Method for forming a charge pattern

An ink jet apparatus deposits a pattern of charged, substantially colorless droplets on an insulating imaging surface. The droplets are dried to leave an electrostatic charge pattern. The charge pattern is developed using conventional techniques and can be electronically read.

BACKGROUND OF THE INVENTION 
This invention relates generally to a method and apparatus for forming a 
charge pattern on an imaging surface and more particularly to a method for 
forming a charge pattern using an ink jet apparatus. 
The formation of charge patterns an imaging surfaces is well known, 
especially in the xerographic arts. In xerographic processes, typified by 
the Carlson process originally disclosed in U.S. Pat. No. 2,297,691, a 
photoconductive insulating imaging surface is first uniformly 
electrostatically charged. The charged surface is then exposed to 
imagewise radiation to which the surface is sensitive, such as light, and 
the charge in the radiation-struck area is dissipated. The charge remains 
on the imaging surface in the non-radiation-struck areas to form a charge 
pattern. Such a charge pattern is commonly referred to as an electrostatic 
latent image. 
The uniform charging of the imaging surface is typically accomplished, for 
example, by the method disclosed in U.S. Pat. No. 2,588,699 to Carlson 
which involves the use of an ion producing filament or filament arrays 
operating on corona discharge principles. However, such uniform charging 
can be accomplished by contacting the surface with a charging electrode as 
disclosed in U.S. Pat. No. 2,774,921 or with a charging brush as disclosed 
in U.S. Pat. No. 3,146,385. Other methods of uniform charging include the 
use of pin electrode arrays as disclosed in U.S. Pat. Nos. 2,934,650; 
3,649,830; 3,655,966 and 3,689,767. 
An alternative method of forming a charge pattern by uniformly charging an 
imaging surface with a charging electrode through a mask is described by 
Gundlach in U.S. Pat. No. 2,912,586. 
The charge pattern thus formed is oftentimes made visible or developed with 
marking material by development processes well known in the xerographic 
arts. A typical such development process is described by Carlson in U.S. 
Pat. No. 2,297,691. 
A method for forming a charge pattern on an imaging surface alternative to 
the method of charging through a mask is desirable. Such an alternative 
method which can produce charge patterns responsive to electrical input 
from, for example, a computer or a remote optical scanning device, is 
especially desirable. 
Ink jets are well known in the art as a means for direct writing on an 
imaging surface. Ink jets normally project a dyed or pigmented liquid onto 
an imaging surface responsive to electrical or mechanical control. Various 
types of ink jets are known. Some produce a stream of liquid which is 
broken into droplets by ultrasonic vibration as the stream emerges from a 
nozzle. Other ink jets rely on an electric field to draw droplets from the 
open end of a small nozzle. Still others use pulsing mechanisms to squirt 
droplets from an orifice. 
In many of the known ink jets direct writing systems the droplets are 
charged as they exit the ink jet orifice. The droplets are most often 
charged so that their trajectory from the ink jet orifice to the imaging 
surface can be controlled by electrons placed along the trajectory. The 
electrodes are usually electrically controlled, for example, by computer 
output or by remote optical scanning of an original image. 
Typical examples of direct writing with charged, colored particles from ink 
jets are shown in U.S. Pat. Nos. 3,596,275 to Sweet and 3,852,772 to Hecht 
et al. Sweet discloses deflection of charged droplets to create an image 
pattern on a surface. Sweet shows the use of electrodes to deflect the 
droplets. Hecht et al shows uncharged droplets impinging on a receiver 
sheet while selectively charged droplets are deflected. 
It is to be noted that the use of deflecting electrodes requires that the 
ink jet orifice be spaced a distance from the imaging surface sufficient 
to permit the electrode to have an effect on the trajectory of the ink 
droplet. Such spacing is sometimes undesirable in compact arrangements of 
apparatus. 
The direct writing ink jets of the prior art generally make use of dyed or 
pigmented liquids. Such liquids are known to dry in the ink jet orifice 
when not in frequent use, causing clogging problems. One attempt to 
overcome the clogging problem common to most direct writing ink jets 
centers around increasing the orifice size. However, increasing the 
orifice size undesirably reduces the resolution of the directly written 
image. The larger orifice size results in the image being written with 
large droplets which are capable of less image definition. 
A method and apparatus for forming a charge pattern on an insulating 
surface using a stream of ionized fluid, such as gas, is disclosed in U.S. 
Pat. No. 3,715,762 to Magill et al. However, the method so disclosed 
requires a second non-ionized, fluid stream to deflect the ionized stream 
when no charge is desired on the insulating surface. Ionized gases are 
known to be unstable and difficult to control with accuracy, and there is 
no way in such a system to visibly inspect the charge pattern prior to 
development, if desired. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to create charge 
pattern on an imaging surface. 
It is a further object of the invention to create a charge pattern on an 
imaging surface responsive to electrical input. 
It is also an object of the invention to generate a charge pattern on an 
imaging surface utilizing an ink jet apparatus. 
It is yet another object of the invention to construct a charge pattern of 
useful resolution on an imaging surface. 
It is an object of the present invention to electronically read charge 
patterns on an imaging surface. 
It is yet a further object of the invention to render visible charge 
patterns created from colorless ink droplets. 
A further object of the invention is to write a latent image on a substrate 
with colorless ink droplets and to develop the latent image with 
electrostatic toner. 
These and other objects are achieved, generally speaking, by a method for 
forming a charge pattern on an insulating imaging surface which comprises 
depositing a droplet layer of charged, substantially colorless droplets in 
a pattern configuration on the surface, the droplets being deposited by an 
ink jet means, and allowing the droplets to dry, leaving a charge pattern 
on the surface. Alternatively, the surface is uniformly pre-charged and 
selective portions of the surface are discharged by such droplets charged 
with a polarity opposite that of the surface. After the droplets are dried 
a charge pattern remains on the imaging surface. 
In further steps, the charge pattern is made visible by development and is 
read by electronic recognition equipment.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring more specifically to FIG. 1 there is shown an ink jet apparatus, 
generally designated 1, which comprises reservoir section 2 and nozzle 
section 3. Ultrasonic vibrator 4 is attached to nozzle 3 so that when 
activated vibrator 4 causes a vibration of orifice 5 at the end of nozzle 
section 3 opposite reservoir section 2. 
Liquid 6 is maintained under pressure in reservoir section 2 so that a 
steady stream of liquid 6 flows from orifice 5. The vibrations translated 
to orifice 5 by vibrator 4 cause the stream of liquid 6 to break up into 
droplets 7. 
Droplets 7 pass through annular ring 8 as they leave ink jet apparatus 1. 
Annular ring 8 and ink jet apparatus 1 are electrically connected through 
power source 9 so that droplets 7 acquire a charge as they pass through 
annular ring 8. 
Control switch 10 is placed in the connecting circuit between power source 
9 and ring 8 so that the potential to ring 8 may be interrupted. Droplets 
7 do not obtain a charge when passing through ring 8 when switch 10 is 
open. 
Switch 10 is connected to switch control mechanism 11 which may be a simple 
manually operated device or may be, for example, an electronic computer or 
a remote optical scanner. 
It is to be understood that the ink jet apparatus of FIG. 1 is not to be 
considered limiting but is illustrative of any of the various kinds of ink 
jet apparatus which are useful in the present invention. Many of the 
useful sorts of such apparatus are mentioned above. Descriptions of other 
useful ink jet apparatus can be found in U.S. Pat. No. 3,747,120 to 
Stemme, in the publication "Ink Droplet Printing Devices" by Robert D. 
Carnahan, TAPPI, Vol. 58, No. 7, July 1975, pages 82-86 and in the 
publication "High Frequency Recording With Electrostatically Deflected Ink 
Jets" by R. A. Sweet, The Review of Scientific Instruments, Vol. 36, No. 
2, Feb., 1965, pp. 131-136. 
Liquid 6 may be any suitable substantially colorless liquid. Because the 
liquid is not used to mark the imaging surface as in direct writing 
techniques, it need not be pigmented or dyed. However, it is sometimes 
desirable to use a small amount of dye in the liquid so that the image 
created by the droplet layer can be observed to development. The liquid 
should be capable of being charged. Typical such liquids are water and 
liquids of a higher volatility such as the alcohols. Volatile solvents 
containing electrolites are especially well suited for use in this 
invention because of their quick drying properties and their ability to 
accept charge. The preferred electrical properties for ink jet printing 
inks described by Kamphoefner in "Ink Jet Printing" IEEE Transactions On 
Electron Devices, Vol. ED-19, No. 4, April 1972. 
Because the colorless liquids such as water and alcohols leave little or no 
residue upon drying, the ink jet nozzle clogging problem mentioned above 
is substantially reduced and smaller ink jets giving higher resolution in 
the charge pattern can be used. An additional advantage of using smaller 
droplets is that charging rates can be increased on droplet streams of a 
fixed flow as described in detail by Schnieder et al in "Stability of an 
Electrified Liquid Jet", Journal of Applied Physics, Vol. 38, No. 6, p. 
2599-2605, May 1967. Charge patterns of high electrical density are 
established by the present invention while avoiding the 
resolution-limiting disadvantages of highly pigmented inks, described 
above. 
The amount of charge placed on each droplet by ring 8 or by any of the 
other useful charging means is any useful amount. The charge placed on the 
droplet should produce an electrostatic charge on the imaging surface 
sufficient for the required purposes. For example, if the charge pattern 
on the surface is to be developed by known xerographic means, the charge 
required on the surface will depend on the effective capacity of the 
surface and the development means. For example, with CZ1900 dielectric 
paper (dielectric constant about 3 and thickness about 5 micrometers) the 
voltage for magnetic brush development should be about twice as high as 
that for electrophoretic development (usually about 150v.). Electronic 
reading of the charge pattern will generally require a charge of less 
magnitude. 
For example, a charge of 6 .times. 10.sup.-13 coul./droplet is used for 
droplets 0.073mm in diameter at a charging potential of 150v and a droplet 
rate of 10.sup.5 droplets/sec. from a single orifice. Such an arrangement 
is equivalent to a current density of about 1.43 .times. 10.sup.13 
amps/cm.sup.2 and results in a charge on a dielectric imaging surface 
producing about 300v. Typically, on a droplet of 0.073mm diameter charges 
of from about 6 .times. 10.sup.-15 to about 1 .times. 10.sup.-12 
coul/droplet are useful. A charge of about 6 .times. 10.sup.-15 
coul/droplet is readily detectable by most reading means and yet is 
sufficiently small to avoid electrostatic interaction between droplets. 
The charge on the droplet is varied sometimes to control the density of 
the image which is made visable in the subsequent development step. 
Referring more specifically to FIG. 2 there is shown in perspective view an 
apparatus for performing the method of the present invention by depositing 
charged particles. on an imaging surface. Ink jet nozzle 12 is arranged to 
scan imaging surface 13 in the direction shown by the arrow. At the end of 
each scan it is indexed and returned to a starting position to begin a 
subsequent scan. 
A colorless liquid is held in reservoir tank 14. As the nozzle 12 scans 
surface 13 nozzle 12 is selectively activated by electronic control unit 
15 to deposit charged droplets on surface 13 to form charge pattern 16. 
Any suitable imaging surface 13 may be used. The surface should be capable 
of holding charge pattern 16 at least until it has been used for its 
intended purpose. For example, surface 13 should hold charge 16 until it 
is developed or "read" by an electronic recognition apparatus 
Typically surface 13 is a dielectric material such as plastic film, rubber, 
or dielectric paper. Other useful materials for surface 13 are deformable 
thermoplastics for use in deformation imaging systems such as those 
described in U.S. Pat. Nos. 3,320,060; 3,338,710; 3,404,001 and 3,615,387. 
Still other useful materials for surface 13 are charge deformable fluids 
such as, for example, liquid crystals. Surface 13 can also be formed from 
a conductive material having a barrier layer overcoating. A dielectric 
paper such as CZ1900 available from Crown Zellerbach is frequently 
preferred because of its subsequent usefulness as a document after 
xerographic development of the charge pattern. 
An alternative method (not shown) for practicing the present invention 
includes pre-charging the imaging surface with any suitable means, such as 
a corona discharge device, and placing in pattern configuration on the 
surface a droplet layer of substantially colorless droplets of the 
opposite polarity from the uniform charge on the imaging surface. The 
charge on the droplets at least partially neutralizes the charge on the 
surface so that a charge pattern remains on the surface in the 
non-neutralized areas. 
Referring more specifically to FIG. 3 there is shown in greatly enlarged 
perspective view a portion of an imaging surface 13 which is being scanned 
by multiple orifice ink jet recording head 17 in the direction shown by 
the arrow. Head 17 deposits liquid droplets in a droplet layer on surface 
13. The droplets are charged in image area 18 and uncharged in non-image 
area 19. The individual droplets on the layer prevent charge spreading 
between droplets. 
A charge pattern is formed on surface 13 in image area 18 by the charged 
droplets deposited there. 
Referring more specifically to FIG. 4 there is shown schematically and in 
cross section an automatic apparatus for producing developed images on an 
imaging surface corresponding to charge patterns placed on the surface by 
an ink jet apparatus in accordance with the method of the present 
invention. 
Continuous imaging surface 20 is unrolled from supply roll 21 and moved in 
the direction shown by the arrow. Multiple ink jet recording head 22 
comprises an array of adjacent ink jet nozzles. The array is substantially 
the same width as surface 20. A charge pattern is established on surface 
20 as it passes head 22 by either the method shown in FIG. 2 or FIG. 3. 
Grounded support means 37 enables an equal and opposite charge to be 
established on the opposite side of surface 20 from the charge pattern. 
Such a grounded support means 37 in effect reduces the capacitence of 
support 20 so that undesirably high voltages are not created by the 
charged droplets. 
A colorless liquid from reservoir tank 14 is selectively charged and 
deposited on surface 20 by head 22 in accordance with input from 
electronic control unit 15. As discussed above, unit 15 can provide head 
22 with input from a variety of sources such as, for example, optical 
scanning devices and computers. 
In FIG. 4, input unit 15 is from optical scanner 38 which scans original 39 
at a speed synchronous with the movement of surface 20 as it passes head 
32. 
The liquid droplets on surface 20 are dried by heat source 23 as surface 20 
moves over support rollers 24 leaving an electrostatic charge pattern on 
surface 20. Any suitable heat source may be used. The heat source should 
be capable of drying the droplets relatively quickly without disturbing 
their location. Typically, such suitable sources of heat include heat 
lamps, electric coils, low-pressure air knifes, heated rollers and the 
like. Radiant or thermally conductive heat sources are preferred over 
forced air drying apparatus because of the reduced opportunity they 
provide for disturbing the droplets during drying. 
The charge pattern on surface 20 is observed by electronic reader 40 and is 
displayed on cathode ray tube 41 for visual inspection prior to 
development at developing station 25. 
After drying of the droplets, the charge pattern remaining on surface 20 is 
developed at developing station 25. Methods of developing charge patterns 
are well known in the art, and any suitable such method may be used. 
Typically such development methods include cascade development, powder 
cloud development, magnetic brush development, polar liquid development, 
donor development, fluidized bed development and the like. Disclosures of 
such well known development methods are found, for example, in U.S. Pat. 
Nos. 2,681,551 and 2,825,814 to Walkup; 2,618,552 to Wise; 2,846,333 to 
Wilson; 3,084,043 to Gundlach and 3,015,305 to Hall. 
The developed charge pattern on surface 20 is then fixed to surface 20 by 
any suitable means such as radiant heat-fixing means 26. Suitable fixing 
methods and apparatus are disclosed in greater detail in U.S. Pat. Nos. 
3,130,064; 3,667,280; 3,655,280; 3,215,116 and 3,591,276. 
After development and fixing of the image, surface 20 is collected on 
rewind roller 27. 
It is to be understood that other uses can be made of the charge pattern on 
surface 20 other than development and fixing as shown in FIG. 4. For 
example, the charge pattern can be read by an electronic recognition 
device. 
However, if it is desirable to, for example, electronically read the charge 
pattern on surface 20 and then to reuse the surface, it can be uniformly 
discharged by such means as an AC corotron. Alternatively, if surface 20 
is photoconductive, it can be discharged by exposure to light while 
grounded. 
The invention enables a variety of alternative methods of operation. In one 
alternative method, surface 20 is uniformly charged to one polarity by a 
charging means such as corona device 42. Droplets carrying a charge of the 
opposite polarity are placed in imagewise configuration on surface 20 by 
head 22. The charge on surface 20 is neutralized by the oppositely charged 
droplets, leaving an imagewise charge pattern which is the reverse of the 
droplet pattern. This charge pattern can be developed at developing 
station 25 or observed by an electronic reader or both. 
In one alternative embodiment, the charge pattern (either a positive or 
reverse pattern) is observed by reader 40 and then erased by erasing 
corotron 43. In such an alternative embodiment, surface 20 can be reused. 
Methods of making charge patterns on imaging surfaces according to the 
present invention will now be described by way of example by which other 
useful variations and procedures will become clear to those skilled in the 
art. 
EXAMPLE I 
An ink jet apparatus similar to that shown in FIG. 1 is arranged to deposit 
droplets on an imaging surface as shown in FIG. 2. The ink jet is scanned 
across the surface and indexed after each scan. While scanning, it is 
selectively activated to deposit charged droplets on the imaging surface 
in a desired pattern. 
Water is used as the colorless liquid and a charge of 3 .times. 10.sup.-13 
coul./droplet is supplied to each droplet. The droplets are deposited on a 
dielectric plastic imaging surface. 
After the scanning by the ink jet is completed, the surface is scanned with 
an electrometer to determine the location and strength of the charge on 
the surface. A charge pattern is observed which is substantially 
equivalent in all dimensions to the pattern of charged droplets placed on 
the imaging surface. The pattern has a strength of about 100v. 
EXAMPLE II 
An ink jet apparatus similar to that shown in FIG. 4 is constructed so that 
it has a length sufficient to reach the width of the imaging surface. One 
thousand individual ink jets are arranged along the apparatus. The imaging 
surface is chosen to be an 81/2 inches wide roll of CZ1900 dielectric 
paper. 
Isopropyl alcohol is selected as the colorless liquid with which the ink 
jet apparatus is loaded. The individual ink jets in the apparatus are 
addressed by a computer which has been programmed to produce droplets in a 
charge pattern on the imaging surface as it moves relative to the 
apparatus. The computer output is synchronized with the speed of the 
surface. 
The computer addressing system is activated and the imaging surface is 
moved past the apparatus at a speed of about 10 inches/sec. The apparatus 
covers the entire surface with droplets, with selected ones of the 
droplets being charged to produce a pattern of words in a Century 
Schoolbook 10pt. italic font. 
After the droplets are deposited, they are dried by radiant heat and the 
remaining charge pattern is developed by a toned magnetic brush passing 
over the surface. The developed image is fused to the paper surface using 
radiant heat from an electric resistance coil. An image of high resolution 
is observed. 
It will be appreciated that other variations and modifications will occur 
to those skilled in the art upon reading of the present disclosure. For 
example, several charge patterns may be sequentially established and 
developed on a single surface with each development being in a different 
color to result in a multicolor composite image. Such variations are 
intended to be within the scope of this invention.