US20070125595A1

US20070125595A1 - Image forming apparatus

Info

Publication number: US20070125595A1
Application number: US11/550,540
Authority: US
Inventors: Shin-ichi Sugiyama
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2005-12-02
Filing date: 2006-10-18
Publication date: 2007-06-07
Also published as: JP4845493B2; JP2007156036A

Abstract

An image forming apparatus has an actuation portion emitting an actuation sound and an internal part provided away from and opposed to the actuation portion. Further the image forming apparatus has a sound absorbing member formed of a foam metal, and provided away from the internal part between the actuation portion and the internal part.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an image forming apparatus such as a copy machine and an LBP (laser beam printer), particularly an image forming apparatus to attenuate echo of actuation sound of an actuation portion of the apparatus.
2. Description of the Related Art
Conventionally, as a countermeasure against noise of the image forming apparatus, it has been considered how to reduce the actuation sound, with regard to the actuation portion (sound source) such as motor, solenoid, and roller, which emits the actuation sound or noise. And it has been considered how to muffle the noise by sealing the actuation portion (Japanese Patent Application Laid-Open No. 2002-307780). Further it has been considered to provide the sound absorbing material near the actuation portion (Japanese Patent Application Laid-Open No. H03-2063).
However, the actuation portion of the image forming apparatus includes a manual sheet feed portion, a movable tray of a sheet discharge portion, and so on, which are disposed outside the housing of the image forming apparatus and are not sealed completely. With regard to the actuation sound of these actuation portions transmits to a person with aggregate sum of the sound directly emitted from an unsealed opening and the sound reflected from exterior walls around the actuation portion and emitted from the opening. There is a structure of applying a small amount of sound absorbing member on the exterior wall to attenuate the reflected sound. As the sound absorbing member, porous members such as polyurethane foam and inorganic fiber such as glass wool have been widely employed.
An aluminum has been employed to absorb sound (Japanese Patent Application Laid-Open No. 2002-244403). Foam metal also has been employed to absorb sound (Japanese Patent Publication No. H05-9036).
However, the conventional image forming apparatus has had a problem of little effect of absorbing reflected sound. That is, the center frequency of the actuation sound of the actuation portion which is difficult to seal is mainly not more than 1 kHz, while the sound absorbing members of porous members such as polyurethane or inorganic fiber sound absorbing members are effective against a frequency band of not less than 1 kHz. Therefore, the conventional reflected sound attenuation mechanism has not been suitable for low frequency.
Further, for example, in the case that a sound absorption degree of not less than 0.8 at a noise center frequency of 500 Hz is achieved, a thickness of the sound absorbing member is required to range from about 50 mm to about 100 mm. For this reason, in the conventional apparatus, it has been difficult to employ the sound absorbing member for the image forming apparatus, because it requires a large space for applying the member. If the sound absorbing member is used, this poses the problem of making the image forming apparatus lager in size.
Although the above explanation is related to the problem with the actuation portion disposed outside the housing, there has been also the same problem with the actuation portion disposed inside the housing and opposed to an opening portion of the housing. That is, there has been a problem in which the actuation sound of the actuation portion is reflected by the internal part of the housing to leak as noise from the opening portion of the housing to outside.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image forming apparatus to enhance effect of reducing reflected actuation sound of an actuation portion.
The image forming apparatus of the present invention includes: an internal part away from a wall of a housing inward of the apparatus; and an actuation portion emitting an actuation sound and provided outside the housing, wherein an opening portion is provided in the wall between the internal part and the actuation portion and a sound absorbing member formed of foam metal is provided in the opening portion.
Also, an image forming apparatus of the present invention includes: an actuation portion emitting an actuation sound and disposed, in a housing, opposite to an opening formed in a wall of the housing; an internal part disposed, in an inner part of the housing, opposite to the actuation portion and away from the actuation portion; and a sound absorbing member formed of foam metal, provided between the actuation portion and the internal part and away from the internal part.
Also, an image forming apparatus of the present invention includes: an actuation portion emitting an actuation sound; an internal part disposed opposite to the actuation portion and away from the actuation portion, wherein the internal part is a part constituting an image forming portion to form an image on a sheet; and a sound absorbing member formed of foam metal, provided between the actuation portion and the internal part and away from the internal part.
In the image forming apparatus of the present invention, a space is formed between the sound absorbing member made of foam metal and the internal part. Therefore, the actuation sound of the actuation portion can be attenuated not only by the foam metal but also the space.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an enlarged view showing a reflected sound reduction structure in an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a right side view of FIG. 1.
FIG. 3 is an illustration explaining the reflected sound reduction structure in the case of propagating sound waves in a direction of plate thickness of a foam metal plate.
FIG. 4 is a graph comparing sound absorption performances of the foam metal plates and those of the other sound absorbing members.
FIG. 5 is a schematic block diagram explaining the sound absorption principle of the foam metal plate.
FIG. 6 is a front sectional view showing a schematic construction of the image forming apparatus according to the embodiment of the present invention.
FIG. 7 is an enlarged view of other reflected sound reduction structure provided in the image forming apparatus according to the embodiment of the present invention.
FIG. 8 is a graph showing a relation between the space behind the foam metal plate and the sound absorption degree.
FIG. 9 is a graph showing the sound absorption performances of the other foam metal plates.

DESCRIPTION OF THE EMBODIMENTS

An image forming apparatus of an embodiment of the present invention will be described hereinafter with reference to the drawings. Numerical values employed in the embodiments of the present invention are reference values and do not necessarily limit the present invention.
FIG. 6 is a cross-sectional front view showing a schematic construction of the image forming apparatus according to the embodiment of the present invention.
In the image forming apparatus 20, an image of an original is read by an image reading portion 8, an electrostatic latent image is formed on a surface of a photosensitive drum 1 as a latent image bearing member by an exposure from an image writing portion 9 in accordance with a command from a controller (now shown) based on the read image data. The surface of the photosensitive drum 1 is uniformly charged to a predetermined potential before the exposure by a charger 2. An electrostatic latent image is formed on the photosensitive drum 1 by a laser beam or the like irradiated on the uniformly charged surface of the photosensitive drum 1 from the image writing portion 9. The electrostatic latent image formed on the photosensitive drum 1 is toner-developed with toner as a developer by a developing device 3 to form a toner image. And then the developed toner image is conveyed to a portion opposed to a transfer device 4 by the rotation of the photosensitive drum 1.
In accordance with the conveyance of the developed toner image, a pickup roller 51 of a manual sheet feed unit 5 feeds a sheet S as a recording medium one-by-one from a sheet tray 25. The sheet is timely conveyed to between the photosensitive drum 1 and the transfer device 4 by a pair of registration rollers 6. And while the sheet S is passing through between the photosensitive drum 1 and the transfer device 4, the transfer device 4 transfers the toner image on the photosensitive drum 1 to the sheet S. The sheet is also fed from a cassette 13 so that the toner image is transferred on the sheet.
The sheet S on which the toner image is transferred is conveyed to a fixing device 7 by a conveying device 12. The fixing device 7 nips the sheet while heating the sheet. The fixing device 7 is heated by a heater (not shown) provided in a fixing roller of the fixing device 7 to fuse the toner image on the sheet S to fix the toner image to the sheet S. And then, the sheet S on which the toner image is fixed is discharged to a tray 11 outside a housing 21 by a pair of discharging rollers 10. In this way, a series of the image forming processes in the image forming apparatus is completed.
Next, a reflected sound reduction structure will be described with reference to FIGS. 1 and 2.
FIG. 1 is an enlarged view of the reflected sound reduction structure 24 in FIG. 6. The manual sheet feed unit 5 constitutes a part of an exterior wall 22 of the housing 21. The manual sheet feed unit 5 comprises a pickup roller 51, a pickup support plate 52 rotatably axially supporting the pickup roller 51 to bring the pickup roller 51 into and out of contact with the sheet S, a receptive stopper 53 to determine a rising position of the pickup supporting plate 52. When the pickup support plate 52 is raised, the pickup roller 51 is separated from the sheet S. A manual sheet feed cover 54 of the sheet feed unit 5 and the receptive stopper 53 are integrally formed with ABS resin material and applied with coating for an exterior part.
FIG. 2 is a right side view of FIG. 1. An opening portion 57 is formed in the center of the exterior wall of the manual sheet feed cover 54 near the receptive stopper 53. The opening portion 57 is provided with a foam metal plate 55 having air-permeability porous member so as to close the opening portion 57. The foam metal plate 55 as a sound absorption material is also applied with coating for an exterior part. The foam metal plate 55 is fixed to the manual sheet feed cover 54 with screws 56 at both lateral ends which are boring processed. Between a foam metal plate 55 and a reflection wall 3 a opposed to the foam metal plate 55 of the developing device 3, a below-mentioned predetermined distance is spaced to form a space 69. This space 69 is a space for attenuating sound. A sheet supply port 26 supplying sheets stacked in the manual sheet feed tray 25 is formed below the foam metal plate 55.
With the above configuration, when the pickup support plate 52 is raised by a solenoid 98, the pickup support plate 52 is brought into contact with a surface of receptive stopper 53 to produce collision noise. The pickup roller 51, the pickup support plate 52, and the receptive stopper 53 are an actuation portion emitting actuation sound.
The collision noise is radially propagated in air. When there is a wall near the pickup support plate 52 and the receptive stopper 53 as a sound source, the collision noise is reflected by the wall so that a sound pressure in an open space opposed to the wall surface is increased. The foam metal plate 55 is located on the left side of the pickup support plate 52 and the receptive stopper 53. This foam metal plate 55 dramatically reduces the reflected sound. Especially, when a line connecting the opening portion 57 and the actuation portion as a sound source is perpendicular to one side surface (the reflection surface 3 a in this case) of the developing device 3, there is a strong possibility that the sound from the actuation portion is reflected by the reflection surface 3 a to leak from the opening portion. However, in the embodiment of the present invention, since the foam metal plate 55 is located between the actuation portion and developing device 3, the reflected sound is reduced by the sound absorption effect of the foam metal plate 55.
Here, a sound absorption mechanism of the foam metal plate 55 will be described. FIG. 3 shows a reflected sound reduction structure 44 when the sound waves are propagated in a direction of plate thickness of the foam metal plate 55. Cells 62 independently formed (black portion in FIG. 3) are communicated with each other by through-holes 64 formed by extending through the foam metal plate 55. The sound waves 66 are diffusely reflected by a metal shell 63 of the cell 62 exposed on the surface to be interference-attenuated by reflected waves. And a part of the sound waves 66 passes through the thorough-holes 64. The passed sound waves are expanded and attenuated within a next cell 62, and likewise, the sound waves are repeatedly expanded and attenuated every time the sound waves move to an adjacent cell. And when the sound waves arrive at the opposite side of the foam metal plate 55, the sound waves are reflected by the reflection wall 3 a, which is a predetermined distance away from the foam metal plate 55 through the space 69, to be made into reflected waves 81, thereby the noise is reduced by the interference, expansion, and friction with the metal shells (cell wall surfaces) 63.
FIG. 4 is a graph showing a comparison of sound absorption performance between the foam metal plates 55 and other sound absorbing members. In FIG. 4, absorption members in the graph represented by reference signs (a), (b), and (c) are foam metal A having a plate thickness of 10 mm. A sound absorbing member represented by a sign (d) is a glass wool (plate thickness: 25 mm). A sound absorbing member represented by a sign (e) is a urethane (plate thickness: 15 mm).
The foam metal A is formed of aluminum, which is an air permeability porous member which is cut after closed cell molding, and subjected to a compression process. The cracks are caused by compressing the aluminum with which the closed cells are formed, so that the closed cells are communicated with each other through the cracks. And the space 69 shown in FIG. 3 is provided at the back of the sound absorbing member represented by (a), (b), and (c) in the graph. A distance of the space 69 is 30 mm for the absorption member represented by the sign (a), 60 mm for (b), and 90 mm for (c).
As shown in FIG. 4, in order to absorb a sound of a relatively low frequency not more than 1 kHz, the foam metal plate of the air-permeability porous member is used as a sound absorbing member and a space is provided at the back of the foam metal plate, thereby sound absorption degree is remarkably increases compared with that of the conventional sound absorbing member.
FIG. 5 is a schematic block diagram of the reflected sound reduction structure. A collision sound repeats the expansion attenuation and the interference attenuation due to the diffuse reflection in the process of passing through the foam metal plate 55 to reach the opposite side of the foam metal plate 55. On the opposite side of the foam metal plate 55, the developing device 3 (See FIG. 1.) is arranged as internal part necessary for the image forming process. Between the foam metal plate 55 and the reflection wall 3 a of the developing device 3 disposed in a location opposed to the foam metal plate 55, a space having a width W (e.g. 20 mm) is provided. Traveling waves having passed through the foam metal plate 55 are reflected by the reflection wall 3 a and propagated to and from in the space 69 of 20 mm. In this process, the traveling waves and reflected waves perform interference attenuation. The attenuated reflected sound again passes through the foam metal plate 55. Therefore, about 80° of the actuation sound (incident sound) emitted from the sound source is absorbed when the sound returns to the sound source side.
In this embodiment, the foam metal plate 55 is arranged in a part of the manual sheet feed cover 54, which is an exterior wall of the manual sheet feed unit 5. The foam metal plate 55 is easy-machinable in machining such as bending and boring. For this reason, the manual sheet feed cover 54 may be entirely formed with foam metal.
Further, the example of using foam metal for the exterior wall is not limited to the manual sheet feed unit, but foam metal can be applied to a portion, where the actuation portion which emits an actuation sound of a relatively low frequency is disposed in an open space, for example, of an exterior cover near the movable tray in a sheet discharging portion to which a sheet is discharged.
Furthermore, in this embodiment, a surface corresponding to the reflection wall is the reflection wall 3 a of the developing device 3. The reflection wall, however, may be a surface of another internal part in the image forming apparatus. And in the case that there is a space between the exterior wall of the housing and the internal part and there is a noise source having an open space outside the exterior wall of the apparatus, the similar effect is obtained.
In addition, as shown in FIG. 7, the present invention is also applicable for attenuating an actuation sound generated by a pair of sheet conveying rollers 41 as an actuation portion inside the housing 21 opposed to the sheet discharging port 27 as an opening portion formed in the exterior wall 22 of the housing 21.
That is, between a pair of sheet conveying rollers 41 and a reflection wall 7 a of the fixing device 7 as an internal part, a foam metal plate 71 as a sound absorbing member is disposed away from the reflection wall 7 a to form a space 72 between the foam metal plate 71 and the reflection wall 7 a. With this reflected sound reduction structure 28, the actuation sound of the pair of sheet conveying rollers 41 is attenuated by the foam metal plate 71 and the space 72 and cannot be amplified by the reflection due to the reflection wall 7 a.
Next, a preferable example of applicable scope of each feature in the present invention will be described hereinafter.
1. Regarding Porosity of Foam Metal
With regard to the sound absorption performance of the foam mental, a high porosity is generally preferable. And a material of the sound absorbing member of the foam metal produced by the typical manufacturing method is mainly aluminum (A cast metal may be used.). When the material of the foam metal is aluminum and the porosity is about 50%, an aluminum specific gravity becomes 2.74 gf/cc, and an apparent specific gravity is 1.37 gf/cc, which is substantially equivalent to a gravity of 1.3 gf/cc. of ABS used as general resin material for an exterior coating.
That means, porosity of not less than 50% is preferable to the conventional performance, not only from a viewpoint of the sound absorption performance but also from an ergonomic viewpoint such as an operability of opening and closing of the manual sheet feed unit, and a physical distribution or an installation cost because of reduction in weight of the body of the apparatus. The present embodiment uses a foam metal of aluminum having porosity not less than 50° and not more than 95°.
2. Regarding Width (thickness) of Space
When the width of the space 69, 72 behind the foam metal plate 55, 71 increases, a degree of sound absorption at a low frequency bandwidth is improved. However, a clearance between the foam metal plate and an internal part (the inner parts in the body of an apparatus) cannot be set without limit. FIG. 8 is a graph showing a relation between a space behind the foam metal plate and a degree of sound absorption. In FIG. 8, sound absorbing members represented by reference signs (f), (g), (h), and (j) are foam metal plates B having a plate thickness of 10 mm. Incidentally, the foam metal plate B has the same thickness of 10 mm as the foam metal plate A, but an expansion ratio of the foam metal plate B is higher than that of the foam metal plate A.
Material of the foam metal plate B is aluminum. The foam metal plate B is air-permeability porous member which is cut after closed cell forming, and through-holes are formed. The closed cells are communicated with each other by forming the through-holes in the aluminum with which the closed cells are formed. With regard to a width of the space behind the foam metal plate B, a graph shown by the sign (f) is 0 mm, a graph shown by the sign (g) is 20 mm, a graph shown by the sign (h) is 40 mm, and a graph shown by the sign (j) is 60 mm.
In FIG. 8, for the foam metal plate of graph (f) with no space, degree of sound absorption is 0.8 at a frequency of 2.5 kHz, and the degree of sound absorption not less than 0.8 cannot be achieved within a range of frequencies not more than 1 kHz. However, for the foam metal plate having the space width of 20 mm shown by the sign (g), degree of sound absorption is 0.8 at a frequency of 1 kHz. Therefore, the actuation sound can be attenuated, if the space width is set to not less than 20 mm. And in the image forming apparatus of the present embodiment, it is realistically possible that an internal part is installed to allow an opening to be spaced about 20 mm away from the interior surface of the exterior wall 22. And effective reflected sound reduction structure can be arranged in the conventional space.
In the applicable places which have affordable space inside the body, it is theoretically preferable that a thickness of space 69, 72 is set to (¼) λ where a wavelength at which a sound of a center frequency of noise is propagated in the air is λ, because the maximum interference cancel effect is achieved.
3. Plate Thickness of Foam Metal
A plate thickness of the foam metal is not specifically limited for convenience of a method of forming a foam metal plate 55, 71 and from a viewpoint of sound absorption performance. FIG. 9 is graphs showing the sound absorption performances of foam metal plates different from the sound absorbing member mentioned above in the embodiment. In FIG. 9, sound absorbing members of graphs represented by the signs (k) and (m) are a foam metal C having a plate thickness of 2.5 mm. A width of the space behind the foam metal C is 50 mm for the graph of the sign (k) and 100 mm for the graph of the sign (m). Although the plate thickness of the foam metal plate in FIGS. 4 and 8 used in the explanation of the embodiment is 10 mm, it is apparent for FIG. 9 that a sound absorption degree not less than 0.8 can be accomplished at a frequency of 1 kHz or less depending on the behind space, even with the plate thickness of 2.5 mm. And generally a plate thickness of resin material used for exterior walls of the image forming apparatus is ranging from about 2.5 mm to about 3 mm, and it is possible to replace with the conventional exterior walls in a viewpoint of the plate thickness.
That is, plate thickness of the optimal sound absorption effect may be selected within the scope where there is no change in a position of an exterior wall and there is no intrusion into an internal part space in the housing.
With the above mentioned configuration, the foam metal plate as a sound absorbing member is excellent in heat resistance to maintain for example a sound absorption degree of not less than 0.8 at a center frequency of 500 Hz, thereby products can obtain the standard of flame retardancy. (A flame retardancy for a sheet material as an exterior is not less than the standard HF-2 and not more than 25 cm²).
The conventional sound absorbing member made of inorganic fiber such as glass wool cannot be self-support as a single unit because of its low mechanical strength, therefore it is difficult to install on the exterior wall without forming a complex structure together with a perforated metal plate or a resin plate. While, the sound absorbing member of the foam metal plate of the present invention has a high mechanical strength and it is easy to install.
The conventional sound absorbing member such as polyurethane foam and glass wool has troubles of easy breakage damage and breakage into dust due to vibration, and occasional time degradation. Further, the performance of sound absorption may become low when absorbing water in the atmosphere. While, the sound absorbing member of the foam metal plate of the present invention has less possibility of breakage damage and time degradation. Further, the sound absorption performance is hardly reduced even though it absorbs water in the atmosphere, and it can maintain the sound absorption performance for a long time.
The conventional sound absorbing member can not be coated with coating, thereby it is not suitable for an exterior member while the sound absorbing member of foam metal plate of the present invention can be coated with coating and can be used as an exterior member.
If the conventional sound absorbing member is made self-supported by forming a space between it and the reflection wall, it is required to support the conventional sound absorbing member by fastening through a spacer to the exterior wall or to support the exterior wall and the sound absorbing member by a duct structure. Therefore, it is impractical in terms of cost, space, and exterior appearance. On the contrary, since the sound absorbing member of the foam metal plate of the present invention is superior in strength to the conventional sound absorbing member, installation structure becomes simple. Therefore, the image forming apparatus of the present invention can be made the practical structure in terms of cost, space, and exterior appearance.
The reflected sound reduction structure for the actuation portion (sound source), which cannot be sealed, emitting the actuation sound in the image forming apparatus of the present invention is provided with the sound absorbing member formed of a foam metal plate in the opening portion formed in the exterior wall as a reflection wall. Therefore, a space for attaching the sound absorbing member between the sound source and the exterior wall is dispensable. As a result, the image forming apparatus can be prevented from made into larger in size.
As the image forming apparatus according to the invention is provided with a space on the backside of the sound absorbing member to increase the degree of sound absorption, the space between the exterior wall and the internal part in the apparatus can be effectively used so as to avoid an increase in size of the image forming apparatus.
In the image forming apparatus of the present invention, the foam metal plate can dramatically reduce the low frequency noise, which is a sound of mainly not more than 1 kHz emitted from a sound source, which cannot be sealed in the vicinity of the exterior, and which noise is difficult to reduce by the conventional method of attaching the sound absorbing member on the exterior wall.
If the present invention is applied to the image forming apparatus, as shown in FIG. 7, in which the actuation portion opposed to the opening portion formed in the exterior wall of the housing is provided inside the housing, the apparatus can improve the attenuation effect. Further it can efficiently utilize the space between the actuation portion and the reflection wall of the internal part, thereby avoiding an increase in size of the apparatus.
Further, in the image forming apparatus as shown in FIG. 7, not only the foam metal but also the space between the internal part and the foam metal contributes the actuation sound reduction, thereby increasing the actuation sound attenuation effect to decrease the actuation sound of the actuation portion, which is leaked from the opening portion. In addition, even if the actuation sound is not more than 1 kHz, the actuation sound can be surely attenuated.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2005-349847, filed Dec. 2, 2005, which is hereby incorporated by reference herein in its entirety.

Claims

1. An image forming apparatus comprising:

an internal part away from a wall of a housing inward of the apparatus;

an actuation portion emitting an actuation sound and provided outside said housing; and

a sound absorbing member formed of a foam metal and provided in an opening portion, which is provided in said wall between said internal part and said actuation portion.

2. An image forming apparatus according to claim 1, wherein a porosity of said foam metal is 50% or more.

3. An image forming apparatus according to claim 1, wherein a space between said internal part and said sound absorbing member is set to substantially one fourth of a wavelength of the actuation sound.

4. An image forming apparatus according to claim 1, wherein said sound absorbing member is formed of aluminum.

5. An image forming apparatus according to claim 1, wherein a space between said internal part and said sound absorbing member is set to 20 mm.

6. An image forming apparatus according to claim 1, wherein said actuation portion comprises a pickup roller lifting and lowering to feed a sheet supported on a tray and said internal part is a component constituting the image forming apparatus for forming an image on a sheet.

7. An image forming apparatus comprising:

an actuation portion emitting an actuation sound, provided inside a housing and opposed to an opening portion formed in a wall of said housing;

an internal part provided in an inner part of said housing, opposed to and away from said actuation portion; and

a sound absorbing member formed of a foam metal, provided between said actuation portion and said internal part, and away from said internal part.

8. An image forming apparatus according to claim 7, wherein a porosity of said foam metal is 50% or more.

9. An image forming apparatus according to claim 7, wherein a space between said internal part and said sound absorbing member is set to substantially one fourth of a wavelength of the actuation sound.

10. An image forming apparatus according to claim 7, wherein said sound absorbing member is formed of aluminum.

11. An image forming apparatus according to claim 7, wherein a space between said internal part and said sound absorbing member is set to 20 mm.

12. An image forming apparatus comprising:

an actuation portion emitting an actuation sound;

an internal part provided away from and opposed to said actuation portion, wherein said internal part is a component constituting an image forming portion for forming an image on a sheet; and