US2394190A - Electrolytic etching of zinc plates - Google Patents

Description

Patented Feb. 5, 1946 s PATENT orrlca ELECTROLYTIC ETCHING 0F ZlNC PLATES Albert R. Kreck, Bel-Wyn, Ill.

No Drawing. Application December 7, 1942,

' Serial No. 468,107

2 Claims.

This invention is directed to a method of electrolytically etching zinc which subject is dealt with in my co-pending application, Serial No. 457,935, filed September 11, 1942. It is particularly applicable to the etching of zinc printing plates where a clean, smooth, even etch is re quired, and although it is particularly adapted -to meet the exacting requirements for printing plates, it is equally adapted for the production of etched plates intended for other purposes.

Heretofore zinc has been etched by nitric acid. Electrolytic etching may prove unsatisfactory in that rough, uneven, and spotty surfaces result from the rapid deposition of a film or coating on the metal which retard or obstruct the current flow, or otherwise interfere with the uniform removal of metallic particles from the exposed areas of the plate which should at all stages present a clean, bright metallic surface which can be etched cleanly and rapidly.

This is particularly true in cases where halftone reproductions are to be taken from photographs and in which the printing surface is afforded by a multitude of minute dot-like elevations which by their grouping produce variations in shading and which are produced by the etching away of the metal exposed in the pattern photographed or otherwise imprinted on the surface of the plate. Unless the etching proceeds with certainty and precision, blurred and fuzzy effects will result which render the plate useless for printing purposes.

'The present invention pertains\ to the method of etching zinc electrolytically using a moderate voltage, approximately 10 to 30 volts, determined according to size of plates, a cathode at an approximate distance of two to three inches away from the zinc plate to be etched and an electrolyte which includes sodium sulphate with the optional addition of one or more of the other sulphates recited below, to which has been added a limited quantity of ammonium thiocyanate which acts as a carrying agent to speed the current flow as it is a strong electrolyte.

I am aware of the fact that heretofore zinc plating has been performed electrolytically, but the problems involved in the etching away of a multitude of minute exposed surface particles to expose in relief the printing surface are problems of a nature quite at variance with those involved in zinc plating in which an even deposition of metal on the surface of the cathode is the end in view and in which the anode serves merely at the positive pole to replenish the metallic content of the electrolyte from which the metallic coating is derived and which would otherwise become depleted as the metal is withdrawn therefrom and deposited on the surface of thecathode.

In zinc etching on the contrary the etched condition of the Zinc anode itself is the subject to which attention must be directed while the metallic particles removed therefrom do not replenish the electrolyte, but rather accumulate as a sludge on the bottom of the tank and which is discarded. The cathode serves merely as the negative pole for the current flow and if any of the zinc particles etched from the plate migrate to the cathode they merely adhere thereto as an easily removable coating which may occasionally becleaned off possibly once a day as occasion may require and do not afford anything in the nature ofa plated surface.

In seeking to provide a bright metallic zinc surface at all stages in the etching operation, I

have found that it'is necessary to employ an electrolyte which will maintain the plate surface smooth and clean and will have no tendency to form a deposition on thezinc anode of any'kind of a surface coating whether in the form of a.

thin soapy film which retards the current flow and requires the voltage to be stepped up, or of a zinc sulphate coating of granular consist ency which tends to retard the etching speed and prevent an even and continuous removal of metallic particles or other scum, coating or oxide of whatsoever character. 1

I have found that electrolytic treatment in .a

solution of sodium sulphate in water will etchthe zinc and carry the current but will result shortly in the deposition of a thin film which slows down the current flow and quickly retards the etching unless the voltage be increased.

It thus becomes desirable to employ an additional agent which does not tend to'cause the deposition on the anode of any film, soapy or otherwise, thus keeping the plate clean and bright at all times and makin it possible to secure reliable and uniform results with substantially constant voltage under a given set of. conditions and with less current flow throughout.

I have found that-the addition to the sodium sulphate solution of a relatively small amount of ammonium thiocyanate as a carrying agent will produce quite satisfactory results although the employment of small amounts of ammonium sulphate and zinc sulphate in addition to the sodium sulphate and the ammonium thiocyanate will improve the result where more exacting requirements are specified.

Satisfactory proportions are stated in the formulas hereinafter given, but it will be understood that considerable variation is permissible since it has been found that an excessive amount of ammonium thiocyanate may speed up the etching action to such an extent that for printing plates it will produce a harsh efiect due to the rapid removal of the metallic surface particles which, however, may not be deemed objectionable for certain industrial requirements.

It will therefore be understood that the invention in its basic aspects is directed to the use of sodium sulphate as the major ingredients with the addition of a minor amount of ammonium thiocyanate in quantity suilicient to act as an additional carrying agent without producing a surface reaction regarded as excessive for the use intended and that the addition of small quantities of ammonium sulphate and zinc sulphate is regarded as a specific refinement to be used in cases where a particularly high standard of quality in the finished plate is demanded.

The following formulae for the production of '72 gallons of solution are given as preferred embodiments of the invention but with the understanding that the proportions given are intended to indicate the general order of magnitude of the various amounts and that the proportions may be varied considerably without materially afiecting the results.

First formula Pounds Sodium sulphate (NazSOO 125 Ammonium thiocyanate (NH4CNS) 3-15 Water in amount sufficient to make approximately 72 gallons of the solution Second formula.

Pounds Sodium sulphate (Na-2804) 125 Ammonium sulphate (NH4)2SO4 3-10 Zinc sulphate (ZnSOr'lHzOl 3-10 Ammonium thiocyanate (NHiCNS) 3-5 Water in amount sufllcient to make 72 gals. of

the solution Third formula Pounds Sodium sulphate (NazSOO 125 Ammonium thiocyanate (NH4CNS) Zinc sulphate (ZNSOr'H-laO) 10 Water in amount sufllcient to make '12 gals. of

the solution Gil surface will under a microscope each present the appearance of a truncated cone enlarged at the base and without the undercutting which is characteristic of acid etching. The conical formation of the points thus permits the electrolytically-etched plate to serve as a master pattern from which to take reproductions in wax or metal which will readily separate from the pattern without any marring of the impression which is likely to occur where undercuts are present.

It has been found that a spacing of about 3 7 inches between the zinc anode and the cathode with a current of 10 to 30 volts according to the size of plate to be etched gives highly satisfactory results with any of the formulae above given although after usage for a considerable period of time the accumulation of waste prodnets in the bottom of the tank in the nature of a sludge a sediment may indicate the desirability of stepping-up the voltage somewhat to maintain the desired etching speed. A copper cath- Water in amount sufficient to make approximate- 151 '72 gallons of the solution and in passing an electric current through the solution from the anode to the cathode.

2. The method of etching zinc plates which consists in employing as an anode a zinc plate having applied upon its surface the pattern to be etched by the removal of the exposed metal to the desired depth and introducing the patterned plate in spaced relation to a cathode within an aqueous solution consisting of the following ingredients in proportions within the general range indicated as follows:

Pounds Sodium sulphate Ammonium thiocyanate 3-5 Ammonium sulphate 3-10 Zinc sulphate 3-10 Water in amount suflicient to make approximately 72 gallons of the solution and in passing an electric current through the solution from the anode to the cathode.

ALBERT R. KRECK.