The molds used for compression molding are all made of metal, The forming mold has a great influence on the quality of refractory bricks products.
The influence of mold on the quality of refractory bricks products.
If the hardness of the part contacting the mold and the blank is not uniform, soft spots will appear during use, resulting in potholes and corrugated grooves, resulting in layered cracks in the brick blank during the brick out process; if the mold hardness is too low, the wear will be severe. The corresponding brick section is enlarged accordingly; when the brick is released, when the enlarged brick section passes through the smaller section, the brick is subject to considerable squeezing force and produces dendritic cracks or mouth cracks; if the rigidity of the mold Insufficient, the brick will be repeatedly subjected to the elasticity of the mold to cause longitudinal cracks;
If the strength of the mold is not enough, not only will it be damaged quickly, but the deformation of the bottom plate or punch will also cause the distortion of the brick; if the mold is not smooth enough, it will increase the resistance of the top brick, and at the same time due to the up and down of the refractory brick The increase in pressure difference increases the density difference of the brick. Therefore, the material selection, design,x, and processing of the mold are very important.
Generally, the mold is required to have high wear resistance, pressure resistance, and impact resistance. From the comprehensive consideration of strength, hardness, and toughness, the materials currently used for steel molds in refractory plants are mainly carbon steel and alloy structural steel, tool steel, bearing steel, etc.
The molds processed with these materials must undergo chemical heat treatment and quenching treatment. The quenching process may cause internal stress in the mold, so under normal circumstances, tempering is performed at a relatively low temperature of 200~350℃. Figure 1 shows the temperature variation curve of annealing, quenching, and tempering of a certain tool steel mold.
In terms of the internal structure of the molded refractory bricks, the most frequently encountered problems are:
(1) The green body is fracturing (or laminar cracking), which is a layered crack perpendicular to the pressing direction formed during the pressing process. In addition to the reasons for the mold, this phenomenon is mainly due to the fact that the air in the blank is not discharged. This type of spallation is particularly prone to occur when the moisture of the blank is too high; in addition, unreasonable process parameters such as excessive fine powder, too little binder, and excessive pressure are also factors that cause the spallation;
(2) The density of the green body is uneven. The reason is that when the billet is pressed, there is friction resistance between the particles and the particles and the mold wall, which prevents the pressure from being transmitted uniformly and causes the parts of the blank to be unevenly pressed, resulting in the compaction of the parts The degree is different. In addition, the size (H/D) between the filling height of the blank and the pressure area is also an important factor that causes uneven density. As shown in Figure 2, the greater the pressure, the more uneven the transmission of pressure. In the pressure direction, the pressure decreases from the pressure surface along with the height of the blank, and the density distribution of the blank changes from tight to loose; if double-sided When pressurized, the density of the green body is close at both ends and thin in the middle. Therefore, to reduce the density difference of the molded blank, it is necessary to pay attention to the design of the mold and the method of molding and pressing.
The formed bricks are dried in an oven at 50~60℃ until the residual moisture is not more than 2.5%, and then they can be transported to the firing section for kiln firing. The firing of brick products is carried out in an inverted flame kiln or tunnel kiln. The kiln installation method uses flat packaging, special products, and large-scale, special-shaped bricks, you need to use specially designed tool bricks. For the firing system, please refer to the firing system for thin-walled products. After firing, it is cooled with the kiln. To speed up the kiln turnover, the first kiln door can be opened after the flame is turned off, the second kiln door can be opened after 8h, and the third kiln door can be opened after 8h. After the flame is turned off for 72h, it can be blown into the kiln for cooling. Of course, this cooling system changes accordingly with the volume of the kiln.
The products after exiting the kiln should be subjected to quality inspection and cold processing of the final shape and size. Especially like assembled corundum crucible bricks, it needs to be assembled closely. Therefore, it has high requirements for dimensional tolerances, flatness, and smoothness. It must be processed accurately according to the drawings.
The products that fully meet the quality requirements are finally stored as finished products.
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