From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Paper on the use of plastics as engineering materials, detailing their properties, classifications, and applications.
| Identifier | ExFiles\Box 61\1\ scan0240 | |
| Date | 12th January 1934 | |
| PLASTICS AS ENGINEERING MATERIALS By HERBERT W. ROWELL Paper read before a joint meeting of the Chemical Engineering Group and the Plastics Group on Jan. 12, 1934 becomes the only reasonable method and in B.S.S. 488 for moulded articles, a moulded cantilever 150 mm. long by 15 mm. sq. is loaded with 450 g.{Mr Griffiths - Chief Accountant / Mr Gnapp} It must not deflect more than 5 mm. after 6 hours at the grade temperature of 140° C. for phenolic powder mouldings. Strength on bolt holes.—Strips of sheet with a hole drilled near each end and held by enclosing shackles and bolts are used as insulators on straining cables on overhead trolley systems. A strip 3/8 in. thick by 1 1/4 in. wide with 3/4-in. holes centred 3/4 in. from end and edges will break across the bolt holes at 2650 lb. equal to 7000 lb. per sq. in. on the material broken. The tensile strength of this Grade I material would be 17,500 lb. per sq. in. and its shear across lamine about 12,500 lb. There is no practical bedding of the bolt in this material when the bolt is 1/16 in. less than the hole diameter but a bolt of smaller diameter may bed and would certainly give a lower breaking load, while a snug fitting pin would give a higher figure. Screw threads.—The Grade I paper base material machines smoothly and takes fine threads. Canvas base rolled and moulded rods are largely used as bolts, although at first sight the material seems coarse and the laminations in the worst direction for the prevention of thread stripping. In fact, using Whitworth standard threads and number of threads per nut, no stripping occurs and the rod breaks at the bottom of the thread. "Rolled and moulded" rods screwed by mass production methods on a capstan lathe, fitted with stock mild steel nuts and pulled without special regard to alignment and flat seating, give average tests on 5/8 in. diameter of : paper base—1100 lb. = 5500 lb. per sq. in. ; canvas base—1700 lb. = 8500 lb. per sq. in. The rods themselves give higher figures in tensile, and higher tests can be obtained by careful seating of the nuts, but these are practical figures for small diameter threads. Tanks and tank linings.—The tanks of a tank steamer have been lacquered with a baked-on coat of phenolic resinoid and porcelain tiles have been fitted to a steel tank with similar cementing composition. Both are successful, but costly and not very practical. An electric accumulator case of nitrocellulose plastic sheet with cemented joints is common practice as the jointing is easy, but there is no successful method of building and sealing a tank with phenolic laminated sheet. Tanks have been made from Grade II cylinders with a fitted and cemented bottom for holding hot transformer oil and the asbestos base "Reebush" material is treated in a similar way to produce round tanks up to about 400 gal. capacity. There is still a long road to travel before we reach practical tanks or tank linings of any size for boiling liquor. Chemical attack.—The phenolic resinoids alone will resist a large variety of attack, their weak spots being caustic alkali (but not ammonia) and strong oxidizing agents like strong nitric or chromic acid. The resistance of mouldings or laminated goods depends largely on their absorbency as it is the cellulosic material which fails below the resinoid. ings with high resinoid content and Grade I laminated material are therefore best in resistance as they are practically non-absorbent. The British “Keebush” and German “Haveg” materials which have an asbestos mineral base are more resistant to chemicals than the cellulose base materials, for example, strong hydrochloric acid. All are proof against acetone, industrial alcohol, petrol, benzene, lubricants, and fatty acids. Very few salt solutions but about 5% caustic soda or 25% sulphuric acid will attack the cellulose base materials. Bleach solutions are without effect and Grade I tubes are used for conveying dry chlorine. So many surprising results have been obtained that it is wise and quite easy to try a sample in the laboratory, but what a job on the plant may consider a decent improvement on the material at present in use. Fabric base gears.—Special mention might be made of laminated fabric base gear stock as this material has been used successfully for a number of years to replace raw hide, compressed paper, and such non-metallic material and also all metal gears except where a high load per inch of face is necessary. An average resinoid fabric gear will replace any machine cut cast-iron gear but has the advantages of higher pitch line speed, one-fifth the weight, silent operation, resistance to water, oil, and most ordinary chemicals, excellent wearing properties when properly mated. They are often steel shrouded in the larger sizes of face which assists the strength of the keyway for small standard keys and they are lubricated and treated exactly like metal pinions and gear wheels. They are largely used as electric motor pinions from fractional h.p. to 300 h.p. at all speeds. Camshaft, speedometer, and wind screen wiper gears are usual in this material on motor vehicles and small wheels for electric clocks, carpet sweepers, and meters are made in thousands. Fibrous mouldings.—The percentage of long fibre in the filler may range from 5% to 100% but cloth and paper fillers are difficult to mould and cost more than wood meal filled powder mouldings. No figures are offered on these materials but some specimens have strength almost equal to laminated goods. They are largely used in place of porcelain, lignum vitæ, compositions for overhead transmission insulation because of their toughness and permanence, and this type of moulding is of interest to the engineer because he can hit it with a hammer without danger. General uses.—There is not space to detail the engineering purposes for which these materials are used but they range from light fittings and components to structural sections, rail road fishplate insulation, and gear wheels for heavy duty, and development is still proceeding. In conclusion I must thank a number of people, including Ellison Insulations, Ltd., Bakelite, Ltd., Imperial Chemical Industries, Ltd., and Kestner Eng Co., Ltd., for permission to exhibit plant and specimens and for the use of figures to enable the compilation of the table. For many generations past the term “strength of materials” has conveyed to the mind of the mechanical engineer no more than “strength of metals.” Their ductility and malleability and the fact that they can be made fluid and cast in simple moulds provide comparatively easy methods of working them into the form required. They have strength and considerable permanence, but in the day of aircraft and motor vehicles the fashion in engineering practice tends toward lightness of structure and a high specific gravity begins to be a handicap. Many of the synthetic resinoid materials have a tensile strength equal to cast aluminium but are half as heavy. They can be formed to intricate shape and to exact size by mass-production methods and fashioned easier than metals by ordinary engineering tools. Such material is used either because of its mechanical strength, small weight, or electrical insulating properties or because it combines all these properties with a low price. No motor car is constructed to-day without several pieces of these materials and they are now used as finished components or semi-manufactured material by many industries. Like all other engineering materials they must be used within their capacity and with a proper knowledge of their properties. Those who visited the Plastics Exhibition at the Science Museum were able to get a good idea of their composition and the uses to which these materials are being put. But considerable development is taking place in manufacturing technique, and as the industry is still in its infancy we can predict more useful products and much wider application. The name “Plastics” represents another American perversion of the English language which the British industry has adopted. These goods are not plastic in any dictionary sense but the material from which they were made was plastic during its formation into the shape in which you receive it. CLASSIFICATION OF PLASTICS These products may be divided into two main groups according to their temperature resistance. B.S.S.488 grades them according to the deflection of a loaded cantilever at a grade temperature. The thermo-plastics are compositions which are pressed to shape while hot and when cooled to atmospheric temperature are hard and strong. There is no permanent setting and they will soften again when sufficiently hot. The gramophone record made from gum shellac and mineral filler is an example, while the electric accumulator box made from a mixture of pitches, bitumens, and mineral fillers is another. An intermediate class contains bitumens, drying oils and fillers and after cold pressing the powdered composition to the desired shape it is stoved and shrinks a little but afterwards does not soften at reasonable temperatures. Nitrocellulose, cellulose acetate, and benzyl cellulose, compounded with a variety of organic plasticizers to reduce brittleness and make them mouldable, and with powdered filling materials to give strength and rigidity, also come within the thermo-plastic class. As structural materials their use is limited to tropical atmospheric temperature but they have many attractive properties in the way of resistance to corrosion, pleasing appearance, and extreme lightness. The celluloid pump for cycle tyres is a well-tested example in light engineering. Casein plastics are decorative and suitable for knobs and small components but are not considered here. The thermo-setting group is of more interest in engineering practice as these articles do not soften to any noticeable extent below the temperature which decomposes them, and with organic fillers they withstand 140° C. comfortably. The chemical change which takes place during the thermo-setting process also produces the useful properties of insolubility and resistance to a large variety of chemical attack. These thermo-setting plastics are organic materials and there is a limit to their resistance to heat but they are not easily combustible. They are heteroge neous in structure and composition, and they do not behave mechanically like homogeneous metals. They are composed of a comparatively brittle and highly resistant resinoid which impregnates and binds together a tough and strong reinforcing material or filler and it follows both that the filler and the method of combining it with the resinoid are of particular importance in the manufacture of engineering material. This thermo-setting group must again be divided into powder mouldings and laminated goods. The phenolic resinoids enter into the composition of over 90% of the powder mouldings made to-day and the carbamide resinoids take the second place. The general filler is wood meal but considerable advance has been made in impact strength by the incorporation of fibrous filler and modification of the resinoid. The engineer receives these mouldings as a finished article or component ready for assembly. Their mechanical properties are detailed in Table I. | ||