Non-impact printer with magnetic ink reorientation

A non-impact printer having a support for magnetic ink particles loosely distributed on its surface in mutually spaced aggregates of irregular height. An electrical field of short duration, established in a print position between the particles and a shaped print electrode, charges the particles and attracts them to an intervening recipient sheet. The printed image is rendered more uniform by magnetic reorientation of the aggregates of greater height before printing, in a field having components normal to the electrical field in the region of the print position.

RELATED APPLICATIONS 
This application has been assigned to the same assignee as copending 
applications Ser. No. 710,282 entitled, "Inks for Pulsed Electrical 
Printing and Methods of Producing Same", Ser. No. 710,280 entitled, 
"Magnetic Inking Apparatus for Pulsed Electrical Printing", and Ser. No. 
710,283 "Structured Donor Sheet for High-Resolution Non-Impact Printer," 
all filed on even date herewith, and Ser. No. 710,292 entitled, "Pulsed 
Electrical Printer with Dielectrically Isolated Electrode", filed Aug. 2, 
1976, and incorporates the disclosures thereof by reference as hereinafter 
specifically noted. 
BRIEF SUMMARY OF THE INVENTION 
This invention relates generally to apparatus for pulsed electrical 
printing, as contrasted to mechanical impact and electrostatic printers. 
Mechanical printers deliver ink to a recipient sheet by mechanical 
movement from a supply or donor sheet or strip. Electrostatic printers 
generally employ multi-step procedures involving sequential selective 
charging of surfaces and transfer of toner particles by electrostatic 
attraction. The present invention relates more directly to printers of the 
general type described in the U.S. Pat. to Robert W. Haeberle, et al. No. 
3,550,153 dated Dec. 22, 1970. The printing process of said patent 
consists generally in providing an electrically conductive ink, a 
receiving or recipient paper or sheet, and a means for producing an 
electric field of a predetermined shape to be printed, in pulses between 
the ink and paper. In a typical application this field may be in the order 
of 1000 volts across a gap of between 5 and 10 mils, this gap being 
measured from the ink through the thickness of the receiving sheet to the 
pulsed field shaping electrode. The ink or pigment is in mobile, 
particulate form. During the brief presence of the electric field, the ink 
particles or pinnacles are first charged by conduction of current from 
other particles closer to a supporting sheet, detached by the electric 
field, and then caused to transfer to the receiving paper by the force 
induced solely by the electric field. As described in said patent, the 
particles of conductive ink are initially deposited upon a surface of an 
ink support described as a donor sheet. In general, the amplitude and 
duration of the electric pulses must be so related as to cause an 
efficient transfer of sufficient ink for the required printing density, 
without causing an electrical breakdown or discharge between the 
electrodes. 
As described in said patent, the surface of the donor sheet closest to the 
recipient sheet includes electrically conductive particles of a printing 
material dispersed in a high resistance medium. The pulsed electrical 
field is applied to charge the printing particles selectively. The charged 
particles are subsequently transferred to the adjacent surface of the 
recipient sheet under the influence of the applied field. This is an 
efficient charging technique, whereby a charge is imparted to the printing 
particles in a very brief space of time. Because these conductive printing 
particles are dispersed in a high resistance medium, the electric field 
lines of the applied field become concentrated upon the conductive 
particles; thus these field lines tend to avoid the high resistance medium 
separating the conductive particles. The concentration of the field lines 
is a consequence of the concentration of induced charge upon these 
particles, and in addition it represents a focusing of lines of force upon 
the charged particles. 
The force on a particle depends on the electric field strength at the 
particle and the charge on the particle, being proportional to the product 
of the charge and the field strength. Both factors are increased when 
charge accumulates on a conductive particle, since the gathering of the 
charge is accompanied by an increase in the density of field lines, which 
means an increase in the field strength, measured in lines per unit area. 
In printers of the type described in said patent, there is a 
non-homogeneous distribution of conductive particles in a poorly 
conducting medium, particularly a depth distribution which leaves the 
particles in mounds or towers, and these particles will be subjected to 
strong forces tending to detach them from their neighbors and transfer 
them from the donor sheet to the recipient sheet. In the practice of the 
printing technique described in said patent, the high resistance medium 
need not be a solid material, and in some cases it can be air. That is, if 
the donor sheet is properly constructed and inked, in such a way that the 
conductive pigmented particles are arranged in mounds and towers, the air 
surrounding and separating these mounds and towers can play the role of 
the poorly conducting medium in which the conductive particles are 
dispersed. 
A donor sheet for non-impact printing, in which the poorly conducting 
medium is a solid dielectric composite material, is described in U.S. Pat. 
No. 3,833,409 to John Peshin, dated Sept. 3, 1974. This donor sheet is 
described as having a high lateral resistivity to aid in confining the 
printing to the immediate vicinity of the printing electrode face. 
A further improvement upon the printing apparatus of said U.S. Pat. No. 
3,550,153 is described in U.S. Pat. No. 3,898,674 to Paul L. Koch dated 
Aug. 5, 1975. This patent describes a shield electrode that confines the 
printing field distribution more narrowly than would be possible with an 
unshielded printing electrode. It has been found that with the printing 
field distribution thus confined, satisfactory high resolution printing is 
obtained with a conductive base or support for the pigment particles, 
provided that the structure of the base or support and the arrangement of 
the pigment particles thereon are such as to produce a partial isolation 
of the conductive pigment particles into mounds and towers that are 
separated by a poorly conducting medium, such as air or a suitable solid 
material. 
When the base material of the ink support is conductive, the hazard of 
electrical breakdown during the printing pulse is increased. This hazard 
can be reduced if the pulsed electrode is encapsulated within a dielectric 
material such as a glass or a plastic such as Kapton, a polyimide sold by 
E. I. duPont de Nemours & Co., which in either case will withstand, 
without breakdown, extremely high electric field strengths. As described 
in said application Ser. No. 710,892, entitled "Pulsed Electrical Printer 
with Dielectrically Isolated Electrode", the pulsed electrode can be 
recessed within the volume enclosed by a shield electrode, the remainder 
of this volume being filled by a dielectric material that can withstand 
without breakdown the high electric fields generated by the printing 
pulses. 
Said application Ser. No. 710,280 entitled "Magnetic Inking Apparatus for 
Pulsed Electrical Printing" describes a magnetic inking process preferably 
using ink particles prepared as described in said application Ser. No. 
710,282 entitled "Inks for Pulsed Electrical Printing and Methods of 
Producing Same". The latter application describes a process whereby 
conductive printing particles are prepared by incorporating iron oxide or 
other magnetizable material in each particle. In the magnetic inking 
station there is an arrangement of magnets that keeps a reservoir supply 
of magnetizable conductive printing particles in a compact strip or bead 
adjacent to the moving base layer, which may be a belt or a rotating drum. 
In either case the surface of the moving base layer or ink support is 
microcavernous, and because of the roughness of this surface there is 
friction between the surface of the support and the outside of the bead. 
The bead rotates as a result of this frictional force coacting with a 
magnetic field so distributed in the region of the bead that it restricts 
the location of the bead and gives it freedom to rotate within this 
restricted location. 
At the same time, the combination of frictional and magnetic forces peels 
off enough particles from the rotating bead to leave the base layer 
covered by a coating of magnetizable conductive printing particles. This 
coating is largely in the form of mounds and towers, as a result of the 
orientation of the magnetic field lines in the region where the 
peeling-off process takes place. These lines are oriented steeply, with 
respect to this surface of the base layer, so that the magnetizable 
particles form into chains that stretch and break as the bead and the base 
layer pull apart. When the chains are stretched they are also made thinner 
and become laterally separated from each other. The broken fragments that 
remain attached to the surface of the moving base layer are consequently 
largely separated so as to form individual mounds and towers. 
It often happens, however, that certain of the towers of magnetizable 
conductive printing particles are so long and tenuous that the printing 
pulse, when the base layer has moved from the inking station to the 
printing station, detaches these towers as whole strings of particles, 
rather than individually as one particle at a time. If this is the case, 
then the printing can have a speckled appearance, which is particularly 
undesirable if the printing is of the type known as facsimile with the 
capability of rendering a range of shades of grey as described in the U.S. 
Pat. to James C. Maxwell No. 3,964,388, dated June 22, 1976. 
It is a principal object of this invention to reduce and minimize the 
speckling effect described above. It is a further and related object to 
provide improvements in the printing apparatus that will permit the 
printing of dark images having uniform fill, high edge definition, high 
resolution and strong contrast or print density. 
According to this invention, means are provided to produce a strong 
magnetic field at the printing station. The lines of this field are 
oriented substantially parallel to the surface of the coated base layer or 
ink support as it moves through this station. By this means the longest 
and most tenuous of the towers of particles, being those most likely to 
print as speckles, will be bent over at their weaker points, so that their 
upper segments are turned substantially parallel to the strong magnetic 
field, hence substantially parallel to the surface of the base layer. 
Preferably, this turning action takes place just before the base layer 
reaches the center of the printing station. Thus the printing pulse will 
not be operative upon easily detached segments of particle chains that 
might print as speckles, but upon relatively stronger towers and mounds, 
from the summits of which the pulse will detach individual particles. As a 
consequence, the printing pulse will detach fewer chains that would 
transfer as clumps of particles, and the printed image will include fewer 
speckles and will have a more evenly printed appearance.

DETAILED DESCRIPTION 
FIG. 1 illustrates diagrammatically a pulsed electrical printer embodying 
the invention. The printer comprises a reinking station 12, a printing 
station 14, a fusing station 16, and other associated components as 
hereinafter described. An endless belt 18 of high electrical resistance 
material, having a roughened or microcavernous outer surface, is driven 
continuously by a drive motor 20. This belt and the other forms of ink 
support described herein are preferably constructed as described in said 
application Ser. No. 710,283 entitled "Structured Donor Sheet for 
High-Resolution Non-Impact Printer", the description of which is 
incorporated herein by reference. Also, other methods may be used in 
particular applications. In particular, base sheets having high lateral 
resistivity, for example, as described in said U.S. Pat. No. 3,833,409, 
and sheets of other forms having high resistance or insulating base 
structures as described in said U.S. Pat. No. 3,550,153, may be used. In 
any case, the surface of the ink support should have a roughened or 
microcavernous surface as hereinafter further described. A hopper 22 
deposits particulate printing particles 24 upon the surface of the belt, 
which then travel past a lower magnet 26, which may be a permanent magnet 
or an electromagnet energized by a variable source 28. In certain 
embodiments, an overhead magnet 30 may also be employed. The printing 
particles contain magnetizable material and are preferably produced by the 
method described in said copending application Ser. No. 710,282 entitled 
"Inks for Pulsed Electrical Printing and Methods of Producing Same", the 
description of which is incorporated herein by reference. In the presence 
of the magnetic field, the printing particles deposited on the belt 18 
form a rotating bead 32 from which a portion of the particles are peeled 
off and travel toward the printing station. 
Details of the operation of the reinking station 12 are described in said 
application Ser. No. 710,280 entitled "Magnetic Inking Apparatus for 
Pulsed Electrical Printing", and are incorporated herein by reference. As 
the belt 18 leaves the reinking station 12, the magnetizable conductive 
printing particles thereon are ordinarily distributed in mounds and towers 
as shown in FIG. 5, but some of these towers will be long and tenuous, 
with one or more locations where the towers are unusually thin and weak. 
These locations are relatively easily broken under the force of the 
electrical printing pulse, so that prior to this invention, the particle 
chains above these locations would be detached by the pulse in some cases 
as whole chains rather than as individual particles detached one after 
another. 
In the printing station, a source 34 of brief electrical pulses applies 
such pulses selectively between one or more print electrodes 36 and a base 
electrode 38. For simplicity, only a single print electrode 36 has been 
illustrated, whereas a practical printer is provided with a plurality of 
electrodes and means for selectively energizing them, as described in said 
U.S. Pat. Nos. 3,898,674 and in 3,733,613 to Paul L. Koch, et al. dated 
May 15, 1973. Also, it will be understood that although the illustrated 
print electrode is shaped for printing a round dot as used in facsimile 
and dot matrix alphanumeric printers, other shapes of electrodes may be 
employed. As shown in FIGS. 1 and 3, and in accordance with the teachings 
of said U.S. Pat. No. 3,898,674, the electrode 36 comprises a metallic 
field shaping electrode 40, and electrically insulating material 42, a 
metallic shield electrode 44 and a supporting body 46. By connections 48 
and 50, the shield electrode and the base electrode are held at the same 
electrical potential. 
By the action of brief electrical printing pulses between the field shaping 
electrode 40 and the base electrode 38, printing particles are transferred 
from the belt 18 to a web or sheet of ordinary untreated paper 52 passing 
from a supply roll 54 to a take-up roll 56. After the deposit of printing 
particles on the recipient paper 52, the latter passes through a fusing 
station 16 which provides sufficient heat to fuse the particles, thereby 
spreading them out and causing them to be more firmly attached to the 
paper. Details of the fusing step are given in said application Ser. No. 
710,282 entitled "Inks for Pulsed Electrical Printing and Methods of 
Producing Same", and are incorporated herein by reference. 
The rotating bead 32 is a loose aggregation of magnetizable conductive 
printing particles, these particles being preferably produced by the 
method described in said last-mentioned application. A portion of the 
contour of the bead is roughly cylindrical in shape, and in cross section 
it approximates a circle that is flattened on the side adjacent to the 
moving belt 18. The friction of the moving belt propels the lower surface 
of the bead toward the printing station 14, but the magnetic field 
distribution within the reinking station 12 is such as to oppose the 
forward motion of the magnetizable grains or particles in the bead, once 
these grains have moved a short distance past a corner 58 of the magnet 26 
and have reached a region of weakened magnetic field. In some embodiments 
the lower magnet 26 is used alone, and in other embodiments the field may 
be produced by the magnet 26 in combination with the overhead magnet 30 as 
described in said application Ser. No. 710,280 entitled "Magnetic Inking 
Apparatus for Pulsed Electrical Printing". Instead of moving forward out 
of the strong field region, most of the grains in the lower part of the 
bead 32 will move upwardly away from the belt surface and participate in 
the rotational motion of the bulk of the grains in the bead. However, at 
the point where most of these grains turn and move upwardly, away from the 
surface of the belt, the orientation of the magnetic lines of force is 
such that the magnetizable grains will be aligned in small chains or 
threads running between the belt surface and the surface of the bead that 
is separating itself from the belt. Some of these chains or threads will 
elongate during the separation process, and will then break in two, 
leaving a portion of each broken chain on the surface of the belt, 
oriented upwardly from the belt surface. 
FIG. 1A illustrates a variant of the embodiment of FIG. 1, in which the 
belt 18 is replaced by an endless belt 60 made of metal or other 
conductive material having a roughened or microcavernous surface. In this 
case a brush 62 or other equivalent means is connected with the source 34, 
whereby the belt 60 itself functions as a base electrode, thereby 
replacing the function of the electrode 38 in FIG. 1. 
In the embodiments of FIGS. 1 and 1A, the printing station 14 is provided 
with a magnet 64 that is operable to reorient some of the mounds and 
towers of printing particles. More specifically, the field produced by 
this magnet is operable at the locations of weakness in the particle 
chains mentioned above and illustrated in FIG. 5, whereby the upper 
segments of certain of the towers can be bent over. More particularly, the 
magnetic field is designed to turn the upper segments of the weaker towers 
until they are substantially parallel to the base layer, and substantially 
perpendicular to the direction of the applied electric field generated by 
the printing pulse as illustrated in FIG. 6. The bent-over segments are 
then no longer strong focal points on which the electric lines of force 
will gather, and there will be less charge drawn to the segments. The 
number of such segments that are detached and printed will be greatly 
reduced, and the printed regions will accordingly be less speckled in 
appearance. 
Thus the magnet 64 generates a strong magnetic field distribution whose 
magnetic lines of force in the vicinity of the print head 36 are 
substantially parallel to the average surface of the donor sheet or belt 
18 where it passes closely opposite to the printing electrode 36. 
Ordinarily, the magnet 64 may be a simple horseshoe magnet located on the 
opposite side of the belt 18 from the print head 36. 
The magnet 64 occupies a position in close proximity to the base electrode 
38. In certain cases it may be mechanically convenient to combine the base 
electrode with the magnet structure, forming a composite structure that 
provides a magnetic ground plane, that is, an electrically grounded 
surface with associated magnetic field lines that are substantially 
parallel to the surface in a central region located directly opposite to 
the print head 36. 
In the embodiment of FIG. 2, many of the elements are the same as those 
illustrated in FIG. 1. However, a thin-walled rotating drum 66 of high 
electrical resistance material serves as the donor sheet or support for 
the printing ink particles, replacing the moving belt 18. The outer 
surface of the drum 66 is microcavernous, providing sufficient frictional 
force to maintain the rotational movement within the bead 32. The inking 
station 12 contains, as in FIG. 1, the lower magnet 26 and the overhead 
magnet 30, establishing a magnetic potential well that restricts the 
forward motion of the bead 32. The inking station 12 also contains the 
hopper 22 with its reservoir of ink or pigment particles 25 by which the 
supply of particles in the bead 32 is replenished. The embodiment of FIG. 
2 also includes the magnet 64, the function of which is the same as in 
FIG. 1. 
The embodiment of FIG. 2A is similar to the embodiment of FIG. 2, except 
that the drum 66 is replaced by a drum 58 of metal or other electrically 
conductive material, and a brush 70 is connected to the source 34, whereby 
the drum 68 replaces the function of the base electrode 38. 
In the embodiments of FIGS. 2 and 2A, the rotating bead 32 acts to meter 
and distribute the magnetizable conducting printing particles over the 
outer surface of the rotating drum 66 or 68. The drum carries its inked 
surface around to the printing station 14. The printing station contains 
the print head or electrode 36. The receiving web or recipient sheet 52 
passes between the print head and the ink surface of the rotating drum. 
Means similar to elements of FIG. 1 move the receiving web 52 from a 
supply roll to a take-up roll through a fusing station. 
FIG. 4 is an enlarged view showing the ground electrode 38 and the 
ground-plane magnet 64. Also shown are some of the magnetic lines of force 
72 generated by the magnet 64. The approximate locations of the print head 
36 and the recipient web 52 are also shown. It is evident that the donor 
sheet, represented by the moving belt 18, will move from left to right in 
the direction of the arrow through regions in which the magnetic lines 
change in their directions, from steeply upward to slightly upward, to 
horizontal, to slightly downward and then to steeply downward. The 
printing takes place from a portion of the donor sheet where the magnetic 
lines are approximately horizontal, parallel to the surface of the donor 
sheet or belt 18 and accordingly perpendicular to the direction of the 
electric field vector between the print electrode 36 and the ground 
electrode 38. 
FIG. 5 is a highly magnified representation of the donor sheet or belt 18 
at a position between the magnetic inking station 12 and the printing 
station 14. There is shown a mound 74 and a tower 76 of magnetizable 
conductive printing particles. Also shown is a more tenuous tower that has 
a sturdy lower section 78, a thin point 80, and an upper segment 82. This 
upper segment would ordinarily be readily detached by a printing pulse, 
and would thereafter be accelerated by this pulse and transported as a 
clump of ink particles, which would print as a speckle. This detachment of 
the upper segment 82 would occur if the particle configuration shown in 
FIG. 5 were to enter without change into the region opposite to the print 
head 36, and to be subjected there to the electric field generated by the 
printing pulse. 
However, with the ground-plane magnet 64 present as shown in FIGS. 1 and 2, 
the particle configuration of FIG. 5 is modified as it approaches the 
print head. As the particle configuration of FIG. 5 enters part-way into 
the magnetic field, at the left extremities of the lines 72 as viewed in 
FIG. 4, these lines are tilted forwardly with reference to the direction 
of travel of the belt. Therefore, the upper segment 82 of the tenuous 
tower with the sturdy lower section 78 will attempt to align itself with 
the changing direction of the magnetic field lines. Because of the 
weakness of the thin point 80, the upper segment 82 will bend over. 
In FIG. 6, this same portion of the donor sheet or belt is shown in 
cross-section, after it has moved to a position directly opposite to the 
printing electrode 36. Here the magnetic lines of force are substantially 
parallel to the ground electrode and to the averaged surface of the donor 
sheet. The upper segment 82 of the tenuous tower with the sturdy lower 
section 78 will here be aligned roughly parallel to the magnetic field 
lines, hence roughly parallel to the averaged surface of the donor sheet, 
and roughly perpendicular to the electric field lines that are generated 
by the printing pulse. The topmost part of this tenuous tower will also 
have been lowered by the magnetic bending action, as is evident by 
comparing FIGS. 5 and 6. 
As a consequence of the magnetic reorientation of the ink particles, the 
electric lines of force associated with the printing pulse will 
accordingly concentrate more strongly upon the competing mound 74 and 
tower 76, leaving the bent-over segment 82 only weakly charged and hence 
much less likely to be detached and printed by the applied electrical 
pulse. 
The magnetic field strength close to the printing electrode is chosen to be 
sufficient to bend over those towers of magnetizable conductive printing 
particles that are so long and tenuous that whole segments, containing 
many particles, are liable to be detached as clumps of particles during 
the printing pulse and to print as speckles whose size is too large to be 
considered acceptable. At the same time, the magnetic field strength close 
to the printing electrode is chosen to be smaller than that which would 
bend over the stronger towers whose presence is needed for the efficient 
operation of the printing mechanism. In a typical application, the field 
strength close to the printing electrode is in the range between 1000 and 
2000 oersteds. 
In cases where the characteristics of the printing particles are subject to 
variability from time to time, the strength of the magnetic field 
generated by the magnet 64 is preferably adjusted empirically. For this 
purpose a variable source 84 is connected with the magnet as shown in FIG. 
1. Thus the magnetic field strength is increased to a magnitude sufficient 
to reduce substantially the number of clumps of particles that are 
detached by the printing pulse and printed as speckles of objectionably 
large size. Some reduction in print intensity will accompany this 
reduction in speckling, but this intensity reduction can ordinarily be 
matched by a compensating intensity increase, obtained by an increase in 
the voltage of the printing pulses, or the duration of these pulses, or 
both. 
It will be understood that while the term printing pulse may refer to a 
single unipolar pulse, which may be square or rounded in waveform, an 
acceptable printed image can also be obtained through the use of a 
printing wave form that is bipolar, and a sequence of bipolar pulses 
comprising a plurality of such pulses can be used for printing a single 
image. Accordingly, the above-mentioned compensating intensity increase 
can also be obtained by an increase in the number of single pulses in the 
bipolar sequence of pulses of alternating polarity that constitute a 
printing waveform.