US2333020A - Process for the preparation of printing plates - Google Patents

Description

Patented on. 26, 1943 PROCESS FOR. THE PREPARATION OF PRINTING PLATES Adrian Eugene Le Roy and Jeannette Le Roy,

New York, N. Y., assignors, by memo assignments, to Lectrocut Corporation Limited, a corporation of the Province of Ontario, Canada No Drawing. Application May 6, 1939, Serial No. 272,140

1 Claim.

This invention relates to printing plates, particularly half-tone zinc plates, including the process of making the same and the sensitizer and etching bath for same.

It is customary to make printing plates of zinc or copper by coating the plate with a sensitizer; impressing the image of the object to be printed on the plate; washing the exposed plate; drying it; burning it in; and then placing. it in an acid bath for etching. The etching process usually involves considerable time and care, including agitating the plate or etching fluid, stopping out with an acid resisting varnish those parts which have been etched to the required extent, and making supplementary etchings when necessary.

The object of the present invention is to devise a process and means for etching the plate in a single quick operation requiring no special skill.

The object of this invention is achieved by using a zinc plate and etching in an acid copper plating bath in which. the zinc plate is the cathode. More specifically the invention involves the useof a special sensitizer, plating or etching bath, and electric current having a. distinctive characteristic, all of which are hereinafter more particularly described.

Ounces Water 96 Ammonium bichromate 3 Ferric ammonium citrate 1 Fish gl 42 Ammonia water".- .5

This emulsion is prepared in the usual way and it is not necessary to describe the manner of preparing the emulsion and applying it to the plate.

The developer and desensitizer for the plate are the same as in general use and need not be described.

when the photographic image has been formed on the plate and it has been processed in the usual manner and is ready for etching it is placed in the etching bath. The etching apparatus consists of, in effect, ordinary electro-plating apparatus, including a copper anode and a supporting bracket for the plate which is to be etched, the apparatus including a source of electrical current and being connected up in such a way that the plate comprises'the cathode.

The etching solution may be said in a general way to be the same as the ordinary acid copper plating bath, with, however, a higher proportion of acid. In this connection it should be pointed out that owing to the action of acids on zinc, an acid bath is not usually used for plating zinc, and in this respect the present process diflers essen- V tiaily from an ordinary plating process.

Preferably also, there is added to the bath cobaltous chloride which has the effect of preventing pitting of the printing plate and a small amount of ferric ammonium citrate for a purpose hereinafter descrlmd.

A typical bath contains the following constituents in the following proportions:

Water (mixed at 140 2-1). 1 gal. or 3,785 mi.

Copper sulphate new... 3 lb. or 1,360.8 Cold water 3 gal. or 11,355 mi. Sulphuric acid 2% oz. or ml.

to which is added i. 02. or 29.6 ml. of the iollowing solution:

Cold water 20 oz. or 591.4 ml. Dye (Hercules red) Ii oz. or 29.6 ml. Cobaltous chloride 10 gr. or 0.648 gm. Ferric ammonium citrate 8 gr. or 0.518 gm.

The following is a suitable replenishing solution for the bath:

When the plate has been suspended in the bath a current oi. substantially 6 volts 6 amperes is passed therethrough for substantially six minutes, for making a 5" x 7" plate, at the end of which time the etching will be complete and the plate may be removed, washed and dried and is ready for use. The amperage per square inch of cathode is appreciably higher than used in the ordinary copper electroplating process.

The eflect of the Process is that the acid etches the zinc around the half-tone dot formed by the emulsion, and copper from the bath is deposited on top 01' the dot, being attracted by the chromium content of the emulsion which forms the dot. In fact, a spongy copper deposit covers the whole plate, but does not interfere with the etching around the dots and when the process is completed the soft copper may be readily scrubbed off leaving a minute but readily observable hard deposit only on top of the dots formed by the sensitive emulsion.

The addition of the ferric ammonium citrate to the sensltizer forms an iron deposit which improves the plating effect, and the addition of ferric ammonium citrate to the plating or etching solution forms a contact between the solution and the iron in the sensitizer. It is thought that the iron in the ferric ammonium citrate increases the conductivity of the sensltizer which forms the dot and of the solution, thus concentrating the plating eflect on top of the dot, whereas between the dots the acid acts on the zinc to etch. it. The result is that there is a simultaneous building up of the dots, or at least a hardening of the top of the dots to protect them from decomposition, and etching around the dots so that a clear cut image is etched in a comparatively brief space of time. The process is remarkable for the uniformity of the etching, as well as the speed with which it is done, and the absence of undercutting of the dots. This makes it unnecessary to remove and treat the plate from time to time and makes it possible to obtain good results without the exercise of the skill which is usually involved in ordinary processes when it is necessary to agitate the bath and watch the plate carefully as the process proceeds and treat it and touch it up.

It should be noted that this process has nothing in common with the known method of producing copper plates by deplating between the dots, in which method the printing plate is the anode.

What we claim as our invention is:

In the process of making a zinc half-tone printing plate, the steps of impressing an image through a half-tone screen on a zinc plate coated with a light sensitive emulsion to which has been added a small proportion of ferric ammonium citrate; preparing the plate for etching by developing, washing, drying, and burning it in; immersing the prepared plate in an acid copper plating bath in which the plate is the cathode, said acid copper plating bath consisting of a solution consisting or the following ingredients in substantially the following proportions:

Water mm 4 Copper sulphate lbs 3 Sulphuric acid oz 24 ADRIAN EUGENE LE ROY. JEANNE'I'I'E LE ROY.