The complexity of controlling the cooking process is due to the speed of transfer of the products as well as the significant heat flux that is given to the product per unit of time. In this stage of production, the design of the thermal curve is very important.
Here, the discussion of temperature transfer to the piece at any moment of time and its control from the beginning to the end of the heat treatment is of particular importance, which creates mechanical strength, transparency and removal of impurities from the manufactured products. Inadequate temperature at any moment causes defective parts and waste of energy. Failure to control the thermal conditions and excessive temperature increase not only causes energy loss, but also more energy is consumed to cool the manufactured products and reduce their temperature.
Today, construction ceramics are produced and marketed in several ways in the industrial world, which can be divided as follows:
1. Double cooking method
In this method, the desired products are used according to their size and design, and non-metallic raw materials (kaolin, feldspar, talc, dolomite, silica, etc.) are used to produce their bodies. The raw production bodies are shaped in the press section and after checking the mechanical and chemical properties, they are transferred to the dryer section and then to the furnace. The product is sintered and baked in the oven under heat treatment for a certain time. This leads to high strength of production parts with special standards of those products. In the first stage, only the body is baked and sintered, but in the next stage, after glazing and decoration, the desired products are transferred to the glaze firing furnace, and then the glaze is sintered along with the body. In fact, at that temperature, the glaze has less thermal tolerance than the body, and the glaze melts on the body and becomes glassy. At this stage, the glaze takes exactly the shape of the body. One of the important principles that must be controlled in this process is the adaptation of the coefficients of thermal expansion of the body and glazing. The coefficient of thermal expansion depends on the material of the glaze and the body and the amount of heat transferred in a certain time.
In both types of ovens used to produce double-baked products (body and glaze), the structure of the ovens is exactly the same, but the difference is in the number of burners and a series of minor items:
1. In body baking furnaces, the length of the furnace is longer and the burners are installed on both sides of the furnace body and along the length of the furnace. These burners are located both under the rollers and on the product, where the number of under rollers is more than the number of burners on the product. The burners in the glaze firing furnaces are placed only on the product and along the length of the furnace, in fact, there is no burner in the lower part of the product.
2. The beginning of the body baking ovens is designed in such a way that the dryer is installed in line with the oven. The work of the dryer is to dry the raw material and bring it to the required temperature and prepare it to enter the furnace. The conditions of drying and reducing the humidity of the product are very sensitive and subject to a specific time and temperature chart, which has a very high sensitivity. If there is no dryer and drying in the glazing furnaces of this department and the glazed product enters the glazing furnace at the ambient temperature of the production hall.
3. In body firing furnaces, the ceramic rollers that move the tile forward play a very important role in transferring heat to the product, but in glaze firing furnaces, this role is very insignificant and can be ignored due to the structure of the furnace. According to the main differences mentioned for both types of furnaces. These differences are not technologically important, but they are important in terms of heat transfer and have a great impact on the process of firing ceramics and creating special properties in the product.
2. Single cooking method
The single cooking system is a newer and more difficult method. In this method, the body and the glaze are baked in one period and at the same time, and the design of the formula of ceramic bodies and glaze will be such that they have relatively the same thermal tolerance conditions. The products in this method have major differences in terms of special properties with the products of the double cooking method. This method is mainly used for floor tiles. In Spain and Italy, this method is also used to produce wall tiles
