How is glass made?

The production process of glass includes: batching, melting, molding, annealing, among others. The specific process is as follows:

1. Batching. All kinds of raw materials are weighed and mixed evenly in a mixing machine according to the material sheet. The main raw materials of glass are: quartz sand, limestone, feldspar, soda ash, boric acid and so on.

2.Founding: The prepared raw materials are then heated at high temperature to form a uniform bubble-free glass liquid. This is a very complex physical and chemical reaction process. The melting of glass takes place in a melting furnace. Basically, there are two types of melting furnaces. One is the crucible furnace where the glass is stored in a crucible and heated outside of the crucible. Small crucible furnace only holds one single crucible, and for a large one it can be up to 20 crucibles. Crucible furnace uses intermittent production process, and now only optical glass and colored glass are produced this way. Another type of melting furnace is tank furnace where the glass material is melted in the furnace and heated by open fire on the surface of the glass liquid. The melting temperature of glass is mostly 1300~1600゜C. Most of them are heated by flame, but a small amount of them are heated by electric current, which is called electric melting furnace. Today, the furnaces are produced continuously. The small furnaces can be several meters long, and the big ones can be more than 400 meters long.

3.Molding. It is the conversion of molten glass into a solid product with a fixed shape. Molding, which must take place within a range of temperatures, is mainly a cooling process in which glass first changes from a viscous liquid to a plastic state and then to a brittle solid. There are manual molding and mechanical molding. For manual molding, it is labor intensive and low efficient, thus is obsolete gradually, except in some special occasions.

Float glass is made by pouring the molten glass from a furnace into a chamber that contains a bed of molten tin. The process is sometimes called the Pilkington Process. The atmosphere inside the chamber is carefully controlled. The glass floats on the tin and forms itself in the shape of the container. It spreads 90 to 140 inches wide at a thickness determined at the time of manufacture. The upper surface of the glass is called the air side or score side. It is polished with fire. The lower surface is called the tin side. It is not fire-polished. From the chamber, the glass enters an oven, called a lehr. There it is slowly cooled at a specific rate. This process, called annealing, relieves the glass of internal stresses. The rate of cooling is crucial to the success of the final product. The glass emerges from the lehr at room temperature as a continuous ribbon. It is flat, fire-finished on the top, and has smooth, parallel surfaces. Automatic cutters trim the edges and cut the glass to length.

Flaws in the founding of glass in crucible furnace.

In the process of glass founding, refractories and glass liquid interact with each other at high temperatures, causing the refractories to be damaged by corrosion. For glass furnaces, the corrosion of refractories by molten glass is unavoidable, and the corrosion intensity mainly depends on the physical properties such as viscosity and surface tension of molten glass.

1. Corrosion of the crucible wall

On the one hand, a large amount of gas and the effect of temperature difference will be released in the process of founding of the complex materials, clarification and homogenization of the glass liquid, which will drive the violent churn of the glass liquid, and the stirring effect will drive the churn of the glass liquid, which will have a scouring effect on the crucible wall and be eroded. These scour and corrosion effects make the surface of the crucible wall constantly peel off and enter the glass liquid and those too late to diffuse and homogenize will cause the glass to produce a large number of stripes and nodules.

2. Corrosion by the churning blade

When the churning blade rotates, the glass liquid is constantly turned over and the small bubbles in the glass liquid can be reconstituted and polymerized into a large bubble rising and discharging, strengthening the homogenization of glass liquid. However, since the churning blade is made of corundum mullite, it is also constantly eroded by the glass liquid. When replacing the blade, we can find the edges and corners of the blades have been ragged by corrosion. This is due to the different granularity of the composition of the churning blade material, different force of the churning blade, and different strength of corrosion. Some coarse-grained refractories fallen off into the glass liquid will form stones and nuclear nodules can not be completely melted.

Corresponding Measures

1. To reduce melting and clarifying temperature appropriately.

The higher the temperature of the glass liquid is, the lower the viscosity becomes, and the more beneficial it is for homogenization. Nevertheless, its corrosion to the crucible wall becomes more series and the glass is more prone to flaws. Therefore, under the condition of ensuring the homogenization quality, the melting temperature should be appropriately reduced to 1320 degrees and the clarification temperature to 1450 degrees to ensure the complete clarification and homogenization of the glass liquid.

2. To repair or replace the top brick in time

In order to avoid the peeling refractory material falling into the crucible, when the furnace is stopped, the surface of the furnace roof brick is warped due to corrosion, and the part that may be peeled off should be removed clean. If the corrosion is serious, the new high-quality refractory brick should be replaced in time.

FYI: Annealing

Annealing is a process of heat treatment of metals, in which a metal is slowly heated to a certain temperature, held for enough time, and then cooled at a suitable rate. The purpose is to reduce hardness and improve machinability. It is to clean up residual stress, stabilize size, reduce deformation and crack tendency. It also serves to refine grain, adjust preparation, and get rid of weakness in preparation. Precisely speaking, annealing is a kind of heat treatment process for materials, including metallic materials and non-metallic materials.