US1709327A - Halftone photomechanical printing plate and method for producing the same - Google Patents

Description

Aprl 1929- A. H. SPALDING ET AL 1,709,327

HALFTONE PHOTOMECHANICAL PRINTING PLATE AND METHOD FOR PRODUCING THE' SAME Filed Sept. 19. 1925 T f L J 2 -s 7 f f f FG. a e

/2 7 7 I ia. 5 e

91 3( EINEENITORS a 77%), %mm

Ptented Apr; 16, 1929.

UNITED STATES ,1309527 PATENT OFFICE.

mos E. semure, or MANHASSET HILLS, AND JAMES u. CHANKALIAN, or :nw

vom, N. Y., ASSIGNORS ro rowms PHOTO-ENGRAVING COMPANY, A GOBZPOBA- TION OF NEW YORK.

HALFTONE PHOTOMECHANICAL PBINTING PLATE AN D METHOD FOB PBODUQING THE GAME.

Oontiuatio o! application Serial' No. 583,910. filed August 23, 1922, and serial No. ?33,058, filed August 20, 1924. This application filed September 19, 1925. serial No.- 57,274.

The invcntion relates to a novel process for making half-tone printing plates, and lIl certain aspects thercof it relates' more especially to a process for making such plates for use in making stereotyped plates or electro-type plates.

The distinctive objects and advanta es of the invention will be set forth lereina ter m part, and in part will be ob vious herefrom, or may be learned by practcng the nvention. i

The invention consists in the novel articles, step or steps, sequence of steps and processes herein described.

This application is a contnuaton of our two earlicr and copending applications, Ser. No. 583910, filed August 23, 1922 and Ser. No. '733,058, filed August 20, 1924.

The accompanying drawings are dagramnatic in character, as the actual surfaces and parts are microscopic. They will b e explained and referred to later in the specfication.

The invention is directed to providing a process for making half-tone printng plates which will have superior printing pro erties, will correctly and unifornly pro uce the varying tones of the original, and which shall bc especially adapted for making matrices for casting stereotype plates and waX nolds for making electrotype plates.

The process produces plates having the merits or advantages described by reason of the form or structure of the screen figures of the plate, that is, plates having in the shadows clean, clear, open cells or depressions of practically the depth between the dots, or other screen figures, in the lighter and lightcst tones. Such plates also have the screen figures, including the dots in the lightest tones with sharp pointed edges and with smooth downwardly and outwardly sloping sides. There is also a uniformity of tone throughout a given tone, due to the unifornity of the shape of the dots or equivalent screen figures.

The invention finds one of its widest and most important applications in the making of zinc half-tone plates for use in making stereotype plates, as in newspaper printing,

and the exemplary manner of carrying out the process is hereinafter described in the frequently but a few minutes are available from the time a photograph is received until the matrix must be delivered to the stereotype room. Irrespective, however, of the speed of production, the half-tone plates must be ,o such high quality and peculiar and characteristic properties as'to permit a perfect matrix to be made therefrom, and then from the matrix stereotype plates which Will give excellent results even under the difficult conditions of newspaper rinting.

For certain reasons, among WlllCh are the great speed at which newspaper half-tones must be produced, and the relatively great depth required in the etched half-tone plate it is customary and desirable to give severa etchings or bites to the plate. One of the principal faults in half-tone plates, especially in zine plates, which are etched at great speed and to the required great depth for newspaper stereotyping, is the action of the etching fluid in undercutting. This is due to the fact that the acid or etching fluid not only corrodes or eats away the metal vertically but also laterally. In a properly made half-tone negative for newspaper illustration, all the transparent dots, as well as the areas representing the half-tones are much wider than intended in the final result. This is because of the allowance which must be made for the lateral corrosion of the acid or solution used in etching the plate, the action of the acid as is well known being lateral as well as downward.

The positive photographic prints made from such negatives on the Zinc plates, in an acid-resisting material is much stronger than required, to make allowance for this lateral etching. This lateral etching causes the undercutting and creates shoulders or irregularities in the sides of the dots, or other screen figures, and creates difliculty and damage in withdrawing the matrices from the finished half-tone plate in stereotyping, and the wax mold in electroty ing. The impurities present in the metal o the plate, and which usually resist the aeid, also contribute to the roughness of the screen figures constituting the design.

By crcatingthe screen figures in the halftone flatc with true and well-defined edges for their top surfaces, and smooth downwardly and outwardly supporting sides, the matrix or the wax mold may be withdrawn from the plate without in any wise marring or distorting the impression made in the matrix or wax mold by the half-tone late.

By the usual process it is generally ound that one etching of the developed plate does not give the requisite depth, and furthermore when the pl-ate is etched twice, While sufi'icient depth is securcd, the dots or screen figures for the high lights and lighter tones will be formed with shoulders or heads. This is due to the dragon s blood powder or other resist running partly down the sides of the dots or screen figures formed by the first etch, the etching fluid then acting only on the bottom parts of the sides of the dots or other screen figures. At the same time,

the second etch has no efect upon the middle tones and deep Shadows for the reason that the dragon s blood or other resist has praetic-ally entirely filled the depressions made by the first etch.

F urthermore, the re-etching of the plate tends to produce ragged and irregularly formeddots or other screen figures due to the adherence to the side wall of the screen figures of the powder of dragon s blood, or other resist, in more or less granulated or disintegrated form and scattered or disposed irregularly -adherent on the side walls of the dots or other screen figures.

In carrying out our process, in the present preferred commercial practice thereof, a metal plate, such as a zine plate is coated with a light-sensitive coating, and' preferably with -an enanel ,l coating, such as the well-known enanel eonsisting of a solution of glue, white of cggs and ammonium bichromate. v

After this enamel coating is baked on the plate, it receives the usual exposure through the screen negative. Thereafter the exposed plate is developed, usually with cold water, and thereafter a suitable hardener is applied to the developed coating, such as a solution of chromie acid, alum, animonium bichromate and water. It is also a common practice to stain the coating with an aniline dye. The foregoing steps are well-known and are usually praeticed in the art and may be employed in any form or manner best adapted or most suitable as desired.

In the further preparation of the plate by our process, it receives preferably a preliminary, or repeated preliminary etchings, and then a final etehing which produces a late having adequate and usually and preerably equal depth in the depressions between the screen figures in all parts of the plate, and' having screen figures which are true and symnetrieal in form, and having snooth side walls which are downw-ardly and outwardly inclined. The screen figures have the surface and the side walls thereof very smooth, uniform and true and without the irregularity and raggcdness commonly resulting .from the usual or ordinary process ot' plate making.

The first prcliminary etch, applied preferably to a baked enamel plate, when zine is used, may be a dilute nitric acid, say in the proportions of one part aeid to twenty parts water, the etch being applied usually from twelve to fifteen seconds, although all of these may be varied as desircd, so far as concerns the main features of the present invention.

After the etching has been completed, the plate is washed and dried and is then powdered four ways with the dragon s blood or other resist, and is preferably baked after each way of powdering so that the resist will run down and become attachcd to the sides of the dots to protect them from the action of the next etch. v

The next etch is generally similar to the first etch, using the same kind and strength of acid, but the plate is usually etched :from thirty to forty-five seconds, although all these steps may be varied to suit the particular case or work. The' plate is then washed, preferably with an alkalinc solut-on, such as a caustie soda solution which will thoroughly clean the plate down to the enamel.

The plate as so prepared, or otherwise properly prepared, is subjected to the action of a strong or a pure acid, that is, for example, nit-rio acid of full commercial strength, which is applied by means of a suitable brush, such as a glassbrush or one of pig s bristles, the aeid being applied by brushing in all directions, and this is usually and preferably continued until the high light dots reach their extreme point or become absolutely sharp, and the light tone dots proportionatey so, and the depressions between the dots of the middle tones and in the Shadows are formed into clean-walled, open cells equal in depth to the depressions between the high light dots. The aeid as so applied to the plate has a uniform action throughout, the side walls of the dots, or other figures of the etched plate, being free from adhering powder and the tendency of the strong aeid is to avoid undercutting. The high light dots also reach their extreme point, that is, are reduced to a-minimum of area while, if so desired in particular cases, maintaining their height in the general level of the plate. The enamel also retains or remains on the surface substantially intact or unimpaired, as conpared to the action llO of the weaker or greatly diluted aeids, as

the water in the latter has a strong solvent efl'ect upon the eamel.

Consdering the invention next from the standpoint of the structure and structural ste )`s and changes eflected in the plates:

n Fig. 1 a glass screen figure negative 1 is diagranmatically shown, with light-obstructing parts 2 and light-transnitting parts 3.

After this enanel coating is dricd on the plate, it 'cceives the usual exposure to the screen-figure negative. Thereafter the ex-, posed plate is developed, usually with cold water, or with alcohol if it is cold enamel It is also 'a common practice to stain the coating with an anilin dye. These stepsare well known and usually practiced in the art, and may be employed in any'form or manner best adapted or most suitable for the particular work and kind of plates being made.

In Fig. 2 a metal plate 6 is diagramnatically shown having a design thereon formed by the development of its enanel light-sensitive surface after exposure under the negative 1. The parts' 7 are acid resistant enamel and define the design and effect the tone gradations, and the parts 8 are the parts of the metal bar-ed bythe development in the usual manner.

As previously stated, it is usually customary or desirable to give the plate a plurality of etchings or btes, usually three or four, although the exact number is not essential. The first etching is for a short period in a Weak acid solution, which may be dilute nitrc acid, say in the proportion of one part acid to twenty parts water, this etch being applied usually from twelve to fifteen seconds, although the mixture and time may be Varied as found desirable, so far as concerns the main features of the present invention.

As previously stated, this etch eats out the metal unprotected by the developed enamel surface constituting -the design. It eats not only downwardly but also laterally slightly beneath the edges of the protective enamel coating; The result is that each isolated dot is corroded or eaten away on the sides, or undercut as it is termed, and is somewhat'rough on its edges( This undercuttng takes place on the edges of the design, and of all the areas protected on their sui-faces by an. acid resist.

Fig. 3 is a diagram of the plate at this stage, the undercuttings being shown at 9.

To prevent further undercutting in this same region during the second etching, and to greatly mininize the total undercutting by the successive ctches or bites, an acid resist, usually a resinous powder or dragon@ blood is applied to the sides of the screen figures so far as created by the first etching.

To cause this acid-resisting material to adhere all over the sides of the screen figure dots,'the plate is usually powdered our ways, am is leated or baked prefe'ably after every powdering.

When the sides of the screen figures are thus protected (as shown at 10 in Fig. 4), the plate is again etched, and this may be done with -the same etching, fluid, but in present preferred practice the etching is continued for a somewhat longer time, say from acted upon by the' first etching having beenprotected, as stated, from the action of the second etching solution by the resist 10.

In the case of a third etching, or any number of further etchings, this same step is repeated. That is, the portion on the sides of the screen figures WlllCll were affected by the undercutting action of the second etch are protected by an acid-resisting material in the manner previously described. Thus the portons undercut by the second etch are protected from the action of the third etch,

and the undercutting action of the third etch is 'thus both circunscribed and localized, and oecurs only in a relatively still smaller area and both fai-ther 'down and fai-ther out .with respect to the sides of the screen figure. v

The second powdering is shown in Fig. 6 and the third etching in Fig. 7, and the undercutting thercfrom in Fig. 8 (all diagranatically).

'When sufficient depth between the screen figures, and especially between the high light dots have been effected, the resinous powder or other resistant material, adherent upon and protccting the sides of the dots or other screen figures is removed by a proper solvent, but the enanel coating or equivalent acid resisting material 7 is left adherent upon the tops of the screen figures, that is, upon the printing surface of the plate. The acid resist is removed by any suitable agjent an alkaline solution, such as a caustic soda solution, being usually en'ployed. I

The cleaned platc With the steps or shoulders formcd on the sides of the screen figures is shown diagraminatically in Fig. 8.

On exan'ination of the etched plate, it will be found thatthe high light dots are not as fine as they should be and the highest light dots could be brought to sharp points nor lUO are the small cavities in the shadows deep and wide enough. The principal fault, however, with the etched plate at this stage is, that on the sides of each dot, line or other screen figure, there is a series of steps or shoulders, each of these being slightly undereut. These ragged edges and sides ot' the dots, particularly the undercut side surfaees,.interfere with and prevent the taking of a. proper papier-neh nold for stereotyping for a wax mold for electrotyping. Not. only are the screen figures ragged and rough, but 'hey tear and deforn the impressinn in the uold as it is withdrawn from the plate after the inpression-has been made.

By our invention the rough edges of the etehel screen figures are made regular and clean-cut, and the side faces of the screen tigures are made s'ooth and outwardly and downwardly sloping, by the elinination of the irregularities due to the undercutting. Also the etching is deepened in all parts of the plate or locally where desired` and the highestliglt dots are brought to sharp conical points. The dots in the highest lights are brought, in some plates, to sharp points, and these points may be Very slightly below the general plane of the plate surface, as shown at the right in Fig. 9. lVhen the high light dots are very slightly below the general surface of the plate, Valuable advantages are realized for some kinds of work, as -for instance less pressure in the press, longer wear of the plate, and brighter high lights are Secured. The plate is also better adapted for producing better results in stereotyping. There is thus produced a half-tone plate with increased contrast for newspaper printing, as well as having the other desirable and advantageous characteristics set forth. The invention further pro- Yides for doing this very expeditiously, as is required in certain classcs of work, such as newspaper work, as already described.

This is done, in the present preferred procedure, by brushing over the whole plate, which is now protected only by the enamel adherent upon its surface, with a strong aeid, a pure acid being preferrcd, as for example, nitric acid of full commercial strength. This is preferably applied by means of a suitable brush, such as a glass brush, or one of pig bristles, the aeid being applied by brushing in all directions.

This will remove quickly the rough steps and shoulders on the sides of the dots or other screen figures, will deepen the shallow ravities in the shadows, will deepen and widn the etching between the lines and dots of the whole plate and bring the dots in the high lights to still finer points, .and the highest light dots'to sharp conical points. Thus all the screen figures in the half-tone plate have firm clear edges, and the depressions between the dots of the middle tones and in the shadows are formed into clean-walled, open cells equal in depth to the depressions between the high light dots. One of the valuable features or effects of the process is to render coniform all the raised dots in the plate and the sloping of all the sides of the lines in place of the rough, undercut steps 'and shoulders which were left after the several etchings.

This vigorous treatment produces an etched plate with deeper, wider and smoother cavities between the dots, or other screen figures, which per-mit the forcing of a soft papier-mch fluug down into the deepest portions of the half-tone plate and allows the withdrawal of thematrix without injury, and the same is true of the Wax mold, which would be impossible if the inpressions were taken with all the rough edges and the undereut steps and shoulders unremoved. It will be understood that this treatment may be applied to the entire surface of the plate or loeally, wherever desired, but usually it will be applied over the entire plate and then frequently it will be further applied locally.

From the standpoint of the hysical and chemical ageneies, changes and processes involved and occurring, to the best of our present knowledge, the nature and operation of our invention is substantially as follows:

Considering the plate after it has been exposed and developed, and the acid resist .applied thereto, the high light portions of the design consist of spots or cones of the resist surrounded by the unprotected metal 'of the surface of the plate. The shadows of theclesgn appear as a coating of resist 'with tiny holes in it, the bottom of these holes being the unprotected -metaL' The various gradations of middle tones between the high lights and deep shadows are variously and proportionately intermediate in form between these two, that is, between the islands of resist with the intervening bare metal of the plate in the high lights, and the uniform coat of resist with the small holes therethrough in the deep shadows.

Considcring the surface of the plate from a somewhat different standpoint, the shadows nay be consider-ed as a coating of resist with capillary holes therethrough down to the metal surface of the plate, while the high light portions of the plate show very largely unprotected metal with a relatively small proportion of the surface thereof protected by the dots or islands of resist. In these high light areas, the openings through the' resist to the metal surface of the plate are of sufiiciently large dimensione to eliminate the effects that are associated with or are present with holes of capillary size, such as exist in the shadows. There will be a gradation in the middle tone from these free or non-capillary openings of the high lights to away. In this part 'of the plate it is believed that capillary action plays a negligible part, f present atall, because the distances between the cones or islands of resist are rela- 4 tively great, and this is especially so when bottoms of these openings.

the coarser half tone screens have been used in making the negative. v v

V In the eep Shadows, on the other extreme,

capillary portions play an important part;

The dilute etching acid enters the holes or capillary openings in the resist and comes into contact with the exposed metal at the tends to hold the dilute acid, which 'is the product of the etching action, in the hole. There is thus not the free removal of the products of the reaction, as in the case of the etching of the high light areas as already described, where capillary action does not enter appreciatively into ,the process. In the shadow areas, capillarity draws the fresh acid in, and after the etching operation has begun, the action or force drawing in the acid is counter-acted by this same capillary attraction tending to hold the spent solution in the hole.

In the variously gradated middle tones, where the openings vary from slightly larger capillary openings up to the free openings representing a large proportion of the local area, the chemical action is similarly gradated and approximately in corresponding proportions.

As a consequence of these facts, eonditons and physical and chemical actions, the actual amount of etching is greatest in the high lights, gradating decreasingly through the middle tones to the least amount in' the.

Shadows. This is manifested in the area of etch, or etching action, but is manifested more especially in the depth of the etching ation. The relative amount of etch per unit surface, for illustration, may be given arbitrary numbers which would symbolize relative depths of etch. In -a given case these may be 60, 25 and 15 for h gh lght, middle tone and Shadows, respectively.

To change this ratio of etch, it is o nly necessary to change the effect ve drvmg capillary force. For instance, f it s desired to decrease the etch in the Shadows, the etching solution should have a surface tension such that it will enter less readly into strong acid is more viscous than dilute.

Capillary action the capillary. This may be accomplished by using a more concentrated acid, which has a surface tension considerably lower than a. more dilute solution of the acid. Having a lower surface tension, it has less tendeno to go into the capillary, and therefore t etches less.

The high lights, lowever,` are not so affected, since capillarity is negligible or of Very little importance in the high light areas. The gradation and action in the middle tones would vary mately proportionately. strong acid it has been the etching is done in the ample, and again using arbitrary numbers for a given case, the ratio ofetching in the various areas may become say 85, 10 and 5, from the high lights, through the intermediate tones to the Shadows.

It is probable that viscosity acid may also Thus by the use of found that most of of the strong play a part in this action, as It would be harder to get the more viscous solution into the smaller hole and, therefore, the etching would be less in the Shadows relatively to the high lights.

This etching action of the strong acid is apparent. moreespecially in depth than in extent. In this it is different from the dilute acid etch. This is occasioned probably, to the best of our present knowledge and observation, by the fact that the gas evolved during the reaction tends to escape at the juncture ofthe acid and of the metal cones, protected by the patches of resist, which constitute the high light areas of the plate. The gas must escape in order that the etching may go on.

lt would not push through the liquid between the cones or screen figures, for it can more easily escape at the contact surface with the metal cones or screen figures. Now, granting a uniform action over the whole surface, all of the gas from the reaction must pass the surface of the metal where the resist lies. Take a point on the plate in the bottom between two cones or screen figures in the high light, only the gas generated by the local etch tends to protect this surface, while this and all the remainder of the gas generated by surfaces between the middle point on the side of the cone or screen figure and the top thereof must pass a point at the surface of the metal plate, that is, the top of the cone beneath the layer of resist.

This gas tends to blanket the metal and to prevent the acid coming in contact with it. As this blanketing is least at the middle point between the cones or screen figures, it follows that' the etching action will be greatest at such middle point. Consequently the etching action is manifested more n depth than in area, as Compared with a dilute acid etch, which is relatively slow correspondingly and approxihigh light. For exnitrogen.

and in which gas evolution' is not such a or factor.

The gases evolved in the case of the strong acid are principally oxides of ntro'gen mixed with some hydrogen; while in the ease of the dilute acid hydrogen is the principal gas evolved. The top of the conesof metal or screen fi be protected by ocal passivity, due-to the action of the exoessve amounts of oxide of There is no tendency to undercut the resist, for as rapidly as the metal s etched away along the side and top of the cones or screen figures, the strong acid funes, being oxidizing, eat away the resist. With dlute acid, however, -this would remain as a shelf extending over the cone, occasioned by the metal having been dissolved from under the fringe of the resist path or dot.

The' final result of the strong acid eteh, or re-etch is a shifting of the relative amount of metal etch toward the high lights, occasioned by a decrease in the force driving the acid into the capillary, which decreases ,the relative amount of. etch in the areas where capillarity is important, that is, in the middle tones and in'cre'asingly into the Shadows. This ratio is changed, in the arbitrary example heretofore given, say from 60, 25 and 15 to 85, 10 and 5.

There is also'a shifting in the extent of the etch, especially in the high lights to depth of eteh alone, rather than depth and extent laterally of the plate. Added to this, there is a uniform oxidation of the fringe of resist, thus preventing shelving or socalled undercutting;

This application is `a continuation of our applications Ser. No. 583,910, filed Aug. 23, 1922, and Ser. No. '733,058, filed Aug. 20, 1924. The praetical shop method has been continuously and very widely and successfully in commercial use since some little time prior to the filling of the earlicr application, and the practical use of the process is fully disclosed herein, the actual commercial plates produced possessing all the commercial excellencies and advantages described.

The physical and Chemical facts, conditions and actions which have been added by way of elaboration and explanation, are observed with considerable difliculty owing to the microscopic minuteness of structure of the surface of the plate and the rapidity of the various actions, which occur beneath and are largely obsc'ured by the etching fluids. However, the added xplicatory matter represents our best present knowledge of what actually occurs during the process, from the ph sical and Chemical standpoint; but it wi l' be understood that the invention as a valuable and active commercial improveres of the plate may also Chemical theories and explanations given. It will be understood that the various proportions, specific reagents and other details do not limit the invention; but that departures may be made from the details, within the scope of the accompanying claims, Without departing from the princi-` ples of the invention and without sacrificing its chiefadvantages.

What we claim is:

1. The process of making a half-tone printing plate which comprises sensitizing a plate, exposing the plate to light through a toned screen-figure negative, applying a preliminaryetch, applying a resist so as to leave the sides of the screen figuresexposed, and etching the plate to deepen it while producing points with smooth downwardly and outwardly sloping sides, to efi'cct a desired relation of the various tones of the half-tone plate.

2. The process of making a half-tone printing plate which comprises sensitizing the surface of a plate, exposing said surface through a half-tone screen figure negative, etching the plate and protecting the tops only of the tone-producing screen figures and leaving the sides thereof entirely unprotected, and i'c-etching the plate, thereby producing screen figures with smooth downwardly and outwardly sloping sides, to efi'ect a desired relation of the various tones of the half-tone plate.

3. The process of making a half-tone printing plate which comprises sensitizing the surface of a-plate, exposing said surface through a half-tone screen figure negative, etching the plate and repeatedly etching with a Weak acid a plate having a surface of tone-producing screen figures, having a resist thereon and finally etching the plate with a strong acid without a resist, to eii'cct a desired relation of the various tones of the half-tone plate.

4. The process of making half-tone printing plate which comprises applying a light-sensitive enamel layer to a plate, exposing the plate to light through a half-tone screen figure negative, etching the plate, removing the residue of the resist and etching the plate with only its residual enamel as a protection, to eflt'e'ct a desired relation of the various tones of the half-tone plate.

5. A half-tone printing plate having aii enamel top surface with a great number of minute screen figures representing the cor-.

in the shadows of uniform tone value with the intervening dcpressions clean and clear and with smooth sidcs entirely bounding the respective figures.

7. The process of producing'a half-tone plate which comprises placing a light-sensitive enamel layer on a plate, exposing the plate to light through a half-tone screen figure negative, and etchingthe plate including the steps of localizing the undercutting entirely about the separated screen figures by repeated etching, and then renov ing the resultant roughnesses from the sides of the screen figures of the plate, to cfi'cct a, desired relation of the various tones of the half-tone plate'.

8. The process of producing a half-tone plate which comprises placing a light-sensitive enamel layer on a plate, exposing the plate to light through a half-tone screen figure negative, and etching the plate including the steps of localizing the undercutting entirely about the separated screen figures by repeated etching, and then removing the resultant roughnesses from the sides of the screen figures of the plate bythe application of a strong aeid, t etect a desired relation of the various tones of the half-tone plate.

9. The process of producing a half-tone plate which comprises placing a light-sensitive enamel layer on a plate, exposing the plate to light through a half-tone screen figure negative, and etching the plate by a plurality of biles and proteoting the undercutting entirely about the separated screen figures made by one bite from the action of a succeeding bite, and; removing the resultant roughnesses from the sides of the screen figures of the plate, to efi'ect a desired relation of the various tones of the half-tone plate.

10. The process of producing a half tone plate which comprises etching the plate by a plurality of bites and protecting the undercutting made by one bite from the action of a succeeding bite, and removing the resultant roughnesses from the sides of the screen figures of the plate by scrubbing the plate with strong acid.

11. The process of producing a halftone plate which comprises creating an enamel design on the surface of the plate, etching the plate by a plurality of bites, and protecting the undercutting made by one bite from the action of a succeeding bite, and removing the resultant roughnesses from the sides of the screen figures of the plate by scrubbing the surface of the plate with acid with all resist removed except the surface enamel.

12. A half tone printing plate having the high light dots with smooth conical sides terminating in points very slightly below the plane of the surface of the plate.

13. The process. of producing a half-tone -printing plate which comprises sensitizing the plate with an enamel layer, exposing the plate to light through a half-tone screen' figure negative, repeatedly etching the plate entirely about the separ'ated screen figures, with solutions having varying surface tensions to regulate the relative depth and lateral extent of the different etches, to eflt'ect a` desired relation of the various tones of the half-tone plate.

14. The process of producing a half-tone printing plate which comprises sensitizing the plate with an enamel layer, exposing the plate to light through a half-tone screen figure negative, repeatedly etching the plate entirely about the separated screen figures, with solutions of differing degrees of viscosity to regulate the relative depth and lateral extent of the different etches, to efl'ect a desired relation of the various tones of the half-tone plate.

15. The 'process of producing a half-tone print-ing plate which comprises sensitizing the plate with an enamel layer, exposing the plate to light through a half-tone screen figure negative, repeatedly etching the plate entirely about the scparated screen figures, with solutions, and varying the character of a succeeding etching solution to secure depth of etch with practically no lateral etch.

16. The process of producing a half-tone printing plate which comprises applying a light-sensitive layer to the surface of the plate, exposing and developing the plate to create screen figures thereon, preliminarily etching the plate with dilute acid, and subse quently etching the plate with strong acid without a resist being applied preliminary to said etching.

17. The process of producing a halt-tone printing plate which comprises applying an enamel light-sensitive layer to the surface of the plate, exposing and developing the plate, to create screen figures thereon, baking the developed enamel coating, preliminar'ly etching the plate with dilute acid, and subsequently etching the plate with substantially undiluted aeid without a resist being a plied preliminary to said etching.

18. T e process of producing a half-tone printing plate which comprises applying a light-sensitive layer to thesurface of the plate, exposing and developing the plate, to create screen figures thereon, preliminarily etching the plate with dilute acid, applying a resist to the surface of the plate, etching the plate with a dilute acid, and subsequently etching the plate with strong acid without a resist being applied prelininary to said etching.

19. The process of producing a half-tone printing plate which comprises applying an enamel light-sensitive layer to the surface of the plate, exposing and developing the plate, to create screen figures thereon, baking the developed enamel coating, prelimnarily etching the plate with dilute acid, applying :L rcsist to the surface of the plate, etching the plate with a dilutc acid, and sul.)- sequcnt-ly etching the ulate with stron aeid without a resist being applierl prelin-unary to said etching.

20. A halt-tone printing plate having its screen figures in the high lights uniformly reduced to points and in the Shadows of uniforn tone value with the intervening depressions clean and clear and with smooth sides.

21. The process of proclucing a photomechanieal printing plate which comprises applying a light-sensitive layer to the surface of the plate, exposing the platc through a halt-tone screen-figure negative and (le- Veloping the plate, prelininarily etching the plate with (lilute acid, and subscquently directly applying a strong etching acid hy frictional means applied on the surface of the plate.

22. The process of 1'roducing a photonechanical printing plate -which eomprises applying a light-sensitive layer to the surface of the plate, cxposing the plate through a half-tone screen-figure negative and (leveloping the plate, prelininarily etehing the plate with dilute acid, and suhsequently directly applying a strong etching aeid (lirectly to all parts of the design except the faces of the screen figures.

23. The process of producing a photomechanical printing plate which comprises applying a light-sensitive layer to the surface ot the plate, cxposing the plate through a halt-tone screen-figure negative and cleveloping the plate, applying an acid resist to the developed surface of the plate, pre-.

lininarily etching the platc with dilute acid, and subsequently directly applying a strong etching acid to the plate.

24. The process of producing a photomeehanical printing plate which conprises applying a light-sensitive layer to thesurface of the plate, exposing the plate through a halt-tone screen-figure negative and de- Veloping the plate, applyin an acid resist to the developed surface ot the plate, preliminarily etching the plate with dilute acid, and subsequently directly applying a strong etching acid by frictional means applied on the surface of the plate.

25. The process of producing a 'photomechanical printing plate which eomprises applying a light-sensitive layer to the surface of the platc, exposing the plate through a halt-tone screen-figure negative and. de- Veloping the plate, applying the acid resist to the developed surface of the plote, prclininarily etching the plate with dilute acid, applying an acid resist to the etched surface, etehing the plate with a dilutc acid, and subsequently directly applying a strong etching acid to the entire surface of the design on the plate except the faces of the screen figures.

26. The process of producing a photoncchanical printing plate which comprises applying a light-sensitive layer to the surface of the plate, exposing the plate througia half-tone screen-figure negative and developing plate, prelininarily etching the pinte with dilute acid, and subsequently directly applying a strong etchiug acid to the plale With the sidcs ot' the screen figures unprotected by any rcsist.

27. The process of producing a halt-tone printing plate which comprises applying an enanel to the surface ot' the plale to create screen figures thercon,exposing and dcvcloping the plate, prelininarily etching the plate with dilute acid, removing all resist except the enanel, and subsequently etching the plate with strong acid without a resist being applied prcliminary to said etching.

28. Ihe process of produeing a half-tone printing plate which comprises applying a light-sensitive enanel coating to the pl-ate, exposing the plate through a halt-tone s'l'ctll figure negative, applying a resist to 'hc platc, and etching and cleaning the plate with a strong aeid to insure the renoval of all resistaut matter except the cnanel surface ot the plate.

29. The process of producing a half-tone plac to he used alone in printing, which conprises the steps of exposing a lightsensitive plote through a halt-tone screen figure negative to truly reproduce the light tones ot the original, repeatedly etching the plate, localizing the uudercutting by repeated etching, and then removing the resultant roughnesses from the sides ot' the screenrfigures of the plate.

30. The process of producing a half-tone plate to be used alone in printing which conprises the steps of exposing a lightsensitivc platc through a half-tone screen figure negative to truly rcproduce the light tones of the original, repeatedly etching the plate, localizing the undercutting hy rcpeated etching, and then removing the resultant roughnesses froni the sides ot the screen figures of the plate by the application ,of a strong acid.

31. The process of producing a halt-tone plate to be used alone in printing, which eonprises the steps of cxposing a lightsensitive plate through a half-tone screen figure negative to truly rcproduce the light tones of the original, repeatedly etching the plate, localizing the undercutting by repeated etching, and then removing the resultant roughnesses from the sides of the screen figures of 'the plate by scrubbing'the plate With aeid.

32. The process of producing a half-tone plate to be used alone in printing, which llO comprises exposing a light-sensitive plate through a half-tone screen figure negative to truly reproduce the light tones of 'the original, repeatedly etching the late, etching the plate by a plurality of bites and protecting the undercutting made by one bite from the action of a succeeding bite, and removing the resultant roughnesses from the sides of the screen figures of the plate by the application of strong acid.

33. The process of producing a half-tone plate to be used alone in printing, which comprises exposing a light-sensitive plate througha half-tone screen figure negative and creating an enamel design on the surface of the plate, etching the plate by a plurality of bites, and -protecting the undercutting made by one bite from the action of a succeeding bite, and removing the resultant roughnesses from the sides of the screen figures of the plate by scrubbing the surface removed except the surface enamel.

34. The process of producing a half-tone -plate to be used alone in printing, which comprises exposing a light-sensitive plate through a half-tone screen figure negative and creating an enamel design on the surface of a plate, etching the plate, deepening the plate and sharpening and smoothing the screen figures ofthe plate by scrubbing it with acid with the plate unprotected except by the enamel of the surface design.

35. A half-tone printing plate to be used alone in printing, having the high light dots with smooth sides terminating in points very slightly below the plane of the surface of the plate.

In testimony whereof, we have signed our names to this specification.

AMOS H. SPALDING. JAMES M.- CHANKALIAN.

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