Abstract:
Cut paper electrophotographic printers with continuous paper feed mechanisms including a pickup roll, a photosensitive drum, a charging unit, an optical unit, a host controller, a developing unit, a transfer unit and fixing rolls. Further included are first and second branch passages for providing alternate feeding paths to continuously feed cut paper sheets to the photosensitive drum.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrophotographic printers and, more particularly, to so-called &#34;cut paper electrophotographic printers&#34; for printing sheets of cut paper with an electrophotographic process, each printer having an improved paper feed mechanism. 
     2. Background of the Invention 
     Before entering into the detailed description of the present invention, an electrophotographic printer according to the prior art will be described with reference to the schematic diagram of FIG. 1. 
     A photosensitive drum 1, a pair of feed rolls 2 and a pair of fixing rolls 3 rotate at a constant speed at all times. For printing operations, the photosensitive drum 1 is uniformly charged by a charging unit 6. Printing signals are transmitted from a host control circuit 11 to an optical unit 7 so that a laser beam 8 carrying the printing signal exposes the surface of the photosensitive drum 1. The portions exposed by the laser beam 8 are developed with toner by a developing unit 9. This toner is transferred to a sheet of paper 10 by a transfer unit 12 and is fixed on the paper 10 while passing through the fixing rolls 3. 
     The relation between the paper 10 and the laser beam 8 will be described in detail in the following in connection with the series process described above. A pair of timing rolls 5 are provided for arranging the top margin of the paper 10, i.e., the distance from the top edge to the first line. The paper 10, picked up by a pickup roll 4, is moved forward by the feed rolls 2 to come into abutment against the timing rolls 5. At this time these timing rolls 5 are held stopped. When the first line on the photosensitive drum 1 exposed by the laser beam 8 reaches a predetermined position, the timing rolls 5 are driven to send the paper 10 toward the transfer unit 12. Since the upper end and first line of the paper 10 have their phases arranged in this way, their distance is arranged for each sheet of paper 10. 
     Here arises a problem in that the temporary halt of the paper 10 results in a gap A (Note: Reference numeral &#34;A&#34; and other alphabetical reference numerals will be underlined throughout the specification to distinguish the reference numerals from word text) between the sheets of paper 10 as they cross the photosensitive drum 1, as is better seen from FIG. 2. This gap A, thus required, is different depending upon the printing rate of the printer, but is about 100 mm for a printer having a printing rate of about 50 sheets of A4 size (i.e., 210 mm×297 mm or 8.27 in.×11.69 in.) paper per minute. This makes it necessary to increase the developing/transferring (i.e., processing) rate to an extent corresponding to the gap A. Specifically, the processing rate of the printer having the printing rate of about 50 sheets of A4 size (i.e., 210 mm×297 mm or 8.27 in.×11.69 in.) paper per minute is calculated as follows: 
     (210 mm (per width of A4 paper)×50 (sheets/minute)+100 mm (width per Gap A)×50 (Gaps/minute)/60 sec (i.e. 60 sec./minute)=258 mm/sec. 
     If the gap A could be reduced to zero, the processing rate at this time would be calculated as follows and shortened to a factor of about 1/1.5: 
     (210 mm (per width of A4 paper)×50 (sheets/minute)/60 sec (i.e., 60 sec./minute)=175 mm/sec. 
     On the contrary, the electrophotographic printer using a roll of continuous paper does not require the gap A so that it can enjoy a relatively higher printing rate. On the other hand, the transferring, developing and fixing techniques become more difficult for the higher processing rate. If the gap A can be reduced, it is accordingly expected that a cut paper electrophotographic printer having a higher printing rate can be produced. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to eliminate the aforementioned drawbacks of the prior art and to provide an electrophotographic printer which is enabled to have a higher printing rate (i.e., number of sheets per minute) for a common processing rate by reducing the gap A substantially to zero. 
     According to the present invention, there is provided a cut paper electrophotographic printer for printing sheets of cut paper continuously with an electrophotographic process. The printer includes a pickup roll made rotatable for picking up the cut paper sheets one by one. A photosensitive drum is made rotatable for temporarily retaining thereon printing optical information when it is exposed to the laser beam bearing the optical information. A charging unit uniformly charges the photosensitive drum before the exposure. An optical unit emits the laser beam in response to printing signals. A host control circuit synchronized with the photosensitive drum emits the printing signals. A developing unit develops the exposed portion of the photosensitive drum with toner. A transfer unit transfers the toner from the photosensitive drum to each of the cut paper sheets. A pair of fixing rolls made rotatable at the same circumferential speed as that of the photosensitive drum fix the toner on each cut paper sheet. 
     The improvement of the invention comprises means for feeding the cut paper sheets one by one from the pickup roll onto the photosensitive drum. The feeding means includes two branch passages for alternately feeding the cut paper sheets such that each has a common margin between the top edge and the first line thereof. Passage switching means alternately switch the two branch passages to feed out one sheet to one of the branch passages and then a next sheet to the other to thereby reduce the gap between the two adjacent sheets substantially to zero. 
     The present invention has been conceived by noting that the printing rate of the cut paper electrophotographic printer is highly influenced by the gap A between the adjacent sheets of paper and accordingly provides redesigned timing rolls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become apparent from the following description taken with reference to the accompanying drawings, in which: 
     FIG. 1 shows the electrophotographic printer according to the prior art; 
     FIG. 2 is a schematic view showing the principle of the flow of paper sheets in the prior art; 
     FIG. 3 is similar to FIG. 1 but is a schematic diagram showing an embodiment of the electrophotographic printer according to the present invention; and 
     FIG. 4 is a schematic view showing another embodiment in which the present invention is applied to a double-side (i.e., duplex) printer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a specific embodiment of the present invention, which is different from the prior art of FIG. 1 in the following points. The paper passage extending from the pickup roll 4 to the photosensitive drum 1 is branched and is equipped with two pairs of feed rolls 2a and 2b and two pairs of timing rolls 5a and 5b. A passage switching mechanism 13 is interposed between the pickup roll 4 and the feed rolls 2a and 2b. In the structure of FIG. 3, the pickup roll 4 and the feed rolls 2a and 2b are rotated at a circumferential speed one and one-half to two times as high as that of the photosensitive drum 1. 
     The paper feed for the printing operations is conducted in the following manner. The first sheet is fed out to a passage a, and the passage switching mechanism 13 is then actuated to feed out the second sheet to a passage b. When the first sheet comes into abutment against the timing rolls 5a, these rolls 5a are driven at the same circumferential speed as that of the photosensitive roll 1 in synchronism with the laser beam 8 so that the first sheet is fed out to the transfer unit 12. At the instant when the first sheet has just passed through the first timing rolls 5a, the second sheet has already abutted against the other timing rolls 5b. At this instant, these other timing rolls 5b are driven to feed out the second sheet subsequent to the first sheet. Thus, the gap A of FIG. 2 can be reduced to zero by switching the passage a and b alternately for every other sheet. 
     FIG. 4 shows another embodiment in which the structure of the present invention is applied to a double-sided (i.e., duplex) electrophotographic printer. A double-sided (i.e., duplex) printer is one which prints on both sides of a sheet. In FIG. 4, the passage b provides for the turning back or side reversal of a paper sheet already having its one surface transferred and fixed. In the return passage b, moreover, there are arranged three pairs of feed rolls 2b 1 , 2b 2  and 2b 3  which also have a circumferential speed 1.5 to 2 times as high as that of the photosensitive drum 1. The printing operations are conducted in the following sequence: 
     (1) A first page of information is printed on the first side of a first sheet and the sheet is then fed to the return passage b by actuating the passage switching mechanism 13; 
     (2) With a gap of about one sheet, a third page of information is printed on the first side of a second sheet and is fed to the return passage b; 
     (3) With another gap of about one sheet, a fifth page of information is printed on the first side of a third sheet and the sheet is then fed to the return passage b. 
     (4) By the time printing of the fifth page has finished, the first sheet is turned back at a reversing section by the feed rolls 2b 3  and is thereafter advanced to the timing rolls 5b, where it is halted, its other or second unprinted surface directed downward in FIG. 4. Thereafter, the timing rolls 5b are driven to print the back or second side of the first sheet with a second page of information. The first sheet bearing the first and second pages on a first side and a second side is then fed to a discharge passage c by driving the passage switching mechanism 13 until the sheet is discharged to the outside of the printer; 
     (5) Immediately after the second page has been printed, a fourth sheet has a seventh page of information is printed on the first side thereon by driving the timing rolls 5a. This fourth sheet is fed to the return passage b by actuating the passage switching mechanism 13; and then 
     (6) A fourth page of information is printed on the back or second side of the second sheet, which had already been printed with the third page of information on a first side, by driving the timing rolls 5b. The second sheet bearing the third and fourth pages on first and second sides, respectively, is fed to the discharge passage c by driving the passage switching mechanism 13 until the sheet is discharged to the outside of the printer. 
     By driving the timing rolls 5a and 5b alternately to sequentially execute the above-specified steps (1) to (6), individual sheets of paper can be printed alternately on their front and back surfaces continuously without any interval between the printing of each front and back surface of sequential sheets of paper. 
     Despite this fact, however, the gap A (of FIG. 2) is required between the adjacent sheets (as in the prior art) when only one side is to be printed with the structure of FIG. 4. 
     In contrast, if each sheet is to have its two sides printed in the prior art using only one pair of timing rolls, the sheet residing in the passage b would have again been guided into the passage a and have been driven by the single pair of timing rolls for each of two sides so that the gap A would have been encountered twice. 
     According to the present invention, there is no need for establishing the gap A (of FIG. 2) between the adjacent sheets. As a result, the processing rate can be reduced from [about 1/1.5 as high as] that of the prior art assuming that [even if] the number of sheets to be processed per unit time is common. In viewing this advantage from another perspective, the number of sheets to be processed per unit time can be increased to about 1.5 times as high as that of the prior art for a common processing rate. 
     In the present invention as well as the prior art, it should be noted that the handling of each sheet has to include a &#34;settling time to allow each sheet to become settled i.e., move to a position where the sheet is abutted against the timing rolls. As another advantage of the present invention, this settling time period can be elongated as an auxiliary effect of the present invention. For example, the settling time periods for the printer having the printing rate of 50 sheets per minute are calculated as follows; 
     According to the prior art; 
     210 mm (per width of A4 paper)÷258 mm/sec (processing rate)=0.8/sec (settling time per sheet) 
     According to the present invention; 
     210 mm (per width of A4 paper)÷175 mm/sec (processing rate)=1.2 sec (settling time per sheet). Thus, a settling time period which is higher than that of the prior art can be attained according to the present invention.