US1976039A - Planographic printing - Google Patents

Description

Patented @ot. Q, Ed

PLANQGRAPHHC PREN'EKNG George S. Rowell, Cleveland, Qhio, assignor to Multigraph Company, Wilmington, DeL, a corpcration of Delaware No Drawing. Application September 2, 1932,

Serial No. 631,589

7 Claims.

This invention relates to an improvement in planographic printing, and more particularly to a process of performing such printing, wherein the integrity of definition of a greasy printing image on a metal plate may be preserved throughout long runs, whereby may be obtained large editions of substantially identical prints for which it is essential that the integrity of definition be preserved, that is, that on the one hand the image be prevented from spreading over the nonprinting area, and on the other hand that the means for dampening the non-printing area be prevented from encroaching upon the image area. It is well known that planographic printing depends upon the immiscibility of oil and water. It is also known to those skilled in the art that the relative wettability of metal surfaces by oils and water may be varied by modifying the surface tension of the water or character of the metal surface or both. Many reagents have heretoiore been proposed and used for the purpose of increasing the water wettability of the metal surface of the non-printing areas of planographic printing plates. The water wettability achieved by such reagents, however, has proven to be somewhat fugitive, and prior to this invention it has been thought that of all the reagents which might be used forthis purpose gum arabic is most satis factory, since it is most firmly adsorbed on and retained by a clean metal surface despite the continued application of water thereto during the printing operation, and despite also the attrition incident to the usual rolling operations of inking and dampening. A surface so treated with gum arabic as to be hydrophilic, or readily wet by water, is less sensitive to grease and when clamp is actively repellent thereto.

Bulletin No. 3 of the Lithograplnc Technical Foundation states on page '2 that gun: arable is the most powerful desensitizing agent known and proposes on page 10 to increase the effective life of said gum arabic on the plate by hardening it with a suitable bichromate, thus providing a thicker layer than can otherwise be retained. It D will be understood that desensitizing and sonsitizing are terms commonly used in this art to low grease receptivity and its converse. Even a gum arabic surface, however, the maintenance of the correct balance between the he iy of in to the image and the supply or" water '50 ee gran arabic layer on the non-image area insult and involves the personal equation.

been proposed heretofore to treat the areas of metal plates pl with solutions containing ((31. MIL-149) and glycerin, and to print with inks containing similar solutions emulsified therein. Such inks, however, have been found to be objectionable for many reasons, among which are: that the emulsions formed therein are unstable and tend so to break that the lycerin is free to wet the inkdistributing rolls and thus prevent uniform distribution of the ink; and that the delivery to the plate of the glycerin and salt solution contained in the ink is uncertain, depending as it does upon the stability of the emulsion. In this connection it is to be noted on the one hand that a certain instability of emulsion is essential in order that an adequate amount of glycerin be delivered to the non-printing areas, and the more particularly when the area of the highlights is extensive and that on the other hand an unstable emulsion tends to break in the ink fountain, causing the uneven ink distribution above-mentioned. Furthermore, the addition of the salts to the ink adversely affects the varnish vehicle component thereof, tending to produce therein that condition of gelatin commonly known as livering.

Moreover, inks containing glycerin and/or hygroscopic salt solutions are very slow: drying, and in fact remain tacky to the touch for weeks or even months after printing. Nevertheless, two great advantages result from this process, one of which is that there is little or no tendency for the image to lose its integrity either by squashing or spreading whereby sharpness of definition is lost, or by the encroachment upon the image area by the dampening fluid, as when too much water is used, in which case the image is said to work sharp. The other advantage accruing from the use of an ink containing emulsified glycerin and salts is that the quality of printing is less dependent on the personal equation. However, these advantages are more than offset by the disadvantages above-mentioned and the further fact that the tinctorial dilution caused by the inclusion of the non-pigmented fluid necessitates heavy pigment coating of the ink with the consequent detrimental effect on its physical characteristics.

It is an object of this invention to eliminate the necessity for the use of water as a dampening means in conjunction with the use of ordinary lithographic ink. Another object of this invention is to obtain all the advantages accruing from the use of emulsoid inks While avoiding the objectionable effects of their inherent disadvantages. A further object is to avoid the necessity for the use of gum arabic either as a hydrophilic layer or as a surface protection against i the effects of drying which are so disastrous when water is used as the dampening fluid. A still further object is to provide a dampening fiuid' which is at once efiective to repel grease and to preserve the wettability of the surface for the dampening fluid. Other objects and advantages of the invention will become apparent from its more detailed description.

' I have discovered that the non-printing areas of a planographic printing plate when wet by and covered with a thin layer of glycerin are thereby rendered grease-repellent and that a continuous and suitably thin layer of glycerin, notwithstanding its viscosity, can be effected on the non-printing areas of a printing plate by means of a foraminous roller charged therewith. As disclosed in my copending application No. 631,588, filed September 2, 1932, I have discovered that the entire surface of the printing plate may be conditioned for glycerin receptivity before the image is applied thereto, but for the purposes of the present invention; while desirable, it is not essential that the plate be rendered glycerin-receptive before the application of the image as it is necessary only that the non-printing areas of the plate be or be rendered glycerinreceptive when the glycerin is applied thereto. I prefer to render the desired surfaces glycerinreceptive by means of the reaction product of the metal of a printing plate and a mixture of ammonium nitrate and ammonium phosphate, which may be applied as a solution of any suitable concentration as for example one between 0.5 M and 0.1 M. The solution may be applied to the surface of the plate in any well known manner such for example as by swabbing. However anyotherwise suitable surface which is repaper.

tentive'of a greasy image and which is naturally, or has been rendered readily wettable and retentive of glycerin, will serve the purpose of this invention. Thus for relatively short editions, a tough parchment paper, such as a paper rendered resistant to the disintegrating effects of water by treatment with concentrated sulphuric acid, 'will serve my purpose.

This invention has very many advantages, among which are the following;

1. The glycerin which is used serves, because of its viscosity, as an effective protective layer minimizing the effects of erosion and abrasion and greatly impeding the oxidation of, and in the case of zincthe formation of basic carbonate on, a metal plate surface, thus avoiding the necessity of frequent gumming up to protect the plate, as is necessary when water is used.

2. The glycerin covering a surface readily wettable thereby so adheres to such surface that it will withstand drastic treatment for the removal of accidental ink smears, as for example the use of grease solvents, without requiring the use of the gum arabic protection previously necessary with water wet surfaces.

3. The plate surface is much less sensitive to chemical reaction with traces of reagents in the This is partly because the glycerin is relatively inert, and partly because an effective glycerin film may be so-thin as to prevent any substantial transfer of we glycerin to the paper.

4. Register, in color or multiple printing, is greatly facilitated because of the elimination of that expansion of the paper which inevitably results from theabsorption of moisture from the dampening water.

5. Softer inks may be used, and their use is of decided advantage since it results in a better lifting of the ink with less pressure; in the picking of less lint and/or clay from the surface of the paper; in there being less strain placed on the gripper fingers; and in minimizing the danger of the papers sticking to the blanket in offset printing.

6. The process prevents toning or scumming 'more effectively than does the use of water and gum arabic, hence highlights surrounded by deep shadows in a half-tone image are kept free from tone for long editions, whereas such fine detail is frequently lost after a few impressions when water is used.

7. .There is substantially no tendency for the half-tone image to work sharp or walk off, and the integrity of definition thereof persists, if not indefinitely, at least for very long editions, because the glycerin does not undermine the colloid base of the image and consequently the half-tone dots do not diminish in size or work sharp, nor do they "walk ofi (that is, be readily torn from) the plate by the action of tacky inks.

8. The tendency commonly exhibited by the paper to stick to the offset blanket is substantially eliminated by the film of glycerin maintained on the printing surface of the blanket even when the latter, under the softening influence of the ink, has developed a decided tackiness.

Moreover, in offset printing where only a limited amount of ink may be transferred to the paper, a better depth of color is obtained byavoiding the degradation of color due to waterdampening; less pressure is required, thereby preventing encroachment on the image area by the dampening means, a very desirable condition whereby, as is well known to those skilled in the art, there results longer life of the blanket, longer life of the image on the plate, greater depth and brilliancy of color, and sharper definition of the printed image; the process may be operated without regard to weather conditions at substantially higher speeds than can be attained with the water-dampening process; the very high degree of skill heretofore pre-requisite to the obtainment of eflicient ordinary lithography is not necessary to theobtainment of an equally efficient operation hereby; and problems of register in the case of color work are very much reduced by the substantial elimination of moisture transferable to the paper.

In carrying out one mode of operation of the present invention, the plate is first provided with a greasy image which may be either formed directly on or etched into the plate or superimposed upon a colloid film by any usual photographic means. The non-image area of the plate is then etched or treated with a solution containing suitable salts, ammonium nitrate and ammonium phosphate being preferred, whereby the surface is rendered readily wettable by glycerin, and finally wet with a film of glycerin, which may be and preferably is extremely thin.

The printing operation is performed in any well known manner using any lithographic ink, preferably one slightly acidic. The linseed oil varnish and other inks commonly used as litho faces, although such inks may be efliciently used ink rollers and lodged on the non-printing areas when the entire surface of the plate-has been coated with a colloid film.

The glycerin coating on the treated plate adheres so tenaciously thereto and is of such viscosity that several clean copies may be obtained before it is necessary to supply additional glycerin to the plate. Hence, when large editions are to be struck off, it is essential that the glycerin film on the treated plate be replenished, and it has been discovered that such replenishment of the glycerin may be accomplished 'by means of a suitable glycerin-charged roller, such roller being brought into contact with the printing surface, preferably but not necessarily between successive inkings. This operation is of course similar to the operation performed. in the usual wet lithography procedure, with the exception that the contacting of the roller need not be effected between each two successive inkings and that there need not be provided a continuous andv regulatedsupply of dampening fluid to the rollers, as is required in the practice of ordinary lithography wherein the supply of water must be nicely adjusted.

It has been found that a roller made of suitable absorbent material, such for example as felt or sponge rubber or the like, may be used, and that when such roller is dampened with glycerin it will continue to deliver to the surface of the plate a film of glycerin suflioiently uniform to maintain the requisite sharpness of definition for a hundred or; more copies before it needs to be recharged.- This obviously is of great advantage. The recharging may be performed in any convenient manner, as by transferring glycerin to the roller from a saturated hand roll.

While .it is not essential that the glycerinapplying roller be provided with auxiliary rolls, it has been found that such are of advantage. The auxiliary roll', however, instead of being of brass as, in ordinary wet lithography, is commonly used in conjunction with the dampening roller, should/be a roll composed of a material not reactiveWith the salt solution and not readily wet by glycerin, such as vulcanized rubber or a molded artificial resin. This auxiliary roll acts not only in the usual way to attract to itself any stray particles of ink which may have been picked up by the glycerin-distributing i'oller, but it also serves to keep the surface of the glycerin-distributing roller wet with glycerin by the outward displacement of the glycerin from the interstices of the roller material as it is compressed by the passage of the auxiliary roll thereover.

While it has been suggestedin connection with the use of a lithographic stone that glycerin might be added to the dampening water for the purpose of facilitatng the maintenance of a thin uniform film of moisture on the non-printing areas, it must be borne in mind that this was for use on stone only and was always avoided in the treatment of metal plates. Rhodes, The Art of Lithography, Second Edition, page 46, states that glycerin must not be used upon metal plates, as its chemical action causes them to become greasy and take on tint. This detrimental effect of glycerin on the metal surface of a printing plate may be avoided, however, by conditioning the metal surface by the combined actionof certain salts such as ammonium nitrate and ammonium phosphate. I

In printing large editions, it is generally found that tinting or toning in the highlights will ultimately occur; such toning takes place when minute traces of ink become detached from the of the plate, from which they are transferred to the paper and impart a more or less faint tint or tone to the non-printed areas, or highlights. This toning may take place by reason of the imperfect desensitization or grease-repellency of the non-printing areas, or the improper consistency of the ink, or may be the result of various mechanical 'causes such as improper or too coarse by swabbing, the non-printing areas will almost immediately become free of any stray particles of ink. Just .how thisfreeing of the glycerincoated non-printing areas from thetone-producing particles of ink takes place is not understood, but the effect is striking, and of great advantage. Alternative to the application of the drops of salt solution to the plate, it may be found'expedient to add a small amount .of a concentrated solution of the salts to the glycerin used in the dampening roller. This is particularly of advantage when the character of the ink or a slight maladjustment of the inking mechanism tends to produce toning and would in consequence require frequent applications of the salt solution.

This invention is not limited to planographic printing where the greasy image is'formed directly on the plate, but is equally applicablewhen the greasy image either overlies andis determined by a photographically produced foundation of a light-hardened bichromated colloid on the plate or is intaglio, that is, etched into the body of the plate. When preparing a plate photographically for printing, the plate, having been cleaned or counter-etched, is coated with a treated with the salt solution and the glycerin in the manner previously described, to render such areas ink-repellent, and the printing image is developed with a greasy ink.

It has previously been-found that when the non-printing areas are treated with a solution of gum arabic and then repeatedly dampened with water during the printing operations, the image tends to walk off the'plate. This is due to the fact "that in the formation of the image the hardening action of.the light is greatest on; the upper surface of the film, and that consequently the lower surface,-which contacts the plate, is still susceptible to water. The dampening. with water tends to soften this still water-susceptible portion of the film and. thus results in loosening the bond between the film and the plate, so that the image is either deformed or extirpated when ened bond between the colloid film and the plate.

This is particularly difficult to control in halftone work where the image is made up of a multitude of minute dots, each dot having a large edge area exposed to the softening or undermining action of the water. A plate treated in accordance-with the present invention shows substantially no such tendency, however, and this is believed to be due to the fact that the glycerin, and particularly glycerin rendered slightly acidic by the inclusion of the mentioned salts, has little or no such softening action'upon the colloid.

It will be understood that the above-described photographic process is not strictly planographic, since the greasy image is raised above the level of the plate by reason of the intervening colloid film. It has been proposed to protect the colloid film by filling the non-printing areas to the level 'of the printing areas with a bichromated and light-hardened hydrophilic layer, such as may be obtained for example from gum arabic, as stated. This treatment forms on the plate a continuous film which-is difierentially hydrophilic. While such treatment helps to maintain the definition of the image by mitigating the softening tendency of the water, the results are not entirely satisfactory and a high degree of skill is still required.

The present invention avoids the necessity of such treatment of the plate since, as pointed out above, the glycerin is substantially non-fugitive and maintains a damp ink-repellent surface over relatively long periods. However, the greasy image is formed upon the plate, whether directly on the metal surface, superimposed upon a colloid film, or intaglio, the treatment of the salt-reacted non-image surface with glycerin in accordance with this invention yields results greatly superior to those obtainable with water and avoids the necessity for gumming up with gum arabic.

Throughout the above description reference has been made to the use of glycerin in the treatment of the plate. 'It will be understood, however, that in place of glycerin other fluids of suitable viscosity and character, such as glycol for example, may be employed. The invention is, of course, not limited to the use of glycerin alone, and it will be understood that the reference to glycerin should be considered as including any well known physical substitutes or equivalents therefor. Thus while glycerin is preferred, triethanolamine may be substituted therefor when the non-printing areas have been coated with a light-hardened hydrophilic colloid.

I claim:

1. The process of printing from a grease-receptive image carried by the surface of a printing plate, comprising the steps of rendering the nonimage area thereof glycerin-receptive, forming a film of glycerin over such area, printing successive copies of the image with a greasy ink, and from time to time replenishing said glycerin film between successive inkings.

2. The process of printing from a grease-receptive image carried by the surface of a metal printing plate, comprising the steps of treating the non-image area thereof with salts to render such area glycerin-receptive, forming a film of glycerin over such area, and printing with a greasy ink and occasionally applying a solution of said salts to said film to prevent toning. I

3. The process of printing from a grease-receptive image carried by the surface of a metal printing plate, comprising the steps of treating the non-image area thereof with salts to render such area glycerin-receptive, forming a film of glycerin over such area, and printing successive copies of the image with a greasy ink, from time to time replenishing said glycerin film between successive printings andoccasionally applying a solution of said salts to said film to prevent ton- 4. The process of printing from a grease-receptive image carried by the surface of a metal printing plate, comprising the steps of treating the non-image area thereof with salts to render such area glycerin-receptive, forming a film of glycerin over such area, printing successive copies of the image with a greasy ink, and from time to time, between successive inkings, replenishing said glycerin film with glycerin containing a solution of said salts.

5. The process of printing, comprising the steps of forming a grease-receptive image on a zinc plate, treating the non-image area of the plate with ammonium nitrate, ammonium phosphate and glycerin, inking the image with a non-alkaline lithographic ink, and applying glycerin to the plate from time to time during the printing operation independent of the inking step.

6. The process of printing, comprising the steps of forming a grease-receptive image on a zinc plate, treating the non-image area of the plate with ammonium nitrate, ammonium phosphate and glycerin, inking the image with a non-alkaline lithographic ink, and applying glycerin, ammonium nitrate and ammonium phosphate to the plate from time to time during the printing operation independent of the inking step.

7. The process of printing, comprising the steps of forming agrease-receptive image on a zinc