Insufficient preheating, temperature gradient defects, and slag prevention and control in low-pressure casting riser | LPDC crucible side operation and maintenance
Insufficient preheating and large temperature difference between the upper and lower parts of the riser resulted in 39% of the riser pipe forming cold skin and oxide film being rolled into the casting.
The initial temperature of the riser is below 620 ℃, and upon contact with the aluminum liquid, a layer of solidified aluminum skin forms on the tube wall, which is carried into the mold cavity with the filling process to form slag inclusions.
The temperature difference between the upper and lower parts of the riser is greater than 75 ℃, resulting in high thermal stress on the pipe wall and an increased risk of cracking. At the same time, the formation of nodules on the inner wall accelerates.
Preheating only relies on natural infiltration of aluminum liquid, without an independent preheating device. Cold pipe problems repeatedly occur during crucible replacement, shutdown, and restart.
The depth of the rising pipe mouth buried in the liquid level is unstable, and after the liquid level drops, the pipe mouth approaches the slag layer, and the filling continues to roll the slag.
The temperature fluctuation of the inner wall of the riser is large, and the aluminum liquid repeatedly condenses and melts, continuously thickening the aluminum nodules and changing the filling cross-sectional area.
Low pressure die-casting molds (LPDC) and wheel hub low-pressure casting molds are equipped with a large number of silicon nitride riser pipes; The same problem exists in reverse pressure casting molds (CPC) and differential pressure casting molds for wheel hubs; Gravity casting molds have no lifting system. Motorcycle wheel hub casting molds, structural component casting molds, aluminum alloy wheel hub molds, aluminum casting forming molds, low-pressure casting tooling molds, many factories ignore the preheating of the riser and only control the mold temperature. Zhejiang Xinfeng Machinery standard startup process: independent gas/resistance preheating, preheating the entire liquid lifting tube to ≥ 620 ℃, temperature difference control ≤ 75 ℃, and then pressing down the aluminum liquid. The crucible liquid level monitoring linkage alarm stops production when the liquid level is below the threshold to avoid slag suction. Every time the crucible is changed, the liquid lifting tube should be preheated again, and it is forbidden to directly pour liquid into the cold tube. Spray isolation coating on the outer wall of the riser to reduce aluminum infiltration and nodules, and extend the maintenance cycle.
FAQ
1. Question: What is the minimum preheating temperature for the riser?
Answer: Preheat the entire product to ≥ 620 ℃ before immersing it in aluminum liquid.
2. Question: What is the warning line for the temperature difference between the upper and lower parts of the riser?
Answer: The temperature difference should not exceed 75 ℃.
3. Question: What are the consequences of direct liquid discharge from the cold riser?
Answer: The cold aluminum skin on the pipe wall forms a slag inclusion defect when it is rolled into the mold cavity.
4. Question: What is the risk of low liquid level?
Answer: The liquid inlet is close to the slag layer and continues to roll the slag.
5. Question: What is the effect of the coating on the outer wall of the liquid lifting tube?
Answer: Reduce aluminum infiltration and delay inner wall nodule formation.
6. Question: Does it need to be preheated again to replace the crucible when shutting down?
Answer: Yes, it is necessary to preheat and meet the standards before production.
