Abstract:
A fixed frame includes a photosensitive drum and a developing unit, and a movable frame is openable with respect to the fixed frame. The developing unit, which has two pairs of protrusions thereon, is detachably mounted to the fixed frame by engaging the protrusions with two pairs of guides provided on the fixed frame. One pair of guides includes a portion for momentarily catching one pair of the protrusions as the developing unit is being mounted to the apparatus, and each of the other pair of guides includes a inclined portion having the shape of an arc centered on the momentarily catching portion, whereby an impact to the developing unit is alleviated. The developing unit has a shutter which moves to a position for covering a peripheral surface of a developing sleeve when the developing unit is being dismounted, and which retracts from the developing sleeve covering position and moves to a position for covering the photosensitive drum when the developing unit is being mounted. The movable frame accommodates a fixing device to move the fixing device rearwardly together with the movable frame when the movable frame is opened, whereby the danger of an operator touching the fixing device when the apparatus is opened is reduced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and in particular, an image forming apparatus utilizing an electrophotographic process that exposes and develops an image on a photosensitive drum and then transfers that image to a sheet. 
     2. Description of the Related Art 
     In recent years, various types of small-sized laser printers, which use the electrophotographic process have been offered. As indicated in U.S. Pat. No. 5,343,223, this type of laser printer is generally a clam shell type that is separated into two portions along a paper feed path of a printer main body to allow the upper portion of the frame to open with respect to the lower portion of the frame for inspections as well as removal of paper jams. 
     Some apparatuses of this type are constructed such that the developing unit is lowered down along guides provided on the frame on the main body side when the operator opens the upper frame to set the developing unit in the apparatus main body. For example, in U.S. Pat. No. 4,460,267, an apparatus is disclosed which mounts a developing unit along guides extending in the direction of the photosensitive drum from the upper portion of the apparatus. 
     In an apparatus of this type, if the operator inadvertently lowers down the developing unit, the developing unit will strike the photosensitive drum or its supporting member, which results in damage to the photosensitive drum and deviations in the supporting position. 
     Further, the above apparatus is provided with a shutter to shield the developing sleeve when the developing unit is removed to the outside. This shutter retracts from the shielding position when the developing unit is set in the main body and the developing sleeve is made to face the photosensitive drum. However, when the upper frame of the main body is opened with the developing unit set in the main body, measures to protect the photosensitive drum against light or objects from the outside are left unanswered. Further, when the developing unit is removed to the outside, measures to protect the photosensitive drum against light or objects from the outside are also left unanswered. 
     Thereupon, in a printer using the above electrophotographic process, a fixing assembly is arranged to fix toner to the terminating portion of the paper feed path. When the power supply is turned ON, this fixing assembly generates a considerable amount of heat. Therefore, if the fixing assembly remains in the paper feed path when the main body of the apparatus is opened to remove jammed paper, there is a possibility that it will interfere with the removal of the jammed paper or the operator may inadvertently touch the fixing assembly and burn himself. 
     SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide an image forming apparatus that can prevent a developing unit from being struck when mounted in the main body of the apparatus. 
     Another object of this invention is to provide an image forming apparatus that shields the developing sleeve when the developing unit is removed from the main body, and that opens the developing sleeve and at the same time protects the photosensitive drum when the developing unit is mounted to the main body. 
     Another object of this invention is to provide an image forming apparatus which is able to easily remove paper jams to and simply execute maintenance and inspections inside the apparatus. 
     Another object of this invention is to eliminate as much as possible the danger of the operator touching the fixing assembly when opening the main body of the apparatus. 
     These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings with illustrate specific embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following description, like parts are designated by like reference numbers throughout the several drawings. 
     FIG. 1 is a cross-sectional view showing the internal construction of the laser printer in one embodiment of this invention. 
     FIG. 2 is a cross-sectional view showing the main body of said laser printer in an open state. 
     FIG. 3 is a cross-sectional view showing the photosensitive unit. 
     FIG. 4 is a cross-sectional view showing the developing unit. 
     FIG. 5. is an elevational view showing the guide channel. 
     FIG. 6 is an explanatory view showing the adjacent state of the de loping unit with respect to the photosensitive drum. 
     FIG. 7 is an explanatory view showing the mounting operation for the developing unit. 
     FIG. 8 is an expanatory view showing the mounting operation for the developing unit. 
     FIG. 9 is an explanatory view showing the mounting operation for the developing unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, an image forming apparatus according to one preferred embodiment of the present invention will be described. 
     The embodiment described below applies this invention to a laser printer. 
     The laser printer has a photosensitive unit 10 (see FIG. 3) and a developing unit 20 (see FIG. 4) in the center portion, a laser scanning optical unit 30 arranged at the right side, a sheet feeder portion 40 arranged at the lower portion, and a fixing assembly 50 arranged at the upper portion in FIG. 1. 
     The main body of the printer is constructed as a clam shell type separated into two portions, one of which is a fixed frame 1 and the other is a movable frame 7, and the direction shown by arrow A is the front. Namely, the operator faces the printer in the direction of arrow A to set the print conditions and remove jams. As described below, the photosensitive unit 10 and developing unit 20 are installed on the fixed frame 1 to be freely mountable and demountable, and the laser scanning optical unit 30 is installed on the front of the fixed frame 1. The fixing assembly 50 comprises a heating roller 51 and a pressure roller 52 and is mounted on the movable frame 7, namely, towards the inside away from the operator standing in front of the printer. 
     The movable frame 7 can be rotated on a support axis 2, provided on the fixed frame 1, that acts as a fulcrum: and by raising and rotating it towards the rear, the main body of the printer can be opened upwardly (see FIG. 2). 
     The photosensitive unit 10 has a photosensitive drum 15 and a charging brush 16 housed inside a casing 11 and the photosensitive drum 15 can rotate in the direction of arrow a. A transfer charger 9 is mounted on the movable frame 7 opposite this photosensitive drum 15. 
     The developing unit 20 has a developing sleeve 25 and a developing agent stirring roller 26 housed inside a casing 21, and toner stored in a tank 27 is suitably supplied to the stirring roller 26. The toner is supplied to the outer peripheral surface of the developing sleeve 25 while the developing agent is being stirred with the toner by the stirring roller 26. 
     The laser scanning optical unit 30 is formed by storing a laser diode (not shown in figure), a polygon mirror 32, lenses 33 and 34, and a mirror 35 inside a housing 31 and, based on image information to be transferred from an external host apparatus (for example, a microcomputer), it modulates the light from the laser diode, deflects laser beam by means of the polygon mirror 32 to irradiate the surface of the photosensitive drum 15 through light path b from the mirror 35. 
     A sheet feeder portion 40 is arranged so as to insert sheets S onto a sheet feeding plate 41 from the front side. The sheets are fed one-by-one from the uppermost layer by the rotation of a sheet pick-up roller 42. A sheet is fed and then the sheet with the transferred image is immediately separated from the photosensitive drum 15 and transported upwardly where fixing of the toner is carried out by passing between rollers 51, 52 of the fixing assembly 50 and then the sheet is discharged from a pair of discharge rollers 55 to a discharge tray 7a. This discharge tray 7a is constructed integrally with the movable frame 7. 
     Further, each image forming process of exposure, developing and transfer in this laser printer is well known so a detailed description will be omitted. 
     Next, the mounting/demounting construction of the photosensitive unit 10 and developing unit 20, and each type of shutter member will be described as well as their operation. 
     The photosensitive unit 10 is mounted and demounted by moving the plate 13, fixed to both sides of the casing 11 by a pin 12, up and down in a guide channel 4 (see FIG. 7) formed on both side walls of the fixed frame 1 and has a handle 11a for mounting and demounting. The guide channel 4 extends in the vertical direction, and when the plate 13 reaches the bottom portion of the guide channel 4, the photosensitive unit 10 is set at a prescribed position. 
     The developing unit 20 is mounted and demounted by moving the guide pins 22 and 23, protruding on both sides of the casing 21, up and down inside the guide channels 5, 6 (see FIG. 7) formed on both side walls of the fixed frame 1 and has a handle 21a for mounting and demounting. 
     The first guide channel 5 comprises an inlet portion 5a, a first inclined portion 5b, and a second inclined portion 5c. The second guide channel 6 comprises an inlet portion 6a and a vertical portion 6b extending in the vertical direction at a position shifted slightly inwardly from this inlet portion 6a. When the developing unit 20 is mounted to the main body, at first, the guide pins 22, 23 are positioned at the inlet portions 5a, 6a of the first and second guide channels 5, 6 (see FIG. 7) and then the developing unit 20 is made to rotate in the downward direction at the inside of the casing 21 by the guide pin 23 acting as a fulcrum. During this time, the guide pin 22 is guided along the first inclined portion 5b moving in the downward direction. Then, for a moment the guide pin 22 becomes caught at the connecting part of the inclined portions 5b and 5c, immediately after which it enters the second inclined portion 5c by the weight of the developing unit 20. Along with this, the guide pin 23 moves from the inlet portion 6a to the vertical portion 6b (see FIG. 8). Thereafter, when the operator gradually loosens his grip on the developing unit 20, the guide pin 22 moves in the downward direction to the second inclined portion 5c along with the guide pin 23 moving in the downward direction to the vertical portion 6b and each pin 22 and 23 reaching the bottom portion of each guide channel 5 and 6, which sets the developing unit 20 at a prescribed position (see FIG. 9). 
     As shown in FIG. 5, in order to ensure the mounting operation described above, the gap L between the guide pins 22 and 23 is set larger than the gap L1 between the guide channels 5 and 6 as well as being set smaller than the gap L2 between the guide channels 5 and 6. Further, the inlet portion 6a of the guide channel 6 has a level drop to catch the guide pin 23 with respect to the vertical portion 6b. Consequently, when mounting the developing unit 20, the guide pin 23 is caught at the inlet portion 6a for a moment, thereby alleviating any impact during the mount without suddenly falling in the guide channels 5 and 6. 
     Furthermore, the first inclined portion 5b of the guide channel 5 curves in a semicircular arc shape centered on the portion with a level drop of the guide channel 6, and the second inclined portion 5c continuously extends the photosensitive drum 15 from the first inclined portion 5b. Therefore, as shown in FIG. 8, the developing unit 20 pivots in the counterclockwise direction and, as shown in FIG. 9, while pivoting in the clockwise direction its position is adjusted and it smoothly approaches the photosensitive drum 15. Lastly, the impact which would otherwise occur when the developing unit 20 is set at the bottom portion of the guide channels 5 and 6 is alleviated. 
     In particular, as shown in FIG. 6, in order to alleviate the impact to the photosensitive drum 15 when mounting the developing unit 20, it is preferable that angle θ at which line X showing the direction of movement of the developing sleeve 25 meets tangent line Y is an acute angle (0°&lt;θ&lt;90°). 
     Thereupon, the photosensitive unit 10 is equipped with a first shutter plate 18 and a second shutter plate 19 as one part of the casing 11 (see FIG. 3). The first shutter plate 18 is installed to be rotatable on a pin 18a, provided on the casing 11, and a torsion spring (not shown in figure) always applies force to the first shutter plate 18 in the counterclockwise direction. When the photosensitive unit 10 is removed from the main body and when the main body is opened, this first shutter plate 18 covers behind the photosensitive drum 15, namely the transfer region, by the spring force of the torsion spring. When the movable frame 7 is closed, a protrusion (not shown in figure) provided on the movable frame 7 presses the first shutter plate 18, moving the first shutter plate 18 in the clockwise direction with the pin 18a acting as a fulcrum and then retracts in the upward direction to open the transfer region (see FIG. 1). 
     The second shutter plate 19 is installed to be rotatable on a pin 19a provided on the casing 11 and a torsion spring (not shown in figure) always applies force to the second shutter plate 19 in the counterclockwise direction. When the photosensitive unit 20 is removed from the main body, this second shutter plate 19 covers the developing region of the photosensitive drum 15 by the spring force of the torsion spring (see FIGS. 3 and 7). Therefore, as shown in FIG. 7, the second shutter plate 19 covers and protects the developing region of the photosensitive drum 15 at the initial stage of the mounting of the developing unit 20. When the developing unit 20 is mounted on the fixed frame 1 along the guide channels 5 and 6, a corner portion 21b of the casing 21 makes contact with and presses against the second shutter plate 19 in the downward direction and the second shutter plate 19 rotates in the clockwise direction with the pin 19a acting as a fulcrum to retract the second shutter plate 19 in the downward direction and open the developing region (see FIGS. 1 and 9). 
     Conversely, a shutter plate 24 is installed on the developing unit 20 as one part of the casing 21 (see FIGS. 4 and 7). This shutter plate 24 has a side plate 24a on both sides and is supported to be rotatable on said guide pin 23 on the edge of the side plate 24a. When the developing unit 20 is separated from the main body (see FIGS. 4 and 7), the shutter plate 24 rotates in the counterclockwise direction by its own weight until the tip makes contact with the corner portion 21c of the casing 21 to cover the developing sleeve 25. When the developing unit 20 is mounted to the fixed frame 1, the shutter plate 24 opens the developing sleeve 25 by its tip making contact with the handle 11a of the photosensitive unit 10 to rise in the upward direction (see FIG. 9). 
     In this embodiment, as stated above, the shutter plate 24 covers the external periphery of the developing sleeve 25 when the developing unit 20 is separated from the fixed frame 1, thus preventing toner scattering and the operator&#39;s hands from becoming soiled. On the other hand, when the developing unit 20 is mounted to the fixed frame 1, the shutter plate 24 retracts upwardly facing the developing sleeve 25 towards the photosensitive drum 15 along with covering the upper portion of the photosensitive drum 15 (see FIG. 1). At this time, the exposure light path b irradiated from the laser scanning optical unit 30 passes between the opposing side plates 24a and below the lower portion of the shutter plate 24 so as to strike the photosensitive drum 15, as shown in FIG. 1. Further, as shown in FIG. 2, when paper jam S&#39; is removed or internal maintenance and inspection is carried out, the movable frame 7 is rotated backwardly to open the main body and, at this time, the photosensitive drum 15 is protected by the shutter plate 24. Thus, this prevents stress on the photosensitive drum due to light from outside or damage due to contact with another body. 
     Moreover, in this embodiment, the photosensitive unit 10 is equipped with the first shutter plate 18 and the second shutter plate 19. When the main body is opened, the first shutter plate 18 protects the transfer region of the photosensitive drum 15 and when the developing unit 20 is separated, the second shutter plate 19 protects the developing region of the photosensitive drum 15. 
     Thereupon, in this embodiment, when a jam of sheets S occurs or the photosensitive unit 10 and/or developing unit 20 must be replaced or an internal inspection becomes necessary, the movable frame 7 is rotated backwardly on the support axis 2 acting as a fulcrum to open the main body (see FIG. 2). At this time, the fixing assembly 50 moves integrally with the movable frame 7 retracting backwardly. Therefore, the upper portion of the fixed frame 1 opens to a large space, which makes it easy to remove paper jam S&#39;. Of course, replacing internal parts, as well as maintenance and inspections are easy also. Furthermore, the fixing assembly 50, which had been operating until then, is heated to a considerably high temperature when removing a paper jam. However, because the fixing assembly 50 retracts backwardly where is separated from the operator who is positioned in front of the apparatus, the danger of the operator touching the fixing assembly 50 is reduced. 
     Moreover, in this embodiment, the transfer charger 9 is mounted on the movable frame 7. Consequently, as shown in FIG. 2, the transfer charger 9 also retracts backwardly when opening the main body so that the paper path is wide opened. This makes it even easier to remove paper jams. 
     Also, when the developing unit 20 is mounted, the guide pin 23 is held for a moment at a level drop of the inlet portion 6a, thereby alleviating any impact during the mount without suddenly dropping the guide pins 22 and 23 in the guide channels 5 and 6. 
     Moreover, the image forming apparatus related to this invention is not limited to said embodiment and can be modified in various ways within the range of the claims. 
     In particular, the clam shell mechanism of the main body or the mounting and demounting mechanism of the photosensitive unit and developing unit can be freely modified. Further, the photosensitive drum can be fixed to the main body as well. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted 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, they should be construed as being included therein.