How to choose between gravity casting molds and low-pressure casting molds for wheel hubs? Adaptation boundary for three working conditions

How to choose between gravity casting molds and low-pressure casting molds for wheel hubs? Adaptation boundary for three working conditions
Gravity molds have low molding costs and are suitable for small batches; Low pressure molds have better density and surface properties, making them suitable for mass production of passenger car wheels. The boundary between the two lies in batch size and wall thickness.
Batch scale is the first boundary line
For modified or retro models with an annual demand of less than 3000 pieces, the investment and recycling of gravity molds are faster. The quotation for a single set of molds is usually 30% -45% lower than that for low-pressure molds.
For passenger car wheels with an annual demand of over 20000 pieces, the single piece cost advantage of low-pressure molds is obvious. The amortization of molds is usually less than 8 yuan per piece.
Orders between 3000 and 20000 pieces need to be calculated separately based on wheel complexity. When there are multiple varieties and small batches, the flexibility of changing gravity molds is more valuable.
The difference between surface quality and machining allowance
The surface roughness of castings produced by gravity casting is mostly between Ra12.5-25 μ m, with a machining allowance of 1.5-2.5mm. The material removal amount is large, and the tool wear is also high.
The surface roughness of low-pressure casting can reach Ra6.3-12.5 μ m, and the margin is usually controlled at 0.8-1.5mm. This difference is very intuitive for the appearance of the wheel rim.
For every 0.5mm reduction in surplus, the machining hours for a single piece decrease by approximately 8% -12%. When mass producing 100000 pieces, this part of the savings is quite considerable.
Density determines whether thin walls can be made
The low-pressure filling pressure is 0.02-0.08MPa, and with sequential solidification, the probability of shrinkage at the root of the spoke is significantly lower than that of gravity. Thin walled wheel shapes rely more on this.
Gravity molds rely on sprues for shrinkage, and the shrinkage distance is generally not more than 4 times the wall thickness. Beyond this ratio, the axis at the hot junction is prone to shrinkage and looseness.
When the wall thickness of the wheel hub reaches 3-5mm, a denser arrangement of cold iron is required for the gravity route. The increase in the quantity of cold iron leads to an increase in mold structure and maintenance costs.
The difference between mold structure and maintenance cost
Gravity molds are mostly a combination of metal molds and sand cores, with a simple structure. The weight of a single set is usually 20% -30% lighter than that of low-pressure molds. Lifting and changing models are more convenient.
Low pressure molds need to be matched with insulation furnaces, liquid lifting pipes, and sealing systems. Seals are vulnerable parts, and it is recommended to inspect them 5000 times and replace them 10000 times.
Taking the gravity mold project of Xinfeng Mold as an example, the density of exhaust plugs in the wheel rim area will be adjusted according to the wheel shape, usually with one exhaust point arranged every 300mm ².
Selection suggestions for three typical working conditions
After sales modification of wheels, 300-2000 pieces per model, priority given to gravity molds. The mold opening fee is low, and it is also more convenient to modify the cavity during the redesign process.
Original wheels for new energy passenger vehicles, with an annual output of over 100000 pieces, priority given to low-pressure molds. The consistency between airtightness and fatigue is easier to control.
For commercial vehicles or construction machinery wheels with large wall thickness and heavy weight per piece, differential pressure or low pressure combined with local pressurization can be considered. The scrap rate of the pure gravity route is relatively high.
Common misconception: Low pressure is always better than gravity
The advantages of low pressure lie in density and surface, but the equipment investment and mold complexity are higher. In small-scale scenarios, the additional costs are difficult to spread evenly.
Another misconception is to ignore machine matching. Low pressure molds require insulation furnaces and lifting systems of corresponding tonnage, and mismatched machines can lead to unstable filling.
When selecting, it is recommended to have three sets of data at the same time: scrap rate of the same wheel type, single piece machining time, and mold maintenance interval. Just looking at the unit price of the mold can easily lead to misjudgment.
Frequently Asked Questions
Q: Can gravity molds be used to make passenger car wheels?
Answer: Yes, but the scrap rate is higher when thin-walled and airtight requirements are high.
Q: How much is a low-pressure mold more expensive than a gravity mold?
Answer: Usually 30% -45% higher, including waterway and sealing system.
Q: Which type should I choose for small batch modifications?
Answer: Priority is given to gravity molds for quantities below 3000, which can be recycled more quickly.
Q: How much can the surface of the gravity mold reach?
Answer: Conventional Ra12.5-25 μ m requires a large machining allowance.
Q: Can two types of molds share the same machine?
Answer: No, the low-pressure insulation furnace needs to be matched with the liquid lifting system.
Question: What is the time difference for changing models?
Answer: Gravity mode takes 0.5-1.5 hours, low-pressure mode takes 1.5-3 hours.
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