Patent Publication Number: US-6711373-B2

Title: Printer and transfer unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printer for performing electrophotographic image printing on a recording paper. The present invention also relates to a transfer unit to be incorporated in such a printer. 
     2. Description of the Related Art 
     Conventionally, an electrophotographic printer is provided with a photosensitive member having a surface for forming an electrostatic latent image, a developer unit for developing the electrostatic latent image into a toner image, a transfer unit for transferring the toner image onto a recording paper, and a fixing unit for fixing the toner image on the recording paper. FIG. 6 illustrates an example of prior art transfer unit incorporated in such a printer. 
     The transfer unit Ue shown in FIG. 6 is arranged in facing relationship to a surface  90   a  of a photosensitive drum  90 . The transfer unit Ue includes a transfer wire  93 , a discharge wire  94 , a first guide  91  and a second guide  92 . A transfer current flows through the transfer wire  93  for transferring a toner image formed on the surface  90   a  of the photosensitive drum  90  to a recording paper P. For example, while the toner image formed on the photosensitive drum  90  is negatively charged, the transfer current passing through the transfer wire  93  positively charges the recording paper P for attracting the toner to the recording paper P. On the other hand, a discharge current flows through the discharge wire  94  for releasing the electric charge on the recording paper P after the image transfer. The discharging operation by the discharge wire  94  prevents the recording paper P from adhering to various parts of the printer in the subsequent paper transport path. The first and the second guides  91 ,  92 , functioning to guide the recording paper P, may be made of an wear-resistant metal for avoiding early wearing due to contact with the recording paper P. The first and the second guides  91 ,  92  are spaced from each other in a recording paper transport direction so as not to shield the transfer wire  93  and the discharge wire  94  relative to the photosensitive drum. Specifically, the first guide  91  is arranged upstream from the transfer wire  93  in the recording paper transport direction, whereas the second guide  92  is arranged downstream from the discharge wire  94  in the recording paper transport direction. 
     In the printer including the above-described transfer unit Ue, the length La of the paper P (the length in the paper transport direction) winding around the photosensitive drum  90  should be as small as possible to obtain a high quality printed image for the following reasons. Since the recording paper P is transported by a transport mechanism (not shown) provided separately from a rotary mechanism (not shown) of the photosensitive drum  90 , there exists a slight difference between the transport speed v1 of the recording paper P and the peripheral speed v2 at the surface  90   a  of the photosensitive drum  90 . Therefore, when the winding length La is large, the recording paper P may become loose, or the drum  90  may rub against the paper P. This causes an improper toner image transfer to the recording paper P, resulting in a degradation of the printed image quality. 
     However, in the prior art transfer unit Ue, the first and the second guides  91 ,  92  are arranged at front positions which entirely avoid the transfer wire  93  and the discharge wire  94 . Therefore, the distance Lb between the first and the second guides  91 ,  92  is relatively large, which inevitably increases the winding length La of the recording paper P about the photosensitive drum  90 . As described above, such a large winding length La causes a deteriorated printed image. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a printer capable of providing a high quality printed image by reducing the winding length of a recording paper about a photosensitive member. 
     Another object of the present invention is to provide a transfer unit which may be advantageously incorporated in such a printer. 
     In accordance with a first aspect of the present invention, there is provided a printer comprises a photosensitive member having a surface for forming a toner image; a transfer member for passing a transfer current for transferring the toner image onto a recording paper, the transfer member being arranged to face said surface of the photosensitive member; a discharge member for passing a discharge current for performing discharge of the recording paper, the discharge member being arranged downstream from the transfer member in a recording paper transport direction to face said surface of the photosensitive member; a first guide arranged upstream from the transfer member in the recording paper transport direction for guiding the recording paper; and a second guide arranged downstream from the first guide in the recording paper transport direction for guiding the recording paper. The second guide includes a contact portion located between the transfer member and the discharge member in the recording paper transport direction for contact with the recording paper, and the contact portion is positioned adjacent an unshielding portion for exposing the discharge member relative to said surface of the photosensitive member. 
     The technical advantages of the printer having the above-described structure will be specifically described later with reference to preferred embodiments of the present invention. 
     In one embodiment, the second guide extends from a first position located between the transfer member and the discharge member in the recording paper transport direction to a second position located downstream from the discharge member in the recording paper transport direction, and an edge of the second guide located at the first position serves as the contact portion. In this embodiment, the unshielding portion may be provided by a single slit formed in the second guide along and adjacent said edge of the second guide. Alternatively, the unshielding portion may be provided by a row of slits formed in the second guide along and adjacent said edge of the second guide. 
     In another embodiment, the transfer member and the discharge member are separated from each other by a partition wall which also serves as the second guide, and the contact portion is provided by a forward end of the partition wall. In this embodiment, the forward end of the partition wall serving as the contact portion may be bent downstream in the recording paper transport direction. 
     According to a second aspect of the present invention, there is provided a transfer unit to be brought into contact with a recording paper, comprising a transfer member arranged to face the recording paper for passing a transfer current; a discharge member for passing a discharge current for performing discharge of the recording paper, the discharge member being arranged to face the recording paper downstream from the transfer member in a recording paper transport direction; a first guide arranged upstream from the transfer member in the recording paper transport direction for guiding the recording paper; and a second guide arranged downstream from the first guide in the recording paper transport direction for guiding the recording paper. The second guide includes a contact portion located between the transfer member and the discharge member in the recording paper transport direction for contact with the recording paper, and the contact portion is positioned adjacent an unshielding portion for exposing the discharge member relative to the recording paper. 
     Other objects, features and advantages of the present invention will become clearer from the detailed description given below with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view illustrating a printer according to a first embodiment of the present invention. 
     FIG. 2 is a sectional view illustrating the transfer unit of the printer of FIG.  1 . 
     FIG. 3 is a front view of the second guide of the transfer unit shown in FIG.  2 . 
     FIG. 4 is a front view of a second guide according to another embodiment of the present invention. 
     FIG. 5 is a sectional view illustrating a transfer unit according to a second embodiment of the present invention. 
     FIG. 6 is a sectional view illustrating a prior art transfer unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below in detain with reference to the accompanying drawings. 
     FIGS. 1-3 illustrate a printer according to a first embodiment of the present invention for printing an image on a surface of an elongated recording paper P. As shown in FIG. 1, the printer A includes a paper hopper  10 , a photosensitive drum  2 , a developer  40 , a transfer unit U, a fixing unit  41 , a stocker  11  and other parts which will be described later. 
     The paper hopper  10  stores an elongated recording paper P as repetitively and alternately folded. The recording paper P pulled out from the paper hopper  10  by a pair of tractors  50 ,  50   a  and a pair of transport rollers  50   b  is transported by a guide plate  53  and a guide roller  52   a  to a space between the photosensitive drum  2  and the transfer unit U. After passing through the space between the drum  2  and the transfer unit U, the recording paper P is transported by a plurality of guide rollers  52  and a pair of transport rollers  51  for storage in the stocker  11 . 
     The photosensitive drum  2  has a surface (circumferential surface)  2   a  on which an electrostatic latent image is formed by an electrostatic latent image forming unit  4 . The image forming unit  4  includes a cleaning brush  42 , a cleaning blade  43 , a charger  44  for uniformly charging the surface  2   a  of the photosensitive drum  2 . The image forming unit  4  further includes a light exposure device  45  including an LED for example for exposing the surface  2   a  of the drum  2  with light based on the printing data after the drum  2  is charged by the charger  44 , thereby forming an electrostatic latent image. The electrostatic latent image is developed by the developer  40  to provide a toner image on the surface  2   a  of the photosensitive drum  2 . The toner image has a negative potential. The fixing unit  41 , which may be of the flash type including a light source for example, functions to fix the toner image on the recording paper K. 
     As shown in FIG. 2, the transfer unit U includes a casing  30  accommodating a transfer wire  31  and a discharge wire  32 , and a first and a second guides  7 A,  7 B. The casing  30  and the first and the second guides  7 A,  7 B are attached to a movable member  33  which is pivotable about a shaft  60  in the arrow N1 direction. The pivotal movement of the movable member  33  enables the transfer unit U to move between a set position close to the photosensitive drum  2  and a release position separated from the photosensitive drum  2 . When the drum  2  is out of a printing operation, the transfer unit U is held at the release position for keeping the recording paper P out of contact with the surface  2   a  of the photosensitive drum  2 . 
     The casing  30 , the transfer wire  31  and the discharge wire  32  are structured similarly to those of a prior art printer. Specifically, the casing  30  has a box-like configuration elongated axially of the photosensitive drum  2  and having a front portion which is open toward the photosensitive drum  2 . The transfer wire  31  and the discharge wire  32 , which are formed of a metal, extend axially of the photosensitive drum  2  while being spaced from each other in the recording paper transport direction indicated by the arrow N2. The discharge wire  32  is arranged downstream from the transfer wire  31  in the paper transport direction N2. Longitudinally opposite ends of each wire  31 ,  32  are supported by the casing  30 . A transfer current for generating potential for attracting toner flows through the transfer wire  31 , whereas a discharge current for removing the charge from the recording paper P flows through the discharge wire  32 . 
     The first and the second guides  7 A,  7 B are formed of a metal (e.g. aluminum) having a high wear-resistance. The first and the second guides  7 A,  7 B are elongated axially of the photosensitive drum  2  and substantially equal in length to the transfer wire  31  and the discharge wire  32 . 
     The first guide  7 A is arranged upstream from the transfer wire  31  in the paper transport direction N2 so as not to shield the transfer wire  31 . 
     The second guide  7 B has a first longitudinal edge  71   a  located downstream from the discharge wire  32  in the paper transport direction N2 and a second longitudinal edge  71   b  located upstream from the discharge wire  32  in the paper transport direction N2. The second guide  7 B is formed with a slit  70  at a portion which is offset toward the second edge  71   b  in facing relationship to the discharge wire  32 . The slit  70  serves as an unshielding portion. Owing to the provision of the slit  70 , the discharge wire  32  is prevented from being shielded by the second guide  7 B. As shown in FIG. 3, the slit  70  extends longitudinally of the second guide  7 B (axially of the photosensitive drum  2 ) to reveal the discharge wire  32  substantially over the entire length thereof. 
     Alternatively, as shown in FIG. 4, a plurality of slits  70   a  may be arranged to make a line. In this case, the discharge wire  32   a  is partially shielded by portions  71  of the second guide  7 B. However, such partial shielding does not considerably deteriorate the discharging effect for the recording paper P. As compared with the structure shown in FIG. 3, the structure shown in FIG. 4 can provide a greater mechanical strength around the slits  70   a.    
     The image printing on the recording paper P using the above-described printer A is performed as follows. First, as shown in FIG. 2, the first and the second guides  7 A,  7 B are moved to the set position close to the surface  2   a  of the photosensitive drum  2 . The recording paper P fed from the paper hopper  10  along the above-described transport path is guided by the first and the second guides  7 A,  7 B into contact with the surface  2   a  of the photosensitive drum  2 . At this time, since the transfer current flowing through the transfer wire  31  charges the recording paper P to attract toner particles, the toner image formed on the surface  2   a  of the photosensitive drum  2  is transferred to the recording paper P. 
     Since the second edge  71   b  of the second guide  7 B is located upstream from the discharge wire  32  in the paper transport direction N2, the distance L between the first guide  7 A and the second guide  7 B is shorter than in the prior art printer. Accordingly, the winding length of the recording paper P about the photosensitive drum  2  (circumferential contact length between the paper P and the drum  2 ) is smaller than that in the prior art printer. This reduces the possibility that the recording paper P becomes loose at the surface  2   a  of the photosensitive drum  2  or that the image transferred to the recording paper P is rubbed by the surface  2   a  of the photosensitive drum  2  due to a difference between the paper transport speed and the peripheral speed of the drum surface  2   a . Thus, a printed image with a high quality can be obtained. Moreover, to reliably transfer a toner image onto the recording paper P, the recording paper P need be stably transported without largely shifting. For this purpose, it is preferable to narrow that the gaps s1, s2 defined between the recording paper P and the photosensitive drum  2  before and after the paper P contacts the drum  2 . The printer of the present invention also meets such a requirement, so that the printing quality can be further enhanced. 
     Since the discharge wire  32  is unshielded through the slit  70  to face the recording paper P, the electric charge of the recording paper P can be properly released by energizing the discharge wire  32 . For smooth removal of the recording paper P from the surface  2   a  of the photosensitive drum  2 , it is preferable that the electric charge of the recording paper P be released when or immediately after the recording paper P begins to separate from the surface  2   a  of the drum  2 . The printer of this embodiment also meets such a requirement. 
     Referring to FIG. 5, a printer according to a second embodiment of the present invention will be described. In this figure, the elements which are identical or similar to those of the first embodiment are designated by the same reference signs as those used for the first embodiment. 
     In the second embodiment, a casing  30  has a partition wall  35  which separates the transfer wire  31  from the discharge wire  32  and which has a bent tip end serving as a second guide  7 C. The second guide  7 C is located upstream from the discharge wire  32  in the paper transport direction. The dimension of the second guide  7 C in the paper transport direction is so set as to avoid shielding the discharge wire  32  at the front. A front portion  79  of the casing  30  serves as an unshielding portion. 
     Also with this structure, the distance L between the first guide  7 A and the second guide  7 C is shorter than that in the prior art printer, thereby decreasing the winding length of the recording paper P about the photosensitive drum  2 . Therefore, similarly to the first embodiment, the printing quality can be enhanced. In this way, the second guide  7 C may comprise a part of the casing  30  supporting the transfer wire  31  and the discharge wire  32 . With such a structure, the number of parts of the transfer unit can be decreased. Moreover, though not illustrated, the first guide may also comprise a part of the casing  30 . 
     The present invention is not limited to the above described embodiments, and the specific structure of the printer and the transfer unit may be modified in various ways. 
     The material of the first and the second guide is not limitative. Further, the photosensitive drum may be replaced with a photosensitive endless belt. Moreover, although the printer of the present invention is most suitable for printing on a continuous recording paper, the present invention is not limited to a specific use of the printer or to a specific kind of recording paper. 
     As described above, by decreasing the distance between the first guide and the second guide, the winding length of the recording paper about the photosensitive member can be decreased. Therefore, the printer or transfer unit of the present invention is capable of enhancing the image quality while preventing a deterioration of the discharging effect by the discharge member.