Radio age research, manufacturing, communications, broadcasting, television (1941)

were developed, little did the ex- perimenters realize the full signifi- cance of their findings and the pos- sibilities of the radio industrj\ Likewise, we cannot predict today the future radio frequency applica- tions in industry. The research and development engineers of RCA have been work- ing on applications of radiothermic principles for use in defense projects. The principle of generating heat by radiothermic means is not new. The two fundamental ways in which this can be accomplished are dielec- tric heating and induction heating. Non-conducting materials are best heated dielectrically. In this case the heat is generated from dielectric losses in an electrostatic field. Most people are familiar with the diathermy machines which are widely used in hospitals, clinics and by physicians. These radio-fre- quency machines generate heat deep within the body tissues be- cause of dielectric losses. The same principle is now being used in in- dustry in various manufacturing processes. Wood, paper, cloth, and various compositions are examples of material that can be heated in this manner. One of the most important ap- plications of this method is the manufacture of wooden aircraft parts such as propellers, spars, ribs, wing structures, etc., of laminated plywood. Laminated wooden pro- pellers and parts are being widely used as a substitute for metal. Since radio frequency generates the heat uniformly within the part, the time required to bring the wood up to temperature is dependent only on the power used and not on the flow of heat from an external source. The use of plywood for airplanes is, of course, not new. Plywood air- planes were used in 1925. All ply- wood had at that time limitations imposed upon it by the animal and vegetable glues then available. Moisture, age, temperature, and fungi growths weakened the glues and, in addition, plywood was a considerable fire hazard. Because of these difficulties, aluminum re- placed plywood. It was not until the development of synthetic i-esin glues in 1935 that plywood became a practical material. Almost all of the objections were removed by the new synthetic resin glues. These glues are fire-resistant, fungi-proof and practically water-proof. The bond is stronger than that of the wood itself. Two different types of synthetic glues are available at present—the cold-setting urea formaldehyde type and the thermo-setting phenol for- maldehyde type. Since rather high setting temperatures are requireil to properly process most phenol types (250° to 300° F) and from four to six hours in time is required for setting the urea type without heat, the advantage of radio fre- quency as a source of generating heat for bringing these glues up to temperature is obvious. Use Saves Time, Labor A lengthy period is required in the manufacture of laminated air- plane propellers for the heat to penetrate when applied with the conventional steam-plate type press. Also, the outer surface heats faster than the center portions, which may set up residual stresses that affect the strength of the finished piece. Radio frequency provides a means of uniformly applying heat throughout and at a much more rapid rate than can be obtained with hot plates. The processing time, when radio frequency is used, becomes minutes instead of hours. In producing other thick lami- nated "compreg" (impregnated and compressed) structures of wood, canvas, paper, etc., by ordinary processes, the time is exceptionally long, sometimes as long as twenty- four hours. With radio frequency the time for heating can be reduced to approximately 30 to 60 minutes. This means not only a material saving in labor but also in press facilities. Where molding processes of the same materials are involved, this also saves duplicating expen- sive presses and dies to secure addi- tional production. In addition to the application of radio frequency power to the wood industry and other laminated ma- terials, it is equally applicable to the plastic industry. Savings of from 15 to 1 in time and even more can be obtained by use of radio fre- i|iiency. The ratio of time saving depends upon the thickness of the jiart. In the fabrication of molded air- craft parts which are built up of thick wooden laminations, each lay- er of these laminations is tacked on the mold by small staples. As the successive layers are put in place, it is necessary that the staples be removed from the preceding layer. This is a tedious and lengthy proc- ess. A simpler and quicker method was devised by our engineers which eliminates the use of staples en- tirely. This method makes use of an o.scillator, together with an ap- plicator called the "gun." The glue that is used in these operations is generally a thermo-setting resin which requires heat to set. As this glue heats and before final polymer- ization takes place, the glue becomes "tacky." Therefore, if heat is ap- plied locally to a small spot, a bond is formed sufficient to hold the lam- inations temporarily until the whole MF.Tnol) OF OLUINC AIRPLANE SPARS WITH RADIO FREQUENCY SHOWN IN THIS SCENE FROM THE TOLERTON LUMBER COMPANY, ALLIANCE, OHIO.