What is the difference between differential pressure molds for wheel hubs and low-pressure casting molds? Key points for CPC process selection
The differential pressure mold is filled by the pressure difference between the upper and lower tanks, resulting in a higher density than the low-pressure mold, suitable for high-strength wheel hubs; Low pressure molds have lower costs and are more suitable for large quantities of conventional parts.
The difference in the magnitude of filling pressure determines the density of the tissue
The pressure difference of differential pressure casting is usually set in the range of 0.05-0.15 MPa, which is one order of magnitude higher than gravity casting. The higher the pressure, the smoother the compression channel, and the lower the probability of shrinkage.
The filling pressure of low-pressure casting is mostly between 0.02-0.08MPa, relying on a rising tube to smoothly fill the mold from the bottom of the cavity. Smooth filling is an advantage, but the allowance for shrinkage in thick and large areas is limited.
The pressure difference between the two is about 2-3 times, which is directly reflected in the density of the casting. Under the same wall thickness conditions, the density of differential pressure components is usually 0.02-0.04g/cm ³ higher.
The air tightness threshold separates the two processes
The pressure for wheel hub airtightness test is usually taken as 0.6-0.8MPa, and the leakage rate is required to be controlled within 1%. Differential pressure components are more likely to meet stable standards at the same wall thickness.
To achieve the same level of low-pressure components, it is often necessary to increase local pressure or adjust the cooling sequence. The process window is narrower, which requires higher requirements for mold waterway design.
For wheels exported to Europe, airtightness and fatigue are two hard thresholds. ECE R124 has clear testing requirements for both bending fatigue and radial fatigue of wheel hubs.
The difference in mold structure is not just about having an additional sealed can
The differential pressure mold needs to be matched with the upper and lower pressure tanks, and the mold body needs to withstand the additional load caused by the pressure difference of the tank body. The requirements for template rigidity and sealing surface processing accuracy are higher.
The structure of the low-pressure mold is relatively simple, with a focus on the interface of the liquid lifting pipe, the sealing of the insulation furnace, and the synchronization of the top out. The cost of molds is usually 15% -25% lower than that of differential pressure molds.
Taking the practice of Xinfeng Mold on the differential pressure mold of the wheel hub as an example, the design phase will first confirm the tank volume and pressure difference establishment time of the customer's machine, and then deduce the gate area.
Selection depends on batch size, wall thickness, and strength target
The cost-effectiveness of low-pressure molds is clearer for conventional wheel hubs with an annual output of less than 50000 pieces and uniform wall thickness. Low mold investment and faster mold replacement.
The advantages of differential pressure molds are more obvious for wheel hubs with wall thickness differences exceeding 3 times or requiring high-strength molding. The increase in density can offset some of the mold premium.
For modified models with a batch size of less than 3000 pieces, gravity molds are actually more suitable. The cost of mold opening is low, although the surface roughness can only reach Ra12.5-25 μ m.
The debugging sequence of the process window is different
Differential pressure debugging starts with setting the differential pressure curve, and then adjusting the mold temperature. If the pressure difference is established too quickly, it is easy to cause air entrapment, and if it is established too slowly, it will result in insufficient compression. It is usually stepped up in 3-5 stages.
Low voltage debugging first sets the liquid lifting speed, and then sets the holding time. Raising the liquid too quickly will generate turbulence, and oxidation inclusions are prone to occur in the inner cavity of the casting.
Both processes are recommended to undergo X-ray sampling during the T0 stage. The R angle between the spoke and the wheel rim transition is a high-risk area for shrinkage, and it is recommended that the sampling ratio be no less than 10%.
Common misconception: The higher the pressure, the better
A pressure difference exceeding 0.2 MPa will not continue to increase the density, but will instead increase the risk of mold sticking. The mold temperature needs to be maintained synchronously at 200-350 ℃.
Another misconception is to directly install differential pressure molds on low-pressure machines for use. Interface mismatch can lead to seal failure, and the pressure difference cannot be established at all.
A more reliable approach when selecting is to have the mold factory provide a trial mold report for the same specification of wheel hub, including three sets of data: density, airtightness, and fatigue.
Frequently Asked Questions
Q: Can differential pressure molds be installed on ordinary low-pressure machines?
Answer: No, it requires the coordination of upper and lower pressure tanks, and the interface and sealing structure are different.
Q: Are differential pressure wheels necessarily stronger than low-pressure wheels?
Answer: Under the same wall thickness, the density is higher, but the strength also depends on the heat treatment.
Q: Which mold is more cost-effective for selecting small batches of wheels?
Answer: Priority is given to gravity molds for quantities below 3000, resulting in lower mold opening costs.
Q: How long is the development cycle for differential pressure molds?
Answer: Regular 45-60 days, including design and T0 trial molding.
Q: Are the maintenance cycles for the two molds the same?
Answer: The sealing surface of the differential pressure mold needs to be inspected every 5000 molds.
Q: If the airtightness is not qualified, should we change the mold or the process first?
Answer: First, look at the location of the leak, and focus on checking for shrinkage and water flow in the mold.
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