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Aviation History
1953
1953 - 0834.PDF
828 FLIGHT, 26 June 1953 PLASTICS PROGRESS Papers of Aeronautical Interest at the British Plastics Convention THE British Plastics Exhibition and Convention, organized by our associated journal British Plastics, was duly held at Olympia, London, from June 8th-i8th. As was expected, the event was, from all points of view, even more successful than that held in 1951. In the exhibition itself were numerous items of interest to the aeronautical world, but the most prominent were two very large mouldings: the Miles plastic glider wing has already been described Flight, May 15th), and beside it was a i4ft-diameter radar reflec tor aerial, constructed (by F. G. Miles, Ltd., for A. C. Cossor, Ltd.) in impregnated asbestos and weighing only 345 lb. The material is completely non-corrosive, and so strong that weight is more than halved for a given strength. Amongst the lectures, those applicable in some degree to the aircraft industry were given on Wednesday, June 10th. In the morning came Surface Chemistry and Adhesion of Glass and Plastics, by H. Cole, M.A., F.R.I.C., F.S.G.T., of Pilkington Bros., Ltd.; Plastics Containing Asbestos as a Reinforcing Filler, by J. E. Gordon, B.Sc, P. H. H. Bishop, B.Sc., and P. L. McMullen, D.L.C. (Aero Eng.), all of the R.A.E., Farnborough; and A Critical Survey of Commercial Low-Pressure Laminating Resins, hv> V. E. Yarsley, D.Sc, F.R.I.C, F.P.I., and A. G. Goodchild, "B.Sc., A.R.I.C. In the afternoon there followed two further lectures which were of sufficiently close application to aviation to merit summarizing here. Fuller details of these two lectures will appear in the July issue of our sister-journal Aircraft Production, and complete details of all the lectures are in due course to be published in book form by British Plastics. Asbestos-Fibre Materials, by D. S. Bancroft, A.F.R.Ae.S., of the Plastics Division of F. G. Miles, Ltd.—Phenolic-asbestos materials, said the lecturer, could be fabricated by four distinct processes. The no- pressure method gave products generally suitable as low-strength com ponents, although its use for more highly stressed parts should not be dismissed. It could be shown that a structure such as an aircraft rib could be made satisfactorily by the process, whereas an attempt to make a similar component by a pressure method would not only be far more expensive and time-consuming (especially if production was on a small scale), but would also end in failure, since the rib, being thinner for a given weight, would itself fail through instability. Vacuum moulding (at approximately 10 lb/sq in) was particularly suited to the forming of large structures, giving a very good strength/ weight ratio, yet requiring relatively simple tooling. One-piece mould ings of over 200 sq ft had already been fabricated, and others up to 600 sq ft were being considered. Material density was of the order of 1.25. Medium-pressure moulding (50-200 Ib/sq in) came into its own when large-scale production was envisaged, as the higher tooling costs were then justified. The method was most applicable where complete structures could be fabricated in one operation. The use of high pres sures (over 200 lb/sq in) had been examined, but, said the lecturer, he himself had not yet encountered a structural problem where such a technique was desirable. Turning next to the design of moulds, Mr. Bancroft said that this almost equalled product-design in importance, so great was the effect of the one upon the other. For no-pressure work, plaster of Paris had proved itself an excellent material, especially for expendable moulds that could be destructively removed, and for small-scale production. An alternative method was to use aluminium, which, besides having a much longer life, was not prone to warping at the temperatures of 80-90 deg C required for curing plastics: plaster of Paris did tend to warp if the moulds were of small cross-section. Aluminium had the further advantage that it could be melted down and re-used. An interest ing point was that a light but important pressure could be applied in no-pressure work simply by wrapping the product tightly with some material, such as Cellophane, which shrank under suitable conditions. Passing on then to the problems associated with vacuum-moulding, Mr. Bancroft reiterated the best applications of the method, and said that a 200 h.p. generating set coupled with a large vacuum pump was sufficient basic equipment for moulds up to 1,000 sq ft in area—although a fair amount of control gear and associated equipment was required. A problem that became particularly acute where large mouldings were concerned was that of controlled heating. First, it was necessary to be able to measure the temperature at various points in the product, and this could only be done by means of thermocouples, which were far from easy to use accurately. Then a suitable number of heating mats must be provided, and so connected that current to each of them could be controlled at will. It should then be possible to arrange a production routine which could often provide a product accurate to within, say, 0.025m of the required profile. Having settled on the vacuum and heating arrangements, one could then proceed with manufacture of the requisite mould or moulds. The number of major and minor problems that could arise was, again, very considerable, but the difficulties had always proved soluble. It should be remembered, went on Mr. Bancroft, that vacuum technique did not give quite such a smooth surface as a pressure method, and that—although the face of the product next to the mould was accurate in profile—the other face, next to the rubber bag, was far more difficult to control in shape. This might lead to difficulties in certain applications. Finally, said the lecturer, there was the question of the moulds required for low-pressure techniques. Whereas for vacuum moulding a semi- porous mould or mat was required to enable the volatiles to be drawn off, when a pressure of 50 lb/sq in or more was used certain phenolic-asbestos materials would form a satisfactory product whilst retaining all the volatiles. This meant that metal male and female moulds could be used if desired. Higher pressures of the order of 150-200 lb/sq in were useful where complicated mouldings were concerned, and where large production was envisaged; but a problem with this type of work was the correct design of the press-tool and loading procedure in order to arrange fibre- orientation in the correct manner. Two further problems were the time taken to produce the required relatively complex tooling, and the extensive alterations required in this tooling for every minor modification of the product. Glass Fibre Materials, by G. C. Hulbert, B.Sc., of Scott, Bader and Co., Ltd.—Glass-reinforced materials, said the lecturer, showed such advantages over earlier plastic materials that it was only too easy to over look their limitations. It must be remembered that the cost of the new materials was extremely high, and that that of the products must be reduced to an acceptable value by various means, such as expanding production and design of structures to take full advantage of the material by reducing the number of components that would otherwise be required in metal. Although a designer rarely had a structure exhibiting uniformity of load, he was constantly striving towards uniformity of stress, this condi tion being the goal of efficiency. With traditional materials he was restricted to rods, bars, sheet, extrusions and forgings and was frequently driven to expensive machining operations to introduce tapers and such like variations of cross-section in the search for stress-uniformity. Frequently also he required most of his strength in one direction and, to obtain it, had to carry redundant strength in complementary direc tions. This necessity entailed excess weight and material waste and, therefore, inefficiency. With glass-reinforced materials, because of* the potential control of fibre orientation and the ease with which infinite variations of cross-section could be achieved (a property inherent in all mouldings), it would be seen that a comparison based upon performance in an actual structure would show the material in a more favourable light than one based on strengths realized as a new material. A serious problem arose when it became necessary to transmit loads from a glass-laminate into or out of a metal structure. Normal bolting or riveting techniques created local stresses which prevented the body of the laminate from being loaded to more than a fraction of its working stress. Adhesion provided a partial answer to the problem—particu larly if it was used to join the glass to a metal insert, and the insert was then riveted or bolted to the main structure. Adhesion between the metal insert and the plastic was by no means easy to effect, but certain epoxide resins had proved satisfactory, either as coatings for the insert, or as the actual laminating resin—and shot-blasting followed by aluminium spraying of the metal surface sometimes provided an adequate surface to which poly-ester resin could adhere. For prototype work, an attractive feature of poly-ester resins was that they could be moulded without heat or pressure; but when production was envisaged the associated problems increased rapidly, and new techniques and special-purpose tools needed developing. Another important point which required attention was the provision of suitable parting agents, so that mouldings could satisfactorily be removed from tools; and yet another was the fact that there was no non-destructive test yet in existence which was sensitive to all the faults that might exist in a laminate. However, as with welding of old, time and usage would probably provide confidence in the process, and it was therefore doubly important that all articles manufactured during this early period in the history of the material should be sound and successful. ROLLS-ROYCE PRODUCTION OUTLOOK IN a message to Rolls-Royce employees in the current issue of the firm's house-journal, Lord Hives (chairman) comments on newspaper reports that, owing to the rearrangement of the Government aircraft programme, there might be a shortage of work at the company's factories, particularly at Glasgow. He writes :— "I would like to allay any fears that we are contemplating laying off workers. The actual position is that we are desperately short of labour to carry out the latest programme. "The Press obtained their information from evidence given before the Select Committee on Estimates. This evidence was given last March, but we had anticipated these adjustments in the programme many months previously, and our production plans have been arranged accordingly. "We must accept that we shall be faced with competition, and we must be prepared to meet it. Your Management is fully alive to this, and we are extending our markets in all countries so that we are less dependent on British Government orders. "Since the beginning of 1953 we have taken firm orders for over eight million pounds' worth of new commercial business. The Rolls- Royce products are fully maintaining their pre-eminent position through out the world. At all "times, however, we must give of our best with hard work and enthusiasm."
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