US1996429A - Formation of printing blocks - Google Patents

Description

April 2, 1935. LUDWGSEN 1 1,996,429

FORMATION OF PRINTING BLOCKS Filed April 24, 1931 (XSAA? Niels Luck/ 9 pNVENTfOR;

all/1 Attorney.

Patented Apr. 2, 1935 UNITED STATES PATENT on-11:

FORMATION OF PRINTING BLOCKS Niels Ludvigsen, Paris, F 1.

Application April 24, 1931, Serial No. 532,532

In France May 2, 1930 6 Claims. (Cl. 101-4013) the same way as the type characters to the paper.

In order to make a typographical block, the screened image, divided into a large number of parts, is reproduced on a metal plate from the photograph which necessitates a special photographic reproduction apparatus, provided with 'a' screen which is generally formed by two transparent glass plates crossed by opaque lines so that the assembly forms a cross hatching.

To obtain the screened reproduction of the desired photograph the screen is placed in the ap-- parts. In the half tones there are small black dots of different sizes since the light projected by the light parts of the image is sufficiently strong to print the more important parts of the sensitive coating. The dark parts of the image are not divided on the negative because these parts during the time of exposure have not projected rays capable of printing the sensitive coating.

When the production of this screened negative or rather of this negative divided into a-large number of small parts by a screen is finished, it

is printed on a sensitive metal plate in the same way as a negative would normally'be printed on sensitized paper. The sensitized coating of the metal plates is generally formed of a bichromatic paste which is hardened at the places where it is printed by the light, while it passes-off into water when being developed at the parts'where the opacity of the negative has prevented the sensitive coating from being printed.

' a metal plate.-

metal plate'as 'abovementioned, it will be seen where it is not protected by enamel and. at the After developing. the parts oftheisensitive coating remaining on the metalare fixed by heating the plate strongly, which transformsthe sensitive layer into a sort of enamel. 1: v

Reference will be made in the following 'de-' scription for clearer explanation of the invention to the accompanying drawing, inwhichz- Fig. 1 shows a screened image reproduced on Figs. 2 and 3 show in sectionparts ofthe metal plate after two'differ'ent stages of etchmg I Fig. 4 shows the screened image'after being etched more than normal.

Fig. 5 shows in-section a metal plate after passing through a further stage of etchingthan the plate shown in Fig. 4. v

Fig. 6 shows in section a blockformed in accordance with the method of the invention;

On examining the image (Fig. 1) obtained onthe that the enamel entirely covers the metal in the dark parts (a) of the image. In the half tones (b) the enamel forms a cross-hatching more or less close up but showingv in thesmall openings (c) of the cross hatching-the bare metal. *The light-parts are theleast 'covered' with enamel there being only-a large number. of smallindependent dots (d) of enamel visible aroundwhich themetalisbare.

As soon as the sensitive coating is fixed by heat etching can be effected by immersing themetal plate in an oscillating metalbath. "During the etching the acid eats into the metal everywhere same time attacking the protective designformed by the enamel. The etching stepshould thus be. stopped before the acid damages the protective coating. All parts of the metal plate-which are covered and protected by enamel ought to. remain intact when the etching is stopped, for it is the totality of this large number'of small parts which forms and reproduces the image; In fact, at the time of printing it is these numerous small parts which receive the ink from the inking rolle'rsin the same way as the typhographical characters and which deposit-the ink onthepaper, thus printing the image at the same time as the typographical characters print the text.

After a stay of six or seven minutes in an os- 50 cillating acid bath the block is finished. It is generally eaten into suflicientlydeep to print especially'if it is a matter of small work. In other cases it is necessary to carry out a .making ready operation, atrade term which-in printing 55 which can be obtained by sticking some paper at the places which correspond to the dark parts either to the back of the block or on the rollers of the printing machine which presses the paper against the block and against the type.

If, with a view to obtaining stronger etching the etching step. is continued, the weakest parts, the small independent dots forming the light parts of the image, would yield tothe action of the acid.

Fig. '2 shows in section the small parts (d) normally etched; Fig. 3 shows the same parts submitted too long to the action of the acid during movement and about to give way owing to being eaten away a little above their bases.

It is absolutely necessary in practice to retain these small dots for they fill a very important function at the time of printing, of preventing the inking rollers and the paper from bearing right on to the metal, which would transform the light parts into dark parts and would render the blocks useless.

After the disappearance of the small dots of the light parts the destruction action exercised by the acid on the design formed by the enamel, would attack the parts forming the half tones. These parts which are larger resist longer but the acid after having enlarged the small openings c (Fig. 1) of the half tones at the same time as itcausesthesmalldots of thelightparts todisappear, commences to eat away the parts forming the half tones and ends by removing them completely.

' The little dots of the light parts disappear after a stay of about ten minutes in an oscillating acid bath, while the parts forming the half tones offer resistance for about twenty minutes before removal. The dark parts comprising masses of metal covered with enamel, resist the acid for a very long time and if, after the half tones have disappeared, the etching is continued the metal would doubtless be pierced through at those places where at the beginning the small dots of the light parts were located before the dark parts would disappear.

During the whole time of etching the action of "the acid always continues. After having eaten away the small dots of the light parts it quickly levels the surface of the metal by removing the bases of the small dots an'd'lthpn eats into the metal more-actively at these places, because there it no longer meets any resistance to its flowing action.

The same action is repeated later when the acid starts to carry away the parts of metal which should form the half tones. The action of the acid above described can be dividedinto three stages.

1st stack-Exercising its destroying action at all points where the metal is not protected by a coat of enamel, the acid hollows the block enough to be able to print typographically and the etching is stopped before any of the parts composing the image have-been damaged.

2nd stage-If the etching is continued the acid carries away the small dots of the light parts and levels the surface of the metal at the places where .the bases of the small dots existed while greatly enlarging .the openings which are in the enamel covering the parts of the metal whichought to print the half tones. Fig. 4 shows the appearance of the protective coating after the not changed in appearance.

3rd staae.while actively attacking the foun-. dation of the metal at the places where the small dots were and where there is no longer any obstacle to its flowing action, the acid eats away the parts forming the half tones and levels the surface of the metal at the places they previously occupied.

After the 3rd stage a metal plate is shown in Fig. where (cl-denotes the places where initially were the small dots forming the clear parts I the half tones and a the comprises a'method of making a printing block including its -making ready step.

This method consists-in'copyin on a metal plate a screened image normally formed and divided by a screen into its light parts and half tones (Fig.1) while on the reverse the other face of the metal plate, the same image is made but formed as if it had passed through the 2nd stage of etching (Fig. 4)

The metal copy thus printed with identical images on the front and on the back is characterized by the difference of the parts forming the image. The image on the front is normally screened in all parts whilst on the image on the back the parts forming the half tones are much .weaker and no traceof the screen or of the protective coating can-be distinguished in the light parts. Only the dark parts of the two images (front and back) are sensibly identical;

On proceeding with the etching of a metal copy thus formed, the image-on the reverse side will be at the end of the-3rd stage of etching (Fig. 5) when the image on the front is at the end of the 1st stage, that is to say, a printing block is obtained which, has as many different thicknesses as there are values (blacks, whites,

and the whole range of half tones) in the image reproduced. On this block the etching in the front is effected normally, while the etching on the back eats away the back of the block so as to form for the latter a making ready of extreme precision. The block has the greatest thickness in the dark parts of the image and the smallest in the light parts. The whole range of half tones forms the intermediate thicknesses. The making ready thus formed allows the production at the moment of printing,'of all the values of the images.

Attempts have been made to obtain this result by the old method of making ready which consists in'covering the back of a block with a large number of small pieces of paper but this method requires much patience and time and cannot be carried out when rapid work is needed. Moreover, this making ready so laboriously obtained generally gets displaced in the course of printing under pressure of the rollersthus increasing its lack of precision. The different In the light parts nothplate'is obtained which has several different thicknesses. Such a methods of metallic making ready hitherto used deform. the image, because they sacrifice the half tones by giving to those parts the same thickness as the light or dark parts. If a half tone is etched on a part of the metal which has the same thickness as the light parts, this half tone loses its value by sinking down at the time of printing. If, on the contrary, the half tone has the same thickness as a dark part this half tone rapidly becomes a dark part by be-- coming smeared with ink and dust emanating 'from the paper and from the metal; obviously this inconvenience can be obviated by stopping the printing machine in order to clean the block but this considerably delays the pulling. Moreover, the screened and hollowed part in the metal forming the half tones is worn out through the friction of the rollers and the paper thus transforming, by premature wear, the half tones into dark parts. i

7 These disadvantages are not present in a block formed in accordance with the method of the present invention. To its advantage of exactitude in the image reproduced, can be added that of quicker printing since the machine need not be stopped for periodical cleaning of the half tones of the block and that of longer duration permitting a stronger pull because the different half tones of the block are etched on parts of the metal having thicknesses corresponding to their, value and giving way slightly at the time of printing instead of offering the same resistance as the dark parts, the wear of which is of less importance because these dark-parts are neither screened nor etched.

The method of the present invention can be carried out in many different ways. The following two methods are given as examples wich are relatively simple.

1st method-On the surface of a metal plate adapted to form the printing block is copied the image of a negative normally screened in all parts and on the other side the image of a negative not divided up in its light parts. If this method is followed, it isnecessary in order that the image copied on the reverse of the metal plate should be formed as though at the end of the 2nd stage of etching, to increase the parts forming the half tones on the negative. This result is easily obtained by immersing the negative in a strengthening bath.

2nd method.-After having copied the image of the negative on the surface (front) of ametal plate which is going to form the printing block, a second metal copy is taken from this same negative and this second copy is submitted to the etching necessary in order that the diiferent parts may be formed as they are after the sec-- 0nd stage of etching (weakening of the half tones and disappearance of the parts forming the light parts). That part of the image still remaining on thiscopy after the second stage of etching is then transferred to the back (reverse) of the first copy before proceeding to the etching of the latter. Then on etching the block the image on the-reverse will be at the end of the third stage when the image on the front is at the end of the first stage of etching.

Other modifications in the method of the invention can be'utilized. In particular the effect produced by the acid on the reverse of the plate can be corrected by protecting certain parts with an anti-acid varnish at those places where it is desired to stop the action of the acid in .order to obtain an effect of darker half tones; or on the other hand the protective coating can be removed prematurely, for example, by scraping in one or other part of the half tone so as to obtain in the corresponding part a weaker print.

According to one variation of the process of the invention two metal copies m and m (Fig, 6) are made separately on each of which the same screened image is caused to appear, such as is represented in (Fig. l)

The plate m which is intended to form the printing face of the block is only submitted to the ordinary etching acid (first stage). I

The other plate m whichis intended to form the making ready" of the blockis submitted on the other hand to a prolonged action of the acid (while going through the three stages described above) which causes the plate to have as many different thicknesses as there are different values in the image (light, dark, and the assembled one against the other for example by adhesives. A compound block is thus obtained which gives advantages similar to those of the block having a single metal plate before described.

The invention comprises not only the abovedescribed process but also as new products printing blocks as obtainedby these processes and particularly to a printing block characterized by having as many different thicknesses as there are values (black, white, and the whole range of .half tones) in the image reproduced.

For example, this block can have three different thicknesses (Fig. 5) ofwhich the thickest corresponds to the darkparts .of the image, the medium thickness to the half-tones and the smallest thickness to the light parts.

This block can be formed of one metal plate as is'shown in Figs. 1 and 5 or of two plates m and m assembled against the other as is shown in Fig. 6.

What I claim is:--

1. A process for the manufacture of printing blocks provided with means for making ready which consists in copying on the frontof a metal plate an image normally formed and divided by a screen into the high light parts and half tones, applying to the reverse side of said metal plate the same screened image, the screen design on said image being weakened according to the depth of shading of portions of the image, no screen design existing in the light portions, and thereafter subjecting the two surfaces of the plate to a single and uniform action of acid.

2. A process for the manufacture of printing blocks provided with means for making ready which consists in copying on the front of a metal plate an image normally formed and divided by a screen in its light parts and half tones, forming a glass negative of the same image said negative being screened in itshalf tones and be-' ing unscreened in its high light parts, immersing said negative in a strengthening bath to strengthen the half tones of said negative, applying an imageof said glass negative to thereverse side of said metal plate and subjecting said metal plate to the etching action of acid.

3. A process for-the manufacture of printing blocks provided with means for making ready" which consists in copying on the front of a metal applying the image of said auxiliary plate in counter manner to the reverse side of said first metal plate and subjecting the latter to the etching action of acid.

4. A process for the manufacture of printing cliches comprising means for a making ready which consists in copying on the front side of a metal platean image normally formed and divided by a screen in the highlight and the halftone portions, establishing an image of the same object having the same contours as the first on another metal plate, attaching the latter, which carries an image having the screen design weakened in the half-tone portion and removed in the highlights, back to back with the first plate and flnallysubiecting the .two surfaces of the combined plate to a single and uniform action of an acid bath. g

5. A printing plate for the manufacture of cliches according to the process claimed in claim 1 and adapted for etching in a single acid bath,

comprising a regular screened image on the front side of the plate and an image of the same object having the same contours as the-first on the reverse side of the plate in which the screen design has been removed in the highlight zones and only slightly visible in the half-tone portions.

6. A printing plate for the manufacture of cliches according to the process claimed in claim 4 and adapted for a single etching operation, consisting of two plates connected to each other, the first metal plate carrying an image regularly screened, the second metal plate pasted back to back-to the first plate and carrying. on its outer surface an image of the same-object having the same contours as the first in which the screen design has been removed in the highlight mnes and is but slightly'visible in the halftone portions.

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