NEW PROCESSES AND TECHNIQUES
By R. D. CULLUM, M.I.E.D.
Editor of Engineering Materials and Design and Engineering Product Data (London) Issued by British Information Services – The Certificated Engineer May 1965.
When we think of engineering materials, iron, steel, brass, and aluminium are those which automatically come to our minds. There are, however, many other materials that play important parts in engineering today: these include timber, textiles, and chemicals. Each has perhaps a more specialised field of application than metals but they make important contributions to engineering as a whole. Chemicals are particular examples of this, as they form the basis of all plastics materials, which are growing rapidly in number to fulfil specific requirements. In the plastics field, improvements in finished products often start with improvements to raw material or to a constituent of that material. Or, again, it may be a process involved in transforming the raw material into the finished product that is improved.
A particular example arises where reinforced plastics are used for building large models, tools, or patterns and where the technique is first to apply a gel coat or surfacing layer consisting of a mixture of resin, hardener, and filler. Resin-impregnated glass cloth is then laminated On the gel coat until the desired thickness or shape is achieved. When dealing with large components, however, there may be a time-lag between the first application of the gel coat and the subsequent laminate. Normally the adhesion between the two is chemical as well as physical, but, if the gel coat has time to set hard before the laminate is bonded, the bond will be only physical. This results in a weakness in construction.
Another weakness may arise when aliphatic amines or derivatives are used as hardeners. The gel coat absorbs water vapour from the atmosphere, giving a tacky and non-glossy surface which will not bond. These weaknesses in construction can now be avoided by using a special composition-'Resimix' HX 60- recently developed by a British company.
In planning production operations, if it is seen that it will be necessary to laminate to a cured gel coat, the new composition is brushed on the surface once it has set hard enough to withstand such treatment. The composition will then prevent water absorption and also act as an intermediate agent to facilitate adhesion with the later laminations.
Sealing for Concrete Floors
Concrete floors are almost universally used in factories and stores and also in many office corridors. They are hard-wearing and will withstand high loadings but readily absorb oil, grease, and other liquids; they also give rise to dust. These disadvantages can be overcome by the use of a sealing or dressing compound, and one such material is known as 'Co-Seal' clear gloss. With the addition of an appropriate amount of special solvent, it will effectively penetrate the pores and voids of the concrete and, on curing, will create an impermeable seal-not merely a surface film. The treatment can also be used for sealing cement rendering on walls and brickwork.
'Co-Seal' is also available as a pigmented matt finish incorporating a special refractory material which gives if high resistance to abrasion. Additionally, the makers have developed heavy-duty 'Co-Seal' which is intended as an extension to the standard grade, for treatments that are required to withstand exposure to chemical attack, and which, in its clear forms, gloss or matt, is particularly suitable for the treatment of timber.
Component Protection
In the production of parts, particularly those made from ferrous materials, care must be taken tv to ensure that they do not corrode while in storage or transit. One method of ensuring protection is to thoroughly grease or oil the parts. This gives useful protection but often necessitates the removal of the oil or grease before the part can be used.
An alternative form of protection can be given by Covering the part with a plastics coating-usually effected by dipping or spraying. On such coaling is 'Crocell,' a hot-dip strippable coating. It is a tough, rubbery plastics which, when applied to metal articles, is said to give complete protection against corrosion and abrasion. The skin is flexible, strong, and transparent, thus enabling identification marks on the part to be easily read. Articles to be protected are simply immersed for five seconds in the coating material, which has been heated to 160 to 185 degrees Centigrade. The coating will solidify within seconds of removing the part from the liquid.
The great advantage of this coating is that it can be easily removed by making an incision and peeling off, leaving a slight film of oil on the part. Where It is required completely to remove this oil film or other grease or oil protective coatings, a cleaner known as 'Magisol' can be used. This can be applied by brush, although dipping is the best process if the parts are small enough. It has low viscosity and surface tension, which assists penetration, and does not give off harmful vapours.
Locking of Fasteners
The danger of fasteners working loose during long periods of vibration cannot always be eliminated by the use of spring washers and locknuts. Recent developments have produced a liquid sealing medium which, when applied to fitted metal parts like nuts and bolts or a shaft and collar, is claimed to provide a tenacious and positive lock.
Such a liquid is 'Metal-lock'; it is made in several grades, developed to give holding torques and viscosities suitable for many different applications. The low-viscosity grades are suitable for treating large numbers of relatively small, close-fitting components by a tumbling technique; they are also suitable for applying to engaged threads when they will penetrate by capillary action. The higher-viscosity grades have been formulated to meet the exacting requirements of filling the threads of larger bolts and the spaces between slack components.
At normal room temperatures 'Metal-lock' resists penetration by oil and grease and has excellent water resistance. Its resistance to oil and water and the unwinding torque falls as the temperature rises but at 85 degrees Centigrade the unwinding true falls by only 30 p r cent of the to given at room temperatures.
Rubber Heating Elements
Silicone rubbers are used where high temperatures are encountered, and conductive silicone rubber is a silicon-based rubber to which has been added a carbon filler. When cured, the resultant rubber forms an electrical conductor which can be manufactured into heating elements. It can also be coated on various cloth materials, including glass-fibre, 'Terylene', and nylon. It can be coated on flexible-base material as well as rigid materials to form electrically conductive areas. Applications include heating panels, cabinets, and specialist industrial applications.
When coated on glass-fibre cloth, the elements can be flexed without altering their electrical resistance, and, when required to heat especially difficult areas, the cloth can be wrapped completely around the object or area to be heated. Conductive silicone rubber has a high, positive resistance temperature coefficient and therefore tends to remain stable over a wide range of conditions-that is changes in voltage conditions. Because it is spread over an area, it produces an even temperature over the entire surface, with no hot-spots. The maximum element temperature for silicone rubber on the glass-fibre cloth is 150 degrees Centigrade with maximum heat dissipation of 250 watts per square foot. The minimum bend radius of such a cloth is one-sixteenth inch.
Silicone Resin-bonded Glass-fabric Laminates
As mentioned earlier, silicones play an important part where heat resistance is required or where a water-repellant material is necessary. Both these properties are important in electrical work and recent development in an electrical insulation material combines silicone resins with laminates of fine-woven glass cloth. Known as 'Hyperlam' 5S1, the material is said to meet the requirements for a laminate having extremely good dielectric properties, excellent punching and machining characteristics, very low water absorption, and suitability for operation in temperatures up to 250 degrees Centigrade.
Standard sheets have nominal dimensions of three feet by three feet in thicknesses from one sixty-fourth inch to two inches.
Soundproofing Material
Noise is an inevitable by-product of mechanisation and, while mass helps reduce machine noise, it is also in the interests of economics to design parts as light as possible. Considerable use, therefore, is made of sheet-metal fabrications. However, as the internal friction losses are lower in sheet metal than in castings, in many cases the use of sheet metal for structures must be considered together with some form of soundproofing.
'Silentium' provides an answer to the conflicting requirements of lightweight and effective sound insulation. Available in several different forms, it is a plastics sheet providing high internal frictional losses and loaded with a high-density filler. When attached by adhesive to sheet-metal assemblies, the material is said completely to eliminate resonance and, at the same time, to provide an effective barrier to the passage of sound.
Typical applications of the material are fan housings and air ducts; for enclosures containing machinery or motors; for furniture, storage units, and partitions. 'Silentium' will resist the action of oil under reasonable conditions but prolonged immersion in oil is not recommended. It may be used at temperatures up to 100 degrees Centigrade without detriment.
Five grades are available: 'BJN' with a density of one pound per square foot for engineering and building insulation; 'BJ,' of similar density, used for folding partitions and curtains; 'RD,' specially designed for partitions constructed of metal, wood or plaster; and 'A' and 'C' for general uses where the sheet remains visible.
Reinforced Thermoplastics
The disadvantage of thermoplastic material is its comparatively low stiffness, but the recent introduction of a reinforced thermoplastics sheet-'Duraform' -will enable designers and constructors to utilise all the advantages of a thermoplastics material with the added advantage of increased stiffness and strength.
'Duraform' is a vinyl plastics reinforced with asbestos and, in addition to increasing stiffness, the inorganic fibre content imparts heat and chemical resistance, impact strength, dimensional stability, low coefficient of expansion and resistance to creep. It is completely self-extinguishing in respect of flame,
The flat, reinforced sheet laminates can be supplied with all types of decorative finishes, thus making them extremely useful for industrialised building applications. It is also available as a sheet moulding material which can be formed into shapes by the application of heat and pressure.
'Duraform' is available with a matt or polished finish in four sheet sizes and a thickness range from a one-sixteenth inch to half-inch. Fabricators can also obtain a grey pressed sheet with resin-rich surfaces suitable for a wide range of finishes.