Graphite Molds For Metal Casting
Low Wetting
In order to prevent the casting removed from the mold from sticking, the material should have good adhesion to the solidified metal. This is achieved by low wetting of the mold by liquid metal. Graphite molds for metal casting cannot be wetted by most molten non-ferrous metals and alloys. Ferroalloys such as cast iron form wet graphite due to reaction with carbon, thereby forming carbide Fe. Graphite is also wetted by an alloy containing nickel (more than 6% of nickel), zinc or tin.
High Thermal Conductivity
The heat released when cooling and solidifying the cast metal should be removed from the mold. The amount of heat transferred through the mold in a time unit determines the solidification speed and pull-out speed of the casting. The thermal conductivity of the mold material should be high enough to provide rapid heat dissipation. The thermal conductivity of graphite Molds for Metal Casting is in the range of 560-960 Btu*in/hr*ft2*F (80-140 W/m*K), which is equivalent to the thermal conductivity of metal: aluminum 1540 Btu*in/hr*ft2 *F (222 W/m*K); steel 450 Btu*in/hr*ft2*F (65 W/m*K). The thermal conductivity of graphite depends on the temperature: when the temperature rises to 1300 °F (704 °C), it will drop to half.
Self Lubrication
Graphite is a solid lubricant. The low friction between the mold surface and the solidified metal ensures that the casting is smoothly removed (extracted) without cracks and minimizes the thickness of the defective skin. The special layered crystal structure of graphite determines its self-lubricating properties and provides low friction without increasing the amount of lubricating oil.
Good Mechanical Strength
The mechanical strength (tensile strength, compressive strength, flexural strength) of graphite increases with increasing temperature compared to other materials.
Good Processability
Graphite molds for metal casting are easy to process with graphite. Through milling, turning, sawing, grinding and surface finishing, complex shape molds with tight tolerances are available. Finishing (grinding or polishing) of the inner mold surface is important to reduce the wet and friction of the material. A good outer surface quality ensures a minimum air gap between the mold and the water jacket for better heat transfer.
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