Flat platen and image forming apparatus

A flat platen is provided in an image forming apparatus having a side edge detector that detects a side edge of a recording medium to be fed in a predetermined direction. The flat platen has a surface on which the recording medium to be fed is supported and is disposed facing the side edge detector. The side edge detector has a light emitting device and a light receiving device, which are disposed such as to face the recording medium. The side edge detector detects a side edge of the recording sheet while moving in a direction perpendicular to the predetermined direction to emit light from the light emitting device. In such a flat platen, an anti-reflective treatment, which reduces or prevents light led to the light receiving device from the light emitting device through the reflection on the surface, is applied to the surface of which corresponds to at least a vicinity of a side edge of a standard-size recording medium.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a flat platen that is provided in an image forming apparatus. The invention also relates to an image forming apparatus that detects both side edges of a recording sheet being fed therein.

2. Description of Related Art

Japanese Laid-Open Patent Publication No.2000-109243 discloses a conventional inkjet recording apparatus such as a printer and a facsimile machine. (Refer to pp 4-6 and FIG. 3.) The inkjet recording apparatus has a flat platen that guides a recording sheet that is fed horizontally therein and a print head that is movable in a vertical direction relative to a sheet feed direction. In the inkjet recording apparatus, a recording sheet is fed on the flat platen and the print head, which is disposed facing the recording sheet, is moved to eject ink onto the recording sheet, thereby printing is made.

The print head is provided with a light emitting device (a light emitting diode, LED) and a light receiving device (a photo transistor) for detecting a width of a recording sheet. When light emitted from the light emitting device reflects on a recording sheet and is received at the light receiving device, a side edge of the sheet being fed parallel to the sheet feed direction is detected. With the detection of the side edge, a print start position and a print end position with respect to a scanning direction of the print head are determined.

SUMMARY OF THE INVENTION

However, according to the above conventional inkjet recording apparatus, a light beam emitted from the light emitting device reflects on the flat platen. As such, the amount of light that is received by the light receiving device may be beyond a specified amount. In this case, a position where a recording sheet does not exist may be improperly detected as the side edge of a sheet. If the side edge of the sheet is improperly detected, ink may be ejected from the print head to a position where there is no sheet. In such a case, the flat platen becomes soiled with ink and dirt is transferred on a succeeding recording sheet while passing on the platen. A consequent problem thus exists with a reduction in print quality of the inkjet recording apparatus.

The invention thus provides, among other things, a flat platen that can improve print quality and an image forming apparatus for use with the flat platen.

According to one exemplary aspect of the invention, a flat platen is used in an image forming apparatus that has a side edge detector that detects a side edge of a recording medium that is fed in a predetermined direction. The side edge detector has a light-emitting device and a light receiving device that are disposed facing the recording medium. The side edge detector detects the side edge of the recording medium while moving in a direction perpendicular to the predetermined direction in order to emit light from the light emitting device. The flat platen includes a surface that faces the recording medium and supports the recording medium thereon, and an anti-reflective treatment that reduces an amount of light, emitted from the light emitting device, that is reflected on the surface to the light receiving device. The surface is processed with the anti-reflective treatment at least in a part irradiated with the light emitted from the light emitting device.

According to the above structure, the side edge detector is driven and moved in the direction perpendicular to the direction that the recording medium is fed. The side edge detector emits light from the light emitting device, receives the light reflected on the recording medium at the light receiving device and detects a side edge of the recording medium. The surface of the flat platen that supports the recording medium facing the side edge detector is applied with the anti-reflective treatment. The light emitted from the light emitting device to the surface of the flat platen is reduced at the light receiving device through the application of the anti-reflective treatment to the surface of the flat platen. Thus, a provision that an error detection is unlikely to occur, and a side edge of a recording medium can be accurately detected is created.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of the invention will be described with reference to the figures.FIG. 1is a perspective view of a multifunction device including an inkjet recording part. The multifunction device1has a facsimile function, a telephone function, a copier function, a scanner function, and a printer function. The multifunction device1is also connectable with a personal computer.

A main body5of the multifunction device1includes a print unit20(FIGS. 2 and 3) having an ink cartridge (not shown), which performs printing by ejecting ink onto recording sheets such as paper and film, which are conveyed thereto. A sheet feed unit2that supplies recording sheets into the main body5is provided at the back of the main body5. An ejection port6through which recording sheets printed at the print unit20are ejected is provided at the front of the main body5. A handset7is disposed at a side of the main body5which enables speaking and listening by telephone.

A reading unit3is disposed on an upper portion of the main body5. The reading unit3appears when an upper cover8is opened and reads an image of a document placed thereon. At the front of the reading unit3, an operation panel4is disposed. The reading unit3is pivotally openable relative to the main body5. By opening the reading unit3, the print unit20and a path where a sheet is conveyed, which are provided in the main body5, become accessible and sheets jammed can be removed.

FIG. 2is a sectional view showing mainly a sheet feed unit and a recording unit in the multifunction device1. A guide plate15and a sheet feed roller12are provided in the sheet feed unit2covered with a sheet feed unit cover11at the front. The guide plate15guides a sheet in a sheet feed direction and supports the back of the sheet. The sheet feed roller12is disposed such as to have face-to-face contact with an uppermost sheet of a stack of sheets stacked on the guide plate15. With a rotation of the sheet feed roller12, the sheets are fed one by one from the top of the stack. Ahead of the guide plate15, a sheet feed pad13that facilitates the separation of a single sheet from the stack and a chute14that guides the sheet separated from the stack from the sheet feed unit2to the print unit20are disposed.

The print unit20is made up of components, such as rollers, a platen, a print head, and drive mechanisms, which are attached to a main frame24that is covered with an upper frame32at an upper portion. A sheet being fed from the chute14is guided into the print unit20via a pressure roller holder21which can be inclined.

FIGS. 3 and 4show a sectional view and a perspective view of details of the print unit20, respectively. A main roller22and a guide shaft30are disposed upstream of the print unit20in the sheet feed direction with both ends supported by the main frame24. The main roller22is disposed in contact with a pressure roller23supported in the pressure roller holder21and is driven by a sheet feeding motor34via a belt35and a speed reduction pulley36. Thus, a sheet sandwiched between the main roller22and the pressure roller23is fed.

The guide shaft30guides a print head31, which is driven by a head drive motor33, in a direction perpendicular to the sheet feed direction. The print head31includes a plurality of nozzle arrays where nozzles are arranged in the sheet feed direction, and forms images by ejecting ink from the nozzle arrays. The print head31also includes an ink cartridge (not shown) containing ink and a media sensor28detecting a side edge of a sheet parallel to the sheet feed direction.

The media sensor28is a reflective sensor having a light-emitting device such as a light-emitting diode (LED) and a light receiving device such as a photo transistor. In the media sensor28, a light beam is emitted from the light emitting device toward a sheet and a flat platen25and received at the light receiving device, thereby detecting a presence or absence of a sheet. The media sensor28detects both side edges of a sheet while moving along the guide shaft30. Thus, a print start position and a print end position are determined with respect to a moving direction of the print head31.

The flat platen25is disposed ahead of the pressure roller holder21, namely, on the downstream side with respect to the sheet feed direction. The platen25supports a sheet to be printed at the time of printing, and guides it to the ejection port6(FIG. 1) horizontally after printing. At both sides of the flat platen25, paper plates40made of metal are placed to adjust the position of a sheet passing on the flat platen25.

The speed reduction pulley36is connected to a transmission pulley38via the belt37. The transmission pulley38has a pulley part, which is rotated via the belt37, and a gear part. The transmission pulley38is connected to an ejection gear39pressed in a ejection roller26disposed ahead of the flat platen25, transmitting rotational force. Thereby, when the sheet feeding motor34is started, the ejection roller26is rotated.

A front frame29having a plurality of spur rollers27is attached to a front portion of the main frame24. As shown inFIG. 10, the spur rollers27make contact with the ejection roller26. The spur rollers27and the ejection roller26sandwich a printed sheet therebetween and feed it to the ejection port6(FIG.1). InFIG. 10, the spur rollers27are drawn as if they have a thickness to some degree, but actually, they are thin plates.

The spur rollers27are constructed of homopolymer acetal polyoxymethylene (POM) where Teflon® is prepared. The spur rollers27made of resin can be manufactured reasonably compared with those of metal. The homopolymer POM can secure durability because it has higher wear resistance compared with a copolymer POM. In addition, by preparation of Teflon®, the adhesion of ink to the spur rollers27can be reduced. For example, Delrin AF-500® by DuPont™ can be used.

The ejection roller26is formed by coating a peripheral surface of a metal shaft with a urethane film having a thickness of approximately 30-35 μm. The coating of the ejection roller25with the thin urethane film can reduce the effects of linear expansion coefficients lower than those of a conventional ejection roller made of a thick rubber, thereby reducing a change in an outside diameter due to environmental changes, and securing a stable amount of feeding sheets. In addition, the urethane coating can reduce the wearing away of the spur rollers27made of resin.

FIGS. 5,6, and7show the flat platen25in a perspective view, a front sectional view, and a side sectional view, respectively. The flat platen25is a resin molded item, and a plurality of ribs52and56protrude from a surface51facing a sheet to be fed. Grid-type ribs54protrude from the back surface of the flat platen25.

The ribs54ensure the strength of the flat platen25, which is formed into a thin plate, and prevent the flat plate25from warping. A protrusion55is provided at a central portion of the back surface of the flat platen25with respect to a length of the flat platen25such as to protrude therefrom deeper than the ribs54. The protrusion55makes contact with the mainframe24(FIG. 3) to prevent warping of the flat platen25due to an excessive load applied to the flat platen25and deterioration with time.

The ribs52,56, which are formed on the surface51of the flat platen25, extend in the sheet feed direction and are aligned in a direction perpendicular to the sheet feed direction. A sheet fed to the flat platen25makes contact with the upper ends of the ribs52,56and is supported thereon. The ribs52,56can reduce a contact area between the flat platen25and the sheet, reduce friction therebetween, and facilitate feeding of the sheet.

The ribs52are disposed at a rear portion of the flat platen25(on the upstream side with respect to the sheet feed direction) and include a range (indicated by D inFIG. 8) where the media sensor28of the print head31scans. The ribs56are separated from the ribs52with a space57, and are disposed at a front portion of the flat platen25(on the downstream side with respect to the sheet feed direction). The ribs56are lower than the ribs52in height. On the flat platen25, the number of the ribs56is lower than that of the ribs52. The ribs56are also located on lines extended from the ribs52with the space57in the sheet feed direction.

When the sheet is printed on the ribs52, the ribs absorb ink and bows. The leading edge of the sheet bowing upward in the back and forth direction passes in the clearance57(FIGS. 3 and 5) to reduce bowing of the sheet and to prevent deterioration of image quality. When the leading edge of the sheet is away from a printing area, it slides on the ribs56which are lower in height, where bowing of the sheet in the printing area is reduced, and the sheet is fed smoothly.

FIG. 8is a plan view of the flat platen25. An anti-reflective treatment portion51a, which is given anti-reflective treatment indicated by hatching, is formed at the rear portion of the flat platen25and includes the scanning area D of the media sensor28that is on the surface51on the upstream side with respect to the sheet feed direction. The light emitted from the light emitting device of the media sensor28reflects on the flat platen25or a surface of a sheet being fed and goes in the light receiving device.

However, much of the light is diffusely reflected and absorbed at the anti-reflective treatment portion51aof the flat platen25. As such, the amount of the light incident on the light receiving device becomes extremely low. Thus, the light beam reflected at the flat platen25is reduced or prevented. On the other hand, a large amount of light reflected on the sheet is incident on the light receiving device. Thus, a control part (not shown) of the multifunction device1is capable of reliably detecting the presence or absence of the sheet based on the amount of light received by the media sensor28, thereby determining both side edges of the sheet with precision.

For example, anti-reflective treatment for the anti-reflective treatment portion51amay be applied by forming a dull surface with matte treatment, such as sandblasting and texturing. Texturing is a process used to apply a textured pattern to a surface of a part to be molded by making a surface of a mold rough by etching. Thus, as the matte treatment is applied concurrently with the molding of the flat platen25, the number of processes to manufacture the flat platen25can be reduced. In this embodiment, HM3013 (matte finish pattern) of Nihon-Etching Co., LTD. is used as a textured pattern.

By applying matte treatment to the scanning area D of the media sensor28, light emitted from the light emitting device of the media sensor28is diffusely reflected at the anti-reflective treatment portion51a. With this structure, the amount of light that is reflected on the surface51and received on the light receiving device is reduced, thereby relieving a detection error of the media sensor28at a position where no recording sheet exists. Namely, the precision of the media sensor28that detects both side edges of a sheet can be improved. As a result, a positioning accuracy of the print start position and the print end position in the moving direction of the print head31can be improved.

The paper plates40(FIG. 4) disposed on both ends of the flat platen25serve to prevent a recording sheet from going up toward the print head31and guide the recording sheet onto the flat platen25. The paper plates40are in the form of a thin metal to be placed in a narrow area between the print head31and the flat platen25. The paper plates41are formed with recesses40a(FIG. 4) in the scanning area D of the media sensor28to block the reflection of light emitted from the light emitting device. The flat platen25is formed with recesses53at positions corresponding to the recesses40a, and matte treatment is applied to side walls defining the recesses53. Thereby, the reflection of light at both ends of the flat platen25can be controlled.

Ribs53aare disposed along the shape of the recesses53of the flat platen25at positions corresponding to the recesses40a, so that the edges of the paper plates40are covered with the ribs53a. This structure can prevent the media sensor28from making an improper detection of both side edges of a sheet which may occur when a light beam emitted from the light emitting device of the media sensor28is reflected at an edge of either paper plate40and received at the light receiving device. With the application of the anti-reflective treatment such as matte treatment to the ribs53a, an effect on the prevention of the improper detection can be improved.

It is preferable that the anti-reflective treatment portion51ais provided in a place that corresponds to at least a vicinity of each side edge of a lower part of a standard-size sheet. As shown inFIG. 11, this position is in an area51cof the flat platen25including a position51bvertically extending downward from a side edge Pe of a sheet P.

The standard size for recording sheets includes letter size, A4size, B5size, A5size, B6size, postcard size, and L-size for photo, as shown inFIG. 9, which are specified by JIS Standards and North American standards. If the multifunction device1is a large-sized one, B4size and A3size are also available.

When the anti-reflective treatment portion51ais formed in close vicinity to both side edges of a standard size sheet on the reverse side, the positioning accuracy of the print head31can be improved while a recording sheet of a standard size often used is printed. At this time, a range to form the anti-reflective treatment portion51acan be determined depending on variations in positioning during the feeding of sheets and the sensitivity of the light receiving device.

For example, the anti-reflective treatment portion51amay be provided in an area falling within at least 2 mm outside or inside with respect to both side edges of a sheet of each standard size to be disposed, in terms of design specification. Thus, an error in detecting both side edges can be prevented even when there is a positioning variation of a recording sheet.

If matte treatment is applied to the top surfaces of the ribs52where the sheet slides, the friction force of the ribs52with the sheet increases, and the sheet is not smoothly fed. For this reason, it is easy for the top surfaces of the ribs52to reflect light. As shown inFIG. 9, the ribs52are arranged such that the top surfaces of the ribs52are not positioned directly under both side edges of any standard size sheets. Thus, light that is reflected on the top surfaces of the ribs52and received at the light receiving device is reduced, thereby improving the positioning accuracy of the print start position and the print end position in the moving direction of the print head31while the sheet of any standard size often used is printed.

If the ribs52are arranged close to the outside from both side edges of a standard-size sheet, light that is emitted from the light emitting device and reached at the ribs52may be received at the light receiving device due to the variations in positioning of sheets being fed and the sensitivity of the light receiving device. Thus, it is preferable not to provide the ribs52within 2 mm outward from positions directly below both side edges of any standard-size sheet.

The ribs52have a height of 2 mm or more. Thus, the amount of light reflected on the surface51and received at the light receiving device can be reduced. It is preferable to form the anti-reflective treatment portion51atreated with either of the above mentioned methods on the surface51and set the height of the ribs52to 2 mm or more.

As shown inFIG. 6, the ribs52are made up of two kinds of ribs52aand52bof different heights. The ribs52aand52bare disposed in parallel to each other in a direction perpendicular to the sheet feed direction. The high ribs52amake sliding contact with and guides a recording sheet being printed. The low ribs52bhold the sheet that has absorbed ink during printing and that bows in a vertical direction relative to the sheet feed direction such as not to contact with the surface51. With this structure, the sheet can be prevented from bowing thereby obtaining high print quality.

When a side edge of a sheet is brought in contact with the surface51due to its bending, ink smudges the surface51and adheres to the sheet. Thus, as shown inFIG. 11, the low rib52bis disposed inside from the side edge Pe of the sheet P, and the side edge of the sheet is raised to prevent ink from staining on the surface51.

A purpose of the anti-reflective treatment portion51aprovided in the scanning area D of the media sensor28is to enable a provision that reduces or prevents light that is emitted from the light emitting device of the media sensor28, to reflect off of the flat platen25and to be received by the light receiving device. Thus, a light absorbent member51d(FIG. 16) in the form of sheet may be affixed onto the surface51instead of the above-described matte treatment. Thus, a light beam emitted from the light emitting device is absorbed by the light absorbent member, so that the reflection of the light beam can be reduced.

Instead of the matte treatment or the affixture of the light absorbent member, openings51e(FIG. 17) may be formed on the flat platen25in the vicinity of both side edges of a sheet of a standard size, which is to be fed. Thus, in printing the sheet of a standard size, light emitted from the light emitting device can pass through the openings to prevent light reflection at the flat platen25.

Additionally, instead of the matte treatment or the affixture of the light absorbent member, the flat platen25may be provided with a groove portion58including an inclined plane58a, which extends in a direction perpendicular to the sheet feed direction in the scanning area of the media sensor28, as shown inFIGS. 12to14. The inclined plane58ais disposed in an area irradiated with light from the light emitting device of the media sensor28, and formed to incline toward the sheet feed direction. The inclined plane58ahas a width of 2 mm to 10 mm and is designed such that an angle between the inclined plane58aand incident light from the light emitting device of the media sensor is 20° to 70°. In the illustrated flat platen25, for example, the inclined plane58ahas a length of 6 mm, and the angle formed with the incident light from the light emitting device is 45°. In this form, it is preferable that the incline plane58ais smooth without matte treatment or texturing.

By forming the groove portion58having such an inclined plane58a, light that is emitted from the light emitting device of the media sensor28reflects on the inclined plane58a. The reflected light goes to a direction completely different from a direction where the light receiving device of the media sensor28is present. With this formation, light reflected on the surface51and received by the light receiving device can be reduced, thereby relieving a detection error of the media sensor28at a position where no recording medium exists. Namely, the precision of the media sensor28detecting both side edges of a sheet can be improved. As a result, a positioning accuracy of the print start position and the print end position in the moving direction of the print head31can be improved.

By formation of the groove portion58in the flat platen25, the strength of the flat platen25in the form of a thin plate is ensured, and the flat platen25can be prevented from bending in its longitudinal direction. Thus, in this form, the grid-type ribs54provided on the back side of the flat platen25can be omitted or replaced with ribs extending only in the sheet feed direction for simplification. Instead of the ribs54, a plurality of groove portions extending in the direction perpendicular to the sheet feed direction may be provided on the flat platen25in parallel to each other in the sheet feed direction. This form also can sufficiently ensure the strength of the flat platen25.

In the above structure, the groove portion58including the inclined plane58ais formed in the scanning area of the media sensor28on the flat platen25. However, as shown inFIG. 15, a protrusion59including an inclined plane59amay be formed in the scanning area of the media sensor28on the flat platen25. The inclined plane59amay be structured with the same position, the same width and the same angle of inclination as the inclined plane58a, however, the protrusion59should be set lower than the ribs52in height such as not to hinder feeding of recording sheets.

While the invention has been described with reference to a specific embodiment, the description of the embodiment is illustrative only and is not to be construed as limiting the scope of the invention. Various other modifications and changes may occur to those skilled in the art without departing from the spirit and scope of the invention. For example, when the sheet sizes available on the above multifunction device1are shown inFIG. 9, and the smallest size is L-size for photo, the anti-reflective treatment portion51a, the groove portion58having the inclined plane58a, and the protrusion59having the inclined plane59amay be provided, not in the area where a L-size-for-photo sheet passes, but on each side only beyond the L-size-photo sheet. When the sizes of sheets to be used can be limited, the anti-reflective treatment portion51a, the groove portion58having the inclined plane58a, and the protrusion59having the inclined plane59amay be provided in predetermined areas having a width of approximately 2 mm to 5 mm outside from each side of the sheets.

According to the embodiment of the invention, the surface51of the flat platen25is formed with the anti-reflective treatment portion51aor the inclined plane58aor59a, such as to reduce or prevent light that is emitted from the light emitting device and received at the light receiving device by reflection. For this reason, both side edges of a sheet can be accurately detected. Thus, the print head31can be positioned with high accuracy thereby providing high print quality.

Anti-reflective treatment can be easily applied to the flat platen25by either of matte treatment, affixture of a light absorbent member, provision of an opening, and formation of the inclined plane58a,59a. If matte treatment is performed by the formation of a textured pattern by texturing process, anti-reflective treatment can be applied to the flat platen25during molding. Furthermore, anti-reflective treatment can be applied to the flat platen25by forming the flat platen25integrally with the opening or the inclined plane58a,59a. The opening, the groove portion58having the inclined plane58a, or the protrusion59having the inclined plane59acan be integrally formed with the flat platen25in plastic molding.

According to the above embodiment, the ribs52,56are provided such as to reduce the contact area between the surface51of the flat platen25and a sheet, enabling a provision that reduces a friction force between the sheet being fed and the flat platen25, and enabling a provision that facilitates feeding of the sheet. The ribs52,56may be integrally formed with the flat platen25in plastic molding.

According to the above embodiment, the ribs52bprotrude 2 mm or more from the surface51of the flat platen25, thereby enabling a provision that reduces the amount of light that is emitted from the light emitting device, reflected at the ribs52b, and finally received at the light receiving device.

In addition, no ribs are disposed directly under both side edges of a recording sheet or within 2 mm outward from positions directly under both side edges. Thus, at the time a recording sheet of a size that is often used is printed, the amount of light that is emitted from the light emitting device, reflected at the ribs, and finally received at the light receiving device can be reduced.

The ribs are made up of at least the two kinds of ribs52,56of different heights. The high ribs52feed a recording sheet with a low friction, and the low ribs56carry the sheet that has absorbed ink during printing and partially bows to reduce bowing of the sheet. With these ribs, a provision that prevents deterioration of image quality is enabled.