A printer for continuous-form recording paper

Abstract: A printer for electrophotographically forming an image on a condinuous-form recording paper (20). A fransfer unit (7) and a heat fixing roll pair (8) are brought into their withdrawn states, respectively, in a print stand-by state, whereby the recording paper (20) becomes out of contact with a photoconductive drum (1) and a heat roll (81A). A dust removing brush pair (40) is also brought into its withdrawn state in the print stand-by state so as to be out of contact with the recording paper (20).

Description: The present invention relates to a printer for electro photographically forming an image on a continuous-form recording paper. There have been known imaging devices using an electrophotographic process wherein the surface of a photoconductive drum is exposed to an optical image to form a latent image thereon, on which toner is applied for development of the image. The toner image is then transferred to a recording paper and is fixed by means of a fixing device. One of such imaging devices is a printer which is adapted to print information on a continuous-form recording paper as used for conventional line printers. The continuous-form recording paper is a so-called fan-fold paper with sprocket holes (referred to simply as "continuous form" in the following. There are perforations at certain intervals for easier cutting the paper into separate sheets of paper. Different processes have been available to fix the toner image to the recording paper (continuous form) in such an electrophotographic process: thermally fusing the toner to the paper, using a solvent or applying pressure. The thermal fusing process is widely used as it is considered to be the safest and most reliable way of fixing. Among several different types of such thermal fusing processes, mostly used is a heat roll system in which the recording paper with an image remaining unset is pressed against a heat roller so that the heat transferred from the heat roller fuses and sets the toner image. The heat roll system uses a fixing roller pair consisting of the heat roller heated to a high temperature and a backup roller, the two rollers being placed in opposition to each other. The recording paper carrying an unfixed image is clamped between the two rollers so that it is pressed against the heat roller with a certain pressure to fix it on the paper by the heat from the heat roller. The usual arrangement is such that the recording paper is travelling forward with the rotation of the fixing roller pair itself. The electrophotographic printer using continuous form usually employs the heat roll system. In printers using continuous form, however, printing has to start with a certain space from the perforation of the continuous form (i.e., top end of the page). On the other hand, the electrophotographic printing system is so configured that operating elements such as exposure, developing and transfer units are all placed along the circumference of a photoconductive drum. As a result, those individual units are brought into operation as the drum rotates so that the exposure and transfer processes take place in different positions. In order to make the exposure start position coincident with a certain point from the perforation of the continuous form with the start of printing (i.e., to start printing at a certain space from the perforation), it is necessary to adjust the print start position by moving the photoconductive drum relative to the continuous form before starting printing (start positioning). If the drum is kept in contact with the continuous form (held in a transfer operative position), the photoconductive material coated on the drum may be damaged or wear to shorten the life of the drum. This may also cause the continuous form to be soiled with toner remaining on the drum surface. Another problem encountered is that when using the heat roll system as mentioned above, the heat roll must be constantly kept heated even in a print stand-by state, because heating the heat roller takes a considerable time. As a result, the recording paper (continuous form) constantly pressed between the heat roller and the backup roller is at a certain section steadily exposed to the heat transferred from the heat roller, resulting in burning of the paper or blister developed on the paper. The assignee of the present application has proposed an arrangement in which one roller of the fixing roller pair can be withdrawn from the other roller to avoid contact of the paper with the heat roller in the printing stand-by state and in which the transfer charger can be withdrawn from its transfer operating position to separate the recording paper from the drum. However, the fabrication costs become high as separate driving means have been employed for withdrawing the fixing roller and the transfer charger. Further, the surface of the continuous form has paper dust particles deposited thereon which are developed during formation of sprocket holes and perforations. Such dust particles may enter the transfer charger or be stuck onto the drum surface, resulting in various troubles. To eliminate these troubles, a cylindrical dust brush unit is provided near a paper inlet portion of the printer. The dust brush, in contact with the continuous form, is rotated in the direction opposite to the paper feed direction for brushing off such dust particles from the paper surface. However, when the transfer charger and one roller of the fixing roller pair are withdrawn from their transfer operative and fixing operative positions during the print stand-by state, the continuous form can no longer be retained at its position. As a result, the continuous form moves in opposite direction by the force exerted by brushing with the dust brush so that the print start position is changed. Moreover, there has been a problem that the sensitized paper is attracted to the drum surface and can not be separated thereform merely by withdrawing the transfer charger from its transfer operative position. Thus, an arrangement has been proposed such that a paper holder is provided on a pivotable member carrying the transfer charger so that the continuous form is passed between the paper holder and the transfer charger, whereby the continuous form is separated from the drum surface by means of this paper holder upon withdrawal movement of the tansfer charger. With this arrangement, however, there has been a difficulty in loading the continuous form because the form has to be passed between the paper holder and the transfer charger. US-A-4 431 180 discloses an electrostatic copying apparatus in which an operating lever is provided to withdraw a back-up fixing roller and a transfer charger mounted to a common frame to allow easy paper removal when a paper sheet has jammed. This device does not solve problems arising with the use of continuous form paper especially in a print stand-by state. It is therefore the problem to be solved by the invention to provide a printer using the continuous form which can be withdrawn from the drum at the transfer unit and from the heat roller at the fixing unit in a print stand-by state to thereby prevent wear of the photoconductive drum as well as troubles caused by heat applied to the continuous form, wherein such withdrawing operation can be accomplished by a single-unit driving means. The invention solves this problem by the features of the main claim. The subclaims specify advantageous further developments of the invention. The printer may be so configured that it comprises transfer charger load means for loading said transfer charger to its transfer operative position, transfer charger lock means for fixedly locking said transfer charger in its withdrawn position and lock release means for releasing said transfer charger lock means, whereby said transfer charger is fixedly locked by said transfer charger lock means in its withdrawn position, and said transfer charger is restored to its transfer operative position by releasing said lock of said transfer charger by means of said lock release means. Furthermore, the printer may be so configured that said transfer charger is carried by a pivotally rockable arm member to allow said transfer charger to move from its transfer operative position to its withdrawn position by pivotal movement of said arm member, paper holder rollers are rockably mounted on the pivotal center of the arm member for pivotal movement relative to said arm member, said paper holder rollers flexing the paper feed path toward a photoconductive drum after the transfer charger, said arm member being adapted to be pivotally driven for its withdrawing movement within a certain range of the pivotal movement of said rollers, and said printer further comprises a common drive means for executing the pivotal movements of said withdrawable roller of the fixing roller pair and of said paper holder rollers. Furthermore, the printer may be so configured that it comprises means for biasing said paper holder rollers to a predetermined position, transfer charger loading means for biasing said transfer charger to its transfer operative position lock means for fixedly locking said paper holder rollers in their withdrawn position, and lock release means for releasing said lock means, wherein said transfer charger is fixed in its withdrawn position by fixedly locking said rollers in their withdrawn position, and wherein said rollers are restored to their predetermined position, while said transfer charger is restored to its transfer operative position, by releasing said lock means from locking by means of said lock release means. When the continuous form is withdrawn from the transfer operative and fixing operative positions in the print standby state, it does not deviate from its specified position, enabling printing to start at the correct start position, if a dust brush means is provided adjacent the inlet of the continuous-form recording paper, said dust brush means being withdrawable from its position contacting the continuous-form recording paper; and said common drive means also withdrawing and restoring said dust brush means at predetermined timings. The continuous form can be separated from the drum surface with rocking movement of the transfer unit for easier insertion of the continuous form during loading the form, if said paper holder rollers are provided with an operating member for effecting pivotal movement independently of said transfer charger. Furthermore, the printer may further comprise a push-up mechanism for loading the photoconducive drum located above the transfer charger in upward direction, and an operating member operatively associated with said push-up mechanism and said paper holder rollers for rocking said paper holder rollers upon push-up movement of the photoconductive drum by means of said push-up mechanism. Moreover, the drum located above the tansfer charger is pressed against a lower structural part of the printer to lock it in a predetermined position upon closure of an upper structural part of the printer which is adapted to be pivotally moved relative to the lower structural part. The invention will now be further described with reference to the accompanying drawings in which Fig. 1 is a side elevation showing a general configuration of a printer embodying the invention; Fig. 2 is a side elevation of a continuous form withdrawing mechanism; Fig. 3 is a plan view of the withdrawing mechanism; Fig. 4 is an enlarged section taken along the line IV-IV of Fig. 3; Fig. 5 is a view illustrating the operative state of the withdrawing mechanism; Figs. 6 and 7 are operational time charts; Fig. 8 is a side elevation of a modified embodiment; Fig. 9 is a plan view of the embodiment shown in Fig. 8; Figs. 10 (A) and 10(B) are partially enlarged views showing the withdrawn state; Fig. 11 is an operational time chart showing a restoring operation; Fig. 12 is a side elevation of a modified continuous form withdrawing mechanism; Fig. 13 is a plan view of the mechanism shown in Fig. 12; Fig. 14 is an enlarged view of a dust brush unit; Fig. 15 is a view illustrating the shifted state of the dust brush unit; Fig. 16 is a view showing the operational state of the mechanism shown in Fig. 12; Fig. 17 is an operational time chart; Fig. 18 is a plan view of a modified embodiment; Fig. 19 is a section taken along the line X-X of Fig. 18; Fig. 20 is a section taken along the line I-I of Fig. 19; Fig. 21 is a section taken along the line II-II of Fig. 19; Fig. 22 is a view in the direction of arrow III of Fig. 19 Fig. 23 is a section taken along the line IV-IV of Fig. 19; Fig. 24 is a side elevation showing a general configuration of a modified embodiment of the invention; Fig. 25(A) is a side elevation of a paper holding mechanism; Fig. 25(B) is a section taken along the line B-B of Fig. (A); Fig. 26 is a section taken along the line III-III of Fig. 25(A); Fig. 27 is section taken along the line IV-IV of Fig. 25(A); and Fig. 28 is a view in the direction of the arrow V in Fig. 25(A) showing a transfer charger being withdrawn and a paper holder pivotally moved. Fig. 1 shows a laser printer 10 which prints out information from computers etc. onto a continuous form paper 20 by means of a so-called electrophotographic process. The laser printer 10 generally comprises a main body 11 as a lower structural part and a clam shell 12 as an upper structural part pivotally secured to the top of the main body 11 to allow opening movement relative to it. The main body 11 has formed therein a feed path for the continuous form 20 running from right to left in the drawing. Along the outer circumference of a photoconductive drum 1 are a toner cleaning unit 2, a decharging unit 3, a charging unit 4, an optical scanning system 5, a developing unit 6 and a transfer unit 7, all arranged in due order in the rotary direction of the drum 1. A fixing unit 8 is placed ahead in feed direction of the continuous form 20, while a tension applying mechanism 9 is located in the feed path of the continuous form 20 from the drum 1 to the fixing unit 8. Near the inlet of the continuous form 20 (right-hand side of Fig. 1) a paper dust brush pair 40 is provided. The laser beam coming from the optical scanning system 5 effects horizontal scanning (exposure) on the photoconductive surface of the drum 1 along its rotary axis, while the drum 1 is rotated (vertical scanning). The latent image built on the photoconductive surface of the drum 1 is developed to a toner image by means of the developing unit 6. It is then transferred to the continuous form 20 at the transfer unit 7 and is fixed on it at the fixing unit 8. The tension applying mechanism 9 comprises a pair of endless belts 91 having projections adapted to be engaged with sprocket holes at the lateral edges of the continuous form 20, the belts 91 being capable of circulating with a certain resistance along the feed path of the continuous form 20. This is provided to prevent paper skew by giving a certain tension to the continuous form 20 drawn by the fixing unit 8. The paper dust brush pair 40 comprises upper and lower cylindrical paper dust brushes 41 and 42 placed opposite to each other, whose lengths are greater than the width of the continuous form 20. Both paper dust brushes 41 and 42 are rotatably driven in the direction opposite to the feed direction of the continuous form 20 passing between the two brushes 41 and 42. The brushes 41 and 42 are thus adapted to sweep dust particles off the top and bottom surfaces (front and back surfaces) of the continuous form 20. The transfer unit 7 includes, as shown in Figs. 2 and 3, a corona charger 71 substantially as long as the axial length of the drum 1, which is carried at both ends by arms 72 with a certain spacing to the surface of the drum 1 in parallel therewith. Each arm 72 is carried by a bracket 101 mounted on the chassis 100 of the laser printer 10 for pivotal movement about a pivot 74. The arm 72 is connected with a certain portion of the bracket 101 by means of a tension spring 75 so that the tension spring 75 preloads the arm 72 to bring the corona charger 71 retained by it to its transfer operating position with respect to the drum 1. Beneath one end of the arm 72 provided is a rotatable cam follower 73 is provided which is adapted to be operated by a cam plate 30 secured to the top surface of a slide plate 103 which is mounted on the chassis 100 to be slidably movable in fore and aft directions in the laser printer 10. This pivots the arm 72 against the tension load of the tension spring 75. Pivotal movement of the arm 72 causes the corona charger 71 to be pivoted away from the drum 1, i.e. from its transfer operating position to its withdrawn position. The leading end of the arm 72 has a paper holding roller 76 rotatably mounted thereon. The paper holding roller 76 is located at a certain distance from the corona charger 71 and parallel thereto and extends across the continuous form 20. When the corona charger 71 is in its transfer operating position, the paper holding roller 76 is not in contact with the upside of the continuous form 20. When the corona charger 71 is withdrawn from its transfer operating position because of pivotal movement of the arm 72, the roller 76 is brought into contact with the upper surface of the continuous form 20 to push the continuous form 20 down in response to the withdrawing movement of the corona charger 71 (i.e., moving the continuous form 20 apart from the drum 1). When in the arrangement mentioned above the corona charger 71 is pulled down due to pivotal movement of the arm 72, the continuous form 20 is charged to a lower degree even though a voltage is being applied to the corona charger 71. Since the continuous form 20 is moved apart from the drum 1 upon push-down movement of the paper holding roller 76, image transfer is made virtually impossible. The fixing unit 8 comprises a fixing roller pair 81 consisting of a heat roller 81A and a backup roller 81B vertically opposite to each other, both extending transverse to the feed direction of the continuous form 20. The upper heat roller 81A is a cylindrical roller both ends of which are rotatably carried by the chassis 100 of the laser printer 10. It houses therein a heating element such as a halogen lamp, not shown, to be heated to a certain temperature. One end of the heat roller 81A is coupled to a rotary drive means, not shown, so that it is rotated by the rotary drive means in the feed direction of the continuous form 20 at the circumferential revolution speed of the roller 81A, which is in synchronism with that of the drum 1. The lower backup roller 81B is carried at each end by an arm 86 which is linked with a lever 84 vertically rockable upon rotation of a cam 83 against the biasing force of a tension spring 85. The lever 84 and the arm 86 are mounted on the chassis 100 of the laser printer 10 for pivotal movement about a common pivot 87. The leading end of the lever 84 is provided with a rotatable cam follower 84A which is adapted to contact the upper surface of the circumferential edge of the cam 83 because of the weight of the lever 84, the arm 86 and the backup roller 81B. The arm 86 is at its leading end linked with a certain portion of the lever 84 by means of the tension spring 85 which in turn applies a preload to the arm 86 and the lever 84 so that the angle between them is decreased. The cam 83 is of a generally heart-shaped eccentric type and is secured to a shaft 11 which is rotatably carried by a U-bracket 102 mounted on the chassis 100. When the cam 83 is in contact with the cam follower 84A at a top dead point (Fig. 2), the lever 84 is swung to upwardly rock the arm 86 via the tension spring 85. The backup roller 81B is thus pressed against the heat roller 81A under a certain force exerted by the tension spring 85, i.e., brought to its fixing operative position. The image-unfixed continuous form 20 clamped between the two rollers 81A and 81B is then heated to allow the toner image to be fused onto the continuous form 20, while at the same time the continuous form 20 is driven in its feed direction with rotation of the heat roller 81A. When the cam 83, on the other hand, contacts the cam follower 84A at a bottom dead point (Fig. 5), the backup roller 81B is moved apart from the circumferential surface of the heat roller 81A at a certain distance, i.e., brought to its withdrawn position where no fixing is effected. As the cam 83 is rotated, the backup roller 81B is pivotally movable between its fixing position and its withdrawn position. The shaft 11 has further mounted thereon a second cam 13 and a gear 12 besides the cam 83. The gear 12 is in mesh with a gear, not shown, which is mounted to the flange-shaped spindle of a gear motor 10 secured to one side of the bracket 102, whereby rocking movement of the backup roller 81B is effected by the gear motor 10 through the shaft 11. The gear motor 10 is in driving connection with a control unit, not shown, for the whole system, which is adapted to control other mechanical and operational parts of the laser printer 10. The second cam 13 is an eccentric cam generally of the same shape as the cam 83 as shown in Fig. 4 in an enlarged scale. In engagement with the circumferential surface of the second cam 13 is a cam follower 14A rotatably mounted on the leading end of a gear arm 14. The gear arm 14 is secured to the side surface of a gear 15 splined to an arm shaft 16 which is mounted on the bracket 102 in parallel with the shaft 11. The gear 15 is thus rotated upon rocking movement of the gear arm 14. The gear 15 is in mesh with a gear 18 which is in turn rotatably splined to a lever shaft 17 mounted on the bracket 102 in parallel with the shaft 11 and the arm shaft 16. The gear 18 carries a lever 19, the leading end of which is provided with an U-shaped cutaway 19A. The cutaway 19A is engaged by an operating lever 103A which is secured to the rear end of the slide plate 103. The slide plate 103 is held in position by means of a guide shaft 104 mounted on the chassis 100, which is received by a guide slot 103b provided in the slide plate 103, so that it is slidably movable in fore and aft directions in the laser printer 10. On the other hand, another (rear) guide shaft 104 and the rear portion of the slide plate 103 are linked together by a tension spring 31, which applies a forward-directed load to the slide plate 103. With the arrangement mentioned above, the slide plate 103 is loaded in a forward direction by means of the tension spring 31 so that an operating bar 103a of the slide plate 103 pushes the lever 19 with its leading end in forward direction. This causes the gear arm 14 to be swung by gears 18 and 15 so that its leading end is urged rearward, whereby the cam follower 14A at the leading end thereof is pressed against the cam surface of the second cam 13. That is, the cam follower 14A is pressed against the second cam 13 by the tension spring 31, so that the slide plate 103 is slidably movable in fore and aft directions upon rotation of the second cam 13. With the configuration of the laser printer 10 as described above, in the print stand-by state, the gear motor 10 is driven to withdraw the backup roller 81B of the fixing roller pair 81 so as to separate the continuous form 20 from the heat roller 81A, while also withdrawing the corona charger 71 of the transfer unit 7 from its transfer operating position. This prevents not only the troubles associated with continuous application of heat from the heat roller 81A to the same portion of the continuous form 20, but also the troubles arising from contact of the continuous form 20 with the drum 1 even when only the drum 1 starts rotation while the continuous form 20 stands still when printing is stated (re-started). When starting (restarting) printing, the corona charger 71 is put back to its transfer operating position and the backup roller 81B to its fixing operative position (i.e., fixing operation and continuous form drawing take place) at the timing shown in Figs. 6 and 7. More particularly, as shown in the time chart of Fig. 6, when a certain length of time (T) elapses since starting exposure with rotation of the drum 1, the gear motor 10 is driven to restore the corona charger 71 to its transfer operative position and the backup roller 81B to its fixing operative position, thus starting the transfer and fixing operations as well as pulling of the continuous form 20. The heat roller 81A should be brought into rotation prior to pivotal movement of the backup roller 81B. In this case, pulling of the continuous form 20 starts at the time when the continuous form 20 is nipped between the heat roller 81A and the backup roller 81B because of pivotal movement of the latter. If the pivotal movement of the backup roller 81B is not coordinated with the start time of the continuous form 20, it is difficult to ensure a constant spacing between the perforation and the print start (re-start) position on the continuous form 20. In order to prevent this and to enable printing to start at a certain fixed distance from the perforation, the power transmission system to the fixing roller pair 81 (power transmission to the heat roller 81A in this embodiment) should preferably include a clutch, not shown, which is operated in accordance with the time chart shown in Fig. 7. Specifically, before a certain length of time (T) elapses after start of exposure with rotation of the drum 1, the gear motor 10 is driven to put the corona charger 71 into its fixing operative position and the backup roller 81B into its fixing operative position. At this time, the clutch in the power transmission to the fixing roller pair 81 is OFF so that the fixing roller pair 81 does not rotate and the continuous form 20 is not pulled. When the certain length of time (T) after start of exposure has elapsed, the clutch is ON to rotate the fixing roller pair 81 for pulling the continuous form 20. Whereas in the foregoing embodiment the cam 83 for withdrawal of the backup roller 81B and the second cam 13 for pivotal movement of the corona charger 71 (withdrawing and restoring) are two different elements, only one cam may be used for effecting both operations. Also, whereas in the foregoing arrangement the heat roller 81A of the fixing roller pair 81 is rotatably driven with the backup roller 81 adapted to be withdrawn, a modification is possible for instance by making the backup roller 81B to be rotatably driven while the heat roller 81A is arranged to be withdrawn. Figs. 8 through 11 show a modified embodiment. The aforedescribed embodiment is so arranged that the withdrawing and restoring movements of the backup roller 81B of the fixing roller pair 81 and of the corona charger 71 of the transfer unit 7 are accomplished by cams (cam 83 and the second cam 13) rotatably driven by a single driving means (gear motor 10). Since the load applied to the driving means (gear motor 10) varies during the withdrawing and restoring movements, this variation may cause a change in timing of restoration of the corona charger 71 especially if a DC (direct current) motor is used as a driving means. This may result in that the corona charger 71 does not reach its operative position in time for print starting or reaches it so soon as to soil the continuous form 20. This modified embodiment therefore provides an arrangement in which the corona charger 71 is restored to its operative position at an accurate timing. In the drawings, like numerals indicate like and identical parts of the first embodiment explained above for simplicity of explanation. There is a slide plate 103 which is fitted in guide elements 104 mounted in front and rear points of the chassis 100 for fore and aft sliding movements. From one end of the slide plate 103 near the leading end of the continuous form 20 extends upright an operating element 103a for operating a pin 77i of a paper holder 77 to be described later. Its opposite end has an arm 103b extending upright therefrom, the tip end of which is provided with a rotatably carried cam follower 103c. The leading end of the slide plate 103 in the paper feed direction is connected with a guide element 104 by means of a tension spring 31 which applies a tension load to the slide plate 103 to bias it backward, so that the cam follower 103c is brought into contact with the cam surface of the second cam 13 placed behind the arm 103b. That is, the slide plate 103 is held in position through contact of the cam follower 103c with the second cam 13 by means of the tension spring 31. Thus, the fore and aft sliding movements of the slide plate 103 are performed by rotation of the second cam 13 as in the first embodiment. The second cam 13 in this embodiment is so shaped that its contact face (cam surface) with the cam follower 103c varies from a smaller-diameter portion to a larger-diameter portion in its rotational direction. It then returns to its small-diameter portion after the larger diameter portion continues through a certain angle. The drive means (DC motor) for rotatably driving the second cam 13 also serves to withdraw and to restore the roller 81B of the fixing unit 8. The operating member 103a of the slide plate 103 is located downstream, slightly behind the pivot center 74 on which the arm 72 of the transfer unit 7 is rockably mounted on the bracket 101 of the chassis 100, so that the operating member 103a is moved forward by a certain degree upon sliding movement of the slide plate 103. The transfer unit 7 is at its both ends carried by the arm 72 which is pivotally mounted on the bracket 101 upright by extending from the chassis 100 for pivotal movement about the the pivot center 74, as in the first embodiment. It is positioned at a certain distance from the drum 1 in parallel therewith. The corona charger 71 is loaded toward its transfer operative position by means of a spring 75 interposed between the arm 72 and the chassis 100 as a transfer charger biasing means. Forward of the position of the arm 72 at which it carries the corona charger 71, there is a paper pressure guide element 78 provided which, together with a roller 77d of the paper holder 77 to be described later, bends the feed path of the continuous form 20 to bring it into close contact with the transfer position of the drum 1 when the corona charger 71 is in its transfer operative position. A rib-shaped guide member 78a which extends forward of the paper pressure guide element 78 and is placed on the coroner charger 71, prevents the leading end of the continuous form 20 from being pulled into the corona charger 71 during loading. A brush 79 made of a conductive flexible material can be brought into contact with the continuous form 20 after image transfer to electrically discharge the continuous form 20. The paper holder 77 is pivotally mounted on the same pivot 74 as the arm 72. The paper holder 77 comprises a plurality of rollers 77d which are rotatably mounted on a shaft 77c connected between roller holders 77b which are provided by upwardly bending both ends of a paper holder bracket 77a. It is pivotally mounted on the pivot center 74 on the bracket 101 by means of a bent attachment portion 77e adjacent the roller holders 77b for pivotal movement independent of the arm 72. The reason why the paper holder 77 is mounted on the same pivot center 74 as the arm 72 (corona charger 71) for pivotal movement independent of each other is that the paper holder 77 (roller 77B) allows the feed path of the continuous form 20 to be bent for close contact of the continuous form 20 with the drum 1 in its transfer position for a maximum transfer effect. The paper holder 77 must be sufficiently withdrawn, to remove, on one hand, the continuous form 20 from its transfer position in the print stand-by state (when the corona charger 71 is withdrawn), and on the other hand, not to be in the way of the continuous form 20 during its loading. However, sufficient withdrawal is not attained because of the difference between the distances from the pivot center 74 if the paper holder 77 is fixedly mounted on the arm 72 for withdrawal together with the corona charger 72. The paper holder bracket 77a is located below the arm 72. The leading ends of the roller holders 77b extend above the arm 72 through a narrower portion formed by partly cutting away its end and through a cutaway of the arm 72 to thereby retain the roller holders 77d in position. Between the paper holder bracket 77a and the chassis 100 there is a compression spring 77f as a pressure guide biasing means so that the paper holder 77 is pivotally biased upward by means of the compression spring 77f. A contact portion 77g between the roller holders 77b of the paper holder bracket 77a and the attachment portion 77e thus abut against a stopper 101a which is formed by inwardly bending the upper end of the bracket 101. The rollers 77d are thus positioned forward and upward of the transfer position of the drum 1 by certain amounts. The paper holder bracket 77a is provided with a bent-down lever 77h, on one side of which a pin 77i horizontally protrudes. When the slide plate 103 is moved in the feeding direction of the continuous form 20 by means of the second cam 13, the operating member 103a is brought into contact with the pin 77i to actuate the same, so that the paper holder 77 is pivoted to move the rollers 77d down. On the side of the lever 77h there is a hook 106 as a pressure guide lock means, which is carried by a bracket 107 mounted on the chassis 100 for pivotal movement in a horizontal direction, as shown in Fig. 9 equivalent to the plan view of Fig. 8. The hook 106 is provided at one end with a hook-shaped contact portion 106a opposite to the lever 77h. The hook 106 is at the other end rotatably mounted on the bracket 107. The back side of the contact portion 106a has an upright-bent rib 106b. Engaged with an upward opening cutaway formed at the rib 106b is the tip end of a rod 108a of a solenoid 108 as a lock releasing means fixedly mounted on the chassis 100. Between the front end face of the solenoid 108 and the rib 106b of the hook 106 a coil spring 108b surrounds the rod 108a, which normally biases the hook 106 to move the contact portion 106a thereof toward the lever 77h. When the solenoid 108 is energized, the hook 106 is rotated away from the lever 77h against the biasing force of the coil spring 108b. When the mechanism is ready to print (when the corona charger 71 is in its transfer operative position), rotation of the hook 106 by the biasing force of the coil spring 108b is prevented by the tip end of the contact portion 106a contacting the lever 77h. During printing, the cam follower 103c of the slide plate 103 contacts the small-diameter portion of the second cam 13. At this time, the slide plate 103 is moved back by the tension spring 31 so that the operating element 103a is separated from the pin 77i of the paper holder 77 to keep it in its inoperative position. Thus, the paper holder 77, the corona charger 71 and the arm 72 are held in their operative positions by the biasing force of the springs 75, 77f. In other words, the corona charger 71 takes its transfer operative position with respect to the drum 1, while at the same time, the continuous form 20 is brought into contact with the drum 1 in a position just opposite the corona charger 71 (transfer position) by means of the paper pressure guide element 78 which is placed rearward of the corona charger 71 and the roller 77d of the paper holder 77 located forward thereof. In a print stand-by state, the transfer unit 7 and the paper holder 77 are withdrawn from the positions stated above upon rotation of the second cam 13. With rotation of the second cam 13, the slide plate 103 is moved forward so that the operating element 103a engages with the pin 77i of the lever 77h for forward actuation thereof. As a result, the paper holder 77 first starts pivotal withdrawing movement by itself, and after moving through a certain angle (5 degrees in this embodiment), the upper end 77j of the narrow portion cutaway of the roller holder 77b of the paper holder bracket 77a is brought into contact with the arm 72. Thereafter, the arm 72 is pushed by the paper holder 77 for common pivotal movement, so that the corona charger 71 begins to withdraw. When the lever 77h is further pivoted to have its rear face positioned forward of the contact portion 106a of the hook 106, the hook 106 is rotated by the biasing force of the coil spring 108b. The contact portion 106a is then moved into the moving area of the lever 77h for contact therewith. This prevents the paper holder 77 and the arm 72 from rocking back to their operative positions so that they maintain the withdrawn states (shown in Fig. 10(A)). The pivoting angles of the paper holder 77 and the arm 72 during withdrawing movements are 15 and 10 degrees, respectively. When starting printing, the cam 13 is rotated in advance so that it contacts the smaller-diameter portion of the cam follower 103c of the slide plate 103. When the solenoid 108 is energized to rotate the hook 106 away from the lever 106 to release the contact, the paper holder 77 and the arm 72 are instantly moved back to their operative positions because of the biasing force of the springs 75, 77f loading them toward the operative positions. Movement of the transfer unit 7 (corona charger 71) and the paper holder 77 back to their operative positions is now described with reference to the time chart shown in Fig. 11. In this embodiment, feeding the continuous form 20 is carried out by the fixing unit 8, but start and stop of rotation of the roller 81A are performed by ON/OFF change of the clutch, not shown, interposed between the roller 81A and the drive means. As has been described above, drive means for rotatably driving the second cam 31 also drive the cam 83 for withdrawing and restoring the roller 81B in the fixing unit 8. A sensor fixedly mounted on the chassis 100 checks a member attached to the drive shaft 11 to detect restoration of the roller 81B as well as the position at which the second cam 31 contacts the cam follower 103c of the slide plate 103. When a certain period passes after starting rotation of the drum 1, the second cam 31 begins to rotate and is brought to a stop when a signal from the sensor for detecting its rotation indicates that it has rotated to a predetermined position. This stop position detected by the sensor is a position where the roller 81B in the fixing unit 8 is returned to its fixing operative position and the second cam 31 contacts the cam follower 103c of the slide plate 103 at its smaller-diameter portion. Exposure starts at a certain time after starting rotation of the second cam 31, and pulling the continuous form 20 starts at a certain time after start of the exposure through the clutch in the fixing unit 8. Furthermore, the solenoid 108 is energized at a certain time after the start of pulling of the continuous form 20 ("t" seconds after start of the exposure: determined by the circumferential speed of the drum 1 and the circumferential distance between the exposure unit 5 and the transfer unit 7), whereby the transfer unit 7 (corona charger 71) and the paper holder 77 are restored to their operative positions while at the same time the corona charger 71 is energized. As described above, withdrawal the continuous form from the drum at the transfer unit and from the heat roller in the print stand-by state can be accomplished by a single driving means, whereby it is possible to provide a low cost printer using a continuous form of recording paper that is able to prevent damage and wear of the drum as well as problems associated with heat applied to the continuous form of recording paper. Also, since the transfer charger is loaded towards its transfer operative position, while the transfer charger is locked in its withdrawn position and the transfer charger is restored to its transfer operative position by releasing the lock of by means of the lock release means, restoring the transfer charger with start of printing can be accomplished at a correct timing. Furthermore, the pressure guide is rockably mounted on the same pivotal axis as the arm member for pivotal movement independent of the arm member, wherein withdrawing movement of the pressure guide causes the arm member to be withdrawn, so that the pressure guide is sufficiently withdrawn during loading of the continuous form of recording paper. Figs. 12 through 17 show another embodiment of the invention wherein the paper dust brush 41 is also withdrawn from its operative portion during the print stand-by state. Like reference numerals indicate like and identical parts corresponding to those in the first embodimant. As shown in Figs. 12 and 13, the lower dust brush 42 is so mounted on the chassis 100 that its shaft 42A is rotatably carried by a bracket 104 extending upright from the chassis 100. The paper dust brush 42 is rotatably driven by rotary means, not shown, in the direction opposite to the feed direction of the continuous form 20 as stated before. The upper paper dust brush 41 has its shaft 41A at its rear end rotatably retained by right and left arms 43 pivotally mounted on the bracket 104. The other end of the arm 43 is connected with the chassis 100 by means of a spring 44 as shown in Fig. 14. The arm 43 is thus loaded by the spring 44 to be pivotally moved downward (toward the lower paper dust brush 42). The arm shaft 104A has a pivot axis by which the arm 43 is pivotally mounted on the chassis 100. It has one end fixedly carrying a lever 45, which is not rotatable and forms a certain angle between them. Below the lever 45 is a lever operating arm 46 whose one end is secured to a connecting shaft 105A rotatably mounted on a bracket 105 fixed to the chassis 100. Rocking movement of the lever 45 is restricted by contact with the top surface of a lever operating arm 46 by means of the spring 44. Therefore, the position of the lever 45, i.e., the position of the paper dust brush 41 carried by the arm 43 is determined by the position of the lever operating arm 46 (the angle through which it swings about the connecting shaft 105a). The other end of the connecting shaft 105a has a crank-shaped follower arm 105b. The tip end of the follower arm 105b is then fitted in a cam slot 103c opening to an upright portion of the slide plate 103 which is pivotally moved by the second cam 13 secured to the chassis 100 in fore and aft directions in the laser printer 10. The cam slot 103c continues on the slide plate 103 in its sliding direction with a positional change. As the slide plate 103 slides, the tip end of the follower arm 105b moves up and down accordingly. This up and down movements allow the paper dust brush 41 to be pivotally moved up and down by means of the connecting shaft 105a, the lever operating arm 46 and the lever 45. When the paper dust brush 41 is in its lower position (as shown in Fig. 14), it grips the continuous form 20 with the paper dust brush 42 to sweep the surface of the continuous form 20 with the paper dust brushes 41 and 42 removing the dust particles deposited thereon. When it is moved up (as shown in Fig. 15), the paper dust brush 41 is completely separated from the surface of the continuous form 20. Rotational movement of the paper dust brush 41 is performed through meshing of a gear, not shown, which is secured to one end of the shaft 41A with the same drive power source which rotatably drives the paper dust brush 42. Thus, as the paper dust brush 41 is pivotally swung upward, the gear comes out of engagement to stop transmission of the rotational force, thereby stopping rotation of the paper dust brush 41. Furthermore, the shaft 11 having cams 83 and 13 thereon is provided with first and second sensor plates 51 and 52 both in disc-shape which are secured to one end projecting outward of the bracket 102. Sensors 61 and 62 each consists of a so-called photo-interrupter which has a light emitting portion and a light receiving portion placed opposite to each other at the ends of the legs of an inverted-C shaped frame. They detect a slot formed at a certain position of the sensor plates 51 and 52 and generate detection signals. Since the sensor plates 51 and 52 are secured to the shaft 11, their rotations are in complete synchronism with that of the cams 83 and 13 secured to the shaft 11. This makes it possible to detect the operating state of the structural parts (the paper dust brush pair 40, the transfer unit 7 and the fixing unit 8 driven by the cam 83 and the second cam 13 when the sensors 61 and 62 detect the slots formed at an arbitrary position of each sensor plate 51 and 52. In one sensor plate 51, the slot is so formed that it generates a signal immediately after the backup roller 81B starts moving up (when the cam 83 and the second cam 13 rotate at an angle of 45 degrees), and when the structural parts (the paper dust brush 40, the transfer unit 7 and the fixing unit 8) driven by the cam 83 and the second cam 13 are all in their operative positions (when the rotary angle of the shaft 11, i.e., that of the cam 83 and the second cam 13 is in the range of 235 to 265 degrees). In the second sensor 52, on the other hand, the slot is so formed that it generates a signal when the backup roller 81 is entirely in its withdrawn position (when the rotary angles of the cam 83 and of the second cam 13 are in the ranges from 0 to 20 degrees and from 340 to 0 degrees). With the configuration of the laser printer as described above, in the print stand-by state the gear motor 10 is driven to withdraw the backup roller 81B of the fixing roller pair 81 so as to separate the continuous form 20 from the heat roller 81A, while also withdrawing the corona charger 71 of the transfer unit 7 from its transfer operating position. Also, the upper paper dust brush 41 of the paper dust brush pair 40 is moved to its withdrawn position. The operational timing of the structural parts will be described with reference to Fig. 17. In the drawing, the ordinate indicates the rotary angle of the shaft 11 (the cam 83 and the second cam 13). When a command for start of printing is received from the control unit for the whole system, not shown, rotation of the motor 10 begins so that the cam 83 and the second cam 13 are rotated with the shaft 11. Around this time, a main motor for rotating the drum 1 also starts driving. When the shaft 11 is rotated by the motor 10, the backup roller 81B of the fixing roller pair 81 operatively driven by the cam 83 begins to move upward from its withdrawn position. At this time, the second sensor 62 sensing the second sensor plate 52 generates a signal. When the rotary angle of the shaft 11 (cam 83) reaches approximately 20 degrees, this signal disappears to indicate that the backup roller 81B begins to move upward from its withdrawn position. With further rotation of the shaft 11 (cam 83) to reach its rotary angle of 45 degrees, the sensor 61 sensing the first sensor plate 51 generates a signal. Receipt of this signal starts the exposure of the drum 1 to the laser beam from the scanning optical system 5. At a certain time following this signal (1.73 seconds), the clutch for rotatably driving the heat roller 81A of the fixing unit 8 is brought into the ON-state. This time of 0.73 seconds after start of exposure is a period in which the position where the surface of the drum 1 exposed to the laser beam reaches the transfer position (transfer unit 7). As soon as the rotary angle of the shaft 11 (cam 83) reaches approximately 190 degrees, the backup roller 81B is pushed against the heat roller 81A into its fixing operative position. In approximate synchronism with this, the corona charger 71 of the transfer unit 7 begins to pivot upward. The corona charger 71 reaches its transfer operative position when the rotary angle of the cam 83 becomes approximately 215 degrees. With the rotary angle of the shaft 11 at about 210 degrees, the upper paper dust brush 41 of the paper dust brush pair 40, driven by the slide plate 103 which is moved by the second cam 13, begins to rock downward, and the paper dust brush 41 reaches its predetermined position (cleaning operative position) when the rotary angle of the shaft 11 becomes 235 degrees. That is, when the rotary angle of the shaft 11 becomes 235 degrees, all the structural parts (the paper dust brush pair 40, the transfer unit 7 and the fixing unit 8) driven by the cam 83 and the second cam 13 are in their operative positions. This state lasts until the rotary angle of the shaft 11 becomes 265 degrees. The rotary angle of the shaft 11 reaches 250 degrees which is about the middle of the range in which all the structural parts are in their operative positions. This time corresponds to the time of 0.73 seconds after a signal output from the first sensor 61, which in turn corresponds to the time at which the rotary angle of the shaft 11 is 45 degrees. At this time, the clutch for rotatably driving the heat roller 81A of the fixing unit 8 is brought into its ON-state, so that the heat roller 81A begins to rotate to start pulling the continuous form 20 into a nip between the fixing rollers 81. That is, the exposure start position on the surface of the drum 1 reaches the transfer unit 7 via the developing unit 6, while at the same time the continuous form 20 is fed to effect printing. The motor 10 is kept inoperative during printing. When printing is complete, at the time of 0.73 seconds after finishing exposure of the drum 1 to the laser beam from the optical scanning system 5, the clutch for rotatably driving the heat roller 81A of the fixing unit 8 is brought into its OFF-state, so that pulling of the continuous form 20 by means of the fixing roller pair 81 is stopped. Rotation of the motor 10 is restored in advance, so that the rotary angle of the shaft 11 reaches 265 degrees when the continuous form 20 stops. With rotation of the shaft 11, the structural parts are brought into their withdrawn positions in the reverse order. That is, when the rotary angle of the shaft 11 reaches 265 degrees, the upper paper dust brush 41 of the paper dust brush pair 40 starts its pivotal upward movement (withdrawal). At 290 degrees rotary angle; the corona charger 71 of the transfer unit 7 starts withdrawing movement, and further at about 310 degrees rotary angle, the backup roller 81B of the fixing roller pair 81 starts withdrawing movement. When the rotary angle reaches about 340 degrees, a signal is generated by the second sensor 62 to indicate the withdrawal of the backup roller 81B. At about the same time as the withdrawing movement of the backup roller 81B, rotational movement of the drum 1 by the main motor is stopped. At a rotary angle of 360 degrees, the motor 10 stops to be ready for restart of printing. Figs. 18 through 23 show a modification of the embodiment stated above with reference to Fig. 12 through 17. Like reference numerals indicate like and identical parts in the first embodiment for simplicity of explanation. As shown in Fig. 18, the fixing unit 8 includes a shaft 88, on which the cam 83, the second cam 13, the first sensor plate 51 and the second sensor plate 52 are fixedly mounted in their predetermined positions. The shaft 88 is rotatably carried by side plates 201 extending upright from both sides of a unit chassis 200. The shaft 88 is rotatably driven by the motor 10 which is carried by a motor bracket 202 extending at right angle to the shaft 88 near one side of the unit chassis 200. There are provided two cams 83 swinging the backup roller 81B of the fixing unit 8, which are generally of the same shape and mounted near both ends of the shaft 88 for stable swinging movement of the backup roller 81B at its both ends. The second cam 13, the first sensor plate 51 and the second sensor plate 52 are arranged in a row approximately in the center of the shaft 88. Behind the position where the first and second sensor plates 51 and 52 are located in the unit chassis 200 there is a vertically extending sensor holder 203 which is provided with the first sensor 61 and the second sensor 62 in the positions corresponding to the first sensor plate 51 and the second sensor plate 52, respectively. Around the center of the shaft 88 beyond the second sensor plate 52 and the second sensor 62 there is a recording paper detecting mechanism 300 whose sensor 301 is also carried by the sensor holder 203 together with the first sensor 61 and the second sensor 62 side by side. The motor 10 is a flange-type pulse motor which is fixedly carried by the motor bracket 202 as mentioned above. The motor 10 has its spindle 10A passing through and projected from the motor bracket 202. On the spindle 10A a gear 10B is mounted which meshes with a gear 12 fixedly mounted on the shaft 88 in its corresponding position. The shaft 88 is thus rotatably driven by the motor 10. Below the portion of the shaft 88 carrying the second cam 13 an L-shaped lever 204, as shown in Figs. 19 and 20, is pivotally carried at its apex point by means of the unit chassis 200 for rocking movement in fore and aft directions. The lever 204 has a cam follower 205 rotatably mounted on the leading end of the substantially horizontally extending lower arm 204A. The leading end of the generally vertically extending upper arm 204B is pivotally linked with the upper end of an upright arm 103d of the slide plate 103 mounted on the chassis 100 for fore and aft movements. The cam follower 205 is located in a position corresponding to the second cam 13 and the slide plate 103 is urged forward by means of the spring 31. This allows the lever 204 to be pushed and to bring the cam follower 205 into contact with the outer circumferential surface of the second cam 205. Thus, displacement of the second cam 13 causes fore and aft sliding movements of the slide plate 103. At its front end the slide plate 103 receives one end of a lever 107 carried by the bracket 106 secured to the chassis 100 for horizontal pivotal movement (rocking movement about a vertical axis). The lever 107 is thus rockably moved by sliding movement of the slide plate 103. The other end of the lever 107 receives an engagement projection 108A projecting at right angle from the shaft 108 at one end thereof, which shaft 108 is rotatably carried by the bracket 106 carrying the lever 107 for rotation in horizontal right and left directions. The shaft 108 is thus rotatably moved by rocking movement of the lever 107. The other end of the shaft 108 is provided with an arm 108B extending forward. The leading end of the arm 108B is thus swung up and down by rotation of the shaft 108. That is, the leading end of the arm 108B is swung up and down by fore and aft movements of the slide plate 103. The leading end of the arm 108B is located opposite to the lateral end of the paper dust brush pair 40. As shown in Fig. 22, the paper dust brush pair 40 has a box-shaped lower holder 43 made of synthetic resin and an upper holder 44 rockably carried by the lower holder 43, while the paper dust brush 41 is rotatably carried by the upper holder 44. At the end of the upper holder 44 opposite to the leading end of the arm 108B there is an operating projection 44A laterally projecting therefrom. An operating portion 108C at the leading end of the arm 108B can contact the arc-shaped lower surface of the operating projection 44A. Thus, when the arm 108B is swung upward, the operating portion 108C at its leading end pushes the operating projection 44A up so that the upper holder 44 (paper dust brush 41) is swung upward toward its withdrawn position. The upper holder 44 is at its either end formed integrally with a J-shaped resilient section 44B. The upper surface of the resilient section 44B is adapted to contact with the chassis frame. This urges the upper holder 44 toward the lower holder 43 with its restoration from resilient deformation to bring the paper dust brush 41 into its operative position. On the other hand, the upper holder 44 is swung upward against the resilient load of the resilient section 44B, whereby the paper dust brush 41 can be brought into its withdrawn position. The transfer unit 7 is rockably moved by sliding movement of the slide plate 103. The corona charger 71 is retained by the arm 72 pivotally carried by the shaft 74 which passes through brackets vertically extending from the right and left sides of the chassis 100 at a certain distance from each other. At the end of the arm 72 opposite to the corona charger carrying portion across its pivot axis is the tension spring 75 with its lower end retained by an upright bracket 105 secured to the chassis 100. The arm 72 is loaded by the tension spring 75 to allow the corona charger 71 carried by it to be brought to its transfer operative position with respect to the drum 1. Under the pivot axis of the shaft 74 of the arm 72 a horizontally extending pin 73 is mounted which is operated by the vertical extending end face of the operating portion 103A on the slide plate 103 to pivot the arm 72 against the tension force of the tension spring 75. With this rocking movement of the arm 72 by means of the operating portion 103A of the slide plate 103, the corona charger 71 is swung by a predetermined amount from the transfer operating position to its withdrawn position. Forward of the position of the arm 72 where it carries the corona charger 71 a paper holder 72A is supported in parallel with the corona charger 71. The paper holder 72A presses the continuous form 20 against the image transfer position of the drum 1. The shaft 74 rockably carrying the arm 72 also has a roller arm 76 rockably mounted thereon, though not shown in detail. The roller arm 76 has a paper holder 76A rotatably mounted on the forward end thereof, with its near end formed with an operating portion 76B. The roller arm 76 is biased by a torsion spring 77 wound around the shaft 74 so that the paper holder 76A is pivotally urged toward the corona charger 71, whereby the paper holder 76A and the corona charger 71 are held in predetermined relative positions. The swinging torque of the roller arm 76 exerted by the torsion spring 77 is set to be smaller than the swinging torque of the arm 72 generated by the tension spring 75. That is, the roller arm 76 and the arm 72 become an integral unit in their certain positional relationship and are pivotable about the shaft 88 as such an integral unit. Therefore, when the arm 72 is swung by the operating portion 103A of the slide plate 103, the roller arm 76 kept integral therewith keeps its position relative to the arm 72. When the operating portion 76B at the rear end of the roller arm 76 is pushed downward in Fig. 23, however, the roller arm 76 alone is swung overcoming the biasing force of the torsion spring 77, so that the paper holder 76A is moved upwardly away from the corona charger 71 (their relative positions are changed). With the corona charger 71 being in its transfer operative position, the roller arm 76 is in a position where its paper holder 76A is located closer to the inlet of the continuous form 20 and to the drum 1 than the corona charger 71. At this time, the continuous form 20 is passed through a space between the paper holder 76A and the corona charger 71 so that it is brought into contact with the transfer section on the drum 1 opposite to the corona charger 71. In a print stand-by state, the sliding movement of the slide plate 103 causes the arm 72 to pivot so that the corona charger 71 is moved into its withdrawn position. As a result, the continuous form 20 takes a lower electrical charge even with a voltage sustained. At the same time, it is pulled down by the paper holder 76A to be separated from the drum 1, thereby disabling image transfer. In a position corresponding to the operating portion 76B of the roller arm 76, a push lever 78A is fixedly mounted on the shaft 78 which is located in front of the shaft 74. The shaft 78 is rotatably driven in association with opening and closing movements of the upper structural part 11 of the laser printer 10 by means of an interlock mechanism. Rotation of the shaft 78 allows the push lever 78A to apply a pressure to the operating portion 76B of the roller arm 76. Upon opening of the upper structural part 11, the paper holder 76A is swung upward with the corona charger 71 remaining in its withdrawn position. This makes a greater distance between the drum 1 and the paper holder 76A and the corona charger 71, thereby facilitating loading of the continuous form 20. Thus, according to this embodiment, as in the foregoing embodiments, the backup roller 81B of the fixing unit 8 is withdrawn by driving the motor 10 to move the continuous form 20 away from the heat roller 81A in the print stand-by state. At the same time, the corona charger 71 of the transfer unit 7 is moved into its withdrawn position through sliding movement of the slide plate 103, while also moving the upper paper drust brush 41 of the paper dust brush pair 40 upward into its withdrawn position. As described above, in a printer embodying the invention, withdrawing the continuous recording paper from the drum at the transfer unit and from the heat roller and withdrawing the paper dust brush in the print stand-by state can be accomplished by a single drive means, whereby it is possible to prevent damage and wear of the drum as well as problems associated with heat applied to the continuous recording paper, while also avoiding positional displacement upon withdrawal of the recording paper. This makes it possible to start printing at a correct position. Figs. 24 through 28 show a further embodiment of the invention, wherein the transfer unit 7 includes arms 72 arranged at a certain distance from the drum 1 parallel therewith for supporting the corona charger 71. The arm 72 is at each end thereof rockably mounted on a shaft 74 which passes through brackets 101 and 102 appropriately spaced from each other and extending upright from the right and left sides of the chassis 100. The arm 72 bears at its end opposite to the corona charger mounting portion across the pivot center the upper end of a tension spring 75 whose lower end is retained by the bracket 105 fixed at its lower end to the chassis 100. The tension spring 75 biases the arms 72 to bring the corona charger 71 retained by it to its transfer operating position with respect to the drum 1. This rocking movement is restrained by contact of a pin 72A provided on a side of the arm 72 with the top end surface of the bracket 101. This position where rocking movement is restrained is the transfer operative position of the corona charger 71. Below the pivot center on which the arm 72 is carried by the shaft 74 a horizontal pin 73 is adapted to be actuated by the vertical end face of an operating member 103A extending upright from the slide plate 103 which is mounted on the chassis 100 for sliding movement in fore and aft directions in the laser printer 10. With this operation of the pin 73, the arm 72 is rockably moved overcoming the tension force of the tension spring 75. With this rocking movement of the arm 72 with the operating member 103A of the slide plate 103, the corona charger 71 is separated from the drum 1 to effect a certain degree of rocking movement from its transfer operative position to its withdrawn position. On the shaft 74 rockably carrying the arm 72 a roller arm 76 is pivotably mounted as shown in Fig. 26, the III-III section of Fig. 25(A). The roller arm 76 rotatably carries at its leading end (closer to the continuous form inlet than the pivotal center of the shaft 74) a plurality of paper holder rollers 76A arranged in series over the entire width of the continuous form 20. The rear end of the roller arm 76 is provided with an operating portion 76B. The roller arm 76 is biased by a torsion spring 77 surrounding the shaft 74 so that the paper holder rollers 76A are moved toward the corona charger 71 (in the direction indicated by the arrow in Fig. 26.) Their relative pivotal movements are restrained by contact of a stopper contact portion 76C formed adjacent the pivotal point of the shaft 74 with a stopper 72B formed by bending the corresponding portion of the arm 72. The relative position of the roller arm 76 and the arm 72 is thus determined by this restricted pivoting relative to each other. Here, the swing torque of the roller arm 76 due to bias of the torsion spring 77 is chosen to be smaller than the swing torque of the arm 72 caused by the tension spring 75. Thus, the roller arm 76 and the arm 72 are biased by the torsion spring 77 and bring the paper holder rollers 76A and the corona charger 71 into their predetermined positions as a single unit. In this single unit state, they are pivotable about the shaft 74. When the arm 72 is moved by the operating member 103A of the slide plate 103, therefore, the roller arm 76 also effects pivoting integrally with the arm 72. If, however, the operating portion 76B at the rear end of the roller arm 76 is pushed down, the roller arm 76 alone is moved against the bias of the torsion spring 77 so that the paper holder rollers 76A are moved upward away from the corona charger 71 (i.e., their relative positions change accordingly). When the corona charger 71 is in its transfer operative position, the roller arm 76 is so positioned that its paper holder rollers 76A are located closer to the continuous form inlet than the corona charger 71 while approaching the drum 1. At this time, the continuous form 20 is passed between the paper holder rollers 76A and the corona charger 71 and is brought to the transfer portion of the drum 1 opposite the corona charger 71. In the print stand-by state, the corona charger 71 is moved into its withdrawn position by rocking movement of the arm 72 through slide movement of the slide plate 103. The electric charge on the continuous form 20 is thus reduced with the voltage maintained at the corona charger 71. The form 20 is at the same time pulled down by the paper holder rollers 76A to separate it from the drum 1 to disable the image transfer operation. Ahead of the shaft 74 in the paper feed direction there is a shaft 78 which extends at a right angle to the paper feed direction and serves as a pivot axis of the arm 72 and the roller arm 76. The shaft 78 is rotatably born between one bracket 102 extending upright from the chassis 100 and an other bracket 104 extending upright from the side opposite to the bracket 102. The portion of the shaft 78 which is opposite the operating member 76B of the roller arm 76 is provided with a pressure lever 78A secured thereto. On the outer side of the bracket 104 a push-up guide 104A as is provided shown in Fig. 28. The push-up guide 104A holds a push-up shaft 79 slidably which extends upright therefrom. A spring 79B is held between a pin 79A which is passed through and secured to the push-up shaft 79, and a bottom plate of the push-up guide 104A. The push-up shaft 79 is biased to project upwardly by means of the resilient restoration force of the spring 79B. All these elements constitute the push-up mechanism. The position where the push-up shaft 79 is located corresponds to the position of a push-up contact portion formed at the underside of the drum unit 13. The bias of the spring 79B for pushing up the push-up shaft 79 is chosen to be capable of pushing up the drum unit 13. Thus, with no pressure force exerted to the drum unit 13 from above (e.g. by means of the clam shell 12), the top end of the push-up shaft 79 is brought into contact with the push-up contact portion of the drum unit 13 to push it up towards a floating position. The pin 79A passed through and secured to the push-up shaft 79 is in engagement with an operating lever 78B internally extending through and secured to the shaft 78 as shown in Fig. 25(B) which is a section taken along the line B-B of Fig. 25(A). With up and down movements of the push-up shaft 79, the operating lever 78B is thus rockably moved to rotate the shaft 78. As the shaft 78 is thus rotated, the pressure lever 78A is swung to push the operating member 76B of the roller arm 76. When the clam shell 12 is now closed so that the drum unit 13 is locked in position under pressure by the clam shell 12, and the push-up shaft 73 is in its pushed-down position by means of the drum unit 13, there is a sufficient distance between the pressure lever 78A and the operating member 76B of the roller arm 76 as shown in Fig. 26. With the configuration of the laser beam printer 10 as described above, in the print stand-by state, the backup roller 81B of the fixing roller pair 81 is withdrawn to separate the continuous form 20 from the heat roller 81A, while also withdrawing the corona charger 71 of the transfer unit 7 from its transfer operating position. The continuous form 20 is also separated from the drum 1 by means of the paper holder rollers 76A. This prevents not only the troubles associated with continuous application of heat from the heat roller 81A to the same portion of the continuous form 20, but also the troubles arising from the contact of the continuous form 20 with the drum 1 even if only the drum 1 starts rotation while the continuous form 20 stands still at the start (re-start) of printing. At times other than printing, the transfer unit 7 and the fixing unit 8 are kept in their withdrawn positions so that loading the continuous form 20 takes place in this condition. When the clam shell 12 lock is released to open it for loading the continuous form 20, the drum unit 13 is relieved from the downward pressure so that it is pushed up by the push-up shaft 79 to its floating position. With this upward sliding movement of the push-up shaft 79, the shaft 78 is rotated by means of the operating lever 78B so that the pressure lever 78A is swung to operate the operating member 76B of the roller arm 76 under pressure. As a result, the roller arm 76 is rockably moved to allow the paper holder rollers 76A to move up. Thus, when the clam shell 12 is opened with the corona charger 71 in its withdrawn position, the drum unit 13 is brought into its floating position by means of the push-up shaft guide 79 as shown in Fig. 28. With the corona charger 71 maintained in its withdrawn position, the paper holder rollers 76A alone are swung upward (the paper holder rollers 76A take the same positions as in transfer operation with this swinging movement) so that a considerable spacing exists between the drum 1 and the paper holder rollers 76A and the corona charger 71 for easier loading of the continuous form 20. As has been described above, according to this invention, the transfer charger is withdrawn and the recording paper is separated from the drum by means of the paper holder element to prevent any possible troubles. In loading recording paper, the paper holder element alone is pivoted to provide a spacing from the transfer charger, thereby ensuring easier loading of recording paper and therefore an improved operability.

Claims: A printer arranged to electrophotographically form an image on a continuous-form recording paper (20), which printer comprises: a pair of fixing rollers (81a,81b), at least one (81b) of which is arranged to be withdrawn from its fixing operative position in the print stand-by state; a transfer charger (71) arranged to be withdrawn from its transfer operative position in the print stand-by state; and a common drive means (10,13,103) for executing withdrawing and restoring movements of said one roller (81b) of said fixing roller pair (81a,81b) and of said transfer charger (71). The printer acording to claim 1, further comprising: transfer charger load means (72,75) for loading said transfer charger (71) to its transfer operative position; transfer charger lock means (106,77h) for fixedly locking said transfer charger (71) in its withdrawn position; and lock release means (108) for releasing said transfer charger lock means (106,77h). The printer according to claim 1 or 2, wherein said transfer charger (71) is carried by a pivotally rockable arm member (72) to allow said transfer charger (71) to move from its transfer operative position to its withdrawn position by pivotal movement of said arm member (72), paper holder rollers (77d) are rockably mounted on the pivotal center (74) of the arm member (72) for pivotal movement relative to said arm member (72), said paper holder rollers (77d) flexing the paper feed path toward a photoconductive drum (1) after the transfer charger (71), said arm member (72) being adapted to be pivotally driven for its withdrawing movement within a certain range of the pivotal movement of said rollers (77d); and said printer further comprises a common drive means (13,103) for executing the pivotal movements of said withdrawable roller (81b) of the fixing roller pair (81a,81b) and of said paper holder rollers (77d). The printer according to claim 3, further comprising: means for biasing said paper holder rollers (77d) to a predetermined position; transfer charger loading means for biasing said transfer charger to its transfer operative position; lock means for fixedly locking said paper holder rollers (77d) in their withdrawn position; and lock release means for releasing said lock means. A printer according to anyone of claims 1 to 4, comprising: a dust brush means (41) provided adjacent the inlet of the continuous-form recording paper (20), said dust brush means (41) being withdrawable from its position contacting the continuous-form recording paper (20); and said common drive means (13,103,105) also withdrawing and restoring said dust brush means (41) at predetermined timings. A printer according to anyone of claims 3 to 5, wherein said paper holder rollers (77d) are provided with an operating member (103a) for effecting their pivotal movement independently of said transfer charger (71). The printer according to claim 6, further comprising a push-up mechanism (78,79) for loading the photoconducive drum (1) located above the transfer charger (71) in an upward direction, and an operating member (78B) operatively associated with said push-up mechanism (78,79) and said paper holder rollers (76a) for rocking said paper holder rollers (76a) upon push-up movement of the photoconductive drum (1) by means of said push-up mechanism (78,79). The printer according to claim 7, which comprises a lower structural part (11) and an upper structural part (12) adapted to rockably open relative to said lower structural part (11), said upper structural part (12) pressing the photoconductive drum (1) against said lower structural part (11) to lock it in a predetermined position upon closure of said upper structural part (12), while upon opening of said upper structural part (12), the pressure exerted by said upper structural part (12) is released to allow said drum (1) to be pushed up by a predetermined amount by said push-up mechanism (78,79).

Inventors: CHIKAMA ATSUSHI C O, NEGISHI KIYOSHI C O, NEGORO IKUO C O, TAKANO MASATOSHI C O, CHIKAMA, ATSUSHI C/O, NEGISHI, KIYOSHI C/O, NEGORO, IKUO  C/O, TAKANO, MASATOSHI C/O, Negoro, Ikuo c/o
Applicants: ASAHI OPTICAL CO LTD, ASAHI KOGAKU KOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: G03G15
IPC codes: G03G15