1. Introduction to Cores

In the процесс литья в песчаные формы, cores are an independent sand – based forming component, mainly used to form specific cavities, holes or complex structures inside castings. It is used in conjunction with the external sand mold (mold) to form a complete casting cavity, ensuring that the liquid metal can form an internal shape that meets the design requirements after solidification. The core is one of the core technologies to solve the forming problems of complex – structure castings.

The core function of the core is to shape the internal cavity of the casting. It can be used to manufacture pipes, various holes (such as the water channels and oil channels of engine cylinders), blind holes and other structures that cannot be directly formed by external molds. At the same time, it can construct complex geometric shapes and realize the forming of precise internal structures such as crankcase connecting rod channels and turbine blade cooling channels. In addition, the core can reduce the wall thickness difference of castings, avoid hot spot defects caused by uneven local wall thickness; and can guide the flow direction of molten metal, optimize the filling effect and improve the casting quality.

2. Typical Performance Requirements of Cores

• High Strength: It must withstand the impact of molten metal (for example, the impact force during cast iron pouring can reach 0.3MPa) to ensure that it does not crack during the pouring process.
• Good Air Permeability: It must be able to discharge gas smoothly in a high – temperature environment, and the air permeability index must be ≥100cm³/(cm²·min).
• Easy Collapsibility: After pouring, it must be able to be broken by vibration or chemical action to facilitate subsequent sand cleaning operations, and the residual strength must be < 0.5MPa.
• High Precision: The dimensional tolerance of key parts must be controlled within the range of CT8 grade (±0.25mm).

3. Manufacturing Process of Cores

(1) Forming Technology

• Core Shooting Method: The mixed sand is shot into the core box at a high speed through a core shooting machine, and the core is demolded after hardening. This method has high production efficiency, up to 120 molds per hour, and is suitable for mass production of standardized cores.
• 3D Printing Method: Adhesive Jetting technology is used to directly print complex cores with a forming accuracy of up to ±0.3mm. No core box is required, which is suitable for the rapid manufacturing of small – batch and complex – structure cores.
• Cold Box Method: Triethylamine is used as a catalyst to make the resin undergo a hardening reaction at room temperature. After forming, the surface roughness Ra of the core is ≤12.5μm, which has both high efficiency and high precision.

(2) Post – treatment Process

• Drying: The resin sand core needs to be baked at a temperature of 200 – 250℃ for 2 – 4 hours to remove the moisture inside the core and improve the strength and stability of the core.
• Coating: The surface of the core is dipped with zircon powder coating, and the coating thickness is controlled at 0.2 – 0.5mm, which can significantly improve the high – temperature resistance of the core and prevent sand sticking defects during pouring.

4. Raw Materials and Manufacturing Methods of Cores

The raw materials of the core are usually the same as the resin sand used in the casting process. During production, the resin sand is filled into the core box matching the internal shape of the casting. According to the technical capabilities and equipment conditions of the production enterprise, the sand can be formed in the core box by compaction, heating or sand blowing. Then the core is taken out of the core box and can be put into the mold for use after post – treatment.
There are two main manufacturing methods for cores: hot box method and cold box method:

• Hot Box Method: A catalyst is added to the resin sand, and the catalyst is activated by heating the core box to promote the hardening and forming of the resin sand.
• Cold Box Method: A catalyst that can be activated without heating is used. When the resin and sand are mixed, the catalyst automatically plays a role, making the sand core harden and form at room temperature.

5. Fixing Methods of Cores

During the pouring process, when the molten metal is injected into the mold, it will impact and buoy the core, leading to easy displacement or floating of the core. Therefore, a reasonable fixing method must be adopted:

• Core Seat Fixing Method: The core seat is made of the same metal as the mold, which accurately fixes the core at the designated position in the mold until the molten metal pouring is completed and cooled. After cooling, the core seat in contact with the molten metal will become part of the casting. After the casting is formed, the core is collapsed and taken out by vibration.
• Extended Core Fixing Method: When the casting has a space beyond the outer wall of the mold that cannot be filled with metal, the core can be made longer than the mold, and the core can be fixed by the groove reserved on the sand mold to prevent it from moving during the pouring process.

Core technology is the core means for sand casting to realize the production of castings with complex internal cavity structures. Its development process has continuously promoted the progress of the casting industry, from traditional manual core making to intelligent 3D printing core making. In the future, with the in – depth application of bionic structure design (such as topology – optimized honeycomb cores) and digital twin technology, cores will play a more important role in realizing casting lightweight and functional integration, providing strong support for the high – quality development of the casting industry.

If you would like to learn more about Cores in Sand Casting, please feel free to contact us.