Method and means for transferring electrostatically charged image powder

A method and apparatus for transferring a powder image, consisting of electrostatically charged developing powder, from an image forming medium to an image receiving medium by bringing the image forming medium with the powder image thereon into contact with the image receiving medium, which is provided with a resiliently deformable surface layer. During contact a pressure is exerted between said image forming medium and the image receiving medium with the powder image therebetween, which transfer pressure is sufficient to transfer the powder image to the image receiving medium. In the contact zone an electric field is applied across the image forming medium and the image receiving medium in such direction that the electrostatically charged developing powder is subjected to a force towards the image forming medium.

FIELD OF THE INVENTION 
The present invention relates to a method for transferring 
electrostatically charged powder images from an image forming medium to an 
image receiving medium, and, in particular, to a method and apparatus 
which in combination with pressure transfers the images to an intermediate 
transfer means while applying an electric field across the mediums. 
BACKGROUND OF THE INVENTION 
Developing powder in which the particles have a charge opposite to that of 
the charge image to be developed is frequently used for developing 
electrostatic charge images formed on a suitable image forming medium in 
electrophotographic or electrostatic image forming processes. Normally, 
the charge on the powder particles is obtained by tribo-electric charging 
against a part of the developing device or against carrier particles mixed 
with the developing powder. Lack of uniformity frequently occurs in the 
development of large solid image areas with such developing powder. Such 
non-uniformity can be due to various reasons, such as vibration and 
impacts in the drive system for the image forming medium and/or the 
developing device, exhaustion of the stock of developing powder on the 
developing means, and divergence of the outgoing electric field in the 
case of large solid image areas (known as "edge field effects"). 
When such a non-uniformly developed image is transferred in an electric 
field (as is conventional) to a receiving material and then fixed thereon, 
the result is an image area with a non-uniform optical density. Typically, 
the edges have a greater optical density than the central part of the 
area. In the case of development with black developing powder this defect 
is less visible than in color developing where the defect is very 
distracting. Accordingly, it is an object of the invention to provide a 
means and method for reducing or eliminating this defect. 
It has surprisingly been found that the visual defect can be eliminated by 
the application of an electric field across the image forming medium and 
the image receiving medium in a direction such that the electrostatically 
charged developing powder is subjected to a force directed towards the 
image forming medium during pressure contact between the two mediums. 
Although the pressure adhesion transfer step is already known from U.S. 
Pat. No. 3,591,276, it has been found that the combination of adhesion 
transfer and a counteracting electric field gives the surprising result 
that a uniform thickness of the developing powder layer is obtained on the 
image receiving medium. The use of an electric field to transfer a toner 
image from an image forming medium to an image receiving medium is very 
well known for instance from U.S. Pat. No. 3,734,724, it discloses a 
conventional transfer method where developed images are transferred by an 
electric field from a transparent photoconductive element, e.g.. 
conductive film, to a metal receiving medium, e.g.. lithographic plate or 
nonmetallic medium wherein an electrical repelling potential is applied to 
the receiving medium, just before the transfer zone to prevent premature 
transfer of toner particles. 
In Japanese Application 59-50474 a supporting electric field is applied, 
directing from the image forming medium to a receiving belt, to improve 
the transfer rate of the pressure transfer step. 
SUMMARY OF THE INVENTION 
Generally, the present invention provides a method and means for applying 
an electric field across the image forming medium and the receiving media 
in a direction towards the image forming media. 
According to the invention, the layer of developing powder is not 
completely transferred at those parts of the image areas (edges) which 
have been too intensively developed, but at the less developed parts (the 
central parts) of an image area the powder layer is transferred intact, so 
that a uniform area filling is obtained on the copy. 
Other advantages of the invention will become apparent from a description 
of an electrophotographic apparatus using a presently preferred embodiment 
of the invention taken in connection with the accompanying diagrammatic 
drawing.

PRESENTLY PREFERRED EMBODIMENT 
Referring to FIG. 1, an endless photoconductive belt 1 is driven at a 
uniform speed by means of drive or guide rollers 2, 3 and 4. The image of 
an original disposed on platen 5 is projected by flashbulbs 6 and 7, 
through lens 8 and mirror 9 onto belt 1 after belt 1 has been 
electrostatically charged by corona device 10. 
After the flash exposure, the latent charge image formed on belt 1 is 
developed by a magnetic brush device 11 to give a powder image which is 
then brought into pressure contact with an endless intermediate belt 12 at 
a first transfer zone. Belt 12 is preferably made of or covered with a 
soft resilient, heat-resistant material such as, for example, silicone 
rubber. At the first transfer zone, powder image is transferred by 
adhesion forces from belt 1 to belt 12. 
After the image transfer, any remaining image residues are removed from 
belt 1 by means of cleaning device 13. Belt 1 is thereafter ready for 
reuse. 
Belt 12 is trained about drive and guide rollers 14 and 15 which are 
preferably located together with belt 12 in a space enclosed as much as 
possible by heat-insulating material 16. This enclosed space and, hence, 
belt 12 are heated by one or more heating elements in combination with 
infrared radiator 17 disposed inside roller 15. While belt 12 is advanced 
together with the powder image thereon, the powder image becomes tacky as 
a result of heating. In a second transfer zone the tacky powder image is 
then transferred under pressure to and fixed on a sheet of receiving 
material, for example paper, fed from reservoir 18 via rollers 19 and 20. 
Finally, the resulting copy is deposited in tray 25 by belt 22 trained 
about rollers 23 and 24. 
According to the invention, in the first transfer zone an electric field is 
applied at the first transfer zone via connections 30 and 31 across 
photoconductive belt 1 and intermediate belt 12. The direction of the 
field is such that the electrostatically charged toner powder is subjected 
to a force directed towards photoconductive belt 1. The thickness of the 
uniform layer of powder image to be formed on the belt 12 is selected by 
adjusting the dimension of the electric field. 
Adjusting the dimension of the electric field is based, on the one hand, by 
the magnitude of the adhesion forces exerted by intermediate belt 12 on 
the powder particles and, on the other hand, by a number of material 
parameters, such as the thickness of the photoconductive layer on belt 1, 
the electrical resistance of the developing powder and the particle size 
of the developing powder. Using a silicone rubber intermediate belt, for 
example, excellent results have been obtained with a photoconductor layer 
thickness of between 0 and 25 .mu.m and a counter-voltage of about 100V 
across the first transfer zone. The developing powder used had a specific 
resistance greater than 10.sup.14 ohms.cm and a particle size of between 2 
and 80 .mu.m. 
It is understood that the invention is not restricted to the above 
combination of material parameters to provide an optimum electric field 
according to the invention. A person skilled in the art will find it 
fairly simple to determine experimentally the optimum value for the 
counter-voltage to be applied for each combination of the relevant 
parameters. 
Thus, the invention is not restricted to insulating developing powders, but 
it is also possible to use more conductive developing powders which have 
become charged by induction. 
The invention can also be used in image-forming processes in which a latent 
image is developed by means of non-charged developing powder, e.g., 
magnetographic processes. The powder particles in the powder image formed 
must then be electrostatically charged prior to the transfer step. 
While a presently preferred embodiment of the invention has been described 
and shown, the invention may be otherwise embodied within the scope of the 
appended claims.