Watch the magic of this science-based process begins to unfold, in a series of stages on a float line that may be nearly half a kilometre long. Raw materials enter at one end. From the other, plates of glass emerge, cut precisely to specification, at rates as high as 6,000 tonnes a week. In between lie six high integrated stages...
Stage 1: Melting and refining
Fine-grained ingredients, closely controlled for quality, are mixed to make batch, which flows as a blanket on to molten glass at 1,500 oC in the melter.
Float makes glass of near optical quality. Several processes – melting, refining, homogenising – take place simultaneously in the 2,000 tonnes of molten glass in the furnace. They occur in separate zones in a complex glass flow driven by high temperatures. It adds up to a continuous melting process, lasting as long as 50 hours, that delivers glass at 1,100oC, free from inclusions and bubbles, smoothly and continuously to the float bath. The melting process is key to glass quality; and compositions can be modified to change the properties of the finished product.
Stage 2: Float bath
Glass from the melter flows gently over a refractory spout on to the mirror-like surface of molten tin, starting at 1,100oC and leaving the float bath as a solid ribbon at 600oC.
The principle of float glass is unchanged from the 1950s. But the product has changed dramatically: from a single equilibrium thickness of 6.8mm to a range from sub-millimetre to 25mm; from a ribbon frequently marred by inclusions, bubbles and striations to almost optical perfection. Float delivers what is known as fire finish, the lustre of new chinaware.
Stage 3: Coating
Coatings that make profound changes in optical properties can be applied by advanced high temperature technology to the cooling ribbon of glass.
On-line chemical vapour deposition (CVD) of coatings is the most significant advance in the float process since it was invented. CVD can be used to lay down a variety of coatings, less than a micron thick, to reflect visible and infrared wavelengths, for instance. Multiple coatings can be deposited in the few seconds available as the glass ribbon flows beneath the coaters. Further development of the CVD process may well replace changes in composition as the principal way of varying the optical properties of float glass.
Stage 4: Annealing
Despite the tranquillity with which float glass is formed, considerable stresses are developed in the ribbon as it cools.
Too much stress and the glass will break beneath the cutter. To relieve these stresses, the ribbon undergoes heat-treatment in a long furnace known as a lehr. Temperatures are closely controlled both along and across the ribbon. Pilkington has developed technology which automatically feeds back stress levels in the glass to control the temperatures in the lehr.
Stage 5: Inspection
The float process is renowned for making perfectly flat, flaw-free glass. But to ensure the highest quality, inspection takes place at every stage.
Occasionally a bubble is not removed during refining, a sand grain refuses to melt, a tremor in the tin puts ripples into the glass ribbon. Automated on-line inspection does two things. It reveals process faults upstream that can be corrected. And it enables computers downstream to steer cutters round flaws. Flaws imply wastage; while customers press constantly for greater perfection. Inspection technology now allows more than 100 million measurements a second to be made across the ribbon, locating flaws the unaided eye would be unable to see. The data drives ‘intelligent’ cutters, further improving product quality to the customer.
Stage 6: Cutting to order
Diamond wheels trim off selvedge - stressed edges - and cut the ribbon to size dictated by computer.
Float glass is sold by the square metre. Computers translate customers’ requirements into patterns of cuts designed to minimise wastage. Increasingly, electronic systems integrate the operation of manufacturing plants with the order book.
