1. Introduction to the core
The core in sand casting is an independent sand component used to form a specific cavity, hole or complex structure inside the casting. It is used in conjunction with the external sand mold (casting mold) to form a complete casting cavity, so that the liquid metal can obtain the expected internal shape after solidification. The core is one of the key technologies to solve the problem of complex structure casting in the casting process.
The core's core function is to form an internal cavity, which is used to manufacture pipes, holes (such as water channels and oil channels of the engine cylinder), blind holes and other structures that cannot be directly formed by the external mold. Construct complex geometric features to realize fine internal structures such as the connecting rod channel of the crankcase and the cooling channel of the turbine blade. At the same time, reduce the difference in wall thickness of the casting to avoid hot spot defects; guide the flow direction of the molten metal and improve the filling process.
2. Typical characteristics of the core
High strength: must withstand the impact of molten metal (such as the impact force of cast iron pouring can reach 0.3MPa) without breaking
Strong air permeability: can discharge gas at high temperature (air permeability ≥100cm³/(cm²·min))
After pouring, it can be broken by vibration or chemical action, which is convenient for sand cleaning (residual strength <0.5MPa)
The tolerance of key parts must be controlled within CT8 level (±0.25mm)
3. Manufacturing process of the core
Molding technology
Core shooting method: use a core shooting machine to shoot mixed sand into the core box at high speed, and demold after hardening (production efficiency can reach 120 molds/hour)
3D printing: Binder jet technology (Binder Jetting) directly prints complex cores with an accuracy of ±0.3mm
Cold core box: triethylamine catalyzed resin hardens at room temperature, with a surface finish of Ra≤12.5μm
Post-processing
Drying: Resin sand cores need to be baked at 200-250℃ for 2-4 hours
Coating: Dip-coated zircon powder coating (thickness 0.2-0.5mm) to improve high temperature resistance
4, what are the cores made of?
The cores will be made using the same resin sand as the casting process. The sand is placed in a core box with the required internal shape of the part. Depending on the manufacturer's capabilities and equipment, the sand is hammered, heated, or blown into the core box. The core is then removed from the core box and placed in the mold.
There are two methods for making cores: hot core box and cold core box. The hot core box method is to add resin to the sand and use a catalyst. The catalyst is activated by heating when the core hardens. The cold core box method uses a catalyst that can be activated without heating, which is activated when the resin and sand harden to form the core.
5. How is the core fixed?
Since the core is inside the mold when the molten metal is poured into the mold, it moves or rises from the designated position. The core seat is made of the same metal as the mold. This metal holds the core in place until the metal is poured into the mold. When the metal cools, the core seat that touches the molten metal will become part of the actual part. The core is then shaken loose and removed.
Another way to fix the core is to make the core longer than the mold itself. This method can be used when the space that cannot be filled with metal exceeds the outer wall of the mold. The grooves placed in the sand mold can fix the core in place.
Core technology is the core means of sand casting to achieve complex inner cavity structures. Its development process has continued to promote the progress of the casting industry from manual core making to intelligent 3D printing. In the future, with the in-depth application of bionic structure design (such as topological optimized honeycomb core) and digital twin technology, the core will play a greater role in lightweight and functional integration.