The Future of Electroforming L. ASHLEY RICH
Hillyer Junior College, Hartford, Connecticut
I
NDUSTRY can look for revolutionary changes in the method of manufacturing wire screen and complicated flat metallic objects. Ten years from now there will probably be little screen made by weaving wire. Continuous rolls of screen as well as small, identical objects are being turned out now by a new process not yet described in the literature. Based on electroforming of metal on a continuously rotating cylinder, practically no labor is involved except in the manufacture of the rolls upon which electrodeposition takes place. In the present war situation many products of this process are of primary importance. Thus, bomb sight parts, fine screen up to 400 mesh, air filters for carburetors, nickel screens for Fourdrinier machines, have tremendous defense value. All of these items and many others are being made now. We may have to wait until peace comes to see more general application of the process. For this reason, cloth printed by means of screenrolls, made by a modification of this process, may not come into common use for the duration. After the war, however, we must be ready for radical changes due to the use of materials and machine parts made automatically by electroforming. Continuous electrodeposition of metal on rollers from plating baths made from scrap copper and nickel will be an important new industry. It is obvious from what has been done already that these new automatic electroformingmethods conserve scrap copper and nickel and save a tremendous amount of trained manpower. The machines now used for this work have been brought to a considerable degree of perfection for continuous operation. Nevertheless, future needs will demand that the work be so greatly expanded that existing facilities can only be considered to be a research pilot plant for this new industry. In the making of the rollers on which the electroforming of screen is accomplished, there is still much handwork required. Briefly, the following steps are involved in the manufacture of the matrix for an SO-$ mesh screen: An accurately ruled glass plate is placed over a piece of copper coated with lithographer's glue, ammonium bichromate, and albumisol. This copper lithography process results (after developing, baking, and washing) in a cross hatch of fine lines on the copper. These lines will not plate with copper, but the hackground will build up. Electroplating this copper plate therefore makes the background stand up in relief. The next process is to make a cylinder of this copper plate with these lines inside. Seams have to be made carefully so that no blemish will show in the final screen. Next, on the inside of this copper cylinder, a heavy nickel coating is deposited. This has tobeabout one-eighth of aninchthick as it constitutes the cylinder which rolls over in the plating machine and from which the screen is peeled. The outside copper is now dissolved by submerging the cylinder in a tank of chromic
acid. The nickel cylinder now has in relief the cross hatch corresponding to the screen wanted. This nickel core is then reinforced and coated with a plastic Product (Heresite). When this is baked, it completely covers the nickel. I t is sanded down carefully by hand so that the resulting smooth-surfaced cylinder has a webbing of nickel lines interspaced by plastic. Obviously, copper will plate now on the nickel and leave open spaces corresponding to the plastic areas. The product need not be screen; it can be any design with open and closed areas, since a photographic positive could have been used instead of the original glass plate.
Most master cylinders for screen manufacture are about 30 inches wide and 12 inches in diameter. Before inserting the cylinder in the screen-plating machine the roller is lightly chrome plated. This has to be done frequently in order to prevent the deposited screen from adhering to the cylinder. In the plating machine the lower half of the rotating cylinder dips in the copper sulfate bath. This bath is supplied by a tower filled with scrap copper screen through which the copper sulfate is circulated. A blower forces air up through the tower. As the cylinder slowly revolves, i t is waxed by a rapidly rotating roller brush. As the cylinder rotates, its lower half which dips below the bath becomes plated, since the roller cylinder is the cathode. At the top of its revolution the screen or other plated coating is peeled off and rolled up. This screen frequently has to be built up with copper or nickel. In another machine for this purpose the screen is first run through a phosphoric acid stripping bath. This etches out the open spaces in the screen. A washing device then cleans it while it runs through an atmosphere of illuminating gas. The screen is now built up to the desired thickness by running i t through a nickel sulfate bath. Of course, adequate provision is made for maintenance of these baths with provisions for testing, continuous circulation, filtration, addition of wetting agents and other materials. The product is far stronger than woven screen. These screens are capable of taking the beating such machines as the Fourdrinier paper machine can give them. A slightly more intricate process is involved in the manufacture of cylindrical rollers by means of which floe Or other cloth printing processes can be achieved. A six-foot-long cylinder of the type described, whose Cross section is dumb-bell shaped, is plated in nickel. coating, because of the shape of the cylinder, can perfect screen. It is now off as a be built up heavier in nickel. A ceresin-paraflin mixture of waxes is used to iill all the pores of this nickel screen and the excess is scraped off, leaving the nickel clean. Coating the 'ylinder with graphite and (Continued on page 392.)
THE FUTURE OF ELECTROFORMWG (Continued from page 386.)
i t with a 1/,6-inch thick continuous copper deposit now completes a solid cylinder of copper free from holes and with a M y attached strong nickel screen beneath it. This copper roll is now glue-bichromate coated, greased, and covered with photographic film having on i t the design wanted on the cloth. Arc lighting exposes the copper and it is developed, baked, and washed. The copper cylinder is now plated over with copper and nickel which cover all except the exposed designs on the copper cylinder. As a result the cylinder
can now be immersed in chromic acid and carefully etched. Only the designs are attacked; the copper covering only is dissolved; and these designs have the firmly attached underlying nickel screen to support them. There is therefore no chance of the usual difficulties in printing from such rollers and many miles of cloth have been run from a single roller of this type. In fact, after printing the cloth, these rollers can be returned and a new design put on them simply by stripping the copper, replating, photographing on it, and etching in the new designs.
