[0001] This invention relates to electrophotography and in particular to a novel method
of preparing by an electrophotographic process multicolor pre-press proofs from negative
color separation films.
[0002] The purpose of pre-press proofs as is well known in the art is to assess color balance
and strength which can be expected from the final press run and accordingly to correct
the separation transparencies before the printing plates are made therefrom. In many
instances it is also required to produce so-called customer proofs for approval of
subject, composition and general appearance of the print prior to press run. Thus
it is essential that the pre-press proof should have the same appearance as the press
print, that is to say in addition to matching the colors of the press print, the
pre-press proof shouId be on the same paper as the press print.
[0003] On the basis of the pre-press proofs the color separation transparencies are accepted
or corrected if found necessary and then used for the preparation of printing plates.
There are so-called positive working and negative working printing plates, as is well
known in the art. A positive working printing plate is exposed to a positive transparency
or film positive wherein the information to be printed corresponds directly to opaque
areas whereas the non-printing background areas correspond to transparent areas contained
on such film positive. By exposing to light through a film positive such positive
working plate the exposed areas contained thereon are rendered removable by chemical
treatment and the underlying usually grained aluminium plate surface forms then the
water receptive non-printing or non-image areas whereas the unexposed areas contained
thereon form the ink receptive printing or image areas during the subsequent lithographic
or offset printing. A negative working printing plate is exposed to light through
a film negative wherein the information to be printed corresponds to transparent areas
whereas the non-printing background areas correspond to opaque areas contained on
such film negative. In this case the exposed areas become photohardened and form
the ink receptive printing areas whereas the unexposed areas are removed by chemical
treatment and the underlying water receptive usually grained aluminium plate surface
forms the non-printing or non-image areas during subsequent lithographic or offset
printing.
[0004] It is known to produce by electrophotographic processes lithographic and gravure
pre-press proofs containing in general four colors, such as yellow, magenta, cyan
and black. Such pre-press proofing processes are disclosed for instance in United
States Patents Numbers 3,337,340, 3,419,411 and 3,862,848.
[0005] It is customary to produce such electrophotographic pre-press proofs by charging
a photoconductive recording member followed by exposure through a separation film
positive corresponding to one color, followed by toning of the exposed photoconductor
with a liquid dispersed toner of the appropriate color, followed by in-register
transfer of the color toned image deposit to a receiving member surface, such as paper,
usually of the same grade as the printing stock. These process steps are then repeated
with separation film positives of the other three or more colors and appropriate color
toners to produce a multi-color pre-press proof of print as required.
[0006] It should be noted that all prior art electrophotographic pre-press proofing processes
are so-called direct reproduction processes that is to say the color separation transparencies
employed comprise film positives wherein the image areas to be reproduced correspond
directly to the opaque image areas on such film positives. Consequently in such prior
art electrophotographic pre-press proofing processes the latent image formed on the
photoconductor upon exposure to such positive separation films is developed by attracting
thereto liquid toner material of opposite polarity to that of the electrostatic charges
constituting said latent images whereby the so formed toner deposits on the photoconductor
surface correspond directly to the image areas to be reproduced. Thus prior art electrophotographic
pre-press proofing processes are employed only for proofing of film positives which
are used for the preparation of positive working printing plates.
[0007] Prior art electrophotographic pre-press proofing processes are not suitable for the
proofing of film negatives used for the preparation of negative working printing plates
that is to say such processes are not suitable for the reversal reproduction of imagery
wherein the transparent areas contained on a film negative are to be reproduced as
the image areas on the pre-press proof. Reversal reproduction per se by electrophotography
is well known in the art but the processes employed for this purpose are not suitable
for multicolor pre-press proofing.
[0008] Reversal image reproduction in electrophotography is normally carried out according
to prior art practices by means of so-called repulsion toning. This process comprises
the steps of electrostatically charging the surface of a photoconductor to a polarity,
typically charging an n-type photoconductor such as zinc oxide to negative polarity,
exposing said surface to a film negative containing the image to be reproduced in
the form of transparent areas and the non-image part in the form of opaque areas whereby
the photoconductor surface becomes discharged in the exposed image areas while retaining
the charge in the unexposed non-image areas and applying to said surface toner materiaI
having the same polarity as that of the charges contained on said surface, typically
applying negative toner material to a negatively charged n-type photoconductor surface,
whereby such toner material is repelled from the charged non-image areas onto the
discharged image areas forming toner deposits thereon corresponding to the image
to be reproduced. The thus formed image deposits in certain instances are fused to
the photoconductor surface whereas in other instances they are transferred to a receptor
sheet.
[0009] Such above described image reversal reproduction by electrophotography is very well
suited to microfilm and microfiche reproduction and reader/printers where the information
to be reproduced generally is in the form of alphanumeric characters and lines and
where complete fill-in of large solid areas and complete absence of fog or stain in
the non-image areas are not absolutely required. In pre-press proofing however in
order to match the image quality of the press printed sheet it is essential to have
on the pre-press proof large solid areas completely filled in and background areas
completely free of fog or stain. These requirements can not be met by the prior art
electrophotographic reversal process, because unlike by attraction toning, by repulsion
toning it is not possible to produce uniformly filled in large solid areas in that
toner repulsion from charged background areas onto uncharged solid image areas is
most effective near the edges of the solid area where the intensity of the field lines
from the charged background area terminating in the uncharged image area is highest
and it diminishes in effectiveness towards the center of the solid image area where
the intensity of the terminating field lines is lowest. This results in solid image
areas characterized by high density near the edges and so-called hollow or lower
density center. For the same reason in replusion toning the background non-image areas
are completely free of fog or stain only near the edges. This so-called edge effect
can not be fully overcome even by using biasing means during repulsion toning, that
is by placing a so-called developing electrode a short distance apart from the photoconductor
surface to thereby enhance toner deposition as is well known in the art.
[0010] Accordingly, the invention provides a method of electrostatographic image reversal
wherein a positive print comprising a receptor sheet having image-free areas and image
areas formed by color toner deposits is produced from a negative film having opaque
areas corresponding to the image-free areas on said print and transparent areas corresponding
to the image areas on said print, said method characterized by the steps of electrostatically
charging a photoconductor to deposit thereon uniformly charges of a first polarity,
exposing said photoconductor to light through a negative film thereby to discharge
said photoconductor in the areas corresponding to said transparent areas of said negative
film while retaining said charges theron in areas corresponding to said opaque areas
of said negative film forming a uniform color toner layer of a second polarity on
a donor member, effecting a virtual contact between said photoconductor and said color
toner layer on said donor member to thereby transfer portions of said color toner
layer to said photoconductor by attraction to said retained charges thereon while
preserving said color toner layer on said donor member in the form of color toner
deposits in the remaining portion thereof, transferring said color toner deposits
from said donor member onto a receptor sheet and affixing said color toner deposits
to said receptor sheet.
[0011] The preferred embodiment of this invention will now be described by way of example,
with reference to the drawings accompanying this specification in which:
Figure 1 is a diagrammatic representation illustrating the exposure of a charged
photoconductor to a film negative in accordance with the method of the invention;
Figure 2 is a diagrammatic representation of the photoconductor of Figure 1 illustrating
the remaining charges carried thereon after exposure; and
Figure 3 is a diagrammatic representation of means for toning the photoconductor of
Figure 2 and forming the reverse image on a receptor material according to the method
of the invention.
[0012] In Figure 1 is shown a photoconductive recording member 1 comprising a photoconductive
layer 2 on a conductive support 3, uniformly charged to negative polarity as indicated
by the negative charges 4. A film negative separation of the first color 5 containing
opaque final background or non-image areas 6 and transparent final image areas 7 is
placed in contact with photoconductive recording member 1 for contact exposure through
light source 8.
[0013] In Figure 2 is illustrated the photoconductive recording member 1 after exposure,
having retained negative electrostatic charges 4 only in the areas corresponding
to opaque areas of negative film separation 5 of Figure 1.
[0014] In Figure 3 is illustrated a donor member 9 comprising for instance a metal cylinder,
partially immersed in liquid toner of the first color 10 containing therein suspended
positive toner particles 11. The liquid toner 10 is contained within tank 12. An electrode
13, which may be in the shape of a knife-edge, is placed within tank 12 beneath donor
member 9 spaced a short distance apart therefrom. Electrode 13 is connected to the
positive terminal of power supply 14, the negative terminal of same being grounded.
Donor member 9 is also grounded, whereby an electrophoretic cell is formed wherein
donor member 9 and electrode 13 constitute the two electrodes of opposite polarity.
As donor member 9 is caused to rotate in the direction shown and a voltage from power
supply 14 is applied to electrode 13, and electrostatic field is established between
donor member 9 and electrode 13, positive toner particles 11 are urged to move electrophoretically
towards donor member 9 held at negative ground potential and to deposit thereon in
the form of a toner layer of first color 15, which layer remains on donor member 9
as it rotates and emerges from liquid toner 10. Photoconductive recording member 1
is caused to traverse in the direction shown at the same linear speed as the circumferential
speed of donor member 9, its photoconductive surface 2 containing after exposure residual
negative charges 4 thereon being in virtual contact with rotating donor member 9,
its conductive support 3 being grounded. At the point of virtual contact between
donor member 9 and photoconductive surface, toner layer 15 is split, in that portion
of toner layer 15 contacting the photoconductive surface 2 where electrostatic charges
4 are contained thereon are attracted and transferred thereto whereas those portions
of toner layer 15 contacting areas on photoconductive layer 2 which, due to preceding
exposure are free of electrostatic charges, remain thereon. As the result of this
the remaining portion 16 of toner layer 15 on donor member 9 corresponds to the transparent
final image areas 7 of film negative 5 in Figure 1. The toner deposits forming said
remaining portion 16 are electrostatically transferred onto receptor member such as
printing stock 17 and the circumferential speed of transfer roll 19 being the same
as the circumferential speed of donor member 9. Electrostatic transfer is effected
by connecting transfer roll 19 to the negative terminal of power supply 20, the positive
terminal of which is grounded. It will be seen that transferred first color toner
deposits 18 on printing stock 17 are contained thereon in areas corresponding to the
final transparent image areas 7 of first color separation film negative 5 as shown
in Figure 1 and thus a reversal reproduction, that is to say a positive print from
a negative film has been produced by toner transfer based on attraction throughout.
[0015] To produce a multicolor print or pre-press proof in accordance with this invention
the above disclosed steps are repeated in succession with negative film separations
of subsequent colors and corresponding color toners. For each subsequent color the
receptor member or printing stock carrying thereon the preceding color toner deposits
is moved around the transfer roll in register with the donor member to ensure that
all color images are transferred to the printing stock in exact register with each
other. For the same purpose, as will be obvious to those skilled in the art, the color
separation film negatives are placed in register with the photoconductive recording
member during contact exposure, and as the photoconductive member traverses the
donor member for transfer toning, registra tion means are provided to ensure exact
coincidence between corresponding image areas on the photoconductive surface and
the donor member surface.
[0016] It should be noted that for illustrative purposes in the foregoing, reference was
made to charging the photoconductive member to negative polarity, that is to the use
of an n-type photoconductor, followed by transfer toning same by attraction thereto
of positive color toner. It is equally possible to employ a p-type photoconductor
which can be charged positively and toned by attraction thereto of negative color
toner. It will be realized of course that in those instances where a negative color
toner is transferred from the donor member onto the photoconductive member and a negative
color toner deposit is transferred from the donor member onto the receiving member,
the polarity of the electrode causing toner layer formation on the donor member will
be negative and grounding polarity positive while transfer role polarity will be positive
and transfer power supply grounding negative.
[0017] It should be further noted that in the foregoing electrostatic transfer from the
donor member onto the printing stock has been illustrated by means of a roll comprising
for instance a conductive core connected to the terminal of the power supply and a
cover layer of semi-conductive elastomer, as is well known in the art, however other
well known means for eleotrostatic transfer such as a corona generator can be employed
equally well to serve the same purpose. Futhermore, transfer of color toner deposits
from the donor member onto the receiving member may be effected by methods other than
electrostatic, such as for instance by pressure, adhesion, heat and/or embedment
in a receptor coating on the receiving member.
[0018] While in the foregoing the photoconductive member has been illustrated as a flat
plate and the donor mem ber as a cylinder, it should be realized that the photoconductive
member may be cylindrical and the donor member flat, or both members can be flat or
cylindrical, or either or both members can be in the form of belts, if so desired.
In like manner the receptor member instead of being a flexible paper sheet as illustrated
can comprise a rigid material such as cardboard or metal plate, in which case of course
appropriate re-arrangement of the transfer from the donor to the receptor member
will be required, as would be obvious to those skilled in the art.
[0019] Although in the foregoing illustrations both the conductive support of the photoconductive
member and the donor member were shown to be at ground potential or of equal polarity
during transfer toning, it may be found desirable in certain instances to apply a
bias voltage between the two members to enhance toning or image quality, as is well
known in the art.
[0020] The embodiment as illustrated in Figure 3 comprises only the essential elements of
the present invention, and it should be realized that in practice it may be desirable
to incorporate other elements as are commonly used in electrostatographic equipment
for improving performance and/or image quality, such as for instance a doctor blade
or corona generator means to control the quantity of carrier liquid over the toner
layer formed on the donor member, means to wet the photoconductor surface with carrier
liquid type solvent before and/or after transfer toning, cleaning means to remove
toner layer portions from the photoconductive surface preparatory to oharging for
a following color, means to wet with carrier liquid type solvent the donor member
and/or the receptor member prior to toner deposit transfer, and the like.
[0021] An important feature of this invention consists in the capability to precisely predetermine
the thickness of the toner layer formed on the donor member as a function of the electrophoretic
mobility of specific toners, toner concentration, the gap between electrode and donor
member, electrode voltage and speed of rotation of donor member. As the toner deposits
forming the portions of the toner layer remaining on the donor member after transfer
toning the photoconductor are virtually completely transferred to the receptor member
and as the optical densities of such transferred toner deposits on the receptor member
are therefore directly related to the thickness of the toner layer formed on the donor
member by the electrode, it is readily possible in accordance with this invention
to produce multicolor prints or pre-press proofs wherein image deposits of specific
colors have specific optical densities precisely as desired by predetermining the
toner layer thickness on the donor member as above described.
[0022] Color toners usable in the process of the present invetion may be for example as
disclosed in U.S. Patent No. 3,998,746 of Tsuneda, U.S. Patent 3,820,986 of Fukashima
et aI., and U.S. Patent 3,419,411 of Wright.
[0023] There has been described a novel electrostatographic method of reversal reproduction,
that is, of producing positive imagery from film negatives. The method employs attraction
development throughout with liquid toners, hence image quality is excellent, and the
method is particularly suitable for the production of multicolor pre-press proofs
on printing stock.And additional feature of the method consists in the capability
of predetermining exactly as required the optical density of each color image deposit
on the final pre-press proof or print. Equipment configurations, materials and proportioning
of materials as disclosed herein are intended to be construed in illustra tive sense
only without restricting the scope of this invention.
1. The method of eleotrostatographic image reversal wherein a positive print comprising
a receptor sheet having image-free areas and image areas formed by color toner deposits
is produced from a negative film having opaque areas corresponding to said image-free
areas on said print and transparent areas corresponding to said image areas on said
print, said method characterized by the steps of:
A. electrostatically charging a photoconductor to deposit thereon uniformly charges
of a first polarity;
B. exposing said photoconductor to light through a negative film thereby to discharge
said photoconductor in the areas corresponding to said transparent areas of said negative
film while retaining said charges theron in areas corresponding to said opaque areas
of said negative film;
C. forming a uniform color toner layer of a second polarity on a donor member;
D. effecting a virtual contact between said photoconductor and said color toner layer
on said donor member to thereby transfer portions of said color toner layer to said
photoconductor by attraction to said retained charges thereon while preserving said
color toner layer on said donor member in the form of color toner deposits in the
remaining portion thereof;
E. transferring said color toner deposits from said donor member onto a receptor sheet;
and
F. affixing said color toner deposits to said receptor sheet.
2. The method of electrostatographic image reversal according to claim 1 characterized
in that plural negative separation films of subsequent colors and appropriate color
toners are used sequentially to produce a multicolor print comprising said receptor
sheet having image-free areas and image areas formed by multiple color toner deposits
in register with each other, the steps of transferring said conductive toner and said
color toner deposits respectively being effected in register and the color toner deposits
are affixed to said receptor sheet subsequent to the last to be applied deposit,
the steps A - F being repeated with the said color separation films and the appropriate
color toners.
3. The method of electrostatographic image reversal according to claim 2 characterized
in that the portions of the first color toner layer transferred to the photoconductor
are removed therefrom before the next to be applied color toner layer is applied.
4. The method of electrostatographic image reversal according to any one of claims
1, 2 or 3 characterized in that the formation of each color toner layer on the donor
member is by electrophoretic deposition.
5. The method of electrostatographic image reversal according to any one of claims
1, 2 or 3 characterized in that said color toner deposits are electrostatically
transferred from said donor member to said receptor sheet.
6. The method of electrostatographic image reversal according to any one of claims
1, 2 or 3 characterized in that the second polarity is opposite the polarity of the
charges deposited on the photoconductor.
7. The method of electrostatographic image reversal according to any one of claims
1, 2 or 3 characterized in that said uniform color toner layer on said donor member
is formed by electrophoretic deposition to be of predetermined thickness.
8. The method of electrostatographic image reversal according to any one of claims
1, 2 or 3 character ized in that the optical density of said colored toner deposits
on said receptor sheets transferred thereto from said donor member is controlled by
the thickness of said uniform color toner layer formed on said donor member.
9. The method of electrostatographic image reversal according to any one of claims
1, 2 or 3 characterized in that said photoconductor is contained on a conductive
support and wherein said conductive support and said donor member are electrically
connected while said photoconductor is contacted with said color toner layer on said
donor member to thereby transfer portion of said color toner layer to said photoconductor.
10. The method of electrostatographic image reversal according to any one of claims
1, 2 or 3 characterized in that said photoconductor is contained on a conductive
support and wherein said conductive support and said donor member are electrically
connected while said photoconductor is contacted with said color toner layer on said
donor member to thereby transfer portion of said color toner layer to said photoconductor,
said conductive support and said donor member being held at different voltage potentials
in relation to each other while said photoconductor so contacted with said color toner
layer.