Note: Descriptions are shown in the official language in which they were submitted.
Background of the Invention
Field of the Inventlon: The present invention
relates to a process for dry printing of information. The
process involves f.'orming a magnetic image on a master followed
by decorating the magnetic image with ferromagnetic toner
particles which are then electrostatically transferred to a
substrate capable of maintaining a charge and fixed in place.
Description_of the P'rior Art: Both xerography and
magnetography are known. Xerography involves: forming an
electrostatic charge on a photoconductive material such as
selenium; imagewise exposing the photoconductive material
to light whereby the exposed areas lose their charge; and
applying a pigmented, finely divided, electrically charged
powder which is attracted to and held on the electrostatic
image. The charged toner image is then transferred to copy
paper either with an opposite electrostakic charge or by
means of pressure.
In magnetography a magnetic image is formed, and
ferromagnetic particles applied thereto which adhere to khe ' '
magnetized areas of the image. The particles are then trans-
ferred to copy paper either by pressure or magnetically. The
pressure technique causes objectionable wear to the imaging
member and can also cause bu~ldup of a film on the imaging
~ member which.causes smudgin~. '
:~ In magnetic transfer it has been ~ound di~ficult
to effect trans~er of toner without erasing the latent magnet
~ image on the im~ging member
: Summ~ry o'f''t'he'Invent'ion
The~present invention reIat.es to ~orming a latent
.; .
~ ~ -2- ~
"~, ~
magnetic image, clecorating the latent magnetic image with
a uncharged ferromagnetic toner, and then transferriny the
toner to a substrate electrostatically whereby the problems
of pressure or maynetic transfer are overcome. By uncharged
toner we mean toner which has not purposely been charged by
means such as corona or triboelectric means but which may
contain small triboelectric charges of either polarity.
Description of the Drawings
Fig. 1 is a schematic side view of a printer used
to perform the process of the present invention.
Fig. 2 is a side vie~ of a printer equipped with
a magnetic printing head used to perform the process of the
present invention.
Fig. 3 is a diagram of the exposure of the magnetic
master by radiant energy.
-~ Fig. 4 is a diagram of the latent ma~netic ima~e.
Fig. 5 is a diagram of the toned magnetic ~mage
superposed adjacent the copy paper.
Fig~ 6 is a diagra~ of the copy paper decorated with
the transferred image.
Fig. 7 is a diagram of the final copy decorated with
` the fused image.
~-~ Fig. 3-7 show the stepwise formation of ~he latent
ma~netic imaye, the decoration thereof with toner, the transfer
of the toner to the copy paper, and the fusion of the toner to
the copy paper. `
~ n aluminized polyester film having a layer of
periodically ma~netized chromium dioxide particles in a binder
adhered to the surface thereof whlch is to be used as- a copying
-
-3-
~'
`: : `:
42
device is exposed to uniform illumination as shown in Fiy. 3.
As can be seen from Fig. 3 the printing on the document pre-
~ents the illumination from reaching the magnetized chromium
dioxide particles, thus, leaving them magnetized in the areas
under the printing. On the other hand, those areas of the
document being copied which contain no printing do not prevent
the illumination from reaching the magnetized chromium dioxide
particles, thus heating them to above their Curie point of
about 116C, thus demagnetizing them. In this way the latent
magnetic image shown in Fig. 4 is prepared. Ferromagnetic
toner particles are appliecl to the latent magnetic image to
form a developed magnetic image. Copy paper is brought into
superposition with the magnetic image as shown in Fig. 5. A
corona discharge device then electrostatically charges the
back of the paper. Upon separation of the paper from the
grounded drum an electrostatic force sufficient to o~ercome
the magnetic attraction between the previously uncharged
toner particles and the latent magneti,c image is generated,
thereby cau$i,n~ the toner particles to transfer to the copy
paper and be adhered thereto wi,th surprisingly high effic~ency
as shown in Fi~. 6. Groundin~ is a means of preventing the
accumulation of eIectrostatic char~es on the surface c)~ th,e
~; drum, which may interfere ~ith the' printi,ng process~ This
electrostatic transfer has no ef~ect on the'laten~ ~agnetic
image which ~ay be reused many times~ The transferre~ tonex ' ,
particles- are then fused to the copy paper as shbwn in ~ig. 7
by heat.
~ he~toner particles prefe~ably are magnetic pigments
encapsulated in a suitable binder. Generally the toner
-4-
: ~ :
i~ .
particles have an average size ranging from 10 to 30 microns
with a preferred average size ranging from 15 to 20 microns.
Spherical particles such as prepared by spray drying are
preferred because of their super:ior flow properties which
can be enhanced by the addition of minute amounts of a flow
additive such as fumed silica. A further description of the
preparation of toner particles may be found in U.S. Patent
3,627,682. When using the apparatus disclosed herein the
toner particles should have a low electrical conductivity.
If the particles have high conductivity, they will be passed
back and forth between the drum and the paper causing a
diffuse image and low transfer efficiency. Generally the
toner powder electrical conductivity is less than 1 x 10 13
mho/cm. The ferromagnetic component can consist of hard
magnetic particles or a binary mixture of hard and soft mag-
netic particles. The magnetically soft particles can be iron
or another high-permeable, low-remanence material, such as
certain ferrites, for example, (Zn, Mn) Fe2O4, or permalloys.
The magnetically hard particles can be an iron oxide, prefer-
ably Fe3O4, ~-Fe2O3, other ferrites, for example, BaFel2Olg,
chi-iron carbide, chromium dloxide or alloys of Fe3O4 and
nickel or cobalt. A magnetically hard substance has a high-
intrinsic coercivity, ranging generally from about 40 to about
40,000 oersteds and a high remanence (20 percent or more o~
::
~- the satuXation magnetization~) ~hen removed from the magnetic
f;eId. Such substances are o~ low permeability and re~ui:re
high f~elds for magnetic saturation. A~magnetioally so~t
substance~has low coerci~ity, for exampler one oersted or
less, high permeab~lityr permitting saturatlon to be obtained
5-
:: y ~ ~
,,
;
with a small applied ~ield, and exhibits a remanence of less
than 5 percent of the saturation ma~netization. A particularly
preferred toner has an average particle size of 20 microns
and contains ~0 weight percent thermoplastic binder 30 weight
percent Fe3O4 (ma~netite) and 30 weight percent soft iron
(carbonyl iron).
Referring to Fig. l, the document which is to be
copied is placed on shelf ll and urged against gate 12. The
copier is then activated to lift gate 12 and lower feed roll
13 into contact with the document. Feed roll 13 feeds the
document into the nip between endless belt l~ and drum 15.
Endless belt 14 is made of a transparent film such as poly
(ethylene terephthalate) about 2-7 mils in thickness. Rollers
16, 17, and 18 serve to drive and guide endless belt 14. The
surface of drum 15 is preferably a poly(ethylene terephthalate)
film about 5 mils in thickness. The convex sur~ace of this
film is coated with a conAuctive layer such as by bein~ alumi-
nized with a layer of aluminum to a surface resistivity of
l to 1,000 ohms per square. The aluminum layer is ~rounded.
The conductive support may also ~e a plastic such as polyoxy-
methylene sleeve coated with aluminum, nickel, copper or other
conducti~e metal. The support may also be the conductive metal
itself. The surface o~ the aluminum is coated with a layer of
ferroma~netic material such as ac~cular chromium dioxide i,n an
al~yd or other suitable binder. Generally the acicular chromium
dioxide layer is from 0.001 to 0.012 mm in thickness and con-
tains from 40 to 85 ~ei~ht percent acicular chromium dioxide
and from 15 to 60 ~ei~ht percent alkyd or other su,itable res~n
binder. Suitable acicular chromium dloxide can be Prepare~d in ,'
.
-6- ,
X
.
accordance with the teachings of U.S. Patent 2J956~955~ issued
October 18, 1960, to Paul Arthur, Jr. However, the preferred
acicular chromium dioxide particl.es are produced by the tech-
niques disclosed in U.S. Patents 2,923,683 and 3,512,930.
Generally the chromium dioxide produced as disclosed in these
patents consists essentially of uniform small acicular parti-
cles whose average length is 1 with aspect ratios of 6:1,
the said oxides containing from 58.9 to 61.9% chromium, and
exhibiting an X-ray diffraction pattern which analysis sho-ws
to correspond in its entirety to a tetragonal structure having
cell constants of aO = 4.~1 + 0.10A and cO = 2.90 ~ 0.10~.
The acicular chromium dioxide layer should ha~e a resistivity
of from about 1 x 10 1 to 1 x 10+9 ohm - cm which insures that
any electrostatic charge imposed thereon will dissipate in less
than a millisecond. The coating of the conductive support may
be accomplished in a variety of ways, e.g., by gravure coating
a slurry of CrO2 and resin in tetrahydrofurancyclohexanone on
a web of aluminized polyethylene terephthalate or by spray-
coating a conductive metal sleeve, etc. It is preferred to
orient the CrO2 by passing the wet coated conductive support
between the pole pieces o~ two bar magnets (approximately 1500
; gauss average ~ield strength) aligned with the same poles facin~
:.; one another. The magnetic flux lines orient the acicular CrO2
all in the same di.rection. Ratios of magnetic remanence to
magnetic saturation (Br/Bs) of up to 0.80 with an intrinsic
coercivity (iHc) of 510 to 550 oersteds have been achie~ed by
~; this method.
.~ Drum 15 rotates in a counterclock.wise direction. The
ferromagnetic coatin~ on~the drum is magnetized by premagnetizer
-
:
: ~ -7-
' ~
i . ~ , . . .
- -
3~
19, which records a spatial periodic pattern. We ~ind 300 to
1000 magnetic reversals per inch on the magnetizable surface
to be a working range and prefer about ~00-600 magnetic
reversals per inch. The technique of roll-in magnetization
can be used to structure the CrO2 surface, wherein a high
permeability material such as nickel, which has been surface
structured to the desired groove width is placed in contact
with the DC magnetized CrO2 sur~ace. A permanent magnet or
an electromagnet is placed on the backside o~ the permeable
material. As the structured high permeability material is
brought in contact with the CrO2 surface, the nickel concen-
trates the magnetic flux lines at the points of contact
resulting in the magnetization of the CrO2 coating. The
CrO2 surface can also be thermoremanently structured by
placing the continuously coated CrO2 surface on top of a
magnetic master recording of the desired periodic pattern.
An external energy source then heats the CrO2 surface above
the Curie temperature. As the surface cools below the Curie
point the periodic magnetic signal from the master film
thermoremanently magnetized it. As little as 20 oersteds
can be used to structure the CrO2 in this way, whereas over
200 oersteds are needed to apply detectable magnetism to the
CrO2 at room temperature. Also, a scanning laser ~eam may
be used to structure a uniformly magnetized CrO2 surface.
Alternatively, a film structured by grooves
containing acicular chromium dioxide can be used for the
surface of dru~ 15 in which case a simple DC magnet can be
used as prema~netizer 19. Generally from 200 to 300 grooves
per i~ch across the drum will be used giving 4no to 600
8 -
, ~
~ ,
73~:
magnetic reversals per inch. Then the magnetized drum surface
in contact with the document is moved past exposure station
indicated generally at 20. The exposure station consists
of lamp 21 and reflector 22. A suitable lamp 21 is a xenon
flashl which has a color temperature equivalent to 6,000C.
The surface of drum 15 is exposed stepwise until the entire
document has been recorded as a latent magnetic image on
the surface of drum 15. The chromium dioxide as used herein
has a Curie temperature of about 116C. The printing of
the document being copied shades the areas of the chromium
dioxide over which such printing is situated during exposure
thereby preventing their reaching the Curie point. Thus,
after exposure, the surface of drum will have magnetized
areas of chromium dioxide correspanding to the printed areas
of the document being copied.
After exposure, the document bein~ copied is
dropped into tray 23.
The imagewise magnetized drum 15 is rotated past
a toner decorator. The toner decorator comprises a trough.
24 fitted with rapidly rotatin~ roll 25, and bar 26, The
toner is a fine powder of a magnetic mater~al such as i.ron
oxide encapsul.ated in a thermoplastic resin having a
relatively lo~ so~tening point of from 75 to 120C. The
toner ~enerally will have an average parti.cle size of ~rom
lQ to 3~ microns. ~ vacuum knlfe 31 is used to remove
~hatever toner particles may have adventitiously become :.
attached to the dema~net~zed:areas o~ the chromium dioxide
on the surface:of drum 15. :~
:
X :' '
The paper 32 on which the copy is to be made is
Eed from roll 33 arouncl idler rolls 34, 35, and 36 to feed
rolls 37 and 38. If desired, other substrates such as
fabrics and films may be used rat:her than paper. Backing
roll 39 cooperates with roll 40 equipped with cutting edges
41. Rolls 39 and 40 are activated by means not shown to cut
the paper 32 to the same length as the length of the document
being copied. The paper is then fed by feed rolls 42 and 43
into physical contact with the surface of drum 15. The paper
32 in contact with the surface of drum 15 is fed past corona
discharge device 44. Corona discharge device 44 preferably
is of the type known as a COROTRON* which comprises a corona
wire spaced about 11/16" (17.5 mm) from the paper and a metal
shield around about 75 percent of the corona wire leaving an
opening of about 90 around the corona wire exposed facing
the paper 32. The metal shield is insulated from the corona
wire. The metal shield is maintained at ground potential.
~ Generally the corona wire will be from 0.025 to 0.25 mm. in
; diameter and will be maintained at from 3,000 to 10,000 volts~
The corona wire may be at either a negative, or a positive
potential with negative potential being preferred. The
COROTRON 44 electrostatically charges the back side of paper
32. This lightly pins the paper to the drum, and upon sepa- -
ration of the paper from the drum causes image-wise transfer
of toner particles to paper 32. At the region in which paper
. ~ .
32 separates from the surface of drum 15 under the action of -
endless vacuum b~elt 50, the toner particles remain heId in
image-wise fashion to paper 32. We have observed that
.:
: . .
* denotes trade mark
~ ,
':' .
~; --10--
'
~: . - : , . . : . . . ~
COROTRON ~ should be placed over the arc of intimate contact
between the paper and the drum for best results. If COROTRON
44 is not so located or if there are forces present preventing
the paper 32 from forming an arc of intimate contact, the
resultant image becomes fuzzy. There is only a light amount
of pressure between paper 32 and the surface of drum 15 (i.e.,
merely enough to hold them adjacent each other). The pressure
between paper 32 and drum 15 is essentially entirely generated
by the electrostatic attraction generated by corona discharge
device or COROTRON ~. Nevertheless transfer efficiency is
surprisingly high and approaches 100% for toners with nontacky
surface characteristics and low conductivity. The paper 32
is then removed from the surface of drum 15 by the action of
the vacuum belt 50 in conjunction with the action of puffer
45 that forces it onto the surface of endless vacuum belt 50
driven by rollers 51 and 52. Endless vacuum belt 50 trans-
ports paper 32 past infrared lamps 53, 54, and 55 which heat
the thermoplastic resin encapsulating the ferromagnetic
material in the toner particles causing them to melt and fuse
to the paper 32. The decorated paper 32 is then fed into ~
; hopper 56. ~`
When multiple copies of the same document are
to be made, a control meansr not shown, is so actuated that
drum 15 is continuously rotated without activating demagne-
tizer 60, vacuum box 61, magnetizer 1~ or lamp 21 because
the electrostatic transfer of the toner particles does not
~ .
affect the magnetic state in the chromium dioxide layer on
the surface of drum 15. Many copies can be printed from a ;
single exposure at speeds of up to 300 feet/minute. Over
~`
.: -
:
-11-
, :
: ~ .
10,000 copies from a slngle image have been demons-trated.
Toner particles which do not transfer, may themselves become
electrostatically charged in the transfer zone adjacent to
the COROTRON 44. Subsequently, these particles will pick up
other particles electrostatically and ultimately transfer
these to produce unwanted markings. To prevent this, a
static eliminator 62 is used. Conveniently, this i.5 an AC
corona discharge bar.
When it is desired to prepare copies from a
different document image eraser 60, which conveniently can
be a DC magnetic head in the case of continuously coated
film, is activated and the chromium dioxide is uniformly
magnetized. Whatever toner particles may be remaining on
the previously magnetized areas of chromium dioxide, are
removed by vacuum box 61 which preferably acts in conjunc-
tion with brushes. The chromium dioxide is then magnetiz d
by magnetizer 19 to provide a periodic magnetic structure
and the process described above repeated.
It is to be understood that substrates other than
paper, such as cloth and dielectric films, can be used.
Fig. 2 shows an alternate form of printer using
a magnetic printing head such as have been reviewed by
W. H. Meiklejohn in A.I.P. Conference, Proc. (Pt. 2) 10,
(1973) pages 1102 to 1114. In the example of Fig. 2 mag-
~- netic printing head 71 is used to form the latent image on
the magnetic surface of drum 72 which has the same structure
as drum 15 described above. Magnetic printing head 71 is a ~
multitrack prlnting head such as have been developed for ~ -
' ~ '; ',
~; , .
~ ~ -12-
~ixed head per track discs. Preferably track density will
be about 200 magnets per inch which is adequate to print
with good resolution. Generally the multitrack write head
will be activated by head drivers which can be activated
' by a read-only memory character generator. The read-only
memory character generator can respond to an in~ormation
storage device such as a magnetic tape which may be part
of the printer or remote therefrom. Alternatively, a
keyboard can activate the multitrack write head r wherein
magnetic structuring is accomplished with the magnetic
write head. Toner particles which do not trans~er may
themselves become elec-trostatically charged in the transfer
zone adjacent by the COROTRON 96. Subsequently, these
particles will pick up other particles electrostatically
and ultimately transfer these to produce unwanted markings~
To prevent this a static eliminator lQ7 is used. Conven- - -
iently, this is an AC corona discharge bar. The thus
~` magnetized drum 72 is rotated counterclockwise past a toner
slinger which comprises a trough 73 fitted with rapidly
rotating rolls 74, and stationary bar 75. A vacuum knife
81 is used to remove whatever toner particles may have
:, . .
adventitiously become attached to the demagnetized areas ~-
of the chromium dioxide on the surface of drum 72.
~ . :
The paper 82 to which the toner pattern is to be
applied is fed from roll;83 around idler rolls 84, 85, and
; 86 to feed rolls 87 and 88. Backing roll 91 cooperates with
roll 92 e~ulpped with cutting edges 93~ Rolls 91 and 92 are
~; activated by means not shown to cut the paper 82 to the
, :
., . .:
: .. ......
; ~ -13-
; :
.:
~ :
73~
desired length. The paper is then fed by feed rolls into
physical contact with the surface of drum 72. The paper 82
in contact with the surface of drum 72 is fed past corona
discharge device or COROTRON 96 which electrostatically
charges the back of the paper. Upon separation of the paper
from the grounded drum an electrostatic forc~e sufficient to
overcome the magnetic attraction between the previously
uncharged toner particles and the latent magnetic image is
generated, thereby causing the toner particles to transfer
to the copy paper and be adhered thereto. The paper 82 is
then removed from the surface of drum 72 by the action of
puffer 97 that forces it onto the surface of endless vacuum
belt 98 driven by rollers 99 and 100. Endless vacuum belt
98 transports paper 82 past infrared lamps 101, 102, and
103 which heat the thermoplastic resin encapsulating the
ferromagnetic material in the toner particles causing them
to melt and fuse to the paper 82. The decorated paper 82
is then fed into hopper 104. The drum can be continuously
rotated to make a plurality of copies.
When it is desired to make a different print,
image eraser 105 is actuated to erase the latent magnetic
image and vacuum box 106 is used to remove any toner
particles remaining on the old latent magnetic image.
The process can also be operated using either a
thermal stylus or an electrical stylus to create the latent
magnetic image, the former by conductive heating and the
latter by electrical resistance heating of the imaging
layer. Either stylus can demagnetize selected areas by
'~
. ~
: ~ . .. ,: '
.~
73~;2
hea-ting previously magnetizecl mat.erial above the Curie point
or it can magnetize selected areas thermoremanently by allowing
the heated imaging material to cool through its Curie point
in the presence of a magnetic field. A field of 20 to 200 Oe
adjacent to the stylus has been found to be sufficient for
thermoremanent magnetization, while a much stronger field of
at least 800 Oe is necessary to magnetize unheated chromium
dioxide sufficiently. It is recognized, of course, that
imaging with electromagnetic or thermal transducers onto a
continuous coating with its surface magnetized with a DC
magnet will require modulation consistent with establishing
magnetic gradients for adequate toner attraction in magnetized
image areas.
-: -
'
~ .
- ~ . . ~ - . .. . .
