Production process of alumina ceramics
Powder preparation
The aluminum oxide powder in the factory is prepared into powder materials according to different product requirements and different molding processes. The particle size of the powder is less than 1μm, if the manufacture of high-purity alumina ceramic products in addition to the purity of alumina in 99.99%, but also need to ultrafine grinding and make its particle size distribution uniform. When extrusion molding or injection molding is used, binders and plasticizers need to be introduced into the powder, generally thermoplastics or resins with a weight ratio of 10-30%. The organic binder should be evenly mixed with the alumina powder at 150-200 temperature to facilitate the molding operation. The powder material formed by hot pressing process does not need to add binder. If semi-automatic or automatic dry pressing molding is used, the powder has special process requirements, and it is necessary to use spray granulation method to treat the powder and make it appear spherical, so as to improve the fluidity of the powder and facilitate the automatic filling of the mold wall in the molding. In addition, in order to reduce the friction between the powder and the mold wall, it is necessary to add 1 to 2% of the lubricant, such as stearic acid, and the binder PVA.
To dry press molding, powder spray granulation is required, in which polyvinyl alcohol is introduced as a binder. A research institute in Shanghai developed a water-soluble paraffin wax as a binder for Al203 spray granulation, which has good fluidity under heating. The powder after spraying granulation must have good fluidity, loose density, and flow Angle friction temperature less than 30℃. In order to obtain higher density of blank, the ratio of particle size is ideal.
Forming method
The forming methods of alumina ceramic products include dry pressing, grouting, extrusion, cold isostatic pressing, injection, casting, hot pressing and hot isostatic pressing. In recent years, at home and abroad have developed a filter molding, direct solidification injection molding, gel injection molding, centrifugal injection molding and solid free molding technology. Different product shapes, sizes, complex shapes and precision products require different molding methods.
Introduction to its common forming:
1, dry press molding: alumina ceramic dry press molding technology is limited to simple shape and inner wall thickness of more than 1mm, length and diameter ratio is not greater than 4:1 objects. The molding methods are uniaxial or bidirectional. The press has two kinds of hydraulic type and mechanical type, which can be semi-automatic or automatic forming method. The maximum press pressure is 200Mpa. The output can reach 15 ~ 50 pieces per minute. Because of the uniform stroke pressure of the hydraulic press, the height of the pressing parts is different when the powder filling is different. The size of the pressure applied by the mechanical press varies due to the amount of powder filling, which is easy to lead to the difference in the size shrinkage after sintering and affect the quality of the product. Therefore, the uniform distribution of powder particles in the dry pressing process is very important for mold filling. Whether the filling amount is accurate or not has a great influence on the dimensional accuracy control of manufactured alumina ceramic parts. Powder particles with more than 60μm, between 60 ~ 200 mesh can obtain the maximum free flow effect, to achieve the best pressure molding effect.
2, grouting molding method: grouting molding is the earliest molding method used for alumina ceramics. Due to the use of gypsum mold, the cost is low and it is easy to form large size and complex shape parts. The key of grouting molding is the preparation of alumina slurry. Usually water is used as the flux medium, and then the degumming agent and the binder are added, after full grinding, the gas is discharged, and then the plaster mold is poured backwards. Due to the absorption of water by the capillaries of the plaster mold, the slurry is solidified in the mold. In hollow grouting, the excess slurry should be poured out when the slurry adsorbed on the mold wall reaches the required thickness. In order to reduce the shrinkage of the body, high concentration slurry should be used as far as possible.
Organic additives should also be added to the alumina ceramic slurry to form a double electric layer on the surface of the slurry particles so that the slurry is stably suspended without settling. In addition, binders such as vinyl alcohol, methyl cellulose, alginate amine and dispersants such as polyacrylamide and gum Arabic are added to make the slurry suitable for grouting molding operation.
