Fundamental studies of molten metal flow in permanent molds have been conducted as part of a recent DOE/ AFS project. The experimental data is used to verify flow and solidification computer simulation models. Different gating methods employed in vertical permanent molds and the effect of filters on the flow have been evaluated. Both real-time X-Ray studies with molten metal and water simulations have been employed.
A project concerning the gating of aluminum castings in permanent molds was recently completed. The main goal of the project was to improve the quality of aluminum castings produced in permanent molds. The approach taken was determine how the vertical type gating systems used for permanent mold castings can be designed to fill the mold cavity with a minimum of damage to the quality of the resulting casting. It is evident that somewhat different systems are preferred for different shapes and sizes of aluminum castings. The main problems caused by improper gating are entrained aluminum oxide films and entrapped gas. The project highlighted the characteristic features of gating systems used in permanent mold aluminum foundries and recommended gating procedures designed to avoid common defects. The study also provided direct evidence on the filling pattern and heat flow behavior in permanent mold castings.
Equipment and procedures for real-time X-Ray radiography of molten aluminum flow into permanent molds have been developed. Other studies have been conducted using water flow and the behavior of liquid aluminum in sand mold using real time photography. This investigation utilized a strong graphite mold where it is possible to observe the flow pattern with X-Rays. The gating included systems that are choked at the base of a rounded vertical sprue and enter vertical gating systems with a variety of different ingate sprues into the bottom of a mold cavity.
When a bottom ingate vertical sprue and bottom horizontal runner are employed, a smooth filling pattern is obtained with a large runner tapered at the blank end and a large bottom ingate. Good filling patterns were obtained in a horizontal ceramic foam filter located on the ingate on top of the horizontal runner entering into the mold cavity. When a vertical riser was used on the side of the mold cavity connected to the mold cavity through a web (3mm thick) a smooth filling pattern was obtained when the bottom of the mold cavity was slightly higher than the bottom of the web. A calculation of the actual velocity could be determined for these systems and this velocity was reduced to nineteen inches per second for all of the gating systems listed above. Studies have shown this as a critical velocity; higher velocities will cause excessive oxide and air entrainement.