Electrophotographic color printer with belt-to-belt toner transfer and top-side paper path

A color electrophotographic printer (100) having a vertical photoreceptor belt (111) and a horizontal transfer belt (120) at the top of the printer. The result is a "T-shape" belt-to-belt toner transfer configuration with a top-side paper path. Color developer cartridges (117) are vertically stacked along one side of the photoreceptor belt (111) and an oversized black developer cartridge (116) is placed on the other side.

TECHNICAL FIELD OF THE INVENTION 
This invention relates generally to electrophotographic printers, and more 
particularly to a belt-to-belt printer having a top-side paper path. 
BACKGROUND OF THE INVENTION 
Electrophotographic (also called xerographic) copiers were first introduced 
in 1959. More recently the electrophotographic technology has been 
extended to printers. An excellent text on electrophotography is 
Electrophotography and Development Physics, by L. B. Schein (2d ed. 1996, 
Laplacian Press). 
A typical modern electrophotographic ("EP") printer uses some sort of 
processor to interpret a program representing the image to be printed. The 
interpretation usually involves conversion of the program into a bitmap, 
which determines a pattern of light that will expose a photoreceptor, such 
as the surface of a drum or belt. A copier may operate in analog fashion 
by imaging light reflected from the document to be copied onto the 
photoreceptor or it may use digital data (acquired by digitizing the 
document) in the same manner as a printer. 
For both copiers and printers, the exposure of the photoreceptor results in 
its acquiring a charge pattern. The photoreceptor then passes a toner 
dispenser and attracts toner particles to the areas corresponding to the 
charge pattern. The photoreceptor transfers the toner to a print medium 
such as a piece of paper. The toner is fused to the paper, usually with 
heat, and the paper exits the printer. 
EP copiers and printers have been developed with a variety of design 
choices. One design choice is with respect to whether the photoreceptor 
surface is on a drum or belt. Another design choice is with respect to the 
paper transport mechanism, which may be a drum or a belt. Regardless of 
the configuration of the photoreceptor and the paper transport, at some 
point, toner from the photoreceptor is transferred to the paper. 
Sometimes, instead of transferring toner directly to paper, an 
intermediate drum or belt surface is used as an intermediate media. In 
theory, four possibilities exist for transferring toner from the 
photoreceptor to either the paper or intermediate media: drum-to-drum, 
drum-to-belt, belt-to-drum, or belt-to-belt. 
The choice of belts versus drums is related to the configuration of the 
paper path. A straight paper path is thought to reduce the likelihood of 
paper jams. The benefits of a straight paper path with top-side access to 
the paper path have been successfully marketed for black and white 
printers. 
SUMMARY OF THE INVENTION 
One aspect of the invention is a "develop and transfer assembly" (as 
defined herein) for an electrophotographic color printer. A photoreceptor 
belt has a photoconductive outer surface suitable for exposure as a latent 
image. The photoreceptor belt is substantially oblong in shape, and is 
vertically positioned within the printer. A number of developer cartridges 
are positioned adjacent to the photoreceptor belt. For a full-color 
printer, cartridges for color developer are vertically stacked on one side 
of the photoreceptor belt, and a cartridge for black developer is placed 
on the other side. A toner transfer belt is situated substantially 
perpendicular to and above the photoreceptor belt, such that the 
photoreceptor belt and the transfer belt form a T-shape. The toner 
transfer belt receives toner from the photoreceptor belt, and either 
carries paper or serves as an intermediate transfer media. For full-color 
printing, the toner transfer belt accumulates toner during four passes of 
the photoreceptor belt. For spot color printing, there is only one pass. 
An advantage of the invention is that it provides the ability to print a 
large surface area with a relatively compact photoreceptor. The printer is 
arranged in a manner that is convenient for user access and that does not 
complicate the operation and interaction of the printer components. The 
top-side paper path permits a number of options for attaching and 
registering the paper during color image accumulations. The top-side paper 
path also provides easy access to the internal parts of the machine for 
repairs, maintenance, consumables removal, and paper path access. All 
subsystems may be easily removed from the printer from the top. 
The printer's developer cartridges may be designed to operate at the same 
angle and position with respect to the photoreceptor belt. This simplies 
the design of the printer and reduces variability from color to color. A 
cartridge for black developer may be designed separate and different the 
cartridges for color developer so as to provide for greater volume.

DETAILED DESCRIPTION OF THE INVENTION 
The invention described herein is primarily directed to those parts of an 
electrophotographic printer that perform the toner develop and toner 
transfer steps of electrophotographic printing. For purposes of this 
description, a "printer" is any electrophotographic hardcopy device, 
regardless of whether it is all-digital, for printing and copying, or 
performs only analog copying. Likewise, "printing" includes any 
electrophotographic production of hardcopy output. As discussed below, 
features of the printer include belt-to-belt toner transfer and a top-side 
paper path. 
FIG. 1 is a side cross-sectional schematic of an electrophotographic 
printer 100 in accordance with the invention. Printer 100 is a four-color 
printer (cyan, magenta, yellow, and black), which, as explained below, 
sequentially accumulates toner onto transfer belt 120. For printer 100, 
transfer belt 120 carries paper, and thus, toner is accumulated on paper. 
This is in contrast to printer 200, discussed below in connection with 
FIG. 2, which uses transfer belt 120 as an intermediate surface for toner 
accumulation. 
Photoreceptor belt 111, toner cartridges 116 and 117, and transfer belt 120 
could be referred to collectively as a "develop and transfer assembly". 
Although the emphasis herein is on the components of this develop and 
transfer assembly, the other components of printer 100 are briefly 
described. It should be understood that with regard to these other 
components, a number of alternative assemblies are available and could be 
substituted. 
Photoreceptor belt 111 is coated with organic photoconductive ("OPC") 
material so as to provide the photoreceptor surface. Photoreceptor belt 
111 is substantially oblong in its cross-sectional shape and is positioned 
within printer 100 such that its long dimension is vertical. 
A platen system 112 supports belt 111 and provides a number of toner 
contact points 112a as well as an exposure point 112b. As an alternative 
to the contact points of FIG. 1, platen system 112 may comprise 
conformable roller surfaces or multiple opposing rollers. In the example 
of this description, where printer 100 has four toner cartridges, there 
are four toner contact points 112a. Photoreceptor belt 111 is removable 
from printer 100 in the vertical direction, after transfer belt 120 is 
lifted or otherwise moved from its operating position above photoreceptor 
belt 111. 
A cleaner 113 cleans excess toner from photoreceptor belt 111 after toner 
transfer to transfer belt 120. A scorotron 114 charges the surface of belt 
111. 
An exposure unit 115 converts digital information into a pattern of light 
that discharges the normally insulating photoreceptor belt 111, thereby 
producing a latent image. A latent image is produced for each of the four 
colors to be printed, with belt 111 rotating once per color. Exposure unit 
115 is suitably registered to photoreceptor belt 111. Exposure unit 115 
may be based on laser scan or on spatial light modulator technology. An 
example of the latter is the DMD (digital micro-mirror) exposure unit 
developed by Texas Instruments Incorporated. An example of a DMD-based 
exposure unit is described in U.S. Pat. No. 5,041,851 to William E. 
Nelson, entitled "Spatial Light Modulator Printer and Method of 
Operation", assigned to Texas Instruments Incorporated and incorporated 
herein by reference. 
Exposure unit 115 may receive images acquired by scanning a scanned 
document to be copied (or otherwise acquiring a digital representation), 
and in this sense, printer 100 can perform copying tasks. Alternatively, 
printer 100 could be easily modified for conventional light-lens copying, 
using light reflected from a document to expose the photoreceptor belt 
111. 
A first developer housing 116 contains black toner. It is high capacity, as 
compared to three other developer housings 117, which contain color toner. 
Developer housings 117 contain subtractive toner colors, such as cyan, 
magenta, and yellow (CMY). Developer housings 116 and 117 are mounted in 
printer 100 and registered to photoreceptor belt 111 so as to achieve 
developer gaps. Each developer housing 116 and 117 is positioned along the 
long dimension of photoreceptor belt 111. An advantage of the invention is 
that the three developer housings 117 for color toner can be stacked 
vertically along one side of photoreceptor belt 111. As a result, each 
cartridge 116 is the same distance from belt 111. Tube 118 has different 
channels that permit addition of toner to each developer housing 117. 
The design of FIG. 1 shows the color developer cartridges 117 stacked on 
one side of photoreceptor belt 111 and the black developer cartridge 116 
on the opposing side. This facilitates the oversizing of the black 
developer cartridge 116 without loss of compactness of the printer size. 
However, other configurations are possible. 
The developer cartridges 116 and 117 may be for either two-component or 
monocomponent development. Also, the toner may be of either the 
non-contacting (toner jumping) type or the contacting type. In the case of 
the latter, proximity to belt 111 may be accomplished with a small 
indexing movement. Either the cartridges 116 and 117 may be moved, or the 
brushes or rollers that carry toner to belt 111 can be moved. In either 
case, if cartridges 117 are stacked and are therefore all the same 
distance from belt 111, the indexing is accomplished in the same manner 
for each. The brushes may be moved from proximity to photoreceptor belt 
111 by dynamic means, such as by reversing rotation to collapse a dual 
component toner magnetic brush. 
A paper input belt 119 initializes the paper path. An alternative to belt 
119 could be a roller. As indicated, the paper travels "face down" along 
its path within printer 100. 
Toner transfer belt 120 is positioned above photoreceptor belt 111. 
Transfer belt 120 is substantially perpendicular to photoreceptor belt 
111. As a result, photoreceptor belt 111 and transfer belt 120 form a 
T-shape. Transfer belt 120 carries the paper in a straight horizontal path 
across the top of printer 100. As indicated above, transfer belt 120 is 
moveable to provide access to photoreceptor belt 111 and to permit 
photoreceptor belt 111 to be removed from printer 100. 
As paper is carried along its path by transfer belt 120, the paper passes a 
toner transfer point, where toner is transferred to the paper from 
photoreceptor belt 111. A transfer corona 121 effects transfer of the 
developed image to the paper. The point of transfer for images developed 
on photoreceptor belt 111 is at the apex (top) of that belt. This results 
in an efficient and reliable paper path. 
For a four-color (CYMB) image, transfer belt 120 recirculates the paper 
four times before releasing the paper to the rest of its path through 
printer 100. In other words, transfer belt 120 causes the paper to make 
four passes past the toner transfer point, each time receiving a 
differently colored image. 
Attachment of the paper to transfer belt 120 can be accomplished with 
several means, such as with mechanical clamping, electrostatic attraction, 
or vacuum force. The appropriate type of attachment is related to the 
radius of curvature of belt 120 as defined by end rollers, with a larger 
radius being more amenable to electrostatic attachment. FIG. 1 
illustrates, schematically, attachment by electrostatic attraction 
mechanism 129. A mechical clamping mechanism (not shown) would have a 
means for lifting the clamp where appropriate. An example of a suitable 
clamping means is a leading edge clamp, such as that used in the Xerox Two 
Roll Transfer Loop system used in the Xerox 5775 color printer. 
After transfer belt 120 has circulated the paper once across the toner 
transfer point once for each color to be printed, the paper is released 
from transfer belt 120. A paper detach mechanism 122 effects this release 
and transfer of the paper to an exit carrier belt 123. In FIG. 1, the 
paper detach mechanism is shown as a separation corona 122, but other 
mechanical or electrical separation means known in the art of 
electrophotography, such as "picker fingers", could be used. 
Fuser 124 is implemented with any suitable EP technology. FIG. 1 
illlustrates fuser 124 in the forms of conventional hot rollers 124. 
Synchronization and alignment of belts 111 and 120 may be accomplished with 
optically readable tracks and read heads. FIG. 1A illustrates a read track 
131 coated onto the inner surface of a portion of photoreceptor belt 111. 
As indicated by the dotted lines, belt 111 is somewhat wider than the 
imaging area, so that track 131 may be placed alongside the imaging area. 
Track 131 has at least one encoded section 132, which indicates the 
position of belt 111 in both the process and the cross-process direction. 
Transfer belt 120 may be similarly encoded. Track 131 can be implemented 
as a high resolution, optically encoded track in an ablatable material, 
similar to those used for optical disks. An example of a suitable track 
131 is one having a 7 micron width with 2 micron features. Tracking 
information can be included to servo the optical readhead. 
Referring again to FIG. 1, read heads 126 monitor both the process and 
cross-process position of belts 111 and 120. This ensures registration of 
the paper during toner accumulation, as well as belt-to-belt 
synchronization if the belts are operating at different speeds. For 
lateral (cross-process) wandering of belt 111 or 120, either the imaging 
area or the belt position could be adjusted. Synchronization can be 
accomplished with control unit 127, which is in electronic communication 
with read heads 126. Control unit 127 has a counter that counts between 
reads of the same or different encoded sections 132, and has appropriate 
logic for comparing belt speeds. 
FIG. 2A illustrates printer 200, which is a variation of printer 100. In 
printer 200, transfer belt 120 is used as an intermediate media for 
receiving toner from photoreceptor belt 111. The paper is delivered to 
belt 111 from a top paper tray (not shown). Known air lifting and bottom 
picking means can be used for delivering individual sheets of paper. 
Rollers 202 direct the paper downward along a vertical "top-to-bottom" 
paper path, such that the paper passes past a toner transfer roller 203 
and through fuser rollers 204. Thus, the point of transfer for images 
developed on photoreceptor belt 111 is at one end of belt 120. The printer 
paper is deposited in an output paper tray 205. 
As an alternative to the design of FIG. 2A, where belt 120 is an 
intermediate belt, printer 200 could have a vertical "bottom to top" paper 
path, as illustrated in FIG. 2B. In this case, paper would be delivered 
from a bottom paper tray 211 via rollers 212. The paper travels up the 
side of the printer 200, through a transfer roller 213 positioned at one 
end of belt 120. A fuser 214 would be at the top of the printer 200 and 
printed paper would be deposited in a top paper tray 215. 
The embodiments of FIGS. 2A and 2B are consistent with the short, straight, 
and acessible paper path provided by the "T-shaped" design of belts 111 
and 120. All embodiments provide easy access to the paper path and the 
fuser. The embodiment of FIG. 2B is especially useful for placing the 
fuser 214 where it is easily vented as well as accessed, thereby 
preventing the printer and the printed material from being "cooked". 
FIG. 3 illustrates a spot color printer 300, which is an alternative 
embodiment to the full-color printers 100 and 200 of FIGS. 1 and 2. The 
structure of printer 300 is similar to that of printer 100 except that 
there is only one color developer cartridge 317 in addition to a black 
toner cartridge 316. The color toner and the black toner are accumulated 
on photoreceptor belt 111 before being transferred to belt 120. 
In variations of printer 300, there could be more than one color cartridge. 
However, a common characteristic of all variations is that each color or 
black is spatially distinct. In other words, toners for different colors 
or for black are not overlaid on top of each other. Photoreceptor belt 111 
makes a single pass past the developer cartridges 317 for each pass of 
transfer belt 120. There is no need to attach the paper to transfer belt 
120 during multiple passes of photoreceptor belt 111. 
Other Embodiments 
Although the invention has been described with reference to specific 
embodiments, this description is not meant to be construed in a limiting 
sense. Various modifications of the disclosed embodiments, as well as 
alternative embodiments, will be apparent to persons skilled in the art. 
It is, therefore, contemplated that the appended claims will cover all 
modifications that fall within the true scope of the invention.