1. Dry pressing:
The dry pressing technology of alumina ceramics is only limited to objects with simple shape, inner wall thickness of more than 1mm, and the ratio of length to diameter of less than 4:1. The forming methods are uniaxial or bidirectional. There are two types of presses, hydraulic and mechanical, which can be semi-automatic or fully automatic. The maximum pressure of the press is 200Mpa. The output can reach 15~50 pieces per minute. Because the stroke pressure of the hydraulic press is uniform, the height of the pressed parts is different when the powder filling is different. However, the pressure applied by the mechanical press varies with the amount of powder filling, which easily leads to differences in size shrinkage after sintering and affects product quality. Therefore, the uniform distribution of powder particles during dry pressing is very important for mold filling. The accuracy of filling volume has a great influence on the dimensional accuracy control of alumina ceramic parts. Powder particles shall be more than 60 μ m. The maximum free flow effect and the best pressure forming effect can be obtained between 60 and 200 meshes.
2. Grouting forming method:
Grouting is the earliest forming method for alumina ceramics. Due to the use of gypsum mold, the cost is low and it is easy to form large and complex components. The key of grouting is the preparation of alumina slurry. Generally, water is used as the flux medium, and then the glue remover and adhesive are added. After full grinding, the air is exhausted, and then poured into the gypsum mold. Because the capillary tube of gypsum mold absorbs water, the slurry is solidified in the mold. During hollow grouting, when the mortar adsorbed on the formwork wall reaches the required thickness, the surplus mortar shall be poured out. In order to reduce the shrinkage of green 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 slurry particle surface so that the slurry can be stably suspended without sedimentation. In addition, adhesives such as vinyl alcohol, methyl cellulose, alginate amine and dispersants such as polypropylene amine and arabic gum shall be added to make the slurry suitable for grouting and molding operation.
