According to different product requirements and different molding processes, the alumina powder entering the factory is prepared into powder materials. The particle size of the powder is below 1μm. If the purity of the high-purity alumina ceramic products is 99.99%, it needs ultra-fine grinding and uniform particle size distribution. When extrusion molding or injection molding is used, the binder and plasticizer need to be introduced into the powder. Generally, the thermoplastic or resin organic binder with a weight ratio of 10-30% should be at a temperature of 150-200 with the alumina powder. Mix uniformly to facilitate molding operations. The powder material formed by the hot pressing process does not need to add a binder. If semi-automatic or fully automatic dry press molding is used, there are special process requirements for the powder, and the spray granulation method is required to treat the powder to make it spherical, which is beneficial to improve the fluidity of the powder and facilitate the automatic filling of the mold 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 the lubricant, such as stearic acid, and the binder PVA.
When dry press molding is needed, the powder is sprayed and granulated, and polyvinyl alcohol is introduced as a binder. A research institute in Shanghai developed a water-soluble paraffin as a binder for Al203 spray granulation, which has good fluidity when heated. The powder after spray granulation must have good fluidity and loose density, and the flow angle friction temperature should be less than 30 ℃. The particle gradation ratio is ideal and other conditions to obtain a larger green density.
Molding method
Alumina ceramic product molding methods include dry pressing, grouting, extrusion, cold isostatic pressing, injection, casting, hot pressing and hot isostatic pressing molding. In recent years, domestic and foreign countries have developed molding technology methods such as filter press molding, direct solidification injection molding, gel injection molding, centrifugal grouting molding and solid free molding. Different product shapes, sizes, complex shapes and precision products require different molding methods.
Introduction of commonly used molding:
1. Dry press forming: The dry press forming technology of alumina ceramics is limited to objects with a simple shape and an inner wall thickness of more than 1mm and a length to diameter ratio of not more than 4: 1. The forming method is uniaxial or bidirectional. There are two types of presses, hydraulic type and mechanical type, which can be semi-automatic or fully automatic molding. The maximum pressure of the press is 200Mpa. The output can reach 15-50 pieces per minute. Due to the uniform stroke pressure of the hydraulic press, the height of the pressed parts is different when the powder filling is different. The amount of pressure applied by the mechanical press varies with how much the powder is filled, which can easily lead to differences in dimensional shrinkage after sintering and affect product quality. Therefore, the uniform distribution of powder particles during dry pressing is very important for mold filling. The accuracy of the filling amount has a great influence on the control of the dimensional accuracy of the manufactured alumina ceramic parts. Powder particles larger than 60μm and between 60 and 200 mesh can obtain the maximum free-flow effect and obtain 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 molds, the cost is low and it is easy to form large-sized and complex-shaped parts. The key to grouting 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 fully grinding, the gas is exhausted, and then poured into the gypsum mold. Due to the absorption of moisture by the capillary of the gypsum mold, the slurry solidifies in the mold. In the case of hollow grouting, the excess slurry needs to be poured out when the slurry absorbed by the mold wall reaches the required thickness. In order to reduce the shrinkage of the green body, high concentration slurry should be used as much as possible.
An organic additive needs to be added to the alumina ceramic slurry to form an electric double layer on the surface of the slurry particles so that the slurry is suspended in a stable manner without precipitation. In addition, binders such as vinyl alcohol, methyl cellulose, alginate, and dispersants such as polyacrylamine, gum arabic, etc. need to be added in order to make the slurry suitable for injection molding operations.