The glass production process includes: batching, melting, forming, annealing and other processes. They are introduced as follows:
1. 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, heat the prepared raw materials at high temperature to form a uniform and bubble-free glass liquid. This 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 contains 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 color 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 is heated on the upper part of the glass surface. The melting temperature of glass is mostly 1300~1600°C. Most of them are heated by flames, and some are heated by electric current, which are called electric melting furnaces. Now, tank kilns are produced continuously. Small tank kilns can be several meters long, and large ones can reach more than 400 meters2.
3. Forming is to transform the molten glass into a solid product with a fixed shape. Forming must be carried out within a certain temperature range. This 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.
A. Artificial forming. There is also (1) blowing, using a nickel-chromium alloy blow tube, pick a group of glass in the mold while blowing. Mainly used to form glass bubbles, bottles, balls (for scratching glasses), etc. (2) Drawing, after blowing into small 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) Pressing, pick a ball of glass, cut it with scissors to drop it into the concave mold, and then press it with a convex mold. Mainly used to form cups, plates, etc. (4) Free forming. After picking the material, use pliers, scissors, tweezers and other tools to directly make crafts.
B. 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 has (1) calendering, which is used to produce 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. This 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 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. This is a very good thermal insulation and sound insulation material. In addition, the forming of flat glass includes vertical pull method, flat drawing method and float method. Float is a method to make glass flow 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, and this change leaves thermal stress in the glass. This thermal stress reduces 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). In order to clean up the phenomenon of cold burst, the glass product must be annealed after being formed. Annealing is to keep the temperature in a certain temperature range or slowly cool down for a period of time to clean or reduce the thermal stress in the glass to the allowable value. In addition, some glass products can be stiffened in order to increase their strength. Including: physical rigidization (quenching), used for thicker glasses, desktop glass, car windshields, etc.; and chemical rigidization (ion exchange), used for watch cover glass, aviation glass, etc. The principle of stiffening is to generate compressive stress on the surface of the glass to increase its strength.