Methods of forming refractory materials – machine pressing forming method

Methods of forming refractory materials – machine pressing forming method

Machine pressing forming method

Machine pressing forming method is the most commonly used forming method in the production of refractory materials. This method uses a brick press and a steel mold to press the mud into a green body. Since general machine pressing forming refers to a semi-dry material forming method with a moisture content of 4%-9%, it is also called semi-dry forming. Commonly used equipment for this method includes friction brick press, lever brick press and hydraulic press.

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, so this method occupies a major position in the production of refractory materials. In order to obtain a dense blank during machine pressing, sufficient pressure must be applied to the blank. The magnitude of this pressure should be able to overcome the internal friction between the blank particles, overcome the external friction between the blank particles and the mold wall, and overcome the uneven pressure distribution caused by the uneven filling of the blank moisture, particles and the blank in the mold.

The proportional relationship between these three depends on the dispersion of the blank, particle composition, blank moisture, size and shape of the blank, etc. Although there are several theoretical formulas for calculating the relationship between pressure and blank densification, such as the change of blank pores in a logarithmic relationship with pressure, etc., the unit area pressure required for the blank is usually determined approximately by experimental methods, and the total pressure of the brick press should be determined accordingly. In addition to the moisture content within a certain range, the requirements for the blank during press forming also include certain requirements for particle size, such as a reasonable particle size ratio and a bulk density as large as possible. The general critical particle size is 3-5mm, and the content of fine powder less than 0.088mm should be within the range of 35%-45%.

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. Too much moisture in the blank, excessive fine powder, too little binder and too high pressure will lead to layer cracking. Therefore, these parameters must be controlled in production. Layer density phenomenon means that the density of the brick after forming reverses along the direction of pressure. Bricks pressurized from above are generally dense on the top and sparse on the bottom, and dense in the middle and sparse on the outside on the same horizontal plane. This is due to the pressure reduction caused by the friction between the blank particles and the friction between the blank and the mold wall. The method of double-sided pressurization and applying lubricating oil on the four walls of the mold to reduce external friction can reduce this phenomenon and reduce the porosity of the blank.

The selection of forming equipment should be comprehensively considered based on the quality requirements, shape, size and production quantity of the product. For general heating furnace bricks or products with simple shapes, large quantities and general quality requirements, a lever brick press with higher production efficiency can be selected; for products with higher quality requirements, a friction brick press with a nominal pressure of not less than 3000kN can be used; for products with larger thickness or more complex shapes and large production volumes, a high-stroke friction brick press can be used for molding. When the product specifies the lower limit of apparent porosity, a press with lower pressure should be selected.