In this process a suitable generator will take current from the mains and transform it into very high frequencies which, when transmitted to the work via suitable platens or electrodes, will excite the molecules of the material which is being glued, distorting them and causing them to rotate, and thereby causing friction and inducing heat. The amount of heat generated depends upon the electrical properties of the material, and the poorer its insulating properties the more heat will be generated. Thus the actual glue itself, which must be of the heat reactive (synthetic resin) type, is purposely made more conductive than the wood (which is a good insulator and, therefore, a poor conductor) and will absorb more energy and, therefore, generate more heat.
Three basic methods of applying RF heating to a glue-line are practised: (a) through or transverse heating; (b) glue-line heating; (c) stray field heating. In 'through' heating metal platens are applied on either side of the work (325:7), and the whole mass of wood is heated in order that the glue-line itself can reach the required temperature for a rapid set. This method is generally adopted for laminated bowed doors, etc. in either plywood or constructional veneers, where pressure can be applied on either side by male and female formers/forms which have been lined with non-ferrous metals (copper, brass, aluminium, etc.). These form platens which act as electrodes for the high-frequency voltages to pass through the total thickness of the laminates, dependent of course on the power of the generator and the rapidity of the set required. As with low-voltage heating, pressure must be positive and high enough that the glue-lines will not foam, otherwise lower temperatures and consequent longer setting times must be tolerated. With 'glue-line' heating (325:8) the electrodes are placed on either side of the glue-line, thus allowing most of the energy to be concentrated in the glue itself and ensuring a faster setting time for the same power, but in practice the glue-line must not be more than about 3 in (7.5 mm) wide, therefore the technique is confined to butt joints, edgings, etc. On the other hand 'stray field' heating (325:9) is usually adopted when neither 'through' heating nor 'glue-line' heating are feasible, and in all cases where it is physically impossible to place the electrodes on either side of the glue-line. The main disadvantage of this method is the longer setting time required, for there is no heating directly under the electrodes but only in the field between the electrodes, therefore the heat generated within the field must be given time to penetrate to the glue. The greater the thickness of material and, therefore, the greater the required heat penetration, the greater the distance there must be between the electrodes; nevertheless faster sets than with most other methods can be obtained, and in complicated assemblies all three techniques can be adopted through the same jig (325:6).
While the operation of RF heating does not call for electronic skill, a thorough knowledge of the basic working priciples is called for, and as the equipment itself (generator, co-axial cable, etc.) is relatively expensive to install only high outputs in quantity production will justify the cost, moreover the generator must be as close to the jig and, therefore, to the method of pressing as possible. Portable outfits for spot setting of small areas of glue are available but their application is fairly limited, for instance in spot tacking veneers in position before pressing. Where large outputs are called for, however, RF heating is probably the cheapest and most efficient process in the long run.
Figure 326A depicts a dielectric RF generator for continuous operation; 326B is an RF carcass press with a small mitred corner open carcass being pressed and cured; 325C is an LV heating unit comprising step-down transformer and
326d RF stray field platen (Figs 326a-d by courtesy of Pye Thermal Bonders Ltd)
leads, mild steel heating strip and heat-resistant strip for softening; 326D is a stray field platen in which the electrodes are placed at an angle of 45°, thus the RF field extends over the full area of the platen and can be used for sub assemblies, fastening tops, etc. The platen is covered with silicone bonded glass-cloth to yield a smooth surface, and mounted on a 23/8 in (60 mm) insulating plywood base with a plastic shield base for the electrodes.
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