US2421607A - Method of making metallic printing screens - Google Patents

Description

June 3, 1947- H. B. FowLER METHOD 0F MAKING METALLIC PRINTING SCREENS 2 sheets-sheet` 1 Filed April 3, 1942 imi/1,1,

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June 3, 1947. H B FOWLER 2,421,67

METHOD OF MAKING METALLIC PRINTING SCREENS Filed April 3, 1942 2 Sheets-Sheet 2 vea/s f1.5@ T t ,fo 55 /f larguer# Patente'd June 3, .1947

UNITED STATES PATENT N osi-ica METHOD OF MAKING BIETALLIC PRINTING SCREENS Harwood B. Fowler, Wayne, Pa. Application April 3, 1942, Serial No. 437,543

3 claims. (ol. ion-128.4)

printing screens, avoiding or reducing saw-tooth ing at the printing edges of the designs.

A further purpose is to avoid the necessity of employing extremely ne mesh screens for minimizing raw-toothing in tscreen printing, and to permit the use of coarse screens for printing radium dial material and the like without causing objectionable saw-toothing.

A further purpose is to mask the non-printing" portions of a metallic printing screen by' a metallic sheet, preferably on the printing face of the screen, to secure the metallic sheet and the metallic printing screen together by a metallic `uniting layer different from the metallic sheet, to etch the design in the metallic sheet, finishing with an etchant which does not substantially attack th'e uniting metal and subsequently `to remove the uniting metal at the design portions of the screen, finishing with an etchant selectively attacking the uniting metal.

A further purpose is to etch a metallic printing screen from the printing face so that the most perfect-etching of the design will be right at the printing face where etching'starts.

A further purpose is to obtain a sharp etched edge in the design on a metallic sheetunited to a printing screen by placing between the metallic sheet and the screen a metallic layernot substantially attacked by the etchant which finishes the etching of the metallic sheet, the metallic layer separating the metallic sheet from the screen at the points of etching.

A further purpose is izo-unite a metallic sheet and a metallic screen by an alloy containing zinc which will be relatively resistant to the etchant which finishes the etching of the metallic sheet, but will be easily attacked by any usual strong acid.

A further purpose is to make a commercial printing screen of metallic screen wire,l soldered to a metallic sheet.

A further purpose is to produce a printing screen from metallic screen wire united by a metal to a sheet made of an alloy predominantly containing copper.

A further purpose is to. laminate together a metallic screen and a metallic sheet by a metal- I lic bond and to coat the face ofthe screen remote from the metallic sheet with a transparent resist, preferably a transparent lacquer, to delineate `a design through the sheetfand subsequently to etch out the design thrcughthe uniting metal while inspecting the face of the screen remote from the metallic sheet through the transparent resist for the end point of the etching operation.

A further purpose is to etch a metallic printing screen in several bites to `avoid break down of the resist.

A further purpose is to regulate the thickness Y of build-up in screen printing by securing on the printing face ofthe screen a metallic sheet of thickness variable with-respect to the `amount of build-up desired.

A furtherpurpose is to reduce the waste of radium dial material used in printing.

Further purposes appear in the specification and in the claims.

In the drawings, I have illustrated a few only of the possible embodiments of the invention, the forms shown being chosen from the standpoint of convenience in illustration and satisfactory operation.

Figures 1 to 7 'are diagrammatic sections illustrating successive steps in the construction of the screen of the present invention.

Figure 8 is a diagrammatic section showing electrolytic etching of themetallic sheet.

Figures 8a. to 8d are diagrammatic fragments showing successive stages of etching in Fig-v ure 8.

Figure 8e is a diagrammaticfront elevation of the anode in electrolytic etching.

Figure 9 is an enlarged diagrammatic plan View of a fragment of a screen, showing how sawtoothing develops. i

Figures 10 and 10a are diagrammatic sections showing alternate techniques for etching of the uniting metal.

Figures 11 and 12 are diagrammatic sections showing further stepsin `completing the screen.

Figure 13 is a bottom plan view showing the printing face of a nished screen.

Figures 14 to 17 are diagrammatic sections showing an alternate procedure for applying resist to the design face of the screen.

Figures 18 and 19 are diagrammatic sections showing printing with different thicknesses of build-up.`

In the drawings, like numerals refer to like parts throughout. y

In printingmany types of pigments, including ceramic pigments `for labeling bottles xand jars, and in printing signs on metal and the like, and printing gauges, dials, indications and `i ctions on instruments, apparatus andiequipment, it is frequently necessary to resort to screen prin ing of` the type frequently described as silk screen printing.

The general process consists in blockingoutall portions of the screenexcept those which are to print the design, and then placing the screen in close proximity to the workand passing the pigment through the screnand onto the work pref-l Efforts have therefore been made toemploy metallic screens. These have met with limited success because of the great difficulty in securing,

at low cost, smooth and even edges where the' blocked out areas meet the design, and the relatively high cost of transferring the design to the screen in any reasonably dunable fo-rm.

At the present time a great deal of radium and other luminous dial materials (herein conven-- tionally referred to as radium dial materials) is lapplied to instrument-s, and this work is almost entirely done by hand because of the impossibility of obtaining the requisite high build-up and desired uniformity and precision by the methods of machine printing heretofore available. This hand application of radium dial materials is not only dangerous to the openator, who must carefully avoid all absorption of the radium by his body, but is also very slow and expensive. Approximately 30 to 40% of the radium dial material used in the manual process hardens on the brushes, is removed by the cleaning pads, and must be recovered and reworked. One of the important resul-ts achieved by the present invention is the avoidance of the danger incident to production of radium dials by the manual method and saving of the 30 to 40% radium dial material which has to be reworked in the manual process.

In order to accomplish these and other purposes explained more in detail below, the present inventor laminates together a metallic screen suitable for screen printing and a metallic sheet suitable for blocking out the non-printing areas, and unites the screen to the sheet by a uniting metal having different etching characteristics from the screen and from the sheet.

. In Figure 1 are shown a metallic printing screen 2li and a metallic sheet 2|. The screen may be of any suitable metal, preferably one of the corrosion resisting steels (often called corrosion resistant irons) such as stainless steel. Very satisfactory results are obtained with a screen of stainless steel containing 18 per cent chromium and 8 per cent nickel. Corrosion resi-stapt steels or irons containing smaller amounts of chromium with or without nickel maybe used with success, for example, 17 per cent of chromium and 2 per cent of nickel, or 16 per cent chromium without nickel.

While it is best to employ a screen of stainless steel or the like, it is possible to use a screen predominantly consisting of copper, and made up, for example, of copper, brass or bronze. This type of screen is less desirable than a screen of `corrosion resisting steel, as it is sof-ter and less immune to corrosion. I

The mesh size for the screen may be of any usual screen printing mesh, for example, 165 mesh per linear inch (Tyler standard). As will be explained later, one of the important advantages of the invention is that much coarser meshes can be employed with satisfaction in the present invention than in the prior art.

The metallic sheet 2| preferably predominantly contains copper. Copper or a copper base alloy, such as brass or bronze, may :be used with success.

The metallic sheet and the screen are joined is obtained as well known in the art.

as 23. In tinning the metallic sheet 2|, it is important to keep the lower surface 24 untinned.

While the screen 20 and the metallic sheet 2| are still very hot and the-solder layers 22 and 23 are still molten or plastic, the screen and sheet are laminated together by pressure between suitable dies 25 and 26, las shown in Figure 3. A pressure of a few hundred pounds, as obtained in a hand press, is sufficient for this purpose. 'I'he dies 25 and 26 will desirably be heated, though the temperature need not be as high as the melting point of solder in every case. Where the dies are heated above the melting point of vthe solder, they will be Iallowed to cool sufliciently to permit the solder to harden before the pressure is released. The result as produced in Figure 3 is soldering together of the adjoining surfaces of the screen and the metallic sheet by a layer of solder 21 which extends between the screen and the metallic sheet and fills the interstices of the screen. In accomplishing this soldering, it is best to have the screen above the metallic sheet, so that solder will not necessarily fully fill the meshes of the screen for the entire thickness of the screen, although lt will extend over the entire area.

The thicknesses of the solder layers 22, 23 and 2'|y are exaggerated in the drawing.

After suitable cooling of the laminated screen and sheet, and inverting of the laminated structure, the front or non-printing surface of the screen is coated with a resist 28, preferablya transparent resist as shown in Figure 4. The best transparent resist is a transparent lacquer such as nitrocellulose lacquer, ester gum lacquer or glycerol-phthalic acid lacquer. Any suitable transparent lacquer, which can later be dissolved away, may be employed.

The printing face 24 of the metallic sheetis then coated with a photosensitive layer 29 (Figure 5) as Well known in the art. The photosensitive layer 29 may actually be several layers applied until the desired thickness of photosensitive layer Any suit.- able photo-engravers sensitizing solution of the cold enamel type may be used in making the photosensitive layer 29, as for example, dichromated gelatine. A highly satisfactory sensitizing solution of the cold enamel type is sold onv the market as Hess special re-etching enamel.

After the photosensitive layer 29 has been properly dried (under protection from light strong enough to cause exposure), it is photographically exposed to the copy which contains the design to be transferred to the screen. The most convenient way to do this is by contact printing technique as shown in Figure 6. The copy 30 suitably reversed (because it is being photographed against the printing surface rather than the so-called front surface of the screen) is placed against the photosensitive layer 29, in a contact printing frame, if desired, and a strong light is applied for a time long enough to complete the exposure. Areas 3| where the design appears on the coply will protect the photosensitive layer immediately beneath them from exposure, but other areas 32, where the design is absent will permit transmission of light to expose the portions of the photosensitive layer immediately beneath. The copy 30 may be either a photos negative or a photo-positive transparency on film or glass.

After exposure the photosensitive layer is developed by any developer well known in the art by the exposure into a resist 35 as shown in Figure 7.

An important feature of the invention is the etching in at` least two steps, the rst of which etches through the metallic sheet and the second of which removes the uniting metal at corresponding points.

For etching through the metallic sheet, it is best to use an electrolytic etching process, though this is of course not essential. It is found, however, that electrolytic etching gives very rapid, sharp and straight-walled etching of the metallic sheet.

In Figure 8, the laminated screen and sheet form the anode 36 in an electroplating bath 31 contained within a vessel 38` A cathode 39 suitably of copper is connected inthe circuit with a direct current source 40.

the screen aids greatly in obtaining a straight and smooth etched edge 4| of the copper.

It has been found that the etchant first produces a gradually iilleted depression 42 near the edge of the resist as shown in Figure 8a. As the etching becomes deeper, this curve becomes sharper as shown at 43 in Figure 8b. Further etching results in a configuration as shown at 44 in Figure 8c. If the solder layer 21 is not present, it has been found that the tendency is` to leave a curved projection 45 not removed by etching. If the etching is prolonged without any solder layer being present, the resultis to obtain undercutting from the back and edge of the metallic sheet before the projection 45 is fully etched away.

The solder layer 21, however, not being substantially attacked by the etchant for the copper, tends to prevent undercutting while the copper etchant is attacking and removing the projection 45 until the comparatively straight; etched wall 4| is obtained as shown in Figures 8 and 8d.

Thus it is evident that the square character of the etched edge of the metallic sheet is improved by the presence of the layer of uniting metal 21 not attacked by the etchant.

The layer of uniting metal 21 performs a-further very important function duringy the copper etch. If the metallic sheet 2| and the screen 20 were in direct contact with one another during the copper etch, without any uniting layer being The plating current l will preferably be about 8 amperes and the volt-` present, the etchant would tend to crawl along the wires 46 of the screen beneath the copper and to follow the squares of the screen mesh in etching, producing a saw-toothed outline 41 (Figure 9), instead of following the line 48 of the design. This saw-toothed `outline 41 attained in the prior art, is very undesirable as it is carried over into the outline of the iinal pigment build-up in printing. i

It has been found that the presence of the solder layer 21, preventing all physical contact between the wires 49 and the vmetallic sheet 2| during the etching lof the metallic sheet, eliminates the saw-toothing entirely and produces a smooth etched wall (which of course may be curved to suit the design) following the line 48.

By virtue of the fact that the uniting metal layer 21 is not copper and is resistant to copper` etchants, the uniting layer of soldernot only assists by uniting the metallic sheet and the metallic screeniirmly together, but also aids -greatly in obtaining a square etched Wall of the copper sheet in depth and a in length.

In order to attempt .to avoid saw-toothing in smooth etched wall screens where the metallic screen and the'metal` lic sheet have been in direct contact during the etch, it has been necessary to employ 'very fine screen meshes. This does not in fact prevent saw-teething, but it makes the saw-toothing so line that it is not very noticeable `in the buildup on the final printing. For this reason, it has been the prior practice of the inventor to employ screen mesh sizes of about 165 Tyler standard mesh per linear inch, and coarser screens could not be used without exhibiting very obvious saw-toothing.

This is a distinct disadvantage in printing certain pigments which are desirably of large particle size. For example, in printing radium dial materials, where the production of eliicient luminous coatings necessitates the employment ofparticles of relatively large size,.it has not been possible to obtain satisfactory printing through the line screens needed in the prior practice to avoid saW-toothing, and accordingly, manual application of the radium dial material has been employed with corresponding great risk to workers who are likely to come into physical contact with the radium dial material.

By the present invention, the screen mesh, no matter how coarse it may be, exerts no undesirable effect upon the squareness or smoothness of the etched wall produced in the metallic sheet, since the solder prevents physical contact between the metallic sheet `and the screen during the etching of the sheet. Thus, it has been possible in accordance with the present invention to produce etched screens of mesh or even coarser without any saw-teething taking place at the printing face. With such coarser screens, satisfactory printing of radium dial materials is possible not only for comparatively crude commercial uses, but more especially to meet the very exacting requirements of military aircraft instruments and those required in mechanized land warfare.

One ofthe important features of the invention is that the etching is started at the printing face.

The more the etching advances into` the depth of the sheet, the more pronounced will its tendency be to deviate from the contour of the design.

Thus, if lin spite of the precautions taken in' accordance with the present invention, some tendency-to saw-toothing is evident, it is likely to develop only in the portions of the laminated screen and sheet which are relatively far from the printing face 24. Thus a certain amount of inaccuracy incident to high production methods can be tolerated without destroying the utility of the invention by marring the accuracy and Vsmoothness of the outline at the printing face.

Such slight inaccuracy will merely affect the etching in the region 44 which is relatively far removed from the printing face.

While the procedure above indicated for making ythe copper etch can be used satisfactorily, it is comparatively slow and ineiective. Much more rapid etching can be obtained if a solution of ferrie chloride is initially employed in the electrolytic bath to etch through the copper until the uniting layer is first exposed. Ferrie chloride etches much more rapidly than chromlc acid and can be used quite efficiently up to about the stage shown in Figure 8c where the uniting layer has rst been uncovered. It is not safe to use ferric chloride for etching beyond the stage of uncovering the uniting layer (except as noted below) as it will attack the uniting layer and will not discriminate between the copper and the uniting layer. Therefore the etching solution should be changed after reaching the stage of Figure 8c. lThe laminated screen and .sheet should be washed, dried and then'further electrolytically etched in chromic acid solution to remove the remaining copper, such as the curved projections 45.

Both ferrie chlorideV and chromlc acid have a tendency to break down the resist 35 if etching is carried on continuously from beginning to end. It seems to be a softening eiect due to the Water present rather than a direct chemical attack. It is found that this diiculty can Ibe avoided by etching in a plurality of short bites, and drying the laminated screen and sheet between bites. Each drying is preferably preceded by a wash in water. Each drying tends to harden the resist lso that it can withstand the action of the etchant during the next succeeding bite. For best results the copper should be removed, using several (preferably four or five) bites with ferric chloride solution and then several (preferably four'or live) bites with chromic acid solution, washing and drying Ibetween each bite; by short bites, I mean bites of about ve, ten or fifteen minutes.-

It is very difficult to regulate the electrolytic etching and obtain uniform etching over the surface of the design, particularly near the end of the etch whenA most of the copper to be removed has been etched out. I have discovered that regulation of the electrolytic etching can be greatly improved if a current bleed is used. In Figure 8e, I show the printing face of the cornposite screen in the electrolytic solution with a current bleeding electrode 50, suitably of sheet iron or of tin plate, surrounding the laminated l surrounding it in the same` plane, and is approximately as far from the cathode as is the composite screen.

As the end of the copper etch approaches, in the condition between that shown in Figure 8c and 8d, the effective current density in the copper to be etched tends to increase to a tremendously high level.

The bleeding electrode maintains a constant relatively large area for current distribution, so that the electrolytic etching conditions change less markedly toward the end of the copper etch than would be the case if the bleeding electrode were not present. The bleeding electrode therefore `operates as a balance or regulating device to maintain relatively constant etching conditions toward the end of the copper etch, when the effective current density from the standpoint of the copper being etched would otherwise increase to an abnormally high value, with a tendency to accelerate the copper etching up to an uncorr trollable rate.

After etching through the copper sheet 2|, the laminated screen and sheet is removed from the etchant, washed with water and dried.

Having removed the copper throughout thel area of the design, it is necessary that the uniting metal layer 21 be removed also at corresponding points. This is accomplished by a second etching as shown in Figure 10` or 10a. The etchant for the second etching is one which will preferentially attack the solder rather than the metallic sheet or the screen. For this purpose, it is preferred to employ one of the usual strong mineral acids forming a soluble lead salt, preferably hydrochloric (muriatic) acid in concentrated form. Instead of hydrochloric acid, nitric acid is suitable, preferably in concentrations between 25 and 40%. Nitric acid is not particularly desirable as an etchant, where the screen predominantly consists of copper, and for a copper screen, hydrochloric acid is much more satisfactory. However, Where a corrosion resistant steel screen is used, nitric acid is fairlyV satisfactory as a solder etchant, although it is less desirable than hydrochloric acid. Any other suitable solder etchant maybeused.

It will be evidentV that lthe solder etchant should preferentially attack the solder ratherl than the metallic sheet or the screen. As already explained, the metallic sheet and the screen will preferably be of different metallic compositions, but this is not necessarily so as they could beof the same metallic composition, but of different composition from the solder. The solder of course protects the metallic screen from attack during the etching of the sheet, no matter what the composition of the metallic screen may be.

While ordinary solder, such as 50 per cent leadtin solder, is very satisfactory for making the solder layer 2l, it has been found that there is a little difliculty in removing ordinary lead-tin solder by etching with an etchant which does not seriously attack the copper during the time of etching.

In order to facilitate removal of the solder layer, it is therefore preferred to use as a solder an alloy containing zinc. Pure zinc is satisfactory for the layer 2l, but very much better soldering results are obtained by an alloy containing about 37% lead, 37% tin and 25% zinc. Even much smaller percentages of zinc incorporated in lead or lead-tin alloys, greatly improve the etching, and advantage will. be obtained by adding any amount of zinc in excess of 1% to the metal ofthe layer 21.

It has been round that the removal ofthe solder layer 21 is a delicate operation, as too prolonged etching will tend to cause undercutting and weaken the joint between the copper sheet and the screen at areas which are blocked out. It is therefore desirable to use special etching technique by which the non-printing side of' the screen is subjected to observation during the latter part of the etching.

In Figure 10, the laminated screen and sheet are placed with the printing surface uppermost on a suitably elevated glass plate 5| desirably surroundedby a confining wall 52. The etchant solution is shown at 53, where it can attack the solder over the area covered by the design, and remove it from between the interstices of the screen at points 54. Over-etching will cause the etchant to travel laterally in the space between the copper sheet and the screen.

To prevent over-etching, the non-printing face of the screen has been made subject to observation by using a transparent resist 28, suitably a transparent lacquer. By the mirror 55,.it is then possible for the operator to view the bottom surface of the screen 20 in Figure 10 through the transparent resist 28. Usually the etching time should be about 20 minutes. As soon as the design is clearly delineated in the mirror and before the spreading of the design is possible, etching is stopped and the composite screen removed from the etchant, washed and dried.

The presence of the layer of lacquer 28 tends itself to preventl creeping since the etchant can not pass around under the screen during etching,

but only attack the solder from the sides in order to spread laterally. Y

Various other techniques may beused to accomplish the etching and` one such valternate technique is shown in Figure a. In this form, the composite screen is etched with its printing face down and the operator watches through the transparent lacquer 28 until the design is clearly delineated on the upper surface, at which time he removes the composite screen from the etchant before the etchant can undercut. The technique of Figure 10 is more efiicient as the etchant will best etch downwardly.

Where very rapid etching is desired, possibly at the expense of a certain amount of accuracy,

it is possible to accelerate the solder etch by employing as electrolytic etch withferric chloride at 'the beginning of the solder etch, and then,after taking a short bite with ferric chloride, washing, drying, and finishing the etching in the manner already described using hydrochloric acid, nitric acid, or some other acid and a plain (not electrolytic) etch. fFerric chloride solution attacks stainless steel and other corrosion resistant steel to some extent, but it attacks the solder much more rapidly and can be used to accelerate the solder etch while doing comparatively little damage to the screen and to the edges of the copper sheet. The ferric chloride bite at the beginning of the solder etch should preferably not be more than about ten minutesV long.

After washing and drying the fully etched composite screen, the lacquer is removed by a suitable solvent, for example acetone, in the case of nitrocellulose lacquer, so that the interstices of the screen 2U are exposed at the design areas as shown at 56 in Figure 11,

In case any difficulty is encountered in clearing the openings of the mesh at points 56, the comproduct, as shown in Figure 12, consists of the screen 20 united tothe metallic sheet 2| `at the blanked out areas by the solder layer 21, and completely exposed in the design areas 56. If desired` the entire composite screen may be lightly plated, for example, copper plated. The view shown in Figure 13 of the printing face illustrates the fact that the blanked out area=is` completely covered by the metallic sheet 2|, the screen being visible only at the printing face in'the design areas 56.`

It should be noted that the outline 50 of the design, as shown in Figure 13, is substantially free from saw-toothing, and is etched smoothly and squarely down, as shown in Figures 8d, 10 and 10a.

In some cases it may be desirable to apply the acid resist to the areas of the metallic sheet which are to be blanked out without the necessity of using a photosensitive layer on the metallic sheet. This can be accomplished, as shown in Figures 14 to 17, by printing the resist through a silk or Wire screen. Figure 14 illustrates thecopy 30 having design areas 3| and areas 32 to be blanked out. The copy 30 is a reverse print of the desired design in that it has light transmitting areas at 32 where it is desired to have the final design located-and light obscuring areas at 3| which are to be nally blanked out. l

The copy 30 is used to make a contactprint on a photographic layer 58 superimposed on a suitable silk or wire screen 51, as well known in the art. After the contact print has been exposed and developed, the screen 51 will be open and free from obstructions at areas 59 (Figure 15) which it is desired to have blanked out in the final product, and will be blanked out at areas 60, Where it is desired to have the,` design in the final product. Y The screen 51 islsuperi-mposed upon the composite screen of the invention whose metallic sheet is uppermost against the screen 51. A suitable resist, such as gelatine, is screen printed in Y the areas 59 to be blanked out, while it is excluded from the areas below the masking layer 6D (Figure 16).

The resultant product after tanning, or other-- Wise suitably hardening the resist, where required, will be the composite screen of the invention as shown in Figure 17, coated with a layer of resist 35 and having the design areas 33eXposed for etching. The product of Figure 17, afteraipplying a suitable transparent lacquer coatingto the exposed face of the screen, is ready for etching according to the procedures indicated in Figures 8 and 10 or 10a and treatment according to the subsequent steps of the invention In the prior art, great diiiiculty has been encountered in obtaining different thicknesses of build-up in the finished printing, and efforts have been made to control the build-up by changing the consistency of the pigment,` changing the completion of printing and lifting ofthe com` posite screen and sheet away from contact with` the surface being printed. The surface on which the printing has taken place is visible at 6l and the raised printing or build-up is shown as B2. In this case the metallic sheet 2i' is relatively thick and therefore the screen 20 has been relatively far above the surface 6 I, on which build-up is taking place, during the printing. Correspondingly there is a high build-up at 62.

In Figure 19 the metallic sheet 2 I2 in the composite screen is much thinner and therefore the build-up 62 on the surface 6| is not nearly so thick.

The solder layer 21 is practically invariable in thickness in all cases and, in fact, its thickness from the edge of the screen adjoining the metallic sheet is almost infinitesimal.. Due to the necessities of drawing technique, the thickness of the solder layer 21 has been exaggerated in Figures 18 and 19.

The thickness of the metallic sheet 2| may vary between 0.0015 and 0.015 inch. For normal sign printing, the metallic sheet will usually be about 0.002 inch thick. For printing of radium dial material, it has been found to be desirable, and in fact, almost necessary, to use a thickness considerably greater and best results are obtained when the metallic sheet is about 0.005 to 0.008 inch thick. Using a metallic sheet of this thickness and with the other conditions as indicated herein, it is possible to obtain very satisfactory radium printing on the faces and dials of various instruments of military importance, with adequate thickness of build-up and adequately large particle size to obtain efcientluminosity, and with accurately defined and sharp edges as required to obtain the precision needed in printing on such instruments of importance to national defense. f

The radium printing pigment employed with the composite screen of the present invention may be any suitable prior art radium printing pigment, but preferably of much higher viscosity than used heretofore for hand application. In such hand application, the radium printing pigment has normally been thinned to the consistency of ordinary paint. For the screen printing of the present invention, the radium printing pigment will normally be used at a viscosity about that of S. A. E. 120 oil. This normally requires no thinner. It is also preferable to employ a slow-drying lacquer, such as a four to seven hour lacquer, in the radium printing pigment used according to the present invention.

One of the advantages of the present invention is that requisite accuracy and control of build-up can be obtained by very rapid operations, but nevertheless the printing face of the screen is extremely resistant to wear and capable of making numerous impressions even on surfaces which are not of perfect regularity.

The user, by stocking various composite screens having different metallic sheet thicknesses, can be prepared to vary the build-up at Will, without changing the .operating technique used in printing, or departing from the usual squeegee procedure.

In view of my invention and disclosure variations and modifications to meet individual whim 4 or particular need will doubtless become evident claim as new and desire to secure by Letters Patent is:

1. The process of producing a printing screen having a design, which comprises laminating together a metallic screen and a metallic sheet by a uniting layer of a solder of different metallic composition covering the exposed face of the screen with a. transparent resist,y blocking out the areas around the design by a resist applied to the exposed face of the metallic sheet, etching through the metallic sheet, finishing by an etchant preferentially attacking the metallic sheet and etching through the uniting layer, finishing by an etchant preferentially attacking the uniting layer, saidtransparent resist functioning to permit observation ofthe etching of the uniting layer'by inspection therethrough.

2. The process of producing a printing screen having a design, Which comprises laminating together a metallic screen and a metallic sheet by solder, coating the exposed face of the metallic screen by a transparent resist, coating the areas of the printing face of the metallic sheet other than the design areas with a resist, etching the design through the metallic sheet, nishing by an etchant preferentially attacking the metallic sheet, and etching the design through the solder, finishing byl an etchant preferentially attacking the solder, said transparent resist functioning to permit observation of the etching of the solder by inspection of the opposite side of the screen therethrough.

3. The process of producing a printing screen having a design; which comprises soldering together in side by side relation a metallic screen of corrosion resistant steel and a metallic sheet predominantly containing copper, coating the exposed face of the screen with a transparent resist, protecting the areas around vthe design on the exposed surface of the sheet which is to be the printing face by a resist photographically applied, etching the design through the copper of the sheet, finishing byv an etchant preferentially attacking the copper, etching the designthrough the solder, nishing by an etchant preferentially attacking the solder, said transparent resist functioning to permit observation of the face of the screen remote from the etching therethrough, and then removing the transparent resist.

.H ARWOOD B. FOWLER.

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

UNITED STATES PATENTS Number Name Date 1,376,365 Wertheirner Apr. 26, 1921 909,831 Strecker Jan. 12, 1909 2,213,237 Brennan Sept. 3, 1940 2,177,877 Pfaffmann Oct. 31, 1939 2,058,365 Stark Oct, 20, 1936 2,233,546 Meulendyke Mar. 4, 1941 2,282,203 Norris May 5, 1942 2,267,787 Ciavola Dec, 30, 1941 2,338,091 Brennan Jan. 4, 1944 1,908,487 Powers May 9, 1930 2,290,554 Hack July 2, 1942 2,288,020 Noland June 30, 1942 f FOREIGN PATENTS Number Country Date 454,162 Great Britain Sept. 26, 1936

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