Graphite material molding is mainly to improve the density of the mixture powder, so that aggregate and binder close contact, in order to obtain a certain size, morphology and processing margin of green. There are many molding methods, the main methods are extrusion molding, molding, vibration molding and isostatic pressing molding. The principle of extrusion, compression and isostatic pressing is shown in the following figure.
Extrusion molding of graphite materials
For the products with large aspect ratio, it is difficult to produce them because of the restriction of the working distance of the vertical die press and the inhomogeneity of the density along the height direction. The extrusion molding is to continuously extrude the powder from the mouth of the die, and then cut off the product according to the required length. The length of the product is not limited by the working stroke of the extrusion, and the quality of the extruded product is uniform along the length direction. Therefore, it is suitable to produce L/D products with large long strip, rod and tube shape. So graphite electrode, graphite block, graphite tube and other products are generally extruded.
Extruded graphite products have low volume density and low mechanical strength and are anisotropic. The density of the compact changes little in the direction of length, mainly in the cross-section of the compact, the center density is the smallest, and increases with the radius from the center, but the density of the same radius layer is the same, the maximum is at the edge. In the extrusion process, the compaction powder contacts with the wall of the device, resulting in greater friction, resulting in a gradient of friction and flow rate, resulting in the product inside the dense, serious will cause cracking or concentric shell phenomenon. This has a very negative effect on the roasting process.
Lin Xiuqin invented a method of forming graphite crucible by extrusion. The method is to produce small volume graphite unit by extrusion, and then splice small volume graphite unit into large volume graphite crucible by thread connection or binder. The graphite crucible with purity greater than 99.99% can be obtained by the invention.
Guo Quangui, Liu Zhanjun, Liu Lang and others have invented a process of isostatic pressing graphite, in which the materials of first mixing and rolling are extruded into thin rods of 6~10mm, and the fine rods are carbonized, broken and sifted, and then mixed, rolled, crushed and sifted twice to obtain compacted powder. Forming. The graphite prepared by the invention has the characteristics of fine structure, good uniformity, excellent mechanical properties and isotropy.
Alfred H. Stiller, John W. Zondlo, Peter G. Stansberry invented a method for producing high quality and high purity isotropic graphite from petroleum coke. The high-purity graphite produced by extrusion molding has good mechanical properties, high purity and good isotropy.
Roy W. Zocher, Lawrenceburg, invented a method for making synthetic graphite, in which the molding phase is extruded, and the mixture is extruded at a maximum speed of 40 inches per minute at a pressure of 3,300 psi to 2,500 psi. The invention has high quality and fine structure of synthetic graphite.
Molding of graphite materials
The molding adopts the vertical press. First, the mold is made according to the shape and size of the product. Then, a certain amount of mixed and kneaded powder is loaded into the mold on the working platform of the press, and the press is started to exert pressure on the powder and maintain a certain period of time to make it into shape. Then the pressed green blank is ejected from the mold.
Mould pressing method can be divided into unidirectional pressing and bi-directional pressing, cold pressing and hot pressing according to different process and equipment. Mould pressing method is suitable for pressing products with small size in three directions, small difference in size in three directions, uniform density and high density, but the products are anisotropic. It is mainly used for the preparation of electro-carbon products and special graphite, relative extrusion molding, molding in the production of special graphite is more widely used.
Wang Ning, Shenke and Zheng Yongping used natural microcrystalline graphite as aggregate to prepare isotropic graphite by moulding and isostatic pressing respectively. The results showed that the anisotropy index of moulding and isostatic pressing were 1.11 and 1.05 respectively, and the anisotropy index of graphite products formed by the two moulding methods was lower.
Ye Youfa has invented a process for producing isotropic isostatic graphite with large size and fine structure. In the forming stage, isostatic pressing graphite semi-finished products were prepared by bi-directional molding followed by cold isostatic pressing. In the invention, the graphite product produced by moulding and isostatic pressing has a larger specification and can reach more than Φ900. The product has good isotropic property, uniform and fine structure and high volume density.
Zhang Quanli, Zhao Wencheng, Han Xinwei and others invented a production process for producing high-purity graphite profiles. In the molding stage, the graphite profiles were molded. The final graphite profiles were of high purity, high density, high strength, good thermal shock resistance, long service life and easy processing, which could meet the needs of large-size and high-specification cylindrical graphite profiles on the market.
Liu Zhanjun, Guo Quangui, Cao Yaxiu and others prepared high strength graphite materials with “secondary coke” as filler and coal tar pitch as binder. The cold molding pressure was 150 MPa and the packing time was 10 minutes. The compressive strength and flexural strength were 310.0 Mpa and 113.7 Mpa respectively after heat treatment at 1300 C. The volume shrinkage of the material decreases with the fineness of the average particle size of the pressed powder, and the porosity reaches its maximum at 850 C. The porosity decreases gradually with the further increase of the heat treatment temperature.
Deng Nian, Yuan Guanming, Bie Wangwang and others to use fine graphite powder as raw material, modified and high-temperature coal tar pitch as binder, using hot-pressing molding process to prepare carbon blocks, and its carbonization and graphitization treatment to obtain graphite blocks. The results show that the bulk density and resistivity of the carbon blocks prepared by hot pressing change obviously with the increase of heat treatment temperature. Compared with the modified asphalt, the fine graphite materials prepared by high carbon residue pitch as binder have higher bulk density and lower resistivity. Obviously, fine grain graphite has obvious orientation in block materials.
Isostatic pressing of graphite materials
The principle of isostatic pressure is Pascal’s law: the pressure applied to a closed container (liquid or gas) can be equally transmitted in all directions, and the pressure on the surface under its action is proportional to the area of the surface.
Isostatic pressing technology is usually divided into cold isostatic pressing (CIP), warm isostatic pressing (WIP) and hot isostatic pressing (HIP) according to the temperature of forming and consolidation. Due to the different temperature and pressure medium, the three different types of isostatic pressing technology are respectively equipped with corresponding equipment and die materials.
Isostatic pressure forming technology is to place the specimen to be pressed in a high pressure cylinder which is packed in an airtight envelope. By using the incompressible property of liquid medium and the property of uniform transmission of pressure, the specimen is uniformly pressed from various aspects. When the fluid medium is injected into the pressure cylinder, according to the principle of fluid mechanics, the pressure strength is transferred unevenly to the cylinder in all directions. The pressure in the high-pressure cylinder is uniform and uniform in all directions.
Isostatic pressing can produce all kinds of homogeneous and heterogeneous products, and its products have uniform structure, high density and strength. Generally used in the production of special graphite, especially the production of large-size special graphite products.
Zhao Kunming and Qu Yongle invented a cold isostatic pressing graphite manufacturing method. In this method, the cold isostatic pressing molding process is that the pressure is kept at 80 MPa, the pressing time is 40 minutes, and the temperature is controlled at 20℃. After one calcining and one graphitization, the bulk density of the green body is greater than 1.95g/cm3，the resistivity is less than 11~13μΩ.m, the compressive strength is greater than 85~95 MPa, the flexural strength is greater than 35~40 Mpa, the Shore hardness is greater than 60~70 MPa, and the ash content is less than 0.1%.Zang Wenping invented a method for preparing isotropic graphite. In this invention, isostatic pressing was used to form graphite. The forming process was: pressure 100-140 MPa, boost time (> 20 min), holding time 5-30 min, relief time (> 15 min). After calcination, impregnation and graphitization, the density, compressive strength and isotropy of isostatic pressing graphite products are above 1.8g/cm3, above 70MPa and below 1:1:1. Qiu Tingju, Xie Zhiyou and Gao Jiang invented a method of making graphite material. The invention first uses extrusion molding and then isostatic pressing molding. The isostatic pressing molding process is: pressure 150MPa, holding time 5min. Bulk density of molding green: 1.60g/cm3. Wang Chenghua, Jia Wentao, Jie Zhiyou and others have invented a method for preparing unbonded graphite products. In this method, the pressed powder is formed by isostatic pressing. The forming process is: the rising pressure rate is 2Mpa~3MPa/min, the total boosting time is 50~50min, the maximum pressure is 100MPa~200MPa, the holding time is 10~30min, the lowering pressure rate is 4MPa~5Mpa, and the total step-down pressure is reduced. Time 30~40min. The bulk density of its isostatic graphite product is greater than 1.72g/cm3，the resistivity is less than 1.42μΩ.m, the compressive strength is greater than 57 MPa, the flexural strength is greater than 25.4 MPa, the Shore hardness is greater than 40 MPa, and the elastic modulus is greater than 7.2 GPa. Fang Dengke, Yang Dongliang, Yang Qiao et al. used calcined petroleum coke as aggregate, high temperature coal tar pitch as binder, used cold mixing kneading ball mill, and prepared isotropic graphite materials under different pressures by cold isostatic pressing process. The microstructure and physical properties of the materials prepared under different molding pressures were analyzed. The results show that when the average size of aggregate particle d50 is about 21 μm and the content of binder is 33%, the suitable forming pressure is 80-100 MPa. The isotropic graphite material should be prepared under different molding pressure, and the heating curves of calcination and graphitization should be different. The forming pressure should be increased, and the large-scale flake formation in aggregate petroleum coke should be reduced. The amount of structure particles and the homogeneity of aggregate and binder can improve the physical properties of the prepared isotropic graphite materials, and the thermal conductivity and electrical properties of the isotropic graphite materials can be improved by using high temperature coal tar pitch as binder. Yang Guangzhi, Li Dongfeng, Qiu Hanxun and others have invented a method for preparing isotropic graphite with coal-based needle coke as aggregate. The graphite is formed by cold isostatic pressing or hot isostatic pressing. The forming process is as follows: cold isostatic pressing pressure is 100-200 MPa or hot isostatic pressing pressure is 50-100 MPa. After calcination, impregnation and graphitization, the anisotropy index of the product is less than 1.1, the bulk density is not less than 1.8 g/cm 3, the compressive strength is not less than 40 MPa, and the ash content is less than 200 ppm. Wu Qiang invented a special device for producing graphite materials by one-step molding. The device for forming graphite materials is a thermostatic press. The molding process is: pressure 25.0 MPa, time 2 h. After pressing, the density of the compact is 1.7g/cm3.
Clarence L. Hoening, Livermore, Calif have invented a method of preparing high density and high purity graphite by hot isostatic pressing with insoluble metal envelope. The hot isostatic pressing process is as follows: temperature 2200 C, pressure 206.8 MPa, pressure medium Ar, pressing time 2h. The density of the obtained blank is 2.10g/cm3.
D.C Carmichael, W.C Chard, P.D Ownby used hot isostatic pressing to produce dense isotropic graphite. The temperature ranged from 1650 2700 C Chard, P.D Ownby, and the internal gas pressure of isostatic press was 68 MPa 204 MPa. 2.00~2.25g/cm3 graphite compacts can be obtained by hot isostatic pressing.
At present, the forming process of graphite material is mainly cold isostatic pressing, and then hot isostatic pressing. Hot isostatic pressing combined with roasting and densification technology. However, the development direction of isostatic pressing graphite production is: sintering-hot isostatic pressing, hot isostatic pressing-impregnation-roasting and non-binder hot isostatic pressing.
Microstructure of extruded, molded and isostatic pressing graphite materials
During extrusion, the distribution of particles is similar to that of molding when the powder is preloaded in the material chamber. But when the powder is extruded through the mouth, the long axis direction of the particle is the same as that of the particle in motion (at this time, the movement resistance is the least). When the particle moves from the chamber to the nozzle, it rotates with the change of the nozzle curve, making the direction of the long axis consistent with the moving direction. The nozzle has a section of equal diameter, and the powder particles pass through the section of equal diameter, so that the long axis direction of the particles is parallel to the center line of the nozzle. On the same circumference, the force and distribution of particles are the same, so the particles are distributed in a concentric circle, and the distribution of layers is parallel to the direction of molding pressure. The laminated structure of extruded products is shown in the figure.
During the molding process, the powder moves and deforms under the action of pressure in the mold, and becomes compact gradually. When the particles move, the direction of the long axis is the same as the direction of movement, and the moving resistance is the smallest. When a certain density is reached, the displacement of the particles decreases, and the particles rotate under the action of pressure, so that the long axis direction (the larger cross-section plane) is vertical to the pressure direction distribution (at this time the particle center of gravity is the lowest, the most stable), and shows a layered structure. At the same time, the direction of the layer is perpendicular to the molding pressure direction. The laminated structure of molded products is shown in the figure.
During isostatic pressing, the powder is placed in the mold, and the liquid or gas outside the mold acts on the mold under the same pressure and transfers to the material, which makes the material compact from the surrounding to the center. The movement of the particles is mainly translational. Because the force is equal in all directions, the particles do not rotate. The particles are in a state of chaos when loading, and are still in a state of confusion after being dense. Therefore, there is no regular layered distribution structure. The structure of isostatic pressing products is homogeneous, as shown in the figure.
The structure formed during forming is still retained after calcination and graphitization. Therefore, the structure and properties of molded and extruded products are anisotropic, while those of isostatic pressing products are isotropic in structure and properties.
Bulk density distribution of graphite material for extrusion, compression and isostatic pressing
The bulk density distribution of graphite is different from that of graphite.
The density distribution of the extruded blank is the same on the equal diameter surface. The density increases with the increase of the diameter. In the axial direction, the density change of the extruded blank decreases along the height of the blank, but the decrease is small. The density distribution map of the blank is shown in the figure a.
The volume density and strength of the blank formed by die pressing increase gradually from center to edge at the same level. Longitudinally, if it is unidirectional pressing, the density and hardness decrease from top to bottom along the height of the blank. If it is bi-directional pressing, the density and hardness decrease first and then increase along the height of the blank. As shown in the figure b.
The density distribution of isostatic pressing blank is uniform in both axial and radial directions, but when the blank is large, the phenomenon of soft core will occur. The density distribution of isostatic pressing is more uniform than that of extrusion and die pressing. The volume density distribution is shown in the figure c.
Characteristics of isostatic pressing technology compared with conventional technology
Isostatic pressing technology, as a new molding process, has the following characteristics compared with conventional molding technology:
The products produced by isostatic pressing are of high density and generally 5%~15% higher than those of one way and two way molding. The relative density of hot isostatic pressing products can reach 99.80%~99.99%.
The density of the compact is uniform. In the molding process, the uneven density distribution will occur whether unidirectional or bi-directional pressing. This change of density can often achieve more than 10% when pressing complex shape products. This is due to the frictional resistance between the powder and the steel die. Isostatic fluid medium transmits pressure in all directions. The sheath and the powder are compressed roughly the same, the powder and the sheath have no relative motion, the friction resistance between them is very little, the pressure only slightly drops, the density gradient is generally less than 1%, therefore, it can be considered that the green body density is uniform.
Because the density is uniform. Therefore, the ratio of length to diameter can not be restricted, which is conducive to the production of rod shaped, tubular thin and long products.
Isostatic pressing process, generally do not need to add lubricant in powder, which not only reduces the pollution to the product, but also simplifies the manufacturing process.
Isostatic pressing products have excellent performance, short production cycle and wide application range.
The disadvantage of isostatic pressing is that the process efficiency is low and the equipment is expensive.
Author: Xie Huixian
