How to investigate if the airtightness of the differential pressure mold for the wheel hub does not meet the standard? Find the cause from the mold side

How to investigate if the airtightness of the differential pressure mold for the wheel hub does not meet the standard? Find the cause from the mold side
The inadequate airtightness of the wheel hub is usually caused by three reasons on the mold side: insufficient shrinkage channels, low local mold temperature, and failure of the parting surface seal.
First locate the leakage point and then discuss the cause
The commonly used pressure for airtight testing is 0.6-0.8MPa, and the pressure is maintained for 30-60 seconds. If the leakage rate exceeds 1%, it is considered unqualified. It is recommended to control the test water temperature at 20-30 ℃.
Immerse the wheel hub into the water tank, and the bubbling point is the leakage point. The R angle at the transition between the wheel rim and spoke, opposite the gate, and around the bolt hole are high-risk areas.
The causes vary in different locations. The R angle is mostly related to shrinkage and looseness, the opposite side of the gate is mostly related to cold insulation, and the area around the bolt hole is mostly related to local looseness.
Insufficient replenishment channels are the most common reason
The shrinkage of differential pressure casting relies on the gate and pressure difference. If the compression distance exceeds 4 times the wall thickness, the end is prone to axial looseness.
When the thickness of the spoke transitions from 12mm to 6mm, if the solidification sequence of the transition zone is reversed, the thicker part will solidify first, and the thinner part will lose its source of shrinkage.
The response on the mold side is to add reinforcing ribs or adjust the gate position. Moving the sprue from the center of the spoke to the center of the hub can shorten the filling distance by 30%.
Low local mold temperature will form cold insulation
When the mold temperature is below 200 ℃, the flowability of the aluminum liquid front significantly decreases. The thin wall is not fused, forming a through gap, and the airtightness is inevitably unqualified.
Temperature measurement should be based on the surface of the mold cavity rather than the mold body. The temperature difference between the two can reach 30-50 ℃, and using mold temperature instead of cavity temperature may lead to misjudgment.
Xinfeng Mold will arrange 6-10 temperature measurement points on the differential pressure wheel hub mold according to the wheel shape, and check each point during debugging to avoid a single point representing the whole.
Hidden leakage between parting surface and exhaust plug
There is a gap of 0.05mm on the parting surface, and a leakage channel will form at 0.8MPa. If there are pits after sanding the seams, it will also damage the seal.
Blocked exhaust plugs can cause an increase in back pressure in the mold cavity and incomplete filling. It is recommended to clean every 2000 molds, and shorten it to 1000 molds when the aluminum liquid is not degassed.
After the sealing surface is developed, a red lead inspection should be conducted, and the contact spot area should not be less than 75%. Below this ratio, leakage will gradually occur after long-term use.
Cross validation of processes and molds
When the air tightness of the same die becomes poor after changing the machine table, the pressure difference establishment time of the machine table shall be checked first. If the pressure difference is established slower than 3 seconds, the compression effect will be significantly weakened.
After replacing the mold on the same machine, the airtightness deteriorates. Prioritize checking the mold water circuit and exhaust. If the waterway flow is below 20% of the design value, it needs to be investigated.
If both items are normal but still unqualified, check the gas content of the aluminum liquid again. When the gas content is higher than 0.2ml/100g, the tendency to shrink significantly increases.
Repair and Prevention
Local shrinkage can be treated with infiltration, but infiltration can only block connected pores and cannot restore mechanical properties, which is a remedy rather than a cure.
The issue of batch airtightness must return to mold design. Modifying the gating and cooling scheme typically takes 15-25 days.
The key to prevention lies in the T0 stage. Perform X-ray and airtight double inspection on the first piece to prevent problems before mass production, with the lowest cost.
Frequently Asked Questions
Q: What is the pressure for the airtight test?
Answer: Commonly used 0.6-0.8MPa, maintain pressure for 30-60 seconds.
Q: What is the leakage rate standard?
Answer: Generally, it is required to be less than 1% and follow the customer's drawings.
Q: What is the reason for R-angle leakage?
Answer: It is mostly due to shrinkage, and it is necessary to modify the shrinkage channel or add local cooling.
Q: Is infiltration treatment considered qualified?
Answer: It is a remedial measure and cannot restore the mechanical properties.
Q: How long does it take to clean the exhaust plug?
Answer: Conventional 2000 cycles, 1000 cycles when the aluminum liquid is not degassed.
Q: How long does it take to modify the mold for airtightness issues?
Answer: It usually takes 15-25 days, including model validation.
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