AU598702B2 - Electrostatic proofing of negative color separations - Google Patents

Description

JCS 36270 COMMONWEALTH OF AUSTRALI5 Q 7 1 Patents Act 1952-1969 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int. Class Application Number Lodged Complete Application No.

Specification Lodged Published Priority: -Icy Related art: Name of Applicant: Address of Applicant: Actual Inventor: TO BE COMPLETED BY APPLICANT STORK COLORPROOFING a Dutch Company Wim de Korverstraat 43a, 5831 AN Boxmeer, The Netherlands JOSEF MATKAN and JOHN THOMAS ALSTON Address for Service: COLLISON CO., Patent Attorneys, 117 King William Street, Adelaide, South Australia. 5000 Complete Specification for the invention entitled: "ELECTROSTATIC PROOFING OF NEGATIVE COLOR SEPARATIONS" The following statement is a full description of this invention, including the best method of performing it known to xex us: A000808 29 II /e PATENT, TRADE MARKS &c DESIGNSJ sUB-OF-FICE 29 INOV 198 SOUTF-I AUSTALIA as ii _ii SELECTROSTATIC PROOFING OF NEGATIVE COLOR SEPARATIONS FIELD OF THE INVENTION This invention relates generally to electrophotography and, in particular, to a novel method of preparing multicolor pre-press proofs from negative color separation films by an electrophotographic process.

BACKGROUND OF THE INVENTION The purpose of pre-press proofs is to enable one to assess the color balance, registration, appearance, among other features, which can be expected from the press run and to correct the separation films before the printing plates are made therefrom. It is also desirable to produce socalled "customer proofs" which tell the customer how the original artwork will appear when printed with plates made I from the separation films. Thus, it is essential that the pre-press proof have the same appearance as the press print.

Accordingly, in addition to matching the color balance of the press print, the customer proof should be on the same paper as the press print.

The separation film can be a positive film or a negative film, depending on the type of printing plate to be used. The priicing plate used can be the so-called positive working and negative working lithographic or offset printing plate as is known in this field. A positive working plate is exposed to light through a film positive on which the information to be printed corresponds to opaque areas and the non-printing background areas correspond to transparent areas. The exposed areas on the plate are rendered removable by chemical treatment and the underlying plate surface, -lausually grained aluminum, forms the water receptive nonprinting or non-image areas, whereas the unexposed areas form the ink receptive printing image areas. A negative working printing plate is exposed to light through a film negative on which the information to be printed corresponds to transparent areas and the non-printing background areas Scorrespond to opaque areas. In this case, the exposed areas on the plate become photo-hardened and form the ink recepjtive printing areas, whereas the unexposed areas are removed by chemical treatment and the underlying water receptive plate surface forms the non-printing or non-image areas.

IIt is also known to produce, by electrophotographic .i 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 example, in United States Patent Nos. 3,809,555 and 3,862,848. An apparatus for the production of electrophotographic pre-press proofs is described, for example, Sin United States Patents Nos. 4,556,309 and 4,557,583.

It is known that electrophotographic pre-press proofs can be produced 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 directly or through an intermediate or offset member to a receptor, 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 multicolor proof.

-2t- After all of the required color toner deposits have been transferred to the receptor paper, it is coated by spraying or other methods with a clear polymer layer to transparentize the color toner deposits and fuse them to the receptor paper sheet.

All of the above referred to prior art elec- Strophotographic proofing processes are so-called direct reproduction processes. Accordingly, the color separation films employed can comprise film positives only, and thus, these processes are not suitable for the proofing of negative separation films wherein a reverse reproduction process is required.

Methods of electrophotographic image reversal, that is, production of a positive image from a negative film, are i 15 known, for example, as taught in United States Patent No.

S3,300,410 and United Kingdom Patent No. 998,599.

United States Patent No. 3,300,410 discloses a photoconductive recording member that consists of a sheet of paper that is coated with photoconductive zinc oxide and charged to negative polarity. The sheet is exposed through a negative film and toned with a positive liquid toner having film forming colloidal size conductive resin particles to form, after evaporation of the carrier liquid of such toner and drying, a permanently fixed conductive and colorless film deposit in the unexposed or non-image areas. The sheet was then re-charged negatively and only image areas free of conductive colorless film deposit accepted charges.

These areas were then toned with a colored positive toner to form visible image deposits, whereby a reversal image or a positive reproduction of the negative film was obtained.

Since the conductive film deposit affixed in the non-image areas was colorless, it did not affect the appearance of the zinc oxide coating.

United Kingdom Patent No. 998,599 discloses an image reversal that was obtained on a sheet of paper coated with photoconductive zinc oxide in a similar manner as described above. However, a positive liquid toner comprising low tinting strength pigment particles was used to form, in the unexposed or non-image areas upon evaporation of the carrier E liquid for such toner by drying, a permanently fixed conductive deposit. The deposit did not accept charge during the subsequent step of re-charging the surface for toning with a colored toner to form visible image deposits. Again, since the conductive deposit affixed in the non-image areas had a Slow tinting strength, it did not affect the appearance of i the photoconductor. The low tinting strength materials used were alumina hydrate, magnesium and barium carbonates, talc, plaster of Paris, conductive zinc oxide, mica and silica, having a refractive index less than about 1.6 or 1.7 and an 9 i electrical volume resistivity less than about 10 ohmcm.

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SIn each of the above cases, the colorless or low tinti ing strength toner deposits were conductive and thus did not accept charges. Since these toner deposits were permanently affixed to the photoconductor surface, these processes are "'i suitable only for single color reproduction on disposable photoconductors and are not suitable for applications Swherein images are produced successively in a variety of colors on a reusable photoconductor and then transferred i therefrom onto a receptor.

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t' 1 This invention provides an image reversal process for the production of electrophotographic color proofs from negative separation films wherein the electrophotoconductive recording member is reusable and wherein the proofs are produced on printing stock paper, very closely matching the appearance of the printed sheet.

The process of the invention includes exposing an electrophotoconductor that is charged to a first polarity through a color separation negative film which may be in contact therewith, developing the unexposel areas on the photoconductor with opposite polarity background toner to form background deposits thereon in areas corresponding to the opaque non-image or background areas on the negative, .1 subjecting the photoconductor and the background deposits ,i 15 thereon to corona discharge of said first polarity to charge the photoconductor in the areas free of said background deposits, that is, in areas corresponding to the transparent image areas on the negative, removing charges of said first polarity from the background deposits, developing the image S" 20 areas on the photoconductor with opposite polarity color l Ba«: toner, and transferring the thus formed color toner deposits e"e i to a receptor such as printing stock paper. Prior to development with the color toner, the charges are removed from the background deposits to ensure that no color toner will be attracted thereto, since any color toner contained on the background deposits would transfer onto the receptor and form thereon objectionable fog in the non-image or background areas. The background deposits are not adhesively affixed to the photoconductor, yet do not transfer to the receptor but can be easily removed from the photoconductor when desired.

For each additional color separation negative film, the process is repeated to transfer of the additional specific color developed image in proper registry. Of course, a proper toner for the specific color image will be used.

The above described process of this invention includes, in essence, the steps of: 1. uniformly charging a reusable photoconductor to a first i polarity; 2. exposing the photoconductor to light through a negative separation film of the first color; 3. toning the photoconductor with opposite polarity liquid toner, henceforth referred to as background toner, to form in unexposed areas thereon a background deposit which: upon drying remains on the photoconductor without being adhesively affixed thereto, is chargeable to positive and negative polarity, has a lower capacitance than the photoconductor, i- is substantially not transferable electrostatically or is transferable only at substantially higher voltages then the color toners used in the process as referred to further below, and 1- upon transfer to the receptor becomes fully transparent when a clear polymer film is formed over same; S4. drying the background deposit; i 5. optionally applying charges of opposite polarity to the photoconductor and the background deposit to thereby induce charges of opposite polarity only on the background deposit; -6-

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6. uniformly charging the photoconductor and the background deposit to the first polarity, wherein the first polarity charges induced on the background deposit are limited by the opposite polarity charges induced thereon in preceding step 7. applying uniformly charges of opposite polarity to the i photoconductor and the background deposit, wherein the magnitude of the opposite polarity charges is selected to substantially reduce the first polarity charges on the background deposit in view of its lower capacitance and optionally induce charges of opposite polarity thereon, without substantially affecting the first i polarity charges on the photoconductor in view of its higher capacitance; 8. toning the photoconductor with opposite polarity liquid i toner of the first color to form color deposits thereon Sin image areas free of the background deposit; 9. transferring such color deposits directly or through an offset member onto a receptor such as proof paper; 10. optionally, while employing the background deposit formed in steps 3 and 4, repeating steps 5 to 9 the required number of times if multiple proofs are needed; 11. removing the background deposit from the photoconductor; 12. repeating steps 1 to 9 and 11, and optionally step with negative separation films of subsequent colors and liquid toners of corresponding colors; 13. drying the receptor; and 14. forming a clear polymer film on the recepto i per, at least in the areas containing color toner deposits thereon.

-7q _IIL i Cii~Fii BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic sectional view taken through a A photoconductor and separation film illustrating the first step of forming a color proof in accordance with a method of the invention; FIG. 2 is a diagrammatic sectional view taken through a photoconductor illustrating the second step of forming a color proof in accordance with a method of the invention; FIG. 3 is a diagrammatic sectional view taken through a photoconductor illustrating a third step of forming a color proof in accordance with a method of the invention; FIG. 4 is a diagrammatic sectional view taken through a photoconductor illustrating a fourth step of forming a color proof in accordance with a method of the invention; FIG. 5 is a diagrammatic sectional view taken through a photoconductor illustrating a fifth step of forming a color Sproof in accordance with a method of the invention; SFIG. 6 is a diagrammatic sectional view taken through a photoconductor illustrating a sixth step of forming a color proof in accordance with a method of the invention; FIG. 7 is a diagrammatic sectional view taken through a photoconductor illustrating a seventh step of forming a color proof in accordance with a method of the invention; FIG. 8 is a diagrammatic sectional view taken through a receptor illustrating an eighth step of forming a color proof in accordance with a method of the invention; FIG. 9 is a bar graph illustrating the surface voltages on the photoconductor and the background deposits when step is included; and FIG. 10 is a bar graph illustrating the surface voltages on the photoconductor and the background deposits when step 5 is omitted.

-8t'i DESCRIPTION OF THE PREFERRED EMBODIMENT Applicant has discovered that particulate materials of the type disclosed in United Kingdom Patent No. 998,599 |I referred to above are not truly conductive, per se, and if Iincorporated in toner compositions as hereinafter described, are useful for making background toners in accordance with this invention to form background deposits which differ very significantly from the low tinting strength toners of United Kingdom Patent No. 998,599. The background deposits formed in accordance with this invention: are non-conductive and are thus chargeable, yet easily dischargeable; S; are not adhesively affixed to the photoconductor; S- are substantially not transferable; and ii can be easily cleaned off the photoconductor to render it reusable.

i i Certain other substances that were found to be useful in making background toners in accordance with this invention include particulate material such as calcium carbonate, |micronic size celluloses such as methyl cellulose and carboxy methyl cellulose, polymeric materials such as polyvinyl pyrollidone, polyvinyl alcohol and calcium resinate, carbohydrates such as starch and dextrin, silicates such as bentonite, asbestine and montmorillonite, clays such as kaolin and attapulgus clay and the like, as well as dielectric or highly insulative polymeric materials in particulate form, which are insoluble in the carrier liquid, such as epoxies, acrylics, polyvinyl chloride, polyvinyl acetate, polyvinyl butyral, polyesters, polystyrene, polyethylene and the like. Mixtures of these materials can also be used.

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r niu~nrr~* c!~ The background toner of this invention is prepared by dispersing particulate materials of the above disclosed type in the toner carrier liquid such as isoparaffinic hydrocarbon in the presence of a soluble dispersing aid or wetting agent such as acrylic polymer, rosin ester and the like. A charge director or polarity control agent can be included in the dispersion. To prevent adhesion of the background deposit to the photoconductor, the proportion of such disi persing aid is kept at a minimum, such as not more than about 25 percent by weight of the particulate material.

SFurthermore, to prevent electrostatic transfer of the background deposit, r.o transfer enhancing materials such as i waxes or lattice forming substances are included in the Sbackground toners of this invention.

The background deposits formed by the above disclosed background toners of this invention remain, upon drying, on the photoconductor surface due to the presence of the small proportion of the soluble dispersing aid, without becoming affixed thereto. Therefore, they can be applied to reusable photoconductors and can be very easily removed therefrom when desired.

Although such background deposits are not affixed to the photoconductor, they are electrostatically substantially not transferable, at least not at transfer voltages normally used in the process for the color toners. At higher voltages some random transfer of the background deposit may occur, without, however, affecting the appearance of the receptor. This is because the above disclosed particulate materials become fully transparent when the aforementioned clear polymer film is formed on the receptor.

A further essential requirement of the background deposit of this invention is that its capacitance must be substantially lower than that of the photoconductor. This is accomplished by the above disclosed toner composition, wherein the proportion of the dispersing aid is insufficient not only to affix the toner deposit to the photoconductor but also to cement together the individual toner particles l and thereby to form a continuous layer. Thus thb deposit is i discontinuous, in that it comprises substantially discrete weakly coherent particles having voids or air pockets therebetween. The capacitance of a background deposit layer having such a structure, irrespective of the layer thickness and of the dielectric constant of the materials contained i therein, is per se lower than the capacitance of the commonly known continuous layer photoconductors.

As stated earlier, the background deposit of this in- I :vention can be charged positively and negatively. However, the rate of decay of the charge accepted by the background deposit is, due to its low capacitance, significantly faster than the rate of dark decay of the charge accepted by the photoconductor. Also, if both the background deposit and A3 the photoconductor are charged to one polarity, application of weak charges of opposite polarity will readily discharge the background deposit, due to its low capacitance and consequently low surface charge density, without significantly J affecting the charge on the photoconductor.

The process of this invention will now be described in more detail with reference to the drawings, where, for illustrative purposes, operation with only a negatively chargeable n-type photoconductor is shown. It is to be understood, however, that the process is equally applicable to -11positively chargeable p-type photoconductors, in which case charges of opposite polarity to those shown in the drawings would be used throughout the process steps.

Referring now to Fig. 1, a photoconductor is designated generally by reference numeral 1. The photoconductor 1 includes a photoconductive layer 2 that is secured to a conductive substrate 3. The photoconductor 1 is uniformly charged to a negative polarity as indicated by negative charges 4. A first color negative separation film 5, containing opaque non-image or background areas 6 and transparent image areas 7, is placed in contact with the photoconductor 1 for contact exposure through a light source o 0 8.

000 D 0 1 Fig. 2 illustrates the photoconductor 1 after exposure 00 0 by the light source 8. The phot;onductor 1 retains the 0 a negative electrostatic charges 4 only in the areas coro 0

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responding to the opaque background areas 6 of the negative film 5 illustrated in Fig. 1.

o 00 0o 0 The photoconductor 1 is then toned with a positive 0 00o0 20 background toner of the invention which forms background toner deposits 9, as illustrated in Fig. 3.

0 Fig. 4 illustrates the step where the photoconductor 1 and the background deposits 9 are charged positively by 0o o means of a corona generator 10. Only the background 25 deposits 9 accept positive charges 11, while the n-type photoconductor 1 remains uncharged. It is to be noted that this is an optional step that can be used to reduce the negative charge which would be accepted by the background deposits 9 in the following step illustrated in the next Figure.

-12- 4 it Fig. 5 illustrates the step where the photoconductor 1 and the background deposits 9 are charged negatively. The negative charges 4 on the photoconductor 1 are of the same magnitude as in FIG. 1 that is needed for toner attraction.

The magnitude of negative charges 12 on the background deposits 9, however, depends on whether or not the optional step illustrated in FIG. 4 has been carried out. Namely, if the background deposits 9 carry the positive charges 11 induced in the preceding optional step, the positive charges on the background deposits 9 at first have to be neutralized by this step of negative charging before the background deposits 9 can be actually charged negatively. In this case, the magnitude of negative charges induced in this step on the background deposits 9 would be considerably lower than in the case where the optional step is omitted.

03o0 Fig, 6 illustrates the step where the photoconductor 1 o and the background deposits 9 are again charged positively.

In this Ltep, the positive charging current is selected to 0 be low enough so as not to appreciably affect the negative o 20 charges on the high capacitance photoconductor i, yet sufficient to substantially neutralize the negative charges 12 on 000.. the background deposits 9. This is possible due to the low capacitance and consequently, low surface charge density, of 0 the background deposits 9. Moreover, if the optional step a o 0 25 illustrated in Fig. 4 is performed, positive charges will be induced in the background deposits 9 to actually repel positive color toner therefrom in the following step of toning.

The photoconductor 1 is then toned with a positive toner of a first color to form first color toner deposits 13 thereon, as illustrated in Fig. 7. Accordingly, no color toner is attracted to the background deposits 9.

-13- Fig. 8 illustrates a receptor 14, such as paper, after electrostatic transfer of the first color image deposits 13 from the photoconductor 1 of Fig. 7 has taken place.

Figs. 9 and 10 illustrate the effects of charging in the steps described in Figs. 4, 5, and 6 corresponding to process steps 5, 6, and 7 respectively. For simplicity, in Figs. 9 and 10 the charging effects are illustrated in terms of the surface voltages Vs corresponding to the surface charges.

F. 9 illustrates the effect of the positive Vs induced on the background deposits 9 in optional step 5. In Sstep 6, the photoconductor 1 is charged negatively to the Stop Vs, while the negative Vs induced on the background j deposits 9 is relatively low. Consequently, at very low positive charging current in step 7, the negative Vs on the background deposits 9 is reductJ to zero, or even a positive Vs is induced thereon, as shown by the dotted lines in Fig.

i 9, while the negative top Vs on the photoconductor 1 remains virtually unaffected.

If optional step 5 is omitted, as illustrated in Fic.

the negative Vs induced on the background deposits 9 in j step 6 is high. In this case a higher current is needed for positive charging in step 7 to reduce the negative Vs on the background deposits 9 to zero. At the same time, this results in a greater drop in the top Vs on the photoconductor.

j Reusable photoconductors which are suitable for a colorproofing process in accordance with this invention can be, for exampls, crystalline sputtered cadmium sulfide as disclosed, for example, in United States Patent No.

4,025,339. Other reusable photoconductors can be used if so desired.

-14- 4I The colorproofing process of this invention can be conveniently carried out in electrophotographic color proofing equipment as described, for example, in United States Patents Nos. 4,556,309 and 4,557,583, which were referred to crystalline cadmium sulfide photoconductor on a stainless steel substrate to prepare the data for the illustrative examples given further below.

It should be noted that in the above referred to colori 10 proofing equipment, electrostatic transfer is effected by means of rollers and the toner deposits are transferred from the photoconductor first to an offset or intermediate member and then to the receptor proof paper. For simplicity, however, in the following examples reference is made only to a single transfer from the photoconductor to a paper receptor.

It is to be noted that double transfer through an offset or intermediate member is equally applicable as well as electrostatic transfer by other means, such as, for example, by *corona discharge.

Liquid toner compositions forming electrostatically transferable color deposits useful in the colorproofing process of this invention are disclosed, for example, in United States Patent No. 3,419,411 and in U.S.

Patent entitled "Method Of Image Fixing In Color Electrostatography", NHo. 4,820,618 filed October 17, 1986 and owned by the same assignee.

*t These are incorporated herein by reference.

The following examples will serve to further illustrate the process of this invention.

ai" l 30 COMPARATIVE EXAMPLE 1 This example is included to illustrate the nonconductive nature of the background deposits 9 of this -A si) invention and the image quality obtainable if positive charging as proposed in optional step 5 and in step 7 is not employed.

The background toner in this and the following examples included a dispersion of pigment grade calcium carbonate and about 20 percent by weight acrylic dispersing aid in isoparaffinic hydrocarbon carrier liquid.

The same color toners were employed throughout all examples, also in isoparaffinic hydrocarbon carrier liquid, and the printing sequence was black, yellow, magenta and cyan.

Throughout all examples colorproofs were produced on a high quality clay coated art paper.

After all of the required color toner deposits 13 were transferred to the receptor paper 14, it was coated by spraying with a clear acrylic polymer layer to transparentize the color toner deposits 13 and to fuse them to the V 111receptor 14, as described earlier. Equal transparentization V and fusion was obtained by spraying the receptor with a pure soivent to thereby dissolve the clear polymeric binder in the color toner deposits 13, without affecting the appearance of the receptor 14 in non-image areas, as disclosed in said aforementioned uJ.S. Patent No. 4,820,618.

25 To match the press printed subject matter on the same art paper, the densities of the colors on the proof had to be within +0.05 tolerance as follows: black 1.80 yellow 0.90 magenta 1.45 *cyan 1.35, -16areas was colorless, it did not affect the appearance of the zinc oxide coating.

-3at 0.00 fog density in the background areas. All densities were measured with a Macbeth 927 wide band reflection densitometer.

For electrostatic transfer of the color toner deposits 13 to the art paper the following voltages were used throughout: for black 500V, for yellow 900V, for magenta and cyan -1500V. At these voltages there was no appreciable transfer of the background deposits 9 to the art paper.

It should be noted that in the previously referred to colorproofing equipment used in these examples, the time lapse between negatively charging the photoconductor 1 and a. commencement of background toning is about 100 seconds.

Also, the time lapse between negative charging in step 6 and .o ocommencement of color toning is about 100 seconds, and the charges or surface voltages on the photoconductor 1 and on a o athe background deposits 9 at such time determine the density which the color toners develop during the following toning step.

a In all examples the photoconductor was charged negaa" 20 tively for background toning and then in step 6 for color aO.: toning with a corona current of 350 microamps. This induced a top surface voltage on the photoconductor 1 of 30V, which in 100 seconds decayed to 28V.

In this comparative example where steps 5 and 7 were S 25 omitted, the negative charging in step 6 induced on the background deposits 9 a surface voltage of 50V, which in 100 seconds decayed to Applying 28V on the photoconductor 1 and 20V on the background deposits 9 at commencement of color toning gave the following densities: -17- Image Fog black 1.90 0.08 yellow 1.00 0.05 magenta 1.50 0.15 cyan 1.43 0.05 The cumulative 4-color fog density was 0.25 to 0.30.

i The high voltage of 20V on the background deposits 9 in view of its low capacitance and consequently low surface -i charge density attracted relatively little color toner, how- 10 ever the thus caused fog level was sufficient to render the proof completely unacceptable.

EXAMPLE 2 Comparative Example 1 was repeated with the exception f that optional step 5 and step 7 were carried out.

S 15 In step 5, the photoconductor 1 and the background S, deposits 9 were charged positively with 200 microamps corona current. This induced a positive surface voltage of about i 50V on the background deposits 9.

Step 6 of negative charging immediately followed step 5. In this instance the negative surface voltage induced on the background deposits 9 was only about In the immediately following step 7, the photoconductor 1 and the background deposits 9 were charged positively with i a corona current of 50 microamps, which reduced the negative voltage on the background deposits 9 to zero. The top suriI face voltage on the photoconductor 1 was reduced by only IV to 29V, which in 100 seconds decayed to 27V.

Applying 27V on the photoconductor 1 and OV on the background deposits 9 at commencement of color toning gave the following densities: -18i Image Fog black 1.85 0.00 yellow 0.95 0.00 magenta 1.48 0.00 cyan 1.39 0.00 The thus produced colorproof was fully acceptable.

EXAMPLE 3 Example 2 was repeated with the exception that in step 7 the positive corona current was 60 microamps. This induced a positive voltage on the background deposits 9 of 12V, which in 100 seconds decayed to 5V. The top surface voltage on the photoconductor 1 was reduced by 2V to 28V, which in 100 seconds decayed to 26V.

Applying 26V on the photoconductor 1 and 5V positive on the background deposits 9 at commencement of color toning I gave the following densities: SImage Fog black 1.82 0.00 yellow 0.92 0.00 magenta 1.45 0.00 cyan 1.36 0.00 SThe thus produced colorproof was fully acceptable.

EXAMPLE 4 Comparative Example 1 was repeated with the exception that step 7 was included.

In step 7, the positive corona current had to be 75 mi- J croamps to reduce the negative charge on the background deposits 9 to zero. However, this reduced the top negative surface voltage on the photoconductor 1 to 26V, which in 100 seconds decayed to 24V.

-19- Applying 24V on the photoconductor 1 and OV on the background deposits 9 at commencement of color toning gave the following densities: Image Fog black 1.77 0.00 yellow 0.86 0.00 magenta 1.40 0.00 cyan 1.30 0.00 The color densities were lower that in the preceding examples, but still within the specified tolerance limits.

The colorproof was fully acceptable.

There has been described a novel electrophotographic process for the production of positive colorproofs from negative color separation films. The materials and equipment disclosed herein are intended to be construed in illustrative sense only without restricting the scope of this invention.

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Claims (15)

1. An image reversal process for the production of positive color imagery from negative color separation films comprising the steps of: a) uniformly charging a photoconductor to a first polarity; b) exposing said photoconductor to light through a negative separation film of the first color; c) toning said photoconductor with opposite polarity liquid background toner to form in unexposed areas a background deposit thereon; d) drying said background deposit; e) uniformly charging said photoconductor and said background deposit to said first polarity; f) uniformly applying charges of opposite polarity to said photoconductor and said background deposit, the magnitude of said opposite polarity charges I being selected to substantially reduce the first polarity charges on said background deposit without substantially affecting the first polarity i 20 charges on said photoconductor; g) toning said photoconductor with opposite polarity [i liquid toner of the first color to form color deposits thereon in image areas free of said back- 1> ground deposit; h) transferring said color deposits onto a receptor; i) removing said background deposit from said i photoconductor; and J) repeating steps a) to i) with negative separation films of subsequent colors and liquid toners of corresponding colors. -21- ;c q t ~i-rr,1Y.r: i :I j rr A I A

2. The process as defined in claim 1 wherein in step f) the magnitude of said opposite polarity charges is selected to substantially reduce the first polarity charges on said background deposit and induce charges of opposite polarity thereon, without substantially affecting the first polarity charges on said photoconductor.

3. The process as defined in claim 1 wherein said dried background deposit on said photoconductor remains on said photoconductor during the required process steps, without being adhesively affixed thereto, until removed therefrom by cleaning; is chargeable to positive and nega- tive polarity; has a lower capacitance than said photoconductor; is substantially non-transferable elec- trostatically at least at the voltages at which the color toner deposits used in the proness are transferred; and be- comes transparent upon random transfer to the receptor when a clear polymer film is formed over said background deposit and said receptor.

4. The process as defined in claim 1 wherein said photoconductor is chargeable to one polarity only. The process as defined in claim 1 wherein in step f) the substantial reduction of said first polarity charges on said background deposit, without substantially affecting said first polarity charges on said photoconductor, is due to the capacitance of said background deposit being lower than the capacitance of said photoconductor.

6. The process as defined in claim 1, wherein after step h) while using said background deposit formed in steps c) and steps e) to h) are repeated to image a multi- plicity of receptors.

7. The process as defined in claim 1 wherein said photoconductor is reusable. -22- t-

8. The process as defined in claim 1 wherein the com- position of said background deposit includes particulate material and a dispersing aid for said particulate material and wherein the proportion of said dispersing aid is about

20-25 percent by weight of said particulate material. 9. The process as defined in claim 8 wherein said director. The process as defined in claim 1 wherein said receptor is dried upon transfer thereto of toner deposits of all required colors. 11. The process as defined in claim 1 wherein after I t transfer of toner deposits of all required colors to said receptor a clear polymer film is formed over said receptor, 1 at least in the areas containing said color toner deposits t II 5 thereon. 12. The process as defined in claim 1 wherein said receptor is proofing stock material for the production thereon of a multicolor pre-press proof. 13. An image reversal process for the production of positive color imagery from negative color separation films comprising the steps of: 4 a) uniformly charging a photoconductor to a first polarity; b) exposing said photoconductor to light through a negative separation film of the first color; c) toning said photoconductor with opposite polarity liquid background toner to form in unexposed areas a background deposit thereon; d) drying said background deposit; -23- i e) applying charges of opposite polarity to said photoconductor and said background deposit to thereby induce charges of opposite polarity only on said background deposit; f) uniformly charging said photoconductor anid said background deposit to said first polarity, wherein said first polarity charges induced on said back- ground deposit are limited by said opposite polarity charges induced thereon in preceding step e); g) uniformly applying charges of opposite polarity to said photoconductor and said background deposit, the magnitude of said opposite polarity charges being selected to substantially reduce the first polarity charges on said background deposit, 4,44 P without substantially affecting the first polarity charges on said photoconductor; h) toning said photoconductor with opposite polarity liquid toner of the first color to form color deposits thereon in image areas free of said back- ground deposit; i) transferring said color deposits onto a receptor; j) removing said background deposit from said photoconductor; and k) repeating steps a) to j) with negative separation films of subsequent colors and liquid toners of corresponding colors. 14. The process as defined in claim 13, wherein in step g) the magnitude of said opposite polarity charges is selected to substantially reduce the first polarity charges on said background deposit and induce charges of opposite polarity thereon, without substantially affecting the first polarity charges on said photoconductor. -24- LL-. 1 The process as defined in claim 13, wherein said dried background deposit on said photoconductor remains on said photoconductor during the required process steps, without adhesively affixed thereto, until removed therefrom by cleaning; is chargeable to positive and negative polarity; has a lower capacitance than said photoconductor; is substantially non-transferable electrostatically at least at the voltages at which the color toner deposits used in Sthe process are transferred; and becomes transparent upon random transfer to the receptor when a clear polymer film is formed over said background deposit and said receptor. 16. The process as defined in claim 13 wherein said photoconductor is chargeable to one polarity only. 17. The process as defined in claim 16 wherein in step Se) the induction of opposite polarity charges only on said i background deposit is due to said photoconductor being chargeable to said first polarity only. 18. The process as defined in claim 13 wherein in step g) the substantial reduction of said first polarity charges on said background deposit, without substantially affecting i said first polarity charges on said photoconductor, is due to the capacitance of said background deposit being lower than the capacitance of 7-id photoconductor. 19. The process as defined in claim 13, wherein after step i) while using said background deposit formed in steps i: c) and steps e) to i) are repeated to image a multi- 2 plicity of receptors. process as defined in claim IS wherein said photoconductor is reusable.

21. The )rocess as defined in claim 13 wherein the composition of said background deposit includes particulate _i L~ 44 material and a dispersing aid for said particulate material and wherein the proportion of said dispersing aid is about 20-25 percent by weight of said particulate material.

22. The process as defined in claim 21 wherein the composition of said background deposit includes a charge director.

23. The process as defined in claim 13 wherein said receptor is dried upon transfer thereto of toner deposits of all required colors.

24. The process as defined in claim 13 wherein after transfer of toner deposits of all required colors to said Sa receptor a clear polymer film is formed over ,aid receptor, a at least in the areas containing said color toner deposits a a 5 thereon. a a a 25. The process as defined in claim 13 wherein said receptor is proofing stock material for the production thereon of a multicolor pre-press proof.

26. An image reversal process for the production of *a positive color imagery from at least one negative color separation film comprising the steps of: a) uniformly charging a photoconductor to a first polarity; b) exposing said photoconductor to light through a 0;4 negative separation film of the at least one color; c) toning said photoconductor with opposite polarity liquid background toner to form in unexposed areas a background deposit thereon; d) drying said background deposit; e) uniformly charging said photoconductor and said background deposit to said first polarity; -26- i. I r$I<9 o I t 42 420 42244 42 42 0 424 42 42A2 4 424 42 a ,t 42 42 4242 4 42 424 42 424 42s r 4224 42 420 42 42 f) uniformly applying charges of opposite polarity to said photoconductor and said background deposit, the magnitude of said opposite polarity charges being selected to substantially reduce the first polarity charges on said background deposit without substantially affecting the first polarity charges on said photoconductor; g) toning said photoconductor with opposite polarity liquid toner of the first color to form color 0 deposits hereon in image areas free of said back- ground deposit; h) transferring said color deposits onto a receptor; and i) removing said background deposit from said 5 photoconductor.

27. The process as defined in claim 26 including, after step applying charges of opposite polarity to said photoconductor and said background deposit to thereby induce charges of opposite polarity only on said background 5 deposit, wherein said first polarity charges induced on said background deposit in step e) are limited by said opposite polarity charges induced thereon.

28. The process as defined in claim 27 wherein in step f) the magnitude of said opposite polarity charges is selected to substantially reduce the first polarity charges on said background deposit and induce charges of opposite polarity thereon, without substantially affecting the first polarity charges on said photoconductor. DATED this 29th day of November 1988. STORK COLORPROOFING B.V., By their Patent Attorneys, COLLISON CO. -27- f