Formation of surface defects in gray iron lost foam castings

A model to predict metal velocity and defects in gray iron lost foam castings (LFCs) was developed through statistically designed experiments. In addition, patterns with different coating thicknesses were evaluated to see the effects of decreased permeability on metal velocity and formation of defects. The effects of coating permeability (or airflow rate), pouring temperature, foam density and carbon equivalent (CE) were studied. Thick, low-permeability coatings reduced metal velocity, the severity of surface carbon defects and metal penetration (burned-on sand). A small increase in metal velocity was obtained by increasing the pouring temperature, but the higher pouring temperature reduced the severity of the carbon defects. Nigh CE of the iron also provided a small increase in metal velocity and more severe carbon defects. The density of the foam pattern had no significant effect on metal velocity, carbon defects or penetration. The effects of the casting variables on metal velocity and defects ale explained in terms of the physical model for mold filling and transport of pyrolysis products from the foam pattern. High-permeability coatings increase the metal velocity because air and gaseous pyre lysis products are quickly removed through the coating. However, this also reduces the time during which liquid pyrolysis products can be transported into the coating, thus leading to surface defects. Increasing the pouring temperature, although increasing metal velocity, can assist in removing the liquid pyrolysis products.