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

Radio Heat Seals Plastics By Wiley D. Wenger Electronic Apparatus Section, RCA Victor Division THE use of high-frequency radio waves to generate heat within thermoplastic materials and join them quickly in strong, air- tight seams, is greatly improving manufacturing processes for many consumer products, bringing the benefits of electronic industrial techniques into the average home. Commonly-used products now being fabricated in this way include dress ELECTRONIC POWER GENERATORS SPEED UP FABRIC SEAMING, PRODUCING BETTER ADHESION. shields, raincoats, bathing caps, shower curtains, baby pants, pelts, tobacco pouches, and other com- modities. Before the age of electronics, lieat sealing of thin thermoplastic materials was accomplished in vari- (lus ways, but electronics revolu- tionized existing sealing methods by furnishing a source of energy that does not exist as heat until it passes into the plastic material. The effect of heat on the surface of a thermoplastic is similar to that of a solvent or cement. It prepares the surface to be sealed, so that if pressure is then applied, an inter- mingling of surface films occurs and a chemical bond is formed. Es- .sentially, the same fundamentals apply to any method for bonding thermoplastic materials. There are disadvantages in using solvents, however, such as the time required for chemical action, the need for extra operations, and the continued effects of solvent penetration. Successful edge or butt welding OPERATORS AT THE DU PAGE PLANT, LOM- BARD, ILL., JOIN VINYLITE STRIPS BY ELECTRONIC HEAT-SEALlNG TO FORM AN AIRTIGHT, WATERTIGHT BALLOON. of various types of plastics has been accomplished in the past by using a "hot blade" applicator. Immedi- ately after heating the two sur- faces, the knife-blade is removed and pressure is applied to the two pieces. If the blade temperature is too high, the plasticizer of the ma- terials is vaporized or burned and no bond results. Conversely, if the blade is too cool, no seal will be made. The speed of the hot-blade operation is restricted by this tem- perature limit, in addition to the fact that the joint must be cooled uniformly after its formation. Since the application of heat di- rectly to the interface of a seam, in thin films or sheet stock, is difficult, critical of control, and of limited scope, this system of "hot blade" sealing is impractical for most pur- poses. The usual method of heat sealing plastic or plastic-coated ma- terials has been the application of heat from the outside by means of hot dies, bars, or rollers. This is satisfactory under certain condi- tions, but extrusion of the outer surfaces of plastic film results, and the process is slowed down by both the time required for heat conduc- tion into the material and the cool- ing period. Radio Heat is Uniform The need for a sealing device which would not be dependent on heat conduction was quite apparent. Turning to electronics, RCA engi- neers applied alternating electrical energy of a high frequency to the plastic material to be sealed. This radio energy, it was found, could generate heat uniformly through- out the material and quickly raise the temperature of the plastic to the softening point. Best of all, the electrodes, used to apply the energy, remain cool and conduct heat away from the outer surfaces of the seam. The cold metal electrodes were used to raise the temperature of the plastic to the bonding point, apply the necessary pressure, and cool the outside surfaces. Demonstrations of laboratory devices of this type [18 RADIO AGE]