Longitudinal cracks are cracks that appear along the longitudinal direction of the fired product. Generally, the causes of longitudinal cracks include:

1. When green bodies are loaded into the firing kiln, if they are placed too close to the heating fire wall, local temperatures rise too rapidly, causing the volatile components on the surface of the green bodies to undergo pyrolysis at an excessively fast rate and resulting in uneven expansion or contraction of the green bodies.

2. If the firing temperature ramp-up curve is improperly designed or the operation is conducted incorrectly—specifically, if the temperature rises too rapidly during the stage when large amounts of volatile matter are being released—this can result in an excessive temperature difference between the interior and exterior of the green body. As a result, the surface of the green body has already begun to char, while the interior continues to undergo pyrolysis and release volatile matter, leading to cracking on the surface.

3. If the cooling rate during the cooling phase is too rapid, the surface and interior of the calcined material will shrink at different rates, also leading to cracking.

4. Local leakage of material from the furnace wall causes certain areas of the green body to heat up too rapidly, resulting in cracks.

For the aforementioned types of situations, if the amount of binder used in the green body is too low or the forming pressure is insufficient, resulting in structural defects within the green body, the likelihood of longitudinal cracking during firing will significantly increase.

 

A circumferential crack is a crack that develops along the circumference of the green body, primarily due to poor green-body quality. The causes of such cracks include:

1. During molding, the paste temperature is low, the pre-compression pressure is insufficient, or the pre-compression time is too short, resulting in inadequate pressure.

2. When the pressurized plunger of the extruder returns, it tears apart the paste in the material chamber. During the subsequent pressing cycle, the paste fails to bond properly. Although the surface appears smooth after extrusion, the internal structure of the green body contains defects, making it prone to transverse cracking during firing.

3. The molded parts from the two batches of paste material differ significantly and do not bond well, resulting in defects in the internal structure of the green body.

4. If the plunger head of the press is lubricated excessively, the two batches of paste will fail to bond properly, and cracks are likely to develop after firing.

5. During the firing process, the temperature rises too rapidly, resulting in an excessive temperature difference between the top and bottom of the green body and causing transverse cracks.

6. During cooling, if the upper layer of packing material is removed too early, the upper part of the fired body will be exposed to the air while the lower part remains buried in the packing material. This significant difference in shrinkage rates between the top and bottom of the fired body can lead to transverse cracking.

7. The raw materials were not sufficiently calcined, or raw material lumps were inadvertently mixed in during batching, leading to secondary shrinkage during firing.

8. The paste material is unevenly mixed or contains water;

9. The binder content in the paste is too low, while the powder content is too high.

10. The forming pressure is too high, resulting in excessive internal stress within the green body and causing the internal stress to release too rapidly during firing.

Warping refers to the degree of bending in the longitudinal direction of the fired green body exceeding the allowable dimensional deviation. Deformation refers to localized bulges or depressions that occur in the longitudinal or circumferential direction of the fired green body and exceed the allowable dimensional deviation. The causes of these phenomena include:

1. If the paste contains an excessive amount of binder, it is prone to bending and deformation during firing.

2. Improper firing and charging procedures, such as excessively high furnace chamber temperature during charging, prolonged charging duration, failure to promptly seal the gaps around the green bodies with packing material after loading them, insufficient compaction of the packing material, and the presence of localized voids around the green bodies.

3. The filler contains too much moisture, the filler includes large particles, the amount of new filler used is excessive, and the filler temperature is too high.

4. During furnace loading, if the green bodies are not vertically aligned or if the upper and lower layers of green bodies are improperly matched—especially when the green bodies in the upper layer vary significantly in length—the green bodies in the lower layer, subjected to greater pressure, are more likely to bend and deform.

5. The furnace chamber is in poor condition—for example, there are cracks in the furnace bottom, causing some of the refractory material to leak out and creating localized voids without refractory filling.

6. The heating rate during the low-temperature stage of roasting is too slow, and the distance between green bodies as well as between green bodies and the furnace walls is too small.