The similarity between lost wax casting and lost foam casting (LFC) is almost the same. The main difference is the use of EPS (expanded polystyrene) instead of wax. EPS is the disposable element. The only difference is the pattern remains in the sand mold. The hot metal will decompose the mold. The origin of this process dates back to 1958. H. F. Shroyer the originator called it the full mold process. In the early days, the patterns were machined on expanded polystyrene blocks and molded in sand that had furan resin as a bonding agent. Luckily, the patent expired in 1980 and it became available in the public domain.
The process of casting using EPS patterns explained in the following steps
- Patterns machined with pre-expanded EPS
- Pattern is coated with refractory
- Pattern is placed in a flask
- Sand is filled into the flask
- Liquid metal is poured into the flask
- EPS melts or vaporizes
- Pattern is shaken loose for the casting to emerge
The alternative for EPS is expanded polymethyl methacrylate (EPMMA). Sometimes the two are mixed together to obtain a blend. Polycarbonate is another material that used in the LFC process.
Forming the Pattern
These are the various steps in the pattern making
- Design the part
- Tooling design for the EPS foam
- Tooling design decides initial dimensions
- Different sections of foam design
- Ultimate foam pattern quality and fusion of foam to refractory coating
Tool design of foam patterns take into account holes in the foam and pullbacks for undercuts. The design also incorporates collapsible cores for making large internal hollow sections in the foam. The complexity of the design involves making several sections of foam that are joined together in a final assembly to arrive at the final pattern To carve each foam section a tool and suitable fixture is designed so that correct alignment of sections are glued.
Tool designs generally are a long process. With the advent of the CNC router tool design and foam section carving is automated. Prototyping involved gluing hand carved foam sections and the resulting cast was inferior in quality that needed careful machining. Today 3d printers have made complex designing of parts user friendly. The same design is transferred via CAD / Cam software to an EPS shaping CNC router machine. The router creates the foam patterns. When all the foam sections are assembled together the pattern is filled with unbonded sand. Hot metal is poured and allowed to cool. The foam vaporizes. The sand mold is shaken loose and reused for the next casting process. The resulting cast is almost perfect that requires very little machining.
Cylinder blocks for automobiles use this process widely. Formula one engine constructors like McLaren and Ferrari have their own pattern making workshops that are state-of-the-art.
OMNI CNC EPS shaping router machine is widely used in die and mold industries. The machine‘s versatility can be gauged because it is used on expanded polystyrene, wood, and other non-metallic molds.
They have found special use in
- automotive foam mold
- wooden ship modeling
- wooden aviation modeling
- model trains
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