TAKING THE DAN R OUT OF GLASS
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The Editor Pays a Visit to the Extensive Works of British Indestructo Glass Ltd., and Records His Impressions of the Production of Safety Glass Made in Two Types— Laminated and Toughened —Indestructo Glass as well as Transparent Plastics, are Being Employed for a Wide Variety of Vital Purposes
ONE of use most successful makers of safety glass in its two main forms-a-laminated and toughened—is British Indestructo Glass, Ltd., Park Royal Road, London; .N.W.10. Previous to the war, the bulk of its business comprised the supplying of these forms of glav to manufacturers of motor vehicles.
Now, it produces safety glass not only for vehicles of every kind, but for many other purposes, such as instrument-cover discs, searchlight segments, marine bridge and deck-house glasses and fire-engine screens; probably the smallest components being the eye pieces for Service gasmask goggles. In some cases, laminations are built up to a thickness of several inches, and this with extraordinary clarity of vision, without distortion.
The numerous activities have led to many problems, which have sometimes cropped up in the process of manufacture, and in others have been put directly before the company for solution. In fact, the technical department is always prepared to consider and advise upon any proposition in respect of safety glass which may be brought to its 'notice. It works upon the principle that no task is too hard to attempt and, in many cases, to solve. For instance, one interesting 'little task was to provide absolutely true laminated-glass discs for moulded-plastic goggles, which would withstand pressure without leakage.
Incidentally, the company is a member of the British Standards Institution, which has done so much to assist production and raise quality. From the Indestructo factory material flows regularly to the Institute for test, so that output is Constantly maintainecrat, or above, the Institute's standards for this material. .
How " Raw ". Glass is Cut
and Sprayed for Laminating A walk around the extensive shops was a, revelation in smooth Working and careful control. Starting with what is known as " raw" glass, supplied accurately ground and • polished, in convenient sizes, this is cut to the required • shape with the least possible waste. Owing to the variety of pieces now necessary, much of the cutting is done manually," but where large contracts for uniform sizes are concerned, automatic cutting machines, working to template, are always used. In cases where drilling is necessary, special drills of appropriate dimensions, sometimes working with carborundum as an abrasive, are employed.
After cutting, each piece of glass passes on a conveyor band through _a machine equipped with numerous reciprocating brushes, and these, in conjunction with hot water, thoroughly clean the surfaces, the glass continuing onwards through a conveyor drying oven. • -Drying completed, the plates are carried, still on the conveyor, through a chamber in which they are sprayed on one side with a special solution. The spraying nozzle is given a figure-of-eight movement so that it covers every inch of tbe surface: A thin acetate sheet of special composition is then laid on to the sprayed glass, and a second piece of glass, of the same size and similarly treated, placed above it, the " sandwich " thus formed being squeezed between rollers, following which it is closely inspected for clarity, bubbles. etc.
These " sandwiches " then proceed to the press department, each of the glasses there being subjected to a pressure of 90 tons or more, according to the area of the glass. For curved laminations, those of other shapes than flat, parts with multiple laminations, or smaller products, such as goggle discs, a different method is employed, the pressure being applied equally all over them by their immersion in fluid heated in pressure chambers., Sealing Edges of Sheets and Grinding Contoured Screens An important operation in the production of laminated glass is edge sealing. The edges are first carefully ground on wet carborundum wheels, following which the material between the plates is routed out by hand to the required depth, sealing then being performed with a bituminous compound. Some of the sealing is done by hand, but a considerable portion on a machine of the company's own design, which maintains the compound at the requisite temperature, arid the sealing is effected under pressure. The company also makes use of a special type of automatic grinder for such parts. as the edges of contoured windscreens, several being dealt with simultaneously, the stones following the contours and exercising constant pressure. Door glass is also treated in this way.
Finally, the sheets are scrupulously cleaned preparatory to being thoroughly inspected by skilled operatives. Polishing then removes any small scratches which may be detected.
. WeJhave already referred to the fact that the 'company manufactures a considerable amount 'of. " toughened." glass. Most people know what this is, but for those who have any doubts upon the matter, we may mention that, in this case, there is no laminating. Plate glass is used.
usually in thicknesses of in to I in. This is heated until it is semi-plastic, and then is cooled on each face by a blast of air. The result is the formation of hard outer surfaces and a soft core, the whole forming a tough, flexible sheet which can be bent or twisted to a remarkable degree, whilst remaining unimpaired and definitely durable, unless hit hard directly by a stone or other medium. The outer hard skin being pierced, the tension of the surfaces and the compression of .the core are released, and the sheet becomes either opaque and rather like a honeycomb or it dissolves into small harmless pieces having no cutting edges or dagger-like spikes, and which can be crumbled in the fingers without any risk of the skin being punctured or scratched.
We witnessed several tests conducted on some pieces of this toughened plate, in which a .1.65 lb. ball was dropped from various heights on to the glass. With 1-in, plate, the British Standards test is to drop the ball from a height of 4 ft. 6 ins., but we observed it to fall from 6 ft. 6 iris., without causing fracture. Similarly, with plate. the official test is 3 ft. 8 ins., but this can be carried to greater heights before fracture Occurs.
For special purposes, where pressure and" shock may be experienced, al in glass panels for ships' bridges, lights for port-holes, etc:, the company has developed a combination type of glass, comprising the laminated and -toughened forms sandwiched together to render it proof against the blows of rough seas or any form of blast.
To indicate the type of glass, the details are stencilled on each sheet in a particularly interesting manner by sand-blasting under pressure. 'It will be realized that with this work the stencils must have a comparatively short life, and the wording is ver"y small. The stencils are, therefore, cut. in an ordinary cinematograph film, using a master stamp, and the sand passes through the perforations in the film, which can be replaced easily and at low cost.
No matter how progressive a company may be, the demands' and urgencies of war initiate experiment, accumulate experience, accelerate improvement,, and often create new products. If a company be far-seeing, it considers how these improvements may be turned to advantage for postwar use, and whether these products may create new trends or markets specifically for this country or for the promotion of our trade overseas.
In an endeavour to anticipate peacetime demands, the company has been closely watching safety-glass trends in the passenger and goods-transport fields, and considering them in relation to toughened glass, whilst the successful production of non-distorting, curved, laminated glass has entailed consideration of its possibilities when peace returns. Transparent plastics, too, with which the company is largely concerned, may, sooner or later, have a growing place in transport, so that it can safely be said that, backed by its accumulation of experience, British Indestructo Glass, Ltd., is well advanced to deal with post-war problems.