Blackhawk Studios Tutorial: Lost Foam Process

For the" loose sand lost foam" process we use expanded bead board. This is the same stuff that styrofoam ice chests are made of. (Extruded board will not cast as well and creates unacceptable fumes and smoke. It can be used with chem-sand, and only out doors, but that's another process!) The foam can be cut with knives and saws, sanded, drilled (gently), and shaped in many traditional ways. Rubber cement (BUT NOT ANTI-WRINKLE) can be used to attatch pieces together, but a tight fit is crucial. Sewing pins can be used to hold parts in place while the glue dries. Thin coatings, allowed to almost dry, on both parts work the best.

Foam working is called "sqeaking." Typical tools include box knives and hacksaw blades.

The foam model will have to be sprued just like in any other replacement process. Usually a square sprue with a cross section of about 1 to 1 1/2" is enough. You want at least six inches of length between the bottom of the cup and the top of the model to insure that the metal will have a pretty high velocity when being poured.

You may want to coat the foam with "StyroCoat" to preserve the surface texture.

These sprued models have been sprayed with StyroCoat and are drying.


Flasking Process

  Shown to the left is a foam model (OK, ok, not much to look at, but stay with me anyway)
  Here is our foam model with a foam sprue attatched. Try to do a better job of joining the sprue to the model than I did here. If loose sand can get into the joint then the sand will become an inclusion in the cast piece.
 

Here the sprued model is held near the top of the metal flask (steel pails work well). Note that if you need more depth the bottom can be cut out of a steel pail and the pail can serve as a sleeve or extension to make a deeper flask. Don't scrimp on depth. Sheet metal or stovepipe can also be used under certain circumstances, but the bottom must be sealed.

 

I haven't shown hands but someone would be gently holding the sprue in the open-top bucket right now.

  A sqare hole is carefully chipped from the bottom of a cheap, ceramic flower pot. The sprue is placed inside this (hopefully tight fitting) hole. Now there are two hands I haven't shown, one holding the sprue, another holding the pot. The top of the pot is flush (even) with the top of the pail.
  While one person holds the model and cup (sprue and pot), someone else, someone with patience, slowly fills the flask with clean, loose, graded sand. Usually a scoop is held in one hand and the other is used to deflect the sand from delicate foam parts. You want a very tight fill. NO bubbles or voids.
 

When the sand is absolutely all the way to the top, you will vibrate the flask to cause the sand to settle firmly around the model in the flask. You can tap the sides gently with a hammer, or place that pail on a thin welding rod and rock it back and forth. Add sand to the tip-top again and then weight the cup. Foundry or theatre weights are good. Bricks may not be dense enough.

 

It is CRITICAL that the expanding gas (when the foam is vaporized) NOT be able to push up through the sand.

 

This model is ready to flask.

 

These artists are finishing flasking the molds. The artist on the left uses a scoop to carefully top off the flask. The artist on the right uses a vacuum to remove any loose sand that has fallen into the cup. Note the pail used as an extension on the left-hand flask.

 

Here a vacuum hose is used to keep the foundry reasonably clear of noxious styrofoam fumes. This is pretty important. Open doors and fans are not enough. If you don't have an air handler then cast foam outside only.

 

Here the casting is cooling. The large cup serves to feed metal to the cooling and shrinking model below. A theatre weight is shown on the grate.

 

When the casting has cooled (2 hours or more) the flask is gently rolled onto its side. The sand will trail out and the casting can be pulled from the sand. This piece is aluminum.

 

This is the finished piece.


There is not enough information here to allow you to go out and start casting using the loose sand lost foam process, but there is enough to get you asking the right questions. Good luck, and make art!

Address questions and comments to: joel@blackhawk-studios.org