Lost-foam casting
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
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]
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 1000 µin) RMS.[3] Typical linear tolerances are ±0.127 mm/mm (0.005 in/in).[4]
Advantages and disadvantages
This casting process is advantageous for very complex castings that would regularly require
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
See also
- Full-mold casting – evaporative-pattern casting process
References
- ^ a b Degarmo, Black & Kohser 2003, pp. 320–321.
- ^ ASM (2002), "Lost Foam Casting", ASM Handbook Volume 15, Casting.
- ^ a b c Degarmo, Black & Kohser 2003, pp. 321–322.
- ^ Top ten lost foam casting questions, retrieved 2009-03-29.
- ^ a b Kalpakjian & Schmid 2006, pp. 297–299.
- ^ Degarmo, Black & Kohser 2003, p. 319.
- ^ Spada, Alfred T. (December 2001), "GM Unveils Latest Lost Foam Success: Setting the bar for lost foam casters, GM's newest venture at Saginaw Metal Casting Operations has ramped up in record time to produce cylinder heads and blocks for the firm's award-winning SUVs", Modern Casting: 1.
Bibliography
- Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4.
- Kalpakjian, Serope; Schmid, Steven (2006), Manufacturing Engineering and Technology (5th ed.), Pearson, ISBN 0-13-148965-8.