What is the production process of alumina ceramics
When using extrusion molding or injection molding, binder and plasticizing agent should be introduced into the powder. Generally, thermoplastic or resin organic binder with weight ratio of 10-30% should be mixed evenly with alumina powder at 150-200 temperature to facilitate the molding operation. There is no need to add binder for powder material formed by hot pressing process. If semi-automatic or full-automatic dry pressing is adopted, it has special technological requirements for the powder. Spray granulation method is needed 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 during the molding. In addition, in order to reduce the friction between powder and mold wall, 1 ~ 2% lubricant, such as stearic acid, and binder PVA should be added.
It is necessary to spray the powder for dry pressing molding, in which polyvinyl alcohol is introduced as binder. A water-soluble paraffin wax was developed as a binder for spray granulation of Al203 with good fluidity under heating conditions. Spray granulation of powder must have good fluidity, loose density, flow Angle friction temperature less than 30℃. The particle size ratio is ideal to obtain the larger density of the green billet.
Molding method
The forming methods of alumina ceramic products include dry pressing, grouting, extrusion, cold isostatic pressing, injection, flow casting, hot pressing and hot isostatic pressing, etc. In recent years, some molding technologies have been developed, such as filter press molding, direct solidification injection molding, gel injection molding, centrifugal grouting molding and free solid molding. Products with different shapes, sizes, complex shapes and precision require different forming methods.
Common forming introduction:
1, dry press molding: alumina ceramic dry press molding technology is limited to simple shape and the inner wall thickness of more than 1mm, length to diameter ratio of no more than 4:1 of the object. The forming method can be uniaxial or bidirectional. There are two kinds of presses: hydraulic and mechanical, which can be semi-automatic or full-automatic forming. The maximum press pressure is 200Mpa. The output can reach 15 ~ 50 pieces per minute. Because the stroke pressure of hydraulic press is uniform, the height of pressing parts is different when the powder filling is different. However, the pressure exerted by the mechanical press varies with the amount of powder filling, which easily leads to the difference of size shrinkage after sintering and affects the product quality. Therefore, the uniform distribution of powder particles is very important for mold filling in the dry pressing process. The accuracy of filling quantity has great influence on the dimension precision control of alumina ceramic parts. The maximum free flow effect can be achieved for powder particles larger than 60 m and between 60 ~ 200 meshes, and the best pressure forming effect can be achieved.
2. Grouting molding: Grouting molding is the earliest molding method used for alumina ceramics. Due to the use of gypsum mold, low cost and easy to shape large size, complex shape parts. The key of grouting is the preparation of alumina slurry. Usually with water as the flux medium, then add the gel and binder, after fully grinding exhaust, and then pour into the gypsum mold. The slurry solidified in the mold due to the adsorption of water by the gypsum capillary. When hollow grouting, when the mold wall adsorption slurry to the required thickness, but also need to pour out the excess slurry. In order to reduce the shrinkage of the body, high concentration slurry should be used as far as possible.
It is necessary to add organic additives in alumina ceramic slurry to form a double layer on the surface of slurry particles to stabilize slurry suspension without precipitation. In addition, the binder such as vinyl alcohol, methyl cellulose, alginate and dispersant such as polyacrylamine and gum Arabic should be added to make the slurry suitable for grouting operation.
