How is glass produced?

The glass production process includes: preparing the ingredients, melting, forming, annealing and other processes.

  1. Prepare the ingredients
    According to the designed material list, weigh various raw materials and mix them evenly in a mixer. The main raw materials of glass are: quartz sand, limestone, feldspar, soda ash, boric acid and so on.
  2. Melting
    The prepared raw materials are heated at high temperatures to form a uniform and bubble-free glass liquid. It is a very complicated physical and chemical reaction process. The melting of the glass takes place in the furnace. There are two main types of melting furnaces: one is a crucible furnace, where the glass frit is contained in the crucible and heated outside the crucible. The small crucible kiln only puts one crucible, and the large one can have as many as 20 crucibles. Crucible kiln is produced in gap type, and now only optical glass and colored glass are produced in crucible kiln. The other is a tank kiln, where the glass frit is melted in the kiln, and an open flame heats the upper part of the glass surface. The melting temperature of the glass is mainly 1300~1600°C. Most of them are heated by flames, and a few are heated by electric current, which is called electric melting furnaces.
  3. Forming
    The molten glass is transformed into a solid product with a fixed shape. Forming must be carried out within a specific temperature range. It is a cooling process. The glass first changes from a viscous liquid state to a plastic state and then into a brittle solid state. The forming method can be divided into two categories: artificial forming and mechanical forming.
    Artificial forming:
    1) Blowing, using a nickel-chromium alloy blow tube, pick a group of glass in the mold while blowing. They are mainly used to form glass bubbles, bottles, balls (for scratching glasses), etc.
    2) Drawing, after blowing into tiny bubbles, another worker sticks it with the top plate, and the two of them are mainly used to make glass tubes or rods while blowing.
    3) Press, pick a ball of glass, cut it with scissors to drop it into the concave mold, and then press it with a convex mold. They are mainly used to form cups, plates, etc.
    4) Free forming, after picking the material, free-forming uses pliers, scissors, tweezers, and other tools to make crafts directly.
    Mechanical forming:
    Because of the high labor intensity, high temperature and poor conditions of artificial forming, most of them have been replaced by mechanical forming except for free forming. In addition to pressing, blowing, and drawing, mechanical forming also includes rolling, casting, centrifugal casting, and sintering.
    1) The calendering method produces thick flat glass, cut glass, and metal wire glass.
    2) Casting method to produce optical glass.
    3) Centrifugal casting method, used to manufacture large diameter glass tubes, utensils and large-capacity reaction pots. It is to inject the glass melt into a high-speed rotating mold. The centrifugal force makes the glass adhere to the mold wall, and the rotation continues until the glass hardens.
    4) Sintering method, used to produce foam glass. It adds a foaming agent to glass powder and heats it in a metal mold with a lid. The glass forms many closed bubbles during the heating process. It is suitable for heat insulation and sound insulation material.
    In addition, the forming of flat glass includes the vertical pull-up method, flat-draw method and float method. Float is a method to make the glass stream float on the surface of molten metal (tin) to form flat glass. Its main advantages are high glass quality (flat and smooth), fast drawing speed, and large output.
  4. Annealing
    The glass undergoes drastic temperature changes and shape changes during the forming process, which leave thermal stress in the glass. Such thermal stress will reduce the strength and thermal stability of the glass product. If it is cooled directly, it is likely to rupture on its own during the cooling process or later during storage, transportation and use (commonly known as the cold burst of glass). To eliminate the phenomenon of cold bursts, the glass product must be annealed after being formed. Annealing is to keep the temperature in a specific temperature range or slowly cool down for a period to eliminate or reduce the thermal stress in the glass to the allowable value. In addition, some glass products can be stiffened to increase their strength, including Physical tempering and Chemical tempering.
    Physical tempering (quenching), used for thicker glasses, desktop glasses, car windshields, etc..
    Chemical tempering (ion exchange), used for watch cover glass, aviation glass, etc..
    The principle of tempering is to generate compressive stress on the surface of the glass to increase its strength.