Float Glass Manufacturing Process
Today, there are three types of flat glass manufactured in the world: sheet, float, and plate. Float glass accounts for over 90 percent of the flat glass currently being produced. It is the fundamental building block in the world of fabricating architectural glass. The float glass process, developed in 1952, is the world standard for high quality glass production. A float glass process consists of five major steps:
- Forming and Coating – StewartFloat® and AcuraCoat®
- Cutting and Packing
I : Batching
The first stage, batching, prepares the raw materials for melting. The raw materials arrive by truck or train continuously: sand, dolomite, limestone, soda ash and salt cake. These materials are stored in the batch house which consists of silos, hoppers, conveyors, chutes, dust collectors, and the necessary controls to properly handle the raw materials and mixed batch. From the minute the materials arrive, they are in nearly constant motion. Inside the batch house, a long, flat conveyor takes the materials in sequenced layers from their silos and sends them to a bucket elevator that carries them up to a scale to check their combined weight. Recyclable glass shards or cullet, which is crushed glass from the plant’s finishing end, is added to these ingredients. Each batch contains approximately 10-30% cullet. The dry materials are added to a mixer that stirs them together to form the batch. The mixed batch is delivered from the batch house to the furnace storage bin by a belt conveyor system, where it is stored and then fed into the furnace at a controlled rate by the batch charger.
Figure 1: Typical glass composition
Figure 2: Stored piles of cullet
Figure 3: A hopper feeds the mixed materials into the entrance
end of the melting furnace, where temperatures reach 1650°C
II : Melting
The typical melting furnace is a Six Port Cross Fired Regenerative Furnace approximately 25 m wide by 62 m in length with a capacity of 500 tonnes per day. The major sections of the furnace are the melter/refiner, working end, regenerators, and ports, as shown in Figure 4, and are constructed of specialized refractory material with an outside steel framework. As the batch is fed by the batch charger into the furnace melter area it is heated by the natural gas burners to approximately 1650°C. From the melter the molten glass flows through the refiner then through the waist area, where stirrers homogenize the glass. It then flows into the working end where the glass is allowed to cool slowly to about 1100°C to reach its correct viscosity before delivery to the float bath furnace.
Figure 4:Typical melting furnace cross section
III : Forming and Coating – StewartFloat® and AcuraCoat®
The process of forming the refined liquid glass into a solid ribbon is one of mechanically manipulating the material around its natural propensity to be 6.88 mm thick. From the melting furnace, the glass pours through the canal area which contains an adjustable gate called a tweel that regulates its flow volume and depth to within ± 0.15mm. It lands atop a bath of molten tin, on which it floats— hence the term “float” glass. The glass and tin do not react with each other but stay separated; their mutual resistance at the molecular level makes the glass perfectly smooth.
The float bath furnace is a sealed unit with a controlled atmosphere of nitrogen and hydrogen. It consists of support steel, upper and lower casings, refractory liner, tin, heating elements, a reducing atmosphere, temperature sensors, a computerized process control system and is approximately 8 m wide and 60 m in length with a line speed up to 25 m/min. The float bath furnace contains nearly 200 tonnes of pure molten tin at an average temperature of 800°C.
Figure 5: Schematic of Float Bath Furnace – Narrow End
As the glass forms a thin layer, called a ribbon, in the entrance end of the float bath furnace, a series of adjustable top roll machines on either side manipulate it. The desired width and thickness is obtained through an operator controlled program which sets the speed of the annealing lehr and top roll machines. The ribbon thickness can range from 0.55 to 25 mm. Zones of glowing overhead heating elements are used to control the glass temperature. As the continuous ribbon moves through the float furnace its temperature is gradually reduced, allowing the glass to become flat and parallel. At this point Reflective, Low E, Solar Control, Photovoltaic, and Self Clean coatings can be deposited using the AcuraCoat®Pyrolytic On-Line Chemical Vapor Deposition system. The glass is now ready to be cooled.
Figure 6: Glass is poured in a thin layer atop a bath of molten tin, from which it stays separate as it takes shape. Heating elements hang above and provide heat. The top roll machine speed and angle controls the glass width and thickness.
IV : Annealing
As the formed glass ribbon leaves the float bath furnace, the glass is at a temperature of 600°C. If the glass ribbon was allowed to cool in free air, the surfaces would cool much more rapidly than the internal body of the glass. This would cause the surface to be in severe compression and would also cause unwanted internal stresses in the ribbon.
The thermal history of glass during and immediately after forming is such that internal stresses are bound to be present. It is therefore necessary to bring the glass to ambient temperature gradually by a controlled thermal treatment – annealing. In practice, annealing is carried out in a long annealing lehr (see Figure 7) approximately, 6 m wide and 120 m long, with a predetermined temperature gradient, through which the glass passes. Controlled electric heating elements and air blowers are provided in the annealing lehr to maintain a consistent temperature profile across the width of the glass ribbon.
Figure 7: Annealing Lehr
The end result of the annealing process is glass that has been carefully cooled to ambient temperature without the induction of undue temporary or residual stresses.
V : Cutting and Packing
The cooled glass ribbon exits the annealing lehr and is conveyed to the cutting area by a system of rollers and drives linked to the annealing lehr drive system. The glass passes the on-line inspection system to eliminate any defects and then is scored by carbide cutting wheels and the edges are trimmed from the outer pieces (the refuse is recycled as cullet). It is then cut into sizes that meet the plant’s customer requirements. The glass receives an application of a separating medium, a fine powder that allows the panels to stack without sticking together or scratching. The defect free glass ribbon is then separated into lites for packaging by unloading personnel or automatic equipment for transfer to the warehouse for storage or shipment to the customer.
Figure 8: After exiting the annealing lehr, a ribbon of fully formed glass drops in temperature further as it moves down the cooling line. As it cools, an on-line inspection system checks it for defects.
Figure 9: The glass as it passes under automated cutters that score it into preprogrammed widths before cutting it lengthwise. The cutting blades are computer-adjusted to fulfill the specified dimensions for different sizes of prefabricated glass sheets.
Figure 10: Workers in the warehouse transport bulk loads of glass, weighing about 7,500 kg