US2135873A - Process of making metal reflectors - Google Patents

Description

Nov. 8, 1938. F. L. JONES ET A1. 5, 3

PROCESS OF MAKING METAL REFLECTORS Filed Nov. 22, 1934 FRANK L. JONES RAYMOND J. KIRCHMAIER INVENTORS ATTORNEY Patented Nov. 8,

UNITED" STATES PATENT OFFICE PROCESS OF MAKING METAL REFLECTOBB Application November 22, 1934, Serial No. 754,316

2 Claims. (01. 204-8) The present invention relates to reflectors and more particularly to metallic reflectors having a reflecting surface of silver and an electrodeposited strengthening back of metal on thesilver reflecting member and to a method and apparatus for forming such reflectors.

In the manufacture of such reflectors, it has heretofore been the practice to grind and polish a glass or metal mold and to deposit chemically a layer of silver upon the mold. This mold with its layer of silver is then placed in an electroplating bath and a heavy back of copper is electrolytically deposited upon the silver. After the back has attached a suitable thickness, the reflector and mold are withdrawn from the bath and subjected to a gentle heating and'the difference in expansion between the reflector and the mold permits the reflector to be stripped from the mold. Such a reflector follows accurately the curvature of the mold and no polishing or grinding are necessary. The silver reflecting-surface is then given a light coating of some nontarnishing metal such as platinum or palladium or rhodium to protect the silver from corrosion.

This process has certain inherent difficulties which make it unsatisfactory. A copper plating bath capable of fast plating at a high current density must be strongly acid and this acid solution readily attacks the sflver. The edge of the silver layer on the mold is particularly vulnerable and any acid at this edge is drawn across the silver reflecting surface by capillary forces during the time the mirror is plating or when the reflector is stripped from the mold. This stains the silver surface and thus spoils the reflector. Pinhole or minute defects in the silver coating entrap small amounts of acid spoiling some mirrors. The present invention eliminates these inherent defects in the prior art.

One of the objects of the present invention is to provide a new and improved metallic reflector and a new and improved method and apparatus for producing same. Another object is to provide a new and improved method and apparatus for electrolytically depositing a metallic back upon a metal reflecting member. A further object is to provide a metal reflector having a metallic back electrodeposited upon a selected portion and to provide a method and apparatus for producing same. A further object is to provide a method and apparatus for electrolytically deositing a metallic back upon a silver reflecting member in which the acid plating bath is kept out of contact with the edge of the silver member. A further object is to provide electroplating method and apparatus in which the article to be plated forms the bottom of the apparatus. A further object is to provide an improved method of electrolytically depositing a heavy back of copper upon a silver reflector which comprises forming a thin protective coat of copper upon the silver from a neutral copper solution before plating the heavy copper coating from an acid copper bath. These and other objects and advantages reside in certain novel features of construction, arrangement and combinations of parts and processes as will hereinafter be more fully set forth and pointed out in the appended claims.

Referring to the drawing:

Fig. 1 is a vertical section of our apparatus for electrolytically depositing the heavy copper back.

Fig. 2 is a vertical section of a mirror made according to our process just after removal from the plating apparatus.

Fig. 3 is a vertical section of a finished mirror made according to our process.

In the drawing l0 indicates a glass mold which is ground and optically polished upon its convex side and a layer ll of silver is chemically deposited upon this polished surface as is. well known to those skilled in the art. The silvered mold Ill is then placed in a neutral copper plating bath and a thin coating I! of copper is electrolytically deposited upon the silver. The current density is kept low, preferably to about one ampere per square foot; By the term neutral copper plating bath is meant a copper electrolytic soluton which is not strongly acid. Such a solution will not attack the silver although only a thin coating of copper can be deposited. Among such solutions are, a water solution of disodium diaquodioxalatocupriate, a mixture of copper sulphate and ammonium sulphate, and a mixture containing equal parts of copper sulphate and Rochelle salt dissolved in water with sufficient ammonia to dissolve the precipitate. Obviously other neutral copper plating baths will be satisfactory and the solutions are given above merely by way of example.

The thin skin of copper formed on the silver in this bath is suflicient to completely protect the silver for some time even in a highly acid plating bath. The mold II] with its silver layer II and the copper coat I2, is then placed in a strongly acid copper plating bath such as a bath containing a saturated solution of copper sulphate and thirty to forty grams of sulphuric acid per liter. In this bath, a heavier current density is used, preferably about thirty amperes per square foot and a thick skin I! of copper is deposited upon the thin copper coat l2. The plated mirror is removed after about minutes, at which time the skin of copper is approximately one one thousandth of an inch in thickness. This length of time in the acid plating solution is so short that the edge of the silver is still adherent to the glass mold and acid has not penetrated under the silver. The mold l0 carry ing the silver I l with its copper coats I 2 and I 3, is washed and dried after its removal from the bath.

A heavy copper reinforcing back I4 is then electrolytically deposited upon a selected area of the copper layer l3, usually upon the central portion. For this step is used a cylindrical tube I! open at both ends having a non-conducting and acid proof lining l8 and having a resilient non-conducting gasket [1 at one end. The coated mold I0 is placed with its convex side against the gasket i1 and is tightly held in that position by a conducting plate It and clamps it. Thus, a fluid tight joint is obtained between the reflector and the tank so that the reflector can serve as the bottom of the tank and the electrolytic solution be kept out of contact with the edge of the reflector.

A reinforcing ring or web 20 may be placed in the tank It on the back of the reflector. An electrolytic solution 21 is then placed in the tank. A copper anode 22 is suspended in the solution 2| and a voltage is impressed between the reflector and the copper anode 22 by a source of electric power 23. In this way a thick copper reinforcing back It is deposited which firmly and permanently attaches the ring or web 20 to the reflector. A stirring paddle 24 is placed in the solution 2| to keep the solution agitated and insure an even deposit of metal and obviously, this paddle could serve as the anode.

When the copper back I! has reached the desired thickness, the reflector is removed from the tank and stripped from the mold ID as shown in Fig. 3. The thin sheet comprising the silver I I and the copper layers l2 and i3, is cut off beyond the reinforced back It as indicated in Fig. 3. The silver reflecting surface is then given a light coating of some non-tarnishing metal such as rhodium and the reflector is ready for use.

While the mirror has been shown and described as forming the bottom of the plating tank in which the back is formed, quite obviously it could be put in other locations and the back could be deposited equally as well. For example, a tank could be made with several openings about the side walls and a mirror clamped againsteachopening. Inthiswaystiifreinforcing backs could be plated onto several mirrors at the same time. The only requisite is that the strongly acid plating solution used in depositing the back should not come in contact with the edge of the mirror and this is best accomplished by keeping the edge entirely outside of the tank.

Copper has been specified as the material used for the two coats and the back but it is obvious that another metal or metals could be used for one or more of these layers in place of the copper. Such metals would readily suggest themselves to those skilled in the art and the process of depositing them would be substantially similar to the process outlined above.

From the foregoing it is apparent that we are able to attain the objects of our invention and provide a new and improved metal reflector and a new and improved method and apparatus for producing such a reflector. Various modiflcations can, of course, be made without departing from the spirit of our invention.

We claim;

1. The method of forming a reflector which comprises depositing a reflective layer of silver upon a mold, electrolytically depositing a thin layer of copper on said silver substantially co extensive with said layer of silver, electrolytically depositing a thick metallic back upon only a selected portion of said copper and removing the copper and silver beyond said selected portion.

2. The method of making a reflector which comprises depositing a layer of silver upon a mold, electrolytically depositing a thin layer of copper upon said silver from a neutral copper solution using a low current density, electrolytically depositing a heavier layer of copper on said thin layer from an acid copper solution using a higher current density, restricting a highly acid copper solution to a portion of said heavier layer remote from the edge thereof, and electrolytically depositing a thick copper back upon said heavier layer of copper.

FRANK L. JONES.

RAYMOND J. KIRCHMAIER.

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