In graphite plate production, transverse or longitudinal cracks longer than 10 mm are not allowed, and for medium and larger specifications, cracks wider than 1 mm and longer than 50 mm are not permitted. Smaller cracks can mostly heal during the product firing process, but larger cracks become channels for volatile matter to escape during firing, causing the cracks to widen.
Reasons for cracks on the surface of graphite plates:
1. Excessive binder content in the paste or high charging temperature can lead to significant elastic rebound after the paste is extruded, and the stress dissipates slowly, which may cause cracks. High charging temperatures can also result in incomplete removal of flue gas, which, when trapped in the paste, can easily cause cracks.
2. Insufficient binder content in the paste or too low a charging temperature results in poor plasticity of the paste and weak bonding between paste particles, making it difficult to compact and thus prone to cracking.
3. Excessive temperature of the extrusion die and pressure chamber causes the paste close to the die and chamber walls to be overheated. This creates a significant temperature difference between the paste near the walls and the paste in the center of the chamber. After compression, the extrusion speed of these two types of paste differs, and the resulting product has inconsistent rebound coefficients between the surface and the center, which easily leads to cracks. Conversely, low heating temperatures of the extrusion die and pressure chamber, while the paste in the center is at a higher temperature, will have the same consequences.
4. Uneven cooling of the paste during cooling, sometimes even adding paste that has already cooled into hard lumps to the pressure chamber; and insufficient heating of the pressure chamber to uniformly heat the paste in a short time, can also cause cracks on the surface of the graphite plate after extrusion.
5. After the graphite plate leaves the extrusion die, it should be supported by a semi-circular (for circular cross-section graphite plates) or horizontal plate (for square cross-section graphite plates) receiving platform. If the receiving platform is improperly positioned, excessive bending and sagging of the graphite plate after extrusion can also cause transverse cracks in the product.