EP0388342A2

EP0388342A2 - Electrophotographic machine with efficient transfer

Info

Publication number: EP0388342A2
Application number: EP90480013A
Authority: EP
Inventors: Roger Wayne Bell; Dean Steward Ehn; Sidney Jared Fox; David Kent Gibson; Michael David Ries; Alfonso Armando Rosati; Ronald Gene Velarde
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1989-03-17
Filing date: 1990-02-02
Publication date: 1990-09-19
Also published as: JPH02278277A; EP0388342A3; CA2007788A1

Abstract

An electrophotographic machine with a transfer corona generator (13) for producing ionic flow to the backside of print paper to electrostatically attach the paper to the photoreceptive image bearing surface (12). The machine includes a flexible support member (40-44) which is movably mounted to mechanically press the print paper against the photoreceptive surface within the transfer zone, preferably at or near the region of peak transfer charge distribution. In that manner, charge from the transfer corona attaches the print paper to the photoreceptive surface at the line of release from the mechanical pressure applied by the flexible support member and thereby eliminates voids in the transfer of toner to print paper caused by air gaps between the paper and the photoreceptive surface.

Description

This invention relates to image producing machines such as electrophotographic printers and copiers and more particularly to improving the transfer of toner from a developed image to an image receiving surface.

BACKGROUND OF THE INVENTION

Image producing machines such as electronic printers and copiers are frequently of the electrophotographic type. In electrophotographic machines, a print is produced by creating an image of the print on a photoreceptive surface, developing the image and then fusing the image to print material. In machines which utilize plain bond paper the electrophotographic process is of the transfer type where a photoreceptive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. In the typical transfer process, photoreceptive material is passed under a stationary charge generating station to place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoreceptive surface. Next, the photoreceptor is moved to an imaging station where, in an electrophotographic printer, it receives light rays which are modulated in accordance with the data to be printed. The light generator may produce laser beams, it may be an array of light-emitting diodes, or it may be any other suitable light source. The light rays are directed to the photoreceptor and cause it to bear a charge pattern which is a latent image of the information used to modulate the light rays. Modulation is usually derived from a character generator which is driven by image pattern data frequently produced by a computer and held in digitized form in memory. Similarly, in an electrophotographic copier, light rays are reflected from an original document to be copied in order to modulate the light rays in accordance with data on that original. The reflected light rays are then directed to the imaging station to create an electrostatic image of the original on the photoreceptive surface.
After producing an image on the photoreceptor, the next step in the electrophotographic process is to move the image to a developing station where developing material called toner is placed on the image. This material may be in the form of a colored powder which carries a charge and is electrostatically attracted to those areas which it is desired to develop.
The photoreceptor, with a developed image, is moved from the developer to a transfer station where image receiving media, usually paper, is juxtaposed to the developed image. A charge is placed on the backside of the paper so that when the paper is stripped from the photoreceptor, the toner material is held on the paper and removed from the photoreceptor. Any toner remaining on the photoreceptor after transfer is removed by a cleaning station before the photoreceptor is reused. The paper is sent to a fusing station for permanently bonding the transferred toner to the paper.
In the transfer operation, it is essential that the print paper be tightly pressed against the photoreceptive surface. Should there be any small air gaps or bubbles between the paper and the photoreceptive surface, poor transfer will result and will show up as a transfer void on the print. There may also be residual images on the next print if the cleaning operation is unsuccessful in adequately cleaning the untransferred toner from the photoreceptive surface. In prior art machines, the means of attaching the image receiving paper to the photoreceptive surface usually has been a transfer corona generator, that is, a means for generating electrical charge which is deposited on the backside of the image receiving paper with a polarity opposite to the polarity of the charge on the toner. The result is to attract the paper to the photoreceptive surface and thereby obtain good transfer. However, air gaps may sometimes be present due to creases in the paper or due to paper distortions caused by the fusing operation. These distortions are sometimes present where fusing heat is accumulated in areas of heavy toner deposition as opposed to lesser heating in white background areas. These differences in heating paper can cause paper distortions which then create poor transfer when the same sheet is returned to the transfer station for the application of print to the reverse side of the paper. Another problem occurs with the use of perforated forms or labels or fanfold paper which contain perforations along which the paper is folded prior to use. Such folding causes a crease along the perforation which can result in the paper bulging away from the photoreceptor surface near the crease. These bulges can cause difficulty in flattening the sheet onto the photoreceptive surface near the perforations, and as a result it may become necessary to inhibit printing near perforations.
This invention is designed to increase the efficiency of transfer by mechanically flattening the sheet inside the transfer corona and pressing the paper against the photoreceptive surface. In that manner, charge deposited on the backside of image receiving media will attach the media to the transfer surface without the presence of air gaps or bubbles either at the leading and trailing edges of cut sheet at or near perforation or fanfold paper, or in the body of the paper.
U.S. Patent No. 4,101,212 relates to the provision of a sheet depressing member in the form of a roller or a plate to forcibly depress the sheet onto the surface of the photosensitive recording element. The depressing member, which may be made of MYLAR, is positioned outside of the transfer corona such that it depresses the paper against the photoconductive surface prior to entry of the paper into the transfer zone. As such, this technique is only marginally effective.
U.S. Patent No. 3,620,617 relates to the provision of a MYLAR (trademark of DuPont and Co.) flap partially covering the transfer corona station. The flap is attached to the shield of the transfer corona, and at no time does it bear against the print paper. The flap is positioned to receive a negative potential so as to provide an electrostatic field between the flap and the photoconductor. The purpose of the flap is to provide a uniform electromagnetic transfer field thereby eliminating streaking.
U.S. Patent No. 4,673,280 relates to the provision of a stationary metal paper guide placed at the voltage of the corona shield. The guide does not bear against the backside of copy paper but the lip of the guide does extend into the region of corona emission. The purpose of the lip is to limit the application of the transfer field to a predetermined angular extent around the photoconductor drum and the purpose of applying voltage to the guide is to prevent charge from leaking laterally off the paper while it is guided toward the photoconductor drum. This is another attempt at producing more efficient transfer.
U.S. Patent No. 4,110,027 relates to a printer in which continuous fanfold paper is used. Two rollers are used with one roller positioned prior to and the second roller positioned subsequent to the transfer corona to hold paper to the photoconductor drum during transfer. These rollers do not bear against the backside of the copy paper at any point in the area of corona emission.
It is an object of this invention to obtain improved efficiency in the transfer of a developed image to image receiving material.
It is a further object of this invention to prevent or minimize the formation of air gaps near the leading and trailing edges of cut sheet print receiving material, or near the perforations present on fanfold type image receiving material.
It is another object of this invention to prevent the formation of air bubbles within the body of image receiving material.
It is still another object of this invention to provide a paper support member to press paper against the photoreceptive surface within the region of peak charge distribution under the transfer corona, such that as the paper leaves the support member it immediately receives charge from the transfer corona, thus attaching the paper to the photoreceptive surface without the formation of air bubbles. It is another object of the invention to provide support members of more than one length so that papers of different size may be firmly pressed against the photoreceptive surface at the transfer station.
It is yet another object of the invention to provide a paper support member which can be moved to a home position out of contact with the photoreceptive surface when image receiving material is not present at the transfer station.
It is another object of the invention to provide a support member made of metal such as spring steel which in turn is covered by a plastic wear resistant insulating member.

SUMMARY OF THE INVENTION

This invention relates to the positioning of a flexible support member near the transfer station of an electrophotographic machine employing a transfer corona generator. The end of the support member must extend into the region of corona emission in the transfer zone and should be positioned near the region of peak current distribution for maximum benefit. The purpose of the flexible support member is to bear against the backside of image receiving media in order to press the media against the photoreceptive surface. By extending the flexible support member into the transfer zone, media is held against the support surface such that it receives charge from the transfer corona as it leaves the line of release from the mechanical pressure exerted on it by the support member. Thus media is electrostatically attached to the photoreceptive surface before air gaps have a chance to form. In that manner, voids or air bubbles between the media and the photoreceptive surface are eliminated, thereby improving transfer and decreasing the amount of residual toner left on the photoreceptive surface after transfer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, the description of which follows.

FIG. 1 is an illustration of an electrophotographic machine in which the principles of the instant invention can be gainfully employed.
FIG. 2 shows the transfer station of the machine of FIG. 1 with the flexible support member of the instant invention positioned within the area of transfer corona emission.
FIG. 3 shows a perspective view of the support member shown in FIG. 3.
FIG. 4 illustrates the support member, together with the photoreceptive surface.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates an electrophotographic copier or printer which can benefit from use of the instant invention. Copy or print image receiving material, such as paper, which may be either cut sheet paper or fanfold paper, is loaded into input bin 10 from which it is sent along a relatively straight through paper path to exit bin 11. Transfer of the image from photoreceptive surface 12 to the image receiving material is accomplished at transfer station 13.
In the machine, shown in FIG. 1, photoreceptive material 12 is mounted on the surface of a revolving drum 14 which moves in the direction A. Charge corona generators 15 place a relatively uniform electrostatic charge across the photoreceptive surface 12 as it rotates to thereunder. An image is placed on the photoreceptive surface through the application of image bearing light rays 16. Note that in the particular machine configuration of FIG. 1, the imaging station is located at the same position as the transfer station 13.
As the photoreceptive material (photoconductor) continues to rotate, the photoreceptive material 12, now bearing an image, moves to a developer station 17 where toner is placed on the image. The now developed image passes under the two charge coronas 15, which are turned off, to a pretransfer lamp 18 and on to the transfer station 13 where the developed image is transferred to image receiving material. Such material has been sent to the transfer station from input bin 10 and is controlled such that the leading edge of the image receiving material mates with the leading edge of the image. As the image receiving material is stripped away from the photoconductor, the toner is transferred to the image receiving material. After transfer, the photoreceptive surface continues to rotate to the developer 17, which is now used as a cleaning station to clean away residual toner left after the transfer operation. The photoreceptive material is then sent to the charge corona 15 for the beginning of a new cycle of a image reproduction.
The machine of FIG. 1, described above, is a two cycle machine, that is, a machine that requires two revolutions of the photoreceptive surface in order to produce one print. On the first cycle of the drum revolution, as explained above, the photoreceptive surface is charged, imaged, and developed. On the second cycle, the developed image is transferred to copy paper and residual toner is cleaned from the photoreceptive surface.
The machine shown in FIG. 1 contains certain other elements located around the drum, including a detector 19 for sensing the failure of paper to detach from the drum after transfer. A preclean erase lamp 20, an edge erase lamp 21, and a sensor 22 for determining the density of toner developed on the photoreceptive surface are also located next to the drum. An electrostatic probe 23 is utilized for sensing the charge level placed on the photoreceptive surface by charge corona 15, and an inter-image erase lamp 24 is used to erase charge prior to development in those areas which are not touched by light imaging rays.
After transfer, the image receiving material is transported on the underside of a vacuum transport 26 to a fusing station 27 for fusing the toner into the image receiving material. If cut sheet paper is being used, duplex copies may be produced, that is, a print with an image on both sides of the paper, by returning the paper along a duplex path 28 to the transfer station 13 for application of an image to the reverse side.
FIG. 2 is an illustration of the transfer station 13 showing a paper inlet guide 30. Paper or other image receiving material is directed to the transfer station on the underside of the paper inlet guide 30 as shown by arrow B. A transfer corona generator is present at the transfer station 13 with the transfer zone defined by the side plate 31 on the inlet side and the side plate 32 on the exit side. Corona wires 33 and 34 extend across the photoreceptive surface 12 and through the application of voltages to these wires, a corona emission occurs which causes a flow of ions from the wires to the charged photoreceptive surface 12 as it moves through the transfer zone. With the interposition of image receiving material between the corona wires and the photoreceptive surface ions strike the backside of the copy paper and electrostatically attach the copy paper to the surface 12.
A flexible member 40 made from a plastic insulating material such as polyetherimide is mounted on a flexible metal member 41 which is made from a material such as spring steel. The combination of the metal member 41 and the insulating member 40 together comprise the flexible member of this invention. The metal member 41 provides a constant spring rate and does not take a set, thereby minimizing the stress on the plastic member 40. The metal member 41 is positioned to make contact with a copy sheet and is the primary member for bearing against the backside of paper to eliminate transfer voids. Plastic insulating member 40 also makes contact with the paper. Metal member 41 is fastened to shaft 44. A similar combination of a metal member 43 with plastic member 42 is also present with the metal member 43 also fastened to shaft 44. The presence of two flexible support members is for purposes of providing for two different size copy sheets. For example, member 40 is slightly less wide than 11 inches and is used to bear against the backside of 11 inch paper. Member 41 is slightly less wide than 3 inches and is used together with member 40 to bear against the backside of 14 inch paper.
FIG. 3 shows a perspective view of shaft 44 with flexible steel members 41 and 43 fastened thereto. Plastic members 40 and 42 are fastened to steel members 41 and 43. Shaft 44 contains an indented area 45 through which members 42 and 43 take a position slightly more removed from photoconductor 12 as shown in FIG. 2. Note that the view of members 40-44 shown in FIG. 2 is taken along section line 2-2 in FIG. 3.
FIG. 3 also shows that plastic insulating members 40 and 42 completely cover metal members 41 and 43 in order to shield these members from corona emission and thereby prevent arcing.
FIG. 4 shows the flexible members 40 and 42, together with shaft 44 in position across the photoreceptive surface 12. In working position, members 40 and 42 are in contact with the backside of image receiving material. Metal members 41 and 43 are bidden from view in Fig. 4, but in working position are also in contact with the image receiving material.
Shaft 44 is connected to stepper motor 46 such that rotation of motor 46 moves the flexible support member 40 from a home position to a working position in which members 40-43 are in contact with the backside of image receiving media. At the home position, members 40 and 42 are out of contact with the photoreceptive surface 12, so that the surface can pass underneath the flexible support members without touching. The tip 47 of member 40 is that portion of the insulating member which bears against image receiving material. Tip 47 defines a "line of release" from which the image receiving material moves out of the influence of the mechanical pressure exerted by member 40 on the material.
In operation, for 11 inch paper, as copy paper moves across photoreceptive surface 12 into the transfer zone, stepper motor 46 moves the flexible support members 40 and 41 against the backside of the copy paper such that contact is made just after the leading edge of the paper has moved beyond the tip 47 of flexible insulating member 40. If 14 inch paper is in use, stepper motor 46 moves slightly further so that flexible insulating member 42 together with member 43 come into contact with the copy paper. Since movement of the support members 40-43 into pressure exerting position does not flexible occur until the leading edge of copy paper is present, the members 40-43 do not contact the photoreceptive surface directly thereby preventing damage to that surface. Like-wise, there is no contamination of the support member with toner.
In addition to eliminating transfer voids in the body of a sheet, the support member has the additional advantage of improving transfer at the leading and trailing edges of the sheet, thus reducing the possibility of paper leaving the photoreceptive surface and crashing into the exit sidewall 32 of the transfer corona. The support member also makes it possible to operate under a wider latitude of temperature and humidity conditions by attaching the paper more firmly to the photoconductor. It should be noted, that in addition to the preferred embodiment described above, a series of holes in the member may be added so that some electrostatic tacking of the paper to the photoconductor can begin even before the paper moves beyond the tip of the support members. It should also be noted that the dimension of the support member must be slightly narrower than the paper dimension to allow for paper registration tolerances. As a consequence, the edges of paper may be subjected to electrostatic tacking prior to the time that the body of the sheet leaves the tip of the support member. As a result, the edges may become tacked to the photoreceptive surface too early, causing paper distortions to be trapped in the copy sheet. To prevent such a condition, a section may be added to the support member to block the transfer current to the sheet at the edges. Such a section must be positioned so that it does not directly contact the photoreceptive surface.
It is preferred that the tip 47 of insulating members 40 and 42 be located near the region of peak corona current, that is, directly under the corona wire 33 as shown in FIG. 2. In that manner, ions produced by corona wire 33 strike the backside of the image receiving material immediately upon its release from support members 40 and 42, thereby electrostatically attaching the material to the photoconductor before any air gaps are allowed to form. However, as long as the support member extends into the transfer zone the benefit of attaching the image receiving material to the photoconductor will be achieved to a degree. The maximum benefit is achieved when the tip 47 is positioned directly under the corona wire.
As noted above, the metal members 41 and 43 are the primary members for mechanically bearing down on the paper to eliminate voids. Insulating members 40 and 42 should also touch the paper, but usually do not apply significant mechanical force. This is the arrangement of the preferred embodiment since plastic insulating material may produce a triboelectric effect when forcibly engaged with the paper. Such an effect electrically charges the paper and may cause premature transfer of toner from the photoreceptive surface to the paper which can create a problem in background areas. Metal members 41 and 43 avoid that problem and also give considerably longer wear which increases the replacement interval. Nevertheless, in some machines the triboelectric effect may not be significant and in such case the advantages of this invention could be realized by plastic member contact only.
It has also been found that plastic material with relatively high glass transition temperatures are desirable, that is to say, polyetherimide is more desirable as the insulating member than is polyester, even though polyester wears longer. The combination of crystallinity and relatively low glass transition temperature for polyester may lead to a softening or melting of abraded wear products which then tend to collect at the edge of the member thus lengthening it in an irregular manner. In some applications this may not be a problem and polyester can be a suitable alternative.
While it is desirable for both the insulating member and the metal member to contact the surface, worst case tolerances may allow for initial contact by the plastic member with metal member contact occurring as the plastic member wears in. Such tolerances may also allow for metal member contact only, but that would chance a small increase in air gaps at the leading edge.
It may be surprising that a metal member in contact with paper does not bleed away transfer charge on the paper, especially under high humidity conditions. Such bleed-off, however, is time dependent and at sufficiently high process speeds does not detrimentally affect image transfer.
While the invention has been particularly shown and described with reference to a preferred embodiment which shows photoconductor mounted on a drum surface, it will be understood by those skilled in the art that a photoconductor mounted in a belt arrangement is also within the ambit of the invention. Also, it should be understood that the flexible nature of the support member can be achieved in various ways. For example, the preferred embodiment described above is made from flexible plastic. An inflexible material could be used with flexibility given to the member by using springs, torsion motors, or other flexible mounting arrangements. The foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

1. An electrophotographic machine for producing reproductions of data on image receiving media comprising;
a photoreceptive surface mounted for movement in a continuous path through various processing stations;
a charge producing processing station for placing a uniform electrostatic charge on said photoreceptive surface;
an imaging station for allowing data bearing light rays to strike the charged photoreceptive surface to produce an electrostatic image thereon;
toner material for receiving a charge;
a developing station for holding said toner material and for placing the charged toner on the image bearing photoreceptive surface to develop said image;
a transfer station comprising corona generator means for producing ions for charging the backside of said image receiving media for attaching said media to said image bearing photoreceptive surface so that when said media is stripped from contact with said surface said toner is removed from said surface through electrostatic adherence to said media; and
a flexible support member means positioned to mechanically bear against the backside of said media within the zone of ionic flow at said transfer station so that charge from the transfer corona attaches said media to said photoreceptive surface at the line of release from mechanical pressure applied by said flexible support member to thereby minimize voids in the transfer of toner caused by air gaps between said media and said surface.

2. A flexible support member for use in an electrophotographic machine with a photoreceptive surface mounted for continuous movement through various processing stations, said stations including a transfer station whereat corona generating means produce ions for charging the backside of the image receiving media, comprising;
a plastic insulating member means to mechanically bear against the backside of said image receiving media positioned within the zone of ionic flow from said corona generating means so that charge from said corona attaches said media to said photoreceptive surface at the line of release from the mechanical pressure applied by said plastic member to thereby minimize voids in the transfer of toner caused by air gaps between said media and said surface.

3. The machine of claim 1 or 2 wherein said flexible support member extends into said zone of ionic flow to a position at or near the maximum level of current distribution.

4. The machine of claim 1 or 2 or 3 wherein said flexible support member means is sectioned into a plurality of support member means for use with a plurality of sizes of image receiving media.

5. The machine of anyone of the preceding claims including motive means connected to said flexible support member for moving said flexible support member from a first position at which said member is out of contact with said photoreceptive surface to a second position at which said member bears against the backside of image receiving media positioned on said photoreceptive surface.

6. The machine of anyone of the preceding claims wherein said flexible support member means is comprised of a spring metal member covered by a plastic insulating member, with both said members used for bearing against said image receiving material.

7. The machine of anyone of claims 1 to 5 wherein said flexible support member means is comprised of a spring metal member covered by a plastic insulating member, said spring metal member used for bearing against said image receiving material to provide the primary force application means bearing against the backside of image receiving material.