Voltage control bar for electrophotography

An electrophotographic apparatus is provided with an electrically conductive voltage control bar between the photoconductor drum or belt and the developer or developer/cleaner roll. The voltage control bar prevents toner filming on said cleaning roll, and makes it possible for that roll to be made of an insulating material such as molded plastic.

TECHNICAL FIELD 
The present invention is concerned with electrophotography. Specifically, 
it is concerned with placing an electrically conductive voltage bar 
between the photoconductor drum or belt and the developer/cleaner roll of 
an electrophotographic apparatus. 
BACKGROUND ART 
U.S. Pat. No. 3,884,572 discloses a roll made of materials which may be 
somewhat insulating, but the roller is used exclusively as a cleaner that 
works by mechanically rubbing toner from the surface of the 
photoconductor. Its purpose is to replace fiber brush or fur brush 
cleaners commonly used at the time of the patent. In the present 
invention, a plastic roll may be used, but it does not touch the 
photoconductor and is used only for transporting mix. 
U.S. Pat. No. 4,478,510 discloses a system for placing toner on a 
substrate, pel by pel, by shooting toner from a developer roll through an 
opening in a strip. This is done with an electric field between the 
developer roll and electrodes in the strip to pop the toner off the roll. 
In the present invention, we develop toner from a toner-carrier mix moving 
between the bar of the photoconductor. The electric field is applied 
between these two, in contrast to the situation in that patent. The toner 
itself jumps from the carrier, not the developer roll. 
United Kingdom Pat. No. 2,065,557 discloses a monocomponent cleaning system 
with a rotating magnet assembly rather than the conventional stationary 
one. The patent is similar to two component magnetic brush cleaning 
systems but for monocomponent toner. Although the drawings superficially 
resembles that of the present invention, it is obvious that the patent is 
totally irrelevant to the present invention. 
DISCLOSURE OF THE INVENTION 
Electrophotographic systems have employed magnetic brush development 
systems in various forms since its invention. It is currently almost the 
only means by which toner is applied to the photoconductor. It basically 
consists of a metal roll surrounding a set of magnets. The rotation of the 
roll and magnetic field together transports the developer mix against the 
photoconductor, allowing the toner to transfer where the voltage between 
the roll and the photoconductor is appropriate. In the present invention, 
we separate the two functions of the developer roll - transport of the mix 
and transfer of the toner - by placing a stationary conductive bar between 
the roll and the photoconductor. 
The present invention has several advantages over the conventional system. 
The bar is used as the developer or development/cleaning electrode 
(two-cycle process). The bar is an electric conductor placed between the 
magnetic cleaning roll and the photoconductor. It should be sufficiently 
wide so that the carrier chains do not simultaneously touch the roll and 
the photoconductor. Since the bar acts as the electrode in place of the 
roll, the bar-photoconductor separation should be comparable to the 
original roll-photoconductor separation in the absence of the bar. For the 
simplest situation, the magnetic cleaner roll and the voltage control bar 
are electrically connected. 
The use of the bar frees the roll from the burden of maintaining the proper 
potential. In this fashion, two needs for the roll are circumvented; (1) 
voltage control and (2) precise fabrication. It is no longer necessary for 
the roll to be fabricated to precise dimensions because the previous 
requirement for uniform development has not been transferred to the bar. 
Roller runout affects the mix volume only very slightly, because the bar 
has a slight averaging affect on the amount that gets to the nip. For 
these reasons it is now possible to make the magnetic cleaning roll of an 
insulating material. In particular, such an insulating material can be 
molded plastic. The ease and cost of producing the roll are therefore 
markedly reduced compared to corresponding machined metal parts. 
The use of an insulating material for the magnetic cleaning roll can lead 
to charge build-up on the roll. Excessive change accumulation is 
eliminated by brushing a metal strip against the roll. This strip is to 
control the charge rather than toner filming. 
It is thus seen that the present invention makes possible the use of an 
inexpensive insulating material instead of an expensive metal conductor 
and it also permits the use of a roll not necessarily machined to close 
tolerances. 
The voltage control bar is also advantageous in a two-cycle 
electrophotographic process. Conventionally the developer/cleaner roll has 
a propensity to film during the cleaning cycle because the process of 
removing toner from the photoconductor develops toner on the roll. This 
toner can then smear on the roll, which is very undesirable. According to 
the present invention, such toner film development on the magnetic roll is 
eliminated by the use of the voltage control bar. 
During the entire electrophotographic cycle, no electric field exists 
between the bar and the roll, and no toner develops on the roll. When 
necessary, the voltage between the roll and the bar can be adjusted to 
achieve this result of no toner accumulation. Because the large electric 
field is confined to the region between the roll and the photoconductor, 
any toner that would have developed on the roll without the bar during the 
cleaning deposits instead on the bar.

FIG. 1 is a side view, not to scale, of the apparatus of the present 
invention. As may be seen by inspection of the drawing, a photoconductor 
roll is separated from a magnetic cleaning roll. Toner carrier mix in the 
form of chains leaves the photoconductor roll and moves toward the 
magnetic cleaning roll. An electrically conductive voltage control bar is 
inserted into the space between the photoconductor drum or belt and the 
cleaning roll. Toward the bottom of the cleaning roll a metal discharge 
strip brushes against it if the roll needs to be discharged. 
The following example is given to illustrate a preferred mode of operating 
the present invention. A stainless steel bar 320 mil wide and 20 mil 
thick, spanning the length of the developer roll, was used on a 
developer-cleaner in a robot with the photoconductor drum. In the absence 
of the bar, the magnetic roll was found to be covered with toner after a 
minute of operation with development/cleaning voltage alternately applied. 
With the bar, absolutely no toner was seen on the roll after ten minutes 
of such operation. The bar itself was also clean, probably from the much 
larger relative motion of the mix in the region of high field. Toner was 
developed on the photoconductor during the development cycle, showing that 
sufficiently large fields existed between the bar and the photoconductor. 
The mix was found to flow in the bar-voltage conductor region as if the 
bar were absent. The leading edge of the bar, however, had to be 
sufficiently sharpened to separate the mix into the two regions.