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Aviation History
1944
1944 - 0714.PDF
372 p i I PLYWOOD AND PLASTICS as well as pressure. This was, however, turned down after the hazards were considered of high-pressuiie steam, neces sitated by 30 deg. F. and the possibility of- leakage with damaging results to the work. - Thus it was finally decided to use dry air for the pres sure medium, as already stated, and to incorporate an in dependent source of "heat comprising an electrical resist ance made up of narrow strips of metal. Such strips cover the area to be heated, and are proportioned in individual width to meet the electrical input and B.T.U. output requirements for a given time of moulding cycle and for a given temperature of the electrodes. The arrangement of the heaters varies according to the class of work being executgd. For example, in some cases the heaters are placed between two or more pieces of work, or in other cases two heaters are used, one on either side of the work, the latter obviously being a most efficient arrangement which from the diagram of the process shown in Fig. 2 imme diately suggests itself. As with the earlier processes, time required for heat penetration by conduction into the work is primarily de pendent on the total thickness of the work involved, the thermal conductivity of the wood veneer species, the mois ture content and the rate of heat input. Heat must, of course, penetrate to the furthermost glue line away from the given source. Thus, as stated above, a heater system on each side of the work could be very efficient. Obviously the hotter the press is maintained, and the more residual heat that is stored in the press, the quicker the work will be brought to the desired temperature. Likewise, the hotter the press is kept, the faster must be the closing of the press or application of pressure to the work to obviate precuring of the near glue lines. Applying Resin to Panels Undoubtedly where flat panels or simple curved panels can be put under pressure in a matter of a few seconds, this new process would permit of a considerable residual heat being retained in the press, resulting in speeding-up the heat transfer to the work, and materially reducing the time of the bonding cycle, facilitated by the ease of con trol of the electrical resistance type of heater which has some desirable advantages in this respect. From experience gained with the "Thaden" process so far, involving moulded units of varying thicknesses from rsin. to Jin., and with pressures varying between 50 to 100 lb. sq. in., the heating cycles have occupied from 12 to 50.minutes accordingly, the wood species that have been successfully moulded being hickory, maple, mahogany, poplar and gum. Other facts which have been brought to light are that for liquid glue a bonding temperature of 250 deg. F. is used, whilst this is increased to 300 deg. F. forjfilm glue: This process has been primarily evolved to cater for moulded units of much larger size than has hitherto been possible with earlier processes, and as a result involving fabricated sheets of such dimensions that applying the liquid glue with a brush is not consistent with modern production methods- Also glue-spreading .machines, even if available in the sizes now required, would be less con venient than the method adopted with this process of spraying the liquid resin on the panels by spray gun. For this operation the panels are held vertically in con venient batches within a suitable jig, allowing the rapid removal of each panel as coated to reveal the next for spraying. Evaluating the "Thaden" process in its present stage of development, it would seem that it affords an economic method of manufacturing single or multiple panels of flat or complex shapes and of much larger overall- dimensions than has hitherto been possible. Greater thicknesses than about fin. are practical, but obviously require increasing curing periods to permit the heat to penetrate by conduc- APRIL 6TH, 1944 tion, and to obviate this it is saggested that the develop^ ment of moulded plywood construction demanding increas ing thicknesses will call for high-frequency internal heat ing. That being so, the all-wood construction of the press and dies—a feature of the "Thaden" process—should have a valuable dielectric quality. At the present moment extraordinary interest is being shown in the application of high-frequency heating to the plastics industry, and undoubtedly in the near future it may be applied to moulded plywood manufacture. This development, however, is a job for the radio engineer, and all the necessary research in respect of the construction and supply of the electrical apparatus is a job for the electrical manufacturer. So far as the operator is concerned, he may have nothing more to do than to " plug in " or " switch on." It is, however, of great interest to review the improve ments, advantages, and possible disadvantages that high- frequency heating will automatically bring about, especially in moulded plywood and laminated plastics manufacturing technique with which the aircraft engineer is mainly con cerned. High-frequency Heating This revolutionary method of heating a non-conducting material comes about in this way. When a dielectric material is placed between the plates of a condenser % which an alternating potential is applied, the material becomes subjected to an alternating state of stress. The dielectric then holds a quantity of potential energy which may be released or discharged in a number of different forms; the major portion of the energy released, however, can appear as heat and the so-called '' loss factor" is a measure of the amount of energy dissipated in this way. The theory as to how an alternating electric field will cause the temperature to rise in the work being produced is beyond the scope of this article, for it involves discus sions on polar and non-polar molecules and similar com plexities. Also experts themselves* differ in their explana tion of. the exact manner in which the molecular excitation takes place, thus causing the heat to be generated inside the material. Briefly, however, high-frequency currents produce heat in each individual particle of a mass of material, and further such mass of material becomes uni formly and homogeneously warmed up throughout its bulk. The molecules of the synthetic resins involved are no^ electrically symmetrical, and when such molecules are placed in an electric field they tend to align themselves in much the same way as small magnets in a magnetic field. In addition to this electrical asymmetry, when the molecule is placed in an electric field it tends to be distorted, or asymmetry is induced in it. Advantages of New Method. When the field potential varies rapidly, molecules lying within the field will tend to vibrate as a whole and also tend to suffer cyclic distortion. Although movement as a whole is not generally possible, parts of these complex molecules can and do vibrate in sympathy with the varia tions in the field, and the amount of displacement induced. In other words, the degree of polarisation determines the value of the dielectric constant of the material. PjCrt of the resistance to this polarisation, or the damping effect, gives rise to the dielectric loss in the material and is responsible for the absorption of energy and consequent rise in temperature. If the materials and conditions are kept constant, this tendency to vibrate depends only on the potential gradient. Therefore providing the potential gradient throughout the material is kept uniform, such material will heat uniformly throughout its mass. Perhaps the essential difference between high-frequency heating and conventional heating by conduction can best be des'cribed by regarding the former method as a process in which the rate of heating of each small portion of the mass depends on the value of the potential gradient through it, and there fore its rate of use in temperature >s independent of the portion next to it. Providing the mass and potential gradient are uniform there will be no variation in tern'- G HT
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