Casting is an energy-intensive manufacturing process within the metal casting and aluminum industries, requiring natural gas to melt aluminum and electricity to run equipment. The higher-than-acceptable faults and scrap rates in the lost foam casting process for the complex L61 engine previously resulted from the inability to control and measure refractory coating thickness and to control particle size and the shape of the unbonded sand. Remelting defective castings adds to overall energy costs, emissions, and use of resources.
The lost foam casting process starts with a foam pattern of the desired endproduct made out of polystyrene beads. The foam pattern is coated with a thin refractory film and placed into dry, unbonded sand that is compacted by vibration. Molten metal, poured into the sand casting, evaporates and replaces the foam, producing a metal casting that is nearly identical to the foam pattern. The foam vapor passes through the pores in the refractory coating and the sand. This process enables the joining of several components within a single casting, thereby reducing downstream machining and assembly.
With the assistance of a NICE3 grant and the New York State Energy Research and Development Authority, General Motors Corporation has developed tools to precisely measure dried coating thickness and pore size distribution, more accurately measure the size and shape of sand used in casting, and better understand the rheology of coatings. Rheology affects both coating thickness and uniformity on foam patterns. Coating thickness controls the permeability of gaseous expanded polystyrene by-products, which is directly related to casting defects such as porosity and folds. Therefore, measuring the rheological properties of the lost foam coating is critical to minimizing casting defects.