Fixing device for fixing an unfixed toner image formed on a sheet-shaped recording medium

A fixing device for fixing an unfixed toner image formed on a sheet-shaped recording medium is disclosed. A first roller incorporates a heat source therein. A second roller is pressed toward the first roller. A part of an endless belt is wound on an outer periphery of the second roller to be circulated. A non-rotatable belt stretcher stretches the endless belt together with the second roller such that the circulated endless belt is slid on a first face extending in a widthwise direction of the endless belt, and such that a tensile force is applied to both widthwise end portions of the endless belt. The circulated endless belt is nipped between the first roller and the second roller to form a first nip portion through which the unfixed toner image is fixed on the recording medium with heat from the heat source. The first face is curved in the widthwise direction.

BACKGROUND OF THE INVENTION

The present invention relates to a fixing device that fixes an unfixed toner image formed on a sheet medium. The fixing device comprises: a heating roller; a pressing roller that is pressed against the heating roller; a belt that moves by being wound around an outer periphery of the pressing roller and nipped between the pressing roller and the heating roller; and a belt stretcher over which the belt is stretched. The present invention also relates to an image forming apparatus incorporating such a fixing device.

An image forming apparatus such as a copying machine, a printer and a facsimile machine incorporates a heating roller type fixing device in which an unfixed toner on a recording medium is subjected to a press contact with heat to thereby fix the unfixed toner on the recording medium. Such a fixing device comprises: a rotatable heating roller coated with an elastic body on the surface thereof and housing a heat source therein; a belt stretched over plural supporting rollers, and a pressing member for forming a nip region by causing the belt to wind around the heating roller by a predetermined angle while causing the elastic body on the surface of the heating roller to undergo deformation by locally applying a larger pressure to the exit of the nip region than the other portions, thereby making it easier for a sheet medium to be discharged from the nip portion. Such a configuration is disclosed in Japanese Patent No. 3084692.

In the fixing device, the surface of the heating roller has been deformed in advance due to the presence of the pressing member, and the deformation on the surface is relieved momentarily while toner is in contact with the surface of the heating roller. For this reason, when a sheet medium is discharged from the nip portion, an contact force between the toner and the heating roller is reduced in inhibiting the sheet medium from winding around the heating roller, so that even a recording sheet of paper that is not very firm can be readily separated at the exit of the belt nip. This device thus eliminates the need for a separating claw member.

However, plural supporting rollers and the rotation bearings are required. Further, not only the peripheral length of the belt is extended, but also the fixing device becomes complex with an increase in both size and cost. The structure such that makes the fixing device complex with an increase in size and cost naturally makes the image forming apparatus equipped with such a fixing device complex with an increase in size and cost.

Moreover, the belt is heated at the nip portion with the rotatable heating roller housing the heat source; however, with the configuration in which the belt is stretched over plural rollers and its peripheral length is inevitably extended, heat energy is absorbed by plural supporting rollers when it moves along a predetermined path. In addition, natural heat release is increased with the peripheral length. This extends a time needed to reach a predetermined temperature, which in turn extends a so-called warm-up time needed for the fixing operation to be enabled since the apparatus has been activated. Hence, this configuration is not preferable.

Further, the configuration, in which deformation of the elastic layer on the heating roller is caused by winding the belt around the heating roller by an angle needed to form the nip and by applying a larger pressure locally at the exit of the nip portion than the other portions, is suitable in inhibiting a sheet medium from winding around the heating roller; however, the sheet medium discharged along the deformation of the elastic layer is distorted, by being curled along the deformation or wrinkled as a result of a local high pressure.

Japanese Patent Publication No. 6-40235B discloses a fixing device in which an unfixed toner image is fixed on a recording medium by passing through a nip region formed between roller members which are in press contact with each other. In this device, the rotating speed of the roller members can be set to either a first speed or a second speed in accordance with the characteristic of the recording medium.

However, since the heat capacity of the roller members are large, the warm-up time is extended. Moreover, the recording medium passing through the nip region formed by deforming the roller members with a pressure undergoes distortion, such as curls and wrinkles, because an applied stress induced by this pressure is large.

Japanese Patent Publication No. 8-262903A discloses a fixing device in which an endless belt stretched over a heating roller, which is coated with an elastic body on the surface thereof and housing a heat source therein, is run while being in contact with the heating roller, and a non-rotatable pressing pad, provided on the inner side of the endless belt to press the endless belt against the heating roller, not only forms a nip portion, but also causes the elastic layer on the surface of the heating roller to undergo deformation. A sheet medium bearing an unfixed toner image is forced to pass through a space between the heating roller and the endless belt for the toner to be fixed on the sheet medium. Since the pressing pad is not rotated, heat transmitted from the heating roller is hardly dissipated. Therefore it has an advantage that a quantity of heat absorbed from the heating roller is small.

Nevertheless, the warm-up time is extended because heat is transmitted to the pressing pad from the heating roller via the endless belt during the warm-up operation. In addition, three or more rollers are necessary to circulate the belt, which raises another problem that the device is increased in size.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a heating roller type fixing device which is simple, compact and cost-saving in the structure, and which is capable of shortening a warm-up time as well as suppressing the distortion of a discharged sheet medium (recording medium), such as the occurrence of curls and wrinkles, by reducing a stress applied to the sheet medium.

In order to achieve the above object, according to the invention, there is provided a fixing device for fixing an unfixed toner image formed on a sheet-shaped recording medium, comprising:a first roller, incorporating a heat source therein;a second roller, being pressed toward the first roller;an endless belt, a part of which is wound on an outer periphery of the second roller to be circulated; anda non-rotatable belt stretcher, which stretches the endless belt together with the second roller such that the circulated endless belt is slid on a first face extending in a widthwise direction of the endless belt, and such that a tensile force is applied to both widthwise end portions of the endless belt, wherein:the circulated endless belt is nipped between the first roller and the second roller to form a first nip portion through which the unfixed toner image is fixed on the recording medium with heat from the heat source; andthe first face of the belt stretcher is curved in the widthwise direction of the endless belt.

Here, a widthwise center portion of the first face of the belt stretcher is made concave or convex from the both widthwise end portions thereof.

Preferably, the circulated endless belt is nipped between the first roller and the belt stretcher to form a second nip portion through which the unfixed toner image is fixed on the recording medium with heat from the heat source.

With the above configurations, since the both widthwise end portions of the endless belt are brought into close contact with the first face of the belt stretcher, the buckling of the endless belt at the second nip portion can be avoided. In addition, the total nipping length for the recording medium can be elongated to realize the stable fixing operation.

Here, it is preferable that the belt stretcher comprises a wall member having a second face adjacent to at least one of the both widthwise end portions of the endless belt, in order to restrict a wobbling motion of the circulated endless belt.

It is further preferable that the wall member is integrally formed with the belt stretcher. More preferably, the wall member is monolithically formed with the belt stretcher.

With the above configuration, the wobbling motion of the circulated endless belt can be eliminated so that the stable fixing operation can be realized.

It is also preferable that the second face of the wall member is continuously extended from the first face of the belt stretcher.

Here, it is further preferable that the second face of the wall member and the first face of the belt stretcher facing one of the widthwise end portions of the endless belt form a first angle. A side end face of the endless belt and the first face of the belt stretcher facing one of the widthwise end portions of the endless belt form a second angle which is smaller than the first angle.

With the above configurations, the bucking of the widthwise end portion of the circulated endless belt can be avoided more effectively.

It is also preferable that a height dimension of the wall member is larger than a thickness dimension of the endless belt.

It is also preferable that the wall member is formed with a chamfered face continued to the second face to guide the circulated endless belt into the first face of the belt stretcher.

With the above configurations, the circulated endless belt can be prevented from surmounting the wall member.

It is also preferable that a part of the wall member is abutted against the first roller to define a gap between the first roller and the first face of the belt stretcher at the second nip portion.

Here, it is further preferable that a distance between the first roller and the first face of the belt stretcher at the gap is larger than a thickness dimension of the endless belt. The distance is determined such that the first face of the belt stretcher presses the recording medium against the first roller through the endless belt when the recording medium enters the second nip portion.

With the above configurations, since the gap serves as a heat insulation layer during the warm-up operation of the apparatus, a heat quantity absorbed from the first roller (i.e., heat loss) via the endless belt is lessened, so that the time period-required for the warm-up operation can be shortened. When the recording medium enters the second nip portion, the gap is eliminated so that the stable contact between the recording medium and the first roller can be realized.

Preferably, wherein the belt stretcher is pivotable. More preferably, an elastic member urges the belt stretcher toward the first roller. Here, a pivot center of the belt stretcher may be identical with or different from a rotational axis of the second roller.

Preferably, the belt stretcher has a semiannular or circular cross section viewed from the widthwise direction of the endless belt.

In a case where an open side of the semiannular cross section is faced the second roller, the belt stretcher can be disposed as close as possible to the second roller. Therefore, the entire length of the endless belt can be shortened, so that the natural heat release for prolonging the warm-up operation can be suppressed. In addition, the fixing device can be made compact.

Preferably, a cleaner is disposed between the second roller and the belt stretcher so as to abut against an inner peripheral face of the endless belt.

Preferably, the belt stretcher is disposed at an upstream side or a downstream side in the circulating direction of the endless belt relative to the first nip portion.

According to the invention, there is also provided an image forming apparatus, comprising:an image forming device, which forms a toner image on a recording medium; andthe above-described fixing device for fixing the toner image on the recording medium.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described with reference to the accompanying drawings.

FIG. 1Ashows a fixing device according to a first embodiment of the invention. A heating roller1is formed by coating the periphery of a base member1b, which is a pipe member having an outer diameter of about 25 mm and a thickness of about 0.7 mm, with an elastic body1chaving a thickness of about 0.4 mm. The heating roller1is rotatable in the direction indicated by an arrow Q and includes two column-shaped halogen lamps1aof 1050 W in the interior of the base member1bas a heat source. A pressing roller2is formed by coating the periphery of a base member2b, which is a pipe member having an outer diameter of about 25 mm and a thickness of about 0.7 mm, with the elastic body2chaving a thickness of about 0.2 mm. The pressing roller2is configured to achieve a press-contact force F between the heating roller1and the pressing roller2at 10 kg or below and the nip length of about 10 mm, and is rotatable in the direction indicated by an arrow R.

A heat-resistant belt3(hereinafter, simply referred as “belt”) is an endless belt made movable by being nipped between the heating roller1and the pressing roller2and stretched over the pressing roller2and a belt stretcher4along their peripheries. The belt3is formed from a metal tube, such as a stainless tube and a nickel electrocast tube, or a tube of heat-resistant resin, such as polyimide and silicon, having a thickness of 0.03 mm or greater.

According to this embodiment, because the outer diameters of the heating roller1and the pressing roller2are set to a diameter as small as 25 mm, a sheet medium5having been subjected to the fixing operation winds around neither the heating roller1nor the belt3, which eliminates the need for a member for forcedly separating the sheet medium5. Also, the rigidity is enhanced by providing a PFA layer of about 30-μm-thick on the surface layer of the elastic body1con the heating roller1. Although the elastic bodies1cand2cdiffer in thickness, they undergo elastic deformation almost identically, and a so-called flat nip is formed. Hence, no difference is generated in transportation speed of the belt3or a sheet medium5with respect to the peripheral speed of the heating roller1, which enables an image to be fixed in an extremely stable manner.

In this embodiment, two heat sources1aare built into the heating roller1such that the halogen lamps are placed in the different positions to be selectively activated, it becomes easier to control temperatures under different positions or conditions, that is, between a portion where the belt3winds around the heating roller1and a portion where the belt stretcher4comes in sliding contact with the heating roller1, and between a wide sheet medium and a narrow sheet medium.

The belt stretcher4is provided upstream of the nip portion between the heating roller1and the pressing roller2in the transportation direction of the sheet medium5, and is provided to be able to pivot in a direction indicated by an arrow P about the rotary shaft2aof the pressing roller2. The belt stretcher4is configured in such a manner that the belt3is stretched over in the tangential direction to the heating roller1while no sheet medium5is passing through the nip portion. When a fixing pressure is too large at the position from which a sheet medium5starts to enter the nip portion, the sheet medium5is not able to enter the nip portion smoothly, and the fixing may be performed while the leading end of the sheet medium5being folded. However, by configuring the belt3to be stretched over in the tangential direction to the heating roller1, it is possible to form an entrance portion through which a sheet medium5is able to enter the nip portion smoothly and stably.

The belt stretcher4is a semiannular member which is disposed in an inner periphery of the heat-resistant so as to provide a tensile force f to the belt in cooperation with the pressing roller2. Accordingly, the belt3slides on the semiannular member. The belt stretcher4is provided at a position where a nip is formed by bringing the belt3into press contact with the heating roller1beyond a tangential line L of the press contact portion between the heating roller1and the pressing roller2.

A protruding wall4ais formed to protrude from one end or both ends of the belt stretcher4, and regulates the leaning when the belt3leans to one side as it abuts on the protruding wall4a. A spring9is provided in a space between the frame and the end face of the protruding wall4aon the opposite side of the heating roller1. The belt stretcher4is thereby slightly pressed against the heating roller1by the spring9and is positioned such that the belt3comes in sliding contact with the heating roller1.

In order to drive the pressing roller2in a stable manner by stretching the belt3over the pressing roller2and the belt stretcher4, it is preferable to set a frictional coefficient between the pressing roller2and the belt3larger than a frictional coefficient between the belt stretcher4and the belt3. The frictional coefficient, however, may become unstable due to intrusion of foreign substances or abrasion. In contrast, by setting a winding angle of the belt3to the pressing roller2smaller than a winding angle of the belt3to the belt stretcher4, and by setting the diameter of the belt stretcher4smaller than the diameter of the pressing roller2, it is possible to shorten a distance over which the belt3slides on the belt stretcher4. Destabilizing factors derived from a change with time or disturbances can be thus avoided, which in turn makes it possible to drive the belt3in a stable manner by the pressing roller2.

A cleaner6is provided somewhere between the pressing roller2and the belt stretcher4to come in sliding contact with the inner peripheral surface of the belt3, and thereby cleans the foreign substances, abrasive powder, etc. on the inner peripheral surface of the belt3. By cleaning the foreign substances, abrasive powder, etc., the belt3is refreshed and destabilizing factors are thereby removed. In addition, a concave portion4fprovided in the belt stretcher4is suitable to store the removed foreign substances, abrasive powder, etc.

A nip entrance is defined as a position at which the belt stretcher4is slightly pressed against the heating roller1, and a nip exit is defined as a position at which the pressing roller2is pressed against the heating roller1. A sheet medium5enters from the nip entrance and passes a space between the belt3and the heating roller1so that an unfixed toner image5ais fixed on the sheet medium5. The sheet medium5is then discharged from the nip exit in the direction of the tangential line L.

A supporting structure of the pressing roller2and the belt stretcher4will now be described. The both ends of rotary shaft2aof the pressing roller2are, as is shown inFIG. 1B, supported by frames7on the right and left via bearings7ato be free to rotate (the right side is omitted from the drawing). On the both sides of the rotary shaft2aof the pressing roller2are fitted arms4bto be free to pivot, and a guiding groove4cis formed in each arm4bso as to face the belt stretcher4. On the other hand, at the both ends of the belt stretcher4are formed guiding portions4dto be fitted into the guiding grooves4cin the arms4bvia springs4e. The belt stretcher4is therefore of a structure such that provides the tensile force f to the belt3as it is pushed by the springs4ein a direction to move away from the pressing roller2.

In this embodiment, the belt stretcher4is used as a non-rotating member for the belt3to slide thereon, a bearing or the like is not necessary, which can make the supporting structure simpler. In addition, by forming the belt stretcher4in a semiannular shape, it is possible provide the belt stretcher4in the closest proximity to the pressing roller2by facing the missing half of the ring toward the pressing roller2. This also makes it possible to form the belt3with a shorter peripheral length. It is thus possible to provide a compact, inexpensive heating roller type fixing device by simplifying the structure.

Also, because the belt3moves along a path of the least necessary length, for the belt3heated at the nip portion with the heating roller1housing the heat source, heat energy absorbed while moving along a predetermined path can be minimized. Also, because the peripheral length is short, a drop in temperature due to natural heat release is small, which makes it possible to shorten a so-called warm-up time needed to reach a desired temperature for the fixing to be enabled since the apparatus has been activated.

Also, because the belt3is brought into press contact with the heating roller1to form the nip portion with the tensile force f provided from the belt stretcher4in cooperation with the pressing roller2, the nip length can be readily extended and the structure can be simplified, which in turn makes the device compact and less expensive.

In order to fix an unfixed toner image5aformed on a sheet medium5in a stable manner, it is essential to melt the unfixed toner image5asufficiently before it is fixed, and predetermined temperature and melting time are needed. In this embodiment, since there is no need to provide a member for extending the nip length by causing the elastic body1cof the heating roller1to undergo considerable deformation, the thickness of the elastic body1ccan be made thin. In addition, since there is no need to set a large press-contact pressure to the pressing roller2in causing the elastic body to undergo deformation. Hence, only a small stress is applied to the sheet medium5bearing the unfixed toner image5awhen it passes through the space between the heating roller1and the belt3, which suppresses the sheet medium distortion, such as the occurrence of wrinkles, when the sheet medium5is discharged after the unfixed toner image5ais fixed thereon.

Thus, not only can the need to increase the mechanical rigidity of the heating roller type fixing device be eliminated, but also the heating roller1can be thinner. It is thus possible to increase the heating speed with which the heat source heats the belt3. The pressing roller2can be thinner, too, and a heat capacity can be smaller, which can reduce absorption of heat energy from the belt3. It is thus possible to shorten the warm-up time.

As shown inFIGS. 2 and 3, the protruding wall4aof the belt stretcher4is positioned by coming in sliding contact with the heating roller1on a slide-contact surface4g. A gap (step difference) G, which is larger than the thickness of the belt3, is provided to the belt stretcher4between the slide-contact surface4gand a pressing surface4hthat presses the belt3and thereby presses a sheet medium5against the heating roller1. The pressing surface4his formed concentrically with the heating roller1. To be more concrete, by forming the gap G from a step difference of about 110 μm, and forming the belt3to have a thickness of about 80 μm, a clearance of about 30 μm is secured, which enables the fixing to be performed in a stable manner even for a sheet medium5as thick as 60 μm.

As shown inFIG. 4, the belt3is nipped between the heating roller1and the pressing roller2, and is brought into press contact with the heating roller by the belt stretcher4so as to define the nip entrance.

In is not practical to bring a speed in the process step of forming an unfixed toner image5aon a sheet medium5, which is the preceding step of the fixing step, into perfect agreement with a speed in the fixing step, when irregularities in specifications of various mass-produced components are concerned. Hence, speeds in the consecutive steps are balanced by setting the speed in the fixing step to one side, that is, either reduced or increased from the speed in the process step of forming an unfixed toner image5aon a sheet medium5in consideration of such irregularities. Although there is a need to specify an entering speed of a sheet medium5by gripping the sheet medium5in a reliable manner at the nip entrance, the need can be satisfied by the configuration described above.

In addition, the surface of the elastic body1cof the heating roller1and the surface of the belt3move at the same peripheral speed to fix an unfixed toner image5aformed on a sheet medium5. However, when the surface of the belt3is corrugated or the leading end of the sheet medium5is corrugated, the initial condition of the fixing operation may become unstable. Hence, by configuring the belt3in such a manner that it is pressed against the heating roller1at the nip entrance, the both components are able to meet in a stable condition, which enables the unfixed toner image5ato be fixed in an extremely stable manner.

In this embodiment, in the foregoing state while no sheet medium5is passing through, the gap G is formed between the belt3and the belt stretcher4. Hence, the clearance in the gap G serves as a heat insulation layer during the warm-up. A heat quantity absorbed from the heating roller1via the belt3is thus lessened, and so is a heat loss. The warm-up time, therefore, can be shortened.

On the other hand, when a sheet medium5is passing through the nip portion, as is shown inFIG. 5, the protruding wall4aof the belt stretcher4is kept spaced apart from the heating roller1, which eliminates the gap G between the belt3and the belt stretcher4. The sheet medium5is thus pressed by the belt3at the nip portion, and hence pressed against the heating roller1, which makes it possible to achieve adequate fixing by adjusting this pressing pressure to a desired pressure with the use of the spring9shown inFIG. 1A.

Also, a heat quantity, accumulated while the belt stretcher4is heated by the heating roller1, is small due to the presence of the gap G. Hence, when the sheet medium5enters the nip portion, the second surface of the sheet medium5opposite to the first surface on which the unfixed toner image5ais formed cools the belt3having a small heat capacity. Since a heat quantity supplied from the belt stretcher4side is small, in a case of double-sided image fixing to fix an unfixed toner image5aon the second surface after the unfixed toner image5aformed on the first surface of the sheet medium5is fixed, the image fixed earlier on the first surface is not excessively heated when the second surface undergoes fixing, and no disturbance occurs in the image fixed on the first surface.

In this embodiment, as is shown inFIG. 1A, the spring9is provided upstream of the press contact portion between the heating roller1and the pressing roller2in the moving direction of the belt3, at a position remote from the pivot center of the belt stretcher4. The belt3is driven by driving one of the heating-roller1and the pressing roller2, and the belt stretcher4starts to pivot toward the heating roller1by this driving force and the sliding-frictional force between the belt3and the belt stretcher4. However, there may be a case where the pivotal moving force alone cannot induce a sufficient fixing pressure to fix an unfixed toner image5aformed on a sheet medium5. To address this inconvenience, the pivotal moving force is set to a desired fixing pressure with the assistance of the spring9, so that the unfixed toner image5acan be fixed in an extremely stable manner.

It is preferable that the tensile force f, applied to the belt3from the belt stretcher4in cooperation with the pressing roller2, is identical across the entire stretched region. However, in some cases, the accuracy of straightness of the pressing roller2and the belt stretcher4, or the accuracy of homogeneity of the inner periphery of the belt3, or an erroneous factor of a quantity of deformation occurred while the tensile force is applied to the pressing roller2and the belt stretcher4, makes it difficult to make the tensile force f identical across the entire stretched region.

In addition, besides the foregoing state, as is shown inFIG. 6A, the belt3runs with wobbling due to the factors, such as the accuracy of parallelism in the pressing roller2and the belt stretcher4provided at their respective positions, and the larger the frictional force between the inner surface of the belt3and the pressing roller2as well as the belt stretcher4, the larger the wobble-leaning force becomes, which makes it difficult to regulate the running position.

The condition to achieve the position regulation in preventing the wobbling of the belt3is to provide a position regulator well capable of responding to the wobble-leaning force of the belt3and to have a sufficient withstanding strength while the position of the belt3is regulated by the position regulator. Hence, it is crucial whether the belt3can secure the withstanding strength.

As is shown inFIG. 1B, by forming the belt stretcher4to be supported at the both ends with the springs4ein providing the tensile force f to the belt3at least at the side edges of the belt3in the width direction, and to have the protruding wall4athat protrudes from each tension applying portion and regulates the side edge face of the belt3, the belt3is brought into a close contact state to the tension applying portion while running along the protruding wall4a.

For example, assume that, as is shown inFIG. 6B, a concave, a step difference, a clearance or the like is present in the vicinity of the side edge of the belt stretcher4provided with the protruding wall4a. Then, the belt3is not able to come in close contact to the sliding surface of the belt stretcher4, and a clearance is formed. Under these conditions, when the belt3runs with wobbling and abuts on the protruding wall4a, a larger wobble-leaning force is exerted, upon which, as is shown inFIG. 6C, the bucking occurs at the side edge of the belt3.

When the inclined angle at the side edge face of the belt3and the extending angle of the protruding wall4arelative to the sliding surface of the belt stretcher4are concerned, as is shown inFIG. 7A, in a case where the inclined angle θ2of the side edge face of the belt3becomes larger than the extending angle θ1of the protruding wall4a, if the belt3runs with wobbling and abuts on the protruding wall4a, a large wobble-leaning force is exerted, upon which the side edge of the belt3is lifted along the protruding wall4aas indicated. On the other hand, as is shown inFIG. 7B, in a case where the angle θ2at the side edge face of the belt3is smaller than the extending angle θ1of the protruding wall4a and a clearance is formed between the sliding surface of the belt stretcher4and the protruding wall4a, if the belt3runs with wobbling and abuts on the protruding wall4a, a large wobble-leaning force is exerted, upon which the side edge of the belt3engages in the clearance portion along the protruding wall4a, thereby giving rise to the bucking at the side edge of the belt3.

In order to prevent the occurrence of bucking at the side edge of the belt3, as is shown inFIG. 8B, the belt3needs to come in close contact to the sliding surface of the belt stretcher4without any clearance at the side edge region of the belt3, and the protruding wall4aneeds to have a wall surface that rises continuously from the sliding surface. The protruding wall4ais preferably provided as a monolithic part of the belt stretcher4. Even when the protruding wall4ais an individual member from the belt stretcher4, it is preferable that the protruding wall4ais provided integrally with the belt stretcher4, so that the belt3slides on the sliding surface of the belt stretcher4in close contact without leaving any clearance. To this end, it is also preferable to support the belt stretcher4at the both ends with the use of the springs4e, so that when the belt3is stretched over, as is shown inFIG. 1B, the belt stretcher4is preferably shaped into an inversed crown shape in which the sliding surface is concave from the both ends to the center portion.

In view of the foregoing, the side edge of the belt3abuts on the protruding wall4awhile coming close contact with the sliding surface of the belt stretcher4, so that the wobbling of the belt3can be regulated effectively and the position regulation of the belt3to be performed in a stable manner. In this case, the belt3is supported by the belt stretcher4at the both side edge regions thereof, thereby generating a bent at the center portion thereof. In view of this, as shown inFIG. 8A, the sliding surface of the belt stretcher4may be shaped into a crown shape in which the sliding surface is convex from the both ends to the center portion, under the condition that the side edge regions of the belt3are brought into close contact with the sliding surface of the belt stretcher4(along the reference line φ).

Further, in order to prevent the bucking at the side edge of the belt3, as is shown inFIG. 8C, it is preferable to make the inclined angle θ2of the side edge face of the belt3less than the extending angle θ1of the protruding wall4arelative to the sliding surface of the belt stretcher (the reference line φ), that is, to set as θ2≦θ1.

As a structure such that applies a tensile force f to the belt3from the belt stretcher4in cooperation with the pressing roller2, by forming the protruding wall4aon either the pressing roller2or the belt stretcher4side, the wobbling side edge of the belt3abuts on the protruding wall4a, thereby the wobbling regulation is performed. However, in a case where the protruding wall4ais provided on the pressing roller2, it is necessary to provide a structure for receiving the protruding wall4aon the heating roller1which is brought into press contact with the pressing roller2. Such a provision may be a limitation or a restriction to the design of the heating roller1. In a case where the protruding wall4ais provided in the belt stretcher4as in this embodiment, the heating roller can be made free from such a design limitation.

Also, the belt3closely adheres to the belt stretcher4and runs with sliding due to the tensile force f, at the portion where it abuts on the protruding wall4aand the position regulation on the running with wobbling is thereby effected. When the tensile force f is greater than the running force with wobbling, there may occur a phenomenon that the side edge of the belt3surmounts the protruding wall4aor the end face of the belt3extends to overlap a part of the protruding wall4a. In order to avoid such phenomena while increasing the degree of the design freedom, such as the edge strength of the belt3, the tensile force f, the running force with wobbling of the belt3, it is preferable to form the protruding wall4a, as is shown inFIG. 9A, to have a belt regulating height h at least twice the thickness t of the belt3.

When the belt3is brought into close contact with the sliding surface of the belt stretcher4, the belt3tends to wobble. Therefore, it is necessary to provide a structure for smoothly subjecting the belt3having been separated from the pressing roller2to the wobbling regulation by the protruding wall4aof the belt stretcher4. In this embodiment, as shown inFIGS. 9B,9C,10A and10B, a chamfered portion4a′ is formed along an inner top edge of the protruding wall4a′ to smoothly introduce the belt3to the region where the protruding wall4ais formed. As shown inFIG. 9A, the belt regulating height h is defined so as to exclude the chamfered portion4a′.

Alternatively, as shown inFIG. 9D, the chamfered portion4a′ may be formed at the entrance of the region where the protruding wall4ais formed such that the width of the sliding surface of the belt stretcher4is gradually reduced.

In this embodiment, the spring9is provided upstream of the press contact portion between the heating roller1and the pressing roller2in the moving direction of the belt3, which is remote from the pivot center of the belt stretcher4. Hence, as is shown inFIG. 11B, it is possible to increase the fixing pressure continuously from the nip entrance toward the press contact portion between the heating roller1and the pressing roller2due to the mechanism of leverage, and a stress having an inflection point is not applied to a sheet medium5. Hence, no fixing irregularities or the like are generated in a fixed image, which not only enables an unfixed toner image5ato be fixed in an extremely stable manner, but also the sheet medium distortion, such as the occurrence of wrinkles, can be suppressed for a sheet medium5discharged after the unfixed toner image5ais fixed.

Incidentally, “H” denotes the case of a thick sheet medium or a layered sheet medium such as an envelop, having a large heat capacity, or a transparent sheet medium such as an OHP sheet, “S” denotes the case of a standard sheet medium, and “L” denotes a thin sheet medium or a sheet medium having a poor heat resistance.

For the comparison purpose,FIG. 11Ashows the pressing pressure variation in a case where the spring9is not provided. In other words, it is a case where the fixing pressure is generated only by the pivot movement of the belt stretcher4without the assistance of the spring9.

The fixing pressure (abutting pressure distribution) between the heating roller1and the belt3reaches the maximum pressure at a portion at which the heating roller1and the pressing roller2come in pressing contact with each other. In a case where it is difficult to melt an unfixed toner image5asufficiently to enable the fixing in a stable manner, for example, when the surface of a sheet medium5is irregular, or the surface is made of a material having excellent hermeticity and is so smooth that a melted toner image hardly permeates inside like an OHP sheet, by providing a higher pressure than that in the melting stage to melted toner in the final stage where the sheet medium5passes through the nip, not only can the surface of the melted toner be smooth, but also the permeation into the sheet medium5can be promoted and the fixed image, therefore, can be more stable.

FIGS. 12A and 12Bshow a fixing device according to a second embodiment of the invention. Members similar to those in the first embodiment are designated by the same reference numerals and the repetitive explanations for those will be omitted.

In the first embodiment, the belt stretcher4is allowed to pivot by a predetermined angle about the common axis with the rotary shaft2aof the pressing roller2. In this embodiment, the belt stretcher4is configured so as to be allowed to pivot by a predetermined angle about an axis7b, which is different from the rotary shaft2aof the pressing roller2.

In other words, on the both sides of the axis7bprovided at a position different from the axial center of the rotary shaft2aare fitted arms4bto be free to pivot, and a guiding groove4cis formed in each arm4bso as to face the belt stretcher4. On the other hand, at the both ends of the belt stretcher4are formed guiding portions4dto be fitted into the guiding grooves4cin the arms4bvia springs4e. The belt stretcher4is therefore of a structure such that provides a tensile force f to the belt3as it is pushed by the springs4ein a direction to move away from the pressing roller2.

This configuration can change the torque acting on the belt stretcher4(in this embodiment, the torque is increased), which makes it possible to adjust the press-contact force between the belt3and the heating roller1in accordance with the position of the axis7b. Also in this embodiment, as is shown inFIG. 2, a gap (step difference) G, which is larger than the thickness of the belt3is provided to the belt stretcher4between the slide-contact surface4gand the pressing surface4hthat presses the belt3to press a sheet medium5against the heating roller1.

FIG. 13shows a fixing device according to a third embodiment of the invention. Members similar to those in the first embodiment are designated by the same reference numerals and the repetitive explanations for those will be omitted.

In this embodiment, the belt stretcher4is provided as a non-rotatable cylindrical member. Also in this embodiment, as is shown inFIG. 2, a gap (step difference) G, which is larger than the thickness of the belt3, is provided to the belt stretcher4between the slide-contact surface4gand the pressing surface4hthat presses the belt3to press the sheet medium5against the heating roller1.

A fourth embodiment of the invention will be described with reference toFIGS. 14A through 17. Members similar to those in the first embodiment are designated by the same reference numerals and the repetitive explanations for those will be omitted.

In this embodiment, the belt stretcher4is provided downstream of the press contact portion between the heating roller1and the pressing roller2in the transportation direction of a sheet medium5, and is allowed to pivot in the direction indicated by an arrow P about the rotary shaft2aof the pressing roller2, as shown inFIGS. 14A and 14b.

The belt stretcher4is a semiannular member which is disposed in an inner periphery of the heat-resistant so as to provide a tensile force f to the belt in cooperation with the pressing roller2. Accordingly, the belt3slides on the semiannular member.

An entrance of the fixing nip is defined as a position at which the belt stretcher4is slightly pressed against the heating roller1, and an exit of the fixing nip is defined as a position at which the belt stretcher4is separated from the heating roller1. A sheet medium5enters from the nip entrance and passes a space between the belt3and the heating roller1so that an unfixed toner image5ais fixed on the sheet medium5. The sheet medium5is then discharged from the nip exit in the direction of the tangential line L.

As is shown inFIGS. 15A and 15B, the protruding wall4aof the belt stretcher4is positioned by coming in sliding contact with the heating roller1on a slide-contact surface4g. A gap (step difference) G, which is larger than the thickness of the belt3, is provided to the belt stretcher4between the slide-contact surface4gand a pressing surface4hthat presses the belt3and thereby presses a sheet medium5against the heating roller1. The pressing surface4his formed concentrically with the heating roller1. To be more concrete, by forming the gap G from a step difference of about 110 μm, and forming the belt3to have a thickness of about 80 μm, a clearance of about 30 μm is secured, which enables the fixing to be performed in a stable manner even for a sheet medium5as thick as 60 μm.

The belt3is nipped between the heating roller1and the pressing roller2, and is brought into press contact with the heating roller1by the belt stretcher4at the exit of the nip region.

In this embodiment, in the foregoing state while no sheet medium5is passing through, the gap G is formed between the belt3and the belt stretcher4. Hence, the clearance in the gap G serves as a heat insulation layer during the warm-up. A heat quantity absorbed from the heating roller1via the belt3is thus lessened, and so is a heat loss. The warm-up time, therefore, can be shortened.

On the other hand, when a sheet medium5is passing through the fixing nip portion, as is shown inFIGS. 16A and 16B, the protruding wall4aof the belt stretcher4is kept spaced apart from the heating roller1, which eliminates the gap G between the belt3and the belt stretcher4. The sheet medium5is thereby pressed by the belt3at the fixing nip portion, and hence pressed against the heating roller1, which makes it possible to achieve adequate fixing by adjusting this pressing force to a desired pressure with the use of the spring9shown inFIG. 14A.

Also, a heat quantity, accumulated while the belt stretcher4is heated by the heating roller1, is small due to the presence of the gap G. Hence, when the sheet medium5enters the nip portion, the second surface of the sheet medium5opposite to the first surface on which the unfixed toner image5ais formed cools the belt3having a small heat capacity. Since a heat quantity supplied from the belt stretcher4side is small, in a case of double-sided image fixing to fix an unfixed toner image5aon the second surface after the unfixed toner image5aformed on the first surface of the sheet medium5is fixed, the image fixed earlier on the first surface is not excessively heated when the second surface undergoes fixing, and no disturbance occurs in the image fixed on the first surface.

The belt3is driven when either the heating roller1and the pressing roller2is driven, and the belt stretcher4starts to pivot in a direction to move away from the heating roller1by this driving force and the sliding-frictional force between the belt3and the belt stretcher4. However, by setting a desired fixing pressure by pushing the belt stretcher4toward the heating roller1by a desired pushing force larger than the pivotal moving force, it is possible to fix an unfixed toner image5ain an extremely stable manner.

Hence, in this embodiment, the spring9is provided downstream of the press contact portion between the heating roller1and the pressing roller2in the moving direction of the belt3, which is remote from the pivot center of the belt stretcher4.

In this embodiment, the spring9is provided downstream of the press contact portion between the heating roller1and the pressing roller2in the moving direction of the belt3, which is remote from the pivot center of the belt stretcher4. Hence, as is shown inFIG. 17, it is possible to increase the fixing pressure continuously from the nip entrance toward the press contact portion between the heating roller1and the pressing roller2due to the mechanism of leverage, and a stress having an inflection point is not applied to a sheet medium5. Hence, no fixing irregularities or the like are generated in a fixed image, which not only enables an unfixed toner image5ato be fixed in an extremely stable manner, but also the sheet medium distortion, such as the occurrence of wrinkles, can be suppressed for a sheet medium5discharged after the unfixed toner image5ais fixed.

Incidentally, “H” denotes the case of a thick sheet medium or a layered sheet medium such as an envelop, having a large heat capacity, or a transparent sheet medium such as an OHP sheet, “S” denotes the case of a standard sheet medium, and “L” denotes a thin sheet medium or a sheet medium having a poor heat resistance.

FIGS. 18A and 18Bshow a fixing device according to a fifth embodiment of the invention. Members similar to those in the fourth embodiment are designated by the same reference numerals and the repetitive explanations for those will be omitted.

In the fourth embodiment, the belt stretcher4is allowed to pivot by a predetermined angle about the common axis with the rotary shaft2aof the pressing roller2. In this embodiment, the belt stretcher4is configured so as to be allowed to pivot by a predetermined angle about an axis7b, which is different from the rotary shaft2aof the pressing roller2.

In other words, on the both sides of the axis7bprovided at a position different from the axial center of the rotary shaft2aare fitted arms4bto be free to pivot, and a guiding groove4cis formed in each arm4bso as to face the belt stretcher4. On the other hand, at the both ends of the belt stretcher4are formed guiding portions4dto be fitted into the guiding grooves4cin the arms4bvia springs4e. The belt stretcher4is therefore of a structure such that provides a tensile force f to the belt3as it is pushed by the springs4ein a direction to move away from the pressing roller2.

This configuration can change the torque acting on the belt stretcher4(in this embodiment, the torque is increased), which makes it possible to adjust the press-contact force between the belt3and the heating roller1in accordance with the position of the axis7b.

In the above embodiments, the heating roller1or the pressing roller2serves as the driving roller. In this case, in order to achieve the safe driving, it is preferable to use either roller whichever is the harder as the driving roller and the other softer roller as the driven roller. The belt3is wound around the pressing roller2to be circulated, and is brought into press contact with the elastic body1ccoated on the surface of the heating roller1. In this case, since the pressing roller2serves as the driving roller and the heating roller1serves as the driven roller, the pressing roller2determines the transportation speed of the belt3, that is, of a sheet medium5bearing an unfixed toner image5a. The pressing roller2is configured to have a harder surface than at least the elastic body1c. This configuration enables the driving with a stable transportation speed to be achieved without causing distortion.

In the above embodiments, the rotational speed of the heating roller1and the pressing roller2may be selectably controlled. The control of the driving speed will now be described. A driving member has two rotational speeds to drive the heating roller1and the pressing roller2, and drives the heating roller1and the pressing roller2selectively at a first rotational speed or a second rotational speed slower than the first rotational speed, according to the sheet medium properties.

In order to set the rotational speed, a detector for detecting the sheet medium properties is provided, and a table or the like for selecting the rotational speed corresponding to the sheet medium properties is also provided. The sheet medium properties are detected while a sheet medium5bearing an unfixed toner image5ais being transported, and the rotational speed is selected according to the detected sheet medium properties through the table when the fixing command is processed.

To select the rotational speed, the members interlocked with the fixing device may be manually operated before the fixing command is processed, or remotely controlled by an electrical signal or the like.

The sheet medium5bearing the unfixed toner image5amust be used in diversified purposes, including a typical sheet medium of paper or the like, a thick sheet medium having a large heat capacity, a transparent sheet medium (OHP sheet), etc. In comparison with a typical sheet medium, for a thick sheet medium and a layered sheet medium, such as an envelope, having a large heat capacity, a transparent sheet medium (OHP sheet medium), etc., a relatively long time period is needed to melt the unfixed toner image5asufficiently before it is fixed thereon. In such a case, by selectively driving the heating roller1and the pressing roller2at the first rotational speed or the second rotational speed slower than the first rotational speed according to the sheet medium properties, the unfixed toner image5ais melted adequately, and the fixing as desired can be achieved.

In addition, even by the selective driving at the first rotational speed or the second rotational speed, a stress, applied to a sheet medium5bearing an unfixed toner image5awhen the sheet medium5passes through a space between the heating roller1and the belt3, varies little and remains small. This suppresses the sheet medium distortion, such as the occurrence of the wrinkles, for the sheet medium5discharged after the unfixed toner image5ais fixed thereon. Hence, not only can the need to increase the mechanical rigidity of the heating roller type fixing apparatus be eliminated, but also the heating roller1can be thinner. A heating speed with which the heat source heats the belt3can be thus increased. In addition, because the pressing roller2can be thinner, too, a heat capacity can be smaller. Heat energy absorbed from the belt3is thus lessened, which makes it possible to shorten a so-called warm-up time needed to reach a predetermined temperature for the fixing to be enabled since the apparatus has been activated. The selective driving may be realized by, for example, selectively changing the rotational speed of a driving motor.

FIG. 19shows an image forming apparatus10incorporating any one of the fixing devices as described the above.

The image forming apparatus10includes a housing10a, a medium discharging tray10cformed on the top portion of the housing10a, and a door cover10battached at the front of the housing10ato be free to open and close. Inside the housing10aare provided an exposure unit W, an image formation unit D, a transfer belt unit29having an image transporter18, and a medium feeding unit30. In the vicinity of the door10bis provided a medium transportation unit11. Each unit is detachably loaded inside the housing10a, so that any one of the units can be individually detached from the housing10aduring a maintenance work or the like for repairing or replacement.

The image formation unit D includes plural (four, in the embodiment) image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black), each forming an image of a different color. Each of the image forming stations Y, M, C, and K includes an image carrier17comprising a photosensitive drum, as well as a charger19comprising a corona charger and a development device20both provided in the periphery of the image carrier17. The respective image forming stations Y, M, C, and K are provided in parallel below the transfer belt unit29along an arch-shaped line with the image carriers17facing upward. The placement order of the respective image forming stations Y, M, C, and K is arbitrary.

The transfer belt unit29includes a driving roller12driven to rotate by an illustrated driving source provided at the lower end of the housing10a, a driven roller13provided diagonally above the driving roller12, a tension roller14, the image transporter18comprising an intermediate transfer belt stretched over these three rollers or at least two of them and driven to circulate in a direction indicated by an arrow S, and a cleaner15that abuts on the surface of the image transporter18. The driven roller13, the tension roller14, and the image transporter18are arranged such that the circulating path of the image transporter18extends obliquely. More specifically, they are arranged such that a belt surface18afaced down in the belt transportation direction is positioned lower and a belt surface18bfaced up in the belt transportation direction is positioned upper.

Thus, the image forming stations Y, M, C, and K are arrayed obliquely. The belt surface18aof the image transporter18comes in contact with the image carriers17along the arch-shaped line, and the image carriers17are then driven to rotate in the transportation direction of the image transporter18as indicated by arrows. The flexible image transporter18in the shape of an endless sleeve comes in contact with the image carriers17at almost the same winding angle to cover the image carriers17from above. This configuration makes it possible to adjust a press-contact pressure or a nip width between the image carriers17and the image transporter18by controlling a tensile force applied to the image transporter18from the tension roller14, placement intervals of the image carriers17, the winding angle (the curvature of the arch), etc.

The driving roller12also serves as a back-up roller of a secondary transfer roller39. On the peripheral surface of the driving roller12is formed a rubber layer having, for example, a thickness of about 3 mm and a volume resistivity of 105Ωcm or less, and the grounding via a metal shaft is used as an electrical conduction path of the secondary bias provided via the secondary transfer roller39. In this manner, by providing the driving roller12with the rubber layer having high friction and impact absorbing ability, an impact when a sheet medium enters the secondary transfer portion is hardly transmitted to the image transporter18, which makes is possible to prevent deterioration of the image quality. In addition, by making the diameter of the driving roller12smaller than the diameters of the driven roller13and the back-up roller14, a sheet medium, having undergone the secondary transfer, can be separated more readily by its own elastic force. Further, the driven roller13is used also as a back-up roller of the cleaner15described below.

Alternatively, the inclined direction of the circulating path of the image transporter18is arbitrary and the array of the image forming stations Y, M, C, K is determined accordingly.

The cleaner15is provided on the side of the belt surface18afaced down in the transportation direction, and is provided with a cleaning blade15athat removes toner remaining on the surface of the image transporter18after the secondary transfer, and a toner transporter15bthat transports collected toner. The cleaning blade15aabuts on the image transporter18at the winding portion of the image transporter18to the driven roller13. On the back side of the image transporter18abuts a primary transfer member16oppositely to the image carriers17of the respective image forming stations Y, M, C, and K described below, and the transfer bias is thus applied to the primary transfer portion16.

The exposing unit W is provided in a space below the image formation unit D obliquely arrayed. Also, the medium feeding unit30is provided at the bottom of the housing10abelow the exposing unit W. The exposing unit W is accommodated in the case entirely, and the case is provided in a space formed diagonally below the belt surface18afaced down in the transportation direction. At the bottom of the case are provided horizontally a single scanner21comprising a polygonal mirror motor21a, a polygonal mirror (rotary multi-faceted mirror)21b. In an optical system B, which reflects laser beams from plural laser light sources23modulated by image signals of their respective colors on the polygonal mirror21bto be deflected and scanned on the respective image carriers17, are provided a single f-θ lens22and plural reflection mirrors24that return scanning light paths of respective colors to the corresponding image carriers17not in parallel with each other.

In the exposing unit W configured as described above, image signals, corresponding to respective colors, from the polygonal mirror21bare emitted in the form of laser beams formed and modulated according to the common data clock frequency, and irradiated to the image carriers17in the respective image forming stations Y, M, C, and K by way of the f-θ lens22and the reflection mirrors24for a latent image to be formed. The scanning light path is bent by providing the reflection mirrors24, and the height of the case can be thereby lowered, which in turn makes it possible to achieve a compact optical system.

Moreover, the reflection mirrors24are set so that the lengths of the scanning light paths to the image carriers17in the respective image forming stations Y, M, C, and K are all equal. By configuring in such a manner that the lengths of light paths (light path lengths) to the respective image carriers17from the polygonal mirror21bin the exposing unit W become nearly equal with respect to the image formation unit D, the scanning widths of light beams scanned via the respective light paths also become nearly equal. This eliminates the need for a special configuration to form image signals. Hence, the laser light source is modulated by different image signals for images of different colors; nevertheless, it can be modulated according to the common data clock frequency, and because the common reflection surface is used, color shifting induced from a relative difference in the sub-scanning directions can be prevented. It is thus possible to fabricate an inexpensive color image forming apparatus with a simple structure.

Further, in this apparatus, by providing the scanning optical system on the lower side of the apparatus, it is possible to minimize vibrations of the scanning optical system induced by vibrations given to the frame supporting the image forming apparatus from the driving system in the apparatus, which can in turn prevent deterioration of the image quality. In particular, by providing the scanner21at the bottom of the case, vibrations given to the entire housing10afrom the polygonal motor21aper se can be minimized, and deterioration of the image quality can be prevented. Also, by limiting the number of the polygonal motor21a, serving as the vibration source, to one, vibrations given to the entire case can be minimized.

The medium feeding unit30is provided with a cassette tray35in which a pile of sheet media are held, and a pick-up roller36that feeds the sheet media from the feed cassette35one by one. The medium transportation unit11is provided with a pair of gate rollers37that regulates the feed timing of a sheet medium to the secondary transfer portion (one of the rollers is provided on the housing10aside), the secondary transfer roller39that is brought into pressing contact with the driving roller12and the image transporter18, a main transportation path38, a fixing unit40, a pair of discharging rollers41, and a double-sided printing transportation path42.

An unfixed toner image secondarily transferred on a sheet medium is fixed at the nip portion formed by the fixing unit40at a predetermined temperature. In this embodiment, it is possible to provide the fixing unit40in a space formed diagonally above the belt surface18bof the transfer belt, which is faced up in the transportation direction, in other words, a space on the opposite side of the image forming stations Y, M, C, and K, with respect to the transfer belt. This configuration can reduce heat to be transmitted to the exposing unit W, the image transporter18, and the image forming means, and the frequency of a color-shift correcting operation for respective colors can be reduced. In particular, the exposing unit W is at the remotest position from the fixing unit40, and distortion caused by heat in the scanning optical system components can be minimized, which can in turn prevent a shift in color.

In this embodiment, because the image transporter18is extended diagonally, a wide space can be secured on the right side of the drawing for providing the fixing unit40. Hence, not only can the apparatus be made compact, but also transmission of heat, generated in the fixing unit40, to the exposure unit W, the image transporter18, and the respective image forming stations Y, M, C, and K, all positioned on the left side of the drawing, can be prevented. Also, because the exposure unit W can be provided in a space on the lower left of the image formation unit D, vibrations of the scanning optical system in the exposure unit W, induced by vibrations given to the housing10afrom the driving system in the image formation unit D, can be minimized, which can in turn prevent the deterioration of the image quality.

Because no cleaner is provided, the corona charger19is adopted as the charger. In a case where the charger comprises a roller, primary transfer residual toner present on the image carrier17accumulates on the roller, albeit in a slight quantity, and gives rise to a charging defect. However, toner hardly adheres to the corona charger19, which is non-contact charger. The occurrence of a charging defect, therefore, can be prevented.

In this apparatus, the intermediate transfer belt serves as the image transporter18and is configured to come in contact with the image carriers17. However, the image transporter18may be a sheet medium transportation belt that attracts a sheet medium on the surface and moves to transport the sheet medium in transferring toner images sequentially to be superposed on the surface of the sheet medium, and is configured to come in contact with the image carriers17. In this case, the belt transportation direction of the sheet medium transportation belt serving as the image transporter18is made upward on the lower surface that comes in contact with the image carriers17.

It should be appreciated that the invention is not limited to the above embodiments, and can be modified in various manners. For example, although a tandem-type color image forming apparatus in which the development devices of respective colors are arrayed has been described the above, the invention can be applied to a rotary-type color image forming apparatus, in which the development devices of respective colors are mounted to the rotary frame, as well as to a monochrome image forming apparatus.