US2516222A - Graining of lithographic and other printing plates - Google Patents

Description

y 1950 J. G. LINDMARK 2,516,222

GRAINING 0F LITHOGRAPHIC AND OTHER PRINTING PLATES Filed April 13, 1949 2 Sheets-Sheet 1 u V r Fig: 2.

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GRAINING OF LITHOGRAPHIC AND OTHER PRINTING PLATES Filed April 13, 1949 2 Sheets-Sheet 2 Patented July 25, 1950 GRAINING F LITHOGRAPHIC AND-OTHER PRINTING PLATES" 1 'Johan Gunnar Lindmark; Stockholm, Sweden,

assignor to Aktiebolaget Johnson & Borsell, Stockholm, Sweden, a corporationof Sweden T Application April 13, 1949, Serial No. 87,313 I, In Sweden JanuaryZfi, 1945 k I .4 Claims. (01; 51 282) This application is a continuation-in-part of I my copending application, Serial No. 643,271, med January 25, 1946, now Patent No. 2,495,269. This prior application is directed to an apparatus which can be used for. conducting the. process which is described and claimed in the present application.v f j 1,

In the lithographic and oflfset printing process it is highly important for obtaining sharp and accurate printed impressions to provide printing plates the porous surfaces of which will absorb uniformly a maximum quantity of character forming media, such as, lithographic ink, for the design and also a maximum of-moisture onv the portion not covered by the design. In order to obtain this maximum absorptivity it is well recognized that the pores in the surface of the plate should be as deep as possible and of uniform depth and size. In the past it has not been possible to produce lithographic plates whose porous surfaces are free from waves, craters and other imperfections which; cause a non-uniform and insufficient absorption of ink or water. Accurate reproduction of grained surfaces was likewiseimpossible.

Various methodsof grai ning plates have'been used heretofore but none of them have been entirely satisfactory.

In the most frequentlyused method a so-called shaking or. graining box is used, this box being furnished with-water, steel balls and a graining 1 material. The plates to be grained are mounted in the box and the box is then shakento produce the grinding effect. Imperfections are frequently present inthe plates produced by this method and the depth of the grain is not wholly satisfactory. Moreover the grain produced by .this method in any given operation cannot bereproduced since the steel balls wear down rather rapidly and-the graining depends upon the sizeand weight of these balls. corrosion and oxidationjoccurs injthese graining boxes due to the presenceof water and air. The method is both slow and dirty.

While various proposals have been made to employ sand blasting equipmentj'to grain lithographic plates, none ofthese have been found sufficiently satisfactory to, have been adopted .to any important extent. Many 0f these machines use a blast of sand and water and in'these machines corrosion occurs. In some fof these blasting machines the work has been mounted flat and this has invariably caused the formation of waves in the. work. In :others'the work has been mounted on drums which rotateslowly Moreover a considerable amount of stantially double that ing boxes.

in front of the blasts. It has been found impossibleinthese machines to produce grains of uniformsize and depth. Numerouspits and craters are always present.

Itis an object of the present invention to provide a method having a rate of production subof the conventional grain- Another object isjto provide a method which is clean, in which no oxidationoccurs, which requires no preliminary lye treatment or cleaning of the plates tobe grained and which produces plates which can be used immediately or stored indefinitely without deterioration.

. A further object is to provide a method in which "the sand or other grit consumedis a small fraction of that consumed-by otherprocesses.

,It'is still another object of. this invention to produce grained printing plates having an extremely uniform texture 'ofjgrain substantially free from,pits and craters whichtexture can be reproduced accurately at any future time merely by the use of the same operating conditions.

I I I have'found that the above and other objects ,can be accomplished and the enumerated dimculties overcome by my novel process which consists in mounting the. work to be grained on a drum or other rotatable support, rotating: said support at a high critical peripheral speed while directing a highvelocity blast of grinding ma- ,terial, against said work and moving saidblast longitudinally back and forth at a relatively'slow speed during the graining operation.

In the tests which led up to the present inyention I made a discoverywhichI believe accounts for the failure of prior sand blasting methods to produce grains of uniform. texture free from pits {and craters. I found that if'the 'plates to be grained are mounted on a drum and .ing the pits and craters substantially disappear this drum is rotated at high speed during blastfrom'the grained surface. The 'minimum pefripheral speed required to thus eliminate pits andcraters is critical and in the neighborhood of 25 inches per second. The cause for this phenomenon is believed to be that at the high peripheral speeds in question the spent particles of sand are removed by centrifugal force from the blast area substantially immediately after they strike the plate. Sand particles which at slower speeds would tend to stick to the'plate are thrown off by centrifugal force and particles which rebound from the plate regain asubstantialvelocity in a tangential direction due to the impelling action qf'therapidly moving plate. The

chances of the high-velocity blast particles striking spent grains and driving them into the plate are substantially reduced and I believe that in prior processes pits and craters are largely produced by this action-at least. that. seems to be a reasonable explanation of the factsistated;

I have also found that the sand blast nozzles should be moved rather slowly in front of the rapidly moving drum in order to produce satisfactory graining for lithographic. work. The" best speed for this type of work I have found tolbe from about 5 to 25 inches per. minute. Of course since my method can be applied to the graining.

of materials for other purposes than lithography the speeds mentioned can be varied outside the range stated. The texture of' the graining' produced depends upon the speed of the drum, the

speed of the blast nozzles, the angle at which- .pheric pressure is,always.maintained in the ma- 4 driven by means of a worm-Wheel gearing or belt drive at a variable speed from an electric motor 24 mounted on the projecting end of the shaft of the cylinder 11. For reversing the direction of. rotation of. the. motor 24 and of the screw spind1e:20'at the turning points of the nozzle there are provided adjustable limit abutments 26 which change the direction of the rotation of the motor under control of the nozzle by mechanicai or' electrical means.

It'sliould be clear that by turning the cylinder l1 and the parts connected therewith about the axis? of the. cylinder the angle between the envelopersur-faceof the drum l l and the blast nozzle l9 may be adjusted to every suitable value. For locking'thecylinder in the adjusted position there is provided a locking device 21.

Theinterior of the housing In is in communication with a suction fan or the like through the intermediary. of a conduit 28 so that. a subatmoschine. whereby dust is preventedfrom emerging into the workroom. To reduce the air cona-recordof these factors. and. reproduce them later Fig. Lisa partial. sectional. View of a modification,. similar to; the. lower portionof Fig... 2 but showing theuse. of a pair ofblast. nozzles,

V Fig. 5 is a. partial vertical transverse section on alarger scale,.simi1ar to the upper portion of Fig.3,v but showing-a modification in which sealing means, are provided. for closing. the slot through which the blast nozzles pass,,whil'e Fig. 6 is. a fragmental. sectional view of the modificationoill'ig. 5, taken, along the line 6--6 Referring. to. the drawings in which like parts are designated by like reference numerals. there is provided in. the upper. part of a closed. housing it a. drum H, on which. a lithographic printing plate.l-2.of zinc or other materialmay be secured by means or. clamping jaws l3, said drum being rotatably journalledby meansof'a suitable shaft 14, which is. driven at a variable speed from an electromotor l5 through the intermediary of a wormewheel gearing l6. lnonelongitudinal wall ofthe housing} right opposite the drum H there is provided a. comparatively narrow rectangular opening. whichis. closedlby means of a cylinder or guide. l1. journalleddn the housing so as to be rotatable around. its own axis. Diametrically extending throughsaid cylinderav slot 18 is provided. through which a reciprocable. sand blast nozzle. is projects into. the housing- The reciprocating. movement of. the blast. nozzle 19 is derived. from:. a. longitudinally extending screw spindle 2a, which is journalled in. end plates 2! united with the cylinder. L1. and cooperates with a. nut 22 which isconnectedwith. aholder 23. in which the nozzle Iii isaxially displaceable. for the purpose'of. setting the nozz1e=at the. desired distance from the drum. The screw spindle 20 is This apparatus is.

sumption. the guide slot l8 for the blast nozzle I?! may be closed, if desired, by means of an elastic sealing device, for. instance consisting of two flexible bands of rubberor the like, as'shown attle and 31) in Figs. 5 and 6 which bands; in their normal position cover the slit opening.

During the sand blasting operation the drum H is preferably rotated at such a high speed thatth'e: stresses in the printing plate l2 clamped to the drum are. increased considerably through the action of the centrifugal force. The drum used mayyary in size from about 40 to '70 centimeters in diameter with a preferred diameter. of from about 50 to 60. centimeters.

To avoid patterning, the velocity of movement of'the nozzle should be so selected with respect to the" speed of rotation of the drum that the nozzle is not periodically directed to exactly the same points of the printing plate. As mentioned previously the best nozzle speed for the graining' of lithographic plates from about 5 to 25 inches per minute.

The distance of the nozzle from the work is also important and best results are obtained when this distance is within the range of about 2 to 6" inches witha' preferred range of about 3 130 4 inches. If two or more nozzles are employed the-blasts should diverge at such an angle that there is no overlapping of the-blasts where they meet the'work. Theair pressurewhich operates the blast must be kept highly constant. Best results are obtained when the pressure at the blast is withinthe range of about 15' to '70 pounds per square inch while the preferred pressure range is from about 25 1D 50 pounds per square inch.

The angle; of incidence of the blast. with the work determines the depth a's Well as the shape of the grains; This angle' can'. be variedfrom about to 45in both directions but, of course, the texture of the grain willdifier considerably as the angle is varied'within this, range. The type of impression desired will largely determine the best angle of incidence to be employed.

I have found. that" the. grainof plates grained by my process. has. an. average. depth at, least 50% greater than plates grained by other processes. A uniformityis obtainedlwhich is not possible by other methods. Whenasingle blast nozzle is used plates can be. grainedin from. 7 to 14 minutes depending ontheirsizewhile with more than one nozzle this time can be reduced still further. These plates are capable of absorbing uniformly a far greater amount of water and ink.

I have found that plates made by my process are entirely free from waves, craters and pits and other imperfections ordinarily present in grained plates produced by other processes. I have also found that the use of my process will not result in oxidation either of the plates or of the metal grinding particles when metal is used as the grinding medium.

It is a further important feature of my invention that the sub-atmospheric pressure under which the process is conducted keeps the operating area as well as the surface of the plates free from dust.

While I have described what I believe to be the most advantageous embodiments of my invention it is evident of course that various modifications can be made in the specific procedures described without departing from the purview of my invention. My process is applicable for the graining of plates made of various materials such as plastic or metals such as stainless steel, copper, aluminum and zinc. Although I prefer to use ordinary sea sand as the blasting material other types of sand are suitable as well as carborundum, saw dust, powdered glass, metal pow ders, alundum and emery. I preferably use air as the propelling means for the sand or other grinding material but other gases may be used if desired such as nitrogen, steam, carbon dioxide or other non-oxidizing gases.

The machine used in my process may be provided with means for moistening the plate to be grained with water or other liquid during the sand blasting operation as shown in 3| in Fig. 5. It is also possible to employ a plurality of blast nozzles Ill, as shown in Fig. 4 of the drawing. It should also be clear that as concerns the use of my novel process for treating the surface of old lithographic printing plates, the process can be used not only for graining the surface of a printing plate from which the old lithographic picture has been removed as by washing with a suitable solvent, but also for the removal of the old impression prior to graining. Other modifications of my invention which fall within th scope of the following claims will be immediately evident to those skilled in the art.

I claim:

1. In the process of graining lithographic and other printing plates as well as plastic sheets used in the art of offset printing, the process which comprises mounting the work to be grained on a rotatable drum, rotating said drum at a peripheral speed of at least about 25 inches per second,

directing a high-velocity blast of grinding material against the work at an angle of incidence within the range of from 45 to and under a nozzle pressure of from about 15 to 70 pounds per square inch, and moving said blast longitudinally at a slow constant speed along said drum.

2. In the process of graining lithographic and other printing plates as Well as plastic sheets used in the art of offset printing, the process which comprises mounting the work to be grained on a rotatable drum, rotating said drum at a peripheral speed of from about 25 to inches per second, directing a high-velocity blast of grinding material against the work, reciprocating said blast longitudinally at a slow constant speed along said drum and maintaining a subatmospheric pressure in the neighborhood of the blast.

3. In the process of graining lithographic and other printing plates as well as plastic sheets used in the art of offset printing, the process which comprises mounting the work to be grained on a rotatable drum, rotating said drum at a peripheral speed of from about 25 to 150 inches per second, directing a high-velocity blast of grinding material against the work under a nozzle pressure of from about 15 to '70 pounds per square inch,

moving said blast longitudinally at a constant speed of about 5 to 25 inches per minute along said drum and maintaining a subatmospheric pressur in the neighborhood of the blast.

4. In the process of graining lithographic and other printing plates as well as plastic sheets used in the art of offset printing, the process which comprises mounting the work to be grained on a rotatable drum, rotating said drum at a peripheral speed of about 65 inches per second, directing a high-velocity blast of grinding material against the work under a nozzle pressure of about 40 pounds per square inch, moving said blast longitudinally at a constant speed of about 10 inches per minute back and forth along said drum and maintaining a subatmospheric pressure in the neighborhood of th blast.

J OI-IAN GUNNAR LINDMARK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,091,036 Bouillet Mar. 24, 1914 2,005,654 Fritsche June 18, 1935 2,276,594 Rowell Mar. 17, 1942 2,332,251 Parrish Oct. 19, 1943

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