How to set the process parameter window for low-pressure casting molds

How to set the process parameter window for low-pressure casting molds
Low pressure casting relies on a four-dimensional window for stable production: filling pressure, liquid rise rate, mold temperature, and holding time.
Lower and upper limits of pressure window
The filling pressure is usually taken as 0.02-0.08MPa. Below 0.02MPa, the aluminum liquid does not rise enough, and the far end is prone to insufficient pouring.
Above 0.08MPa, the filling speed is too fast, and the risk of gas entrapment and mold sticking increases. Thin walled components can be set to a higher value, while thick walled components can be set to a lower value.
The pressure also needs to match the diameter of the riser. As the diameter of the pipe increases while the pressure remains constant, the flow rate will increase, and the actual filling speed has exceeded the set value.
The liquid lifting speed determines the degree of gas entrainment
It is recommended to control the liquid level rise speed during the liquid lifting stage at 50-150mm/s. If it is too fast, turbulence will occur and the oxide film will be rolled into the casting.
The inclusions formed by turbulent flow are mostly distributed on the surface of the inner cavity of the casting, and are only exposed after machining, which belongs to implicit losses.
Segmented liquid lifting is a common practice: slow exhaust in the front section, acceleration in the middle section, and smooth filling during deceleration in the end section. Splitting into three sections can balance efficiency and quality.
The mold temperature window is more sensitive than pressure
The mold temperature for aluminum alloy casting is usually maintained at 200-350 ℃. Below 200 ℃, the risk of cold insulation significantly increases.
Above 350 ℃, both mold sticking and shrinkage become more pronounced. The lifespan of the mold coating will also be significantly shortened.
The uniformity of mold temperature is more important than absolute value. If the temperature difference on the surface of the mold cavity exceeds 15 ℃, the solidification sequence between thin and thick areas will be disrupted.
Algorithm for holding time
The holding time is often estimated based on wall thickness, ranging from 1-1.5 seconds per millimeter. When the wall thickness is 6mm, hold the pressure for 6-9 seconds.
Insufficient pressure holding, gate not solidified, aluminum liquid refluxed after pressure was removed, forming shrinkage cavities. The holding pressure is too long, the rhythm is wasted, and the gate is difficult to cut off.
Thick and large parts need to be separately extended for pressure maintenance. The unified holding time of the entire mold often sacrifices the efficiency of the thin-walled area.
How to calibrate a four-dimensional window
First, use simulation to provide the initial window, and then use T0 trial to make single factor adjustments. Change only one parameter at a time and record the defect changes.
Suggest conducting a 3x3 orthogonal experiment covering three factors: pressure, mold temperature, and holding pressure. Nine sets of experiments are sufficient to identify trends.
When delivering low-pressure casting molds, Xinfeng Mold will attach a parameter recommendation table, indicating the initial window and adjustment direction of each wheel type.
Troubleshooting of window drift
After the same mold runs for a period of time, the parameters become invalid. First check the water path for scaling. If the flow rate decreases by 20%, the mold temperature will significantly shift.
Check the liquid level of the insulation furnace again. Fluctuations in liquid level can alter the static pressure head, resulting in deviations between the actual filling pressure and the set value.
Finally, check the composition and temperature of the aluminum liquid. The fluctuation of composition will change the liquidus temperature, and the original window is no longer applicable.
Solidification and handover of parameters
After the window is confirmed, it should be written into the process card, indicating the applicable wheel type, machine and mold temperature conditions. The process card is transferred with the mold, and mass production has a basis.
Do not directly use the old card when changing the wheel type. After the changes in wall thickness and wheel rim width, the filling time and holding time need to be recalculated.
Frequently Asked Questions
Question: What is the filling pressure?
Answer: Normally 0.02-0.08MPa, adjusted according to wall thickness.
Q: What is the appropriate liquid lifting speed?
Answer: Suggest 50-150mm/s, segmented control is better.
Question: What is the mold temperature controlled at?
Answer: 200-350 ℃, with a temperature difference controlled within 15 ℃.
Question: How is the holding time calculated?
Answer: Estimated at 1-1.5 seconds per millimeter of wall thickness.
Q: What to look for when parameters suddenly fail?
Answer: First check the water flow rate, and then check the liquid level inside the furnace.
Q: Do we need to conduct an orthogonal experiment?
Answer: It is recommended to cover pressure, mold temperature, and holding pressure with a size of 3 × 3.
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