Core Gas Model
In sand casting, resin-bonded cores are used to create internal cavities and complex passages in the final part. When molten metal surrounds a core, the heat drives thermal decomposition of the organic binder, releasing gas. If that gas cannot escape fast enough through the core and its vents, the pressure builds until it overcomes the ferrostatic pressure of the metal at the core surface, and gas enters the casting as porosity or blow defects.
The Core Gas Model in FLOW-3D CAST predicts the generation and flow of binder gas within permeable sand cores and molds during and after mold filling. It gives foundry engineers a detailed, quantitative view of where gas is generated, how it moves through the core, and where it is most likely to enter the metal, enabling better core venting design before any metal is poured.
How the Model Works
The Core Gas Model treats the sand core as a porous medium through which compressible gas can be generated, flow, and exit. The conversion of solid binder to gas is described by an Arrhenius reaction rate equation, meaning gas generation accelerates rapidly as the core heats up. The generated gas is treated as compressible and ideal, governed by the ideal gas law with a gas constant determined from the binder chemistry.
Gas flow through the core is governed by Darcy’s law for flow in porous media. The metal pressure distribution at the outer surface of the core acts as a boundary condition for the gas flow: gas can escape into the metal when its pressure exceeds the local metal pressure, but once the metal has solidified at the core surface, no further gas exchange occurs at that location.
Because gas flow in cores can be much faster than the metal filling, FLOW-3D CAST employs a sub-time-stepping scheme for the core gas solver, allowing accurate gas computation without slowing down the overall filling simulation. A sub-cell resolution method is also used to accurately capture the steep thermal gradients in the thin binder decomposition zone at the core surface, which is typically only 0.25 to 0.5 mm wide.
Modeling Capabilities
The Core Gas Model in FLOW-3D CAST computes:
- Binder gas generation rate as a function of local core temperature, using an Arrhenius decomposition model
- Gas flow through the core based on Darcy’s law, with permeability defined by average sand grain diameter or directly by Darcian and non-Darcian permeability values
- Gas pressure, density, and velocity distribution within the core, including thermal expansion of initial air as the core heats up
- Gas flux at the core surface, distinguishing between gas that escapes to void regions, gas that enters the metal, and gas that exits through print vents
- Venting at print surfaces, where gas escapes to atmosphere through the parting plane or through dedicated mold vents
- Metal pressure as a boundary condition for gas flow, updated in real time as filling progresses
- Total gasification rate and mass and volume flux history for each core component, available for postprocessing
Applications
Core gas defects are a persistent challenge in sand casting of both ferrous and non-ferrous alloys. The model is used to:
- Predict blowhole and gas porosity defects before committing to tooling
- Evaluate and optimize core vent placement and sizing
- Assess the effect of binder type and content on gas generation
- Compare vented and unvented core designs for complex assemblies such as water jacket cores in engine blocks
- Diagnose gas defects in existing castings by simulating the as-built core geometry and venting configuration
- Design core print geometry to ensure adequate gas escape paths