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.
FLOW-3D Cast enables users to account for variable density foam patterns. The lost foam model contained in FLOW-3D Cast 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.
Behavior of Lost Foam Patterns
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.
A qualitative comparison with an X-ray test case.
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.
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.
When Do Density Patterns 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.