The Effects of Foam Density and Metal Velocity on the Heat and Mass Transfer in the Lost Foam Casting Process

Abstract

As an innovative technique, the lost foam casting (LFC) process has drawn great attention from both academia and industry in recent years. The key feature of LFC process is that a desired shape pattern made of expandable polystyrene (EPS) foam is buried in unbonded sand and replaced by advancing molten metal. The heat and mass transfer between the molten metal front and the EPS foam pattern plays an important role in the soundness of the product in the LFC process. The present study focuses on determining the characterization of heat and mass transfer during the EPS pattern degradation process. A unique experimental system using a cylindrical quartz window and heated steel block simulating the hot molten metal front has been constructed to make measurements and visualize the process. The foam pattern is 88 mm in diameter and 254 mm long. It is coated twice with DCH Ashland refractory material and the average coating thickness is 1.2 mm. The heat flux and pressure between the moving steel block and the EPS pattern are measured. The process variables studied during this experiment include foam density and steel block speed. It was found that unlike the fluidity of the molten metal which is highly dependent on the density of the foam patterns, foam density has marginal effect on the heat flux from the steel block to the foam pattern. The heat flux increases about 37% during a one-minute process under steel block velocity of 4.4 mm/s using different EPS foam density of 24 kg/m3 and 27 kg/m3. Flow visualization shows a gaseous gap formed between the steel block and the foam pattern. The phase change and degradation of EPS foam pattern and the heat and mass transfer in the gap are crucial to characterize the mold filling process which decides the quality of casting products. The maximum pressures measured in the gap using steel block velocity of 4.4 mm/s are 1.1 kPa and 1.4 kPa for EPS foam density of 24 kg/m3 and 27 kg/m3, respectively. Under a slower steel block velocity of 3.6 mm/s the gap peak pressure using 24 kg/m3 density EPS foam pattern is 0.43 kPa. It is concluded that higher foam density and faster steel block speed give rise to larger gas pressure between the steel block and foam pattern. The measured pressure values confirm data reported in literature.