The production process of ceramic parts in general can be divided into three stages: powder → molding → firing. Today, we will discuss the powder molding process, the raw material of ceramic parts is loose powder material, the process of powder molding is to make loose powder raw material or its aggregate into a certain size, and strength of the body or product. In the forming process of ceramic parts, ceramic parts powder raw materials need to be processed to make billets suitable for specific forming methods. According to the flow and rheological characteristics of the blank, the forming methods of ceramic parts can be divided into three categories: pressing forming method, plastic forming method and slurry forming method.

Stamping process, also known as stamping process, is to add a small amount of binder to granulation powder (moisture content must be controlled within 4%-7%, or even up to 1%-4%), and then place the granulated powder in the mold (dry pressing molding is generally steel mold, cold isostatic pressing is rubber film). The body is pressed on a pressure machine to form a certain shape. The characteristic of pressing forming method is that the binder content is low, it can be directly roasted without drying, and the shrinkage rate of the billet is small. The method greatly improves the density of the billet, and then improves the strength of the parts, and the mechanization level of pressing forming is high. Dry pressing and isostatic pressing are widely used in pressing forming. The principle, process and characteristics of dry pressing and isostatic pressing are introduced, and the advantages and disadvantages of the two forming methods are compared.

First, dry pressing

Dry pressing is a process of pressing powder billet in the model based on a large pressure. Its essence is that the particles are close to each other in the mold under the action of external force, and the particles are firmly linked and maintain a certain shape with the help of internal friction. The process includes feeding, pressure forming, demoulding, billet production and mold cleaning.
1, feeding: the powder particles into the mold frame, in order to ensure the specifications and quality of the body, feeding should be uniform and quantitative.
2, pressure molding: the use of the relative movement between the mold to put pressure on the loose powder, so that the powder pressed into a dense body. This process is the key process in the pressing process, need to control the size of the pressure, pressing time and pressing method and other factors, any change in conditions may lead to changes in the quality of the body.
3, release: the molded body from the mold cavity out.
4, billet out: the shape of the ejecting body moved to the billet surface or conveyor belt.
5, clean the mold: if necessary, it is also necessary to spray oil in the inner wall of the mold cavity to lubricate.

Second, isostatic pressing

Isostatic pressing, also known as hydrostatic forming, is a forming method that utilizes the incompressibility of the liquid medium and the uniform transfer of pressure. That is to say, the pressure of the sample in the high-pressure vessel is the same as the pressure of the static water at the same depth, so it is called hydrostatic or static pressure, and the forming process obtained according to this principle is called hydrostatic or isostatic pressing. Generally, the isostatic pressure of ceramic parts materials can be divided into wet bag isostatic pressure and dry bag isostatic pressure.
Compared with the dry pressing method, the biggest feature of the isostatic pressing method is that the powder material is in a three-way compressive stress state, so the performance and quality of the forming part are significantly improved.

The advantages of isostatic pressing are specifically shown as follows:

1. Dry press molding has only one to two pressure surfaces, while isostatic press molding is multi-axial pressure, that is, multi-directional pressure and multi-surface pressure, which is conducive to compactingthe powder to a comparable density. At the same time, the linear displacement of powder particles is small, and the friction work consumed in the movement of powder particles is relatively small, which improves the pressing efficiency.
2. Compared with other pressing forms with roughly the same pressure strength, isostatic pressing can obtain a higher density of green billet, and it is uniformly dense in all directions, and does not change greatly because of different shape thickness.
3. Due to the small difference in the pressure direction of isostatic pressing, the friction between powder particles and between particles and the model is significantly reduced, so the phenomenon of stress in the green body is rarely seen.
4. The green body formed by isostatic pressing has high strength, uniform internal structure and no particle orientation arrangement.
5. The water content of the powder is very low (generally in 1%-3%), and the adhesive and lubricant are not necessary or rarely used. This is beneficial for reducing drying shrinkage and firing shrinkage.

The pressing molding method is also called die molding, among which dry pressing and isostatic pressing are the most widely used. Compared with dry pressing, the biggest feature of isostatic pressing is that the ceramic parts powder is in a strong three-way compressive stress state, so the compact body is more dense, the anisotropy is small, and the mechanical properties are good. In the production of ceramic parts phone backplane and high-end ceramic parts bearing ball, the two pressing methods have their own uses.