Ten Questions Asked About Lost Foam Casting

1. What is lost foam casting?

The lost foam casting process originated in 1958 when H.F. Shroyer was granted a patent for this cavityless casting method, using a polystyrene foam pattern imbedded in traditional green sand. The polystyrene foam pattern left in the sand is decomposed by the molten metal. The molten metal replaces the foam pattern, precisely duplicating all of the features of the pattern. Like investment casting (lost wax process), a pattern must be produced for every casting made.

First a foam pattern and gating system are made using a molding press. Secondly, the foam pattern and gating system are assembled into a cluster. Then the cluster is coated with a permeable refractory coating. The next step is to invest the dry coated cluster in loose unbonded sand that is vibrated in and around the foam cluster. The final step in the casting process is to pour the molten metal directly into the foam cluster, decomposing the foam in the system and replacing it with metal. The balance of the casting cleaning operations (cut off, grind, heat treat, straightening and blast) are straight forward and quite similar to other casting processes.

2. What metals can be poured as an lost foam casting?

Generally, all ferrous and nonferrous materials can be successfully cast using the lost foam process. Because the foam pattern and gating system must be decomposed to produce a casting, metal pouring temperatures above 1000¡ F are usually required. Lower temperature metals can be poured, however, part size is limited. In addition, very low carbon ferrous castings will require special processing.

3. What size range of parts can be produced by the Lost Foam Casting Process?

Lost foam castings can be produced in most all metals from a fraction of a pound to thousands of pounds. Slightly different techniques are used for extremely large castings.

4. What "as-cast" tolerances can I expect?

Typically, a linear tolerances of +/- 0.005 in./in. is standard for the lost foam process. This varies depending on size, complexity and geometry of the part. Subsequent straightening or coining procedures often enable even tighter tolerances to be held on one or two specific dimensions. A concerted three-way effort between the foam pattern producer, the casting producer and the casting user will often result in a lost foam casting that substantially reduces or completely eliminates previous machining requirements.

5. What type of surface finish can I expect from a lost foam casting?

Because a thin permeable refractory shell is applied around the smooth foam pattern, the resultant finish is excellent (see comparison on chart). Each casting facility may differ slightly from the standards shown. This must be discussed and agreed upon based on the function and cosmetic requirements of the part prior to release of the tooling order.

6. Aren't lost foam casting expensive? How can they save me money?

While lost foam castings are generally more expensive than forged parts or parts made by other casting processes, they make up for the higher cost through the reduction in machining due to the tight, near-net shape tolerances achieved as cast. Many parts that require milling, turning, drilling and grinding can be produced as lost foam components with only 0.020--0.030 in. finish stock. Again, it is imperative that all involved discus what features can be cast to determine final finished product cost.

7. What order quantity do I need to make buying a lost foam casting practical?

Not as many as you may think! Tooling amortization is a key factor in this determination. Potential overall savings for your application will aid in your decision. Generally, a minimum of 500--1000 pieces per year is a good place to start. On the high end, several hundred thousand automotive parts per year are currently cast in the lost foam process economically.

8. What about casting Integrity? Will I have porosity and shrinkage problems?

Lost foam castings are used for many critical applications in the automotive, marine, aerospace and construction industries, requiring x-ray and other soundness testing.

9. What type of tooling is necessary? How much will tooling cost?

Typically, a split cavity aluminum die is made that is the negative mold from which the positive foam pattern is produced. This tooling is highly specialized and must be done by expert tooling manufacturers familiar with the requirements of the casting producers. Remember: The casting can be no better than the pattern used to produce it. Most tooling for lost foam casting patterns will compare very favorably with permanent mold and diecasting: about $1000 on the low end to several hundred thousand dollars on the high end for automotive components like engine blocks, cylinder heads and manifolds. Lost foam casting tooling life is at least three to four times that of permanent mold or diecast tooling.

10. What lead times can I expect when ordering a lost foam casting?