Transfer system for a color printer

An apparatus in which successive toner images are transferred from a photoconductive drum to a sheet at a transfer zone. The sheet is transported by a perforated belt entrained about a stationary sleeve through a recirculating path. The sleeve has a plurality of spaced ports extending in and through the periphery thereof and connected to a vacuum source. In this way, the sheet is vacuum tacked to the belt to move in unison therewith in synchronism with the photoconductive drum. At the transfer zone, the sheet separates the belt and is interposed between the photoconductive member and a corona generator. The corona generator applies an electrostatic charge to the sheet to attract successive toner images thereto. The sheet moves with the belt in a recirculating path so that successive different color toner images are transferred thereto in registration with one another.

This invention relates generally to a color electrophotographic printing 
machine, and more particularly concerns a system for transferring 
successive toner images from a photoconductive member to a sheet. 
In an electrophotographic printing machine, a photoconductive member is 
charged to a substantially uniform potential to sensitize the surface 
thereof. The charged portion of the photoconductive member is exposed. 
Exposure of the charged photoconductive member selectively dissipates the 
charge thereon in the irradiated areas. This records an electrostatic 
latent image on the photoconductive member corresponding to the 
informational areas contained within the original document being 
reproduced. After the electrostatic latent image is recorded on the 
photoconductive member, the latent image is developed by bringing toner 
into contact therewith. This forms a toner image on the photoconductive 
member which is subsequently transferred to a copy sheet. The toner image 
is heated to permanently affix it to the sheet in image configuration. 
Multi-color electrophotographic printing is substantially identical to the 
foregoing process of black and white printing. However, rather than 
forming a single latent image on the photoconductive surface, successive 
latent images corresponding to the different colors desired in the copy 
are recorded. Each single color electrostatic latent image is developed 
with the appropriately colored toner. The single color toner images are 
transferred to the copy sheet in superimposed registration with one 
another. This creates a multi-layered toner image on the copy sheet. 
Thereafter, the multi-layered toner image is permanently affixed to the 
copy sheet creating a color copy. The developer material may be a liquid 
material or a powder material. 
Generally, transfer systems used in multi-color printing machines move the 
copy sheet in a recirculating path so that successive toner images may be 
transferred thereto in superimposed registration with one another. One 
type of transfer system previously used has an electrically biased drum 
rotating in synchronism with the photoconductive drum. The copy sheet is 
secured to the drum by either gripper bars or vacuum tacking and rotates 
therewith. Another type of system uses gripper bars to secure the sheet to 
a belt moving in a recirculating path. The belt transports the sheet 
between a corona generator and the photoconductive drum at the transfer 
zone. Still another type of system uses a drum having a plastic mesh to 
push the sheet against the photoconductive drum. A corona generator 
applies an electrostatic charge to the sheet through the mesh. However, in 
time the plastic mesh gets dirty and builds up a charge which suppresses 
the transfer field. It is thus clear that various types of transfer 
systems have been devised for multi-color printing. The following 
disclosures appear to be relevant: 
U.S. Pat. No. 4,251,154 Patentee: Russel Issued: Feb. 17, 1981 
U.S. Pat. No. 4,537,493 Patentee: Russel Issued: Aug. 27, 1985 
U.S. Pat. No. 4,712,906 Patentee: Bothner et al. Issued: Dec. 15, 1987 
U.S. Pat. No. 4,724,458 Patentee: Roy et al. Issued: Feb. 9, 1988 
U.S. Pat. No. 4,739,361 Patentee: Roy et al. Issued: Apr. 19, 1988 
U.S. Pat. No. 4,740,813 Patentee: Roy Issued: Apr. 26, 1988 
The relevant portions of the foregoing patents may be briefly summarized as 
follows: 
U.S. Pat. No. 4,251,154, U.S. Pat. No. 4,537,493 and U.S. Pat. No. 
4,712,906 disclose electrophotographic printing machines for producing 
colored copies on a sheet. The transfer systems have a transfer drum which 
secures sheets thereto by a vacuum. Different color images are transferred 
to the sheet while it is secured to the transfer drum. 
U.S. Pat. No. 4,724,458, U.S. Pat. No. 4,739,361 and U.S. Pat. No. 
4,740,813 all describe a transfer roll which may be used to form 
multi-color copies. The sheet is vacuum tacked to the transfer roll. 
Pursuant to the features of the present invention, there is provided an 
apparatus for transferring successive toner images from a photoconductive 
member to a sheet at a transfer zone. The apparatus includes a flexible 
member. Means are provided for vacuum tacking at least a portion of the 
sheet to the flexible member with successive portions of the sheet being 
separated from the flexible member at the transfer zone. Means move the 
flexible member to transport the sheet in a recirculating path. Means 
apply an electrostatic charge to the sheet at the transfer zone to attract 
successive toner images thereto. 
In another aspect of the present invention, there is provided a printing 
machine of the type in which successive toner images are transferred from 
a photoconductive member to a sheet at a transfer zone. The printing 
machine includes a flexible member. Means are provided for vacuum tacking 
at least a portion of the sheet to the flexible member with successive 
portions of the sheet being separated from the flexible member at the 
transfer zone. Means move the flexible member to transport the sheet in a 
recirculating path. Means apply an electrostatic charge to the sheet at 
the transfer zone to attract successive toner images thereto.

While the present invention will hereinafter be described in connection 
with a preferred embodiment thereof, it will be understood that it is not 
intended to limit the invention to that embodiment. On the contrary, it is 
intended to cover all alternatives, modifications and equivalents as may 
be included within the spirit and scope of the invention as defined by the 
appended claims. 
For a general understanding of the features of the present invention, 
reference is made to the drawings. In the drawings, like references have 
been used throughout to designate identical elements. FIG. 1 depicts a 
color electrophotographic printing machine incorporating the features of 
the present invention therein. It will become evident from the following 
discussion that the present invention is equally well suited for use in a 
wide variety of printing machines, and is not necessarily limited in its 
application to the particular machine shown herein. 
Turning initially to FIG. 1, there is shown a color electrophotographic 
printing machine employing a photoconductive drum 10. Preferably, 
photoconductive drum 10 is made from a photoconductive material such as 
selenium. However, any suitable photoconductive material may be used. Drum 
10 rotates in the direction of arrow 12 to advance successive portions of 
the photoconductive surface sequentially through the various processing 
stations disposed about the path of movement thereof. 
Initially, a portion of photoconductive drum 10 passes through the charging 
station. At the charging station, a corona generating devices, indicated 
generally by the reference numeral 14 charges photoconductive drum 10 to a 
relatively high, substantially uniform potential. 
Next, the charged photoconductive surface is rotated to the exposure 
station. The exposure station includes an electronic subsystem that 
transmits a set of signals corresponding to a series of raster scan lines 
of different colors for the copy. These signals are transmitted to a 
raster output scanner (ROS) 16. ROS 16 includes a laser with rotating 
polygon mirror blocks. Preferably, a nine facet polygon is used. The ROS 
illuminates the charged portion of photoconductive drum 10 at a rate of 
about 400 pixels per inch. The ROS will expose the photoconductive drum to 
record three latent images. One latent image is adapted to be developed 
with cyan developer material. Another latent image is adapted to be 
developed with magenta developer material with the third latent image 
being developed with yellow developer material. The latent images formed 
by the ROS on the photoconductive drum correspond to the signals from the 
electronic subsystem. 
After the electrostatic latent image has been recorded on photoconductive 
drum 10, drum 10 advances the electrostatic latent image to the 
development station. The development station includes four individual 
developer units generally indicated by the reference numerals 18,20,22 and 
24. The developer units are of a type generally referred to in the art as 
"magnetic brush development units." Typically, a magnetic brush 
development system employs a magnetizable developer material including 
magnetic carrier granules having toner particles adhering 
triboelectrically thereto. The developer material is continually brought 
through a directional flux field to form a brush of developer material. 
The developer particles are continually moving so as to provide the brush 
consistently with fresh developer material. Development is achieved by 
bringing the brush of developer material into contact with the 
photoconductive surface. Developer units 18, 20 and 22, respectively, 
apply toner particles of a specific color which corresponds to the 
compliment of the specific color separated electrostatic latent image 
recorded on the photoconductive surface. The color of each of the toner 
particles is adapted to absorb light within a preselected spectral region 
of the electromagnetic wave spectrum. For example, an electrostatic latent 
image formed by discharging the portions of charge on the photoconductive 
drum corresponding to the green regions will record the red and blue 
portions as areas of relatively high charge density on photoconductive 
drum 10, while the green areas will be reduced to a voltage level 
ineffective for development. The charged areas are then made visible by 
having developer unit 18 apply green absorbing (magenta) toner particles 
onto the electrostatic latent image recorded on photoconductive drum 10. 
Similarly, a blue region is developed by developer unit 20 with blue 
absorbing (yellow) toner particles, while the red region is developed by 
developer unit 22 with red absorbing (cyan) toner particles. Developer 
unit 24 contains black toner particles and may be used to develop the 
electrostatic latent image formed from a black and white original 
document. Each of the developer units is moved into and out of the 
operative position. In the operative position, the developer unit is in 
the development zone with the magnetic brush being closely adjacent the 
photoconductive drum, while, in the non-operative position, the magnetic 
brush is spaced therefrom. During development of each electrostatic latent 
image only one developer unit is in the operative position, the remaining 
developer units are in the non-operative position. This insures that each 
electrostatic latent image is developed with toner particles of the 
appropriate color without comingling. In FIG. 1, developer unit 24 is 
shown in the operative position with developer units 18, 20 and 22 being 
in the non-operative position. The developer units in the non-operative 
position are sealed to prevent the escape of toner therefrom and to 
prevent contamination of the developer material in each developer unit. A 
shield or cover 54, located on one side of the development zone, seals the 
non-operative developer units located on that side of the development 
zone. A similar shield or cover 56, located on the other side of the 
development zone, seals the non-operative developer units located on that 
side of the development zone. A motor 48 coupled to developer carriage 50 
translates the developer units in a horizontal direction, as indicated by 
arrow 26, between the non-operative positions and the operative position. 
A housing 52 having a chamber therein, is mounted beneath developer 
carriage 50 for storing waste toner. 
After development, the toner image is moved to the transfer station where 
the toner image is transferred to a sheet of support material, such as 
plain paper amongst others. At the transfer station, the sheet feeder, 
indicated generally by the reference numeral 28, separates the uppermost 
sheet from a stack of sheets 30 supported on tray 32. The sheet is 
advanced to the transfer system, indicated generally by the reference 
numeral 34, and vacuum tacked to a perforated, flexible belt wrapped about 
a drum. The flexible belt moves about the drum in the direction of arrow 
58. At the transfer zone, portions of the sheet are released from the belt 
and pass between a corona generator and the photoconductive drum. The 
corona generator applies an electrostatic charge to the sheet to attract 
the toner image from the photoconductive drum to the sheet. As the sheet 
moves in a recirculating path with the belt, each of the differently 
colored toner images are transferred, in superimposed registration with 
one another, to the sheet to form the multi-color copy. The transfer 
system will be described hereinafter in further detail with reference to 
FIG. 2. 
After the last transfer operation, the sheet is released from the belt and 
advanced by a conveyor 36, in the direction of arrow 38, to the fusing 
station. At the fusing station the sheet passes through the nip defined by 
a heated fuser roll 40 and a pressure roll 42. The toner image contacts 
fuser roll 40 so as to be affixed to the sheet. Thereafter, the sheet is 
advanced to catch tray 44 for subsequent removal therefrom by the machine 
operator. 
A blade cleaner, indicated generally by the reference numeral 46, is 
periodically moved into and out of contact with drum 10. The blade cleaner 
is moved into contact with the photoconductive drum when there is no toner 
image thereon so as to remove residual toner particles remaining after the 
transfer operation. Any residual charge remaining on the photoconductive 
drum is also removed therefrom prior to the start of the next successive 
cycle by illuminating drum 10. 
Referring now to FIG. 2, there is shown further details of the transfer 
system. As depicted thereat, transfer system 34 has a tubular sleeve 60 
with a plurality of spaced ports 62 in and extending through the 
peripheral surface thereof. The sleeve is mounted stationarily on the 
printing machine frame. Sleeve 60 has slots 64 and 66 therein. An idler 
sprocket gear 68 is mounted internally of sleeve 60 in the region of slot 
64. A sprocket gear 68 is mounted resiliently, e.g. on leaf springs, to 
resiliently hold belt 74 against sleeve 60. A drive sprocket gear 70 is 
mounted internally of sleeve 60 in the region of slot 66. Motor 72 drives 
sprocket gear 70. A flexible transport belt 74 having a plurality of 
perforations, i.e. holes, therein is wrapped about sleeve 60. Belt 74 has 
a sprocket hole pattern along a side marginal region which meshes with 
sprocket gears 68 and 70. Belt 74 passes through slots 64 and 66 and the 
sprocket holes in the side marginal region therein meshes with idler 
sprocket gear 68 and drive sprocket gear 70. Idler rollers 76 and 78 are 
located on opposed sides of slot 64 to support belt 74 as it passes around 
sprocket gear 68. Similarly, idler rollers 80 and 82 are located on 
opposed sides of slot 66 to support belt 74 as it passes around sprocket 
gear 70. A vacuum source 84, e.g. a blower, is coupled by conduits to 
ports 62 and, in turn, to the perforations in belt 74 wrapped thereabout 
to vacuum tack the sheet to belt 74. A corona generator 88 is located in 
slot 66, opposed from photoconductive drum 10, defining the transfer zone. 
Baffle 86 guides the lead edge of the sheet to a location where it is 
acquired by belt 74. Motor 72 drives drum 10. Gear 75 is mounted on the 
shaft of drum 10 and rotates in unison therewith. Gear 75 drives gear 73, 
which, in turn, drives gear 70. Gear 70 moves belt 74 around the 
peripheral surface of sleeve 60 in the direction of arrow 58. In this way, 
belt 74 is driven in synchronism with drum 10, i.e. belt 74 and drum 10 
have the same tangential velocity at the transfer zone. The sheet, vacuum 
tacked to belt 74, moves in unison with belt 74 in a recirculating path. 
Thus, the sheet moves in synchronism with drum 10, i.e. at the same 
tangential velocity in the transfer zone. In this way, the sheet and the 
toner image enter the transfer zone in registration with one another. As 
the sheet enters the transfer zone, the lead edge thereof separates from 
belt 74 and is interposed between corona generator 88 and photoconductive 
drum 10. It is clear that the transport system for the sheet does not 
require lead edge gripper bars. Corona generator 88 sprays ions onto the 
back side of the sheet to apply an electrostatic charge to the sheet for 
attracting toner images thereto. As the sheet passes through the transfer 
zone, the lead edge is re-acquired by belt 74 and vacuum tacked thereto. 
This process is repeated for each cycle until all of the different color 
toner images have been transferred to the sheet in superimposed 
registration with one another. After the last toner image is transferred 
to the sheet, stripper finger 90 separates the sheet from belt 74 and 
guides it onto conveyor 36. Conveyor 36 is a vacuum transport and the 
sheet is vacuum tacked thereto. The sheet with the toner images thereon is 
advanced by vacuum transport 36 to the fusing station. 
In recapitulation, the apparatus of the present invention transfers 
successive toner images from a photoconductive member to a sheet. The 
sheet is vacuum tacked to a belt entrained about a stationary tubular 
sleeve. In this way the belt transports the sheet in a recirculating path 
through a transfer zone. At the transfer zone, a corona generator applies 
an electrostatic charge to the sheet to attract the toner image from the 
photoconductive drum thereto. Successive different color toner images are 
transferred to the sheet in superimposed registration with one another to 
form a color copy. 
It is, therefore, evident that there has been provided in accordance with 
the present invention, an apparatus for transferring successive different 
color toner images from a photoconductive drum to a sheet that fully 
satisfies the aims and advantages hereibefore set forth. While this 
invention has been described in conjunction with a specific embodiment 
thereof, it is evident that many alternatives, modifications and 
variations will be apparent to those skilled in the art. Accordingly, it 
is intended to embrace all such alternatives, modifications and variations 
as fall within the spirit and broad scope of the appended claims.