Mold side solution for shrinkage and looseness defects in low-pressure casting wheel hubs

Mold side solution for shrinkage and looseness defects in low-pressure casting wheel hubs
Hub looseness often occurs in the hot zone between the root of the spoke and the center of the hub. The core strategy on the mold side is to shorten the shrinkage distance and establish sequential solidification.
The position of shrinkage is regular
The wall thickness at the junction of the spokes and the hub undergoes a sudden change, with the highest thermal modulus and the latest solidification. Once the contraction channel is interrupted, there will inevitably be shrinkage here.
In X-ray detection, shrinkage appears as a low-density area in the form of clouds or sponges. In ASTM E505 standard, grades 1-2 are acceptable, while grades 3 and above require rework.
The shrinkage and looseness of the wheel rim can directly lead to airtightness failure, and the tolerance for such defects is zero, which must be resolved from the mold design stage.
The compensation distance should be calculated according to the wall thickness
The shrinkage distance of low-pressure casting generally does not exceed 4 times the wall thickness. When the wall thickness is 10mm, the effective range of compensation is about 40mm. If it exceeds this range, another compensation source needs to be set up.
The spoke length often exceeds 80mm, and it is not enough to rely solely on the center gate for shrinkage. At this point, it is necessary to set up reinforcing ribs or thickened transitions at the end of the spokes.
Changing the sprue from a single point to a circular sprue and changing the direction of filling from unidirectional to radial can increase the effective filling distance by about 50%.
Sequential solidification is achieved through cooling
By placing a cooling rod with a diameter of 8-12mm at the hot spot and inserting it into the mold, the local solidification time can be shortened by 20% -30%.
The thin-walled area of the wheel rim should be insulated to avoid premature solidification and cutting of the shrinkage channel. Thermal insulation coating or reducing the water flow in the area can be achieved.
Xinfeng Mold will perform solidification sequence simulation on low-pressure wheel hub molds, controlling the solidification time difference at the hot spot position within 3 seconds.
The role of pressure curve is limited but necessary
The filling pressure is 0.02-0.08MPa, and the holding time is usually 1-1.5 seconds per millimeter of wall thickness. Insufficient pressure holding, unable to complete the shrinkage repair.
During the pressurization phase, an additional 0.01-0.02MPa can be added after the filling is completed, but excessive pressure can cause mold sticking and flash.
Pressure can only solve macroscopic shrinkage, while microscopic shrinkage still depends on temperature gradient. Putting all hope on pressure often leads to disappointment.
Common Misconceptions
The first misconception is that increasing the gate can solve the problem of shrinkage. If the gate is too large, the gate itself becomes a hot spot, and the location of shrinkage only shifts rather than disappears.
The second misconception is to lower the pouring temperature. Low temperature will cause the shrinkage channel to solidify prematurely, resulting in increased shrinkage. 680-720 ℃ is a more stable range.
Misconception three is to ignore the gas content of aluminum liquid. When the gas content is higher than 0.2ml/100g, the combination of gas shrinkage and porosity increases the difficulty of interpretation significantly.
Verification and Closed Loop
At T0 stage, X-ray sampling should be conducted, covering the root of the spoke, the center of the hub, and the transition of the wheel rim, with no less than 3 samples in each area.
After the first piece is qualified, run 20 molds continuously and recheck once. The defect morphology after the mold temperature stabilizes represents the mass production state.
During the batch stage, it is recommended to randomly inspect 1 piece per 500 pieces. Shrinkage is a process drift type defect, and a low sampling frequency can easily result in missed judgments.
Frequently Asked Questions
Question: Where does the shrinkage occur the most?
Answer: The root of the spoke and the hot zone at the center of the hub.
Question: How is the distance of compensation calculated?
Answer: Generally not exceeding 4 times the wall thickness.
Q: How much can a little cold improve?
Answer: The local solidification time can be shortened by 20% -30%.
Q: Can pressure eliminate shrinkage and looseness?
Answer: Only macroscopic shrinkage can be improved, microscopic temperature gradient is still needed.
Q: What are the criteria for X-ray judgment?
Answer: Commonly used ASTM E505, grades 1-2 are acceptable.
Q: What is the appropriate sampling ratio?
Answer: It is recommended to draw 1 piece for every 500 pieces during the batch stage.
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