Molding is one of the very important links in the production process of electrode paste.

The paste or paste made by kneading is pressed and powdered and mixed with dry powder. It has not a fixed shape and size, nor has it reached a high bulk density. There is no overall strength. In order to obtain a carbon product with a certain shape, size, density and mechanical properties, the kneaded paste must be molded.

The so-called “forming” is the process of pressing the kneaded paste or powder through a certain method and a certain pressure in a mold into a block (rod) object with a certain shape, a certain size, a certain density and a certain mechanical strength. operate.

www.graphite electrode.com Electrode paste production process forming mechanism and method 1

The forming mechanism of electrode paste

The material is densely packed in real time. In order to deform the material, the shear stress in the material must reach a certain value. The shear stress at this time is called the material flow limit stress, which is represented by σs. The size of σs is related to the characteristics of the powder particles in the paste, the characteristics of the binder and the amount of binder. When the material is subjected to various forces during the deformation process, σs is only related to the shear stress with the largest absolute value and the smallest absolute value, σs=σmax-σmin. Generally, for extruded paste, σs=1.8~2.5MPa, and for molded powder, σs=2.0~2.9MPa.

During the pressing process, both the pressed powder and the paste have a certain degree of plasticity, which is related to the physical properties of the material, the softening point of the binder, the amount of the binder added, and the molding temperature. The plasticity of the material is good, the pressure required during molding is small, and the density of the raw product is high, and the mechanical strength is high. However, when the plasticity is too large, the raw product will be easily deformed and the mechanical strength of the product will be reduced. Therefore, the plasticity of the paste must be controlled during molding. The plasticity of the material can be measured with B.

B=d2σc/d1P

B — the plasticity index of the material;

d2 — the bulk density of the material, g/cm3;

d1 — the bulk density of the raw product after pressing, g/cm3;

σc — compressive strength of the raw product after pressing, MPa;

P — pressure during molding, MPa.

The pressed powder and paste must have certain fluidity. When the material is pressurized, it can transmit pressure in all directions at the same time, so that the pressure distribution in the whole chamber can be evenly distributed, reducing pressure loss, and increasing the uniformity of the density of the raw product . On the other hand, due to the fluidity of the material, it can also fill all parts of the material chamber with the material during the pressing process, and also make the density of the raw product uniform. The fluidity of the material is related to its particle shape, size and particle size ratio.

All particles that can move freely have the property of being perpendicular to the direction of the force with their wider and flatter side, which means that the particles can naturally be in the position with the smallest moment of force, which is called the natural orientation of the particles. The particles of paste and pressed powder are not spherical. In the plastic deformation during molding, its extension direction is consistent with the natural orientation, resulting in structural anisotropy. Therefore, the particle arrangement direction and the anisotropy ratio in the green products obtained by different molding methods are also different. The pellets of the green products made by the extrusion molding method are arranged parallel to the direction of the extrusion force, and the anisotropy ratio is large. Isostatically pressed green products are isotropic in structure.

www.graphite electrode.com Electrode paste production process forming mechanism and method 2
www.graphite electrode.com Electrode paste production process forming mechanism and method 3

Method for forming electrode paste

In the production process of the electrode paste, there are many molding methods, the main methods are compression molding, extrusion molding, vibration molding, and isostatic pressing.

  • Compression molding method

This method is to use a vertical press, first make a mold according to the shape and size of the product, and then load a certain amount of kneaded paste or powder into the mold on the worktable of the press, and then start the press to adjust the paste Pressure is applied to the material or pressed powder and maintained for a certain period of time to make it shape, and then the pressed blank is ejected from the mold.

The molding method can be divided into one-way pressing and two-way pressing, hot pressing and cold pressing according to different processes and equipment. The molding method is suitable for pressing products with small dimensions in three directions and the same three-dimensional dimensions, uniform density, and high-strength structure.

However, the products are anisotropic and are mainly used to produce electrode paste, anode paste and various specialties graphite products.

  • Extrusion method

This method is to use a vertical press, first make a mold according to the shape and size of the product, and then load a certain amount of kneaded paste or powder into the mold on the worktable of the press, and then start the press to adjust the paste Pressure is applied to the material or pressed powder and maintained for a certain period of time to make it shape, and then the pressed blank is ejected from the mold.

The molding method can be divided into one-way pressing and two-way pressing, hot pressing and cold pressing according to different processes and equipment. The molding method is suitable for pressing products with small dimensions in three directions and the same three-dimensional dimensions, uniform density, and high-strength structure.

However, the products are anisotropic and are mainly used to produce electrode paste, anode paste and various specialties graphite products.

  • Vibration molding method

Vibration molding is to put the paste into the mold placed on the vibrating table of the vibrating molding machine, and then place a heavy hammer on it, and use the high-speed vibration of the vibrating table to achieve the purpose of compacting and molding the paste.

The vibrating molding machine has a simple structure. As long as a small molding pressure is applied to the paste, it can produce larger-sized products. It is especially suitable for the production of thick and short products and some special-shaped products with the same dimensions in the three directions of length, width and thickness, such as Pre-baked anodes, cathodes, carbon blocks and large or extra large carbon electrodes and crucibles.

  • Isostatic pressing

Isostatic pressing is done in an isostatic pressure vessel. It fills the compressed powder into an elastic mold made of rubber or plastic, seals it and puts it in a high-pressure vessel, and uses an ultra-high-pressure pump to drive the high-pressure liquid medium (oil Or water), the pressed powder is pressed and formed.

Isostatic pressing includes cold isostatic pressing and hot isostatic pressing. Isostatic pressing can produce isotropic products and special-shaped products with uniform structure, high density and strength. Generally used in the production of special graphite, iron type is the production of large-size special graphite products.

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