Abstract:
An image forming apparatus includes several image forming units which are vertically arranged, paper connecting units which connect the paper discharging section of one image forming unit and the paper feeding section of another image forming unit where the two units are vertically adjacent, and a paper separating mechanism to switch the employed paper conveying path from one path to another. Such a structure requires floor space only as much as that required for one image forming unit, and yet provides various functions, e.g. double-sided printing, mulitple copying on one side of a paper with different originials, high-speed printing, multi-colored printing, printed side up discharging, printed side down discharging, job stocking, and sorting. The invention results in a compact and inexpensive image forming apparatus with various high quality fuctions.

Description:
This application is a continuation of now abandoned application, Ser. No. 07/174/8865 filed on Mar. 29, 1988. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to image forming apparatuses installed in electrophotographic printers, including electrophotographic copying machines. 
     Recently, there have been two rather conflicting requests from the users of electrophotographic printers whose light sources are either lasers or light-emitting diodes (LED). One is for high quality functions such as double-sided printing, composite copying on one side of a paper, high speed printing, multi-colored printing, and sorting. Another is for compactness, less weight, and inexpensiveness. These are usually considered conflicting since compactness leads to slower printing speed and to fewer functions. In fact, no option unit to upgrade compact electrophotographic printers by adding functions such as double-sided printing and/or composite copying on one side of a paper with different originals is provided. 
     It has been necessary, therefore, for users to purchase high quality electrophotographic printers besides the compact ones if high quality printing is required. There is indeed a device which comprises two photocopying machines juxtaposed in the paper feeding direction so that papers may flow through the copying machines continuously for double-sided copying or composite copying on one side of a paper with different originals (such as two-colored printing) as disclosed in Japanese Patent Laid Open No. 59-10958 and No. 60-195576, respectively. Each function, however, is limited to double-sided copying or to composite copying on one side of a paper respectively, and also, because the device is no more than two conventional photocopying machine connected, it requires floor space as large as that used for two copying machines. 
     The present invention, however, provides compact image forming apparatuses that are inexpensive and have several sophisticated functions. 
     SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide an image forming apparatus which consists of compact printers, with various functions such as double-sided copying and composite co on one side of a paper with different originals, and yet that are inexpensive. 
     The second object of the invention is to provide an image forming apparatus with the above-mentioned various functions that requires little floor space. 
     In order to accomplish the two objects, an image forming apparatus that incorporates the principles of this invention comprises: plural image forming units which are vertically arranged; connecting units which connect the paper discharging section of one image forming unit and the paper feeding section of another image forming unit, where the two units are vertically adjacent; a mechanism for conveying papers from the paper feeding section of one image forming unit to that of another, which consists of several selective path provided in image forming units and in connecting units; and a mechanism for selecting a single paper conveying path from several possible conveying paths. 
     The third object of the invention is to provide an image forming apparatus with various functions, which consists of compact printers, requires little floor space, and discharges papers through the upper part of the apparatus. 
     In order to accomplish the above object, an image forming apparatus that incorporates the principles of this invention comprises a paper discharging unit which is attached to the paper discharging section of a top image forming unit, in addition to the above components described as requirements to fulfill the first and the second objects. 
     Further features and advantages of the invention will be better understood by reference to the following description, and to the drawings forming a part thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the first exemplary image forming apparatus that incorporates a principles of this invention. 
     Similarly, FIGS. 2, 3, and 4 are crosssectional views of second, third, and fourth exemplary image forming apparatuses, respectively, that incorporate the principles of this invention. The subscripts `a` and `b` in the figures show that the parts belong to the lower or to the upper image forming unit, respectively. A short-cut path 31 in FIG. 2 traverses two vertically adjacent image forming units, and thus, neither `a` or `b` accompanies the number that designates the path. 
    
    
     The invention will now be described by reference to the four preferred embodiments of the present invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The first embodiment which incorporates the principles of the invention is described below referring to FIG. 1. 
     The embodiment comprises two image forming units 1a and 1b, each of which is actually a compact printer, a connecting unit 21a, and a paper discharging unit 21b. They are stacked from bottom to top as follows: image forming unit 1a, connecting unit 21a, image forming unit 1b, and paper discharging unit 21b. 
     In the lower unit, a laser beam, which is emitted from a semiconductor laser 2a and controlled by an image controller not shown in the figure, i scanned with a polygon mirror 3a to reach a lens 4a and then a reflecting mirror 5a so that an image is formed on a photoconductive drum 10a. The drum 10a is electrified by a charger 11a to form an electrostatic latent image by the laser beams, which is developed by a developing device 12a, and finally, a visible image is transferred onto a paper by a transferring charger 9a and residual toner is removed by a cleaner 13a. Papers are fed from a paper feeding cassette 6a by a paper feeding roller 7a to a transferring section and synchronized with the image formed on the photoconductive drum 10a by a timing roller 8a. The paper, which has the image transferred thereto at the transferring section, has the printing fixed by a fixing roller 14a and is discharged from the image forming unit 1a by a paper discharging roller 15a. 
     In the connecting unit 21a, the paper discharged from the paper discharging roller 15a is sent to a first paper separating claw 23a, via a vertical path 22a, and is further guided to either a paper reversing path 24a or a conveying path 25a. In case the paper is guided to the reversing path 24a, it is reversed and sent to the conveying path 25a. After going through the conveying path 25a, the paper is sent to a second paper separating claw 26a and further to either of two paths, which are described below, via a first discharging roller 27a. One of the paths discharges the paper to the paper discharging tray 28a, which forms an intermediate discharging section. Another sends the paper to the paper circulating roller 16b in the upper image forming unit 1b. 
     Similar to the lower image forming unit la, the upper image forming unit 1b consists of a semiconductor laser 2b, an image controller not shown in the figure, a polygon mirror 3b, a lens 4b, a reflecting mirror 5b, a paper feeding cassette 6b, a paper feeding roller 7b, a timing roller 8b, a copying charger 9b, a photoconductive drum 10b, a charger 11b, a developing device 12b, a cleaner 13b, a fixing unit 14b, and a paper discharging roller 15b, and forms an image in an exactly the same way as the lower image forming unit. The upper image forming unit 1b, unlike the lower 1a, has a paper circulating roller 16b, whose purpose is to guide the paper discharged from the connecting unit 21a to the timing roller 8b. However, the lower image forming unit 1a may be equipped with a paper circulating roller to make the two units interchangeable. 
     The paper discharging unit 21b consists of a vertical path 22b, a first separate claw 23b, a conveying path 25b, and a first discharging roller 27b, as in the connecting unit 21a; the shapes of the casings, the structures of the paper discharge areas and the connecting units are almost identical. It should be noted, however, that the connecting unit further comprises a second paper separating claw 26a and a second discharging roller 29a, the discharging unit comprises only one paper separating claw 23b and one discharging roller 27b. It is not that the paper discharging unit should be devoid of a second paper separating claw and a second discharging roller. Both of them may be equipped with these features, so that the paper discharging unit would be identical to, and thus interchangeable with, the connecting unit. 
     Using such an apparatus, the paper discharged from the lower image forming unit 1a has two options. It can be guided to the conveying path 25a via the first paper separating claw 23a in the lower image guiding unit 1a. In this case, the printed side of the paper is faced down. Or, it can be guided to the reversing path 24a to be reversed so that the printed side faces up, and then to the conveying path 25a. In both cases, the paper may either be sent to image forming unit 1b by the second paper separating claw 26a or be discharged to the paper discharging tray 28a. The papers discharged on the paper discharging tray 28a have their printed side facing up if they came through the reversing path 24a, and printed side facing down if they did not come through the path 
     Now the papers requiring high quality printing, such as double-sided printing, are sent to the upper image forming unit 1b. Those that went through the reversing path 24a have their printed side down when they are circulated to the upper unit 1b in order to have both sides printed. The others that did not go through the reversing path 24a will be printed twice on one side: once in the lower image forming unit 1a and again the upper 1b; but the data to be printed in the upper apparatus may be different from that printed in the lower apparatus. Also, two-colored printing can be accomplished by applying toner of a different color to each image forming unit. Similarly, three vertically arranged image forming units will yield three-colored materials. 
     As described above, the flow of papers through vertically arranged image forming units make various functions possible: discharging papers printed side up or down, whichever desired; double-sided printing; printing two different data on one side as a single operation; and multi-colored copying. 
     There is another attractive feature of the invention; when the two image forming units 1a and 1b are operated simultaneously but independently, they may serve as two printers engaged in completely different jobs. That is, high-speed printing and job stocking and sorting become possible by using two units which have their own discharging trays 28a, 28b. The assembly of plural independent units also allows one of the units to be out of order while others perform their tasks; the reliability of the copying machine as a whole is increased. 
     Finally, the vertical arrangement of image forming units requires floor space only as large as that required for one unit. Such an arrangement may also turn out to be useful, as the users can collect all the printed copies without moving around. 
     The principles of this invention can be applied as shown in FIG. 2. The second embodiment is identical to the first, expect that the second embodiment has four paper separating claws 23a, 23b, 26a, 30a, while the first has only three 23a, 23b, and 2a, it has a short-cut path 31 while the first embodiment does not. 
     The connecting unit 21a is now equipped with a third paper separating claw 30a in such a place that the papers may encounter the claw 30a after the first paper separating claw 23a. The connecting unit 21a and the lower end of the vertical path 22a of the paper discharging unit 21b are connected with the short-cut path 31. The papers that were printed in the lower image forming unit 1a are sent to the connecting unit 21a and eventually either to upper paper discharging tray 28b via the short-cut path 31 and a paper discharging unit 21b or to lower paper discharging tray 28a. Since the papers are discharged to the upper discharging tray 28b without going through the image forming unit 1b due to the shortcut path 31, desirable sorting can be accomplished speedily. 
     The third embodiment illustrated in FIG. 3 has an identical structure to the first embodiment except that the embodiment has several paper separating claws 32b along the conveying path 25b of the upper image forming unit 1b and an accompanying discharged paper tray 33b for each claw (the trays 33b are named sorting section 34b as a whole). Therefore, a sorting function becomes available and the printed materials are sorted into as many groups as the number of pairs of separating claws and trays. 
     Although the above three embodiments were provided with vertical paths, conveying paths, and first and second paper separating claws as parts of paper guiding units, not all of them are required to obtain the merits of the invention. For example, in a fourth embodiment shown in FIG. 4, the connecting unit 21a has to be equipped only with a second paper separating claw 26a, a first discharging roller 27a, a second discharging roller 29a, and a paper discharging tray 28a, and the discharging unit only with a discharging roller 27b and a discharging tray 28b, if each of the image forming units 1a and 1b has a vertical path 22a, 22b, a reversing path 24a, 24b, a conveying path 25a, 25b, and a first paper separating claw 23a, 23b. In other words, the connecting and the paper discharging units are reduced to exclude paper paths, and the excluded paths are included in the image forming units. However, if there is no need to reverse the paper with a reversing path, the image forming units 1a and 1b should be equipped with vertical paths 22a, 22b and conveying paths 25a, 25b but not necessarily with first paper separating claws and reversing paths. 
     In addition, an image forming unit can be made into one with either a connecting or a paper discharging unit, and, more than two image forming units may be further vertically arranged and connected. 
     While the present invention has been described in connection with certain specific embodiments, it is to be understood that it is not to be limited to those embodiments. On the contrary, it is intended to cover all alternatives and modifications falling within the spirit and scope of the invention as set forth in the appended claims.