Dry pressing of pressureless sintered silicon carbide is to add a small amount of binder to the powder, after granulation, placed in the mold, pressurized to form a certain shape of the blank, suitable for pressing the diameter of φ5 ~ 50mm, the height of 0.3 ~ 60mm of the parts.
The essence of mold forming is that under the action of external force, the particles are close to each other in the mold, and the internal friction on the thin layer of bonding agent on the periphery of the particles close to each other firmly connects the particles and maintains a certain shape. Due to the friction between the powder particles and between the powder and the die punch and die wall, the pressing pressure is lost, resulting in uneven density distribution. The pressing process for this kind sintered silicon carbide can be divided into three stages: ① under the action of pressure, the particles begin to move, forming a relatively close stack; ② particles undergo brittle and elastic deformation, at this time the compression curve is stepped; ③under the action of the ultimate pressure, the densification of the billet is no longer increased.
Unidirectional compression, the density along the height of the direction of the decrease, in order to improve the distribution of pressure-free sintered silicon carbide billet density, on the one hand, can be changed to bi-directional compression (including the use of floating negative mold), on the other hand, can be added to the powder lubricant, in order to reduce frictional resistance. Dry press forming is a relatively common forming method in the production of silicon carbide ceramics due to its simple process, easy to operate, short cycle, high efficiency, and easy to realize automated production. In addition, the prepared blanks have high density, accurate dimensions and low shrinkage. However, dry pressure forming is difficult for the production of large blanks, mold wear, processing complexity and high cost. Secondly, the pressurization can only be up and down, the pressure distribution is uneven, resulting in uneven density of sintered silicon carbide, prone to cracking, delamination and other phenomena. With the development of new molding methods, this shortcoming has been overcome by isostatic pressure molding.