US2753298A - Method and apparatus for continuous plating - Google Patents

Description

July 3, 1956 w. R. GRAY 2,753,298

METHOD AND APPARATUS FOR CONTINUOUS PLATING Filed Oct. 2, 1952 INVENTOR W. R. GRAY,

BY KA MW ATTORNEY United States Patent METHOD AND APPARATUS FOR CONTINUOUS PLATING William R. Gray, Sarver, Pa., assiguor to Pittsburgh Plate Glass Company, Allegheny County, Pa., a corporation of Pennsylvania Application October 2, 1952, Serial No. 312,807

7 Claims. (Cl. 20428) This invention pertains to electroplating, and particularly to a method and apparatus for continuous copper plating, having special features which are adapted for continuously applying a protective copper plating to the silver deposit on mirrors.

The continuous electroplating of metallic plates, sheets or similar shapes has been accomplished in various ways, and many efforts have been made to adapt such systems to the application of a copper layer upon the silvered sides of mirrors. In the manufactureof mirrors, sheets of plate glass are silvered on one side by methods well known in the art, and the resulting silver layer is extremely soft and sensitive to abrasion as well as chemical deterioration upon exposure to atmospheric elements. Even the application of protective varnishes, which usually have substantial viscosity, involves considerable chance of mechanical deterioration of the silver deposit, and thus the manufacture of high quality mirrors has been complicated and rendered unduly expensive by reason of the care which must be exercised with reference to the silver layer, or by reason of the considerable number of rejects which are produced.

It has therefore become common practicein the mirror manufacturing industry to apply to the freshly deposited silver layer a protective layer of another metal, copper being the usual choice. Such a copper layer can readily be formed by electroplating, and with comparatively little chance of damaging the silver reflecting layer.

One great objection to the copper plating of mirror surfaces has been the fact that the pieces being plated have had to be handled as individual articles, one or more being positioned in the plating tank and electroplated and thereafter removed from the tank for further processing. This procedure has the usual undesirable features of intermittent or batch operations, including the use of considerable skilled labor and the requirement for a sufficient number of separate plating tanks and other machinery to handle the desired production rate.

As a result of the above situation, mirror manufacturers have long sought for some process or apparatus which would enable them to achieve the desired copper plating in a continuous manner, but such efforts have been unsuccessful because of the difliculties in handling the fragile silver layer carried .bythe glass sheets, as well as the difiiculty of controlling the rate of copper deposition so that a uniform coat of sufficient thickness is obtained.

It is therefore a principal object of the present invention to provide a method and apparatus for the continuous electroplating of a protective metallic layer upon the silvered side of partially completed mirror sheets.

A further object of the invention is to provide a method and apparatus of the type indicated in which the desired coating of protective metal is obtained at a speed sufficiently great to enable a single unit of apparatus to pace existing commercial silvering production lines.

Still another object of the invention is to provide a method and apparatus for the continuous copper plating of the silver deposit on mirrors or the like, without injury 2,753,298 Patented July 3, 1956 to the silver layer, whereby a maximum output of mirrors of uniformly high quality is assured.

Still another object of the invention is to provide a meth- 0d and apparatus of the kind indicated above which will permit efficient utilization of the chemicals utilized in the plating process.

With the above and other objects in mind, the invention will best be understood from the following detailed specification of the method employed, and of one preferred form of apparatus for carrying out the method, reference being had to the appended drawings, forming a part hereof, and in which:

Fig. 1 is a side elevation, partly broken away and partly schematic, of a preferred apparatus in accordance with the invention,

Fig. 2 is a fragmentary sectional view taken on the line 2-2 of Fig. 1, and

Fig. 3 is an enlarged isometric view showing one of the cathode rollers of the previous views and a preferred means for mounting the same in the apparatus.

Briefly, the method of the present invention depends for its success upon maintaining a relatively thin layer of the plating solution in contact with the silver film which .is to be plated, and passing the plating current from an anode which defines one surface of said thin film of electrolyte to the silvered surface, which then completes a return path to the source of current. The return current is collected by making contact to the silver layer in such a Way that no sliding contact is involved, such as might result in damage to the silvered layer. Inasmuch as only a small quantity of plating solution or electrolyte is actually in use at any one instant, due to the thinness of the electrolyte film, it is necessary that the same be continuously replenished. Therefore, the method also contemplates the continuous circulation of plating solution, so that the chemical composition of the part represented by the thin film can be kept relatively constant.

The plating solutions employed may be those ordinarily used in known intermittent plating processes. Thus, such a conventional solution may contain 10% of copper sulfate and 1% sulfuric acid, by weight, the balance being water. Such a solution has a pH of about 2. As is well known, the acid content may be reduced, and the pH thus raised, to reduce the corrosive effect of the solution on the Pounds Copper sulfate 50 Rochelle salts 60 Triethanolamine 45 Water to make gals.

Such a solution will have a pH of about 7, and has the advantage that the corrosion problem is greatly simplified.

In accordance with the invention, the plating solution is maintained in a film about inch thick on the silver layer. This is accomplished by supporting the silvered glass sheet in a horizontal position for movement beneath a transverse copper anode so arranged that a space of about /s inch is provided therebetween, and the plating solution is flowed upon the surface of the glass sheet in such quantity so that it fills the space between the sheet and the anode. An excess of solution is supplied, and the run-ofi can readily be collected and recirculated through a reservoir of such capacity that the composition of.the solution varies only slightly with time. Fresh solution "or make-up chemicals may of course be added from-time to time,-as required.

It was found that silvered sheets of commercial size, for example, as wide as 96 inches in the directionof anode length, could be treated at-satisfactoryspeeds-by'moving the silvered sheets continuously beneath the anode in such a way as-to maintain the thickness of the film at approxi- 'mately inch, as indicated above.

"However, it was also found that while the rate ofdeposition could beincreased by increasing the dimension of the anode in the direction of plate travel, this dimension must be kept to a value not over 20 inches. The use of wide anodes resulted in inability to maintain the solution as a continuous film, so that certain portions of thesilver-layer received insufficient copper deposit.

The return path for the plating current is provided by a multiplicity of highly polished copper cathode rollers Time,

I Copper seconds Deposit, Gms./sq.' it.

Amperes Inasmuch as a satisfactory copper film requires an average deposit of the order of 0.1 gram of copper per square foot, a practical apparatus capable of maintaining a commercially satisfactory linear speed requires a somewhat wider anode, a larger current, or the use of more than one anode. A preferred embodiment of commercial apparatus for carrying out the method is shown in the appended drawings, to which reference is now made.

The drawings illustrate, partially in schematic fashion, a preferred embodiment of an apparatus by which the novel method may conveniently be practiced. InFig.

1, there is shown a plurality of spaced-apart rollers 10,

which may be coated with rubber or like resilient material, and arranged with their axes lying in a common horizontal plane so as to form a support for a sheet or sheets of glass 12, these sheets having on their upper surfaces the silver reflecting coating needed for mirror operation. In Fig. 1, the sheet or sheets 12 travel from left to right as indicated by the directional arrow, and

are propelled by any conventional means.

In the apparatus shown, there are two anodes and two sets of cathode rollers, the rollers of each set being carried by transverse beams 14 and 16, and the rollers themselves being shown at 18. Each roller is formed as a highly polished disc of copper, and the rollers comprising each set are maintained substantially in coaxial alignment by means to be described. Inan apparatus suitable for handling sheets 12 of the order-of 48 inches wide, for instance, there may be eighteen ofthese rollers 18, spaced about 2 /2 inches apart.

The anodes in Fig. 1 are shown as flat plates having their leading and trailing edges upturned, this construction aiding in the maintenance of an uninterrupted film of electrolyte or plating solution on the glass sheets 12. As shown, the anodes 20 are located and suitably 'supported so that their under surfaces "are about /8 inch "above the surfaces being plated, and they extend (in the tance at least equal tothe width of saidsheets.

are two of these copper anodes in the apparatus shown,

indicated in the drawings.

and two sets of copper cathode rollers 18, one set of rollers being disposed in advance of the first anode and the second set being disposed behind the last anode.

A tank 22, which may p'referably be disposed above the table rollers 10 for gravity feed, contains a quantity of the copper plating solution or electrolyte, which is fed through a conduit 24 to a distributing pipe 26 extending across the apparatus, and thence via pipes 28 and 30 to the upper surfaces of the sheetsbeing plated. As shown, the distribution pipes may include valves for controlling the rate of feedingof the. solution, and the lower ends of said pipes preferably terminate at points just ahead of the respective anodes 20. There are as many of these pipes 28 and 30 as required, widthwise of the machine, to ensure a suflicient supply of the plating solution for film formation beneath anodes 20.

At the right hand end of the machineanupper roll 32 is provided, and preferably coveredwith a rubber layer-or the like, to act as a squeegee and thereby to prevent excess solution from being carried beyond the desired point. A tank 34 is disposed beneath and partially encompassing table rolls 10, and solution leaving the plated sheets 12 runs into this tank and thence into a collector pipe 32 from which it is recirculated by pump 38 and conduit 40 to the tank 22.

The arrangement of the cathode rollers '18 across the machine is best illustrated in Fig. 2 which also illustrates one way in which the table rolls 10 can be disposed'between side'frarnes'which also carry the tank 34.

In order to prevent physical damage to the sensitive silver surface'entering the plating apparatus, the cathode rolls 18 which contact this surface are arranged for selfalignment in a'manner bestshown in Fig. 3, which shows one of these rollers and its supporting means in enlarged perspective view. In that figure, a representative roller 18 is illustrated as carried by cross beam 1'4 by a sort of universal connection comprising a yoke 42 in which roller 18 is .journalled as by bearing pins 44, whose axial positions may be adjusted nicely (for free rotation of the roller) and secured by set screws or the like. Each yoke 42 is similarly pivoted for horizontal swinging movements in a second yoke 46, as by bearing pins 48 similar to those of yoke 42, and this second yoke 46 is in turn carried by a yoke block 50 in pin bearings 52 for free vertical swinging movement.

Each block 50 is mounted for vertical adjustment in a block 54, as by a dovetail arrangement 56, and the vertical position of each block50 can be adjusted by means of a screw operating in a threaded "hole in a lug carried by a pillar 58 secured to block 50, said screw 60 having its end in engagement with the upper surface of block 54. In thisway, the vertical position of each block 54 can beset as desired for equality with the other rolls 18 in a set, and the roll 18 will be free for slight movements vertically and horizontally so that a minimum of drag will be placed on the silver surface beneath each roller.

To ensure good electrical contact between the roller 18 and its support beam or bar 14, a conductive jumper 62 may be connected between block 54 and yoke 42, as Theplating current is then connected to the anodes 20 and the cathode rollers 18 in the known way.

In one embodiment of v a commercial machine, the anodes 20 .have a width of 8 inches and are approximately 96 inches long and extend entirely across the mirror plates, and the linear speed of travel of the plates is about 36 inches per minute. This speed of travel could be increased, of course, by increasing either the anode area or the plating current, or both. Other modifications in the disclosed apparatus, and minor variations inthe disclosed method, may be made-by those skilled in the art without departing from the spirit of the invention as defined in the appended claims.

What is claimed is:

1. The method of electroplating a metallic protective layer upon the silvered surface of a mirror, comprising passing a silvered sheet of glass continuously in a substantially horizontal plane beneath an anode plate extending crosswise of said sheet and being of substantial width as measured in the direction of sheet travel, the silvered surface of said sheet facing said anode plate and being closely adjacent the same, continuously flowing plating solution into the space between said sheet and said anode plate only in sufficient quantity to maintain a constantly changing unbroken single thin layer of solution filling the space between the anode and the sheet, and passing a plating current from said anode plate through said layer of solution and into the silver surface.

2. The method in accordance with claim 1, including the steps of collecting plating solution flowing off of said sheet and continuously recirculating said solution into position between said anode and said sheet,

3. Apparatus for the continuous electroplating of sil vered sheets to provide a protective metallic coating over the silver layer, comprising means for supporting and guiding a silvered sheet for movement in a horizontal plane, with its silvered surface uppermost, a flat anode electrode extending both widthwise and lengthwise of said sheet and supported in closely spaced parallel relation above the silvered surface of said sheet, a plurality of freely rotatable cathode contact rollers distributed crosswise of said sheet adjacent said anode and swingably mounted for horizontal and vertical motion to rest lightly in contact with the silvered surface of said sheet and to distribute plating current substantially uniformly widthwise thereof, and liquid distributing manifold means having outlets adjacent said anode distributed crosswise of the sheet and above the same to direct a continuous stream of plating solution onto the silvered surface and into the space between said surface and said anode, in suflicient 6 quantity to maintain a thin unbroken layer of solution filling said space.

4. Apparatus in accordance with claim 3, including means for wiping excess solution off of said sheet after passage past said anode, and means for collecting runoff solution and recirculating the same onto the said surface.

5. Apparatus in accordance with claim 3, in which the space between said anode and the silvered surface of said sheet is of the order of 4; inch.

6. Apparatus in accordance with claim 3, including universal mounting means for said cathode contact rollers, for free horizontal and vertical movements thereof upon said silvered surface.

7. Apparatus in accordance with claim 3, in which there are a pair of said anode electrodes, spaced apart in the direction of sheet travel, and in which a set of said rollers is mounted adjacent each of said anodes.

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,456 Rieger Jan. 22, 1952 936,472 Pfanhauser Oct. 12, 1909 1,115,671 Hermann Nov. 3, 1914 2,244,423 Hall June 3, 1941 2,271,736 Hall Feb. 3, 1942 2,345,356 Owen Mar. 28, 1944 2,372,599 Nachtman Mar. 27, 1945 2,372,665 Egli et al. Apr. 3, 1945 2,476,286 Cox July 19, 1949 2,490,055 Hoff Dec. 6, 1949 2,512,328 Hays June 20, 1950 2,513,515 Powers July 4, 1950 2,540,602 Thomas et al. Feb. 6, 1951 FOREIGN PATENTS 20,237 Great Britain of 1899 345,676 Great Britain Mar. 27, 1931 331,930 France Aug. 11, 1903

Claims (1)

1. THE METHOD OF ELECTROPLATING A METALLIC PROTECTIVE LAYER UPON THE SILVERED SURFACE OF A MIRROR, COMPRISING PASSING A SILVERED SHEET OF GLASS CONTINUOUSLY IN A SUBTANTIALLY HORIZONTAL PLANE BENEATH AN ANODE PLATE EXTENDING CROSSWISE OF SAID SHEET AND BEING OF SUBSTANTIAL WIDTH AS MEASURED IN THE DIRECTION OF SHEET TRAVEL, THE SILVERED SURFACE OF SAID SHEET FACING SAID ANODE PLATE AND BEING CLOSELY ADJACENT THE SAME, CONTINUOUSLY FLOWING PLATING SOLUTION INTO THE SPACE BETWEEN SAID SHEET AND SAID ANODE PLATE ONLY IN SUFFICIENT QUANTITY TO MAINTAIN A CONSTANTLY CHANGING UNBROKEN SINGLE THIN LAYER OF SOLUTION FILLING THE SPACE BETWEEN THE ANODE AND THE SHEET, AND PASSING A PLATING CURRENT FROM SAID ANODE PLATE THROUGH SAID LAYER OF SOLUTION AND INTO THE SILVER SURFACE.

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