Image forming apparatus

Abstract: The present invention provides an image forming apparatus having an electrophotographic image forming means for fixing a toner image formed on a sheet to the sheet by heating the toner image, wherein an interval between the sheets when the sheets are continuously sent to the image forming means to form images on both surfaces of plural sheets is selected to become smaller than an interval between the sheets when the sheets are continuously sent to the image forming means to form images on one surfaces of the plural sheets.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an image forming apparatus in line with the preamble of claim 1. Related Background Art Productivity (index for indicating the number of recordable copies per unit time) which is one of performances of an image forming apparatus such as a copying machine is generally determined by a process speed and a conveying distance (interval) between recording sheets. The process speed is a conveying speed for the recording sheet in an image forming part. The conveying interval of the recording sheet is a distance between a trail end of a preceding recording sheet and a tip end of a succeeding sheet when images are formed continuously. That is to say, the productivity of the copying machine can be improved by increasing the process speed or by decreasing the conveying interval. However, the process speed greatly depends upon a processing ability of the image forming part. Accordingly, in order to increase the process speed, an image forming apparatus having a high speed processing ability must be adopted or developed, which leads to increase in the manufacturing cost and which is difficult to realize for a short time. In order to decrease the conveying interval of the recording sheet, there is a danger of arising the following problems. In case of an image forming apparatus of electrophotographic type, an image is fixed to a recording sheet by heating toner transferred to the recording sheet by means of a fixing device. If the conveying interval is decreased, since the number of recording sheets fixed per unit time is increased, greater heat amount is absorbed from the fixing device. As a result, the recording sheet cannot be heated sufficiently, thereby causing poor toner fixing. On the other hand, in case of an image forming apparatus having a first page processing function, a rear surface discharge mode in which recording sheets on which the images were formed are turned up or reversed and then are stacked on a discharge tray is used. In this case, when a switch-back system in which the recording sheet is temporarily into a reverse path is used as a recording sheet reversing system, since the succeeding recording sheet cannot be conveyed until the preceding recording sheet is discharged from the reverse path, a predetermined conveying interval is required. Further, in a case where a retard system in which sheets are supplied one by one by means of a supply roller and friction of a separation roller is used, in an area where the sheet is conveyed only by the supply roller and the separation roller, a conveying speed of the recording sheet becomes unstable. Thus, the conveying interval must be increased accordingly From document US-A-5337135 a variable speed duplex copier is known having an image forming apparatus comprising electrophotographic image forming means including means for fixing a toner image formed on a sheet to the sheet by heating the toner image. Further a sheet supply means is provided for feeding out a sheet from a sheet containing part and re-conveying means for returning the sheet on which the image was formed by said image forming means to said image forming to form an image on the sheet again. The sheet re-conveyed by the sheet conveying means to form images on both surfaces and the sheet fed out by said sheet supply means are sent to the image forming means alternately. SUMMARY OF THE INVENTION Due to the above limitations, the conveying interval cannot merely be decreased, and, thus, it is difficult to improve the productivity by decreasing the conveying interval. In consideration of this, an object of the present invention is to provide an image forming apparatus in which a conveying interval can be decreased regardless of the above limitations thereby to improve productivity. This object is achieved by the features of claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of an entire image forming apparatus according to the present invention; Fig. 2 is a view showing a reverse part of the image forming apparatus for explaining rear surface discharge; Fig. 3 is a view showing the reverse part of the image forming apparatus for explaining re-convey; Fig. 4 is a flow chart for explaining timings for feeding out a recording sheet and a re-conveyed sheet; and Figs. 5A, 5B and 5C are views for explaining a conveying interval. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, an image forming apparatus according to a preferred embodiment of the present invention will be explained with reference to the accompanying drawings. The image forming apparatus A shown in Fig. 1 is a digital copying machine in which an image forming means C is disposed at an upper part and a plurality of cassettes 1 for stacking recording sheets P are mounted at a lower part. A supply means B is associated with each of the cassettes 1 to convey the recording sheet P to the image forming means C. A re-convey means D is connected to the image forming means C so that the recording sheet P having a first surface on which an image was formed by the image forming means C can be subjected to reverse of front/rear surface and then be discharged or re-supplied. Incidentally, by discharging the recording sheet P while reversing front/rear surface, the recording sheets can be stacked in a page sequence (page order). This is referred to as "first page processing function". On the other hand, a system in which, after the recording sheet is reversed, the recording sheet is re-supplied from the supply means B to a convey path for the image forming means C is referred to as "through-path both surface system". [Supply means] A pick-up roller 2 is urged against the recording sheets P stacked on each cassette 1 so that an uppermost recording sheet P can be fed out. The supply means is of retard type, and double-feed of the recording sheets P is prevented by supply rollers 3 driven in a recording sheet conveying direction and separation rollers 4 rotated in an opposite direction. The separated recording sheet P is brought to a supply path 6 by a corresponding pair of convey rollers 5 and then is fed out to the image forming means C of electrophotographic type. [Image forming means] In the image forming means C, an electrostatic latent image is formed on a photosensitive drum 7 in response to an image signal emitted from a computer (not shown), and the latent image is visualized as a toner image. The toner image formed on the photosensitive drum 7 is transferred, by a transfer charger 8, onto the recording sheet P existed between the photosensitive drum 7 and the transfer charger 8. The recording sheet P is supplied by the supply means B in a timed relation to the photosensitive drum 7. The recording sheet P to which the image was transferred is fed out, by a convey belt 9, to a fixing device 10, where the toner image is fixed to the recording sheet by heat and pressure. After the fixing, the recording sheet is sent to the re-convey means D through a first pair of discharge rollers 11a, and is discharged out of the apparatus by a second pair of discharge rollers 11b or re-supplied while reversing front/rear surface. [Re-convey means] In the re-convey means D, a flapper 12 for switching an advancing direction of the recording sheet P is disposed between the first pair of discharge rollers 11a and the second pair of discharge rollers 11b, so that the recording sheet P conveyed by the first pair of discharge rollers 11a can be directed to a switch-back path 13. The switch-back path 13 is branched on the way to a downward path connected to a lower convey path 17. Hereinafter, a path of the switch-back path 13 disposed toward the flapper 12 from the branched position is referred to as "switch-back path 13a" and the other path of the switch-back path 13 is referred to as "switch-back path 13b". As shown in Fig. 2, a first valve 16a is attached to a lower portion of the flapper 12, which valve is disposed in a convey path from the first pair of discharge rollers 11a to the switch-back path 13a. With this arrangement, the recording sheet P is prevented from returning from the switch-back path 13a to the first pair of discharge rollers 11a. Further, a second valve 16b is disposed at the branched position between the switch-back paths 13a and 13b, so that, as shown in Fig. 3, the recording sheet P fed out from the switch-back path 13b is directed to the lower convey path 17. The lower convey path 17 is connected to the supply path 6 for feeding the sheet from the supply means B to the image forming means C. [Reversing operation] In the image forming apparatus having the above-mentioned arrangement, when the image is recorded only on one surface of the recording sheet P, first of all, by the action of the flapper 12, the recording sheet P is fed out to the switch-back path 13a. When a predetermined time period is elapsed after the trail end of the recording sheet P is detected by a reverse sensor 14a, a switch-back roller 15a is rotated in a reverse direction. Consequently, under the action of the first valve 16a, the recording sheet P is directed to the second pair of discharge rollers 11b, with the result that the recording sheet is discharged onto a tray T out of the apparatus while reversing front/rear surface. When the images are recorded on both surfaces of the recording sheet P, after the fact that the recording sheet P was contained in the switch-back path 13b is detected by a reverse sensor 14b, a switch-back roller 15b is driven in a reverse direction. Consequently, under the action of the valve 16b, the recording sheet P is fed out to the lower convey path 17 and then is directed to the image forming means C through the supply path 6. After the image is recorded on the rear surface of the recording sheet, the recording sheet is directly discharged onto the tray T out of the apparatus through the first and second pairs of discharge rollers 11a, 11b, since the front/rear surface of the sheet was already effected. Now, feeding of the recording sheet will be explained. In this case, the recording sheet fed out from the cassette 1 is referred to as "recording sheet P1" and the recording sheet re-conveyed from the lower convey path 17 is referred to as "recording sheet P2". When the images are recorded continuously, as shown in the flow chart of Fig. 4, the re-conveyed sheet P2 which is re-conveyed (step S1) is temporarily stopped (step S3) when the tip end of the sheet is detected by a stop sensor 18 (step S2). When the trail end of a recording sheet P1 newly fed out from the cassette 1 is detected by a vertical path sensor 19 (step S4), conveyance of the re-conveyed sheet P2 in the lower convey path 17 is re-started (step S5). Further, when the trail end of the re-conveyed sheet P2 is detected by the vertical path sensor 19 (step S6), a new recording sheet P1 is fed out from the cassette 1 (step S7). Such a cycle is repeated. [Productivity] Next, the productivity of image recording will be explained with reference to Figs. 5A to 5C while comparing with the conventional case. In the illustrated embodiment, it is assumed that a conveying speed V of the recording sheet P1 is 300 mm/s and a standard size of the recording sheet P1 is A4 (length in the conveying direction = 210 mm). First of all, a case where the image is recorded on one surface of the sheet and the sheet is discharged while reversing front/rear surface (first page processing) will be explained with reference to Figs. 2 and 5A. In the fixing device 10, after the fixing operation was finished, the recording sheet P1 enters into the switch-back path 13a. After the trail end of the sheet is detected by the reverse sensor 14a, the sheet is conveyed by 26 mm and is stopped there. In this case, a distance between the trail end of the recording sheet P1 and the switch-back roller 15a becomes 20 mm. Immediately after the stoppage of the recording sheet, the switch-back rollers 15a, 15b are rotated in the reverse direction, thereby discharging the recording sheet P1 out of the apparatus. In this case, a time period required for separating the recording sheet P1 from the switch-back roller 15a becomes. L/V = (210 [mm] - 20 [mm])/300 [mm/s] = 0.63 [s]. And, the time period is at least 0.63 second. In actual, margin is required for poor conveyance. It is assumed that the margin is 30 mm, the required time period becomes L/V = (210 [mm] + 30 [mm] - 20 [mm])/(300 [mm/s]) = 0.73 [s]. In this case, the conveying interval becomes 210 [mm] + 30 [mm] - 20 [mm] = 220 [mm]. In this case, in consideration of the length of the recording sheet P1 itself, the productivity becomes 300 [mm/s] × 60 [s]/(210 [mm] + 220 [mm]) = 42 [sheets/min]. Further, as shown in Fig. 5B, in the conventional case, even when the images are formed on both surfaces of the re-conveyed sheet P2, the conveying interval was set to be equal to that in the one surface recording. Accordingly, in this case, the productivity became 42 [sheets/s]/2 [surfaces] = 21 [sheets/min]. By the way, in the rear surface recording of the reversed and re-conveyed sheet, since the recording sheet P2 was once passed through the fixing device 10, the sheet was already heated in comparison with the recording sheet P1 fed out from the cassette. Thus, since a heat amount of the recording sheet P2 absorbing from the fixing device 10 is smaller than that of the recording sheet P1, the conveying interval can be reduced. Further, when the images are recorded on both surfaces of the recording sheet continuously, the recording sheet P1 having the front surface on which the image is to be recorded and the recording sheet P2 having the rear surface on which the image is to be recorded are supplied alternately. In this case, when the image is recorded on the rear surface, since the recording sheet is not reversed by the re-convey means D, the time period required for the reversal does not relate to determination of the conveying interval, and, thus, the conveying interval can be reduced. Further, since the recording sheets are supplied alternately, regarding the recording sheet P1 newly supplied from the cassette 1, there is the conveying interval corresponding to at least the length of the re-conveyed sheet P2. Accordingly, even if the conveying speed for the recording sheet P1 is not stable at the region where the recording sheet is conveyed only by the supply roller 3 and the separation roller 4, it is not required to increase the conveying interval accordingly, thereby decreasing the conveying interval. In the illustrated embodiment, as shown in Fig. 5C, in the both surface recording, the conveying interval is decreased as much as possible. In consideration of the remaining factor for determining the conveying interval (poor conveyance due to dispersion in supply), the convey interval of 90 mm is provided. In this case, the productivity regarding the recording sheet having A4 size becomes 300 [mm/s] × 60 [s]/(210 [mm] + 90 [mm])/2 [surfaces] = 30 [sheets/min]. Similarly, regarding various recording sheet having standard sizes, the convey interval of 90 mm is provided. In comparison with the productivity between the one surface recording, conventional both surface recording and the present invention both surface recording, results as shown in the following Table 1 were obtained. Sheet size Productivity (sheets/min) sheet interval in both surface printing one surface printing both surface printing in prior art both surface printing in present invention prior art present invention A4 (210mm) 42 21 30 220 90 B4 (364mm) 24 12 20 374 90 A3 (420mm) 21 11 18 430 90 From the above Table 1, in the conventional case, the ratio of productivity of both surface recording with respect to one surface recording was 50%. To the contrary, the ratios of productivity in the present invention becomes as shown in the following Table 2. one surface printing both surface printing in prior art both surface printing in present invention A4 100% 50% 71% B4 100% 50% 83% A3 100% 50% 85% As apparent from the above Table 2, in the both surface recording, the productivity can be improved greatly in comparison with the conventional case. Incidentally, in the illustrated embodiment, while an example that the conveying intervals for various size sheets in the both surface recording are selected to be the same was explained, the optimum conveying interval may be selected for each size sheet. Further, while an example that the conveying speed of the recording sheet P1 is selected to be constant was explained, even when the conveying speed in the reconvey path D and the entire conveying speed for the re-conveyed sheet P2 are set to be faster, the effective result can be achieved by the present invention. Further, in the illustrated embodiment, while the image forming apparatus of through path both surface type in which the front surface recording and the rear surface recording are effected alternately was explained. However, so long as the present invention is applied to image forming apparatuses in which a recording sheet on which an image was once formed is re-conveyed to an image forming means, effective result can be achieved. Accordingly, the present invention can be applied to image forming apparatuses of other both surface recording type or multi recording type (for example, batch processing through path type or intermediate tray type).

Claims: An image forming apparatus having electrophotographic image forming means (C) including means for fixing a toner image formed on a sheet to the sheet by heating the toner image, said image forming apparatus comprising: sheet supply means (B) for feeding out a sheet from a sheet containing part (1); and re-convey means (D) for returning the sheet on which the image was formed by said image forming means (C) to said image forming means (C) to form an image on the sheet again, wherein the sheet re-conveyed by said re-convey means (D) to form images on both surfaces and the sheet fed out by said sheet supply means (B) are sent to said image forming means (C) alternately, characterized by control means arranged to select the sheet interval when the sheet re-conveyed by said re-convey means (D) and the sheet fed out by said sheet supply means (B) are sent to said image forming means (C) alternately to become smaller than the sheet interval when the sheets are continuously sent to said image forming means (C) to form images on one surface of the plural sheets. An image forming apparatus according to claim 1, wherein the control means is arranged to feed out the sheet from said sheet containing part when an image shall be formed on one surface of the sheet, and to send the sheet to said image forming means again through said re-convey means when images shall be formed on both surfaces of the sheet. An image forming apparatus according to claim 1 or 2, further comprising a first reverse mechanism (12,13a) disposed at a downstream side of said image forming means (C) for discharging the image recorded sheet while effecting reverse of front/rear surface, and a second reverse mechanism (13b,15b) disposed in said re-convey means (B) on the way for re-conveying the image recorded sheet while effecting reverse of front/rear surface. An image forming apparatus according to claim 3, wherein, when the sheets are continuously fed out by said sheet supply means (B) and the images are formed on the sheets by said image forming means, the sheet is discharged out of the image forming apparatus by said first reverse mechanism while effecting reverse of front/rear surface, and, when the sheet re-conveyed by said re-convey means (B) and the sheet fed out by said sheet supply means (B) are sent to said image forming means (C) alternately to form image on the sheet, the sheet is sent to said image forming means (C) by said second reverse mechanism while effecting reverse of front/rear surface. An image forming apparatus according to claim 1, wherein said sheet supply means (B) comprises a pick-up roller (2) for feeding out the sheets from said sheet containing part (1), and a pair of separation rollers (3,4) for separating the sheets fed out by said pick-up roller one by one. An image forming apparatus according to claim 2, wherein said re-convey means (B) has a through path for continuously sending the sheets having one surface on which the image was formed. An image forming apparatus according to claim 2, wherein said re-convey means (B) has an intermediate tray on which the sheets having one surface on which the image was formed are temporarily contained.

Inventors: USHIO KENJI, USHIO, KENJI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1998
Main classification: G03G15
IPC codes: B65H29, G03G21, B65H85, G03G15