Float Glass Melting Process and Defects

January 06, 2023

Glass melting process

The glass melting process is a series of physical, chemical and physicochemical reactions of the compound in a high temperature heated environment, which is a crucial process in glass production. The result is the transformation of the compound into a complex molten material, i.e., glass liquor. From the heating of the compound to the fusion into glass, it can be divided into five stages:

  1. Silicate formation stage. The components of the compound react during the heating process, releasing most of the gaseous fugitives, and the compound is transformed into a sintered mass consisting of silicate and quartz sand. It is basically in the solid state, and this stage ends at 800-900°C. The mating material is added directly to the high temperature zone of the pool kiln fusion, and the reaction is very rapid and is completed within 3 to 5 minutes.

  2. Glass formation stage. Continuing to increase the temperature, the sinter begins to melt until the mating material particles are completely dissolved and the sinter becomes a clear liquid. However, the glass liquid will contain a lot of bubbles, streaks, etc., and is not homogeneous in chemical composition. This stage ends at 1200°C. However, the glass formation rate will be very slow affected by the dissolution and expansion rate of quartz sand grains.

    For soda lime silica glass, it takes about 32 minutes from the beginning of silicate formation to the end of glass formation, where silicate formation takes only 3 to 4 minutes, while glass formation takes 28 to 32 minutes. Moreover, there is no clear boundary between the two processes, and the glass formation starts before the end of silicate formation.

  3. Glass liquid clarification stage. Continue to heat up the temperature, the viscosity of the glass liquid to reduce the bubbles in the glass liquid growing, rising to the liquid surface to escape, until the bubbles are all removed from the process that is the clarification process. The temperature of the clarification stage is 1400 ~ 1500 ℃. The gas is mainly present in the glass solution in three forms: visible bubbles, physically dissolved gas and chemically combined gas. Therefore, the clarification process of glass liquor is the process of removing visible gas bubbles.

  4. Glass liquid homogenization stage. Glass liquid in a high-temperature environment for a long time, the streaks in the glass liquid by stirring diffusion, dissolution and elimination, the entire glass liquid chemical composition gradually tends to uniformity, the temperature also tends to uniformity, which is the homogenization process of glass liquid. The homogenization temperature of ordinary soda lime silica glass can be lower than the clarification temperature.

  5. Glass liquid cooling stage. The viscosity of the clarified and homogenized glass liquor is smaller than the viscosity needed for molding, so it cannot be used for molding immediately. Different molding methods require different viscosity of the glass liquid, so the glass liquid must be cooled to achieve the viscosity required for molding. Ordinary soda lime silica glass usually has to be cooled to about 1000 ℃ to enter the tin bath molding through the runner.

    According to the process is divided into five stages that are interrelated and affect each other, often at the same time or alternately, there is no obvious boundary between them.

Defects in the vitreous humor

The various substances in the vitreous that affect homogeneity can be called vitreous defects. They can be divided into three main categories according to their state of existence.

  1.  Gas bubbles: visible gas inclusions in the glass body, it is melting, clarification, homogenization process of various gas residues, is the most notable defects in the glass body, affecting the quality of the appearance of glass, reducing the mechanical strength and transparency of glass products.

  2. Calculus: crystalline solid inclusions that appear in the glass body, is the most dangerous defect in the glass body, the appearance of glass products and optical homogeneity has a serious impact, is the main factor causing the blow-up.

  3. Lines, stripes and nodules: the presence of foreign glassy inclusions within the glass body, mostly protruding from the glass body, where lines and nodules have a particularly serious impact on the quality of the glass.