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.