GB2023501A

GB2023501A - Electrostatic colour printing method and apparatus

Info

Publication number: GB2023501A
Application number: GB7921636A
Authority: GB
Original assignee: Coulter Systems Corp
Current assignee: Coulter Systems Corp
Priority date: 1978-06-22
Filing date: 1979-06-21
Publication date: 1980-01-03
Also published as: GB2023501B; SE439846B; IT1117237B; DE2924912A1; AU4828679A; IT7949496A0; DK261679A; CA1142994A; IL57608A0; NL185963B; SE7905496L; JPS556391A; FR2429457B1; NL185963C; FR2429457A1; JPH0150907B2; BE877131A; LU81413A1; CH657244GA3; AU527113B2

Abstract

The apparatus has multiple independent stations each having a master sleeve with an image in semipermanent fixed primary dielectric toner formed thereon. The image is transferred directly or by way of an intermediate transfer roll. The master sleeve is charged in light, the toned parts retaining charge. The resulting now charged image is toned with a pigmented secondary toner and this double toned image is pressed against the passing substrate or brough very close to it either directly or through the transfer medium. An electrical field is established between the substrate and the sleeve or transfer medium, the toner transferred to the substrate and the sleeve or transfer medium circulate back to pass through the same process of charge, tone and transfer. The transferred images are fixed between or during movement through the stations. Image density information can be used to control change in the electrical field conditions or other parameters. The impressions from each station are applied in registration.

Description

SPECIFICATION Printing method and apparatus This invention relates generally to printing apparatus and more particularly is concerned with electrostatic printing apparatus for color printing.

Conventionally, color printing is effected by mixing the colors on a final receptor through the use of multiple registered impressions. For example, a color object or image is photographed through several filters to make color separations in the printed colors comprising cyan, magenta, yellow and black, each of which are made into plates, mounted in a printing press and the receptor passed through the press and impressed with each of the plates in turn.

Present available electrostatic techniques for multicolor printing are complex, expensive and unreliable. Formation of the color separated plates of different basic colors is still normally required.

The use of xerography or electrofax techniques for multiple printing is contemplated.

The prior application of electrostatic techniques require a direct transfer or imaging for each example printed. It would be advantageous electrostatically to form images to be printed semipermanently on sleeves that remain so imaged throughout the entire printing run. With use of such technique, it also would be desirable to achieve the different parameters which may be required for each color without interference with the imprinting of other colors, say at other stations.

Accordingly, the invention provides a method of electrostatically printing composite color images on an elongate substrate wherein the colored images are produced repeatedly and spaced along the substrate and each is composed of a plurality of impressions of different color which comprises providing a plurality of printing stations, -each station being arranged to produce an impression of a different color, providing a master sleeve of electrophotographic material at each station and rotating the sleeve continuously while simultaneously moving the substrate through the station, the sleeve carrying a semipermanent fixed dielectric toner image thereon capable of being reproduced as the said impression, forming a secondary unfixed toner image upon the fixed toner image during said master sleeve rotation, transferring the secondary unfixed toner image to the surface of said substrate from the master sleeve as the substrate passes said each station, fixing the unfixed toner image after transfer on said substrate as the substrate moves, and repeating steps of forming the unfixed secondary toner image, transferring same and the unfixed toner image at each station with the impressions from each station being applied to said substrate in registration.

Additionally, the invention provides an apparatus for printing a composite pattern repeatedly upon a moving substrate, said pattern being made up of registered impressions of different images having different colors, said apparatus comprising a plurality of image printing stations, each being substantially similar in construction and adapted to produce a different color impression upon a substrate, means for moving an elongate substrate through the apparatus from station to station, each station comprising a rotating master sleeve, said sleeve having an image thereon in the form of a component of a composite image, said sleeve being an electrophotographic member including an outer coating of photoconductive material, said image being a fixed dielectric toner image which has been electrostatically formed on said outer coating, means for charging and toning said fixed toner image with a secondary toner to achieve a secondary toned image of said fixed toner image which is developed but not fixed, means for transferring the secondary toned image to a surface of the substrate while the substrate is moving through said station, means for fixing the image to said substrate surface before said substrate moves to the next following station to achieve an imprinted impression of a certain color, and means for removing any excess toner which is not transferred to said substrate surface if any had remained, the stations being arranged continuously to repeat the imprinting on the substrate as it passes through the apparatus whereby to achieve a continuous series of the same composite image along the substrate as it leaves the apparatus.

The preferred embodiments of this invention now will be described, by way of example, with reference to the drawings accompanying this specification in which: Figure 1 is a diagrammatic view of apparatus for carrying out the method of the invention and printing composite color images on a moving substrate; Figure 2 is a diagrammatic view showing a method of making a master sleeve for use with the invention; and Figure 3 is another diagrammatic view showing a second method making a master sleeve for use with the invention.

The printing method provided herein involves the making of a master sleeve upon which an image is carried. The master sleeve rotates preferably in cylindrical form and either contacts the substrate directly in which case it rotates in the same direction as the substrate or has a peripheral direction of movement contra that of the substrate. In the latter, the master sleeve serves as a medium through which the secondary toned image on the master sleeve is transferred onto a second sleeve or blanket which in turn is transferred directly to a moving substrate from the second sleeve and indirectly from the master sleeve. It is important to note that the described apparatus includes a plurality of stations each of which is independently controlled as to the various parameters required to produce the necessary impression or image on the substrate.

Referring to the drawings, in Figure 1 there is illustrated a printing apparatus 70 having four stations 72, 74, 76 and 78, it being assumed that said apparatus 70 is to be used for color imprinting on paper.

The substrate 80 passes through apparatus 70 and moves through various rollers, tensioning devices, etc., none of which need be shown herein. At the first station there is a master sleeve 82 which is mounted on a roller 84, the master sleeve 82 in this case being cylindrical although it could be in the form of a belt. The sleeve 82 need not be transparent. Preferably, the sleeve 82 comprises a coating of photoconductive material applied to a thin, metal cylinder maintained in its cylindrical condition upon the roller 84 by suitable means.

The sleeve 82 is formed of an electrophotographic material or member comprising a substrate which, if formed of resin such as transparent polyester, carries an ohmic layer and a coating of photoconductive material on the outside thereof. If the substrate is formed of metal, the photoconductive material comprises a wholly inorganic compound, preferably cadmium sulfide, which has been sputtered onto the substrate as a microcrystalline, transparent, high quantum yield deposit.

The sleeve 82 carries an image preferably in the form of a digital coded color separation which has been applied to the sleeve in a manner to be described in connection with Figures 2 or3, i.e., the image having been toned with a dielectric or primary toner and fused so that it is semipermanently affixed to the exterior of the master sleeve 82.

The station also includes a pressure roller 86 which can assist in driving the substrate 80 from left to right and/or can assist in the transfer of the toned image from the exterior surface of the master sleeve 82 to the bottom surface of the substrate 80, In use the imaged master sleeve 82 is charged by any suitable means at 88 and after charging or even at the same time is exposed to light.

As a result of the exposure to light after the sleeve 82 has been charged, the charge remains on the dielectric toned parts and leaks off those parts which have not been toned thereby providing a secondary charged image which then is passed to a secondary toning bath 90 where the secondary image is now developed (toned). This secondary toned image is passed around by rotation of the roll 84 into engagement with the passing substrate 80 at the point where the roller 86 is juxtaposed. Transfer can be effected either by pressure or by an electric field furnished by the power supply 92 connected to the roller 86 and to the toner bath 90 or to the roller 84. In any event, there is a bias voltage field across the space between which the substrate 80 is moving and this assists in the transfer of secondary toner to the lower surface of the substrate.An intermediate blanket or transfer roll can be provided.

After the said secondary toned image is applied to the substrate, it passes over a heating device 94 at which point the image is fixed.

All of the remaining stations operate in the same way and are constructed in the same way, the difference between them being the parameters of heat, temperature and so on. It has been found that different toners require different voltages, temperatures, etc., so that the use of a sequence of stations operating independently is more likely to give best results. It has even been found that certain cases of transfer require biases of opposite polarity for optimum results.

Information on density can be sensed after each image is applied, for example at 64 which comprises an optical transducer, to a control signal converter 66 from which the bias voltage generated in 92 will be varied for optimum density condition.

When the substrate 80 emerges from the station 78, all four images are compositely applied to the bottom surface thereof, it being assumed that registration is maintained by suitable means which are not shown. From here the substrate 80 moves to slitters, cutters, folders, supply rolls, etc., none of which are shown but presumed to comprise structure using the oncoming substrate.

In Figure 2 there is shown a roll 96 of the transparent electrophotographic material from which a sleeve is made. This material may be welded together to form a flexible belt which is indicated at 20 either before or after the image has been applied. The belt 20 is capable of being formed into sleeve 82 and mounted cylindrically.

The image can be projected onto the belt 20 optically, this being done by charging followed by imaging, toning with dielectric toner and -fixing. It is preferred however, that the image be applied in a digital fashion so that there is no need for a screen and no problem deriving from optical projection means.

A suitable multi-colored pattern is scanned and its colors separated and digitized into bits which are stored in an electronic store 98. The store 98 then is used to modulate a low power laser, such as for example a helium-neon laser 100. The laser 100 projects its modulated beam by way of reflecting means 102 and 104 onto the surface of the electrophotographic material of the roll 96 at 106 as this material is being unrolled. By suitable drives, the reflectors 102 and 104 are capable of correcting and compensating for the conversion from the continuous flow of bits to application to a surface as the beam swings back and forth. The material first is charged at 108, imaged at 106, toned at 110 and fixed at 112. The resulting image is now semipermanently fixed and the length of material can be cut from the roll 96 and formed into the sleeve 20 in the form of a belt by welding the ends together. The belt also is capable of being in sheet form and clamped to a mandrel.

In Figure 3 there is illustrated apparatus which can be used to make a sleeve directly as a cylinder such as shown at 82 in Figure 1. The sleeve 82 is in the form of a thin cylinder of metal or the like having a coating of the photoconductive material described mounted on a mandrel 114 which is driven. The same type of iaster 116 as shown in Figure 3 at 100 fed from an electric store 11 7 directs a modulated beam of light 11 8 composed of digitized bits to a travelling mirror 120 mounted on a suitable driven guide rod 122 so that the digitized bits are properly laid down upon the surface of the sleeve 82 to produce the desired image representing a single color of a composite.

A corona device 124 is arranged to charge the surface and a toning device 126 is provided to tone it after the image has been laid down. At 128 one can see a representation of a toner fixing device which is on the opposite side of the view in Figure 3 semipermanently to affix the dielectric toned image to the master cylinder 82.

After the master cylinder has been toned and the image fixed it is removed from the mandrel 114 and installed in an apparatus such as 72.

In the operation of the apparatus 72 after the secondary toner has been transferred to the substrate 80, any toner which remains on the surface of the sleeve 82 can be removed by suitable cleaning apparatus such as shown at 87.

It is feasible to have the sleeve 82 imaged on line, that is, permanently secured in place in the apparatus 72 but provided with means for applying the image from the store and fixing the same for use. Thus, on station 76 there is illustrated a block 1 30 at which location the same functions are performed as performed by the laser 116 and the moving mirror 120 of Figure 3. The charging means 88' charges the sleeve 82', and the toning apparatus 90' accomplishes the toning, all of this being done in a series of revolutions before the substrate 80 is brought against the surface of the sleeve 82'. The cleaning apparatus 87' is not used in this preliminary operation but a fixing device is required at 132 after toning.

Once the image has been semipermanently applied to the sleeve 82' the apparatus 130 and 132 are rendered inoperative and the toning device 90' then works in conjunction with the charging device 88' and the cleaning device 87'.

In referring to the images as semipermanent, it is meant that the images can be removed by suitable chemicals which will not affect the photoconductive surface. For example, even fused images of dielectric toner can be removed by acetone or in some instances warm ethyl or methyl alcohol.

The dielectric toned images need not be pigmented. The secondary toned images are required to be pigmented.

Dielectric toner is defined herein as toner which, when fixed, is highly insulating so that charge will not leak off.

Claims

1. A method of electrostatically printing composite color images on an elongate substrate wherein the colored images are produced repeatedly and spaced along the substrate and each is composed of a plurality of impressions of different color which comprises: A. providing a plurality of printing stations, each station being arranged to produce an impression of a different color, B. providing a master sleeve of electrophotographic material at each station and rotating the sleeve continuously while simultaneously moving the substrate through the station the sleeve carrying a semipermanent fixed dielectric toner image thereon capable of being reproduced as the said impression, C. forming a secondary unfixed toner image upon the fixed toner image during said master sleeve rotation, D. transferring the secondary unfixed toner image to the surface of said substrate from the master sleeve as the substrate passes said each station, E. fixing the unfixed toner image after transfer on said substate as the substrate moves, and F. repeating steps C., D., and E. at each staiton with the impressions from each station being applied to said substrate in registration.

2. The method according to claim 1 in which the secondary unfixed toner image is transferred directly from the said master sleeve to said substrate.

3. The method according to claim 1 in which the secondary unfixed toner image is transferred to a transfer medium from said master sleeve and directly from the second sleeve to said substrate.

4. The method according to any one of claims 1, 2 or 3 and the step of measuring density of the fixed toner image of said substrate after fixing and using measurement to control the transfer.

5. The method according to any one of claims 1 to 4 and the step of aiding the transfer to said substrate electrically.

6. The method according to any one of claims 1 to 4 and the step of effecting the transfer to said substrate electrically.

7. The method according to any one of claims 1 to 6 in which the image on the master sleeve is formed prior to use by digitizing a pattern and storing same, modulating a laser beam with the digitized information from said store and writing on said master sleeve, toning said sleeve and fixing the resulting developed image.

8. Apparatus for printing a composite pattern repeatedly upon a moving substrate, said pattern being made up of registered impressions of different images having different colors, said apparatus comprising: A. a plurality of image printing stations, each being substantially similar in construction and adapted to produce a different color impression upon a substrate, B. means for moving an elongate substrate through the apparatus from station to station, C. each station comprising i. a rotating master sleeve, said sleeve having an image thereon in the form of a component of a composite image, a. said sleeve being an electrophotographic member including an outer coating of photoconductive material, b. said image being a fixed dielectric toner image which has been electrostatically formed on said outer coating, ii. means for charging the toning said fixed toner image with a secondary toner to achieve a secondary toned image of said fixed toner image which is developed but not fixed, iii. means for transferring the secondary toned image to a surface of the substrate while the substrate is moving through said station, iv. means for fixing the image to said substrate surface before said substrate moves to the next following station to achieve an imprinted impression of a certain color, .v. and means for removing any excess toner which is not transferred to said substrate surface if any had remained, D. the stations being arranged continuously to repeat the imprinting on the substrate as it passes through the apparatus whereby to achieve a continuous series of the same composite image along the substrate as it leaves the apparatus.

9. The apparatus according to claim 8 in which the sleeve is arranged to rotate into a position juxtaposed relative to said substrate and at a peripheral speed which is the same as the linear speed of the substrate in moving through said station, and said station having means for effecting said transfer directly from the sleeve to the surface of the substrate as it passes through said station.

10. The apparatus according to claim 1 in which the means for transferring the secondary toned image comprise a second rotating sleeve, the second sleeve being arranged to bring the secondary toned image in juxtaposed relation to the moving substrate as it passes through the station and means for effecting the transfer from the second sleeve.

11. The apparatus according to any one of claims 8, 9 or 10 in which the sleeve is in the form of a cylinder.

1 2. The apparatus according to claim 11 in which the cylinder is formed of thin metal and the coating of photoconductive material is adhered directly to said metal.

1 3. The apparatus according to claim 9 in which the means for effecting transfer comprise a pressure roller at said position on the surface opposite the substrate from said sleeve.

14. The apparatus according to claim 10 in which one of the sleeves is a belt.

1 5. The apparatus according to any one of claims 8 to 14 in which the means for removing excess toner comprise cleaning apparatus adjacent the sleeve after the position where the transfer is effected.

1 6. The apparatus according to any one of claims 8 to 1 5 in which the fixing means comprise a heater adjacent the surface of the substrate following the location in the station where the transfer is effected.

17. The apparatus according to any one of claims 8 to 1 6 in which the charying of the fixed toner image is carried out in ambient light

18. The apparatus according to any one of claims 8 to

1 9 in which the means for effecting transfer include electric field producing means for establishing an electric bias between the sleeve and the substrate effective to move the unfixed secondary toner from sleeve to substrate.

1 9. The'apparatus according to claim 18 in which the field producing means comprise an electric power supply, a roller disposed at said sleeve but on the opposite side of said substrate and engaging the same, and said power supply is electrically coupled to said sleeve and roller.

20. The apparatus according to any one of claims 8 to 1 9 in which means are provided to sense the density of said impression produced by a station and generate a signal for controlling at least one of the parameters of transfer of said station to maintain the said density at a predetermined value.