Fig. 143.—The Metal Sprayer.
In Fig. 143 is shown very clearly the construction of the sprayer, which it will be understood comprises a combined melting and spraying jet and a feed mechanism. The metal, in the form of rod or wire, is fed to the melting flame. This, as already stated, is formed by coal gas burned in the air, or oxygen, water gas, acetylene, hydrogen, etc., may be employed instead of the coal gas. The gases are supplied at such a pressure as to prevent blowing out and to ensure a highly deoxidizing flame. The spraying jet can be of carbon dioxide, nitrogen, air, steam, etc., and it must be fed at such a pressure as to produce a sufficiently high velocity for successful coating. The usual gauges and reducing valves will, of course, be employed. The feeding of the wire is accomplished by a small pneumatic motor, driven by the spraying medium, either in series or parallel with the main jet. The dimensions of the wire nozzle, and feed mechanism vary with the different metals. To obtain a good adhesion between the metals being sprayed and the surface to which it is to be applied, the latter must be thoroughly clean and of an open nature, to give a key for the deposit. Sandblasting is sometimes employed to effect this.
Fig. 145 shows an enlargement of the nozzle with the different parts marked. The cost of the process is not prohibitive; the cost of the metal only on one square foot of a thickness of 0·001 inch is quite small with the cheaper metals. The process is put on the market by the British Metal Spray Co., Ltd., Queen Anne's Chambers, Tothill Street, Westminster, London, S.W.
In a paper read by Mr. R. K. Morcom before the Institute of Metals, the following interesting information was given:—
Fig. 144.—Sectional Drawing of the Metal Spraying Machine or "Pistol."
With a given design of jet there is only a certain volume left by the air-jet which can be filled with flame, and this flame has a limiting temperature which cannot be exceeded. The wire, passing through this cone of flame receives heat, partly by radiation, but chiefly by conduction, and becomes melted; but there is a definite limit to the amount of heat which can be picked up by the wire passing through the flame, and a definite limit to the rate at which it can be melted. This cannot be increased by forcing more gas into the flame, as the extra gas is merely blown away by the air-jet. It is possible to increase the rate of melting by shaping the nozzles so as to leave room for a larger cone of flame, and experiments are in progress on this point. There is, therefore, a most definite economical quantity of gas which should be used in the pistol, this quantity being about 1·5 cubic foot of hydrogen per minute, and 0·5 cubic foot of oxygen; or about 0·8 cubic foot of coal-gas to 0·65 cubic foot of oxygen for the present standard designs.
In refractory metals these quantities may be increased slightly, as a slightly higher temperature can be obtained if the burning gases are under a pressure greater than atmospheric, and this occurs if the gas quantities are increased, the inner surface of the air-jet acting to some extent as an enclosing wall to the flame. On the other hand, for the more easily fusible and oxidizable metals, such as tin, lead, and zinc, it is advisable to keep the gas quantities rather below the figure given, so as to avoid any possibility of overheating and burning any portion of the wire.
The outer jet performs a threefold purpose: it keeps the nozzles and wire cool, it cools the object, and it produces the requisite velocity.
