Lost Foam Castings

Evaporative pattern casting uses molds which will melt or evaporate upon contact with hot molten metal to generate intricate, complex metal parts. Polystyrene foam frequently serves as a mold material in a &#;lost foam&#; casting process. 

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Today, the majority of evaporative pattern casting conducted within manufacturing environments involves foam casting. Automation and mass-production help make this type of casting a very efficient choice in some metal parts fabrication settings.

A Brief History of Lost Foam Casting

Evaporative pattern casting has contributed to manufacturing for centuries. However, during most of this period, artisans relied upon wax (not polystyrene foam) to form a mold.

Lost foam casting represents a fairly recent innovation. The work of H. F. Shroyer contributed to the development of this technology. He received a patent in for devising a way to perform casting using polystyrene foam embedded within casting sand. 

Manufacturing firms around the world quickly appreciated the commercial benefits of this newly invented casting process. Today, lost foam casting has become an important way to form many types of metal objects and parts.

Foam Casting Process

The process of lost foam casting strongly resembles lost wax investment casting methods. However, foam (typically polystyrene foam) substitutes for wax during casting. 

Today manufacturers utilize this process widely to create metal parts comprised of aluminum alloys, cast irons, nickel alloys or steels. Infrequently, they also cast work pieces in copper alloys or stainless steels this way.

The process of lost foam casting in a mass production environment typically involves creating a detailed polystyrene mold inside an aluminum die. A manufacturer applies steam to heat polystyrene beads placed within the metal mold. The polystyrene will assume the contours and dimensions of the interior of the aluminum die as it expands under the influence of the moisture and heat. Mold makers remove the completed polystyrene mold to finish it, typically by attaching any additional desired features, such as gates.

Manufacturers can also prepare lost foam molds by carving polystyrene foam directly into desired shapes. They may create prototype molds using this technique, for instance.

The manufacturer then paints or sprays the finished polystyrene mold completely to encase it within a &#;refractory coating&#; of ceramic material without obscuring any of the mold details. The coated mold sits with an enclosing shell. This finished mold will fit inside a vented flask or other vented container. The manufacturer packs the mold securely inside the container using heavily compacted sand to hold it in position before adding molten metal.

The hot metal causes the polystyrene mold to evaporate on contact. Evaporating gases escape through the vents. When the metal cools, the manufacturer removes it from the sand and brushes away any clinging debris to reveal the work piece. Manufacturers remove excess metal from gates during the machining process. They frequently rely on foam casting to create ornate, decorative and complex metal work pieces using this technology.

Applications

Lost foam casting enjoys many industrial applications. Its comparative simplicity and low cost make this technology a popular addition to hobby foundries, also. Artists sometimes rely on this form of casting to create statues, for example.

Since manufacturers employ foam casting to create ornate, complex and detailed parts comparatively inexpensively, this process frequently assists prototype manufacturers. It has found wide application within many fields, including the automotive industry. 

Manufacturers sometimes use lost foam casting to create cast cylinder heads or other complex shapes, for instance. Since they can produce metal parts in many different kinds of metals and metal alloys using this process, this casting method holds wide appeal for companies seeking efficient, cost-effective and versatile manufacturing services.

Key Advantages of Lost Foam Casting

Currently, lost foam casting offers a number of important advantages during metal parts manufacturing. Although not always a preferred method for casting certain types of metals or precision parts, it does offer some key advantages.

1. This process of casting can permit the generation of complex, ornate shapes. Polystyrene foam will assume the shape of an aluminum die reliably with the addition of steam.
2. This form of casting often proves highly cost-effective. Particularly when a manufacturer requires the production of a complex or unusual shape, lost foam casting may supply the most affordable production method.
3. Depending upon the size and dimensions of the part, some other casting techniques may not achieve comparable results. Lost foam casting can reproduce parts of many different sizes.
4. This process usually does not require extensive machining. Manufacturers frequently need to extract excess metal from some areas of the part, including any gates. However, the ability to cast complex shapes may enable companies to avoid some other frequent tooling expenses.
5. Depending upon the selection of the metal alloys, this form of casting often permits a manufacturer to generate large runs of cast parts with excellent surface finish properties. 
6. Lost foam casting finds widespread utility in &#;low tech&#; environments also, making the process easily adaptable for use in the construction of artistic works and prototype models. Lost foam casting may permit a manufacturer to create several initial prototype models of a proposed product affordably, for instance.
7. In industrial settings, it frequently becomes possible to use a single mold to generate multiple lost foam castings. This ability to combine castings helps maintain the cost-efficiency of production. Additionally, industrial manufacturers typically re-use and recycle the majority of the sand used to support the mold, a money saving practice.
8. Skilled lost foam casting will permit the effective casting of narrow or thin dimensional shapes under some circumstances. This aspect contributes to the utility of this manufacturing process.

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Type of evaporative-pattern casting process

Lost-foam casting (LFC) is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of polymer foams to simplify the investment casting process by removing the need to melt the wax out of the mold.

Process

First, a pattern is made from polystyrene foam, which can be done by many different ways. For small volume runs the pattern can be hand cut or machined from a solid block of foam, or a sheet of foam core board if the geometry is simple enough it can even be cut using a hot-wire foam cutter. If the volume is large, then the pattern can be mass-produced by a process similar to injection molding. Pre-expanded beads of polystyrene are injected into a preheated aluminum mold at low pressure. Steam is then applied to the polystyrene which causes it to expand more to fill the die. The final pattern is approximately 97.5% air and 2.5% polystyrene. Pre-made pouring basins, runners, and risers can be hot glued to the pattern to finish it.[1]

The foam pattern does not need to be coated with investment if high detail is not needed, simply putting the foam pattern in a box, filling with sand and vibrating will do. However, when detail is needed, the foam cluster is coated with ceramic investment, also known as the refractory coating, via dipping, brushing, spraying or flow coating. After the coating dries, the cluster is placed into a flask and backed up with un-bonded sand which is compacted using a vibration table. The refractory coating captures all of the detail in the foam model and creates a barrier between the smooth foam surface and the coarse sand surface. Secondly it controls permeability, which allows the gas created by the vaporized foam pattern to escape through the coating and into the sand. Controlling permeability is a crucial step to avoid sand erosion. Finally, it forms a barrier so that molten metal does not penetrate or cause sand erosion during pouring. [1][2] Once the sand is compacted, 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.[citation needed]

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There is no bake-out phase, as for lost-wax. The melt is poured directly into the foam-filled mold, burning out the foam as it pours. As the foam is of low density, the waste gas produced by this is relatively small and can escape through mold permeability, as for the usual outgassing control.

Details

Commonly cast metals include cast irons, aluminium alloys, steels, and nickel alloys; less frequently stainless steels and copper alloys are also cast. The size range is from 0.5 kg (1.1 lb) to several tonnes (tons). The minimum wall thickness is 2.5 mm (0.098 in)[citation needed] and there is no upper limit. Typical surface finishes are from 2.5 to 25 μm (100 to  μin) RMS.[3] Typical linear tolerances are ±0.005 mm/mm (0.005 in/in).[4]

Advantages and disadvantages

This casting process is advantageous for very complex castings that would regularly require cores. It is also dimensionally accurate, maintains an excellent surface finish, requires no draft, and has no parting lines so no flash is formed. The un-bonded sand of lost foam casting can be much simpler to maintain than green sand and resin bonded sand systems. Lost foam is generally more economical than investment casting because it involves fewer steps. Risers are not usually required due to the nature of the process; because the molten metal vaporizes the foam the first metal into the mold cools more quickly than the rest, which results in natural directional solidification.[3][5] Foam is easy to manipulate, carve and glue, due to its unique properties. The flexibility 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.[6]

The two main disadvantages are that pattern costs can be high for low volume applications and the patterns are easily damaged or distorted due to their low strength.[3] If a die is used to create the patterns there is a large initial cost.[5]

History

Lost-foam casting was invented in the early fifties by Canadian sculptor Armand Vaillancourt. Public recognition of the benefits of LFC was made by General Motors in the mid s when it announced its new car line, Saturn, would utilize LFC for production of all engine blocks, cylinder heads, crankshafts, differential carriers, and transmission cases.[7]

See also

• Full-mold casting &#; evaporative-pattern casting process

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References

Bibliography

• Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471--4
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• Kalpakjian, Serope; Schmid, Steven (), Manufacturing Engineering and Technology (5th ed.), Pearson, ISBN 0-13--8
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