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Aviation History
1944
1944 - 1745.PDF
AUGUST 24TH, 1944 FLIGHT 201 Plasties for Aircraft Engineers Basic Principles of Moulding, with Hints on Design By W. NICHOLS, A.R.Ae.S. A 1. Airscrew blades and spinner. 2. Jettison tanks. 3. Landing light shield. 4. Wing tips. 5. Navigation light covers. 6. Trimming tabs. 7. Tank fillers. 8. Hydraulic liquid header tank. TIE illustration in Fig. 1 gives an excellent impression of the application of plastics to a modern aircraft, and leaves one in no doubt that the aircraft industry is at the present time one of the largest users of plastic materials One of the most prominent materials is the transparent plastic known in this country by the trade name of "Perspex." Also various laminated plastic materials are now being employed for various components with success, whilst modern plywood, using plastic adne- sives and moulded plywood construction indicate a return once again to the oldest of all materials in the history of *rcraft engineering, the principles of design and the manu facturing technique now being well known and appreciated. In every modern aircraft, however, are to be found, in addition, a host of small plastic mouldings involving quite a different manufacturing technique, and such parts are constantly being increased as this modern drama of raw materials, being transformed rapidly and economically into a multitude of useful gadgets and decorative objects, becomes more generally appreciated by enterprising de signers and technicians. Some .knowledge of the basic fundamentals of the moulding processes for such parts is advisable prior to projecting an aircraft equipment unit 10 be made wholly or in part of plastics, and is the primary purpose of this article/concluding with notes on design which the aircraft draughtsman in particular may find &l soma value. Cast Plastics Of the various processes, cast plastics will be dealt with first. These comprise those thermosetting materials which may be poured as liquids into suitable moulds to fix their shape, and then hardened by means of an after-baking operation. Inexpensive lead moulds are used, and where standard shapes or sections are required these can be cast U> long rods and cut off or sliced into small sections, having the shape or contour provided by the casting process. Phenolic resins used in this process are prepared differently from the phenolic moulding materials; the hardening makes use of the same basic chemical reaction of polymen- Radio panel. Table tops. Battery case. Control wire guards. D F aerial hous ing. Turret. Nose window. Fig. 1. Diagram indica ting the uses of plastics in aircraft. sation, changing the molecular structure and producing a new material which cannot be melted again. The resins, however, are modified to meet the special requirements for casting and hardening. Compression Moulding The compression moulding process is used for thermo setting materials such as the urea and phenolic compounds which not only require heat for plasticisation but also the application of heat for a period of time to polymerise or set them. Thermoplastic materials require cooling in the mould and can be moulded more economically in injection- moulding machines. By the compression process the • material is loaded directly into the mould cavity, as shown in Fig. 2, and compressed to the required density by the pressure, the material being confined to the shape of the mould. Continued heat and pressure produce the chemical reaction which hardens the compound into the finished moulded product. Economical commercial moulding by this process depends mainly upon ease of loading, correct heat and proper application. For example, the heat must be applied simultaneously to the material charge and to the part while undergoing hardening. Proper pressure is MOULD PLUNGER GUIDE PINS MOULDING COMPOUND MOULD CLOSED MOULD CAVITY MOULD OPEN Fig. 2. Compression Moulding. The moulding compound takes shape under heat and pressure and is maintained in this form until the material hardens.
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