Key points of mold design for low-pressure casting with sand core

Key points of mold design for low-pressure casting with sand core
The difficulty of adding sand cores in low-pressure casting lies not in filling, but in the sand cores: positioning, exhaust, and collapse determine the consistency of wall thickness and cleaning costs.
Double point constraint is required for sand core positioning
Single core head positioning is prone to drift under filling pressure. Suggest using a dual core head with anti rotation structure, selected according to H8/f7, with a measured gap of about 0.15mm.
When the offset reaches 0.3mm, the thickness of the thin sidewall will fall below the lower limit. The wall thickness of structural components is mostly around 5mm, and the fault-tolerant space is very narrow.
After prolonged use, the gap between the core holder will widen. Measure every 5000 molds and replace the insert if it exceeds the tolerance.
Sand core exhaust cannot be saved
The sand core will produce gas during pouring. Poor exhaust can cause gas to enter the aluminum liquid, forming pores.
An exhaust channel should be installed inside the sand core to lead out the mold cavity through the core head. The diameter of the exhaust channel is usually taken as 3-6mm.
Blockage of exhaust channels is a common problem. Clean each mold once or use breathable sand to reduce the probability of blockage.
Balance between sand core strength and collapsibility
Insufficient strength, sand core fractures during filling. Excessive intensity makes it difficult to remove sand and increases cleaning time.
The compressive strength at room temperature is controlled at 3.5MPa. The collapsibility is adjusted by the ratio of sand and the amount of resin added.
Xinfeng Mold will require the sand core supplier to provide both strength and collapsibility data for the structural component sand core mold, both of which must meet the standards.
The mating surface between sand core and mold
It is inevitable to leave gaps on the mating surface. When the thickness exceeds 0.25mm, the cleaning time increases and cracks may be covered.
Make rounded corners at the junction between the core and the parting surface to avoid sand accumulation; It is easy to crack the sand core when demolding with a right angle transition.
The exhaust plug should be avoided facing the sand core, otherwise it will suck sand particles into the channel and cause blockage.
The function of paint
Coatings can improve the surface strength of sand cores and prevent sand sticking. The coating is applied in two coats, with a total thickness of approximately 0.25mm.
Excessive coating can block the exhaust passage. After painting, it needs to be fully dried, and a moisture content higher than 0.5% will increase the risk of porosity.
The type of coating should match the alloy. The interface reactions of different alloys are different, and selecting the wrong coating can cause surface roughness.
Correspondence between common defects
Uneven wall thickness is mostly due to sand core displacement. Ultrasonic thickness measurement or CT can indicate the direction of deviation.
Sand holes are often caused by insufficient strength of the sand core or insufficient sand injection, and sand particles can be seen on the fracture surface, which is different from the morphology of shrinkage porosity.
Sand sticking is often related to thin coating or overheating of aluminum liquid. Lowering the pouring temperature by 10-20 ℃ can often improve it.
Storage of sand cores
Sand cores stored for more than 72 hours will absorb moisture and need to be dried before use. The drying temperature is usually set at 120-150 ℃ for 2-4 hours.
Practical tips
Measure the moisture content of sand cores before storage, and dry those above 0.5% before use to avoid concentrated gas production during pouring.
After assembling the sand core, conduct a gap check, and the position where the plug gauge can be inserted is the potential source of the seam.
Frequently Asked Questions
Q: How to locate the sand core?
Answer: The dual core head is equipped with anti rotation, and the measured gap is about 0.15mm.
Q: Does the sand core need to be vented?
Answer: Yes, the channel diameter is often taken as 3-6mm.
Q: How is the strength of the sand core determined?
Answer: The compression resistance at room temperature is controlled at 3.5MPa, taking into account both collapse and dispersion.
Q: What is the limit for the thickness of the seam?
Answer: It is recommended not to exceed 0.25mm.
Question: How thick is the coating?
Answer: The total thickness of the two layers is about 0.25mm, and they need to be fully dried.
Q: How to distinguish between sand holes and shrinkage porosity?
Answer: The sand hole fracture has sand particles, and the shrinkage is sponge like.
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