is attached at an underside of the base member134. An operation of the release mechanism130will be described.

FIG. 5Aillustrates a change of the drag force for a time period from a time the leading edge of the recording radium hits the transfer nip to a time a space which allows the recording radium P to go into the transfer nip is created. InFIG. 5A, the drag force of the coil spring150is neglected because the inertial force of the transfer roller110and the support mechanism120to the recording medium P at the transfer roller side is large enough.

FIG. 5Billustrates a change of a moving distance of the transfer roller110for a time period from a time the recording medium hits the transfer nip to a time the space which allows the recording medium to go into the transfer nip is created. A point “a” inFIG. 5Ais a time the plate-shaped spring131is released and a point “b” inFIG. 5Ais a time the coil spring133is constricted.

When the recording medium P having a thickness greater than a reference thickness value goes into the transfer nip, the transfer roller110, the transfer roller110, the arm121and the release mechanism130are pushed down by a pushing force of the recording medium P. At the same time, the moving member132is moved downward as shown inFIG. 4B.

The drag force against the pushing force which the leading edge of the recording medium P pushes the transfer member110is a little bit smaller value than a value expressed by a formula (m+M1+M2)α, where m is mass of the transfer member110, M1is mass of the arm121, M2is mass of the release mechanism130and α is an accelerated velocity of these three m, M1and M2. This is because the transfer member110, the arm121and the release mechanism130are not moving together exactly as one piece and the transfer member110is only pushed down with the downward movement of the moving member132.

When the moving member132moves downward more than a designated distance, the moving member132is free from the support of the plate-shaped spring131and the coil spring133works dominantly. Thus, the spring constant of the release mechanism130rapidly decreases. The arm121is moved towards the transfer roller110by the pushing force of the coil spring150and the transfer roller110is moved towards the arm121by the pushing force of the recording medium P. The coil spring133of the release mechanism130is shrunk by these pushing forces.

Due to the rapid decrease of the spring constant of the release mechanism130, a length of a supporting portion of the support mechanism120which supports the transfer roller110decreases rapidly in a direction of the thickness of the recording medium P. In this example embodiment, a length of the coil spring133of the release mechanism130decreases. As a result, a similar situation where there is no support member to support the transfer roller110is generated and a space between the arm121and the transfer roller110is created.

Thus, the transfer roller110does not move together with the arm121. While the coil spring is shrunk from the point “a” to “b” shown inFIG. 5A, the drag force to the recording medium P at the transfer roller side becomes the inertial force of the transfer roller110i.e., the force is expressed by a formula mα, where m is mass of the transfer roller110and α is an accelerated velocity of the m. Strictly speaking, an inertial force of the of the moving member132and a drag force of the coil spring133are existing. However, the inertial force of the of the moving member132and the drag force of the coil spring133are negligible small.

Thus, the coil spring is shrunk and the drag force is mα while a period between the points “a” and “b”. A space H which is equal to the thickness of the recording medium P is quickly created by the pushing force of the leading edge of the recording medium P to the transfer roller110. The leading edge of the recording medium P may not stack at the position immediately before the transfer nip for a relatively long time. The recording medium P is smoothly passing through the transfer nip. The increase of the load to the intermediate transfer belt can be avoided.

As the transfer roller110employs a roller having a smaller mass and a lower inertial force, the drag force to the recording medium P at the transfer roller side can be made smaller in a period the coil spring is shrunk. As a result, the transfer roller110can be retracted quickly so that the necessary space for the thickness of the recording medium P is formed quickly. Therefore, the recording medium P enters the transfer nip smoothly without stacking at the position immediately before the transfer nip.

After the leading edge of the recording medium P have been entered the transfer nip, the coil spring133is fully compressed and may not work as a spring. As a result, the arm121, the release mechanism130and the transfer roller110are pushed as one piece by the coil spring150to the intermediate transfer belt5while the recording medium P is passing through the transfer nip.

As shown inFIG. 5A, the drag force to the recording medium P at the transfer roller side becomes a value expressed by a formula (m+M1+M2)α, where m is mass of the transfer member110, M1is mass of the arm121, M2is mass of the release mechanism130and α is an accelerated velocity of these three m, M1and M2. As a result, a transfer pressure can be maintained stably with a designated value and without jittering of the transfer roller110having a low inertial force.

A threshold to release the plate-shaped spring is to be determined by experiments using actual equipment with repetition of trial-and-errors. It is repeated to print an image on recording mediums having different thicknesses using the actual equipment in which the release mechanism130is fixed not to work. Every printing result on the recording medium is observed checking whether the shock jitter is occurred. The thinnest recording medium is selected from among the recording mediums on which the shock jitter is occurred. Using the thinnest recording medium with which shock jitter is observed, a threshold at which the plate-shaped spring131releases is searched. With the example embodiment, the shock jitter is observed when the recording medium having a basic weight of equal to and more than 60 [g/m2] is used.

After a trailing edge of the recording medium has passed the transfer nip, the moving member132is moved towards the intermediate transfer belt5and pushes the transfer roller110up. If the transfer roller110is lifted up by a threshold distance, the moving member132is supported and fixed again by the plate-shaped spring131as shown inFIG. 4A.

If the recording medium P is thinner than the reference thickness value, the recording medium P can pass the transfer nip keeping the configuration shown inFIG. 4Abecause the necessary space for the thickness of the recording medium P is quickly obtained by the reform of the rubber layer of the transfer roller110without release of the plate-shaped spring131. The drag force to the recording medium P at the transfer roller side is a value expressed by a formula (m+M1+M2)α, where m is mass of the transfer member110, M1is mass of the arm121, M2is mass of the release mechanism130and α is an accelerated velocity of these three m, M1and M2. Therefore, a transfer pressure can be maintained stably with a designated value and without jittering of the transfer roller110having a low inertial force.

FIGS. 6A and 6Billustrate an absorbing mechanism230according to a second example embodiment.FIG. 6Ais a schematic of the absorbing mechanism230when a thick recording medium P is not passing through the transfer nip.FIG. 6Bis a schematic of the absorbing mechanism230when a thick recording medium is passing through the transfer nip.

The absorbing mechanism230includes a moving member232, a solenoid coil spring233and a base member234. The moving member232is attached on the base member234via the solenoid coil spring233. The moving member232is configured to move up and down by a guide member (not shown). On the upper surface of the moving member232, a bearing is arranged to engage with the shaft110aof the transfer roller110. The base member134is attached at the arm121.

The solenoid coil spring233is attached to a base surface234aof the base member234at a left end portion of the base member234as shown inFIG. 6A. The solenoid coil spring233has a shape of a circular arc. A middle of the circular arc of the solenoid coil spring233slightly deviates from a straight line to a right direction with an almost fully compressed condition. Thus, the solenoid coil spring233is not fully compressed when a thick recording medium P is not passing through the transfer nip. If the solenoid coil spring233is fully compressed, a small positional change of the transfer roller110may cause a buckling of the solenoid coil spring233. A spring constant of the solenoid coil spring233is determined to be a larger number than the spring constant of the coil spring150which presses the transfer roller110and the support mechanism120to the intermediate transfer belt5.

When the recording medium P having a thickness greater than a reference thickness value passes through the transfer nip, the leading edge of the recording medium P pushes the transfer roller110downward ofFIG. 6A. The moving member232which is attached to the bearing110aof the transfer roller110is moved downward as shown inFIG. 6B.

If a distance of the moving member232to be pushed down is smaller than a threshold, the solenoid coil spring233maintains a designated drag force to a compression force. The drag force to the recording medium P at the transfer roller side becomes almost equal to a value expressed by a formula (m+M1+M2)α, where m is mass of the transfer member110, M1is mass of the arm121, M2is mass of the release mechanism130and α is an accelerated velocity of these three m, M1and M2.

If the moving member232is moved down more than the threshold, the solenoid coil spring233is fully compressed and does not work as a spring. The solenoid coil spring233buckles to a right side as shown inFIG. 6Band looses the drag force. The transfer roller110experiences a similar situation in which the transfer roller110looses any support. The arm121moves to the transfer roller110by the pushing force of the coil spring150. The transfer roller110moves to the arm121by the pushing force of the recording medium P. The solenoid coil spring233contacts with a right side wall234bof the base member234as shown inFIG. 6B.

Thus, the solenoid coil spring233which supports the transfer roller110buckles and a length of the portion of the solenoid coil spring233decreases rapidly in a direction of the thickness of the recording medium P. Until the solenoid coil spring233contacts a right side wall234bof the base member234as shown inFIG. 6B, the transfer roller110experiences a similar situation in which the transfer roller110looses the support. The drag force to the recording medium P at the transfer roller side becomes the inertial force expressed by a formula mα, where m is mass of the transfer roller110and α is an accelerated velocity of the m.

When the solenoid coil spring233buckles by pushing the transfer roller110downward with the leading edge of the recording medium P, a space which is equal to the thickness of the recording medium P is quickly created. The leading edge of the recording medium P may not stack at the position immediately before the transfer nip for a relatively long time. The recording medium P is smoothly passing through the transfer nip. A threshold with which the solenoid coil spring233buckles is to be determined by experiments using actual equipment after repeating trial-and-errors.

When the solenoid coil spring233buckles and contacts with the right side wall234bof the base member234, the solenoid coil spring233can not bend further. Therefore, after the necessary space for the thickness of the recording medium P is formed, the transfer roller110, the arm121and the release mechanism130are pushed as one piece by the coil spring150to the side of the intermediate transfer belt. Therefore, while the recording medium P is passing through the transfer nip, a transfer pressure can be maintained stably with a designated value and without jittering of the transfer roller110having a low inertial force.

When the trailing edge of the recording medium P passes through the transfer nip, the solenoid coil spring233is forced back to straighten up. When the length of the solenoid coil spring233becomes above a threshold, the solenoid coil spring233is released from the buckling so as to get back gain to the condition ofFIG. 6A.

A release mechanism330according to a third example embodiment will be described. The release mechanism330includes a power detection sensor and an actuator333. The power detection sensor detects a power from the recording medium P to push the transfer roller110. When the power detection sensor detects a certain power, the actuator333is driven to rapidly decrease the length of the actuator to generate a similar condition in which the transfer roller110looses the support with the transfer roller110and the arm121. The drag force (equal to the inertial force) is reduced when the leading edge of the recording medium P pushes the transfer roller110.

FIG. 7illustrates the release mechanism330according to the third example embodiment. The release mechanism330further includes a moving member332and a base member334in addition to the actuator333. Similarly to the first and second example embodiment, a bearing with which the shaft110aof the transfer roller110is engaging is attached. The base member334is fixed to the arm121. The moving member332is connected to the base member334via the actuator333.

As for the actuator333, a various sorts of actuators can be selected from among, for example, a piezoelectric device using electric and magnetic strain, an electromagnetic actuator such as voice coil and an electrostatic actuator using an electrostatic force. In this third example embodiment, the piezoelectric device is used. The piezoelectric device is connected to a control circuit335by electric wires (not shown).

FIG. 8illustrates a cross sectional view of the power detection sensor. A pressure-sensitive resistance film is used to form the power sensor. As shown inFIG. 8, a pressure-sensitive resistance film layer114is formed on a surface of a metallic film layer113which is arranged on a rubber layer112formed on a core metal111of the transfer layer110. The pressure-sensitive resistance film layer114is connected to a measurement circuit336by electric wires (not shown).

When the recording medium P pushes the transfer roller110, a resistance value of the pressure-sensitive resistance film layer114changes. By detecting the resistance value of the pressure-sensitive resistance film layer114, a pushing force of the recording medium P is detected. The actuator333besides the pressure-sensitive resistance film layer114may be used as the power sensor.

If the piezoelectric device is installed, an electric and magnetic strain is occurred when the pushing force of the recording medium P is transmitted to this piezoelectric device. A pushing force of the recording medium P can be detected by detecting the electric and magnetic strain.

If the electromagnetic actuator such as voice coil is installed, an electric signal is changed when the recording medium P pushes the transfer roller110. A pushing force of the recording medium P can be detected by detecting the change of the electric signal. If the electrostatic actuator is installed, a capacitance is changed when the recording medium P pushes the transfer roller110. A pushing force of the recording medium P can be detected by detecting the change of the capacitance signal.

It is an indirect measurement via the transfer roller110to use the actuator333to detect the pushing force of the recording medium P. Therefore, the measurement using the actuator333may have a slightly longer time rag in comparison with the direct measurement to use the pressure-sensitive resistance film layer114to detect the pushing force of the recording medium P. It may delay more to drive the actuator333in comparison with the direct measurement of the pushing power using the pressure-sensitive resistance film layer114. The direct measurement to detect the pushing power for the transfer roller110using the pressure-sensitive resistance film layer114may be able to prevent the recording medium P more steadily from stacking at the position immediately before the transfer nip in comparison with the indirect measurement using the actuator333.

FIG. 9illustrates a flow chart to control the release mechanism330according to the third example embodiment. The power sensor detects the pushing power of the recording medium P to the transfer roller110. (S1) The resistance of the pressure-sensitive resistance film layer114is detected. (when the pressure-sensitive resistance film layer114is used as the power sensor) A signal, for example, a voltage generated by the actuator333is detected. (when the actuator333is used as the power sensor) After the detection, it is checked whether the detected value (voltage, resistance and so on) exceeds the threshold. (S2)

The threshold is a power detected by a power detection sensor when the leading edge of the thinnest recording medium contacts and pushes the transfer roller110based on experiments using a thinnest recording medium on which the shock jitter is observed. If the power is the above threshold, (YES in S2) the measurement circuit336is shut off for a safety reason of the measurement circuit336of the power detection sensor. (S3) A certain voltage is applied for a designated period to the actuator333. (S4) The piezoelectric device rapidly shrinks and forces the moving member332and the base member334to get closer.

During the shrink of the piezoelectric device, the transfer roller110moves to a side of the arm121and the arm121moves to a side of the transfer roller110. The transfer roller110and the arm121do not move together as one piece. A similar situation in which the transfer roller110looses the support is generated due to the rapid shrunk of the piezoelectric device. The drag force to the recording medium P at the transfer roller side becomes the inertial force expressed by a formula mα, where m is mass of the transfer roller110and α is an accelerated velocity of the m. Figuratively speaking, a situation as if the leading edge of the recording medium kicks off the transfer roller110with no weight to the arm side is generated.

The necessary space for the thickness of the recording medium P is formed quickly so that the recording medium P may not be stacked at the position immediately before the transfer nip. If the piezoelectric device shrinks more quickly, the drag force to the recording medium P at the transfer roller110can be reduced more rapidly.

A maximum stroke of the piezoelectric device is defined to be a distance equal to the space between the intermediate transfer belt5and the transfer roller110for a maximum thickness of the recording medium which the image forming apparatus can handle. For example, if the maximum thickness of the recording medium P which the image forming apparatus100can handle is a thickness similar to a containerboard, the stroke of the piezoelectric device may be enough to be 1 [mm].

After a time period, the piezoelectric device is stretched by shutting off with the power to the piezoelectric device. The transfer roller110is contacted to the recording medium P. The power sensor and the measurement circuits are connected again. (S5) These steps are repeated. While the recording medium P is passing through the transfer nip, the transfer roller110does not jitter because the arm121and the transfer roller110moves as one piece and pushes the recording medium P by the coil spring150.

The maximum stroke of the piezoelectric device can be a different value flexibly depending on the thickness of the recording medium P. A threshold corresponding to a recording medium to be used and a suitable voltage to be applied to the piezoelectric device may be input in a table and stored in a memory. If the recording medium P is thicker, a power which the leading edge of the recording medium P pushes the transfer roller110is stronger.

When the power is detected by the detection sensor, the threshold corresponding to the detected power is searched. A voltage corresponding to the threshold is applied to the piezoelectric device. The transfer roller110moves down to the arm121by a corresponding distance to the thickness of the recording medium P. The space between the intermediate transfer belt5and the transfer roller110is to be equal to the thickness of the recording medium. Further, the voltage to be applied to the piezoelectric device is controlled with a waveform so as to avoid undesired vibration of the piezoelectric device.

When the trailing edge of the thick recording medium P passes through the transfer nip, the actuator333may be driven to rapidly increase the length of the actuator333. During a time period from a time the trailing edge of the thick recording medium P passes through the transfer nip to a time the transfer roller110contacts the intermediate transfer belt5, the intermediate transfer belt5experiences a similar situation to have no load. The speed of the intermediate transfer belt5may change to cause an uniformity of the color density.

With this reason, the transfer roller110may be configured to quickly contact the intermediate transfer belt5by driving the actuator333immediately after the trailing edge of the thick recording medium P passes through the transfer nip. Thus, the time period from a time the trailing edge of the thick recording medium P passes through the transfer nip to a time the transfer roller110contacts the intermediate transfer belt5again is made short. The time period the transfer roller110has no load becomes short. As a result, it is possible to avoid an uniformity of the color density due to the speed change of the intermediate transfer belt5.

More specifically, the resistance of the pressure-sensitive resistance film layer114becomes a lower value by the transferring pressure while the recording medium P passes through the transfer nip. After the trailing edge of the thick recording medium P has passed through the transfer nip, no pressure is applied to the pressure-sensitive resistance film layer114and the resistance of the pressure-sensitive resistance film layer114changes. When the change is detected, the actuator333is stretched by driving the actuator333.

The distance to be stretched is determined to be the equal thickness of the recording medium P. Thus, the transfer roller110moves rapidly to a side of the intermediate transfer belt5so as to contact the intermediate transfer belt5. Thus, the time period from a time the trailing edge of the thick recording medium P passes through the transfer nip to a time the transfer roller110contacts the intermediate transfer belt5is made short. The change of the load of the intermediate transfer belt5which causes the speed change of the intermediate transfer belt5can be made relatively small. As a result, it is possible to avoid an uniformity of the color density due to the speed change of the intermediate transfer belt5.

If the control described in the third example embodiment is performed only for a time period from a time the leading edge of the recording medium P is expected to enter the transfer nip to a time the trailing edge of the recording medium P is expected to pass through the transfer nip, the durability of the release mechanism will be improved. When the recording medium P is fed from the resist roller23, a power sensor has been detecting a power for a time period. If the power is not detected during the time period, an alarm signal may be output to inform an user of an occurrence of paper jam. Thus, the power detection mechanism can be used both as the power detection mechanism and the jam detection mechanism.

A release mechanism430according to a fourth example embodiment will be described. The release mechanism430detects with the transfer voltage or the transfer current whether the leading edge of the recording medium contacts the transfer roller110. The actuator433is driven by this detection result.

FIG. 10illustrates the release mechanism430according to the fourth example embodiment. Similarly to the release mechanism330according to the third embodiment ofFIG. 7, the release mechanism430includes a moving member432, an actuator433and a base member434. The moving member432is attached to the shaft of the transfer roller110and the base member434is attached to the arm121. The actuator433connects the moving member432and the base member434.

The actuator433is connected to the control circuit435by electric wires (not shown). The transfer roller110is connected to a power control apparatus436which controls a transfer electric field. The power control apparatus436generally uses a voltage control method or a current control method. When the voltage control method is used, the power control apparatus436always observes the transfer voltage and controls so as to apply a constant voltage. When the current control method is used, the power control apparatus436always observes the transfer current and controls so as to supply a constant current. In the fourth example embodiment, the power control apparatus436is controlled by the current control method.

When the leading edge of the recording medium P is conveyed near an entrance of the transfer nip, a transfer electric field is temporally changed due to difference of dielectric constants between the recording medium P and the air. When the leading edge of the recording medium P contacts the transfer roller110and the intermediate transfer belt5, the transfer current changes because the transfer current flows through the recording medium P.

When the leading edge of the recording medium P pushes the transfer roller110and a space is created between the transfer roller110and the intermediate transfer belt5, the transfer current changes due to cutoff of the current flow. These phenomena are detected as a series of changes of the transfer current. The changes of the transfer current become relatively large at a time the leading edge of the recording medium contacts the transfer roller110and at a time the space is created between the transfer roller110and the intermediate transfer belt5. It is easy to detect these relatively large changes.

There may be a delay if the actuator433is driven after the detection of the current change at a time the space is created between the transfer roller110and the intermediate transfer belt5. The intermediate transfer belt5may stack at the position immediately before the entrance of the transfer nip. Therefore, as for the release mechanism430in the fourth example embodiment, the actuator433is driven by the detection of the change of the transfer current when the recording medium contacts the transfer roller110and the intermediate transfer belt5.

FIG. 11illustrates a flow chart to control the release mechanism430according to the fourth example embodiment. A transfer current is detected. (S1) It is detected whether the leading edge of the recording medium P contacts the transfer roller110and the intermediate transfer belt5. This is judged by checking whether the current is larger comparing to a reference current stored in a memory. The reference current values is obtained by measurements using the thinnest recording medium with which the shock jitter is occurred.

When the leading edge of the recording medium contacts the transfer roller and the intermediate transfer belt5(i.e., When the detected current is larger the reference current value stored in a memory), (YES in S2) a voltage is applied for a time period. (S3)

The transfer roller110is pulled down to the side of the arm121. After a time period, the actuator433is stretched by shutting off with the power to the actuator433. The transfer roller110is contacted to the recording medium P. While the recording medium P is passing through the transfer nip, the transfer roller110does not jitter because the arm121and the transfer roller110moves as one piece and pushes the recording medium P by the coil spring150. The transfer pressure can be maintained stably with a designated value and without jittering of the transfer roller110.

A power is to be detected in the release mechanism330of the third example embodiment as previously described. The actuator333is driven after the leading edge of the recording medium P contacts the transfer roller110and the intermediate transfer belt5and pushes the transfer roller110down by some distance. In the release mechanism430of the fourth example embodiment, the actuator433can be driven immediately after the leading edge of the recording medium P contacts the transfer roller110and the intermediate transfer belt5by detecting an instantaneous change of the transfer current.

Therefore, the actuator433of the fourth example embodiment can be driven more quickly than the actuator333of the third example embodiment so as to form the space for the recording medium P. It is possible more steadily to prevent the recording medium P from stacking at the position immediately before the transfer nip in comparison with the release mechanism330of the third example embodiment.

The stroke of the actuator433can be a different value flexibly depending on the thickness of the recording medium P. The voltage to be applied to the actuator433may be controlled with a waveform so as to avoid undesired vibration of the actuator433.

The actuator433of the fourth example embodiment may be driven when the trailing edge of the thick recording medium P passes through the transfer nip. In the fourth example embodiment, when the trailing edge of the recording medium P passes through the transfer nip, the transfer current changes by cutting a current path off due to the creation of the space between the transfer roller110and the intermediate transfer belt5.

When the change of the transfer current is detected by the power control apparatus436, the actuator433is stretched by the thickness of the recording medium P by driving the actuator433. The transfer roller110moves rapidly to a side of the intermediate transfer belt5so as to contact the intermediate transfer belt5. Thus, the time period from a time the trailing edge of the thick recording medium P passes through the transfer nip to a time the transfer roller110contacts the intermediate transfer belt5is made short. As a result, it is possible to avoid an uniformity of the color density due to the speed change of the intermediate transfer belt5.

The release mechanism may be arranged with a tilt of an angle relative to the vertical direction as shown inFIG. 12. The transfer roller110is pushed down to the side of the arm121and to obliquely downward from an upstream to a downstream of the convey direction of the recording medium P.

With this arrangement of the release mechanism, the recording medium P can keep moving to the convey direction without stacking at a position of entrance of the transfer nip even while the leading edge of the recording medium P is still creating a space for the recording medium P. The recording medium P can be conveyed more stably without stacking and with no jitter of the transfer roller110in comparison to the release mechanism which moves only in a vertical direction.

Further, it may be allowed to take a longer time that the transfer roller110moves downward by the equal distance with the thickness of the recording medium P because the transfer roller110moves together with the recording medium P to the convey direction of the recording medium P. Because slower movement of the transfer roller110can be acceptable to create the space equal to the thickness of the recording medium P, the transfer roller having a lower inertial force may be applicable.

With passive release mechanisms such as the first and second example embodiments, the transfer roller110can be pushed down with a smaller force so as to be pushed down by the recording medium P more smoothly. With active release mechanisms such as the third and fourth example embodiments, it may be allowable to push the transfer roller110down with slower speed.

The release mechanism described above can be applied to the fixing apparatus8which includes a fixing roller and a pressuring roller. A fixing nip is formed with the fixing roller and the pressuring roller which are movable devices configured to move around endlessly. When a thick recording medium is conveyed to the fixing apparatus8having the fixing nip, the recording medium may stack at an entrance of the fixing nip until a space equal to a thickness of the recording medium is formed. If a recording medium being conveyed is staying at the transfer nip which locates upstream to the fixing nip in a convey path of the recording medium, a convey speed may change and cause an uniformity of a color density.

The release mechanism described in the first to forth example embodiments may be used to avoid the problem. With passive release mechanisms such as the first and second example embodiments, the release mechanism changes a spring constant of the release mechanism so as to push down easily with the fixing roller or the pressuring roller when the recording medium contacts the fixing roller and the pressuring roller.

As a result, a necessary space between the fixing roller and the pressuring roller is formed without a stack of the recording medium at the entrance of the fixing nip so that the recording medium passes through the fixing nip. After the leading edge of the recording medium passing through the fixing nip, a toner image is fixed with a designated pressure because a spring is fully shrunk.

With active release mechanisms such as the third and fourth example embodiments, the fixing roller or the pressuring roller moves to a direction to release the fixing nip so as to form the space equal to the recording medium. As a result, it is avoided that the recording medium stacks at the entrance of the fixing nip. After the leading edge of the recording medium passing through the fixing nip, a toner image is fixed with a designated pressure by cutting a drive voltage for the actuator off.