Lost Foam Casting:
Variable Pattern Density
Making foam patterns for use in the lost foam casting
process is a difficult business. To make a pattern, foam beads
are blown into a mold containing discrete vent locations for the displaced air
and steam. This makes the
density of the packed beads difficult to control. Patterns typically show
final density variations of ±20%. And, larger variations are not uncommon.
X-ray observations of the metal filling process reveal interesting information
about the behavior of foam patterns. In particular, when the foam has a
low degree of fusion, metal is observed to move very fast into the foam (e.g.,
4 to 5 times faster than in normal fusion foam). The advancement of the
metal is typically in the form of fingers, which subsequently spread sideways
causing the meeting of metal fronts that result in many fold defects. Further,
the location of the fingering is significantly affected by density variations
in the foam pattern.
In contrast, when the foam patterns consisted of normal fusion foam, the metal
front moved smoothly (i.e., no fingering) and considerably fewer fold
defects occur. What remains unsettled is the effect of varying densities
of normal fusion foam. If such variations cause the same fingering effects as
low-fusion foam, fold defects could occur.
Density variations in foam typically
occur at walls of the pattern mold where foam beads are cooled most rapidly and
near blow holes where the foam material is injected into the pattern mold. Any
model for density variations in a pattern must be general enough to incorporate
these and other possible sources of density variation.
FLOW-3D enables users to account
for variable density foam patterns. The lost foam model contained in the FLOW-3D program
makes use of the foam density in two ways. First, it uses the product of
the density times the specific heat of the foam as a heat capacity variable,
which controls the amount of heat energy needed to raise the temperature of the
foam and subsequently melt and possibly vaporize it. Second, the density
of the foam is used in the foam-residue defect model to characterize the mass
of defect material produced when the foam is degraded.
Snapshots of computed filling pattern compared to metal front
shapes taken from an X-ray video. Arrows indicate dimples
in the front caused by increased density regions.
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