HOW SAFETY GLASS IS MADE
A VISIT TO TRIPLEX AT KING’S NORTON
SAFETY HARNESS is not worn by all motorists but few of us would care to drive behind a plain glass windscreen. Yet it occurred to me, when thinking about this, that I hadn’t much notion of how safety glass, now a legal compulsion in car manufacture, is made. The remedy was simple—a telephone call to John Passmore, the Triplex PRO, a pleasant drive to Birmingham in the Alfa Romeo (although from Oxford to Stratford I would have been about as quick had I been in a ten-tonner), and I was being shown round the King’s Norton factory of the Triplex Safety Glass Co., after meeting the Managing Director, Mr. Geoffrey Iley, whom I last encountered amongst MGs at Abingdon.
Triplex was advertising safety glass, in those days a glass sandwich with celluloid forming the “meat” (and thus liable to discolour with age), during the First World War, not only as safe windscreens but as safety goggles. With the coming of inexpensive mass-produced small cars in the 1920s, accidents inevitably increased and safety glass came into its own as a means of avoiding very nasty injuries and safe guarding the eyesight when a collision occurred. Today the annual output of Triplex safety glass is some 70-million sq. ft. a year. By the late 1930s toughened glass arrived, in which the internal stresses are such that it shatters into small safe fragments should a blow release the internal tension, and now only about 3-million sq. ft. of the aforesaid output is of laminated glass, much of which is exported.
The King’s Norton factory of Triplex makes all the laminated glass, a considerable quantity of toughened glass car-ware, houses the important aviation and aerospace division, where some very complex glass manipulation is undertaken for aircraft like the Concorde, and here the Research and Development department is situated and also the modern office block, built on stilts and rising to ten storeys. Some 1,500 staff are employed there. The financial affairs of the Company are looked after from Piccadilly, but the main weight of Triplex’s output comes from the Lancashire factory at Eccleston, where new equipment for making heated rear windows is located. The work force here numbers around 2,000. The original Triplex works were in Hythe Road, London, close to the Rolls-Royce Service Station. Today many of the older Triplex employees are installed in a small plant at near-by Willesden, where 400 people work. There is another small Triplex plant at Larkhall in Lanarkshire, employing 100 persons.
Triplex have had a virtual monopoly of the safety glass business in this country, since they took over British Indestructo a few years ago. Now, apart from Triplex only a few very minute companies make such glass. The task of supplying nearly every car and commercial vehicle maker in the land with not only windscreens but rear windows, side windows, vent panels, etc., is a staggering one but all runs smoothly at King’s Norton and Eccleston. A week’s output is planned in advance to meet these extremely diverse requirements of the many different customers and the manufacturing processes are geared to these ends. Sometimes the total requirements can be met in three working days and if a hold-up at a major car plant slackens the need for screens at a given time, Triplex can warehouse the finished products and deliver them later. Most of their glass is obtained from Pilkington’s, but a small quantity arrives from Belgium, where tinted glass has been something of a speciality.
The first stage in the process of making toughened safety glass is to cut the raw glass into manageable panels of roughly the size of the finished article, this being done on steel and fibreglass templates; some 300 different glasses is quite a normal requirement. The edges of the sheets are trimmed on diamond-wheel tables, and they then go on conveyor belts into a washing machine. The toughening operation consists of lowering the glass sheets into under-floor electrically-fired furnaces, in which they are heated to 650°C and then quenched in air jets to around 200°C. While under heat treatment the required shapes are formed over templates, each of which is special to the make of car and the area of it for which the glass is intended. The complexity of coding can be imagined!
The bigger areas of safety glass, as for windscreens, are made in these press furnaces, which are of Triplex’s own devising. Smaller glasses, such as vent panels, etc., are formed in smaller sag-bending furnaces, the required shape being attained as the glass sags under the heat treatment which toughens it, instead of being pressed into shape on glass-fibre covered moulds as in the larger furnaces. Overhead conveyors convey sheets of glass about the spacious and well-lit factory, these conveyors crossing the service road through an enclosed bridge. Each completed glass is subjected to a rigorous inspection, before the famous Triplex trade mark (the triple X) and Safety coding is stamped on. Correct size is ensured by placing each screen or window, etc., on a jig of fibreglass or cast alloy. It has to be within a 120-thou. tolerance all round, although door vents get away with a 60-thou. tolerance! The quality of glass in the finished product is ensured by looking at it through coloured screens, which show up imperfections, and by passing it through polaroid screens, so that imperfect stressing is revealed. Any scratches are polished out on Sun machines and the tiniest bubble or other blemish in a laminated job causes it to be scrapped, even when Triplex know that it would in fact behave perfectly. The inspections in their own factories, to meet the stringent stipulations of the Motor Industry, who are their customers, ensure an extremely high standard for the finished Triplex products.
Laminated glass, although in comparatively small supply, being expensive, is exported in worthwhile quantities and the demand for it is thought likely to increase. Its cost is understandable after seeing the complexity of its manufacture. Glass of 3 mm. thickness is used for this type of screen, whereas 5 to 6 mm. sheets are selected for toughened safety glass. After the same polishing and washing processes, glass destined for a laminated screen is sprayed on its inner surface with a chemical paste which allows separation of the two sheets which will make the laminated “sandwich”, after they have been bent to shape. After the glasses have been formed to shape, the inner glass being slightly smaller than the outer according to the degree of bending required, the “meat”, consisting of polyvinyl butyral sheeting, is added. This is immune from the discolourisation of the old celluloid fillers and is also more flexible, an advantage if the glass should have to resist impact.
The filler for a laminated screen has to be festooned in an air-conditioned room by operatives wearing surgical-type clothing, as dirt and dust are fatal to effective bonding. The material is supplied to Triplex in sheets, cut to handleable size, and then dried in these rooms at a maintained humidity for five hours.
The bending of the outer glasses is done in nine Triplex furnaces, of which the two most modern were installed last August. These furnaces can handle 60 different shapes of glass at a time. After the glasses and inner lining have been formed into a “sandwich” there is the problem of extracting air before the edges are bonded. This is achieved by first drawing out some of the air by a vacuum process, after which the screen is heated at 100°C and pressurised at 520 lb./sq. in. for 2 1/2 hours. This conditions the plasticised inner layer and clear screen emerges, ready for sealing.
Every laminated screen goes through the final inspection processes. It is interesting that screens for Rolls-Royce are made in single-stage furnaces and that three inspectors, working in shifts, each spend 15 minutes in checking them. Incidentally, Riley was one of the last makes to specify laminated screens as a standard fitting, but laminated screens are in growing demand for motor coaches. Naturally, modern GT cars offer a challenge to Triplex due to the often complicated curvatures of their windscreens and windows, but while advice is available to the stylist, his requirements are never rejected, no matter how complex. Indeed, Triplex enjoy making “awkward” objects in glass, for the domestic and aircraft industries as well as for cars—their Ogle experiments and their pleasingly non-claustrophobic Rover 2000 with its roof entirely of Triplex glass are examples of this. Quite a lot of flat screens are still fitted, however, which reduce costs and are more easily cleaned by the wiper blades.
A separate bay of the King’s Norton Triplex factory is set aside for the new departure of making heated rear windows, a significant contribution to road safety and happier motoring. Pioneered as an extra for Rover owners, the adoption of electrically-demisting rear windows is growing apace and Standard-Triumph fit them as standard on the Triumph GT6. It is amusing that some car manufacturers like the bus-bar to be visible, for the snob value of being seen to have such a window, but Jaguar insist that it shall be hidden by the sealing round the window’s edges. . . . Wiring is to the car-maker’s specification, the wire being wound on Triplex glass-encased looms. Girls fit the wire element circuits, the bonding being by vinyl and chloroform, to the glasses which then have to go right to the other end of the factory for lamination. They return for inspection, which includes an electrical check of the circuit. Already 2,000 of these heated rear windows are being produced each week and now Triplex have introduced the less expensive Hotline product, in which the circuit is printed onto one side of a toughened glass window. In this the lines of the circuit are fully visible. A further development in this direction is heated windscreens, which some coach manufacturers are already specifying.
W. B.