General refractory products are mostly formed by semi-dry method, the moisture content of the blank is low, and various types of brick presses can be used to press. The pressed blank has high strength and can be moved at will without being damaged.
In addition to the common mechanical pressing method, different refractory materials are also suitable for other molding methods.
1. Plastic and grouting molding method
Plastic molding method refers to the method of making blanks with plastic mud with a moisture content of more than 16%. Usually, the prefabricated blank is put into the mud extruder, extruded into mud strips, then cut, and then made into rough blanks according to the required size. The rough blanks are pressed by a re-pressing machine to make the blanks have the specified size and shape. They are mostly used to produce large or particularly complex refractory products.
Grouting molding method is a method of injecting mud into a gypsum model and making a blank with the help of the model to absorb water. Hollow casting is the main casting method. The blanks formed in this way have basically the same internal and external geometric shapes, which are suitable for thin-walled hollow products, such as thermocouple sleeves, crucibles, ball mills, etc.
2. Machine pressing method
Machine pressing method is the most commonly used molding method in the production of refractory materials, and the moisture content is generally 4%-9%. The machine pressing process is essentially a process of making the particles in the blank dense and the air discharged to form a dense blank. The bricks formed by machine pressing have the advantages of high density, high strength, small drying shrinkage and firing shrinkage, and easy control of product size.
In order to obtain a dense blank during machine pressing, sufficient pressure must be given to the blank to overcome the internal friction between the blank particles and the uneven pressure distribution caused by the uneven filling of the blank moisture, particles and mold in the mold.
The most common defects of machine pressed bricks are layer cracking and layer density. Layer cracking is a layered crack perpendicular to the direction of pressure formed during the pressurization process. Excessive moisture in the blank, excessive fine powder, too little binder and excessive pressure will lead to layer cracking. In order to reduce the occurrence of defects, the main factors should be paid attention to during the production process and strictly implemented in accordance with the standards.
3. Vibration molding method
The vibration molding method is to use vibration to make the clay into a green body. The principle is that under the vibration of a very high frequency (generally 3000-12000 times/min), the particles collide with each other, and the dynamic friction replaces the static friction between the particles. The clay becomes particles with fluidity, which gradually accumulate and compact under the action of self-weight and external force to form a dense green body.
During vibration molding, due to the energy output by vibration, the particles have the ability to move in three-dimensional space. The particles are dense and filled in every corner of the model and the air is squeezed out. Therefore, even at a very small unit pressure, a product with a high density can be obtained. In the molding of a variety of products, vibration molding can effectively replace heavy high-pressure brick presses, such as lever brick presses and hydraulic presses.
4. Ramming molding method
The ramming molding method is a method of compacting the clay with a manual, pneumatic or electric ramming hammer.
The power of the pneumatic ramming hammer is compressed air, which is a common power source in factories. The ramming molding method is generally used to mold products with complex shapes and large volumes. The moisture content of the ramming clay is mostly controlled within the range of 4%-6%, and the critical particle size of the clay is larger than that of the machine pressing, which is conducive to improving the density of the green body. During ramming, the materials are generally added in layers, and attention should be paid to the close bonding between the layers. Because the ramming molding method is very labor-intensive, this molding method is currently used less.
5. Hot pressing molding method
Hot pressing molding method is a method of molding and sintering the clay into products under the conditions of heating and pressurization. The hot pressing process combines the molding and sintering of the clay into one process. The characteristic of the hot pressing method is that it can obtain special products with very high density, and its density value can almost reach the theoretical value. Pressurization at high temperature helps the contact and diffusion between the clay particles of the green body, thereby reducing the sintering temperature and shortening the sintering time.
In the refractory industry, hot pressing is mostly used for the manufacture of pure oxide products, such as beryllium oxide, magnesium oxide, calcium oxide, aluminum oxide and other refractory compound products.
6. Isostatic pressing molding method
Isostatic pressing molding method is a method of molding by applying equal pressure in all directions of the clay. Commonly used pressure transmission media are liquids, such as oil, water or glycerin. Since brake oil or anhydrous glycerin has very low compressibility, almost all pressure can be transmitted to the elastic mold, so these two liquids are often used as pressure transmission media.
The biggest feature of isostatic pressing is that each part of the clay is evenly pressed and the pressure is very high, so the resulting green body has high and uniform density, which greatly reduces the deformation and shrinkage of the green body during the firing process, and does not cause the firing cracks caused by stress generated by density difference in the green body formed by the general molding method.
7. Extrusion molding method
The extrusion molding method is a method of using strong extrusion of plastic clay to form it through a hole mold. Extrusion molding and plastic molding both use plastic clay. The difference between the two is that extrusion molding requires a strong extruder, and the power source is hydraulic, compressed air or mechanical pressure, which is mostly used in the production of special refractory materials.
The advantage of extrusion molding is that it can manufacture products with a length much larger than the cross-section, such as hollow tubes or solid bars of various cross-sections, and the length is cut according to needs, with high production efficiency. The disadvantage is that the clay material is required to have greater plasticity, and a large amount of binder is added to the clay material, thereby increasing the firing shrinkage of the product and reducing the density of the product.
8. Melt casting method
The melt casting method is a molding method in which the batch material is directly cast into a product after being melted at high temperature. Since the batch material is generally melted in an electric arc furnace, the melt-cast products are also called electric fused products.
The biggest feature of melt casting is that the high-temperature melt is directly poured into the model for molding. This requires the model to have high refractory performance, good air permeability, high impact strength and thermal shock resistance, and does not react with the melt. Commonly used models are made of quartz sand, corundum sand or graphite plate.
Melt-cast refractory products have the characteristics of large grains, dense structure, high mechanical strength, good corrosion resistance, etc., and are mainly used in the glass industry.
HOME