Problems of surface carbon in lost foam cast parts
When it comes to lost foam casting defects, surface carbon is an obstacle. This is because the lost foam casting process uses foam that contains carbon, whether it is EPS (expandable polystyrene) or STMMA.
It is especially an obstacle in the casting of ductile iron. During the pouring process, most of the pyrolysis product of the foam pattern has already been drawn out via vacuuming, leaving a small amount of the product in the mold.
Pouring ductile iron, the temperatures cannot be as high as that of steel, as it will result in decay in the spheroidization process, resulting in an incomplete gasification - something fully achieved in the lost foam steel casting process.
Ductile iron that has a high carbon content (in general more than 3.6%), cannot absorb the excess carbon created, resulting in the carbon from the foam pattern being left on the surface of the casting.
This 'leaving behind' of the carbon on the casting's surface leads to flow marks.
Carbon inclusion is caused by the pyrolysis product left in the casting itself, as it cannot be discharged or absorbed by the high carbon content ductile iron. This results in carbon inclusion formed by the carbonization of the foam in the product.
Expandable Polystyrene (EPS) and STMMA
EPS is the most widely used pattern material in lost foam method because:
- The gasification of low density expandable polystyrene is rapid.
- The raw material of expandable polystyrene is plentiful, making it an inexpensive material to use
However, EPS does cause lost foam casting defects such as pores and flow marks because of its high carbon content (up to 92%). The pyrolysis of EPS is also disorderly and slow fracturing, resulting in the formation of a solid pyrolytic carbon residue.
In 2001 a lost foam casting specific copolymer material (shortened to STMMA) was developed, consisting of 30% styrene and 70% methyl-methacrylate (with a carbon content of 69.6%). With a significantly lower carbon content, STMMA also contains oxygen, able to further reduce the amount of residual carbon that combines with carbon atoms.
STMMA has shown to be effective in reducing the castings defects in ductile iron castings, including the formation of flow marks, carbon surface defects as well as carbon inclusion.
Back to Main Page: Lost Foam Casting
Further Suggested Reading:
- How Lost Foam Casting Works
- Lost Foam Casting Tolerances
- Metals that can be Lost Foamed
- Surface Finish for Lost Foam Cast Parts