The birth of glass: A comprehensive introduction to the manufacturing process of float glass

The birth of glass: A comprehensive introduction to the manufacturing process of float glass

[ Zhongbo.com ] There are three types of flat glass in the world today: flat pull, float, and clendering. Float glass takes more than 90% of the total glass production and is the basic building material in the world's architectural glass. The float glass production process was founded in 1952 and sets the world standard for high quality glass production. The float glass process consists of five main steps:

● Ingredients

● Melting

● Forming and coating

● Annealing

● Cutting and packaging

• Ingredients

The ingredient is the first stage of preparing the raw material for melting. Raw materials include sand, dolomite, limestone, soda ash and salt cake, which are continuously transported by truck or train. These raw materials are stored in the mixing room which has silos, hoppers, conveyor belts, chutes, dust collectors, and necessary control systems, which control the conveying of raw materials and the mixing of the batch materials. From the moment the raw materials are delivered to the warehouse, they are in constant motion.

In the interior of the mixing room, a long flat conveyor belt carries the raw materials in sequence from the silos of various materials, one layer after another to the bucket elevator, and then sent to the weighing device to test its compound weight. Recycled glass shards or production line backfills are added to these ingredients. Each batch contains approximately 10-30 % cullet. The dried material is added to the mixer and stirred into the mixture. The stirred mixture is sent to the kiln head warehouse for storage through the conveyor belt and then added into the melting kiln at a controlled rate by the feeder.

Picture 1 Ingredients of the Typical glass

Picture 2 broken glass yard

Picture 3 Using a hopper to send the mixture into the inlet of the melting furnace with 1650℃

• Melting

A typical furnace is a horizontal flame furnace with six regenerators, approximately 25 meters wide and 62 meters wide, with a daily production capacity of 500 tons. The main parts of the furnace are the melting /clarifying pool, the working pool, the regenerator and the small furnace. As shown in picture 4, constructed of special refractory materials with steel frame. The mixture is sent by the feeder to the melting pool of the melting furnace, which is heated to 1650 degrees by a natural gas spray gun.

The molten glass flows from the melting pool through the clarification pool to the neck area and is stirred evenly. It then flows into the work section and slowly cools down to approximately 1100 degrees, allowing it to reach the correct viscosity before reaching the tin bath.

Picture 4 Typical Section of Melting Furnace

• Forming and coating

The process of forming the clarified liquid glass into a glass plate is a mechanically manipulated process according to the natural tendency of the material, which has a natural thickness of 6.88 mm. Liquid glass flows from the furnace through the flow channel area, and is controlled by an adjustable door called a flashboard which penetrates approximately ± 0.15 mm of liquid glass. It floats on the molten tin liquid - hence named as float glass. Glass and tin do not react with each other and can be separated; the molecular forms of them repel each other and make the glass extremely smooth.

  The tin tank is a unit sealed in a controlled atmosphere of nitrogen and hydrogen. It comprises a steel support, a top cover and the bottom case, refractories, tin and heating element, a reducing atmosphere, temperature sensors, computer process control system, approximately 8 meters wide and 60 meters long, the production line speeds up to 25 yards / minute. The tin bath contains nearly 200 tons of pure tin with an average temperature of 800 °C . When the glass forms a thin layer at the end of the tin bath inlet, which is called a glass plate, there are a series of adjustable edgers on each side for operation. The operator sets the speed of the annealing kiln and the edger with a control program. The thickness of the glass plate can be between 0.55 and 25 mm. The upper partition heating element is used to control the glass temperature. As the glass sheet continues to flow through the tin tank, the temperature of the glass sheet will gradually decrease, making the glass flat and parallel. At this point, the AcuraCoat ® pyrolytic CVD equipment can be used to plate reflective films, LOW E films, solar control films, photovoltaic films, and self-cleaning films. At this point the glass is ready to cool.

Picture 5 Cross Section of the Tin Tank

Picture 6, the glass is spread into a thin layer on the molten tin liquid, and is kept separated from the tin liquid to be formed into a plate shape. The heat is supplied by the hanging heating element, and the width and thickness of the glass are controlled by the speed and angle of the edger.

• Annealing

The temperature of the glass is 600 degrees when the shaped glass leaves the tin tank. If the glass plate is cooled in the atmosphere, the glass surface will cool faster than the interior of the glass, which will cause severe surface compression and harmful internal stress on the glass plate.

The heating process of the glass before and after forming is also a process of internal stress formation. Therefore, it is necessary to reduce the temperature of the glass to the ambient temperature gradually – that is annealing. In fact, the annealing is carried out in an annealing kiln (see picture 7 ) with a pre-set temperature gradient of approximately 6 meters wide and 120 meters long . The annealing kiln includes an electrically controlled heating element and a fan to maintain a constant distribution of the lateral temperature of the glass sheet.

In a word, the result of the annealing process is that the glass is carefully cooled to room temperature without causing temporary stress or stress.

Picture 7 Annealing kiln sectional view

• Cutting and packaging

The glass plates cooled by the annealing kiln are conveyed to the cutting zone by a roller conveyor connected to the annealing kiln drive system. The glass passes through an online inspection system to eliminate any defects, cut with a diamond cutting wheel to remove the edges of the glass (the edge material is recycled into cullet). It is then cut to the size per the customer needs. The surface of the glass is sprinkled with a powder medium so that the glass sheets can be stacked and stored to avoid sticking or scratching. The unblemished glass panels are then stacked manually or automatically packed, transferred to the warehouse for storage or shipment to the customer.

Picture 8 After the glass plate is out of the annealing kiln, the glass plate is fully formed and moved to the cooling zone to continue cooling.