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The process of shaping clay into a body or product of a certain size, shape, and strength with the help of external forces and models is called shaping. There are many methods for forming refractory materials, which are generally divided into three types according to the moisture content of the billet.
Semi-dry method: the moisture content of the blank is about 5%
Plastic method: the moisture content of the blank is about 15%
Grouting method: The moisture content of the steel billet is about 40%
In addition, there are vibration molding, 500~1500℃ hot pressing, and isostatic pressing molding.
1. Grouting method
The grouting molding method injects a slurry with a moisture content of about 40% into a water-absorbing model (usually a gypsum type). The plaster mold absorbs the slurry moisture and forms a mud film on the surface. When the mud film reaches a certain thickness requirement over time, pour out the excess mud and leave it for some time. When the green body reaches a certain strength, it is demoulded, dried, damaged, and then sent to the next process. This method is mainly used to produce thin-walled hollow high-end products and special products, such as thermowell, high-temperature furnace tubes, stacks, etc.
2. Plasticity method
The plastic method refers to making undesirable bodies from clay with a moisture content of more than 16%. Typically, prefabricated blanks are fed into the extruder. The mud rod is extruded and then chopped, and then the scrap is made according to the required size, and then the blank is pressed with a re-press so that the bad valve body has the specified size and shape.
3. Semi-dry molding
This is a widely used forming method in the production of refractory materials. There are two forming methods, namely mechanical pressing and manual forming. The mud used is a semi-dry granular material with a moisture content of 3-10%. Manual (or vibration) molding is mainly used to produce some large or special products, and the slurry has a high moisture content of 10-12%.
The physical essence of semi-dry forming is: that the blank particles are redistributed under pressure. Under the action of mechanical binding force, electrostatic attraction, and friction, the blank particles are tightly combined, elastic deformation and brittle deformation occur, air is discharged, and undesirable particles are produced and combined into products of certain size, shape, and strength.
In semi-dry molding, attention should be paid to “layer density” and “elastic aftereffect”. The so-called layer density refers to the phenomenon that during the pressing process, because the internal pressure of the green body does not comply with Pascal’s law, the density near the compression surface is relatively large, and as the distance from the compression surface increases, the porosity gradually increases and the density decreases. The so-called elastic sequelae refer to the bad external force generated by the elastic body after extrusion. Elastic aftereffects are often the direct cause of waste products produced during jade processing.
4. Vibration modeling
Vibration molding is a molding method for high-density refractory products. Under the vibration of 3,000 times per minute, particles collide with each other to obtain high-density refractory products. It is generally supplemented by pressure, so it is also called the pressurized vibration method. This method has been widely used in the production of foreign refractory products. In my country, it is mainly used to produce converter tar dolomite lining bricks.
The results show that the vibration force, type and amount of binder, proportion of particles, moisture content, and pressure in the vibration molding method have an impact on the performance and quality of refractory products. For example, when vibrating clay bricks, high alumina bricks, and tar magnesia bricks, there must be enough fine powder, generally ≤0.088mm and fine particles account for 30% or 35% of the material, otherwise the ideal bulk density cannot be achieved.
5. Isostatic pressure
Isostatic pressing is a new forming technology developed later. It mainly applies basic principles to pressurize liquids and evenly transfer the pressure to the powder through the rubber mold. The distinctive feature of this forming method is that it eliminates the layer density phenomenon of other mechanical pressings. Due to the non-directional nature of its pressure, a solid body with uniform density can be obtained. At the same time, because the density of the green body is uniform, there is no directionality in shrinkage during the firing process, and sintering cracks will not be caused by stress due to density differences. In addition, it is easy to process products with complex shapes.
6. Hot-pressure electrofusion casting
Hot press forming: Refractory products produced by sintering take a long time to forge, but the porosity is still very high, because during the sintering process, on the one hand, the air pressure in the pores increases, which offsets the driving effect of the interface energy; on the other hand, Closed pores can only use material to diffuse inside the crystal, but internal diffusion is much slower than interfacial diffusion. Compression molding applies pressure during the sintering process to ensure sufficient driving force to obtain high-density special products with a density value of almost the theoretical value.
In the electrofusion casting method, the raw materials are melted at high temperature in an electric arc furnace, then cast into a mold, and then slowly cooled. This method is mainly used to produce zirconium mullite bricks, zirconium corundum bricks, and quartz bricks. The disadvantages are high power consumption and high production costs.
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