1. Field of the Invention
This invention relates to an image forming apparatus which has the function of detecting a jam of a recording material.
2. Description of the Related Art
FIG. 7 illustrates a sheet conveying path from a sheet-feeding unit to a sheet-discharging unit in a conventional image forming apparatus. In FIG. 7, there are shown sheet-feeding cassette 1 of a sheet-feeding unit, and transfer paper P, serving as paper accommodated within the sheet-feeding cassette 1. Sheet-feeding roller 2 is rotatably driven in response to a print start signal, whereby sheets of transfer paper P are individually taken out from within sheet-feeding cassette 1, and each sheet is fed to a transfer process portion of image forming unit 10 passing along a sheet path 6.
In this case, image forming unit 10 has an electrophotographic process mechanism. A drum-shaped electrophotographic photosensitive member (hereinafter termed a "photosensitive drum") 7 is rotatably driven in the clockwise direction indicated by the arrow at a predetermined circumferential speed (process speed).
Primary charging by charging unit (charging roller) 11, image exposure L by exposure means (not shown), such as a laser scanner or the like, and development by developing unit 12 are performed for rotating photosensitive drum 7, and a toner image corresponding to object image information is formed on the circumference of rotating photosensitive drum 7.
The toner image is sequentially transferred onto a sheet of transfer paper P fed from sheet-feeding cassette 1 at transfer nip portion 9 provided between photosensitive drum 7 and transfer unit (tranfer roller) 8.
The sheet of tranfer paper P passing through transfer nip portion 9 is then separated from the surface of rotating photosensitive drum 7, and is guided to fixing unit 17 passing along a sheet path 16. The surface of rotating photosensitive drum 7, after separation of the sheet, is cleaned by cleaning unit 13 to be repeatedly used for image formation.
The fixing unit 17 of the present case comprises a tensionless film-heating-type fixing unit disclosed, for example, in Japanese Patent Laid-open Application (Kokai) Nos. 4-44075-44083 (1992). FIG. 8 is an enlarged view of fixing unit 17.
The fixing unit 17 includes a heat-resistant film guide member 18 having a substantially semicircular cross section, a plate-like ceramic heater 19 having a low heat capacity disposed along the longitudinal direction at a central portion of the lower surface of film guide member 18, a cylindrical (endless) thin film (fixing film) 20 made of heat-resistant resin loosely fit around film guide member 18 having the ceramic heater, and a pressing roller 21 disposed below film guide member 18 so as to be in pressure contact with ceramic heater 19 across film 20 by always being pressed upwardly by upwardly-pressing spring 22. That is, ceramic heater 19 and pressing roller 21 are in pressure contact across film 20, to form a fixing nip portion N.
Ceramic heater 19 is heated by a current supplied from a current supply system (not shown), and is controlled at a predetermined fixing temperature by a temperature control system (not shown).
Pressing roller 21 is rotatably driven by a driving unit (not shown) in the counterclockwise direction indicated by the arrow at a predetermined circumferential speed. The cylindrical film 20 is rotatably driven around film guide member 18 by the rolling frictional force of pressing roller 21 in the clockwise direction indicated by the arrow at a predetermined circumferential speed while sliding along the surface of ceramic heater 19 in a state of firmly contacting the lower surface thereof at the fixing nip portion N.
The sheet of tranfer paper P conveyed to fixing unit 17 after image transfer is guided at entrance guide 23 to enter a space between the rotatably driven cylindrical film 20 and pressing roller 21 at the fixing nip portion N provided by the temperature-controlled ceramic heater 19 and the rotating pressing roller 21, and passes through the fixing nip portion N together with film 20 in a state of firmly contacting the lower surface of ceramic heater 19 across film 20.
While the sheet of transfer paper P passes through the fixing nip portion N, the unfixed toner image on the sheet is heated across film 20 by the heat of ceramic heater 19. Thus, the image is fixed.
The sheet of transfer paper P passing through the fixing nip portion N is separated from the surface of film 20, and is output outside the apparatus after passing through exit guide 24, a sheet path 29 and a pair of sheet-discharging rollers 30.
Referring again to FIG. 7, registration sensor S1 for detecting the leading edge of the sheet is disposed at a side upstream from transfer unit 8 in the conveying direction of the sheet. Discharged-sheet sensor $2 four detecting the discharge of the sheet is disposed at a side downstream from fixing unit 17 in the conveying direction of the sheet.
As shown in FIG. 8, the discharged-sheet sensor S2 of the present case includes a sensor lever 26 swingable around supporting shaft 25, and a photo-interrupter 27. Usually, sensor lever 26 has a substantially vertical posture by the function of gravity as indicated by the solid lines, with the upper-end portion thereof protruding in the sheet path and a lower-end shutter portion 26a thereof blocking the optical path of photo-interrupter 27.
When the sheet is fed from the fixing nip portion N to exit guide 24, sensor lever 26 is pushed by the sheet to be rotated around supporting shaft 25 in the counterclockwise direction as indicated by the two-dot chain lines, whereby shutter portion 26a rotates to open the optical path of photo-interrupter 27.
Thus, the presence/absence of the sheet is detected by the opening/blocking of the optical path of photointerrupter 27. The registration sensor S1 has the same configuration as the discharged-sheet sensor S2.
Signals from registration sensor S1 and discharged-sheet sensor S2 are input to a CPU (not shown) for controlling the main body of the image forming apparatus. Based on these input signals, control of the main body of the apparatus, such as timing control of exposure by a laser when exposure means for the rotating photosensitive drum 7 comprises, for example, a laser scanner, timing control of sheet feeding, control of the fixing sequence, jam detection and the like, is performed.
A heat-roller-type fixing unit comprising a fixing roller, in which a halogen-lamp heater is provided within a roller having the shape of a hollow pipe, and a pressing roller in pressure contact with the fixing roller, a pressure fixing unit comprising a pair of pressing rollers, or the like may be used as the fixing unit 17. However, the film-heating-type fixing unit used in the present case is advantageous over other fixing units in that, for example, waiting time can be shortened (quick-start property) or electrical power consumption can be reduced, since a low-heat-capacity heating member, in which temperature rises rapidly, and a thin film can be used.
In the above-described image forming apparatus, if a paper jam occurs, the CPU for controlling the main body of the apparatus detects the jam from the relationship between the sheet-feeding timing and the signals from the sensors S1 and S2, stops the driving of the main body of the apparatus, and displays the occurrence of the jam.
The operator then opens an openable/closable cover of the main body of the apparatus, removes the jammed paper within the apparatus, and closes again the cover, whereby a door switch linked with the cover is closed. The CPU resets the jammed state by a signal from the door switch. By this reset, the main body of the apparatus executes pre-rotation driving as a mode of forcedly discharging paper, and returns to a state in which image formation can be executed.
In the above-described image forming apparatus, when the openable/closable cover of the main body of the apparatus is first opened after the driving of the apparatus has stopped based on the detection of a jam, and the cover is closed again carelessly or due to some other reason without removing the jammed paper, or when the jammed paper is insufficiently removed, and the cover is closed again without recognizing the presence of a remaining part of the jammed paper, the jammed state is reset based on a signal from the closed door switch, and the pre-rotation driving of the apparatus is executed, whereby the jammed state of the paper still remaining in the apparatus becomes worse, causing, for example, damage to the apparatus. Such problems also arise in the case of reset of the apparatus by the turning-on and turning-off of the power supply of the apparatus.
This is because when, for example, the paper produces an accordion jam, or a jam P' is produced by the paper becoming wound around the fixing member (comprising rotating film 20, pressing roller 21, the heat roller, the pressing roller and the like) of fixing unit 17, as indicated by the two-dot chain lines in FIG. 8, the jammed paper P' is then present between registration sensor S1 for detecting the leading edge of the paper and discharged-sheet sensor S2 for detecting the completion of sheet discharge. The CPU, therefore, cannot detect the presence of the jammed paper P' within the main body of the apparatus when the door switch is closed by closing the cover again, or when the power supply is turned on or off. Thus, the CPU resets the jammed state, and the apparatus moves to the pre-rotation driving in spite of the presence of the jammed paper P'.
Due to the pre-rotation driving, an accordion-jammed paper remaining in the main body of the apparatus may be more severely jammed, or a jammed wound paper P' may be further wound in the fixing member, making jam removing processing by a user difficult or impossible. When the fixing unit 17 is a film-heating-type unit as in the present case, the thin fixing film 20 may be broken.