Pressure curve setting and debugging of differential pressure casting molds

Pressure curve setting and debugging of differential pressure casting molds
The core of differential pressure casting is a differential pressure curve, consisting of four stages: establishment, filling, holding, and releasing. The slope of each stage determines the density and surface quality.
Pressure difference magnitude and equipment capability
The pressure difference for differential pressure casting is usually set at 0.05-0.15 MPa. Above 0.15MPa, the sealing burden between the mold and the tank body is significantly increased.
Below 0.05MPa, the shrinkage capacity is close to that of low-pressure casting, and the process advantage of differential pressure cannot be reflected.
Before setting, it is necessary to confirm the ability of the machine to establish pressure difference. The larger the tank volume, the higher the gas flow rate required to establish the same pressure difference.
Slope of the four segment curve
The first paragraph establishes a pressure difference, and the slope should not be too steep. Establish smoothly within 30-60 seconds to avoid severe fluctuations in the aluminum liquid level.
In the second stage of filling, the pressure difference continues to rise. The charging speed is controlled by adjusting the opening of the intake valve, and the common charging time is 5-15 seconds.
Maintain pressure in the third section and keep the pressure difference at the set value. The holding time is calculated based on wall thickness and is usually 10% -20% shorter than low-pressure casting.
The fourth stage of pressure relief should be done slowly. If the pressure is released too quickly, negative pressure may form inside the casting, and the thin wall may be deformed by suction.
Sealing is a prerequisite for the validity of a curve
Any leakage in the sealing rings of the upper and lower tanks, the interface between the mold and the tank body, or the flange of the liquid lifting pipe will cause the pressure difference to fail to establish.
During debugging, first conduct a short tank pressure test. Maintain the pressure difference for 60 seconds, and if the pressure drop exceeds 5%, search for the leakage point.
The sealing ring will age at high temperatures. It is recommended to replace every 5000 molds, or based on the change in hardness, replace when the hardness increases by 15%.
Key points of coordination on the mold side
The parting surface of the mold must be able to withstand pressure difference. If the gap between the parting surfaces exceeds 0.05mm, the aluminum liquid will penetrate under the pressure difference.
The exhaust system needs to be redesigned. Under differential pressure conditions, the back pressure of the mold cavity is higher, and the exhaust area needs to be increased by 20% -30% compared to low-pressure molds.
Xinfeng Mold will consider both the exhaust channel and the pressure difference curve on the differential pressure mold to avoid insufficient exhaust and material shortage at the filling end.
The relationship between common defects and curves
Curling often occurs when the slope of the filling section is too steep. Extending the filling time by 2-3 seconds significantly reduces the probability of gas entrapment.
Shrinkage often occurs when there is insufficient pressure difference in the pressure holding section. Increasing the pressure by 0.01-0.02MPa or extending the holding time can improve shrinkage.
The surface cold insulation is mostly related to low mold temperature and has little relationship with the curve. This type of problem should return to mold temperature and coating, rather than repeatedly adjusting the curve.
Debugging sequence
First, empty the tank and maintain pressure to confirm sealing. Close the mold again and confirm that the parting surface is in contact. Finally, test the mold with the material and adjust the curve in sections.
Adjust only one section at a time. Simultaneously changing the charging type and maintaining pressure, it is impossible to determine which parameter is working.
After determining the curve, write it into the parameter library and label the wheel type, machine, and mold temperature conditions. When replacing the machine, recalibration is required and cannot be directly applied.
Frequently Asked Questions
Question: What is the pressure difference?
Answer: Normally 0.05-0.15MPa, depending on the wall thickness and machine.
Q: How long does the filling time take?
Answer: Commonly 5-15 seconds, depending on the wheel shape and gate.
Q: Why is pressure relief slow?
Answer: If it is too fast, negative pressure will form, and thin-walled areas are prone to deformation.
Q: How long does it take to replace the sealing ring?
Answer: It is recommended to use 5000 molds or when the hardness increases by 15%.
Q: Do we need to increase the exhaust?
Answer: Yes, it increases by 20% -30% compared to low-pressure molds.
Q: Do I need to recalibrate when changing machines?
Answer: Yes, the curves for different tank volumes are not universal.
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