GB2039069A - Printing plate for lithography and method of making same from an electrophotographic material - Google Patents

Abstract

A sheet member for making a printing plate for lithography and the resulting printing plate comprises a substrate of sheet metal having sufficient flexibility to enable attachment to a printing press roller. The plate has an r.f. bias sputtered coating of a photoconductive material which has a high photonic quantum efficiency, is microcrystalline, wholly inorganic, flexible and capable of being imaged by an electrostatic technique which includes charging, exposing and toning. The toned parts and untoned parts are rendered respectively hydrophobic and hydrophilic by suitable processes. The hydrophobic parts are oleophilic and the hydrophilic parts are oleophobic. A method of making the coated sheet metal member is described involving an r.f. bias sputtering process.

Description

SPECIFICATION Printing plate for lithography and method of making same.

This invention generally concens lithographic printing plates where the image to be transferred is carried on a surface which is made up of oleophilic parts which carry ink of a greasy or oily nature and oleophobic parts which repel such ink.

There is a negligible difference in height above the carrier surface between these parts. The plate surface normally is bathed in waster during the - printing process which wets the oleophobic parts of the surface but is repelled by the oleophilic parts. When the oily ink is applied to the plate by an ink roller or the like which engages over the entire surface of the plate, such ink is picked up and adhered to the oleophilic parts which have an affinity for such ink but the ink is repelled by the other parts which carry the water so that no ink is deposited on the latter parts. The water repelling parts also are called hydrophobic and the water accepting parts also are called hydrophilic.

The imaged parts on the printing plate normally will comprise toner that is adhered to the surface of the plate. The background or nonimaged parts will comprise the areas of the surface which do not carry toner. It is feasible to make reverse images, however, in which case the image will comprise the parts which carry no toner and the background will be the areas which carry toner. In every case, however, the toned parts will be rendered oily ink carrying while the untoned parts will be rendered water carrying.

The basic lithographic technique has been known for over a hundred years. It was originally practiced upon special stone surfaces. Presently it may be practiced on high speed printing presses, but using the same basic technique.

In modern lithography the imaging is done photographically upon flexible aluminium sheeting which can withstand considerable rough handling and will wear well in a printing press. The sheeting is made up into rectangular plates punched with holes and/or slots on their ends to enable securement to the printing roller or rollers of a printing press.

There is universal use of these aluminium plates in lithography at the present time notwithstanding their shortcomings which are many, because heretofore there has been no practical alternative. Considerable advantage will ensue should such shortcomings be overcome by providing a wholly new type of sheet metal printing plate for lithography which opens a completely new field in lithography for high quality and economical printing.

Known lithographic plates based upon substrates of aluminium must be imaged photographically.

They are carried through multiple step processes which involve applying an ultra-violet sensitive (diazo) coating to the aluminum surface after a prior treatment, exposing the coating through a negative by means of UV light, fixing the image onto the plate by means of chemical treatment and dissolving parts which were not exposed to the UV light by etchants to remove the coating at these parts down to the aluminum surface. The etched surface will readily lose it hydrophilic nature, hence, if the plate is removed from the press and stored or shipped, it must be protected by treating the plate with gum arabic and various other materials to preserve the differential hydrophobic-hydrophilic character of the plate surface.The unexposed plate must be kept in darkness and the fixing of the image also must be carried out in darkness to prevent chemical changes which will ruin the plate and render it incapable of being used to acquire an image of varying density. Many corrosive and light-sensitive chemicals are required to hand!e the processing which places stringent specifications on the apparatus needed to achieve a lithographic plate.

Some of the most expensive processes to which conventional aluminum lithographic plates must be subjected aside from their chemical treatments are brush graining and anodizing. The brush graining is a roughening of the surface so that the photosensitive coating applied to the plate will adhere. The anodizing is to ensure that the aluminum surface will be hydrophilic after the plate has been made. Anodizing prevents corrosion and oxidation of the aluminum surface.

When the conventional process of making a lithographic plate has been completed, the image is carried on the plate surface chemically and it is barely visible to the naked eye. Accordingly, in order to inspecfthe plate it must be used to make a proof. Thus, the plate has ink applied to it and must be cleaned if it is to be handled again as for example in storing or shipping. It is very expensive and time consuming to proof directly upon the press.

If there are any errors in the plate, the conventional lithographic printing plate must be made all over again because the changes wrought by the imaging process upon the UV sensitive coating are irreversible. It is not practical to attempt to remove the coating from the aluminum surface and start all over again because these aluminum plates are normally sold in light-proof packages with the photosensitive material already coated thereon and it is not common for a printer of high quality graphics to have the equipment needed to apply the photosensitive coating.

One of the greatest of shortcomings of the conventional plate is that it is sensitive to light of a very narrow part of the spectrum. Regardless of the material which it is desired to print, such material must be converted into a projected ultra violet image. The production of color in photolithographic technique is most difficult.

Obtaining the needed color separations and working out the process so that the projected light for each color to be ultra violet is time consuming, expensive and complicated. The range is narrow and exposure requires critical parameters of time and intensity to prevent overexposure or underexposure.

Once the image has been developed by the application of toner, it is fused in place by heat.

Where the lithographic plates are based upon polyester substrates however, the fusing of toner must be done at a low temperature, i.e., less than about 1 500C. in order to prevent softening of the substrate and distortion of the images. It would be of considerable advantage to employ toners of greater tenacity but such require higher fusing temperatures and longer duration of fusing to enable complete and tenacious fusing of the toned image to the photoconductive surface. The more rugged toners requiring much higher fusing temperatures could extend press wear to a degree greater than that of the polyester substrate based plates.

It has been necessary to proof the image and handle ink in the inspection of the plate. Further advantage would ensue if the inspection of such plates could be carried out before fusing of the toner in which case an undesirable image can be wiped off the surface with cotton waste or washed off in a bath of isoparaffinic hydrocarbon solvent; or if the fused toner images could be removed chemically without harm to the photoconductive coating and the plate reused.

Additionally it would be highly desirable if, once a plate has been made with a certain image and the image has been fused in place, modification of the image could be possible, even after some use of the plats.

Another desirable advantage would ensue over a lithographic plate based upon a polyester substrate is that in the printing process, if in order to help transfer the ink from the plate to the receptor, the metallic conductive base could comprise an electrode to achieve electrostatic assist. The plate formed with a polyester substrate, for example, is required to have an ohmic layer sandwiched between the photoconductive coating and the polyester substrate in order to enable its initial charging, besides making it difficult to manufacture because of the extra sputtering step does not provide the benefit of a well contacted electrode.

There are some disadvantages of sputtering upon a polyester substrate. These include the need for outgassing the polyester substrate before use and getting rid of the aromatic plasticizers normally used in making the sheeting; the need for keeping the temperature of the sputtered material at the point of sputtering below the melting temperature of the polyester substrate; and the need for complex driving and reeling equipment with sensitive tension controls in order to prevent undue streching and distortion of the resulting material.

Accordingly, the invention provides a flexible member for use in the manufacture of a lithographic printing plate comprising a flexible metal sheeting substrate and a thin coating carried by and bonded to the surface of said sheeting, said coating formed as a thin, flexible transparent, sputter-deposited layer of wholly inorganic, microcrystalline, photoconductive material having an electrically anisotropic surface, high photonic quantum efficiency and said layer being capable of being charged and of retaining the charge to enable imaging electrostatically and toning.

Further, the invention provides an image on said surface, said image being formed of toner material fused to said surface in the form of printing and nonprinting parts, the printing parts being hydrophobic and the nonprinting parts being hydrophilic.

Additionally, the invention provides a method of making a printing plate for lithographic use comprising the steps of: sputtering a photoconductive material upon a flexible metal sheet to form a coating upon said metal sheet, said coating being transpa rent per se, wholly inorganic, microcrystalline, flexible, capable of accepting a rapid charge and retaining the same sufficient to enable toning, having a substantial difference between light and dark decay, a high photonic quantum efficiency and a surface that is electrically anistropic, the coating being of a thickness between about 2000 Angstroms and somewhat less than 2 microns and the sheeting being a fraction of a millimeter thick, charging the surface of the coated sheet in darkness and exposing same to a light pattern made up of individual dots to achieve a latent image of the pattern, toning the latent image to achieve a developed image utilizing a resinous toner that is inherently hydrophobic fusing the toner to the coated sheet to acquire printing and nonprinting parts forming the developed pattern of individual dots, and treating the surface to render the nonprinting parts hydrophilic whereby to maintain the hydrophobic nature of the printing parts.

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 fragmentary sectional view on a greatly enlarged scale through a printing plate according to the invention; Figure 2 is a diagrammatic view generally illustrating the type of apparatus used to produce the printing plate of the invention; and Figure 3 is a diagram showing the electric plasma conditions under which the coating is deposited onto the substrate of the invention.

The invention is concemed with a printing plate of the type which primarily is intended for use in lithography and with the method of making the same.

The basic concept of the invention is the coating of a sheet of thin metal such as tin plated cold rolled steel or other metal with a coating formed of a photoconductive material, preferably cadmium sulfide. The coating is extremely dense and resists abrasion; it has high photonic quantum efficiency compared to known photoconductors; it is microcrystalline with the crystals highly oriented vertically relatively to the substrate; it is electrically anisotropic with respect to its surface and provides extremely high resolution; the resistivity in darkness and during charge of the surface is of the order of 1 o20 ohms per square and it has a light resistivity which is substantially lower so that it can be discharged to practically zero charge and thereby will provide a wide range of grey scale extending from practically zero density to almost maximum for black; it is wholly inorganic and hence not affected by moisture, temperature, fungus and the like; and it is flexible and transparent per se.

Photoconductive materials other than cadmium sulfide can be employed per the teachings herein but the cadmium sulfide of the preferred printing plate gives panchromaticity making this type of printing plate superior in the printing of color. As a matter of fact, another disadvantage of known printing plates which use diazo type coatings to achieve their images is that they must be exposed with ultraviolet light whereas the plate herein described does not require ultraviolet light for exposure.

This coated sheet of metal forms the basis for the manufacture of the plate by the printer who uses it electrostatically to appiy an image to the plate and then treats it so that it acts like the usual lithographic plate with areas that are hydrophobic and areas that are hydrophilic. Thereafter the plate is capable of being mounted on a printing press roller and used to print upon paper and the like.

The manufacturer of the basic material will produce photoconductive-material-coated sheets of metal. These will have the proper dimensions and be punched or slotted so that they can be attached to conventional printing rollers in a lithography press.

The purchaser of the coated sheets can be either the printer himself or an intermediate processor who makes the actual printing plate from the coated sheet. The printer or processor charges the surface of the sheet, exposes it to a light pattern to achieve a latent image on the surface, tones the surface, fixes the toned image and treats the surface to render the nonprinting parts hydrophilic. Since the plate may be imaged for pigment on white or white on pigment printing, the discernible image may not in each case be that which is printed; hence the parts which are intended to carry the ink are the toned parts mentioned above and will be called the printing parts herein and the parts intended to repel the ink which are nontoned will be called the nonprinting parts.

The intermediate processor, if there is one, will produce a printing plate ready to be used by the printer and carrying on the plate specific information in the form of printing and nonprinting parts which the printer has ordered him to image onto the plate.

The printer installs the finished plate in the printing press and operates the press to print upon some form of receptor, principally paper.

In Figure 1 there is illustrated in cross section a completed printing plate 10 that is based upon a substrate 1 2 of tough sheet metal such as tin plated cold rolled steel. Steel and aluminum in thicknesses of a fraction of a millimeter are hardened by working and are dimensionally strjble.

Such sheets are easily punched or slotted along their edges so that they can be engaged upon pins or other projections that may be carried by a printing roller of a printing press. These punches or slots will retain their integrity even though the plate is subjected to considerable tension, especially in the case of steel. Plates can even be clamped to rollers without using holes or slots, or in the case of steel, held in place magnetically.

It is preferred to use cold rolled sheet steel of about .15 mm thick having electroplated coating 13 of pure tin about .013 mm thick. Such steel plates have several advantages over aluminum which will be enumerated below.

A photoconductive coating 1.4 is bonded to the clean surface of the substrate 12. This is done by sputtering in a manner which presently will be described, and the thickness of the coating is of the order of one micron or less.

It is possible to provide plates with coatings of greater than a micron, but some of the characteristics change with thickness. The voltage achieved during charge and remaining after imaging is proportional to the thickness of the coating and should be chosen on the basis of the type of toner which is to be used. Since the voltages which are involved are substantially lower than those of conventional photoconductive materials such as selenium and zinc oxide, toners which give good results at lower voltages are required to be used, but should be related to the thickness of the coating and the voltages achieved.

Assuming a thickness for the coating 14 between about .3 of a micron and a micron, the plate 10 is quite flexible. The coating 14 is quite coherent and will not crack, craze or chip when engaged around even the very smallest diameter printing roller.

The plate 10 is made by charging the surface of the coating 1 4 by means of corona or the like, this being negative type since the coating is n-type material. Because the substrate is conductive there are no problems of achieving good contact during charging. Further, the charging can be effected through a layer of isoparaffinic hydrocarbon which keeps the surface clean by preventing fog during toning. The surface then is exposed to a pattern of light and there results a latent image of the pattern on the surface in the form of charged and uncharged parts. After charging, the surface can be engaged mechanically in order to transport the plate without losing the latent image. The plate surface then is bathed in liquid toner whose particles are electroscopic, that is, carry positive charges which are capable of discharging the eiectrons which combine therewith. Where there are no electrons, the particles are not attracted. As a result, the toner particles will adhere to the charged parts and will not adhere to the uncharged parts. The substrate 1 2 can be connected to suitable polarity voltages to aid in the toning, if desired.

The toner is organic and usually comprises fine resinous particles that preferably include pigments. Usual pigments are of the type used in so-called electroscopic marking particles comprising different colors. Carbon is a usual ingredient. The carbon is preferably incorporated in resin globules suspended in isoparaffinic hydrocarbon to provide an ink-like material that is easily handled as a liquid. The particle size is normally chosen with relation to the voltages achieved during the imaging and for the preferred coating, which is ultrapure cadmium sulfide, the particle size is preferred to be much smaller than commercial so called toners. It may range from 100 or so microns down to less than 10 microns.

When the toner has been applied, there result islands of toner indicated at 1 6 and areas which are not toned as indicated at 1 8. The surface of the plate 10 now is subjected to heat. For example, the plate can be passed under an infra red lamp or by any other suitable means, to fuse the toner islands 1 6 in place upon the surface.

Prior to this time, the developed image is visible and can be easily seen by the processor. If unsatisfactory the image or parts thereof can be wiped off with cotton waste or washed bff with the solvent that suspended the toner particles.

Such solvent commonly is a paraffinic hydrocarbon such as the product made by the Exxon Company called by the trademark "Isopar".

Once the toner islands 1 6 have been fixed, the untoned areas preferably are treated chemically or physically to ensure that they are hydrophilic. The preferred treatment comprises bathing the surface in a chemical solution which is at least as oxidizing as acid dichromate. The chemical should have an oxidation-reduction potential of 1.33 or higher.

The dilution of chemicals used is not critical. For example, potassium permanganate is an excellent conversion medium and its strength can vary twenty-fold and still be effective. Another conversion medium which has been successful is a mixture of hydrogen peroxide with phosphoric acid. A wide range of conversion chemicals is available, their prime qualification being their oxidizing potential as given above. Any one of these will render the nonprinting parts hydrophilic without affecting the printing parts which are naturally hydrophobic. Thus, the treatment of the nonprinting parts is represented by the areas 20 in Figure 1. The photoconductive surface is normally oleophilic.

Thereafter the plate is ready to use. After use, the parts 1 6 can be dissolved in suitable chemicals and the coated metal sheet used over again. This can be done by treating same with ketones, for example.

One important advantage of the lithographic plate of the invention deserves special mention.

As well-known, a printing plate is incapable of producing a continuous tone printed image because the application and transfer of ink is binary in character. It is either applied and transferred or it is not. In order to achieve the appearance of a grey scale, it is necessary to use images in projecting the light onto the sensitive coating of the plate which are broken up by screens into fine dots. The size, frequency of occurrence and distribution of the dots enables one to simulate by printing a good reproduction of a photograph, the latter being a continuous tone article.

In the case of the conventional lithographic plate, the coating is only UV sensitive which means that the plate has ability to produce chemical response in a very limited part of the spectrum. It's optimum exposure range is narrow, making it easy to overexpose and underexpose.

Furthermore, the images produced cannot be made out of dots which are too fine because in the exposure and the chemical etching baths to which the plate is subjected, the detail will be lost because of loss of the dots which are too small, through undercutting of the etchant and the like.

To prevent smeariness, the dots have to be large.

In addition, the material itself does not have the ability to resolve microscopic dots of the order of a few microns.

In the case of the plate 10, especially where the coating of the plate is cadmium sulfide, the panchromaticity gives the same response over the visible spectrum which enables all intelligence to be reproduced with equal intensity. Exposure is relatively simple, achieving a wide range of response especially to red and blue light.

Furthermore, in view of the fact that the coating 14 is electrically anisotropic, there is no surface migration of charge. Thus, the finest microscopic dots can be used to make images of outstanding quality. The only limitation on the fineness of the dots reproduced is the size of the toner particles and the build-up oftoneronto a given increment.

It has been established that the size of dot in the plates of the invention is not attainable by any known commercial lithographic plate making process. Further the plate 10 can be exposed at camera speed without loss of detail.

As indicated above, the preferred metal for the substrate 12 is cold rolled sheet steel with a plating on the surface thereof which is tin as at 1 3.

This material is extremely economical since it is produced in large quantities by the steel tin plate mills primarily for use in the fabrication of canisters for food and other so-called canned products. The plates 10 may be degreased, if desired. Dimensions are usually well maintained by the mills.

In addition to economy, a steel substrate of the character described, has another important advantage. If the surface is highly reflective as is the case with the tin plating of the cold rolled steel, when the coating 14 is imaged by projecting light directly at the coating. since the coating is transparent per se any light that is not absorbed in the coating and passes through is reflected from the surface of the substrate and enhances the illumination, hence providing an increased exposure. This means that intense light is not needed to get good exposure. An advantage of low intensity exposure is that the coating does not heat up as much as with higher intensity exposure, but in the case of the coating 14, even this is not a problem because the coating does not respond to infra-red as it does to the visible parts of the spectrum.

Some of the other metals and/or their platings will be highly reflective and give the enhanced exposure advantage described above, but others may be less reflective. Platings such as zinc, for example, while effective for the operation of the invention may not have the reflectivity of tin and will not enhance the exposure as much if at all by reflection.

The diagram of Figure 2 is more symbolic than intended to represent illustrations of actual apparatus. For disclosures of the type of apparatus intended, reference can be had to U. S.

Patent Nos. 3,829,373 and 3,884,787. The principal purpose for the diagram of Figure 2 is to explain the method of manufacturing the coated metal sheet.

The vessel 24 is a pressure vessel whose walls are normally of metal and grounded. Inlet ports 26 and 28 provide for the connection of the chamber 30 with vacuum pumps 32 ahd background gas source 34, respectively through suitable valves and regulators as indicated at 36 and 38. A drum 40 whose axle 42 is grounded is mounted for rotation in the chamber 30, either driven or idling, depending upon the mechanical nature of the drive for the substrate but not requiring applied force because of the fact that the tension can be maintained on the substrate without concern of stretching or distortion.

Slight modification of the apparatus normally used and fully capable of sputtering a photoconductive material onto a substrate of polyester film that had been previously coated with an ohmic layer as taught in United States patent 4,025,339 can be made to sputter sheet metal substrate. Such an apparatus as shown in Figure 2, would have the drum 40 provided with an outer metallic skin 44 that is conductive but electrically insulated, d.c. wise, from the remainder of the drum by a suitable insulating ring, for example. The drum skin 44 constitutes the anode in a diode-type eiectrical configuration within the vessel 24, the cathode comprising a series of targets 46 surrounding the outer surface of the drum skin 44 and spaced therefrom.The targets 46 are mechanically supported by the walls of the vessel 24 by suitable means (not shown) which enables them to be adjusted in relation to the drum skin 44 and are physically insulated from the walls of the vessel 24.

The targets 46 and anode 44 are coupied to the output of a radio frequency power supply 48 of 13.56 megahertz so that the plasma which is produced as a discharge will occur in the cylindrical space 50 between the targets 46 and the anode 44. Particles of the target material will be driven to deposit on the anode 44, but because there is an intervening substrate (which will be described) the deposit occurs onto the substrate.

In the process of sputtering, in this machine, onto an insulating substrate the sheeting itself insulates the ohmic layer from the body of the drum 40. The skin 44 then can be at a different voltage than ground effect a bias which is producing the second or Langmiur dark space at the anode and the benefits of the resulting photoconductive coating. In a large sputtering machine, there may be some additional capacitive coupling effect between the ohmic layer and the drum skin and/or the drum which assists in the establishment of this bias. Thus, the bias is capable of being achieved without an actual connection between the skin 44 and the bias circuit of the power supply, but for the purpose of this explanation it will be assumed that such a connection is desirable.

In the case that a sheet metal substrate is to be sputtered in the vessel 24, the engagement of such a substrate with the skin 44 directly provides a metallic direct connection to the skin 44. To achieve the effects desired and to assure the presence of the second dark space, it has been found essential to insulate the substrate from the skin 44. For this purpose a thin layer of insulating material such as polyester or some high temperature synthetic resin such as tetrafluorethylene is placed onto the exterior surface of the skin 44 as shown at 45. This may be simply a few warps of sheet material around the drum 40 which can be removed when other tasks are assigned to the sputtering machine or it can be in the form of a molded cover or coating sprayed or otherwise secured to the skin of the drum 40.Obviously the drum construction could be permanently designed to have an outer insulating layer in place of the skin 44 in which case the insulating layer will be a dielectric layer between the metal body of the drum 40 and the metal substrate.

Coupling of the r.f. power to the interior of the chamber 30 and the targets 46 and anode 44 is effected by suitable apparatus and connections that provide for matching and tuning of the individual parts of the system and the system itself. Most important are the matching of the targets 46 to the power supply 48 because the geometry of the system will affect the various r.f.

paths, parasitic leaks, inductance of leads and connection, etc. As an example to indicate all of this, there is illustrated a series of tuning match boxes 52 each connected on one side to an individual target 46 through a coupling 54 which insulatedly passes through the walls of the vessel 24. The other side of each match box 52 is connected by a common lead or series of leads 56 to the output of the power supply 48. There may be other matching and tuning components in these leads.

The r.f. power supply also provides a bias voltage to the anode 44 by way of a lead 58, the means for establishing and controlling the bias being indicated at 60 by the block marked "bias circuit". There may be a structure used which achieves a self-bias through parasitic couplings.

The lead 58 connects to a wiping contact 62 on the interior of the chamber 30 that engages the skin 44 at one end of the drum and establishes its voltage relative to ground. The r.f. power supply 48 has a ground connection at 64 and the targets 46 as well as the majority of the drum 40 which is freely exposed otherwise are well shielded by suitable shields 66 that are all grounded. The exact configuration of the shield is dependent upon the construction and geometry of the apparatus, that which is shown being only by way of example. It should be understood that connection 48, block 60, contact 62 need not be required where the construction and geometry provide a self-bias to produce the second dark space. This can be ascertained by suitable measurements made on any given apparatus.

At 68 there is shown a supply roll of sheet metal such as cold rolled steel which has been tin plated which has been wound onto a spool 67 which can be replenished from time to time by opening the vessel 24. This supply forms the source for an elongate sheet of steel that can be considered foil, but is somewhat thicker than foil as it is commonly known, this comprising the substrate 12 upon which the cadmium sulfide coating is to be sputtered. The substrate 1 2 passes around a guide or feed roller 70 through a suitable opening 72 in the shielding 66 (if necessary in the particular construction) and on to the insulated covering 48 of the drum skin 44 as it is carried into the space 50 between the targets 46 and the drum 40.It passes around the drum in the direction indicated by the arrows, is sputtered with the material of the five targets shown (any number of targets may be utilized), passes through another opening 74 in the shielding 66 as a coated sheet 1 2-14. It is led around the guide roller 76 onto a take-up spool 78 upon which it accumulates.

When the spool 78 has been filled with all of the coated metallic sheeting 12-14 it can practically hold, the apparatus is shut down and the vessel 24 is opened to enable removal of the roll of coated metallic sheeting 1 2-14, and the replacement of the supply roll 68 on the spool 67.

Consideration should be given to the need to maintain the substrate 12-14 and all mechanisms with which it comes in contact insulated from ground. This is to prevent modifying the capacitive coupling of the substrate and skin 44.

Thereafter the elongate strip of coated substrate 12-14 can be formed into individual sheets for further processing and may be punched, slotted, etc., so that the processor can finish making plates like 10.

In the course of the sputtering action, a heavy inert gas such as argon is admitted into the chamber 30 after the chamber has been pumped down to provide ionization and heaving bombarding ions. At the same time hydrogen sulfide and perhaps a small quantity of oxygen are admitted to achieve the stoichiometry of deposit which is essential for the coating 14 and to provide the surface barrier layer that is believed responsible for the extremely high surface resistivity and electrical anisotropy.

In Figure 3 the target is shown spaced from the anode surface comprising the insulating layer 45 and the skin 44 by the intervening space 50 in which the plasma is generated as a result ofthe ionization and bombardment which occurs. The majority of the plasma is a belt 80 through which the molecules of cadmium sulfide, for example, are driven. These have been bombarded or splashed out of the target 46 which is preferably made from ultrapure cadmium sulfide. A Crookes darkspace forms in the vicinity of the target 46, this being a cathodic phenomenon which is wellknown, as shown at 82. This dark space will normally prevent redeposit of the particles from the face of the target and by arranging shields such as 66 around the sides and backs of the targets 46 which the Crookes dark space will follow such redeposit is prevented.It is of interest to note that the background gas is preferably admitted directly in the vicinity of the targets, and the construction of shielding which substantially encloses the backs and sides of a target provides a small chamber with selective exit ports into which the background gas (for instance, argon, hydrogen sulfide and oxygen) can be admitted and directed to flow immediately over the face of the target while sputtering.

By keeping the anode 44 at a small voltage removed from ground there is produced what has been termed a bias voltage on the anode. It will be recalled that considering the diode action, the cathode is negative and the anode positive. If the anode is normally at effective ground in a configuration of this kind, the estabiishment of a small effective negative voltage for the anode places it slightly negative with respect to ground.

The cathode then is substantially negative relative to ground. This gives rise to the production of a second dark space 84 adjacent the anode 44 and hence adjacent the surface of the substrate 12.

While known, this dark space is not generally as familier to those skilled in this art as the Crookes dark space and is sometimes called the Langmuir dark space or probe. It has been found that the presence of this second dark space is essential to the deposit of a coating 14 which is chargeable and imageable. Dimensions in Figure 3 are not to scale, the diagram being intended only for explanation.

Achievement of the second dark space is feasible in several ways, principally in small spluttering machines by the actual connection of a separate conductor to the inner skin of the drum under the insulating layer 45 from the r.f. power supply, using a voltage divider with a potentiometer to control the so-called bias voltage difference. The preferred bias has been found to be from about negative ten volts to as much as negative 100 volts where the cathode is at about 2000 volts r.f. and the shielding is grounded. In large sputtering machines, parasitic capacitive coupling, the inductance of leads and general geometry of the mechanical parts of the apparatus may generate a self-bias of as much as 30 or 40 volts negative and a second dark space without the need for any direct connections.It should be understood that the frequency of the r.f. power at which the sputtering occurs is 13.56 megahertz such that the effects of parasitic coupling and the inductance of simple wire or other conductive connections can be quite substantial.

As stated above, preferably the lithographic plates of the invention is made out of cold rolled steel with a thin plate of tin because this type of substrate is economical and strong. Punched slots or holes retain their integrity longer and more readily than aluminum or sheet plastic or paper.

No brush graining or anodizing is required either of steel or of the aluminum that can be used also to make the plates of the invention. When sputtered onto the tin plating of cold rolled steel, the photoconductive coating is very tenaciously adhered. Steel can be held and handled magnetically. The steel lithographic plate as well as any plate made out of other metal according to the invention is dimensionally stable and enables very accurate reproduction and printing.

Other metals could be used if capable of being manufactured in thin flexible sheets with economy. Such metals are titanium, nickel, nickelchromium, stainless steel and the like. Tin or zinc plating increases the bonding strength. Tests have shown that the bonding strength of metals can be increased with platings of metals such as gold and rhodium which will accept good sputtered coatings, but in commercial printing plates might be expensive.

Cadmium sulfide is preferred as coating 14 because of its panchromaticity and other characteristics mentioned above.

In the manufacture of the plates of the prior art, the methods of coating the aluminum sheeting give rise to pinholes, dust specks and the like which deteriorate the quality of the resulting plate.

The coating 14 is sputtered in a chamber which is dust free and by a process which lays down a uniform smooth coating that is totally devoid of any discontinuities. In addition to the fact that the photoconductive coating is so uniform and perfect, it is extremely smooth and this gives rise to another unobvious advantage. When toner is fused to the smooth surface of the cadmium sulfide, for example, the resin is melted evenly because the underlying surface has none of the roughness and interstices which are inherent in the processed lithographic plate as conventionally known. Even other known electrophotographic coatings of the prior art, such as amorphous selenium and zinc oxide in a matrix, have rough surfaces compared with that of the photoconductive coating 14.

On account of the above, the dots which are produced on the surface of the plate of the invention are microscopically sharp and not fuzzy, and although not readily visible to the naked eye, this characteristic overall produces a sharper, crisper and more pleasing printed image.

Exposure of the plate 10 need not be effected at one time by projecting a pattern onto the charged surface 20. It could be applied by a fine beam of radiant energy such as a laser, suitably modulated from a store of information to lay down a pattern of dots which compose the desired image. Toning can be done when the dot pattern is completed or as it is being laid down.

As used herein, the expression "ultrapure" as applied to the description of the photoconductive coating material is intended to signify a material which is sputtered with extremely good stoichiometry. The targets 46 would be made out of the material with a purity that runs as high as 99.99999%. Every effort is made to sputter the cadmium sulfide, for example, in a manner to achieve the extremely good laid down stoichiometry. It is expected that the photoconductor material may be doped for certain.

purposes, sometimes by incorporating a dopant such as copper into the targets or by introducing carbon by way of a gas such as methane into the chamber and sometimes by applying it to the targets or one or more of them as a paint or spray.

Principally the purpose for doping is to change the chromatic response. In any case, the expression "ultrapure" is intended to describe the stoichiometrica lly correct photoconductive material whether it is doped or not.

Claims (24)

1. A flexible member for use in the manufacture of a lithographic printing plate comprising a flexible metal sheeting substrate and a thin coating carried by and bonded to the surface of said sheeting, said coating formed as a thin, flexible transparent, sputter-deposited layer of wholly inorganic, microcrystalline, photoconductive material having an electrically anisotropic surface, high photonic quantum efficiency and said layer being capable of being charged and of retaining same to a degree to enable imaging electrostatically and toning.

2. The member according to claim 1 in which the thickness of the coating is of from a few thousand angstroms to the order of one micron.

3. The member according to claims 1 or 2 in which the surface of said substrate is highly reflective.

4. The member according to any one of claims 1,2 or 3 in which said sheeting substrate is cold rolled steel.

5. The member according to claim 4 in which said sheeting substrate has a tin plating on the coating surface to aid in bonding.

6. The member according to any one of claims 1,2 or 3 in which the sheeting substrate member is aluminum.

7. The member according to any one of claims 1,2 or 3 in which the sheeting substrate is stainless steel.

8. The member according to any one of claims 1 to 6 in which said photoconductive material is ultrapure cadmium sulfide.

9. The member according to any one of claims 1 to 7 in which an image is formed on said surface, said image being formed of toner material fused to said surface in the form of printing and nonprinting parts, the printing parts being hydrophobic and the nonprinting parts being hydrophilic.

10. The member according to claim 9 in which the toner material is pigmented with a pigment of contrasting appearance relative to the surface of the sheeting substrate member such as to cause a discernible difference in appearance to an observer between the printing and nonprinting parts.

11. The member according to claims 9 or 10 in which the toner material is resinous in character.

12. The members according to any one of claims 1 to 10 in which the thickness of the photoconductive coating is from a few thousand angstroms to the order of one micron.

13. The member according to any one of claims 1 to 11 in which the sheeting substrate is a fraction of a millimeter in thickness.

14. A method of making a printing plate for lithographic use comprising the steps of: sputtering a photoconductive material upon a flexible metal sheet to form a coating upon said metal sheet, said coating being transparent per se, wholly inorganic, microcrystalline, flexible, capable of accepting a rapid charge and retaining the same sufficient to enable toning, having a substantial difference between light and dark decay, a high photonic quantum efficiency and a surface that is electrically anisotropic, the coating being of a thickness between about 2000 Angstroms and somewhat less than 2 microns and the sheeting being a fraction of a millimeter thick, charging the surface of the coated sheet in darkness and exposing same to a light pattern made up of individual dots to achieve a latent image of the pattern. toning the latent image to achieve a developed image utilizing a resinous toner that is inherently hydrophobic, fusing the toner to the coated sheet to acquire printing and nonprinting parts forming the developed pattern of individual dots, and treating the surface to render the nonprinting parts hydrophilic whereby to maintain the hydrophobic nature of the printing parts.

1 5. The method according to claim 14 in which the treatment comprises bathing in a suitable' chemical solution.

16. The method according to claims 14 or 1 5 in which the photoconductive material is ultrapure cadmium sulfide.

17. The method according to any one of claims 14 to 16 in which the metallic sheet is cold rolled steel.

1 8. The method according to any one of claims 14 to 1 6 in which the metallic sheet is tin plated cold rolled steel.

1 9. The method according to any one of claims 11 to 1 5 in which the metallic sheet is stainless steel.

20. The method as claimed in any one of claims 12 to 1 6 in which the metallic sheet is aluminum.

21. The method according to any one of claims 14 to 1 9 in which the treatment is effected by subjecting the imaged surface of the plate to a bath of an oxidizing conversion chemical.

22. The method according to claim 1 5 in which said bath has no substantial effect upon the photoconductivity of the photoconductive coating.

23. A flexible member substantially as herein described with reference to and as illustrated in the accompanying drawings.

24. A method of making a printing plate substantially as herein described with reference to and as illustrated in the accompanying drawings.