1. Field of the Invention
The present invention relates to a fixing device for fixing toner to a sheet and an image forming apparatus for forming and fixing a toner image on and to a sheet.
2. Description of the Related Art
An electrophotographic copier, printer, facsimile machine or a complex machine provided with functions of these may be illustrated as an image forming apparatus. Such an image forming apparatus is provided with a fixing device for fixing toner to a sheet. A certain type of the fixing device is provided with a fixing roller including a heater and a pressure roller to be pressed into contact with the fixing roller. A sheet having a toner image formed by an electrophotographic process transferred thereto is passed through a nip defined between the fixing roller and the pressure roller. In the nip, the toner on the sheet is melted to fix the toner image to the sheet. In order to control the temperature of the fixing roller, this type of the fixing device includes a temperature detector.
Japanese Unexamined Patent Publication No. H05-119664 discloses a fixing device suitably used to fix toner to a laminated sheet (e.g. envelope). In the fixing device disclosed in Japanese Unexamined Patent Publication No. H05-119664, a pressing force between a fixing roller and a pressure roller is variable and reduced while a laminated sheet passes. Meanwhile, this fixing device reduces a conveying speed of the laminated sheet and the temperature of the fixing roller. In this way, this fixing device creates an equivalent condition for fixing toner to the laminated sheet to that for fixing toner to one sheet. As a result, the fixing device disclosed in Japanese Unexamined Patent Publication No. H05-119664 may fix the toner to the laminated sheet without wrinkles or lateral displacement of the sheet due to a difference in circumferential speed (linear speed) between the fixing roller and the pressure roller while the laminated sheet passes between the fixing roller and the pressure roller. On the other hand, toner may be fixed to one sheet using a high pressing force, wherefore the warm-up time of the fixing roller can be shortened and the fixing device can be miniaturized.
A complicated mechanism for automatically switching a pressing force generally results in a high production cost and a high failure rate. A mechanism for manually switching a pressing force is inconvenient to a user. For example, if one user tries to print an ordinary copy sheet after another user printed some image on an envelope, he/she has to operate an image forming apparatus to switch the pressing force before he/she starts his/her printing.
An object of the present invention is to provide a method and an apparatus enabling a pressing force to be switched and enabling toner to be suitably fixed to a laminated sheet using a weak pressing force.
In order to accomplish this object, one aspect of the present invention is directed to a fixing device, comprising a fixing roller configured to be heated by a heat source; a pressure roller configured to be pressed into contact with the fixing roller; a switcher for switching a pressing force between the fixing roller and the pressure roller between a first pressing force and a second pressing force lower than the first pressing force; a pressing force detector for detecting the pressing force between the fixing roller and the pressure roller; a temperature detector for detecting the temperature of the fixing roller; a storage device for storing control data indicating a control temperature for the fixing roller predetermined in accordance with the number of sheets passing the fixing device; and a controller for controlling the temperature of the fixing roller to the control temperature by controlling the heat source in accordance with a signal sent from the pressing force detector.
Another aspect of the present invention is directed to an image forming apparatus, comprising an image forming unit for forming a toner image on a sheet; and a fixing device for fixing the toner image formed by the image forming unit to the sheet, wherein the fixing device includes a fixing roller configured to be heated by a heat source; a pressure roller configured to be pressed into contact with the fixing roller; a switcher for switching a pressing force between the fixing roller and the pressure roller between a first pressing force and a second pressing force lower than the first pressing force; a pressing force detector for detecting the pressing force between the fixing roller and the pressure roller; a temperature detector for detecting the temperature of the fixing roller; a storage device for storing control data indicating a control temperature for the fixing roller predetermined in accordance with the number of sheets passing the fixing device; and a controller for controlling the temperature of the fixing roller to the control temperature by controlling the heat source in accordance with a signal sent from the pressing force detector.
Hereinafter, one embodiment of the present invention is described in detail with reference to the drawings. A term “sheet” used in the following description means, for example, a copy sheet, a coated paper, a cardboard, a tracing paper or another piece capable of bearing an image by toner fixing.
The photoconductive drum 30 shown in
An electrostatic latent image is formed on the circumferential surface of the photoconductive drum 30 by an electrophotographic process. Toner is supplied onto the circumferential surface of the photoconductive drum 30 from a developing device (not shown), whereby a toner image is formed on the circumferential surface of the photoconductive drum 30.
The image forming apparatus 1 shown in
The image forming apparatus 1 shown in
The fixing roller 10 of the fixing device 500 shown in
The pressure roller 12 may include, for example, a core made of aluminum, iron or the like of approximately 14 mm in diameter, an elastic layer (for example, silicon rubber layer) of approximately 5.5 mm in thickness formed along the outer circumferential surface of the core and a mold release layer containing fluoroplastic such as PFA and PTFE and formed on the outer surface of the elastic layer. Thus the pressure roller 12 may be, for example, 25 mm in outer diameter.
The heater 14 is incorporated into the core of the fixing roller 10 shown in
While the sheet passes the nip between the fixing roller 10 and the pressure roller 12, the toner forming the toner image on the sheet is melted and fixed to the sheet. The sheet having the toner image fixed thereto is, thereafter, discharged to the outside (e.g. a discharge tray (not shown)) of the image forming apparatus 1.
The image forming apparatus shown in
The sheet 64 may be, for example, a copy sheet of A3 size. The sheet 60 may be, for example, of a copy sheet of A4 size or letter size. The sheet 62 may be, for example, a laminated sheet formed by laminating a plurality of sheets such as an envelope.
Referring back to
The switcher 600 includes the pressure lever 42 extending from an inner wall of a housing of the image forming apparatus 1 and held by a corresponding one of a pair of supporting members 56 located near the opposite ends of the pressure roller 12, a guide 44 attached to the pressure lever 42, a spring 46 coiled around the guide 44, the L-shaped plate 48 connected with the corresponding guide 44 and the pressure plate 50 including a leading end portion engageable with a slit 49 defined in the corresponding plate 48. The pressure lever 42 including a shaft 54 is attached to the corresponding supporting member 56 via the shaft 54. An external force exerted to the pressure lever 42 to rotate the shaft 54 in a direction of arrow shown in
The pressure plate 50 includes a bearing 52 configured to be engaged with a shaft 26 of the pressure roller 12. By pressing the pressure plate 50 downward, the pressure roller 12 is pressed into contact with the fixing roller 10. Upon exerting a high pressing force (first pressing force) between the fixing roller 10 and the pressure roller 12 (see
When a body cover (not shown) of the image forming apparatus 1 is opened, a user may access the pressure lever 42 to be operated. When the pressure lever 42 is operated to rotate the shaft 54 in a direction opposite to the one shown by arrow in
When the pressure release lever 40 is moved to the right (see
Referring back to
A method for determining the control data illustrated in
For example, in this embodiment, the fixing roller 10 is rotated at a rotating speed (first rotating speed) higher than a fixing condition for the sheet 62 and the pressing force between the fixing roller 10 and the pressure roller 12 is set high (first pressing force) in the case of performing a fixing process for the sheet 60. For example, if the sheet 60 is a paper (plain paper) of 90 g/m2 and a multitude of sheets 60 successively passes the fixing device 500, the control circuit 24 receives a signal indicating that the pressing force between the fixing roller 10 and the pressure roller 12 is the first pressing force from the optical sensor 18. In accordance with the signal from the optical sensor 18, the control circuit 24 sends a command signal to the driving mechanism 22 to adjust the rotating speed of the fixing roller 10 and/or the pressure roller 12 so that a conveying speed of the sheet reaches 170 mm/s. The control circuit 24 further sends a command signal to the heater control circuit 20 before image formation, whereby the temperature of the heater 14 is controlled so that the temperature detected by the thermistor 16 held in contact with the second outer circumferential surface 102 reaches 150° C. Thereafter, the control circuit 24 sends a command signal to the heater control circuit 20 to control the temperature of the heater 14 so that the temperature detected by the thermistor 16 reaches, for example, 158° C. when the first sheet 60 passes the fixing device 500.
The control circuit 24 may receive a signal indicating the successive passage of sheets 60, for example, from an operation panel 28 of the image forming apparatus 1. The control circuit 24 reads the first control data indicated in
If the laminated sheet 62 (e.g. envelope) formed by laminating a plurality of sheets passes the fixing device 500 under the first pressing force, it may be wrinkled. Thus, through the operation described with reference to
The second control data may be prepared for the sheets 60 under the second pressing force condition. Under the control in accordance with the second control data set for the sheets 60, the control circuit 24 sends a command signal to the heater control circuit 20 so that the thermistor 16 detects a temperature of 144° C. before image formation is performed to the sheets 60, and the heater control circuit 20 controls the heater 14 in accordance with the command signal. When the first sheet 60 passes, the heater control circuit 20 controls the heater 14 based on the second control data, for example, so that the temperature detected by the thermistor 16 held in contact with the second circumferential surface 102 reaches 152° C. When the second pressing force lower than the first pressing force is exerted between the fixing roller 10 and the pressure roller 12, the control circuit 24 reduces the conveying speed of the sheet 60 and a fixing temperature for fixing a toner image to the sheet 60 as compared with a control based on the first control data.
The control circuit 24 receives a signal indicating the successive passage of sheets 60, for example, from the operation panel 28 of the image forming apparatus 1. The control circuit 24 reads the second control data indicated in
When the envelopes (sheets 62) pass the fixing device 500, the control circuit 24 first receives a signal indicating the successive passage of the envelopes (sheets 62) from the operation panel 28. The control circuit 24 or the heater control circuit 20 reads the control data stored in the storage device 29 and the heater control circuit 20 controls the temperature of the heater 14 based on the second control data prepared for the sheets 62. In the embodiment shown in
Although the conveying speed of the sheets 60 and that of the sheets 62 are equal under the second control data in the above embodiment, the control circuit 24 may control the driving mechanism 22 to differentiate these conveying speeds. Generally, the temperature control values are set lower as the conveying speeds of the sheets 60, 62 decrease, but the present invention is not limited thereto. When the conveying speed of the sheets 60 is set to be below the above conveying speed condition of 170 mm/s and higher than 120 mm/s under a control based on the second control data prepared for the sheets 60, the heat quantity deprived of the fixing roller 10 is reduced because the passage time of the sheets in the nip between the fixing roller 10 and the pressure roller 12 is shortened, whereas the heat quantity transferred to the sheets 60 is also reduced. Thus, depending on the type of the sheets 60, the control temperature for the first sheet 60 and the final control temperature in the second control data may be set higher than those in the first control data. At this time, since the heat quantity deprived of the fixing roller 10 is small, a difference between the control temperature set for the first sheet 60 and the final control temperature may be reduced and a temperature increment until the final temperature is reached and/or the number of the sheets 60 until the final temperature is reached may be set smaller.
Although the temperature increment until the final control temperature is reached is fixed in the above embodiment, the present invention is not limited thereto. For example, it is also possible to, in the first control data, set a difference between the control temperature set for the first sheet 60 and that set for the second sheet 60 to 10° C., set a difference between the control temperature set for the second sheet 60 and that set for the third sheet 60 to 6° C., set a difference between the control temperature set for the third sheet 60 and that set for the fourth sheet 60 to 3° C. and set a difference between the control temperature set for the fourth sheet 60 and that set for the fifth sheet 60 to 1° C. In this way, an increment of the set control temperature may be reduced as the number of the sheets 60 passing the fixing device 500 increases. The optimization of the control temperature increments results in preferable fixing of toner images.
The above embodiment mainly contains the following features.
A fixing device according to one aspect of the above embodiment fixes a toner image to a sheet. This fixing device comprises a fixing roller configured to be heated by a heat source; a pressure roller configured to be pressed into contact with the fixing roller; a switcher for switching a pressing force between the fixing roller and the pressure roller between a first pressing force and a second pressing force lower than the first pressing force; a pressing force detector for detecting the pressing force between the fixing roller and the pressure roller; a temperature detector for detecting the temperature of the fixing roller; a storage device for storing control data indicating a target temperature for the fixing roller predetermined in accordance with the number of sheets passing the fixing device; and a controller for controlling the temperature of the fixing roller to the target temperature by controlling the heat source in accordance with a signal sent from the pressing force detector.
According to the above feature, a toner image may be fixed, for example, under two pressing force conditions for most frequently used sheets. Further, one of the two pressing force conditions may be compatible, for example, for another type of sheets required to fix toner images with a lower pressing force. In this way, a highly convenient fixing device may be provided without requiring any operation to change the pressing force condition every time the sheet type is changed.
In the above feature, the controller preferably further controls at least one of the rotating speeds of the fixing roller and the pressure roller. In this feature, the controller controls the rotating speed to a first rotating speed when the pressing force detector detects the first pressing force while controlling the rotating speed to a second rotating speed lower than the first rotating speed when the pressing force detector detects the second pressing force. According to this feature, toner images may be suitably fixed, for example, to both of one layer sheet and to a sheet formed by laminating a plurality of sheets, wherefore neither a fixing failure nor an offset of a toner image occurs.
In the above feature, the control data may include first control data indicating the control temperature under the first pressing force and second control data indicating the control temperature under the second pressing force. In this feature, the first control data may indicate the control temperature under the first rotating speed condition and the second control data may indicate the control temperature under the second rotating speed condition. According to this feature, a suitable control temperature may be set according to a conveying speed of the sheet.
In the above feature, the fixing roller includes a first circumferential surface to be held in contact with the sheet and a second circumferential surface not to be held in contact with the sheet; and the temperature detector is so arranged as to be held in contact with the second circumferential surface. According to this feature, damage of the first circumferential surface to be held in contact with the sheet may be prevented, wherefore a proper fixing process may be performed.
In the above construction, the first control data includes a first maximum control temperature indicating the control temperature and constant in relation to an increase in the number of passed sheets, the second control data includes a second maximum control temperature indicating the control temperature and constant in relation to an increase in the number of passed sheets, and the second maximum control temperature is lower than the first maximum control temperature. In this feature, the control temperature of the first control data may increase stepwise until the first maximum control temperature is reached. In this feature, the control temperature of the second control data may increase stepwise until the second maximum control temperature is reached. In this feature, the control temperature of the second control data is below the control temperature of the first control data. In this feature, an increment of the control temperature of the first control data may be fixed. In this feature, an increment of the control temperature of the second control data may be fixed. In another specific embodiment, the increment of the control temperature of the first control data may decrease as the first maximum control temperature approaches. Further, the increment of the control temperature of the second control data may decrease as the second maximum control temperature approaches. In this way, suitable temperatures may be set under the respective fixing conditions.
In the above feature, the switcher may be manually operated. According to this feature, the structure of the fixing device may be simplified as compared with the automatic switcher.
An image forming apparatus according to another aspect of the above embodiment is for forming and fixing a toner image on and to a sheet, and comprises an image forming unit for forming a toner image on a sheet; and a fixing device for fixing the toner image formed by the image forming unit to the sheet. The fixing device includes a fixing roller configured to be heated by a heat source; a pressure roller configured to be pressed into contact with the fixing roller; a switcher for switching a pressing force between the fixing roller and the pressure roller between a first pressing force and a second pressing force lower than the first pressing force; a pressing force detector for detecting the pressing force between the fixing roller and the pressure roller; a temperature detector for detecting the temperature of the fixing roller; a storage device for storing control data indicating a control temperature for the fixing roller predetermined in accordance with the number of sheets passing the fixing device; and a controller for controlling the temperature of the fixing roller to the control temperature by controlling the heat source in accordance with a signal sent from the pressing force detector.
This application claims priority to Japanese Patent Application Serial Number 2008-246073, filed on Sep. 25, 2008 by at least one common inventor, and which is incorporated herein by reference in its entirety.
Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein.
Number | Date | Country | Kind |
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2008-246073 | Sep 2008 | JP | national |