Note: Descriptions are shown in the official language in which they were submitted.
~77~4~
BACKGROUND OF THE INVENTION
Thls inventlon relates to electrostatlc prlntlng and
photocopylng~ partlcularly at hlgh speeds.
Electrostatlc printers and photocoplers share a number o~
common feàtures as a rule, although they carry out di~ferent
processes. Electrostatlc prlnters and photocoplers which are
capable of produclng an image on plaln paper may generally be
contrasted in terms of the method and apparatus used to create
a latent electrostatic image on an intermediate member.
Coplers generally do so by uniformly charging a photoconductor
electrostatlcally ln the dark, and optically exposlng the
charged photoconductor to an lmage corresponding to the image
to be reproduced. Electrostatic printers use non~optical means
to create a latent electrostatic image on a dielectric surface,
in response to a signal indicative of an image to be createdO
In theory, after creation of the electroskatic latent image,
the same apparatus could be used to carry out the common steps
of toning the image, transferring lt to plain paper~ and
preparing the member bearing the electrostatic latent image for
a subsequent cycle, usually by erasure of a residual latent
electrostatlc image. It would, in fact, be desirabie to
standardlze the apparatus to perform these functions.
One commonly employed principle for generating ions is the
corona dlscharge from a small diameter wlre or a polnt source.
~L8~7g~4
Illustrative U.S. Patent Nos. are P. Lee 3,358,289; Lee F.
Frank 3,611,414; A.E. Jvlrblls 3,623,123; P~J. McGill
3,715,762; H. Bresnik 3,765,027; and R~Ao Fotland 3,961,564.
Corona discharge3 are used almost excluslvely ln electrostatic
coplers to charge photoconductors prlor to exposure, as well as
for dlscharglng. These appllcatlons require large area blanket
charging/discharglng, as opposed to formation of discrete
electrostatic images. Un~ortunately, standard corona
discharges provide limlted currents. The maxlumum discharge
current density heretofore obtained has been on the order of 10
microamperes per square centimeter. This can impose a severe
printlng speed limitation. In additlon~ coronas can create
signiflcant màlntenance problems. Corona wlres are small and
fragile and easlly bro~en. Because o~ their high operating
potentials they collect dirt and dust and must be frequently
cleaned or replaced.
Corona dlscharge devlces which en~oy certaln advantages
over standard corona apparatus are dlsclosed ln Sarid et al~,
U.S. Patent No. 4,057,723; Wheeler et al. 4,068,284; and Sarld
4,110,614. These patents disclose various corona charglng
devices characterized by a conductlve wire coated with a
relatively thlck dielectric material, in contact wlth or
closely spaced from a further conductlve member. A supply of
posltive and negatlve lons is generated ln the air space
surroundlng the coated wlre, and ions of a particular polarlty
~37~
are extracted by a dlrect current potentlal applled between the
further conductive member and a counterelectrode. Such devices
overcome many of the above-mentioned disadvantage~ Or prior art
corona charglng and dlscharging devlces but are unsuitable for
electrostatlc imaglng~ Thls limitation ls inherent ln the
feature o~ large area charging, which does not permit formation
of discrete, well-deflned electrostatic images. This prior art
corona device requlres relatively hlgh extraction potentials
due to greater separation from the dielectric receptor.
Various toner lmage trans~er methods are known in the prior
art. The transfer may be accomplished electrostatlcally, by
means of a charge of opposite polarity to the charge on the
toner partlcle~, the former charge being used to draw the toner
particles off the dielectric member and onto the image
receptor. Patents lllustratlve o~ this transfer method include
U.S. Patent Nos. 2,944,147; 3,023,731; and 3,715,762.
Alternatively3 the image receptor medium may be passed between
the toner-bearlng dielectric member and a transfer member3 and
the toner lmage transferred by means o~ pressure at the polnt
of contact. Patents lllustrative of thls method lnclude U.S.
Patent Nos. 3,701,966; 3,907,560; and 3,937,571. Usually, the
toner image is fused to the image receptor subsequently to
transfer of the lmage, at a further process statlon.
Postfusin~ may be accomplished by pressure, as ln U.S~ Patent
No. 3,874,894~ or by exposure of the toner particles to heat,
as ln U.S. Patent No. 3,023,731, and Re. No. 28,693.
7~
It ls posslble, however, to accompllsh transfer and fusing
of the lmage slmultaneously J as shown for example ln the
patents cited above as illustrative of pressure transfer. This
may be accomplLshed by a heated roller, as in Re~ No. 28~693,
or simply by means of high pressure between the lmage-bearing
dielectric member and a transfer member, between which the
image receptor passes.
A problem whlch is typically encountered in transferring a
toner image solely by means of pressure is the exlstence of a
resldual toner image on the dlelectrlc member a~ter image
transfer, due to inefficiencles in toner transfer. The
residual toner particles require scraper blades or other
removal means, and accumulate over time at the various process-
stations associated with the dlelectric memberg including the
apparatus for forming the latent electrostatic image. These
toner accumulations decrease the reliability of the apparatus,
necessltating service at intervals. Furthermore~
lnefficlencies in toner transfer may lead to mottllng of the
images formed on the image receptor sheets~ These problems
have not been overcome in the prior art through the use of
extremely high pressures at the transfer nip.
A phenomenon which is commonly observed when sub~ecting
rollers to high pressures is that of "bowing" of the rollersO
This phenomenon occurs when the rollers are sub~ected to a high
compressive force at the ends, thereby imparting a camber to
77~
each roller. The ef~ect ls to have hlgh pressure at the ends
of the rollers but lower pressure at the center. It is known
ln the prlor art to alleviate thls problem when encountered ln
pressure fusing apparatus by skewing the pressure rollers, i.e.
by adJustlng the mountlng Or the rollers to create an obllque
orlentatlon of the roller axes. Representative Unlted States
patents lnclude U.S. Patent Nos. 3,990,391; 4,188,104;
4,192,229; and 4,200,389. This technique has the dlsadvantage
of causlng "walking" of a receptor sheet fed between the rolls.
In addltlon, this apparatus commonly encounters the problem Or
wrinkllng of the receptor sheets.
Hardcoat anodlzation of aluminum and alumlnum alloys ls an
electrolytlc process which is used to produce thlck oxide
coatlngs with substantlal hardness. Such coatings are to be
distlnguished from natural fllms of oxlde whlch are normally
present on aluminum surfaces and from thln, electrolytically
formed barrler coatings.
The anodlzatlon of alumlnum to form thlck dielectric
coatings takes place in an electrolytlc bath contalnlng an
oxlde, such as sulfurlc or oxallc acld, ln whlch alumlnum oxlde
ls sllghtly soluble. The productlon techniques, pr~pertles,
and appllca~ions of these alumlnum oxlde coatln~s are described
ln detail in The Surface Treatment and Finishlng of` Aluminum
and Its Alloys by S. Wernlck and R. Plnner, fourth edltion,
1972, published by Robert Draper Ltd., Paddlngton, England
(chapter IX page 563). Such coatlngs are extremely hard and
3'77~
mechanically superior to uncoated alumlnum 9 However, the
coatlngs contain pores in the ~orm of flne tubes wlth a
porosity on the order o~ 101 to 1012 pores per square
inch. Typlcal porosities range from 10 to 30 percent by
volume. These pores e~tend through the coatlng to a very thln
barrler layer of alumlnum oxide, typically 300 to 800
Angstrom3.
U.S. Patent No. 3,664,300 discloses a process for surface
treatment of xerographic lmaging cylinders whereln the surface
ls coated with zlnc stearate to provide enhanced sur~ace
lubricatlon and lmproved electrostatic toner transfer. This
treatment technique does not, however, result in a permanent
dielectric sur~ace of requisite hardness and smoothness ~or
pressure transfer and ~using of a toner lmageO
For lmproved mechanical properties as well as to prevent
stainlng, it ls customary practice to seal the pores. One
standard sealing technique involves partlally hydrating the
oxlde through immersion in boillng s~ater, usually containlng
certain nickel salts, which form an expanded boehmlte
~tructure at the mouths of the pores. Oxide 3ealing ln thls
manner will not support an electrostatic charge due;to the
lonic conductlvity of molsture trapped ln the pores~
It is o~ten deslrable in electrostatlc prlntlng and
copying to create an lmage on both sides of a sheet of paper or
other receptorc In electrophotography, the most accurate
791~
reproductlon of a two sided origlnal document would require
this ~aculty. In electrographic printlng, duplex imaging
affords slgniflcant savlngs in paper costs and permlts a
greater flexlbility in prlntlng ~ormats.
A crlterion which should be considered in modifying an
exlsting slngle-slded printlng or copylng system to permit
duple~ lmaging is the extent to which the system must be
modl~ied or supplemented. It is advantageous to employ a
system whlch is structurally compatible with two-slded image
productlon requlrlng only minor changes.
Another factor of some lmportance ls the speed and
efficlency with whlch the system transfers the two images. In
particular, it ls deslrable that such a system allow the
simultaneous fusing o~ the two lmages onto a receptor
medlum.
The lnvention provldes compatibillty o~ design for
electrostatlc prlntlng and photocopying apparatus. It also
provides high speed printlng and photocopylng wlth excellent
lmage quallty.
The invention further provides a plaln paper photocopying
system which ls slmple, compact, and low ln cost. The
photocopylng system requires fewer processing steps than
those o~ conventlonal copylng systems9 wlth an extremely short
and slmple paper path.
The lnventlon ls further able to reduce critlcal
mechanlcal tolerances ln provldlng a latent electrostatic lmage
~877~
in an electrostatic prlnter. It thus reduces the maintenance
problems assoclated wlth the formatlon o~ such an lmage, and it
can ~acllitate the generation of lons, partlcularly at high
current densltiee, ~or use in electrostatic prlntlng and
photcopylng, as well as other applicatlons~
A particular fabrlcation technlque is given ~or ion
generators character~zed by a laminate of mica and foil
electrodesO ~his deslgn ls durable, reslsting delamination due
to molsture and erosion due to ozone, nitric-acid and other
envlronmental substances. Such a laminate ls physic~lly stable
over a wlde range o~ temperatures, and can carry hlgh peak
voltage RF slgnals over a long ser~lce life.
The invention also provides an alternatlve lon generator
design based upon a corona electrode, which achieves high
current densitles with an easily controllable source o~ ions~
mis apparatus does not requlre the critlcal periodlc
maintainance normally characterlstic o~ such corona devices,
and avolds the ob~ectionable operational characteristics of
corona wires~
The inventlon provldes electrostatic imaging apparatus ~or
pressure transfer of a toner lmage from a dlelectric sur~ace to
plain paper and the like. Such apparatus ef~ects simultaneous
fusing o~ the toner lmage, and i5 characterized by a high
ef~iciency o~ toner trans~er.
A pre~erred embodiment of the invention lncorporates an
impregnated alumlnum layer ~or the dielectric member. This
dlelectric sur~ace possesses smoothne~s and hardness properties
whlch ~acllLtate toner transfer, whlle po~sessing suf~iclent
resistlvity to obtain a latent electrostatic image untll
tonlng. The dielectrlc ~ur~ace created by this preferred
method malntalns the above propertles at elevated humidities.
The apparatus o~ the lnvention may be employed ln duplex
lmaging onto plain paper and the llkeO Thls dup~ex lmaglng
enjoys the advantage of the avoidance of of~set lmages and
other problems often associated with duplex imaging. It also
achieves a slmultaneous transfer and fusing of two lmages onto
a receptor medium
SUMMARY OF THE INVENTION
=
The lnve`ntlon encompasses both electrophoto~raphy and
electrostatlc printlng9 as well as preferred components to be
employed ln these processes. The lnventlon also encompasses
two alternative ion senerator designs, the flrst of whlch may
be used to precharge a photoconductor or to form a latent
electrostatic lmage, as well as other appllcatlons. The second
lon generator i3 speclfically adapted to the formatlon of an
electrostatic image~
A flrst aspect of the invention relates to the structure
of the lon generators, whlch are characterlzed by the use of a
glow discharge to generate a pool of posltlve and negative
lons, whlch may be extracted for appllcatlon to a ~urther
member~ In the flrst ion generator, a varying potential is
applled between two electrodes separated by a solld dielectrlc
77~
member to cause an electrlcal alr gap breakdown adJacent the
~unctlon o~ the edge surface of at least one of the electrode~
and the solid dielectric member. In the second ion generator,
a varylng potential ls applied between an elongate conductor
having a dielectrlc sheath and a transverse conductlve member
ln order to generate lons at a crossover polnt o~ these
structures. ~oth lon generator embodiments may be
characterlzed as lncludlng a control electrode and a driver
electrode; an extraction potentlal applied to the former
electrode ls used to extract ions from the glow dlscharge
created by the varying potentlal.
Another aspect of the invention ls seen ln the shared
processing stages in the electrostatic copler and printer
apparatus of the lnventlon. After an electrostatlc latent
lmage has been ~ormed on a dlelectrlc cylinder, the lmage is
toned and pressure transferred to plain paper or any sultable
image receptor. Preferably, thls transfer is achieved by
ln.sertlng the image receptor between the dlelectrlc cylinder
and a transfer roller under high pressure. Advantageouslyg
this pressure trans~er is effected with slmultaneous fuslng of
the toner lma~e. Provlsion may be made for cleaning the
surface of the dielectric cyllnder and transfer roll, and for
dlscharging any resldual electrostatic image on the dielectric
surface.
In a preferred embodiment of the invention, the pressure
transfer of the toner image effected by dielectrlc and transfer
74~L
rollers may be enhanced by provldlng a skew between the
dielectric and transfer rollers. In the nip between the
rollers, the ratlo of the dlelectric surface speed to the lmage
receptor speed is advantageou~ly ln the range Or about l~01 to
l.l, most advantageously between 1.02 and 1.04. Best results
are achieved where the dielectrlc sur~ace has a smoothness ln
e~cess of 20 mlcrolnch rms, and a hlgh modulus of elasticity.
The transfer roller ls preferably coated with a stress-
absorblng plastics materlal. The roller materlals are
advantageously chosen so that the image receptor will have a
tendency to adhere to the surface of the transfer roller in
pre~erence to that of the dielectric roller. The apparatus
provldes effectlve toner trans~er and fuslng wlthout wrinkllng
of the receptor medium.
In the preferred version of the first lon generator, this
devlce comprises a plurallty o~ foil electrodes bonded to
opposite ~aces of a mic~ dielectric sheet. The inventlon
provides a preferred method for ~abricatlng laminations o~ mlca
and conductlve materlals, which technique may be ad~antageously
employed to produce such an ion generator. Such laminatlons
include a sheet o~ mica, one or more metalllc sheets, and
bondlng layers of pressure sensltive adhesive. The conductive
layer or layers may be selectively removed as by etching to
create a desired electrode pattern.
Another aspect o~ the invention relates to a pre~erred
method of ~abricating a dielectrlc member having a smooth, hard
11
~77~
surrace wlth a resistlvlty in excess of 1012 ohm-centlmeters;
such a technlque may be employed to advantage in produclng a
suitable dielectric cylinderD This method provldes for the
prellminary dehydratlon of an anodlc aluminum member followed
by lmpregnatlon of surface pores of the dehydrated member with
a metallic salt of a fatty acid. After completion of` the
impregnating stage, any excess impregnant ls removed from the
member's surface~ In the preferred version of this technlque,
the surface is then polished to a better than 20 microinch
finish. The impregnant materlal conslsts essentially of a
Group II metal wlth a fatty acid containing between 8 and 32
carbon atoms, saturated or unsaturated.
774~
The above and additional aspects of the invention
are illustrated with reference to the detailed descrlptlon
which follows, taken in conJunction wlth the drawlngs in
which:
FIC-URE 1 ls a sectlonal schematic vlew of
electrophotographlc apparatus in accordance with ~ preferred
embodiment of the lnvention;
FIGURE 2 is a partlal sectional schematic view o~ the nlp
area of the upper rollers Or Flgure l;
FIGURE 3 is a sectional schematic view of
electrophotograhic apparatus in accordance with an alternative
embodlment of the inventlon;
FIGURE 4 is a sectlonal schematic view of electrostatic
printing apparatus in accordance with a preferred embodiment
of the invention;
FIGURE 5 ls a partial sectional schematlc view of an
illustratlve charge neutrallzing device for the dielectric
roller o~ Flgure 4;
FIGURE 6 ls an elevatlon vlew of a preferred mounting
arrangement for electrostatlc printing apparatus of the type
illustrated ln Flgure 4;
FIGURE 7 is a schematic view of the rollers of Flgure 7
as seen from above;
13
FIGURE 8 ls a geometric representatlon of the contact
area of the rollers of Figure 6;
FIGURE 9 ls a plot of resldual toner as a function o~ end
to end skew for the apparatus of Example IV-3;
FIGURE 10 ls a sectlonal vlew of ion generatlng apparatus
ln accordance ~ith the preferred embodiment;
FIGURE 11 ls a sectional view of the ion generatlng
apparatus of Flgure 10, further showing ion extraction
apparatus and an lon receptor member;
FIGURE 12 is a plan view of dot matrix prlntlng apparatus
of the type lllustrated in Flgure 11;
FIGURE 13 is a schematlc sectional view of a mica foil
lamlnate in accordance wlth the lnventlon;
FIGURE 14 ls a partial perspective view of an
electrostatic imaging device in accordance with an alternative
embodiment of the lnventlon;
FIGURE 15 ls a schematlc sectional view o~ the apparatus
of Figure 14, further lncludlng lon extractlon apparatus and an
lon receptor member;
FIGURE 16 is a cutaway perspective view of an alternative
version of the lmaging apparatus of Figure 14;
FIGURE 17 ls a cutaway perspective view of a further
alternative version Or the electrostatlc lmaging apparatus of
Figure 14;
FIGURE 18 is a plan vlew of matrlx imaging apparatus of
the type shown in Figure 14;
14
8~
FIGURE 19 is a sectlonal schematic vlew of a three
electrode embodlment o~ the lmaglng device of Flgure 16;
FIGURE 20 is a perspectlve vlew o~ an electrostatic
lmaging devlce ln accordance wlth yet another embodiment of the
lnvention;
FIGURE 21 ls a plan vlew of a serlal prlnter incorporatlng
an electrostatlc lmaging devlce o~ the type illustrated ln
Figure 15;
FIGURES 22-27 are sequentlal schematlc vlews of
eIectrostatic imaglng apparatus of the type lllustrated in
Figure 4, adapted to duplex lmaging in accordance wlth the
lnventlon;
FIGURES 28~32 are partlal perspectlve views of
electrostatlc imaglng apparatus of the type illustrated in
Figure 4 9 showing an electrostatlc latent image and a resulting
toner lmage for varlous stages of the duplex transfer process
ln accordance wlth the invention;
DETAILED DESCRIPTION
I. Introduction
Two maln embodlments of the lnvention are described,
namely the double transfer electrophotographic apparatu~ which
is the sub~ect o~ Section II, and the electrostatic transfer
prlnter which is the sub~ect o~ Section III. These two
embodlments dlffer in the means by whlch a latent electrostatic
image is created on a dielectric imaging roller; thereafter,
identlcal apparatus may be employed.
The skewed roller apparatus of Section IV is pro~ltably
employed to provlde enhanced toner trans~er and fusing in
either of the main embodlments~ m e ion generator and
extractor of Section V may be used in either of the main
embodiments. Sectlon VI discloses an alternatlve ion generator
and extractor which may be incorporated in the printing
apparatus of Sectlon III. The impregnated anodized aluminum
members of Section VII are suitable for applications requiring
good dielectric properties and a hardl smooth surface. These
are qualitles which are preferred in the imaging roller of both
baslc embodlments. m e apparatus of e~ther maln embodiment may
be modified to provide duplex lmaging capability, as dlsclosed
in Section VIII.
II. Double Transfer Electrophotographic System
Figures 1 to 3 show double trans~er electrophotographic
apparatus 10 comprised of three cyllnders, and varlous process
stations.
16
~1877~
The upper cylinder ls a photoconductive member 11, whlch
includes a photoconductor coating 13 supported on a conducting
substrate 17, wlth an intervenlng semiconductlng substrate 15.
Advantageous materlals for the photoconductor ~urface layer 13
include cadmium sulphlde powder dispersed ln a resln binder
(photoconductive grade CdS is employed, typlcally doped with
activating substances such as copper and chlorine), cadmlum
sulpho~elenlde powder dispersed in a resln binder (defined by
the formula CdSxSey, where x+y=l), or organic
photoconductors such as the equlmolar complex o~ polyvinyl
carbazole and trlnitrofluorenone.
The photoconductor is electrostatically charged at
charglng station 19 and then expoqed at exposlng statlon 21 to
form on the surface of the photoconductor an electrostatic
latent lmage of an original. The photoconductor may be charged
employing a conventional corona wire assembly, or alternatively
it may be charged using the ion generating scheme described in
subsectlon V below (Figure 14)o The optical image which
provides the latent image on the photoconductor may be
generated by any of several well known optlcal scannlng
schemes. Thls latent image is trans~erred to a die~ectric
cylinder 25 formed by a dlelectrlc layer 27 coated on a metal
substrate 29. The latent electrostatic image on the dielectrlc
cyllnder 25 is toned and transferred by pressure to a receptor
medium 35 which is fed between the dielectric cyllnder 25 and a
transrer'roller 37. There are means 43, 45, 47 to remove
17
resldual toner from cyllnder 25 and roller 37 and to erase any
electrostatic image remalning on cyllnder 25 after transfer.
Apparatus for ef~ectlng toning and subsequent steps, shown
generally at 30 ln Figure 1, is discussed in detall in
subsection IIIB below~
The method by which a latent electrostatic lmage ls
transferred from the photoconductive cyllnder 11 to the
dlelectrlc cylinder 25 employs a charge transfer by air gap
breakdown. The process of uniformly charglng and exposing the
sur~ace of the photoconductor coating 13 re~ults ln a charge
density distrlbutlon correspondlng to the exposed image, and a
varlable potential pattern of the surface o~ the photoconductor
coating 13 with respect to the grounded conductlve substrate
17O With reference to Figure 2, the charged area of the
photoconductor 11 is rotated to a positlon of close proxlmity
(less than 0.05 mm) to the dlelectric surface. An external
potential 33 is applled between electrodes in the conductive
substrate of the photoconductlve cyllnder 11 and the metal
substrate 29 of the dielectric cylinder 25, with a typical
lnltial charge of about 1,000 volts on photoconductive layer
13, to whlch an additional 400 volts are added by the
externally applled potentlal 33. The aggregate charge o~ 1,400
volts is decreased by about 800 volts during the exposing
process~
It is possible to maintaln the photoreceptor 11 ln direct
contact with the dielectric roller 25~ an arrangement whlch
18
7~
provldes the advantase of slmpliclty ~n mountlng and drivlng
the cylinders. An effective TESI process may be achleved under
these condltions, but this will result ln toner trans~er to the
upper cyllnder and there~ore wlll requlre additlonal cleaning
apparatus.
The charge transfer process requlres that a sufficlent
electrical stress be present in the air gap to cause lonization
of the air. The required potential depends on the thlckness
and dielectric constants o~ the insulatlng materials, as well
as the width of the air gap (see Dessauer and Clark,
~ , the Focal Press, London and
New York, 1965, at 427?. Electrlcal stress wlll vary accordlng
to the local charge density, but if sufflcient to cause an air
gap breakdown lt will result in a transfer o~ charge ~rom
photoconductor surface 13 to dielectric surface 27, in a
pattern duplicating the latent image. Thls means that a
certain threshold potential must be generated across the alr
gap. Roughly half the charge wlll be transferred, leaving a
potential of around 600 volts on the dlelectrlc surface 27.
The nece~sary threshold potential may exlst as a result o~
the uniform charging and exposure o~ the photoconductor sur~ace
or an externally applied potentlal may be employed ln addltion.
Image quality ls generally enhanced through the use of an
externa~ potentlal.
It is important to maintain the lntegrity of the latent
electrostatlc image, ln the face of disruptive charge transfer,
19
7~
which occurs under certaln condltion~ when charge transfer is
effected on the approach of the two insulatlng surfaces. It
has been observed that the addltlon of a semlconductlng layer
15 between the photoconductive surface layer 13 and the
conductlve substrate 17 considerably reduces thls e~ect as
compared wlth using the usual two-layer photoconductor.
Although the pheno~enon by whlch the semiconductlng layer
ellminates the dlsruptive breakdown ls not completely
understood, lt is believed that the tlme constant introduced by
this semiconducting layer has the e~fect o~ smoothing or
reducing the preclpitous behavlor otherwise associated wlth
dlsruptive breakdown. me employment of this pre~erred
constructlon of the photoconductor member 11 avoids a mottling
and blurring Or detall in the transferred image. A typical
range o~ air gap distances ~or charge transfer using this
configuration would be on the order of 0.0125 to 0.0375 mm.
The use of this method o~ charge trans~er allevlates some
of the problems resultlng from undeslrable dlscharge
characteristics o~ the photoconductive member. The employment
o~ an external potential in achievlng a threshold potential
leaves a hlgher voltage on the dlelectrlc cylinder than would
be the case of a single trans~er system relying on the contrast
potential of the photoconductor sur~ace. This, ln turn,
results in a greater`contrast between the light and dark
portions o~ the toned, vlslble image.
In order to provide uniformlty ~rom copy to copy,
particular~ly with certain photoconductors whlch exhiblt
~atig~e, lt is advantageous to dlscharge the residual latent
lmage remainlng on the photoconductor after the latent lmage
has been transferred to the dielectric surface 27. This
erasure may be convenlently carrled out by an erase lamp 23
whlch provldes surficient illuminatlon to dlscharge the
photoconductor below a required level. The erase light 23 may
be either ~luorescent or incandescent.
Example II-l
In a speclfic operative example of an electrophotographic
system of the construction described, the cylindrical
conducting core 29 of the dielectric cylinder 25 was machined
from 7075-T6 aluminum to a diameter of 76 mm. The length of
this cylindrical core, excluding machined Journals, was 230 mm.
The ~ournals were masked, and the aluminum anodized by use of
the Sanford process (see S. Wernlck and R. Pinner, The Surface
Treatment and_ inishin~ of Aluminum_and its Alloys, Robert
Draper Ltd., 4th Edition 1971/72, Vol. 2, Page 567). The
flnished aluminum oxide layer was 60 micrometres ( m) in
thickness. The cylinder 25 was then placed in a vacuum oven at
30 inches mercury. After half an hour, the oven temperature
was set at 150C. The cylinder was maintalned at thls
temperature and pressure for four hours. The heated cylinder
was brush-coated with melted zinc stearate and returned to the
vacuum oven for a few minutes at 150C, 30 inches mercuryl
The cylinder was removed from the oven and allowed to cool.
21
7~
27 of the dlelectrlc cyllnder 25 was then flnished to 0~125 to
0.25~m rms u~lng 600 grit silicon carbide paper.
The pressure roller 37 consisted of a solld machined 50 mm
dlameter core 41 over whlch was press fitted a 50 mm lnner
dlameter, 62.5 mm outer diameter polysulphone sleeve 39.
me conductlng substrate 17 of the photoconductor member
11, comprlslng an alumlnum sleeve, was ~abrlcated of 6061
alumlnum tubing with a 3 mm wall and a 50 mm outer diameter.
The outer sur~ace was machined and the aluminum anodlzed
(again, using the Sanford process) to a thickness of 50 mO In
order to provide the proper level of oxide layer conductivity,
nlckel sulphlde was precipltated in the oxlde pores by dipping
the anodized sleeve in a solution of nickel acetate (50 g/l, pH
of 6) for 3 minutesO To form the semiconducting layer 15, the
sleeve was then lmmediately immersed into concentrated qodium
sulphlde for 2 minutes and then rinsed in distllled water.
This procedure was repeated three times. The impregnated
anodic layer was then sealed in water (92 Celcius~ pH of
5.6) for ten minutes. The semiconducting substrate 15 was
spray coated with a blnder layer, the photoconductor coating 13
consisting o~ photoconductor grade cadmium sulphoselenide
powder milled with-a heatset ~eSoto Chemlcal Co. acrylic resln,
dlluted with methyl ethyl ketone to a viscosity sultable for
spraying. The dry coating thickness was 40 m, and the cadmium
pigment concentration in the resin blnder was 18% by volume.
The resin was crosslinked by flrlng at 180C for three
hours.
22
7~
The dlelectric cylinder 25 was gear driven rrom an AC
motor to pro~lde a surface speed of twenty cms per second. The
pressure roller 37 was mounted on plvoted and spring~loaded
slde ~rames~ causing it to press against the dielectrlc
cyllnder 25 with a pressure o~ 55 kg per linear cm of contactO
The side frames were machined to provlde a 1.10 end-to-end skew
between rollers 25 and 37.
Strips o~ tape 0.025 mm thick and 3 mm wide were placed
around the circum~erence of the photoconductor sleeve 11 at
each end in order to space the photoconductor at a small
interval from the oxide sur~ace o~ the dielectric cyllnder 25.
The photoconductor sleeve was freely mounted in bearings and
frictlon driven by the tape which rested on the oxlde surface.
The photoconductor charging corona station 19, single
component latent image toning apparatus 31, and optlcal
exposing station 21 were essentially identical to those
employed ln the Develop KG Dr. Eisbein & Co~ (Stuttgart) No~
41l4 copler.
The toner removal means 43 and 45 comprlsed ~lexible
stalnless steel scraper blades and were employed to malntaln
cleanllness Or both the oxide cylinder 25 and the polysulphone
pressure roll 37. The resldual latent image was erased using a
semiconducting rubber roller ln contact with the dielectric
sur~ace 27 (see Flg. 5).
Wlth reference to the photoconductor-dlelectrlc cylinder
embodiment of Figure 29 a DC power supply 33 was employed to
23
~377~4~
bias the photoconductor sleeve 11 to a potentlal of mlnus 400
volts relative to the dielectrlc cylinder core 29, which was
maintained at ground potenttal. The photoconductor surface 13
was charged to a potential of mlnus 1,000 volts relatlve to lts
substrate 17. An optical exposure of 25 lu~-seconds was
employed ln dlscharging the photoconductor ln highlight areasO
In undlscharged areas, a latent image of minus 400 volts was
transferred to the oxide dielectric 27. This image was toned,
and then trans~erred to a plain paper receptor medium 35 which
was in~ected into the pressure nlp at the appropriate time from
a sheet feeder.
Copies were obtained at a rate of 30 per minute, having
clean bacXground, dense black images~ and a resolution ln
excess of twelve line pairs per millimetre. No image fusing,
other than that occurring during pressure transfer, was
required.
Example II-2
In another embodlment of the double transfer copier, the
photoconductor sleeve 11 was replaced wlth a flexible belt
photoconductor 11'~ as shown ln Figure 3. The photoconductor
11' was comprlsed of a photoconductor layer 13' whiGh was formed
from a one to one molar solution o~ polyvlnyl carbazole and
trinitrofluorenone dissolved in tetrahydrafuran, and coated
onto a conducting paper base 15' (West Virginia Pulp and Paper
45 No LTB base paper) to a dry thickness of 30 m. The
photoconductor rollers 17'a and 17'b were friction drlven from
24
the dlelectrlc cyllnder 25. The lower roller 17'b was biased
to minus 400 volts. The photoconductor was charged to 19000
volts wlth the double corona assembly 19' shown ln Flgure 3.
me electrostatic latent lmage was generated by a ~la~h
exposure 21' so that the entlre image frame was generated
wlthout the use of scannlng optics.
The rest of the system was ldentlcal to the prevlous
example wlth the exception of the dielectrlc cyllnder 25, whlch
was fabricated from non-magnetlc stainless steel coated with a
m layer of hlgh density aluminum oxide. The coating was
applied uslng a Union Carbide Corp~ (Linde Dlvlslon) plasma
spray technique. After spraying, the oxide surface was ground
and pollshed to a 0.25 m rms finish. Again, high quality
coples were obtained, even at operating speeds as high as 75
cms per second.
III. Electrostatic Trans~er Printing
The electrostatic transfer printing apparatus to be
described includes apparatus for forming a latent electrostatlc
image on a dlelectric ~urface (e.g. an lmaglng roller) and
means for accomplishing subsequent process steps.
A. Latent Electrostatlc Image Formation
Apparatus for generat~ng charged partlcles and for
extractlng them to be applied to a further surface is disclosed
in detail in sectlon V below. Any of the embodiments of such
apparatus which are suitable for forming a latent electrostatlc
image on a dielectric surface may be employed in the
electro~tatlc prlntlng apparatus discussed in this sectlon; for
example, see the embodiments of Figures 11, 12, and 13 and
partlcularly the pre~erred matrix printlng apparatus of Flgure
13, whlch may be employed ~or multiplex printing.
Alternatively, the prlnting apparatus may lncorporate any
embodlment Or the electrostatic lmaging device discussed ln
section VI belowO
All of the above charging devlce~ are characterlzed by the
production of a "glow discharge," a silent dlscharge formed ln
alr between two conductors separated by a solid dlelectric.
Such discharge~ have the advantage of being self-quenchlng,
whereby the charglng of the solid dielectric to a threshold
value will result in an electrlcal discharge between the solid
dielectrlc and the control electrode. By appllcation o~ a
time-varying potential, glow dlscharges are generated to
provlde a pool of ions of both polarities.
It is useful to characterlze all of the charglng device
embodlments in terms of a "control electrode" and a "drlver
electrode." The control electrode is maintained at a given DC
potential in relatlon to ground, while the driver electrode i~
energlzed around this value using a tlme-varying potential such
as a high voltage AC or DC pulse source. In the apparatus of
section V, the apertured conductor comprises the control
electrode; in the lllu~trated embodiments of section VI, the
coated conductor or wire constitutes the drlver elestrode. In
an alternative drivlng scheme ~or the latter device, the coated
conductor may be employed as the control electrode.
26
1'1877k4
Bl Subsequent Processlng
Identlcal apparatus may be employed for both
electrophotography and printlng to carry out process steps
subsequent to the creation on the dielectrlc cylinder of a
latent electrostatic lmage tcompare Figures 1 and 4). The
apparatus of Figure 4 will be consldered for lllustrative
purposesO
In Flgure 4, the dlelectric layer 75 of the dielectrlc
cylinder 73 should have sufficiently hlgh resistance to support
a latent electrostatic image durlng the period between
formatlon of the latent image and toning, or, in the case of
electrophotographic apparatus, between image transfer and
toning. Consequently, the resistivity of the layer 75 must be
in excess of 1012 ohm centimeters. The pre~erred thickness
f the insulating layer 75 ls between 0.025 and 0.075 mm. In
addition, the sur~ace of the layer 75 should be highly
resistant to abrasion and relatively smooth, with a flnish that
is preferably better than 0.25~ m rms, in order to provide for
complete transfer of toner to the receptor sheet 81. The
smoothness of dlelectrlc surface 75 contributes to the
efflciency of toner trans~er to the receptor sheet 81 by
enhancing the release propertles of this surface. The
dielectric layer 75 additionally has a high modulus of
elasticity, typically on the order of 107 PSI, so that it is
not distorted slgnlficantly by high pressures in the trans~er
nlp.
27
4~L
A number of organlc and inorganlc dielectrlc materials are
sultable for the layer 75. Glass enamel, for example, may be
deposited and rused to the surface of a steel or aluminum
cyllnder~ Flame or plaæma sprayed high density alumlnum oxide
may also be employed ln place of glass enamel7 Plastics
materlals, such as polyamldes, polyimides and other tough
thermoplastlc or thermosettlng resins, are also suitable.
A preferred dielectric coating ls anodized aluminum oxlde
impregnated wlth a metal salt of a fatty acldg as described in
section VII, infra.
The latent electrostatic image on dielectric surface 75 is
transformed to a visible lmage at toning statlon 79. While any
conventional electrostatic toner may be used, the preferred
toner is of the single component conducting magnetic type
described by J.C. Wllson, U.S. Patent No. 2,846,333, issued
August 5, 1958, Thls toner has the advantage of simplicity and
cleanllness.
The toned lmage is transferred and fused onto a receptlve
sheet 81 by hlgh pressure applied between rollers 73 and 83.
It ha~ been observed that providing a non-parallel orientatlon,
or skew, between the rollers of Figure 4 has a number of
advantages ln the transfer/fuslng process. An image receptor
81 such as plaln paper has a tendency to adhere to the
compllant surface o~ the pressure roller 83 in pref~rence to
the smooth~ hard surface of the dielectric roller 73. Where
rollers 73 and 83 are skewed, this tendency haæ been observed
28
~779~
to result in a "slip" between the image receptor 81 and the
dlelectric surface 75. The most notable advantage ls a
surprlslng improvement in the efriclency of toner transfer f'rom
dielectrlc Yurface 75 to image receptor 81. Thls efficiency may
be expre~sed in percentage terms as the ratlo o~ the welght of
toner transferred to that present on the dielectric roller
be~ore transfer. Apparatus o~ this nature is disclosed ln
section IV.
The bottom roller 83 conslsts of a metallic core 87 which
may have an outer coverlng o~ englneering plastics 85. The
surface material 85 or roller 83 typlcally has a modulus of
elasticity on the order of 200,000-450,000 PSIo The image
receptor 81 wlll tend to adhere to the surface 85 in pre~erence
to the dielectric layer 75 because of the relatively hlgh
smoothness and modulus of elastlclty of the latter surface. In
the embodlment of section IV, one functlon of this surrace 85
ls to bond image receptor 8l when the latter i~ subJected to a
slip between the roller surfaces. Another function o~ the
plastlcs coatlng 85 is to absorb any high stresses introduced
into the nlp ln the case of a paper ~am or wrinkle. By
absorbing stress ln the pla3t1cs layer 85, the dielectric
coated roller 73 will not be damaged durlng accldental paper
wrinkles or ~ams. Coating 85 is typically a
nylon or polyester sleeve having a wall thickness ln the range
f 3 to 12.5 mm~
29
37~
The pre~sure required ~or good ~using to plaln paper is
governed by such factors as, for example, roller diameter, the
toner employed, and the presence o~ any coatlng on the surface
o~ the paper. It has been dlscovered, in addltion, that the
skewlng Or rollers 73 and 83 will decrease the transfer
pressure requirements. See section IV, below. Typical
pressures run from 18 to 125 kg per llnear cm of contact.
Scraper blades 89 and 91 may be provided in order to
remove any residual paper dust, toner accidentally lmpacted on
the roll, and alrborne dust and dlrt rrom the dielectric
pressure cyllnder and the back-up pressure roller. Since
substantially all of the toned lmage is transferred to the
receptor sheet 81, the scraper blades are not essential, but
they are deslrable ln promotlng reliable operation over an
extended period. The quantity Or residual toner is markedly
reduced ln the embodiments Or sectlon IV, infra.
The small residual electrostatic latent image remaining on
the dlelectric surrace 75 after transfer of the toned lmage may
be neutralized at the latent image discharge station 93. The
actlon of tonlng and transferring a toned latent lmage to a
plain paper sheet reduces the magnltude Or the electrostatic
image, typically rrom several hundred volts to several tens o~
volts. In some cases where the toning threshold is too low,
the presence of a resldual latent lmage will result ln ghost
lmages on the copy sheet, whlch are ellminated by the discharge
station 93.
~377~
At very hlgh surface velocities o~ dlelectrlc coatlng 75,
the remalning charge can again result ln ghost images. In this
case, multlple dlscharge stations will further reduce the
resldual charge to a level below the tonlng threshold. Erasure
of any latent electrostatlc lmage can be accomplished by using
a hlgh frequency AC potentlal between electrodes separated by a
dielectrlc, as descrlbed ln sectlon V below.
The latent resldual electrostatic image may also be erased
by contact discharging. The sur~ace of the dielectric must be
maintained ln intimate contact with a grounded conductor or
grounded semiconductor in order effectlvely to remove any
residual char~e from the surface of the dielectric layer 75,
for exampleg by a heavlly loaded metal scraper blade. The
charge may also be removed by a semiconductlng roller whlch ls
pressed lnto lntimate contact wlth the dielectric surface.
Figure 5 shows a partial sectlonal view o~ a semiconductor
roller 98 ln-rolling contact with dielectric surface 75.
Roller 98 advantageously has an elastomer outer surface.
EXAMPLE III-l
In a speclfic operative example of an electrographlc
prlnter ln accordance with the lnventlon, the cylindrical
conductlng core 5 of the dlelectrlc cylinder 1 was machlned
~rom 7075-T6 alumlnum to a 3 inch diameter~ The length of the
cyllndrlcal core9 excluding machlned ~ournals, was 9 inches~
The journals were masked and the aluminum anodized by use o~
31
7~4~
the Sanford Proce~s (see S. Wernick and R. Plnner, The Surface
~ , Robert
Draper Ltdo fourth editlon, 1971/72 volume 2, page 567). The
finl~hed alumlnum oxide layer was 60 microns ln thic~ness. The
conductlng core 5 was then heated in a vacuum oven, 30 inches
mercury, to a temperature o~ 150C which temperature was
achleved in 40 mlnutes. The cylinder was maintained at thls
temperature and pressure ~or four hours prior to lmpregnationO
A beaker of zinc stearate was preheated to melt the
compoundO The heated cylinder was removed from the oven and
coated wlth the melted zinc stearate using a paint brush. The
cylinder was then placed ln the vacuum oven for a few mlnutes
at 150C, 30 lnches mercury, thereby forming dielectric
sur~ace layer 3. The cylinder was removed ~rom the oven and
allowed to cool. After cooling, the member was pollshed with
successively ~iner SiC abrasive papers and oll. Flnally, the
member was lapped to a 4.5 mlcroinch finish.
The pressure roller ll consisted of a solid machined two
lnch diameter alumlnum core 12 over which was press flt a two
inch inner diameterl 205 lnch outer diameter polysul~one sleeve
13. The dielectric roller 1 was gear driven from an AC motor
to provide a surface speed o~ 12 inches per second. The
trans~er roller 11 was rotatably mounted in spring-loaded side
frames, causing it to press against the dlelectric cylinder
wlth a pressure of 300 pounds per linear lnch o~ contact. The
slde ~rames were machined to provide a skew of 1.1 between
rollers l and 11.
32
A charglng device o~ the type descrlbed ln U.S. Patent No.
4~160j257 was manu~actured as follows. A 1 mil ~talnless steel
roil was lamlnated on both sides of a 1 mll sheet of Muscovite
mica. The bondlng material and technlque ls detailed ln
Example V-l, lnfra. The stalnless ~oil was coated wlth reslst
and photoetched wlth a pattern slmllar to that shown in Figure
22, wlth holes or apertures in the flngers approxlmately .oo6
lnch in diameter. The complete prlnt head consisted o~ an
array of 16 drlve lines and 96 control electrodes whlch formed
a total of 1536 crossover locatlons capable of placlng 1536
latent image dots across a 7.68 inch length of the dielectric
cylinder. Corresponding to each crossover location was a .oo6
lnch diameter etched hole ln the screen electrode. Blas
potentials of the various electrodes were as ~ollows (with the
cyllnder's conductlng core maintained at ground potent~al):
screen potentlal -600 volts
control electrode potential -300 volts (during
the application of a -300 volts print pulse, this voltage
becomes -600 vol ts)
driver electrode bias -600 volts
Ihe DC extractlon voltage was supplied by a pulse
generator, wlth a print pulse duratlon of 10 microseconds~
33
7~4~
Charglng occured only when there was slmultaneously a pulse o~
negative 300 volt~ to the fingers 44, and an alternating
pokentlal of 2 kllovolts peak to peak at a freque~cy of 1 Mhz
supplled between the flngers 44 and selector bars 43~ The
prlnt head was maintalned at a spaclng o~ 8 mils from
dlelectric cyllnder 3~
Under these condltlons it ~-as ~ound that a 300 volt latent
electrostatic lmage was produced on the dielectrlc cylinder in
the form Or dlscrete dots. The lmage was toned using slngle
component toning apparatus essentially identical to ~hat
employed in the Develop KG Dr~ Eisbeln and Company (Stuttegart)
No~ 444 copier. The toner employed~s ~ 1186 (a trade mark of
Phillip A. Hunt Ch~cal Corporation of New Jersey, U.S.A.).
The printlng apparatus 70 included user-actuatable sheet-
feeding apparatus (not shown) for feedlng individual sheets olof paper between cyllnders 73 and 83. The paper feed, toning
apparatus, and cyllnder rotation were driven from a unitary
drive assembly (not shown). Paper feed was synchronlzed wlth
the rotatlon o~ dlelectric cylinder 73 to ensure proper
placement o~ the toned image.
Dlgltal control electronics and a digital matrix character
generator, deslgned accordlng to principles well known to those
skilled in the art, were employed in order to form dot matrix
character~. Each character had a matrlx size of 32 by 24
points. A shaft encoder mounted on the shart of the dlelectric
cylinder was employed to generate appropriate tlmlng pulses for
the digltal electronics~
34
~8~774~
Flexlble steel scraper blades 89 and 91 were employed to
maintaln cleanllness o~ dlelectrlc cyllnder 73 and transfer
cylinder 830 With reference to the electrostatic image erasing
embodiment shown at 98 ln Flgure 5 the resldual latent image
was erased uslng a semlconducting rubber roller ln contact wlth
the dlelectric surface 75.
IV. Toner Transfer Apparatus Wlth Skewed Rollers
Flgure 6 shows in a plan vlew illustrative transfer
printlng apparatus 70 of the type shown schematicall~ in Figure
4, includlng details of a preferred mountlng arrangement
Side frames 5S and 69 house bearlng retainers 57 and 67, which
are fitted to rollers 73 and 83 ln order to allow the rotation
of the rollers while constralning their horizontal and vertical
movement~ Substantially identical side frames and bearlng
retainers are located at the other end of rollers 73 and 83.
Bearlng retalners 57 and 67~ which advantageously are of the
type known as "self-alignlng", fit within llps 51 and 61 on the
respectlve side frames, and against shoulders (not shown) on
the respectlve rollers. The side ~rames are mounted on one
side to superstructure 55, and are mounted on the other end in
spring-loaded ~ournals 58 in order to provlde a prescribed
upward pressure agalnst roller 73. Roller 73 is driven at a
deslred rotatlonal velocity by means not sho~n, while roller 83
is frictionally driven due to the contact of the rollers at the
nip.
~87~
The mountlng lllustrated in Figure 9 ls machined in order
to provide a specified "skew", or devlatlon of the axls of
rollers 73 and 83 from a parallel orlentation. Rollers 73 and
83 may be ad~u~table around a pivot polnt at one end, by
varyln~ the angular relationshlp ~ln the vertlcal plane) o~ the
rollers at the other end~ Alternatively, the rollers may plvot
around a central point o~ contact, by ad~usting the offset o~
one of the rolls about the axis o~ the other, this ad~ustment
belng equal at both ends. This latter, "end-to-end'~ skew wlll
be assumed hereinafter for illustrative purposes.
The mounting arrangement shown in Figure 6 may be easily
adapted to electrophotographic apparatus of the type shown in
Figure 1. In a further embodiment, the dielectrlc imaglng
roller (upper roller) may comprise a photoconductive surface
layer over a conduct~ng substrate. With reference to the
sectional view of Figure 4, the imaging apparatus 71 may be
replaced with any sultable apparatus known ln the art for
depositing a uni~orm charge on surface 75, and ~or exposing ~he
sur~ace to a pattern o~ light and shadow whereby the charge is
selectively disslpated to form a latent electrostatlc lmage.
As in the dielectrlc embodiment, photoconductive surface 75 is
advantageously smooth and abrasion reslstant, with a high
modulus of elasticlty. See Example IV-4.
As shown in Figure 6, axle 50A is disposed in end~to-end
skew, whlch may be measured as an of~set L ln the plane of side
~rame 59. A more signl~lcant measure o~ skew, however, ls the
36
7~
angle between the pro~ected axes of rollers 73 and 83 as
measured in the horizontal plane, or plane o~ paper feed. An
lllustrative value of skew to effect the obJects o~ the
lnventlon ls OolO lnch, measured at the center of roller
bearings 57 and 67, which are separated by a distance o~ 10.375
lnch ~or 9 inch long rollers. Thls represents an angle o~
roughly 1~1D
Figure ? schematically lllustrates s~ewed rollers 73
(with axis B-B) and 83 (wlth axis C-C) as seen from above~
Roller 83 is skewed at the bearing mounts by horizontal of~set
L from the vertlcally proJected axis B'-B' of roller 73. Thls
corresponds to an angle ~ between axes B-B and C-C. Axls B-
B is perpendlcular to the dlrection A of paper feed.
Flgure 8 is a geometrlc representation of the surface of
contact of the rollers at the nlp, showin~ the direction of
paper feed before and after engagement by the rollers. As a
sheet of paper 81 travelllng ln dlrection A enters the nip, it
is sub~ected to dlvergent forces in directlon D (perpendicular
to the pro~ected axis B"-B" of roller 3) and E (perpendicular
to the proJected axis C'-C' of roller 21)~ Because of the
relatlvely high smoothness and modulus of elasticity of the
surface 75 of roller 73, the paper will tend to adhere to the
lower roll, and therefore to travel ln dlrectlon E. This
results ln a surface speed dif~erentlal or "slip" between the
surfaces of paper and roller.
Due to the compresslon of the lower roller 83 at the nip,
paper 81 will contact both roller surfaces over a finlte
37 `
377~L~
distance M in direction D. The width of the contact area, M,
can be calculated using a formula found ln Formulaq For Stress
and Straln (4th edltion) by Ronald J. Roark, publlshed by
McGraw-Hlll Book Company~ The formula for the case of two
cylinders ln contact under pressure with parallel axes can be
~ound on page 320 of the Roark Text, table XIV, sectlon 5. The
transaxial wldth in lnches of the contact area of the two
cylinders ls ~lven by:
~ O-~D~ L
where:
P represents the cylinder loading in pounds per linear
lnch;
Dl and D2 represent the diameters of the cylinders in
inches;
Vl and V2 represent Poisson's ratio in compresslon
for the materials of the cylinders, and
El and E2 represent the modulus o~ elasticlty ln
compression for the materials of the cylinders, in pounds per
square inchO With reference to the resultant triangle ln
Flgure 8, the surface of receptor 81 will undergo a;
proportlonal slde travel N with respect to the surface of
roller 73, the factor of proportionality belng the surface
speed differential~
The skewing of rollers 73 and 83 in the above described
manner results ln a surprlslng improvement in the efficlency of
toner transfer from dlelectric surface 73 to image receptor 81.
38
Thi~ efflciency may be expressed in percentage terms as the
ratio of` the welght of toner transferred to khat present on the
dlelectrlc roller before trans~er. This bears a complementary
relationshlp to the weight of residual toner on the dielectric
roller a~ter transfer. The increase ln transfer efflciency,
whlch is the most notable advantage of the inventlon, ~lnimizes
the service problems attributable to the accumulation of
residual toner at the process stations associated with the
image roller 73, lncludlng scraper blades 89 and 91, erase head
93, and lmage generator 71. This e~fect depends on the choice
of surface material 75 and tonerO
It is another surprlslng advantage of thi~ technique that
thls enhanced toner transfer is achieved without wrinkllng of
the receptor medium 81. These advantages accrued even in the
case of nonflbrous substrates 81, such as
Example IV-l
Apparatus of the type illustrated in Figures 4 and 9
lncorporated a 9 lnch long, 4 inch outer dlameter roller 73
havlng a dielectric surface 75 of anodlcally formed porous
aluminum oxlde, which had been dehydrated and lmpregnated with
zinc stearate (~ee section VII) and then surface polished. The
dielectrlc surface of roller 73 was polished to obtain a finish
of better than 10 microinch rms.
The pressure cyllnder 83 included a 9 inch long steel
mandrel with an outer dlameter of 3.125 inches over whlch was
39
~7~
pressed a 0.375 lnch thlck sleeve of polyvinylchloride. The
roller3 were pressed together at 350 pounds of pre~ure per
llnear inch of nlp.
A latent electrostatlc image was formed on the dlelectrlc
surface of roller 73 by means o~ an ion generator of the type
dlsclosed in sectlon V. The varlous voltages to the lon
generator 71 were malntalned at constant valuesO The tests
were conducted under the same ambient conditions throughoutO
k The toner employed was ~ 1186 (a trade mark of the Phillip A.
Hunt Chemlcal Co ~ ration)~ The single component latent image tonlng
apparatus was essentially ldentical to that employed in the
Develop RG Dr. Elsbein & Co., (Stuttgart) No. 444 copler~
me toner was trans~erred onto Finch white bond paper, #60
vellum o~ Finch, Pruyn and Co. Thls paper was ~ed into the nip
between the dielectric and pressure rollers at a constant speed
throughout the testsO
h Using the above specl~ications, the apparatus was operated
at 0 skew, .55 skew, and lol skew, where the skew was
measured as a 0.10 lnch offset at the bearing retainers of the
9 lnch long pressure roll. The results shown in Table IV-A
were obtained by collectlng the resldual toner and comparing
lts weight to the known welght o~ toner be~ore trans~er. No
a~ter trans~er prlnting was present on the upper cyllnder
durlng the tests wlth 0.55 and 1.1 skew. However,
transfer was so poor durlng the test without skew that printing
was plainly vlslble on the upper cyllnder after trans~er.
~0
7~
PERCENTAGE OF
~N D-50- ~IID 9~
none 12 . 60
o55 ~10
1 .1 ~10
Exam~le_IV-2
The apparatus of Example IV-l was employed with
DESOTO toner 2949 5 (a trade mark of DeSoto IncO, of Illinois,
ll.S.A.). The toner was transferred onto coated OCR IMAGETROLL
paper (a trade mark of S~D. Warren). The rollers were pressed
together without ~kew at 420 pounds per linear inch, resulting
in a transfer efficiency of 92.6 percent, measured by
comparing the weight of toner before image transfer to the
weight of residual toner. The rollers were then pressed
together at l.l~ skew, with a pressure of 200 pounds per
linear inch, and all other parameters unchanged, resulting in
a transfer efficiency of 99.95 percent.
Example IV-3
The apparatus of Example IV-l was employed with the
following modifications. The pressure cylinder 83 comprised a
9 inch long steel mandrel with a 1.945 inch outer diameter,
over which was pressed a 9 inch long Celcon sleeve with a 3.50
77~
lnch outer dlameter. (Celcon ls a trademark of Celane~e
Chemlcal CoO ror thermoplastlc linear acetal resln~). The two
rollers were pressed together at 200 pounds of nlp pressure per
linear inch of nlpO
The toner employed was COATEs RP0357 (a trade mark of
the Coates BrosO and Co. Ltdo of Pennsylvania, U.S.A.). The
toner was transferred onto Finch white bond paper #60 vellum.
U lng the above speclficatlons, the apparatus was operated
with end-to-end skew, varled over a range of angles from 0.0
to lol~ The apparatus was operated using a constant welght
o~ toner prlor to transfer, and the resldual toner present on
dielectric roller 73 was collected and weighed. The results
are shown ln Table IV-B~ and are graphed ln Flgure 9~ In the
case of the test uslng no skew, the resldual toner was vlslble
as prlntlng remaining on the upper roller.
These tests showed a dramatlc lmprovement ln the
ef~lclency o~ toner transfer when the skew was lncreased from
OrO to .42, this resulted ln a decrease ln the welght o~
residual toner by a ~actor of 53. Increases ln skew ~rom
.42 to .85 and from ,55 to 1.1 further reduced the
welght of residual toner by factors of somewhat better than 2.
42
. . .
7~D~
TA~ _
TONER TRANSFER EFFICIENCY,_EXAMPLE IV-3
END-TO-END SKEW RESIDUAL TONER (GRAMS)
o 6~034
.42 0.114
055 o.o~6
85 0.050
97 o.o36
` 1.1 0.031
~,
:. 10 Example IV~4
. .
The apparatus of Example IV-4 was employed wlth the
modiflcatlon that the imaging roller 73 comprised a
photoconductive roller. An aluminum sleeve was fabricated of
6061 alumlnum tubing with a 1/8" wall and 4" outer dlameter.
The sl~eve was spray coated with a bi~der layer
photoconductor consisting of photoconductox grade SYLVA~IA
PC-100 (a trade mark of GTE Products Corporation of Connecticut,
U.S.A.) cadmium sulfide pigment of Sylvania Comp. Electronics
Corp., dispersed in a melamine-acrylic resin, diluted with
methyl ethyl ketone to viscosity suitable for spraying. The
re~in was cros~linXed by firing at 600 for three hours.
A photoconductor char~in~ corona and optical exposing
system were essentially ldentlcal to those employed in the
Develop KG Dr. Eisbein & Co. (Stuttgart) No. 444 Copler. The
toner trans~er efficiency under~ent improvements comparable to
.i' those of Example IV-l for increa~ing skew angles of 0.0;
0.55, and 1.1~
43
,,
~IB7~
V. Ion Generatlon and Extraction
Figure 10 depicts an ion generator 100, which produces an
air gap breakdown between a dielectrlc 101 and respectlve
conducting electrodes 102-1 and 102 2 using a source 103 of
tlme-varylng potentlal, illustratively a periodlcally
alternatlng potentlal. When electrlc fringing ~ields EA and
EB in the air gap 104-a and 104-b exceed the breakdown field
o~ air, an electric discharge occurs whlch results ln the
charging of the dielectric 101 in regions lOl-a and 101 b
ad~acent the electrode edges. Upon reversal of the a,lternatlng
potential of the source 103, there is a charge reversal ln the
brea~down regions 101-a and 101-b. The generator 100 of Figure
19 there~ore produces an air ~ap breakdown twice per cycle of -
applied alternating potential from the source 103 and thus
generates an alternating polarlty supply of ions.
The extractlon of ions produced by the generator lOG of
Flgure 10 ls illustrated by the generator~extractor 110 of
Flgure llo The generator llOA lncludes a dielectrlc ll between
conducting electrodes 112-1 and 112-2. In order to prevent alr
gap breakdown near electrode 112-l, the electrode 112-1 ls
encapsulated or surrounded by an lnsulating materiai 113.
Alternating potentlal ls applled between the conducting
electrodes 112-1 and 112-2 by a source 114A. The second
electrode 112-2 has a hole 112-h where the desired air gap
breakdown occurs relatlve to a reglon lll-r o~ the dielectrlc
111 to provlde a source of ions.
~7~
The lons formcd in the gap 112-h may be extracted by a
dlrect current potential applied from a source 114-B to provlde
an external electrlc fleld between the electrode 112-2 and a
grounded auxlliary electrode 112-3. An lllustrative lnsulatlng
surface to be charged by the lon source 1M Figure 20 ls an
electrographic paper 115 conslYtlng of a conducting base 115-p
coated with a thln dielectrlc layer 115-d.
When a swltch 116 is swltched to posltion X and is
grounded as shown, the electrode 112-2 is also at ground
potentlal and no external ~leld is present in the re~ion
between the ion generator llOA and the dielectric paper 115
However, when the switch 116 ls swltched to position Y, the
potential of the source 114B ls applied to the electrode 112-
2. Thls provides an electric field between the ion reservolr
111-4 and the backlng of dielectrlc paper 115~ The ions
extracted from the alr gap breakdown region then charge the
surface of the dlelectrlc layer 115-d.
A number of materials may be used for the dlelectric layer
lllo Possible choices include aluminum oxide, glass enamels,
ceramlcs, plastlcs fllms, and mica. Aluminum oxlde, glass
enamel~ and ceramics present difficultles ln ~abricatlng a
suf~iclently thln layer (i.e. around 00025 mm) to avold undue
demands on the drlving potentlal source 114A. Plastlcs films,
lncludlng polylmides such as that known by the Trade Mark
Kapton, and Nylon, tend to degrade as a result of exposure to
chemlcal byproducts of the alr gap breakdown process ln
1~ B7749L
aperture 112 h (notably ozone and nltric acid). Mlca avoids
theæe drawbacks, and is therefore the preferred material for
dielectrlc 111. Especlally preferred is Muscov1te mica,
H2KAl3 (S104)3-
The generator and lon extractor 110 of Flgure 11 is
readlly employed; for example, in the formation of characters
on dlelectrlc paper ln hlgh speed electrographic prlntlng.
Devlces embodylng this prlnciple may be used for charglng and
dlscharging a photoconductor as in the apparatus of section II;
suitable embodiments are disclosed in U.S. Patent No.
4~155,0930 To employ ion e~tractlon in the formatlon of dot
matrix characters on dielectric paper, the matrlx ion generator
130 of Figure 12 may be employed~ m e generator 130 makes use
of a dlelectric sheet 131 wlth a set of apertured air gap
breakdown electrodes 132-1 to 132-4 on one side and a set Or
selector bars 133-1 to 133-4 on the other slde, with a separate
selector 133 being provided for each dlfferent aperture 135 in j!
each different finger electrode 132.
When an alternatlng potential ls applied between any
selector bar 133 and ground, ions are generated ln apertures at
the lntersectlons of that selector bar and the fing;er
electrodes. Ions can only be extracted from an aperture when
both its selector bar ls energlzed with a hlgh voltage
alternating potentlal and lts finger electrode is energized
with a direct current potentlal applled between the finger
electrode and the counterelectrode of the dielectric surface to
~6
3774~
be charged. Matrlx locatlon 13523, ~or example, ls printed by
simultaneously applylng a hlgh frequency potential between
selector bar 133-3 and ground and a dlrect current potentlal
between finger electrode 132-2 and a dielectrlc receptor
member's counterelectrode. Unselected fingers as well as the
dlelectrlc member's counterelectrode are malntalned at ground
potential.
By multiplexlng a dot matrlx array ln thls manner, the
number o~ required voltage drlvers ls slgni~lcantly reduced.
I~ ~or example, it is desired to print a dot matrix array
across an are~ 200 mm wlde at a dot matrlx resolution of 80
dots per cm, 1600 separate drivers wouid be required i~
multiplexlng were not employedO By utilizlng the array of
Figure 12 wlth, for example, alternatlng frequency driven
~ingers, only 80 ~inger electrodes would be required and the
total number of drivers is reduced ~rom 1600 to 100.
In order to prevent alr gap breakdown from electrodes 132
to the dielectric member 131 ln regions not assoclated with
apertures 135, it ls d~slrable to coat the edges o~ electrodes
132 wlth an lnsulating material. Unnecessary air gap breakdown
around electrodes 132 may be ellminated by potting these
electrodes.
In constructlng and operatlng a matrix ion generator of
thls constructlon, it is deslrable that the lon currents
generated at various matrlx crossover points be maintalned at a
sub~tantlally unlform level. Thlckness variatlons in the
47
.. ~ . . , , .. . ~
~L8~4~
dielectrlc layer 131 wlll result ln commensurate variatlons ln
the lon current output~ ln that a lower lon current wlll be
produced at an aperture 135 at which the dlelectric 131 is
thlcker. It 18 a partlcuarly advantageous property of mica
that lt has a natural tendency to cleave along planes of
extremely uniform thlckness, maklng lt especlally sultable for
the matrlx lon generator lllustrated ln Flgure 12. In thls
regard, the unlformity of thlcknes~ o~ layer 131 ls much more
lmportant than the actual value o~ that thickness.
Ion generators of the type lllustrated ln Flgures 11 and
12 may be ~abrlcated uslng a layer of mlca laminated to thin
sheets o~ metalllc foil, by etchlng the foil to create an array
of electrodes on each side o~ the micaO Electrodes 102-1 and
102-2 (Flg. 11) are formed by lamlnatlng a thin sheet of
conductive foll to each face of the mica sheet 101. Wlth
re~erence to the sectional view of Flgure 25, a mlca sheet 171
o~ unl~orm thickness is bonded to two layers o~ foil 174 and
175. me bondlng is achleved uslng thln layers of pressure
sensit~ve adheslve 172 and 173.
The preferred dlelectrlc material i8 Muscovite mlca,
H2KA13(SiO4)3. It ls desirable to have a sheet of
unlr~rm thlckness in the range from about 2 ~ - 75 ~ , most
preferably 10 ~ - 15~ . The thinner mlca sheets are generally
harder to handle and more expensive, while the thlcker mlca
requires hlgher RF voltages between electrodes 102-1 and 102-2
(see Figure ll)o me mlca should be free o~ cracksl fractures,
and slmilar defects.
48
7'~
The foll layers 174 and 1~5 advantageously comprise a
metal which may be easily etched in a pattern Or electrodes
132, 133. Illustrative materlals include nickel, copper,
tantalum, and titanlum; the preferred material, however, ls
stalnless steel. A foil havlng a thickness ~rom about 6 ~ - 50
is desirable, wlth the preferred thickness belng around 25~n
A wide varlety of pressure sensltive adheslves are
suitable ~or layers 172 and 173. A number of characteristlcs
should be considered ln chooslng an approprlate pressure
sensltlve adheslve. The adhesive should be thermoplastlc, and
be resistant to molsture and chemlcals. It should be able to
~ithstand the hlgh temperatures resultlng ~rom high voltage
alternatlng potentials 3 on the order o~ kilovolts. The
adheslve should be sultable for bonding of metal to mlcaO
Illustratlve adheslve ~ormulatlons which satisfy the above
criterla lnclude solutions o~ organopolyslloxane reslns, as
well as pressure sensitive adheslves.
The m~ca ls coated wlth a pressure ~ensitive adhesive
~ormulatlon using any well known technique whlch permlts
precise control over the coatlng thicknes~. The adheslve
layer~ deqlrably have a thickness in the range 0.5 ~ - 5 ~ 9
most preferably in the range o.6~ - 2.5~ . The thickness may
be determlned after laminatlon by subtractlng the known
thickness of the mica and foil sheets ~rom the total thlckness
of the lamlnate. The adheqive may be applled manually, as by
brush coatlng, spraying, and dipping. The preferred method o~
49
~37~
coatin~ is that of dlpping the mica lnto a bath Or pre~sure
sensltive adhesive~ followed by wlthdrawal o~ the mica at a
calibrated speed. Generally, a faster speed of withdrawal
re~ults ln a thlcker pressure sensitive adheslve coating on
each slde of the mlca sheet 171.
In the preferred embodlment of the lnvention, the pressure
sensiti~e adhesive 18 applled to the mica in solution. The
resln may be diluted to a desired vlsc031ty uslng a varlety of
solvents, well known to those skilled in the artO In general,
higher vlscosity formulatlons wlll result in a thicker layer of
pressure sensitive adhesive for a given method of appllcatlonO
Advantagecusly, the pressure sensltive adhesive ~ormulation has
a vlscosity in the range from about 10 cps. - 100 cps. m e
mlxture advantageously is flltered prior to coating onto the
mica sheet 171.
The coating of mica sheet 171 preferably lnvolves dipping
the ~heet into the pressure sensitive adhesive bath to
completely coYer both sides; lt is not necessary, however, to
coat the edges o~ the mica sheet ln the preferred embodlment,
whlch calls for a separate protective medium for the edges of
the laminatlon. In lieu of or ln addition to a protective
coating around the edges o~ the mica sheet 171, a protective
layer of tape may be applled to the edges of the mica-foil
lamlnation. me tape provldes protection agalnst migration of
moisture between layers of the mica. Alternatively, the tape
may be removed after processlng of the mica~ durlng whlch it
~77~8
provldes a protective layer, aq further dlscu~sed herein.
Preferably, the tape ls coated on one face wlth pres3ure
sen~itive adhesive whlch may be the same type as used to bond
the mlca-foll layer~,
In the case of certain pre~sure sensitlve adhesives, the
adhe~ive coatlng ls cured in order to cross-link the
formulation and thereby enhance its adhesive character. Thls
may be done uslng any suitable technique for the glven adhesive
formulatlon, such as heat or radiatlon curing~
me ~oll sheets 174 and 175 are cut to desired dimensions,
and cleaned prior to applicatlon to the mica sheet 171. Each
~heet ls placed in registratlon with one ~ace of the mlca
sheet, and then bonded to the mica by applylng pressure evenly
over the foil layers.
After lamlnation o~ the foll layers 174 and 175 to mlca
sheet 171, the foil is selectlvely removed to create a desired
pattern, as for example the pattern Or electrodes 132 and 133
shown in Flgure 12J In the preferred embodlment, the desired
pattern is created by a photoetching process. This involves
coating the foil with a photore~lstant materlal; co~ering the
coated foll wlth a photomask to create the deslred patterns;
exposing the masked laminate to ultraviolet radlatlon; and
etchlng the irradlated foll ln order to remove those portions
which have been rendered soluble durlng the precedlng steps.
The preferred versions Or this process uses a posltlve
photoreslst, which is characterlzed in that those areas which
51
.. .... . . . .
37749~
are exposed to ultravlolet radiation wlll be rendered soluble
and later dlssolved~
In the case o~ solvent based photoresist, there ls a
tendency of the solvent to leach out the pressure sensltive
adhesive around the edges of the laminatlon. In addition, the
photoreslst wlll not coat well due to edge effects, creatlng a
danger of etch-through. For these reasons, lt is advlsable to
tape the edges to provlde a protectlve layer during these
processlng steps; the tape may be removed a~ter etching.
Alternatively, one may employ a dry film photoreslst, which
will adequately protect the edges of the lamlnation lf applied
ln a thlckness of around 35 ~ .
In accordance with a partlcular embodlment, a heat sink
may be appended to the mlca-foll laminate. The heat sink ls
applled to the lamination face containlng selector bars 133
ln order to absorb heat resultlng from hlgh voltage alternatln~
potentials. A variety of materl~ls are sultable as well known
in the art; ln the case Or electrically conductive makerials,
an ln~ulatlng layer must be lncluded to lsolate the heat sink
from selector bar~ 133.
In the examples which ~ollow, all proportions given ~re by
weight unless otherwlse noted.
5~
,. ~ ..... ... ..
EXAMPLE V-l
220 parts Methylphenyl polysiloxane resin solution
1 part 294 Dlchlorobenzoyl peroxide
1 part Dibutyl phthalate
A pressure-sensltlve adheslve composition as set rorth in
the above table was formulated, then dlluted to 90 cps. wlth
butyl acetate. The resultlng llquld was ~iltered under a
? pressure of approximately 30 PSI, and poured into a graduateO
The ~ollowing steps were carried out in a dust-rree
environment~ A sheet of mica having a thickness in the range
20-25 mlcrons wa~ cleaned using lint-~ree tlssues and methyl
ethyl ketone (MEK). A~ter drying, the mlca sheet was suspended
from a dlpplng ~ixture and lowered lnto the pressure-sensltive
adhesive formulatlon until all b~lt two millimeters was
submersedO The mica was then w~thdrawn from the adhesive bath
at a speed of 2 cm/mlnute, providlng a layer o~ adheslve
approximately 3 mlcrons ln thlckne~s. The coated mica was
stored ln a dust-free ~ar and placed ln a 150C. oven for
~lve minutes ln order to cure the pressure-sensitlve adheslve.
Two sheets G~ stalnless steel 25 mlcrons thlck were cut to
the deslred dlmensions and cleaned using MEX and lint-~ree
tissues. One of the sheets was placed ln a reglstratlon
~ixture, ~ollowed by the coated mlca and the second ~oll sheet.
Bonding was e~ected by appllcation o~ light ~inger pressure
~rom the mlddle out to the edges, ~ollowed by moderate pressure
~ 7~4
using a ~ubber roller. Any adhe~ive remaining on expose~ mica
~uxfaces was removed using MEK and lint-free tissuesO The
edges of the lamination were then covered with a .6 mm wide
KAPTON Tape (a trade mark of E.I. DuPont de Nemours and Co.,
of Delaware U.S.A.) coated with th~ above pres~ure s~nsitive
adhesive formula~ion.
The foil layers were respectively etched in the
patterns of ele~-trodes 132 and 133 (Figure 22) using a
positive photoresist.
; EXAMPLE V-2
An lon generator was fabrlcated in accordance with Example
V-l, modlfled as ~ollows: The pressure sensitlve adhesive
was formulated from an acrylic copolymer of vinyl acetate. The
adheslve was diluted to 50 cpsO uslng butyl acetate.
~s EXAMPLE V 3
An ion generator was ~abrlcated ln accordance with Example
V-I, and placed in a mountlng fixture wlth the selector
bars 23 upwardO A capacitor glass mountlng block of dimenslons
compatible with the mlca was prepared for mountlng ~y
applicatlon of a layer o~ sllicon adhesive resln in accordance
with the table of Example V-l, ~ollowed by smoothing of the
adhe~ive using a meterlng blade. The mountlng block was
clamped ln registration with the laminate, and an~ e~cess
adheslve at the edges was removed using cotton swabs. The
completed structure was set aside for 24 hours to allow the
adheslve to set.
~L8~ J~
Vl. Electrostatic Imaging Device Using Dielectric-
Coated Wire
Figure 14 shows ln perspectlve a basic embodiment of an
electrostatic imaglng devlce which may be utllized, for
example, ln the prlnting apparatus o~ Flgure 4. Prlnt device
180 includes a 3erles o~ parallel conductive strip~ 184, 186,
188, etc~ laminated to an insulating support 181. One or more
dlelectrlc coated wires 193 are transversely oriented to the
P conductive strlp electrodes. The wlre electrodes are mounted
in contact with or at a minute distance above (i.ec ~ess than 2
mlls) the strip electrodes. Wire electrode 193 consists of a
conductlve wire 197 (which may consist of any suitable metal)
encased in a thiek dielectrlc materlal 195~ In the preferred
embodiment, the dlelectric 195 comprises a fused gla~s layer,
which is fabricated in order to minimize voids. Other
dielectric materlals may be used ln the place o~ glass > such as
slntered ceramlc coatingsO Organlc lnsulating materials are
generally unsuitable ~or thls application, as mo~t such
materlals tend to de~rade wlth time due to oxidizlng products
formed ln atmospheric electrlcal di~charge~. Although a
dlelectric-coated cylindrical wlre is lllustrated in the
preferred embodiment, the electrode 193 18 more generally
defined as an elongate conductor of Indeterminate cross~
sectlon~ wlth a dlelectric sheath.
Crossover points 185, 187, 189, etc. are found at the
intersectlon of coated wlre electrodes 193 and the respectlve
~L~87~
strip electrodes 184, 186, 188, etcO An electrlcal dlscharge
ls formed at a glven croqsover polnt as a result of a hlgh
voltage varylng potential supplied by a generator 192 between
wire 197 and the correspondlng ætrip electrode, Crossover
regions 185, 187, 189, etc. are preferably posltioned between 5
and 20 mlls. ~rom dielectrlc receptor 200 (see Flg~ 15).
The current~ obtainable from an lon generator o~ the type
illustrated in Figure 14 may be readlly determlned by mountlng
a current senslng probe at a small distance above one o~ the
crossover locations 185) 187, 189, etc. Current measurements
were taken using an illustratlve AC excltation potentlal of
2000 volts peak to peak at a frequency of 1 MHz., pulse width
o~ 25 microseconds, and repetitlon period o~ 500 microseconds.
A DC extractlon potential of 200 volts w~s applied between the
strip electrode and a current senslng probe spaced 8 mils above
the dlelectrlc coated wlre 193. Currents ln the range from
about .03 to .08 microamperes were measured at AC e~citation
potentials above the alr gap breakdown value, which ~or this
geometry was approximately 1400 volts peak to peak. At
excitation voltages above the breakdown value, the extraction
current varled linearly with excltatlon voltage. The
extraction current varled llnearly wlth extractlon voltage, as
well. ~or probe-wlre spacings ln the range 4-20 mils, the
extractlon current was inversely proportional to the gap width~
Under 4 mils, the current rose-more rapldly. Wlth the above
excltatlon parameters, the lmaglng device was ~ound to produce
56
7~
latent electrostatic dot images in periods a short as 10
microseconds~
In the sectional vlew of Flgure 15, lons are extracted
from an ion generator of the t~pe shown in Flgure 14 to form an
electrostatic latent lmage on dielectrlc receptor 200. A high
voltage alternating potential 192 between elongate conductor
197 and transverse electrode 184 results in the generation of a
pool of positlve and negative ions as shown at 194. These lons
are extracted to form an electrostatic image on dielectrlc
surface 200 b~ means Or a DC extractlon voltage 198 between
tran~verse electrode 194 and the backing electrode 205 Or
dielectric receptor 200, Because of the geometry Or the lon
pool 194, the e~tracted ions tend to form an electroætatic
image on sur~ace 200 ln the shape of a dot.
A further lmaging device embodiment is illustrated in
Figure 16 showing a print head 210 similar to that illustrated
in Figure 14, but modifled as follows. The dielectrlc coated
wire 213 is not located above the strip electrodes, but instead
i~ embedded ln a channel 219 in insulating support 2110 The
geometry of thls arrangement may be varied in the separation
(i~ any) of dlelectrlc coated wire 213 ~rom the side walls 212a
and 212b of` channel 219; and in the protrusion (if any) o~ wire
electrode 213 from channel 219.
Figure 17 is a perspectlve view of` ion generator 220 o~
the same type as that illustrated in Flgure 16 with the
57
'44
modlficatlon that the strlp electrodes 224, 226, ~nd 228 are
replaced by an array of wlre~ In thls embodlment wlres havlng
~mall dlameters are ~ost ef~ective and best results are
obtained with wlres having a diameter between 1 and 4 mils.
~le alr breakdown in any of the above embodirnent~ occurs
in a reglon continguous to the ~unction o~ the dielectric
sheath and transverse conductor (see Fig. 15). It is therefore
easier to extract lons from the prlnt heads o~ Flgs. 14 and 17
than from that of Flg. 14 in that thls reglon i~ more
acces~lble in the former embodiments. The ion pool may extend
as ~ar as 4 mlls from the area o~ contact, and therePore may
completely surround the dielectrlc sheath where the latter has
a low diameter.
In the pre~erred embodlment, the transverse conductors
contact the dielectrlc sheath. As the separat~on of the~e
members has a critlcal e~ect on lon current output, they are
placed in contact in order to maintaln consistent outputs among
varlou~ crossover pointsO This also has the bene~lt o~
mlnlmizing drivlng voltage requlrements. It ls ~easible,
however to separate these structures b~ as much a3 1-2 mll.
It ls use~ul to characterlze all Or the above embodiments
ln terms o~ a "control electrode" and a "driver electrode".
The electrode exclted wlth the varying potentlal is termed the
drlver electrode, whlle the electrode supplled with an ion
extractlon potential is termed the control electrode. The
energlzlng potentlal ls generically described herein as
58
8'7 d 4 gk
"varylng," referring to a tlme-varylng potentlal which provides
air breakdown in opposite directions, and hence lons of both
polarities. Thls is advantageou~ly a periodically varying
potential wlth a frequency ln the range 60 Hz. - 4 MHzD In any
of the illustrated, preferred embodiments, the coated conductor
or wlre constltutes the driver electrode, and the transverse
conductor comprise~ the control electrode. Alternatlvely, the
coated conductor could be employed as the control electrode~
Flgures 14, 16, and 17 illustrate various embodiments
lnvolvlng linear arrays of crossover points or print locations.
Any of these may be extended to a multiplexible two-
dimensional matrix by addlng addltional dielectric-coated
conductors. Wlth re~erence to the plan view of Figure 18, a
two-dlmensional matrlx print head ls shown utlllzln~ the basic
structure shown in Flgure 14, with a multlpliclty of
dielectric coated conductors. A ma~rix prlnt head 230 is shown
F having a parallel array of dlelectrlc-coated wires 231A9 231B,
231C etc. mounted above a crosslng array of flnger electrodes
232A, 232B, ~32Cg etc. A pool of ions is formed at a given
crossover locatlon 233X y when a varylng excitatlon potential
is applied between coated wire 231X and flnger elec~rode 232Y.
Ions are extracted from thls crossover locatlon to form an
electrostatic dot image by means of an extraction potential
between finger electrode 232Y and a further electrode (see
Figure 15).
59
~:~L8~
In any o~ the two-dimensional m~trix print heads, there is
a danger Or accldentally erasing all or part o~ a prevlou~ly
formed electrostatlc dot image. This occurs ln the ion
generator illustrated in Figure 18 when a crossover location
233 is placed over a previously deposlted dot lmage, and a high
voltage varying potential 1~ supplied to the correspondlng
coated wire electrode 231. If in such a case no extractlon
voltage pulse ls supplied between the correspondng finger
electrode 232 and ground, the prevlously established dot image
will be totally or partially erased. In any of the emb~diments
o~ Figures 14-17, this phenomenon may be avoided by the
inclus~on o~ an additional, apertured "screen" electrode,
located between the control electrode and the dielectrlc
receptor surPace 200, The screen electrode acts to
lS electrically isolate the potentlal on the dielectric receptor
200 J and may be additionally employed to provide an
electrostatlc lenslng actlonq
Figure 19 ~hows in section an lon generator 240 o~ the
above-described type. The structure o~ Figure 16 is
supplemented with a screen electrode 255, which ls isolated
~rom control electrode 244 by a dielectrlc spacer 252. ~he
dlelectrlc spacer 252 de~ines an alr space 253 which is
sub~tantially larger than the crossover region 245 o~
electrodes 242 and 244. This is necessary to avold wall
charglng effects. The screen electrode 255 contalns an
aperture 257 which ls at least partially posltioned under the
crossover reglon 245.
. ....... ..
37~4~
The lon generator 240 may be utilized ~or electrographic
matrix prlnting onto a dlelectric receptor 258, backed by a
grounded au~lliary electrode 259. When the switch 1~ closed at
posltlon Y, there ls slmultaneously an alternatlng potentlal
acros~ dielectrlc sheath 2425 a negatlve potential Vc on
control electrode 244, and a negative potential Væ on screen
electrode 2551 Negative ions at crossover region 245 are
sub~ected to an acceleratlng field whlch causes them to form an
electrostatic latent image on dlelectric sur~ace 258. The
presence of negatlve potential Vs on screen electrode 2559
whlch 18 chosen so that Vs is smaller than Vc ln absolute
value, does not prevent the formatlon Or the image, whlch wlll
have a negatlve potential ~1 (smaller than Vc in absolute
value).
15When the swltch is at X, and a previously created
electrostatlc image o~ negative potential Vi partially under
aperture 257, a partial erasure o~ the image would occur in the
absence Or screen electrode 255. Screen potentlal ~9,
however, ls chosen so that Vs ls greater than Vl ln
absolute value, and the presence of electrode 255 there~ore
prevents the paæsage of positlve lons from aperture 257 to
dlelectrlc surface 258.
Screen electrode 255 provides unexpected control over
image size, by varying the size o~ screen apertures 257. Uslng
a con~lguration such as that shown ln Flgure 19, a larger
61
screen potential has been ~ound to produce a smaller dot
dlameter. This technique may be u~ed ~or the ~ormatlon o~ ~ine
or bold images. It has also been found that proper cholces of
Vs and Vc will a7low an lncrease ln the di~tance between
ion generator 240 and dielectrlc sur*ace 258 while retalning a
constant dot lmage diameter. Thls is done by lncrea~lng the
absolute value o~ Vs while keeplng constant the potential
dlfference between V5 and VcO
Image shape may be controlled by using a given screen
electrode overlay. Screen apertures 257 may, for example,
assume the shape of fully formed characters which are no larger
than the corresponding crossover reglons 245~ Thls technique
would advantageously utllize larger crossover regions 245. The
lensing actlon provlded by the aper~ured screen electrode
generally results in lmproved image de~inition, at the cost o~
decreased lon current output~
Flgure 20 illustrates yet another electrostatic imaging
device 260 for use ln a high speed serial prlnter. An
insulating drum 261 ls caused to rotate at a high rate of
speed, illustratively around 1200 rpm. To thls dru~ is bonded
a dlelectrlc-coated conductor 262 in the form of a helix. The
drum ls disposed over an array of parallel control wires which
are held rigid under ~pring tension~ The dlelectric-coated wire
is malntalned in gentle contact wlth or closely spaced from the
control wire array. By rotatlng the drum, the helical wire
provldes a serlal scannlng mechanism. As the helix scans
62
aCrO88 the wlres with a hlgh frequency high voltage excitation
applied to dielectrlc-coated wlre 262, printing 1~ ef~ected by
applylng an e~traction voltage pulse to one of the control
electrode wlre3 263.
Flgure 21 lllu~trate~ an alternatlve scheme for provldlng
a relative motion between the print device of the inventlon and
a dielectric receptor surface. A charglng head 270 ln
accordance wlth Figure 18 ls slldably mounted on gulde bars
275. Any ~uitable means may be provlded for reclprocating
print head 270, such as a cable drlve actuated by a s,tepplng
motor. Thls system may be employed to form an electrostatic
image on dielectric paper, a dielectric transfer member, etc.
The electrostatic printing device of the invention is
further illu~trated wlth re~erence to the following speclfic
embodiments.
E:XAMPLE: VI-l
An lmaging device of the type lllustrated in ~igure 14 was
fabricated as ~ollowsO The insulating support 181 comprised a
G-10 epoxy fiberglass circuit board. Control electrodes 184,
186, 188 9 etc~ were formed by photoetching a 1 mil stainless
steel foil whlch had been lamlnated to insulatlng substrate
181, providlng a parallel array o~ 4 mll wide strips at a
separation of 10 mils. The driver electrode 193 conslsted of a
5 mil tungsten wlre coated wlth a 1.5 mll layer of fused glass
to form a structure havlng a total diameter of 8 mlls~
63
~87~
AC excitation 192 was provlded by a gated Hartley
osclllator operating at a resonant frequency of 1 MHz~ The
applied voltage was ln the range of 2000 volts peak-to-peak
wlth a pulse wldth of 3 microseconds, and a repetltlon period
S of 500 mlcroseconds. A 200 volts DC extractlon potential 198
was applied between ,selected control electrodes and an
elec~rode supportlng a dlelectric charge receptor sheet. The
ion generatlng array was positioned 0.01 lnches from the
dlelectrlc-coated sheet~
This apparatus was employed to form dot matrlx characters
in latent electrostatic form on dielectrlc sheet 200. After
conventional electrostatlc tonlng and fusing, a permanent hlgh
quallty image was obtained.
EXAMPLE VI-2
An ion projectlon print device of the type illustrated in
Figure 16 was fabricated as follows. A channel 219 of 5 mlls
depth and lO mils wldth was milled ln a 0.125 lnch thick G-10
epoxy ~lberglass circult board. A driver electrode 213
ldentical to that of Example VI-l was laid in the channel.
Photoetch~d stainless ~teel foil electrodes 214, 21~, 218, etc.
were laminated to circult board 211, contactlng dielectric 215
The device exhibited equlvalent performance to the imaging
device of Example VI-l when excited at the same potential.
EXAMPLE VI-3
The electrostatic print devlce o~ Example VI-2 was
modified to provide lmaging apparatus o~ the type shown ln
64
,, .,, . ~ ~ . ........ .. . .. .. . . ... . .
~37~
Flgure 17. The control electrodes comprlsed a serles o~ 3 mll
diameter tungsten wires cemented to support 221, This device
achleved approxlmately double the lon current output as
compared ~lth the devlces o~ Examples VI-l and VI~2.
In all three examples, the glass coated wlre was not
firmly bonded in place, but was allowed to move ~reely along
its axis. This provided a ~reedom o~ motion to allow for
thermal expanslon when operating at hlgh driving potentialsO
VII. Fabricatlon Of Dlelectric Members
This section descrlbes a serles o~ steps for fabrlcating
and treating anodiæed aluminum members which results in members
partleularly suited to electrostatlc lmaglng. The treated
- member ls adapted to recelve an electro~tatic latent image, to
carry the lmage with mlnlmal charge decay to a toning statlon,
and to impart the toned image to a further member preferably by
pressure trans~er. A number o~ properties of partlcular
concern ln this utlllzation are the hardness and abraslon
reslstance o~ the oxlde sur~ace; the potential acceptance and
dlelectrlc strength oP the dielectrlc layer; the res1stlvity o~
the dlelectric layer; and the release properties o~ the sur~ace
wlth respect to electrostatic toner.
Thls method is advantageously employed ln ~abrlcating the
dielectrlc cylinders of -the apparatus described above ln
~ections II and III. Thls method provldes a simple and
reliable technique for fabrlcating alumlnum oxlde layers o~ a
~137~4
thickness as great as 4 mils and capable of supporting several
thousand volts. Such cyllnders are charactrlzed by a hard,
~mooth surface whlch is ~ultably employed in the simultaneous
pressure transfer and fuslng of a toner lmage.
In order to provide a member of sultable conflguration, an
lnitlal step entalls the fabricatlon of an aluminum member of
desired form. In the preferred embodlment, the member consists
of a cyllnder of alumlnum or aluminum alloy, machlned to a
desired length and outside diameter. The surface ls smoothed
preparatory to the second step of hardcoat anodization.
In the second processlng stage, the machlned alumlnum
member ls hardcoat anodlzed preferably according to the
teachlngs o~ Wernlck and Pinner; see The Surface Treatment and
~ by S. Wernlck and R.
Pinner~ ~ourth edltion, 1972, publlshed by Robert Draper Ltd.,
Paddington, England. The anodlzatlon is carrled out to a
deslred surface thlckness, typically one to two milsO This
results ln a relatively thlck porous surface layer of aluminum
oxide characterlzed by the presence of a barrier layer
lsolatlng the porous oxlde from the conductlve substrateO
Followlng anodlzatlon, the member's surface ls thoroughly
rlnsed ln de-lonlzed water ln order to remove all anodlzlng
bath and other resldual substances from the surface and the
pores. The rinsed Qurface may be wiped dry to minlmize surface
moisture.
66
7~
After anodlzlng the member, and prlor to impregnating of
the pores with a sealing materlal, the method of the inventlon
requlres a thorough dehydration of the porous ~urface layer.
For best re~ul~s, the dehydration ls accomplished lmmedlately
after anodizationO If there is a long delay between these two
steps, however, lt i~ advisable to maintaln the member in a
moisture-free environment ln order to avold a reactlon with
amblent moisture which leads to the formatlon of boehmite
[AlO(OH)2~ at pore mouths9 ef~ectively partially sealing the
porous oxlde so that subsequent impregnation is lnco~plete and
dielectric propertles degraded. This partlal seallng can occur
at room temperature in normal ambient humidity ln a perlod of
several days.
Removal of absorbed water from the oxlde layer of an
anodlzed alumlnum structure may be realized by uslng either
heat, vacuum, or storage of the artlcle ln a desicator, The
dehydration step requires thorough removal of water ~rom the-
pores. Although all three technlques are efrectlve, best
re~ult~ are reallzed by heatlng in a vacuum, ~or example in a
vacuum oven. A prellminary step o~ dehydrating the member ln a
vacuum oven ls especlally preferred where the member has been
stored ln a molst envlronment for a perlod a~ter anodization.
Heating o~ the member in alr, as compared wlth vacuum heatlng,
results ln only a sllghtly lower level of charge acceptance.
It ls preferable that any thermal treatment of the oxide prior
to impregnation be carried out ~t a temperature in the range
67
~77~L
from about 80C to about 300C, wlth the preferred
temperature belng about 150C~ Where precautlons have been
taken after anodlzlng to minlmize the retention and
accumulation of molsture, the dehydration step may be
accompllshed in conJunction wlth the impregnation step, as
e~plalned belowO
After removal of absorbed water from the oxlde coatlng lt
ls sealed with an impregnant material. In the present
inventlon, the impregnant material consists essentlally of a
compound of a Group II or III metal with a long chain fatty
acid. It has been dlscovered that a particularly advantageous
class of materlals includes the compounds of Group II metals
wlth fatty acids containing between 8 and 32 carbon atoms
saturated or unsaturated, The impregnant materials may
comprise elther a slngle compound or a mlxture of compounds~
Due to the water repellant nature of these alkallne earth
derlvatives, the product of the inventlon has superlor
dlelectric propertles at high humldities.
In order to avoid lntroduction o~ moisture lnto the
dehydrated porous surface layer, the member should be
malntalned in a sub~tantially moisture-free state durlng
impregnation. Thls will occur as a natural consequence of the
preferred method of applylng the impregnant mater~als of the
inventlon. At room temperature these materlals take the form
of powders, crystalllne sollds~ or other solld forms. In the
preferred embodlment of the lnvention, the member is malntalned
68
7~
at an elevated temperature (above the melting point of the
impregnant materlal) durlng the lmpregnatlon step in order to
melt the materlal or to avoid solidlfylng premelted materlal.
These materlals have suf~lciently low viscosity arter meltlng
to readily impregnate the pores of the oxide surface layerO In
this embodiment the perlod of heating the member from room
temperature to the lmpregnating temperature may provide the
prellminary dehydration which is required to avoid trapped
molsture in the pores, often wlthout a prlor separate
dehydrating step. Thls preheating stage may take minutes or
hours depending on the mass and volume o~ the aluminum member,
See Examples VII-l, VII-2. In the alternative embodlment o~
the invention discussed below, ln which the lmpregnant
materlals are applied in solution to the anodized member, it is
advisable to heat the member or take other steps in order to
avoid reintroduction o~ moisture during the lmpregnation
process.
It has generally been ~ound unnecessary to maintain the
heated member in a vacuum environment during lmpregnatlon,
either to avoid absorptlon o~ molskure or to assist the
impregnatlon o~ the pores tXrough capillarity. In the
pre~erred embodlment, the lmpregnant material may be applied to
the oxlde surface under molst ambient conditions because the
heatlng of the aluminum member wlll tend to drive o~f any
absorbed moisture from the oxlde surface. Optlonally, a vacuum
may be employed ln order to provide an extra precautlon against
relntroductlon of moisture. Special measures may be requlred,
69
7~
however, in the alternative embodiment in which the lmpregnant
materlal ls dissolved prior to appllcation to the anodized
memberO
In the preferred embodlment of the invention9 the
impregnant materlal ls applled to the surface of the aluminum
member after heatlng the member to a temperature above the
meltlng polnt Or the material. In one verslon of thls
embodlment, the material is applied to the surface ln solid
form (as by dusting or blowing lt onto the surface), whereupon
the materlal will meltO In an alternatlve verslon, the
materlal is premelted and applled to the oxide sur~ace ln
llquld form (as by brushlng the material onto the member or
immerslng the member ln melted materlal). In either case, the
material should then be allowed to spread over the oxide
surface layer. This may be done by permitting a flow of the
meltcd materlal, or by manually spreadlng the materlal over the
surface uslng a clean implement. The member should be
malntalned at thls elevated temperature for a perlod of time
sufflcient to allow the melted materlal to completely
lmpregnate the pores o~ the oxide surrace layer. Thls period
wlll be shorter when using a vacuum to asslst impregnation.
In the preferred embodiment, if the member is allowed to
cool prior to complete fllllng o~ the pores wlth the lmpregnant
material~ the materlal wlll tend to solldl~y leaving
undeslrable alr pockets ln the pores. It ls a particularly
advantageous aspect of thls method that this problem may be
7~
remedled simply by reheatlng the alumlnum member and allowlng a
more complete ~illing of the pore~. The member may be reheated
~or a subsequent impregnatlon step at any time subsequent to
the lnitlal impregnation 9 as the lmpregnant material of khe
lnventlon is not permanently cured.
In an alternatlve embodiment o~ the invention, the
impregnant material is dlssolved prior to appllcation of the
oxlde sur~ace layerO Materlals of the lnventlon susceptible to
application in this manner lnclude the compounds o~ Group III
metals wlth fatty acids, as well as the compounds of ~roup II
metals with some of the longer chain fatty acids (those havlng
around 32 carbon atoms)O Solvents whlch are sultable for thls
purpose include, for example, benzene, and butyl acetate.
After the mater1al ls dissolved~ ik may be applied to the
member by spraying or brushing it onto the oxide surface layer.
The solution is allowed to penetrate the pores Any excess
lmpregnant is removed by wlping the member's sur~ace. In order
to avold reintroduction o~ molsture into the dehydrated porous
surface layer, the member may be impregnated ln a vacuum oven
or ln alr at a temperature ln the range ~rom about 4oc to
55C. Alternatlvely, the member may be lmpregnated in a
desicant dry box. Advantageously, this method would reflect
that employed in the prior dehydration step.
It is deslrable subsequent to precLpltatlon of the
impregnant material ln the alternative embodlment to heat the
member to a temperature above the meltlng point o~ the
71
~1~37~
material~ This fuses the material in the pores, and minimizes
the occurrence of air pockets which are deleterious to
dielectric properties. The member may be reheated as in the
preferred embodiment in order to prove a more complete
impregnation.
Subsequent to impregnation of the pores, the aluminum
is allowed to cool. The member is then treated (as by wiping or
scraping) to remove any excess material fro~ the surface leaving
only the material in the pores. In order to provide a surface
with good release properties for electrostatic toner, a
preferred embodiment of the invention includes a final step of
polishing the member's surface to a better than 20 microinch
finish9 and preferably better than a 10 microinch inish.
The advantages of this method will be further apparent
from the ollowing non-limiting examples.
EXAMPL~ VII 1
_
A series of panels (1.5 inch X 1.5 inch X .Ob7 inch)
fabricated o aluminum alloy 7075-T6 were hard-coat anodized in
sulphuric acid by the Sanford "Plus" process* to a depth of 1.5
mil. The panels were rinsed with deionized water and wiped free
of surface moisture. They were then wrapped in moisture
absorbant paper an~ stored for about one day~
The anodized panels were unwrapped and heated to a
temperature above the melting point of the material to be
7~
applied (see Table VII) and maintained at this temperature for
one minute prior to application of the impregnant material.
The material was dusted onto the heated panel where it melted
rapidly and was allowed to flo~ over the oxide surface layer.
TABLE VII
IMPREGNANT IMPREGNATING CHARGE (Valts/Micron~
10 MATERIAL _ TEMPERATURE(C) ACCEPTANCE
Barium Stearate 300 22
Zinc Stearate 150 34.5
Magnesium Stearate 150 25
Zinc Octanoate 150 33
lS Zinc Behamate 150 41.5
Zinc Oleate 150 7
Zinc Octanoate: 300 19
Barium Stearate
* Sanford Process Corp: 65 North Avenue, Natick, Mass.
~137~
The coated member was malntained at the elevated temperature
~or another minute, and then allowed to cool to room
temperature. This process was repeated with a number o~
dif~erent lmpregnant materlals lncludlng in one case a mixture
of two dlfferent compounds - see Table VII.
After cooling, the samples were ground with 240 grit
sandpaper and water to a th~ckness of between 40 and 45
micronsn They were then heated on a hot plate at 150C for
approximately 30 seconds ln order to rapidly evaporate the
surface moisture, and then allowed to cool.
The plates were placed over a negative ion discharge and
charged to a maximum voltage. This voltage was measured by a
Monroe Electronics electrostatic voltmeter~
~ EXAMPLE VII-2
A hollow alumlnum cyllnder of extruded 7075-T651 alloy was
machlned to an outer diameter o~ 4 inches and 9 inch length,
wlth 0.75 inch wall thicknessr The cylinder was machined to a
30 miGrolnch finish, then polished to a 2.25 microinch flnish.
The cyllnder was hardcoat anodlzed by the Sanford "Plus"
process to a thlckness between 42 and 52 mlcrons, t~en rinsed
in delonlzed water and packed in plastic bagsa
On the followlng day, the cyllnder was unpacked and placed
in a vacuum oven at 30 lnches mercury. After half an hour, the
oven temperature wa~ set at 150C., which temperature was
achleved in a further forty mlnutes. The cyllnder was
74
77~a~
malntained at this temperature and pressure for four hours
prlor to impregnation.
A beaker of zinc stearate was preheated to melt the
compound. The heated cyllnder was removed from the oven, and
coated with the melted zinc steQrate using a paint brushO The
cylinder was then placed back in the vacuum oven for a few
mlnutes at 150C., 30 lnches mercury, The cylinder was
removed from the oven and allowed to cool.
After coollng, the member was polished with successively
finer SiC abrasive papers and oil~ Finally~ the member was
lapped to a 4.5 microinch finish by application of a lapplng
compound and oil with a cloth lap.
Uslng the testing method of Example VII-l, the cylinder's
charge acceptance was measured at 980 volts.
VIIIo Duplex Imaging
This section describes a duplex lmaging technlque
employing either the electrophotographic apparatus of Flgure 1
or the electrostatic printing apparatus o~ Figure 4. The
apparatus o~ either of these embodiments may be adapted as
discussed below to effect simultaneous pressure transfer and
fusing of toner images to opposite sides of an image receptor
medium. Reference should be had to Figure 4 and to the
discussion at section IIIB. In the duplex imaging method
utllizing this apparatus, receptor sheet 81 is inserted between
rollers 73 and 83 only during the second of two toner image
transfer~. An inltial transfer takes place directly ~rom first
lmage drum 73 to second image drum 83, with no receptor
lnserted between the two. Such transfer should be
substantially complete, leaving a toned lmage on second image
drum 83 which ls the mlrror lmage of that formed on flrst
imaglng drum 73 durlng prevlous processing stages~
Second image roller 83 serves a number of functions in the
dup~e~ lmaglng processO Initially, it recelves and carries
the toned image transferred from roller 73. During the second
transfer, lt should effect as complete as possible a transfer
of toner to receptor sheet 81. It is therefore deslrable that
bottom roller 83 have a relatively smooth surface,
advantageously better than 0.25 microinch rms. In a preferred
embodiment, the second, two-sided transfer to receptor sheet 81
is accompllshed slmultaneously with a fuslng of the toned lmage
due to hlgh pressure applied between the two rollersO Such
pressure may be provlded by pressure drum 83 comprlsing
a metalllc core 87 having an outer coating of engineering
plastic 850
The pressure required for good fusing to plain paper is
governed by such factors as, for example3 roller dlameter~ the
toner employed 3 and the presence of any coating on the surf`ace
of the paper. Typlcal pressures run from 18 to 125 kg per
linear cm of contactl Roller 83 desirably has a surface 85 of
engineering thermoplastic or thermoset material, which will
absorb any hlgh stresses in the transfer nip in the case of a
paper Jam or wrlnkle. By absorbing stress in the plastlcs
76
37~4
layer, the dlelectric coated roller wlll not be damaged during
accidental paper wrlnkles or ~ams. Surface 85 prererably has a
relatlvely low modulu~ o~ elastlcity as compared with
dlelectric 75, ln order to provide efficient toner transfer
from roller 73 to roller 83~ Illustrative values are a modulus
Or ela~ticity on the order of 107 PSI for dielectric 75, and
approxlmately 400,000 PSI for layer 85. Illustratively,
surface 85 comprises a nylon or polyester sleeve having a wall
thickness in the range 3 to 12~5 mm~
~le efflciency of toner transfer ~rom surface 75 to
surface 85 depends prlmarily on the relative modulus o~
elasticity of the two sur~aces, as discussed above. A second
factor to be considered ln choosing suitable materials is the
relative roughness of the two surfaces~ Advantageously, roller
73 has a relatlvely smooth surfa e as compared with roller 83.
Exemplary values would be a roughness of around 30 mlcroinch
rms for sur~ace 85, as compared with around 10 microinch rms.
for surface 75.
Drums 73 and 83 are advantageously rotated from a common
drive source. Flrst lmage drum 73j for example~ may be
directly drlven at a given angular veloclty, and second image
drum 83 frlction driven by contact with the first lmage roller.
Due to the high pressure with which the drums are held
together, they move at virtually the same linear surface
velocity wlth or without a receptive sheet lnserted between
them.
77
~L877~
The various stages of the two-sided imaglng process are
illustrated ln the Qchematic vlews of FIGURES 22 through 27.
In FIGURE 22, a first latent electrostatlc lmage Ills formed
on flrst image drum 73 by lmage generating station 71. Image
Il iq toned at toning station 79 (FIGURE 23), and rotated to
a position of contact with second image drum 83 to whlch lt is
pressure transferred (FIGURE 24). The -first image, now
lnverted (-Il), continues to rotate on second lmage drum
while a second latent electrostatlc image I2 is formed on
first image drum 73 (FIGURE 25), During this period, any
residual electrostatic lmage on first lmage drum 73 may be
erased at erasing station 93, This second image I2 is toned
(FIGU~E 26), and the two toned images are rotated to the nip,
~here they are pressure transferred to receptive sheet 81
~FIGURE 27). If lt is deslred to match the positions o~ images
-Il and I2 on receptive sheet 81, it ls necessary to tlme
the formation of image I2 q that the circumferential
distance from the nlp on roller 73 of leadlng edge of image
I2 equals the clrcumferentlal dlstance from the nlp on roller
83 of the leading edge of image -Il. The time inter~al
between successive image formations should equal the period of
rotation of bottom roller 83. Thls ls calculable by the
formula T = Roller 83 Diameter
Surface Speed o~ ~oll`ers
78
7~
In order to counteract the mlrror reversal of flrst image
Il that results from the double transfer o~ the lmage, it ls
necessary to provide an inverted latent electrostatic lmage at
lmage generatlng statlon 71. FIGURE 28 shows the case of one-
sided print~ng from the top roller 73. In order to transfer arow o~ toned characters onto receptor 81, image generatlng
station 71 forms an inverted row o~ latent electrostatic
characters along the circumference of roller 73. In FIGURE 29,
the toned characters have been transferred to bottom roller 83.
In FIGURE 30~ the toned characters have been further
transferred to the bottom slde o~ receptive sheet 81~ As a
result o~ the double tran~fer, they are printed in an lnverted
orientation. Thus, as shown in FIGURE 31, it is necessary to
reverse the orlentatlon (l.e. back to normal orientation) Or
the latent characters on drum 73 for transfer to the second
side of receptor Bl~
Image generating statlon 71 may comprise a photoconductor
member on whlch a latent electrostatic lmage is formed
correspondlng to a scanned optical lmage, with a trans~er o~
the latent lmage to lmage roller 20 by TESI (Flgure l). As will
be apparent to ækilled artisans 3 the scanning optics 21 may be
simply modi~ied to provide an lnverslon of alternate imagesO
In the case o~ electrographic printing apparatus, the
latent electrostatic image on lmage roller 73 is formed by ion
generating means in response to a signal indicatlve o~ the
79
7~7~4
deslred image. Image generatlng statlon 71 may comprlse, for
example, the ion generator and extractor discussed ln sectlon V.
FIGURE 32 shows in a plan vlew a multiplexed lon generator o~
thls type~ The lon generator 130 lncludes a series of flnger
electrodes 132 and a crossing serles of selector bars 133 with an
intervening dlelectric layer 131. Ions are generated at apertures
135 in the finger electrodes at matrix crossover pointsO Ions
can only be extracted from an aperture 135 when both its selector
bar is energlzed by a high voltage alternatlng potentlal supplled
by one of gated oscillators 137, and its ringer electrode is
energized by a direct current potential supplied by one o~ pulse
generators 1360 The timing Or gated oscillators ls
advantageously controlled by a counter 138. -
If axis A-A of the print head is orlented along the
clrcumference of upper roller 73, one may invert the latent
electrostatic lmage as required by the inventlon by reverslng
the order of signals to selector bars 133 from gated oscillator
137. This may be done by reversing the sequence of actuating
signals from counter 138.
