Sheet feed device
This application is a continuation of application Ser. No. 07/309,960 filed Feb. 9, 1989, now abandoned which was a continuation of application Ser. No. 160,191 filed Feb. 25, 1988 also abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to sheet feed devices for use mainly in copiers, laser beam printers, facsimile devices, dot printers and ink-jet printers and other image forming apparatuses for producing images on sheets. 
     2. Description of the Related Art 
     Conventionally, the sheet feed device for the imaging forming apparatus described above has been made to operate with selection of interchangeable sheet cassettes of different size or paper quality according to necessity. As examples of the cassette mention may be made of one having a separation pawl as disclosed in U.S. Pat. No. 4,017,181, or another one without it as shown in Japanese Laid-Open Utility Model Application No. Sho 61-200838. 
     The prior known sheet feed device of the cassette without a type using the cassette without a separation pawl-less cassette will be described below. 
     In the past, an example of the sheet feed device for the copier or like image forming apparatus is shown generally at 1&#39;, having a feed roller 3 in cooperation with the releasably attached cassette 5&#39;, as shown in FIG. 6(a). Sheets S . . . stacked in the cassette 5&#39; are fed one at a time in separation from the others by the feed roller 3 and a separation pad 6 positioned in confronting relation to the feed roller 3. Also, the cassette 5 is provided with a middle plate 7 bearing many sheets S . . . at once. The middle plate 7 is raised upward by a lifter 9 so that the top one of the sheets S . . . is pressed on the feed roller 3. Thus, the sheets S . . . are smoothly sent out in sequence. 
     By the way, because the remaining sheets S . . . in the cassette 5&#39; continue being raised by the lifter 9, a certain number of sheets S 1 , which are counted from the top, stand out from the upper edge of the front side wall 5&#39;a of the cassette 5&#39;. Therefore, when the cassette 5&#39; is taken out of a main body 2 of the sheet feed device 1&#39;, the upper sheets S 1  are left behind in the main body 2 by the action of inertia or the like, and they later slip down from the cassette 5&#39; (see FIGS. 6(b) and 6(c)). 
     With another type of cassette having the separation pawl, because the leading side edge of the sheet S is retained by the separation pawl, such a problem scarcely arises. But, if, as successive two or more of the sheets are partly overlapped with each other in going out, there remain a number of sheets overrunning the separation pawl, these remaining sheets will occasionally slip down from the cassette when it is taken out. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a sheet feed device which enables exchanging of sheets to be carried out smoothly. 
     Another object is to provide a sheet feed device capable of preventing sheets from slipping down from sheet containing means when the sheet containing means is detached from the sheet feed device. 
     To achieve such objects, according to the invention, the sheet feed device is provided with movement limiting means for limiting the relative movement between the sheets and the sheet containing means only when the sheet containing means is moved from the sheet feeding position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an embodiment of a sheet feed device according to the invention applied to a laser beam printer. 
     FIGS. 2(a)-2(c) are fragmentary side sectional views of the sheet feed device of the invention with the cassette in three positions where the cassette is loaded on the sheet feed device, about to be detached, and fully detached from the sheet feed device, respectively. 
     FIG. 3 is a side sectional view of the front part of the cassette. 
     FIG. 4 is a perspective view of that front part. 
     FIG. 5 is a side sectional view of the cassette with a shutter in the pushed position. 
     FIGS. 6(a)-6(c) are side sectional views of the prior known sheet feed device with FIG. 6(a) illustrating the sheet feed device when the cassette is loaded, FIG. 6(b) illustrating the cassette just after detachment from the sheet feed device, and FIG. 6(c) illustrating the cassette as fully detached from the sheet feed device. 
     FIG. 7 is a side sectional view of an example of variation of the cassette shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a laser beam printer as an example of the image forming apparatus to which the sheet feed device of the invention is applied. 
     A photosensitive drum 31 serving as an image bearing member rotates about a shaft 31a. In the course of rotation, the drum 31 is uniformly charged to the positive or negative polarity in its peripheral surface by a charger 32. Then, in an exposure portion 33, the drum 31 is scanned by a laser beam L issued from a laser scanner 34 and reflected by a mirror 34a, so that an electrostatic latent image of the subject image is formed on its peripheral surface. The electrostatic latent image formed on the surface of the photosensitive drum 31 is developed with a toner by a developer 35 and comes to a confronting position to a transfer discharger 36. 
     Meanwhile, the sheet S is sent out from the sheet feed device 1, and, after its leading side edge has once been latched by a registration roller 37, starts to enter between the photosensitive drum 31 and the transfer discharger 36 at a predetermined timing synchronized with the toner image formed on the photosensitive drum 31. The toner image is then being transferred serially to the surface of the sheet S. 
     The toner image-transferred sheets S are separated serially from the surface of the drum 31 by a separation member (not shown), going along a guide 38 to a fixing device 39, where the image is fixed. Then, each sheet S is expelled as an image-formed member U by a delivery roller 40 to a copy tray portion 41. 
     The surface of each drum 31 from which the sheet S has been separated is cleansed by a cleaner 42 to remove the residual toner, and is pre-exposed by a preconditioning lamp 45 to make ready for the next exposure. 
     Next, an embodiment of the sheet feed device according to the invention is described in detail by reference to FIGS. 1 and 2(a)-2(c). 
     The sheet feed device 1 has the cassette 5 releasably attached thereto in the opposite position to the feed roller 3. The cassette 5 is provided with a sheet stopper 10 constituting a first sandwiching member, and a shutter 11 or extruding member constituting a second sandwiching member. An edge 11a of the front portion of the shutter 11 is arranged to face the sheet stopper 10 which is fixed to the cassette 5. The shutter 11 as shown in greater detail in FIG. 3 which, in the side sectional view, illustrates the front part of the cassette 5 and in FIG. 4 which is a perspective view of that front part, has a front portion 11b confronting the front wall 5a of the cassette 5, and also has a body 11c pivotally supported by a fulcrum pin 12 on the side wall 5b of the cassette 5. A leaf spring 13 is fixed at its one end to the lower portion of the shutter body 11c, urging the shutter 11 so as to resiliently move upward. Further, the shutter 11 is provided with a shutter movement cam 15. When the cassette 5 is loaded in the sheet feed device main body 2, the cam 15 is pushed by a head camming surface 16a of a position determining arm 16 provided in the device main body 2, so that the shutter 11 is pushed down against a spring 13 until the head camming surface 16a of the position determining arm 16 falls in a groove 5c of the cassette 5, as shown in FIG. 5. On the other hand, when the cassette 5 is taken out, the shutter 11 is released from the depression of the position determining arm 16 and is resiliently moved upward by the spring 13 until the lower end of a slot 17 of the shutter 11 abuts on a stopper pin 19, so that the upper edge 11a of the front portion of the shutter 11 protrudes toward the sheet stopper 10, as shown in FIG. 3. 
     Since this embodiment has the features described above, when the cassette 5 is loaded in the sheet feed device main body 2, the sheets S contained in the cassette 5 are raised by the lifter 9 and the upper sheets S 1  are pressed against the feed roller 3. By the feed roller 3 and the separation pad 6, the sheets S 1  are sent out one by one in separation. During this time, the upper sheets S 1  contained in the cassette 5 take their positions above the front wall 5a of the cassette 5, and the shutter 11 is pushed by the camming surface of the position determining arm 16 so that the front upper edge 11a of the shutter 11 lies down so as not to hinder sending-out of the sheet (see FIG. 2(a)). 
     Meanwhile, when the cassette 5 is taken out of the sheet feed device main body 2, the shutter 11 is released from the depression of the position determining arm 16 and protrudes the front upper edge 11a resiliently toward the sheet stopper 10, so that the upper sheets S 1  which have so far partly slipped off the cassette 5 by inertia or the like are caught by being sandwiched in between the front upper edge 11a and the sheet stopper 10. Therefore, even when the cassette 5 is pulled out or taken out from the sheet feed device main body 2, the sheets S contained in the cassette 5 never fly out from the cassette 5 to be scattered (see FIG. 2(c)). 
     It should be noted though that, in the above-described embodiment, the front upper edge 11a of the shutter 11 has been made to move toward the sheet stopper 10 which has been positioned above the shutter 11 to sandwich the sheets S 1  therebetween, the invention is not confined thereto. The shutter 11 may be made to move from above to below. Also, two movable shutters may be used as arranged on either vertical side of the mouth of the cassette to protrude in such a way as to approach each other. 
     Also, though the shutter 11 has been made to be controlled by the position determining arm 16, the invention is not confined thereto. It may be controlled by any other member than the position determining arm. 
     Further, though the protruded sheets S 1  . . . have been sandwiched at a portion of the front wall 5a of the cassette 5, the invention is not confined thereto. The sandwiching may be done at, for example, the center of the cassette 5. 
     Also, a pressor member may be made to move from above to below to press the upper surface of the sheet. In this case, the sheets are sandwiched between the pressor member and the middle plate 7. 
     Further, as shown in FIG. 7, the sheet stopper 10 may be omitted and the shutter 11 may be made to protrude to a position opposite to the edges of the sheets so that the sheets are limited from moving. 
     In addition, in the shutter 11 shown in FIG. 3, the lower end of the slot 17 abuts on the stopper pin 19, so that there is a little space between the front upper edge 11a and the sheet stopper 10. However, the front upper edge 11a may abut on the sheet stopper 10 to stop the shutter 11 moving upward. Essentially, the sheet stopper 10 and the shutter front upper edge 11a may take up their positions to sandwich the sheets S.