To be presented at 1996 Casting Congress

April 20-23, 1996, Philadelphia, PA USA

MOLD FILLING ANALYSIS FOR DUCTILE IRON LOST FOAM CASTINGS

O. Gurdogan, H. Huang, H. U. Akay

Technalysis Incorporated
Indianapolis, Indiana

W. W. Fincher, V. E. Wilson

Lufkin, Texas

Abstract

A mold filling model for lost foam casting process has been developed for a finite element method based casting simulation software. The model includes the resistance of foam pattern to molten metal flow and formation of foam pattern degradation related defects. The principle of this model is described. Experiments were performed to obtain the experimental data used to derive foam pattern resistance parameters to ductile iron and validate the model.

Introduction

The lost foam casting process offers several advantages over conventional sand casting processes, such as simplified production techniques and reduced environmental waste due to binder system emissions and sand disposal. The process is well-suited for castings with complex geometries, tight tolerances, and smooth as-cast surface finish requirements. When the castings are designed to fully exploit these advantages, cleaning and machining times are dramatically reduced if not completely eliminated. Therefore, the lost foam casting process is viewed as a value-added process rather than a substitute for sand casting.

Lost foam castings are produced by pouring molten metal into a foam pattern contained in a flask filled with loose sand that is compacted through vibration. Generally speaking, a foam pattern is coated with a refractory slurry and dried before being placed in the flask and surrounded by large grain fineness sand. The foam pattern degrades immediately after molten metal is introduced, leaving a casting that duplicates all features of the foam pattern. The degradation products are vented into the loose sand. In lost foam casting process, mold filling, thermal transport, and solidification are strongly influenced by the foam pattern degradation. There are three phenomena which are inherent in lost foam casting process: slow molten metal flow, reducing atmosphere, and degradation products. The first and second phenomena help reduce oxides or slag defects. The last one, however, may become casting defects if they remain in the cast parts. To improve lost foam casting design, it is essential to understand the interactions between the foam pattern and molten metal as well as the displacement of degradation products.

In recent years, considerable progress1-5 has been made to investigate the interactions between foam patterns and molten metal. The foam pattern materials used in the experiments are expandable polystyrene (EPS) and polymethyl methacrylate (PMMA). The cast metals poured are aluminum alloys and gray irons. Walling2 found that the lost foam filling of aluminum alloys is different from that of gray irons. The former is controlled by the rate of foam pattern degradation. The latter, however, is not limited by the foam pattern degradation. In a statistical analysis of experimental results, Wang et al.4 pointed out that the most important factors affecting filling time and molten metal velocity are foam pattern material, coating, and pouring temperature. In addition, the metallostatic head has an interaction effect with foam pattern material and coating. Efforts on modeling this complex process have also been made by Wang et al.5 The mold filling was simulated according to foam pattern recession rate for an aluminum alloy, using the finite difference method. So far, the information about mold filling of ductile iron lost foam castings has not been found in the literature.