Powder preparation
The alumina powder entering the plant is prepared into a powder material according to different product requirements and different molding processes. The particle size of the powder is below 1 μm. If the purity of the alumina is 99.99%, the ultra-fine pulverization is required to make the particle size distribution uniform. When using extrusion molding or injection molding, it is necessary to introduce a binder and a plasticizer in the powder, generally a thermoplastic resin or a resin having a weight ratio of 10-30%. The organic binder should be 150-200 with the alumina powder. Mix evenly at temperature to facilitate the molding operation. The powder raw material formed by the hot pressing process does not need to be added with a binder. If semi-automatic or fully automatic dry pressing is used, there is a special process requirement for the powder. It is necessary to treat the powder by spray granulation to make it appear spherical, in order to improve the fluidity of the powder and facilitate the automatic filling of the molding. wall. In addition, in order to reduce the friction between the powder and the mold wall, it is necessary to add 1 to 2% of a lubricant such as stearic acid and a binder PVA.
Powder spray granulation is required for dry press molding, in which polyvinyl alcohol is introduced as a binder. In recent years, a research institute in Shanghai has developed a water-soluble paraffin as a binder for Al203 spray granulation, which has good fluidity under heating. The powder after spray granulation must have good fluidity, loose density, and a flow angle friction temperature of less than 30 °C. The particle gradation ratio is ideal and the like to obtain a larger bulk density.
Molding method
Alumina ceramic products are formed by various methods such as dry pressing, grouting, extrusion, cold isostatic pressing, injection, casting, hot pressing and hot isostatic pressing. In recent years, molding techniques such as pressure filtration molding, direct solidification injection molding, gel injection molding, centrifugal grouting molding and solid free forming have been developed at home and abroad. Different product shapes, sizes, complex shapes and precision products require different molding methods.
Picking up its usual molding introduction:
1. Dry pressing: The alumina ceramic dry pressing technology is limited to objects with a simple shape and an inner wall thickness of more than 1 mm and a length to diameter ratio of not more than 4:1. The forming method is uniaxial or bidirectional. The press is available in hydraulic or mechanical form and can be semi-automatic or fully automatic. The maximum pressure of the press is 200Mpa. The output can reach 15 to 50 pieces per minute. Since the hydraulic press has a uniform stroke pressure, the height of the pressed parts is different when there is a difference in the filling of the powder. However, the pressure applied by the mechanical press varies depending on how much the powder is filled, which may cause a difference in dimensional shrinkage after sintering, which affects product quality. Therefore, the uniform distribution of powder particles during dry pressing is very important for mold filling. The accuracy of the filling has a great influence on the dimensional accuracy control of the manufactured alumina ceramic parts. The powder particles have a maximum free-flowing effect of more than 60 μm and between 60 and 200 mesh, and the best pressure forming effect is obtained.
2, grouting molding method: grouting molding is the earliest molding method used in alumina ceramics. Due to the use of plaster molds, the cost is low and it is easy to form large-sized, complex-shaped parts. The key to grout molding is the preparation of alumina slurries. Usually, water is used as a flux medium, and then a debonding agent and a binder are added, fully ground, and then vented, and then poured into a plaster mold. The slurry is solidified in the mold due to the adsorption of moisture by the gypsum mold capillary. In the hollow grouting, when the mold wall adsorbs the slurry to the required thickness, the excess slurry needs to be poured out. In order to reduce the shrinkage of the green body, a high concentration slurry should be used as much as possible.
An organic additive is also added to the alumina ceramic slurry to form an electric double layer on the surface of the slurry particles so that the slurry is stably suspended without precipitation. In addition, a binder such as vinyl alcohol, methyl cellulose, alginic acid amine, or a dispersing agent such as polyacrylamide or gum arabic is added, and the purpose is to make the slurry suitable for slurry molding operation.
