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Lost foam casting (LFC) is a type of investment casting process that uses foam patterns as the investment. The method takes advantage of the properties of foam to simply and cheaply form castings that would be difficult or impossible, using normal "cope and drag" techniques.
Process
Shaping Investment
The original Polystyrene investment either carved or molded.Carving Polystyrene
Polystyrene may be carved with traditional carving tools or hot-wire cutting tools. It can also be easily sanded.Injecting Polystyrene in a Mold
Polystyrene, which contains pentane as a blowing agent, is commonly used for beads. The beads are first pre-expanded and then stabilized, after which, they are blown into a mold to form pattern sections. A steam cycle then causes them to expand fully and fuse together, following which, it undergoes an in-mold cooling cycle. If the final shape is too complex for a single mold, it is molded in sections. The shaped foam sections are aged, and then glued together to form a cluster.Preparing Final Investment for Casting
Risers and gates are also likewise attached. (They must be part of the casting to minimize shrinkage.) Next, the foam cluster is coated with ceramic, either by dipping, spraying or pouring. The coating forms a barrier so that molten metal does not penetrate or cause sand erosion during pouring. Coating also helps to protect the structural integrity of the casting.After the coating dries, the cluster is placed into a flask and backed up with un-bonded sand. Mold compaction is then performed, using a vibration table to ensure uniform and proper compaction. Once compacted, the cluster is packed in the flask, and the mold is ready to be poured.
Automatic pouring is commonly used in LFC, as the pouring process is significantly more critical than in conventional foundry practice. With LFC process, cleaning is easier and requires fewer operations, since there are no fins or parting lines to remove.
Patent
LFC originated on April 15, 1958, when H.F. Shroyer patented the use of foam patterns, imbedded in traditional green sand, for metal casting. In his patent, a pattern was machined from a block of expanded polystyrene (EPS), and supported by bonded sand during pouring. This process is now known as the "full mold process".
The polystyrene foam pattern left in the sand is filled by the molten metal, precisely duplicating all of the features of the pattern. Like the lost wax process, a pattern must be produced for every casting made.
With the full mold process, the pattern is usually machined from an EPS block and is used to make a one-of-a kind casting. The "full mold process" was originally known as the "lost foam process".
In 1964, M.C. Flemmings used unbonded sand for the process. Flemming’s method is today known as "lost foam casting" (LFC). LFC is differentiated from full mold by the use of unbonded sand, as opposed to bonded sand that is used in the full mold process. Currently, more foundries in North America use the LFC process than the full mold process.
Foam casting techniques (both “full mold” and LFC) have been referred to by a variety of generic and proprietary names. Among these are “full mold”, “cavityless casting”, “lost foam”, “evaporative foam casting”, “evaporative pattern casting”, foam vaporization casting, Styrocast™, Foamcast™, and Policast™. The use of these terms has led to much confusion among design engineers, casting users, and casting producers.
Benefits
The advantages of LFC include:
- Flexibility of foam: Foam is easy to manipulate, carved and glued, due to its unique properties.
- Dimensional accuracy: The patterns are accurate representations of the desired casting, as compared to sand casting and there is no tool wear. There is also less finishing work required for an LFC casting, as there are no fins or parting lines.
- Elimination of cores: This allows for more complex casting designs, well-controlled wall thickness of castings, and no core-prints. This process also eliminates fins or shifts, core defects and sand mixing.
- Elimination of parting line: There is no draft and multiple levels of casting are possible. Proper gating and riser placement can be achieved, and there are no shifts or fins. Forms that are not typically possible with traditional cope and drag methods can be produced since the mold does not need to be parted to remove a pattern.
- Part consolidation: The flexibilty of LFC often allows for consolidating the parts into one integral component; other forming processes would require the production of one or more parts to be assembled.
- Lower cost: Because the process is very much simpler, as compared to traditional sand-casting methods, production cost is very much lower.
Public recognition of the benefits of LFC was made by General Motors in 1993. By 1998, LFC production had reached approximately 140,00 tons in the United States alone. And in 2005, it is forecast that LFC will account for 29% of the aluminum, and 14% of the ferrous casting markets.
Common metals cast
Commonly used metals include:
- gray iron
- ductile (or nodular) iron
- aluminum alloys
- copper-based casting alloys
Casting tolerance
| Dimensions (in.) | Tolerance (in.) |
|---|---|
| <1 | +/- .007 |
| 1 to 3 | +/- .010 |
| 3 to 5 | +/- .012 |
| 5 to 7 | +/- .017 |
| 7 to 10 | +/- (in. x .003) |
| >10 | +/- (in. x .002) |
See also
"Lost-foam casting"