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zh What is Core Sand in the Foundry Industry?
2025-01-03 09:28:00 hits：0
In the foundry industry, core sand plays a vital role. It works in tandem with molding sand and is a crucial element in producing high-quality castings.
What is core sand?
Core sand, simply put, is the material used to make cores. Cores bear the significant responsibility of shaping the complex cavity structures inside castings. Take the engine block of an automobile as an example. There are numerous precise inner cavities within it, and the forming accuracy of these cavities directly depends on the quality of the core. As the “foundation” of the core, the quality of the core sand is decisive. Another example is various valve bodies, such as the flow control valve in industrial pipelines. The precise construction of its internal passages also hinges on high-quality cores and, consequently, suitable core sand.
When it comes to performance requirements, core sand has several key indicators.
First is collapsibility
After the casting solidifies and takes shape, the core must be able to break and fall off smoothly so that it can be easily removed from the interior of the casting without causing hindrance to subsequent processing. In the field of aluminum alloy automotive wheel casting, data shows that if the collapsibility of the core sand fails to meet the standard and the weight of the sand core remaining inside the wheel exceeds 50 grams, not only will the cleaning time increase by more than 30%, but it is also very likely that during the vehicle’s operation, vibrations caused by the collision between the sand core and the inner wall of the wheel will generate abnormal noises and cause the dynamic balance deviation of the wheel to exceed 20 g·cm, severely affecting the performance of the wheel. Under normal circumstances, after the casting is demolded, by applying an external vibration with a frequency of 50 Hz and an amplitude of 3 mm using a simple vibration device, high-quality core sand can become loose and separate from the casting body, enabling efficient cleaning.
Strength cannot be overlooked either.
During the process when liquid metal is injected into the mold and flows and solidifies around the core, the core sand must maintain its shape stability to resist the impact and enveloping pressure brought by the metal liquid, preventing the core from collapsing and deforming, thus ensuring the accuracy of the inner cavity size of the casting. For large and heavy castings like the propeller of a ship, the pressure borne by the internal core sand can sometimes reach as high as 5 MPa. According to the statistics of a large shipbuilding enterprise, if the strength of the core sand does not reach 2 MPa, during the casting process, about 40% of the cores will experience collapse and deformation problems, resulting in the inner cavity size deviation of the propeller casting exceeding 5 mm, unable to meet the assembly accuracy requirements and having to be scrapped and recast. Therefore, for such castings, the strength of the core sand used must reach 2 MPa or more to ensure that the core remains stable and reliable under long-term and high-intensity casting conditions.
Permeability is also a key point in evaluating the quality of core sand.
When the metal liquid solidifies, a large amount of gas will be released, and these gases must be discharged smoothly through the core sand. Otherwise, if the gases accumulate at the interface between the core and the metal liquid, porosity defects are likely to occur, affecting the internal quality of the casting. For example, when producing a certain model of automobile engine cylinder head, initially, the selected core sand had a permeability of only 60 air holes per square centimeter. As a result, dense pores appeared around the internal air passages of the cylinder head, with a porosity rate as high as 15%, severely impeding the gas flow efficiency of the air passages and reducing the engine power output by about 10%. Later, after adjusting the core sand formula and increasing the permeability to 100 air holes per square centimeter, the pore problem was effectively controlled, and the engine power output returned to normal.
Selection of core sand raw materials
In terms of raw material selection, phenolic resin sand is currently one of the most widely used core sand types. With the excellent bonding performance of phenolic resin, it can make sand grains bind tightly together, endowing the core with sufficient strength. At the same time, under high temperature, the phenolic resin can decompose at a certain rate to meet specific collapsibility requirements. However, different castings have different characteristics, and the core sand formula needs to be adjusted flexibly. For some high-precision, thin-walled castings in the aerospace field, such as the attitude adjustment component of a certain model of satellite, its thin-wall thickness is only 2 mm. To prevent the core sand from generating excessive thermal effects on the thin-wall part during the casting process and causing deformation, foundries will add no more than 5% of special heat dissipation additives to the phenolic resin sand, effectively optimizing the comprehensive performance of the core sand and ensuring that the core can meet the strength and collapsibility requirements while maximizing the protection of the thin-wall structure.
Preparation process of core sand
The preparation process of core sand has a significant impact on the final effect. Mixing, compacting, and other links must be strictly controlled to ensure the uniformity and compactness of the core sand meet the standards. Currently, most advanced foundries adopt automated sand mixing equipment. These devices can accurately control the addition amounts of resins, curing agents, etc., with an error within ±0.5%, ensuring the stable performance of each batch of core sand and providing a solid and reliable material foundation for core production.
In conclusion, although core sand does not directly display the results like castings, it is like a behind-the-scenes hero, silently influencing the internal quality, precision, and overall performance of castings. Only by thoroughly understanding the characteristics of core sand and precisely formulating and carefully preparing it according to the requirements of different castings can foundry practitioners smoothly produce one perfect casting after another and promote the foundry industry to move towards higher quality development.
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