Common deviations in the fit of sand cores in suspension bracket molds

Common deviations in the fit of sand cores in suspension bracket molds
The sand core offset is the primary reason for the wall thickness deviation of the suspension bracket, and the precision of the fit is determined by the gap between the core heads, the arrangement of the core supports, and the quality of the sand injection.
Deviation starts from the gap between the core heads
The recommended clearance between the core head and the core seat is 0.1-0.2mm. Assembling less than 0.1mm is difficult, and sand cores larger than 0.2mm will drift under filling pressure.
Drifting by 0.3mm, the thin sidewall thickness may be lower than the minimum value. The minimum wall thickness of the suspension bracket is usually 4-6mm, and the margin is not spacious.
After the core seat wears out, the gap will gradually widen. It is recommended to check the size of the core seat every 5000 molds, and replace the inserts in a timely manner if they exceed the tolerance.
The more core supports, the better
Core supports are used to support sand core cantilevers. Insufficient quantities can cause sagging, while excessive quantities can form cold gaps or inclusions within the casting.
The sand core cantilever of the suspension bracket often exceeds 100mm. It is recommended that the spacing between support points should not exceed 80mm and avoid critical stress areas.
The core support material should match the casting. When using steel core supports, it is necessary to confirm that they can be completely removed during machining.
The quality of sandblasting determines the strength of sand cores
Insufficient sand injection pressure, low sand core density, and easy collapse during filling. The common sand injection pressure is 0.4-0.6MPa.
The solidification time of the sand core is insufficient, and it is prone to deformation after demolding. The curing time needs to be adjusted according to the thickness of the sand core, with thicker sand cores extending by 20% -30%.
Xinfeng Mold will require the sand core supplier to provide compressive strength data for the suspension bracket mold project, and it is recommended that the room temperature strength be no less than 3MPa.
Control of stitching and flying edges
The mating surface between the sand core and the mold will form a gap. If the thickness of the seam exceeds 0.3mm, the cleaning time will significantly increase, and it may also cover up cracks.
The transition between the parting surface and the core head needs to be rounded. A right angle transition is prone to sand accumulation, which can cause the sand core to crack during demolding.
The exhaust of the mold should avoid the sand core area. The exhaust plug facing the sand core will suck sand particles into the exhaust channel, causing blockage.
Common Defects and Causes
The first type is uneven wall thickness, often caused by sand core displacement. Ultrasonic thickness measurement or CT can be used to locate the offset direction.
The second type is sand holes, which are often caused by insufficient strength of sand cores or insufficient sand injection. Sand particles can be seen at the fracture surface, which is different from the morphology of shrinkage porosity.
The third type is sand sticking, which is often caused by insufficient coating or high pouring temperature. The recommended coating thickness is 0.2-0.3mm, applied twice.
How to suppress the deviation
Conduct sand core assembly simulation during the mold design phase to confirm the stress and deformation of the core head. Simulation can detect the problem of cantilever sagging in advance.
The first CT scan of the sand core position is more intuitive than destructive sectioning. CT can provide three-dimensional offset with an accuracy of 0.05mm.
During the mass production stage, special gauges are used to check the wall thickness, with a sampling of 5% per batch. The trend of wall thickness is more valuable than a single point value.
Frequently Asked Questions
Q: What is the gap between the core heads?
Answer: It is recommended to use 0.1-0.2mm, as excessive sand core will drift.
Question: What is the appropriate spacing between core supports?
Answer: It is recommended not to exceed 80mm for cantilever components.
Q: What is the required strength for sand cores?
Answer: It is recommended that the compressive strength at room temperature should not be less than 3MPa.
Question: How much is the seam control?
Answer: It is recommended to increase the cleaning time within 0.3mm if it exceeds the tolerance.
Q: How to distinguish between sand holes and shrinkage porosity?
Answer: Sand particles can be seen on the fracture surface of the sand hole, which have shrunk and become sponge like.
Question: How to detect offset?
Answer: CT is used for the first piece, and ultrasonic thickness testing is used for mass production.
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