Lost Foam CASTING PROCESS:
The Lost Foam casting process
is a relatively new method of producing castings developed around 1960. The
process is primarily used for aluminum, gray iron, and ductile iron
castings, although it is envisioned that the process will be used by other
segments of the casting industry as the technology matures.
Lost Foam casting uses patterns
made from polystyrene (the same material that is used in foam coffee cups)
or polymethylmethacrylate (PMMA), placed in un-bonded sand. The use of foam
patterns can help improve both the casting process and the final cast
product by eliminating the need for cores and improving the accuracy of the
casting. Cores can be eliminated because un-bonded sand fills internal
spaces of the foam pattern, forming the cavity in the casting.
By eliminating cores, foam
patterns can be used to cast complex internal geometries, including multiple
passages and curved walls of variable thickness. Unlike most other casting
methods, the pattern does not need to be withdrawn from the mold prior to
casting. The foam pattern must be coated to obtain a high-quality casting.
The patterns are made from expandable polystyrene or PMMA beads, which
evaporate from the heat of the molten metal during pouring.
The use of low-density,
pre-expanded beads minimizes the amount of vapors to be vented. A single
pattern may be made of several sections or multiple patterns may be combined
into a cluster and glued together to allow complex or multiple castings per
mold. Once the pattern is made, it is coated with a permeable ceramic or
other refractory wash by dipping, spraying, or pouring. As with other
casting methods, gating and risering systems are included, but in Lost Foam
casting, the gating and risering systems are glued to the pattern. The
assembly is suspended in a flask with side vents, and un-bonded sand is
poured around the pattern while the flask is being vibrated, which allows
the sand to pack tightly around the pattern.
During melting and pouring
operations, the vapors from the polystyrene escape through the coating, into
the mold, and out of the flask. The coating and the rigidity of the sand
allow the mold to stay in place. After the casting has solidified, it is
transferred to the shakeout area and proceeds through cleaning and finishing
operations like other casting processes. The shakeout process is easy with