Last Updated on June 16, 2021 by You Ling
My clients often asked me about this questions since in the office of He-Shuai Ltd , it’s hard to say it very clearly in a short time. But here you should know it more.
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 less than 1 μm. If the purity of the alumina is 99.99%, the high-purity alumina ceramic product needs to be ultra-finely pulverized and the particle size distribution is uniform. When extrusion molding or injection molding is used, a binder and a plasticizer are introduced into the powder. Generally, a thermoplastic plastic or resin organic binder having a weight ratio of 10-30% should be at a temperature of 150-200 with the alumina powder. Mix evenly underneath 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 spray the granulation method to treat the powder to make it spherical, so as 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. A research institute in Shanghai 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.
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.
Common 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 molding 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 the powder filling is different. 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 the slurry molding operation.
4 firing technology
The technical method of densifying a granular ceramic body and forming a solid material is called sintering. Sintering is to remove the voids between the particles in the body, remove a small amount of gas and impurity organic matter, and make the particles grow and combine with each other to form a new substance.
An electric furnace is most widely used for heating devices used for firing. In addition to normal pressure sintering, that is, pressureless sintering, there are also hot press sintering and hot isostatic pressing. Although continuous hot press sintering increases the yield, the cost of equipment and mold is too high, and the length of the product is limited due to the axial heat. The hot isostatic pressing uses high-temperature and high-pressure gas as the pressure transmission medium, and has the advantages of uniform heating in all directions, and is suitable for sintering of complex shapes. Due to the uniform structure, the material properties are improved by 30 to 50% compared to cold press sintering. It is 10-15% higher than ordinary hot press sintering. Therefore, some high-value-added alumina ceramic products or special parts required for defense industry, such as ceramic bearings, mirrors, nuclear fuel and barrels, and the field use hot isostatic pressing.
In addition, microwave sintering, arc plasma sintering, and self-propagating sintering are also under development.
Finishing and packaging process
Some alumina ceramicmaterials need to be finished after sintering. For example, articles that can be used as artificial bones require a high surface finish, such as a mirror surface, to increase lubricity. Due to the high hardness of the alumina ceramic material, it needs to be finished with a harder abrasive polishing brick material. Such as SIC, B4C or diamond drill. Grinding is carried out step by step from coarse to fine abrasive, and the final surface is polished. Grinding and polishing can generally be carried out using <1 μm micron Al2O3 fine powder or diamond paste. In addition, laser processing and ultrasonic processing methods of polishing and polishing can also be used.
Alumina ceramic strengthening process
In order to enhance the alumina ceramics and significantly improve its mechanical strength, a new alumina ceramic strengthening process has been introduced abroad. The process is novel and simple, and the technical means adopted is to apply a layer of silicon compound film on the surface of the alumina ceramic by electron beam vacuum coating, sputtering vacuum coating or chemical vapor deposition, and heat at 1200 ° C to 1580 ° C. Treatment to temper alumina ceramics.
The mechanical strength of the reinforced alumina ceramic can be greatly increased on the basis of the original, and an alumina ceramic having an ultrahigh strength is obtained.
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