LPDC pressure holding curve design, how to match the parameters of pressure rise, holding, and release with the solidification rhythm

LPDC pressure holding curve design, how to match the parameters of pressure rise, holding, and release with the solidification rhythm
Pressure holding is the core process variable in low-pressure casting, and the pressure curve is not a simple straight line; Excessive pressure rise leads to gas entrainment and turbulent flow on the liquid surface; Insufficient holding time leads to shrinkage failure; The timing of pressure relief is incorrect, resulting in the return of castings and the recurrence of shrinkage cavities.
1、 Quantitative standard for three-stage pressure curve
1. Liquid lifting section: Low speed pressure rise, pressure rise rate 0.8-1.5kPa/s, smoothly lift the aluminum liquid to the gate, avoiding liquid level rolling, slag rolling, and gas rolling; The lower limit for high and thin-walled components is 0.8-1.0kPa/s.
2. Filling section: The filling pressure is converted according to the height of the casting, and the conventional filling pressure is 15-35kPa; After filling the cavity, switch to holding pressure.
3. Pressure holding section: The holding pressure is generally 5-12kPa higher than the filling pressure; The holding time is equal to the solidification time of the gate, and holding pressure after the gate solidifies is meaningless.
Pressure relief: It must be slowly relieved at a rate of 0.5-1.0kPa/s; Rapid pressure relief, aluminum liquid flowing back into the crucible inside the mold cavity, resulting in negative pressure and looseness inside the casting.
The change in crucible liquid level will change the pressure benchmark, causing a decrease in liquid level and a change in the height of aluminum liquid lifting under the same pressure, which requires dynamic compensation.
Xinfeng Machinery delivers molds with synchronized matching pressure curves and provides liquid level compensation gauges to distinguish between thin-walled and thick walled benchmark curves.
2、 Adjustment strategies for different casting curves
Thin walled shell: The filling speed is prioritized to be stable, the pressure rise is slow, the holding pressure is moderate, the holding time is short, and the mold temperature is too high to prevent aluminum adhesion.
Thick walled hot section components: increase the holding pressure appropriately, extend the holding time, and ensure continuous shrinkage of the hot section; But it cannot exceed the upper limit of the mold seal.
High drop shell: segmented pressure rise, with a stabilizing platform set in the middle to avoid turbulent flow of aluminum liquid splashing during high-level filling.
Long term shutdown and resumption of production, low temperature of aluminum liquid, appropriate reduction of pressure rise rate to prevent cold insulation.
3、 Differences in pressure curve control for overseas customers
Most Southeast Asian production lines fix a simple linear pressure curve without liquid level compensation, resulting in large fluctuations in defects and requiring manual adjustment based on experience.
The North American Tier 1 pressure curve is solidified, with locked pressure, time, and rate parameters, real-time storage of curve data, and automatic alarm for process deviation, which belongs to PPAP control items.
European customers use a closed-loop pressure control system that automatically compensates for pressure based on crucible liquid level, with extremely high curve consistency and stable batch yield.
The filling path of large castings in the Middle East is long, and multi-level stabilizing platforms are set up to reduce turbulent flow at the filling front and minimize the risk of slag inclusion.
4、 Abnormal defect chain of pressure holding curve
Rapid pressure rise → turbulent rolling of aluminum liquid surface → gas entrainment, slag entrainment, and slag inclusion in pores;
The holding time is shorter than the solidification time of the gate → insufficient shrinkage compensation → loose shrinkage holes in the hot section;
Rapid pressure relief → aluminum liquid reflux, negative pressure inside the casting → secondary looseness;
The holding pressure is too high → the sealing of the parting surface and the insert is broken, causing aluminum burrs to run.
5、 Long tail traffic layout ideas
Low pressure casting pressure holding curve, pressure rise rate, LPDC pressure process, aimed at process engineers and equipment debuggers.
keywords LPDC pressure curve control、low pressure casting pressurization rate and dwell time；
Question class: Pressure holding time for A356 low pressure casting;
Comparison class: Ramp up slow vs fast filling pressure LPDC.
Suitable for special pages on process parameters and equipment debugging.
6、 Common misconceptions in the industry
Misconception 1: The higher the holding pressure, the better. If the pressure is too high, it will break through the seal, cause aluminum leakage, and increase the load on the mold.
Misconception 2: The longer the holding time, the better. Continuing to hold the pressure after the gate solidifies will not have a shrinkage effect and will only damage the mold.
Misconception 3: Once the curve is adjusted, it remains permanently unchanged. After the liquid level drops and the aluminum temperature changes, the reference pressure needs to be compensated.
FAQ
Q: What is the rate of pressure increase in the liquid lifting section?
A：0.8–1.5kPa/s。
Q: How much higher is the holding pressure generally than the filling pressure?
A：5–12kPa。
Q: Pressure release rate?
A：0.5–1.0kPa/s， Slowly release pressure.
Q: What is the core criterion for determining the duration of pressure holding?
A: The solidification time of the gate.
Q: How to choose the rate of pressure rise for thin-walled components?
A: Low, 0.8-1.0kPa/s, to prevent air entrapment.
Q: North American pressure curve control?
A: Parameter solidification, data traceability, and out of tolerance alarm.
Q: What are the consequences of rapid decompression?
A: Aluminum liquid reflux generates negative pressure and looseness.
Q: How to set the curve for high drop castings?
A: Step up the voltage in sections and increase the voltage stabilization platform.