What is Lost Foam Casting?

Lost foam casting (LFC) is a type of metal casting process that uses foam patterns to produce castings.  Lost foam casting uses a cavityless mold with the pattern remaining in the mold during metal pouring.  The foam pattern is replaced by molten metal, producing the casting.
 
 

LOST FOAM CASTING

The use of foam patterns for metal casting was patented by H.F. Shroyer on April 15, 1958.  In Shroyer's patent, a pattern was machined from a block of expanded polystyrene (EPS) and supported by bonded sand during pouring.  This process is known as the full mold process.

With the full mold process, the pattern is usually machined from an EPS block and is used to make primarily large, one-of-a kind castings.  the full mold process was originally known as the lost foam process.  However, current patents have required that the generic term for the process be full mold.

In 1964, M.C. Flemmings used unbonded sand with the process.  This is known today as lost foam casting (LFC).  With LFC, the foam pattern is molded from polystyrene beads.  LFC is differentiated from full mold by the use of unbonded sand (LFC) as opposed to bonded sand (full mold process).  Currently, more foundries in North America use the LFC process that the full mold process.

Foam casting techniques have been referred to by a variety of generic and proprietary names.  Among these are lost foam, evaporative pattern casting, cavityless casting, evaporative foam casting, full mold, Styrocast™, Foamcast™, Styrocast™, Policast™, and foam vaporization casting.  the use of these terms has led to much confusion about the process for the design engineer, casting user and casting producer.

THE LFC PROCESS

The LFC casting procedure starts with the pre-expansion of beads, usually polystyrene, which contain pentane as a blowing agent.  After the pre-expanded beads are stabilized, they are blown into a mold to form pattern sections.  When the beads are in the mold, a steam cycle causes them to fully expand and fuse together, this process is followed by an in-mold cooling cycle.

After aging of the molded foam sections, the sections are assembled with glue, forming a cluster.  The gating system is also attached in a similar manner.

Next, the foam cluster is covered with a ceramic coating.  The coating forms a barrier so that the molten metal does not penetrate or cause sand erosion during pouring.  Also, coating helps protect the structural integrity of the casting.

The coatings are typically applied to a foam cluster by dipping, spraying or pouring.  After the coating dries, the cluster is placed into a flask and backed up with bonded sand.

Mold compaction is then achieved by using a vibration table to ensure uniform and proper compaction.  Once this procedure is complete, the cluster is packed in the flask and the mold is ready to be poured.

Metal pouring in the LFC process is significantly more critical than in conventional foundry practice, and automatic pouring is commonly used.  With the castings produced with the LFC process, cleaning is easier and requires fewer operations since there are no fins or parting lines to remove.
 
 

WHAT ARE THE BENEFITS OF LFC CASTING?

With all of the benefits of the LFC process, it is easy to see why the future is cast in foam.  Advantages include:

1.  Dimensional accuracy.  the patterns are accurate representations of the desired casting.  There is no tool wear.  There is less finishing work required for an LFC casting, as there are no fins or parting lines.

2.  Cores are eliminated.  This allows for more complex casting designs, well-controlled wall thickness of castings and no coreprints.  This process also eliminates fins or shifts, core defects and sand mixing.

3.  The elimination of the parting line.  There is no draft.  Multiple levels of casting are possible.  Proper gating and riser placement can be achieved.  And, there are no shifts or fins.

4.  Part consolidation.  The flexibilty of the LFC process often allows for consolidating of parts into one integral component that, with other forming processes, would require the production of one or more parts to be assembled.

Estimated Tolerances of Lost Foam Castings

Dimensions (in.)	Tolerance (in.)
<1	+/- .007
1 to 3	+/- .010
3 to 5	+/- .012
5 to 7	+/- .017
7 to 10	+/- (in. x .003)
>10	+/- (in. x .002)

Rough Estimate:	<7 in. +/- .015
	>7 in. +/- .003 in./in.