To obtain carbon products with a specific shape, size, density, and mechanical properties, the kneaded paste must be formed. Numerous forming methods are available, with compression molding, extrusion, vibration molding, and isostatic pressing being common in the carbon industry.
During the pressing process, both the pressed powder and the paste exhibit a certain degree of plasticity. The extent of this plasticity depends on the material's physical properties, the softening point of the binder, the amount of binder added, and the molding temperature. Good plasticity reduces the required molding pressure, resulting in a high density and mechanical strength for the resulting green product. However, excessive plasticity can easily cause the green product to deform, reducing its mechanical strength. Therefore, the paste's plasticity must be controlled during molding.
The pressed powder and paste must exhibit a certain degree of flowability. When the material is compressed, it can simultaneously transmit pressure in all directions, ensuring uniform pressure distribution across the entire chamber, both vertically and horizontally, minimizing pressure loss and increasing the uniformity of the green product's density. On the other hand, the material's fluidity allows it to fill every part of the chamber during the pressing process, ensuring uniform density in the raw product. The material's fluidity is related to its particle shape, size, and particle size ratio.
All freely movable particles possess the property of aligning their wider, flatter side perpendicular to the direction of the applied force. This means that the particles naturally assume a position of minimal torque, a characteristic known as their natural orientation. Paste and pressed powder particles are not spherical. During plastic deformation during molding, their extension direction aligns with their natural orientation, resulting in structural anisotropy. Therefore, different molding methods produce different particle alignment directions and anisotropy ratios within the raw product. In raw products produced by extrusion molding, particles are aligned parallel to the extrusion force and exhibit a high anisotropy ratio. In raw products produced by compression molding, particles are aligned perpendicular to the direction of the molding force and exhibit relatively low anisotropy. In contrast, raw products produced by isostatic pressing are structurally isotropic.