US3443517A

US3443517A - Electrostatic duplicating system employing relief printing plate

Info

Publication number: US3443517A
Application number: US607175A
Authority: US
Inventors: Robert W Gundlach
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1967-01-04
Filing date: 1967-01-04
Publication date: 1969-05-13
Anticipated expiration: 1986-05-13
Also published as: GB1212370A; DE1671577A1

Description

May 13, 1969 R. w. GUNDLACH ,5

ELECTROSTATIC DUPLICATING SYSTEM EMPLOYING v RELIEF PRINTING PLATE Filed Jan. 4. 1967 i l JI INVENTOR. ERT W. GUND ACH A T TORNEYS United States Patent York Filed Jan. 4, 1967, Ser. No. 607,175 Int. Cl. B41f 5/04, 13/02, 5/00 US. Cl. 1012l9 18 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process of relief printing which substitutes the use of dry powders for the conventionally used printing inks. As a result of established electrostatic fields of force toner particles are utilized as the printing medium.

This invention relates to an imaging system and, more specifically, to a relief printing system.

Relief or typographical printing is a contact form of printing whereby the image areas of the printing plate are substantially raised above the flat reference plane of the plate. An ink roller coated with a conventional printing ink is passed across the surface of the relief printing plate. As a result of the raised relief nature of the image characters the printing ink contacts only the raised surface depositing ink in the image areas. The ink image is then generally transferred directly to a copy sheet through pressure. This form. of printing is usually referred to as letterpress printing.

Although the most classical form and widely used method of printing, letterpress printing is not without its disadvantages. When printing from a relief imagearea there is generally required very high pressures, a need for a uniform application of ink to the printing surfaces and the necessity in most cases of a very high relief of the characters over the background areas, generally prepared by a chemical etch technique, or mechanical engraving, in order to produce the desired results. In practice, the high pressures employed give rise to what is referred to in the art as ink splash at the character edges. Furthermore, if the printing areas are very small the pressure Will be extremely high. If the printing areas are large, then the pressure is quite small. This pressure differential requires a high degree of makeready. A further disadvantage of letterpress printing is that the inks generally used are yisco elastic and exert considerable pressure on the copy paper when the paper is pulled away from the printing plate. This places severe requirements on the surface strength of the copy paper. In addition, with fluid inks the compatibility ofthe materials utilized in the way of absorbency is a controlling factor as to the amount of ink which is transferable. To alleviate some of thesedifiiculties a form of printing otherwise known as letterset printing was introduced which uses an intermediate rubber covered cylinder for transferring the image from the plate to the final copy sheet. However, this process is also not without its disadvantages, requiring the use of the intermediate surface, thereby increasing the number of image transfers that must be made which inherently leads to a loss in image quality.

' A printing process has been suggested whereby liquid ink is applied to the printing surface utilizing electrostatic lines of force and the ink thereafter transferred from the printing surface to the print receiving material. However, due to the nature of the materials used, this technique has been found to be extremely limited. The printing inks are charged by induction and once out of the field of force the ink droplets will no longer retain a charge. Furice thermore, the surface tension forces of the ink are generally greater than any suitable or operable electrostatic force. As a result it is generally felt that unless contact is made, transfer of the ink is not possible. In addition, special steps must be taken in order to control the selective development of the printing surface.

It is, therefore, an object of this invention to provide a relief imaging system which will overcome the above' noted disadvantages.

A further object of this invention is to provide a novel method for imaging from a relief printing plate. I

An additional object of this invention is toprovide a novel method for developing a relief printing plate.

Another object of this invention is to provide a substantially pressureless imaging system utilizing a relief printing plate.

Still a further object of this invention is to provide a substantially dry printing process.

Yet, still a further object of this invention is to'provide a novel relief printing system utilizing the desirable properties of low relief while eliminating the need for added process steps.

Yet, an additional object of this invention is to provide an apparatus for executing a novel relief imaging system.

The foregoing objects and others are accomplished in accordance with the present invention, generally speaking, by bringing the printing surface of a relief printing plate into close proximity with the surface of a donor electrode or other developer means which has dusted on its surface a layer of electroscopic toner particles, and establishing an electric field such that the toner particles are deposited on the relief area of the printing plate. The fields generated are inversely proportional to the surface spacing, thereby giving transfer of the electroscopic particles only to the relief character areas. The relief plate With the toner particles deposited on the relief printing areas is then contacted with the surface of an electroded copy sheet or web such that the field of forces is reversed and the toner particles are now transferred from the relief characters to the copy sheet in an imagewise pattern. The steps are then repeated in sequence until the desired number of copies of the original are produced.

In a second embodiment of the present invention, an induction transfer system may be used similar to the process disclosed in US. Patent 3,004,860. A reliefimage representing the printing surface of the letterpress plate is coated with a thin layer of an insulating material having a resistivity of the order of about 10 ohm-cm. ,or greater, such as polyethylene terephthalate. The toner powder may be transferred from the surface of the donor electrode, brush or other carrier of charged particles to the relief areas of the letterpress plate by applying the proper biasing potentials as more fully described hereafter. The then toned surface of the letterpress plate is charged such as by a corona discharge device and brought into contact with a grounded transfer sheet which is substantially a conductive material, and image transfer affected.

In still another alternate embodiment of the present invention, the printing surface may be coated .with a low conductivity material such as having a resistivity of the order of about 10 to 10 ohm-cm. with optimum results achieved at about 10 ohm-cm. The presence of this semi-conductive coating delays the build-up of the fields of force between the printing surface and the copy sheet thereby preventing what is ordinarily referred to as developer explosion at the transfer interface. i

The invention is illustrated in the accompanying drawing which shows a diagrammatic sectional view of an imaging apparatus for carrying out the process of the present invention.

Referring now to the drawing there is seen a relief printing master generally designated 1, represented for purposes of the present discussion as a letterpress plate cylinder consisting of raised printing surfaces 2 and recessed non-printing areas 3. In close proximity to the relief printing plate 1 with the axis parallel thereto is a rotary donor electrode 11 made up of a semiconductive layer 12 coated on a metallic drum surface 13. Located in a juxtaposed position to the donor electrode 11 is a development electrode 14 which is preferably curved as shown to conform to the curvature of the donor cylinder. The development electrode may consist of a solid sheet, a screen, a series of wires or a series of points suspended or located over the area or near the surface of the donor electrode and is connected to power source 16. Situated in an elevated position above "both the development electrode 14 and donor cylinder 11 is a discharge receptacle 15 from which cascade developer carrying electroscopic toner particles is discharged onto the surface of the donor cylinder 11 and which gravitates downward between the surface of the donor drum and the development electrode 14 so as to substantially cover the entire surface of the donor electrode. The surplus particles which are not retained on the surface of the donor electrode fall into receptacle 18 from which they may be recharged to the discharge receptacle 15. The developer material 17 preferably consists of a combination of finely divided pigmented electroscopic particles with a coarser carrier material, the carrier being triboelectrically charged with a polarity opposite to that of the electroscopic marking particles upon frictional engagement therewith. The toner layer charge may be enhanced by an optional corona discharge unit Which imparts a charge similar in polarity to that imparted to the toner particles triboelectrically by the toner-carrier relationship. The development electrode 14 which is included in the system to assist in total development of the donor surface is maintained at a potential of from about 400 to about 1000 volts with a polarity similar to the triboelectrically charged toner particles.

The letterpress or relief printing drum 1 is brought into low pressure contact with the donor electrode 11. The toner particles tend to be attracted away from the surface of the donor electrode to the relief areas of the letterpress plate in response to the electrical field established by ,power source 21 as a result of the positioning of the relief printing plate in respect to the surface of the donor electrode. The toner particles 17a now adhering to the raised areas 2 of the relief plate 1 are carried so as to contact copy web 31 which is supplied from feed roller 51, passed over guide roller 32 and brought into intimate contact with the surface of the letterpress plate 1 by conductive transfer roller 33 which is connected to power source 41. The toner particles are transferred in imagewise pattern to the surface of copy web 31 in response to the electrical field established between the conductive transfer roller 33 and the surface of the relief printing plate 1. The potential applied by power unit 41 to the transfer roller with respect to the relief printing electrode 1 is opposite in polarity to the charge of the toner particles 17a and of sufficient magnitude so as to overcome the attractive forces of the relief printing plate and to effect transfer of the toner particles to the copy web. After passing over transfer roller 33 the transferred toner image is guided by a second guide roller 34 to a fixing station represented in the present illustration as a heat fixing unit 45. The copy web carrying the fixed image is then directed by roller 47 to be rewound on take up roller 52.

The surface of the relief plate continues around and contacts cleaning brush 61 which removes any residual toner material remaining on the plate surface and prepares the master or plate for recycling. Alternatively, toner not transferred could simply be recharged to the desired polarity and recycled. A driving means 62 is provided to rotate-the parallel members of the system. Although the voltage power source supplied to the relief printing plate is not necessarily critical it is preferred that it be maintained in a range of from about 200 to about 1500 volts with respect to the donor drum with the specific effective valuebeing determined by the gap which exists between the chargedtoner particles and the relief printing surfaces of the letterpress plate. For purposes of the present invention the surfaces ofthe relief printing plate and the donor electrode are spaced in such a manner such that it varies from actual contact of the surfaces to a distance of approximately 3 mils. For optimum results to be obtained it is desirable to operate at an applied potential difference of about .1000 volts with a transfer space ranging from virtual contact to about 1 mil being the most desirable spacing.

The letterpress or relief I plateused in conjunction with the present invention may be cylindrical in form, as illustrated in the accompanying drawing, or it may also take the form of a-flat bed cylinder press orplaten, with a relief image ranging in height between the printing and non-printing surfaces of from about 1 to about 30 mils.

Although thedevelopment of the donor electrode of the present invention has" been described in conjunction with cascade development, a process more fully described in US. Patents 2,618,551 and 2,618,552, other suitable development techniques-may be used such as powder cloud development, a process more fully described in US. Patents 2,725,305 and 2,918,910, and magnetic brush development more fully described in US. Patents 2,791,949 and 3,015,305. Furthermore, any suitable toner or developer material may be used such as disclosed in US. Patents 2,788,288, 3,079,342 and Reissue Patent 25,136. The toner is generally a resinous material which, when fixed, has the necessary properties to produce a lasting print. Typical developer powders are styrene polymers, includingsubstituted styrenes such as the Piccolastic resins commercially available from the Pennsylvania Industrial Chemical Corporation, phenol formaldehyde resins as well as other resins having similar properties. A developer particle is so chosen such that it is attracted electrostatically by fields at the surface of the donor electrode and held thereon by electrostatic attraction. I

Although the surface of the donor electrode may consist of a conductive material such as aluminum, brass,

steel, copper, nickel, zinc and combinations thereof, it is preferred in order to insure the retention of the electrostatic field and to support the developer particles on the surface of the donor cylinder or electrode, that the donor electrode consists of a conductive material overcoated with a resinous material exhibiting low conductivity properties. Typical semiconductive materials having a low electrical conductivity include polyvinylacetates, polyvinylalcohol, cellulose acetate, ethyl cellulose and mixtures thereof. Other suitable materials having resistivities of about 10' ohm-cm. or greater may also be used. Typical materials include cellophane, polystyrene, polytetrofluoroethylene, polyethylene terephthalate, polyethylene coated papers, polyvinylidene chloride, polyvinylfluorides and mixtures thereof. A further advantage in the use of these materials is to reduce the possibility of shorting in the system.

Any suitable material may be used as the copy web for the developed'image which is transferred from the relief plate. The transfer material may be insulating in nature or it may be partially or completely conductive. Typical materials are polyethylene; polypropylene; polyethylene terephthalate; ordinary bond paper; ethyl cellulose; cellulose acetate; and Tedlar, a polyvinylfiuoride. When the relief areas of the letterpress plate are coated with a thin layer of dielectric material, as mentioned above, then the developed or toned image may be transferred, if desirable, to a conductive substrate such as copper, aluminum, zinc and brass, utilizing the process disclosed in US. Patent 3,004,860.

When utilizing the induction transfer process which entails coating the relief printing plate with a thin dielectric coating having a thickness ranging from about 0.1 to about mils, preferably about 0.8 to about 1.5 mils, any suitable insulating material having a resistivity of the order of about 10 ohm-cm. may be used such as Krylon, an acrylic ester resin commercially available from the Krylon, Inc.; Viken, a vinyl plastic commercially available from the Chemical Rubber Co.; Mylar, a polyethylene terephthalate resin commercially available from E. I. du Pont de Nemours & Co., Inc.; polyethylene; polypropylene; and other similar thermoplastic materials as well as combinations thereof. In addition, any suitable thermosetting resin may also be used such as phenolic resins as for example phenol formaldehyde, urea formaldehyde resins, melamine resins, alkyd resins and the various epoxy resins. The preferred thickness of coating will be determined by the fields generated.

When coating the printing surface with a thin coating of poorly conductive or semiconductive material having a resistivity of the order of about 10 to 10 ohm-cm. any suitable composition may be used. Typical semiconductive coatings are polyvinylacetates; polyvinylalcohols; cellulose acetate; ethyl cellulose; and various dielectric materials doped with conductive additives such as Staybelite, an amine resin, treated with nigrosine; and VYNS, a vinyl chloride-acetate resin, treated with rhodamine B as well as combinations thereof. The thickness of this coating will be governed by the fields of force generated having a range generally similar to that of the dielectric coating applied in the induction transfer system. Optimum results are obtained with materials having a bulk resistivity of about 10 ohm-cm. As a result of the presence of the semiconductive overcoating it is hypothesized that the build-up of the strong fields and charge density at the sharp corners of the characters which generally results is appreciably reduced in time to a period longer than that required for the image transfer to take place, thereby preventing what is ordinarily referred to as developer explosion when the copy web is contacted with the image characters.

Although the fixing mechanism has been represented for purposes of illustration by a heat fixing unit, any suitable technique may be used such as vapor fusing, treatment of the developed image With a regulated amount of heat, as shown, applying or spraying the toner image with an adhesive film to form an overcoating or by placing a lamination of plastic material over the top surface of the transfer image.

To further define the specifics of the present invention, the following examples are intended to illustrate and not limit the particulars of the present system. Parts and percentages are by weight unless otherwise indicated.

Example I A sheet of Baryta paper, a BaSO coated paper, is dusted by an electroded cascade technique with a uniform layer of toner particles comprising polystyrene to a thickness of about 30 microns with the paper-toner relationship being such that the toner particles retain a triboelectric negative charge. The charge on the surface of the toner particles is enhanced by subsequently exposing the toner particles to a corona generated spray of negative ions. A section of letterpress type biased to a potential of about +1000 volts with respect to the donor substrate is brought into close proximity, i.e., about 1 mil separation, to the donor sheet which is grounded from behind. Following separation of the type from the donor, a negative image is left on the donor paper support while the toner particles are transferred in a positive pattern to the relief type of the letterpress plate. The dusted typeface is then brought into contact with a receiver paper sheet and a negative potential of about 1000 volts with respect to the grounded paper support applied to the type. The toner particles are transferred from the type to the surface of the receiver paper sheet in an imagewise pattern corresponding to the relief image of the letterpress plate.

Example II A section of relief type is developed according to the process of Example I excepting the relief type is precoated with a layer of polyethylene terephthalate to a thickness of about 30 microns prior to the contacting of the relief plate with the charged toner particles on the donor sheet. Following transfer of the toner particles to the faces of the relief characters the toned type face is charged to a potential of about -600 volts by a corona discharge device and the particles subsequently transferred to an aluminum copy sheet by contacting the latter support to the grounded printing member bearing the charged toner particles. The procedure is more fully described in US. Patent 3,004,860. The toner particles are transferred to the aluminum support in an imagewise pattern.

Example III A section of relief type is developed according to the process of Example I excepting the relief type is precoated with a composition comprising grams toluene, 30 grams Staybelite and .15 gram nigrosine to a thickness of about 30 microns prior to development. The remaining steps of the process are the same. Results obtained are similar to those of Example I with a somewhat higher quality image noted.

Although the present examples were specific in terms of conditions and materials used, any of the above typical materials may be substituted when suitable in the above examples with similar results. In addition to the steps used in the imaging system of the present invention, other steps or modifications may be used, if desirable. For example, the toned relief image of the relief plate may be initially contacted with a transfer web in a manner so as to effect complete contact with the web prior to transfer of the toner particles. In addition, other materials may be incorporated in the donor electrode, developer particles, the tive relief printing plate comprising the steps of: hance, synergize or otherwise desirably affect the properties of the present system. For example, additives may be incorporated within the insulating layers used in the present invention so as to increase the conductivity, when desirable, of the respective material-s.

Anyone skilled in the art will have other modifications occur to him based on the teachings of the present invention. These modifications are intended to be encompassed within the scope of this invention.

What is claimed is:

1. A method of electrostatic printing from a conductive relief printing plate comprising the steps of:

(a) providing a conductive relief printing plate having raised i-mage printing areas and recessed nonprinting areas which are each formed of the same material,

(b) developing only the raised image printing areas of said relief printing plate with dry developer particles, and

(c) contacting said developed printing plate with the surface of a copy sheet such that the developer particles are transferred to said copy sheet in an imagewise pattern.

2. The process as disclosed in claim 1 further including the step of precoating the entire surface of said relief printing plate, prior to development, with a film of dielectric insulating material.

3. The process as described in claim 2 wherein said dielectric material has a bulk resistivity ranging from about 10 to about 10 ohm-cm. and said copy sheet is insulating in nature.

4. The process as disclosed in claim 2 further including the step of applying a charge to the developer particles retained on the raised printing areas prior to the transfer of said developer particles to the copy sheet, said film of dielectric material having a resistivity of at least about 10 ohms-cm. and said copy sheet being a conductive material.

5. A method of electrostatically producing multiple copies from a relief printing plate comprising the steps of:

(at) bringing the printing surface of a conductive relief printing plate comprising raised image printing areas and recessed nonprinting areas which are each formed of the same material into close proximity with the surface of a donor electrode, said electrode having dusted on its surface a layer of electroscopic toner particles,

(b) establishing an electric field across said relief plate and said donor electrode such that said toner particles are transferred from said donor electrode exclusively to the raised printing surfaces of said relief printing plate,

(c) contacting said toned relief printing plate with the surface of a copy sheet thereby transferring the toner particles from the raised printing surface of said relief printing plate to said copy sheet in an imagewise pattern; and

(d) repeating steps (a) through (c) until the desired copies are produced.

6. The process as described in claim 5, further including the step of precoating the entire surface of said printing plate, prior to development, with a film of dielectric insulating material.

7. The process as described in claim 6, wherein said dielectric material has a bulk resistivity ranging from about 10 to about 10 ohm-cm. and said copy sheet is insulating in nature.

8. The process of claim 5 wherein said relief printing plate and donor electrode are spaced in virtual contact with one another such that the spacing ranges from about to about 3 mils.

9. The process of claim 5 wherein said donor electrode comprises a conductive substrate having superimposed thereon a thin semiconductive layer.

10. A relief electrostatic printing apparatus comprising in combination a rotatably mounted conductive letterpress printing plate comprising raised image printing areas and recessed nonprinting areas which are each formed of the same material, a donor means for presenting to said plate charged developer powder, said donor means rotatably mounted in close proximity and parallel to the axis of said letterpress plate, means for establishing an electric potential between the surface of said letterpress plate and said donor means such that the charged developer powder is transferred from said donor means only to the raised printing surface of said letterpress plate, means to rotate said letterpress plate and said donor means and means to transfer the developer powder from said raised printing surfaces of said letterpress plate to the surface of a copy web.

11. The apparatus according to claim wherein said letterpress plate and donor means are situated such that at the position of transfer of the developer powder the surfaces of said parallel members are spaced less than 3 mils from each other.

12. The apparatus as described in claim 10 further including a development electrode situated in a juxtaposed position to said donor means.

13. The apparatus as described in claim 12 wherein said donor means comprises a conductive base having superimposed thereon a thin semiconductive layer.

14. The apparatus as described in claim 10 wherein said conductive printing plate hascoated on the entire surface thereof a film of dielectric insulating material.

15. A method of developing a conductive relief printing plate for use in electrostatic printing comprising the steps of:

(a) providing a printing plate having raised image areas and recessed non-image areas each formed of the same electrically conductive materials;

(b) bringing the printing surface of said conductive relief printing plate into close proximity with the surface of a donor electrode said electrode having dusted on its surface a layer of electroscopic toner particles;

(c) establishing an electric field across said relief plate and said donor electrode such that said toner particles are transferred from said donor electrode exclusively to the raised printing surfaces of said relief printing plate.

16. The process as disclosed in claim 14 further including the step of precoating the entire surface of said relief printing plate, prior to the development step, with a film of dielectric insulating material having a bulk resistivity ranging from about 10 to about 10 ohm-cm.

17. The process as disclosed in claim 14 further including the step of precoating the entire surface of said relief printing plate, prior to the development step, with a film of dielectric insulating material having a bulk resistivity of at least about 10 ohm-cm.

18. A method of producing multiple copies from a relief printing plate comprising:

(a) bringing the printing surface of a relief printing plate having coated on the entire surface thereof a dielectric material having a resistivity of at least about 10 ohms-cm. into close proximity with the surface of a donor electrode, said electrode having dusted on its surface a layer of electroscopic toner particles,

(b) establishing an electric field across said relief plate and said donor electrode such that toner particles are transferred from said donor electrode exclusively to the raised printing surfaces of said relief printing plate,

(c) applying a charge to the developer particles retained on the raised printing surfaces of said relief printing plate,

((1) contacting said toned relief printing plate with the surface of a conductive grounded transfer sheet thereby transferring the toner particles from the raised printing surfaces of said relief printing plate to said transfer sheet in an imagewise pattern, and repeating steps (a) through (d) until the desired copies are produced.

References Cited UNITED STATES PATENTS 2,573,881 11/1951 Walkup et a1. 3,145,655 8/1954 Hope et al. 3,152,012 10/1964 Schaffert. 3,160,091 12/1964 SchwertZ. 3,244,546 4/ 1966 Cranch. 3,299,809 1/1967 Javorik et al. 3,306,198 2/1967 Rarey.

EDGAR S. BURR, Primary Examiner.

US. Cl. X.R.