Alumina ceramics are also known as precision ceramics, special ceramics, or high technology ceramics. It is a ceramic that uses highly selected raw materials and is produced according to a special manufacturing process that allows precise control of chemical composition and excellent performance.

Currently, alumina ceramics are mainly used in high-tech and cutting-edge industries such as microelectronics, nuclear reactors, aerospace, magnetic fluid power generation, artificial bones and artificial joints. Alumina ceramics should meet the following three requirements in the manufacturing process.

① The selected raw materials should be of high purity and the particles should be as fine as possible.

② Strict control of chemical composition. The manufacturing process should prevent the mixing of impurities and the volatilization of the components themselves, and the granularity, interface, and porosity of the sintered parts should be strictly controlled in order to achieve stable quality and reproducibility.

③ Precise shape and size. Alumina ceramic parts are generally not processed and used directly, especially ceramic electronic devices require high precision.

Alumina ceramics and ordinary ceramics are very different in composition and manufacturing process. Ordinary ceramics are made through three processes: raw material preparation, billet forming and kiln firing; alumina ceramics are mostly manufactured by the powder sintering method. In terms of forming technology, due to the extremely high hardness of ceramics, it is difficult to cut and process, especially for the complex shape of non-conforming products, such as the supercharger rotor in the car engine, bones, teeth and other biological ceramic products, after forming and sintering that is the finished product, no further processing. In order to meet this requirement, people imitate the polymer materials industry injection molding technology to produce plastic parts, processing alumina ceramic products, and achieved satisfactory results.

Ceramic injection molding technology, is added to the ceramic powder thermoplastic resin, thermosetting resin, plasticizer and friction reducer, so that the ceramic powder into a viscous elastic body, and then heated and mixed slurry from the spout into the metal mold, cooling and curing is ready. Commonly used thermoplastic resins are polyethylene, polystyrene, polypropylene, with an addition of 10-30%. This technology has greatly improved the accuracy and reliability of forming complex products.

1 Injection molding equipment

The injection molding machine is generally composed of a plasticizing device (or injection device), a mold closing device, a hydraulic device, and electronic and power control devices. The types can be divided into plunger type and parallel propeller type according to the different internal structure of the plasticizing device. In recent years, the latter is generally considered to have more advantages.

The injection molding machine is centered on the electronic and power control device, which drives the hydraulic device while making the plasticizing device and the mold closing device work in sequence. The working procedure is: the ceramic raw material is put in by the funnel and enters the cylinder, the raw material is melted and stirred while feeding the end of the cylinder, and the blank is injected into the cavity of the metal mold by the nozzle at the end of the cylinder. The advanced foreign control system adopts the screen display method (such as using oscilloscope, plasma, electroluminescence, liquid crystal), as well as the design of graphic console method or the combination of the two composite console method.

Mold materials generally use high cleanliness, wear resistance, corrosion resistance are excellent alloy steel. The mold design should conform to the flow characteristics of the ceramic polymer system. In order to reduce the shrinkage of the forming body and avoid the air inside the mold to be involved in the forming body, so the mold should consider controlling the release outlet. On ring-shaped products, the opposite side of the grille is prone to fusion patterns, so attention should also be paid to the position of the grille. In order to optimize the injection conditions, it is necessary to rely on the temperature management of the mold, barrel, etc. and the pressure sensor inside the mold for careful management. In addition, there should be cooling troughs on the mold for cooling and heating, relying on thermostats to keep the mold temperature constant, which is effective for improving the accuracy of the formed body. Since a large amount of organic material is to be used in the raw material, degreasing without carbon residue left is also an important issue in order not to produce thermal cracks in the blank.

2 Process

The injection molding principle for ceramics is basically the same as that for plastics. Only the plastic is mixed with a large amount of ceramic powder. In order to improve the injection molding conditions, it is necessary to select the organic material that matches the raw material used and to select the amount to be added. In order to obtain a dense and homogeneous injection molding body, the concentration of ceramic powder should be higher. But too high will make the molding performance worse. In order to improve the flowability of the blends, the viscosity of the dispersant polymer system should be reduced. As a pre-treatment is very important to improve the dispersion of ceramic powder, in order to improve the fluidity of the polymer, need to add appropriate plasticizers and lubricants. The particle size of ceramic raw material is generally 1μm, add binder (or called additive), after thorough mixing and stirring. 

In the mixing process, the ceramic powder is wetted and encapsulated by the binder, and all become a uniform compound before injection molding. And need to be cooled, dried, crushed, before obtaining suitable for injection molding machine funnel feeding particles.

1, the fluidity of raw materials
Injection molding of ceramic particles used in general by 80-90% (important ratio, the same below) of the powder and 10-20% of the binder composition. The binder is removed in the degreasing process, so the amount added to the minimum appropriate, but it should be noted that if the amount added is not enough will affect the forming effect. In addition, the flowability of ceramic particles is worse the smaller the particle size and the more the shape deviates from spherical. Therefore, the flowability should be tested in the simplest way possible.

2. Defects caused by forming conditions
If the forming conditions are not correct, various defects can occur. One of the most critical is the fusion welding line, if the forming body with through-hole or blind hole, it is easy to have this defect. Therefore, it is necessary to pay attention to the design of the mold, especially the type, location, size and number of openings. Attention should also be paid to the balance between injection temperature and speed of injection molding.
In addition, in order to avoid defects such as surface roughness, cracks, long stripes, deformation, etc. In the case of forming difficulties, an adaptive controller can be installed in the injection molding machine for subtle control.

3、Degreasing
This process is also called removing the binder, usually the rate of temperature rise is 3~5℃/b, about 5~10 days, but when it is carried out under the protective atmosphere of 0.5MPa pressure, 40 hours can end the degreasing.

4、Sintering
Thermal and other parameters can be determined according to the type of ceramic. Line shrinkage in sintering is about 15-20%, the shape is more complex or thicker wall work, easy to produce cracks in the sintering, should be careful to prevent.
Additives

This is one of the most important issues in injection molding technology. Different forming methods require different additive properties. Such as injection molding requires degreasing, fluidity, gum solubility, strength, shrinkage; extrusion requires plasticity, gum solubility, strength, lubrication; cold isostatic pressing (rubber molding) requires lubrication, granulation, strength; mechanical pressing requires lubrication granulation, strength, demoulding, etc. At the same time, different bonding agents should be used for different products.

Injection molding technology requirements for additives are as follows.
1, gum solubility: all kinds of forming methods to try to use less additives is appropriate, can be used to decoupling (gum solubility) additives, which reduce the production cycle and product costs are beneficial. In particular, it is beneficial to remove the binder later.

2, fluidity: injection molding requires fluidity under high pressure. The use of resin-based binder injection effect is good, the reason is that when using resin-based, attention should be paid to maintaining viscosity and slow injection.

3, expansion and shrinkage: injection molding due to the use of a large number of binder, resulting in large shrinkage of the blank, affecting the dimensional accuracy and geometry, and easy to form pores. Therefore, you can choose a class of paraffin expansion - shrinkage of small substances for additives. This kind of material than rosin and other non-crystalline material expansion - shrinkage effect is small. 

Injection molding technology, Japan as early as the 1950s used A12O3 ceramics for internal combustion engine ignition plug. Later, it was used in the production of small and complex parts one after another. In recent years, people are actively engaged in a variety of heat-resistant, wear-resistant parts development research, for example, the diesel engine vortex chamber head has been ceramicized, is the use of injection molding technology manufacturing.

It is reported that the Institute of Production Technology of the University of Tokyo in Japan has successfully tested the "freeze injection molding method" using only water. This method uses the fluidity and freezing properties of water to solidify and demold ceramics. When the internal freezing reaches the release strength, the blank can be removed from the mold. Since it is not necessary to use a large number of organic additives as mentioned before, there is no need for a long degreasing process, so it can greatly; shorten the production time, thus creating favorable conditions for reducing costs and increasing production.

Article source: Electronic Materials Circle

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