lostfoam

machine#construction: casting#techniques: lostfoam

The Lost Foam Technique

Description of the process:: A pattern is made out of expanded polystyrene. In industry, styrene beads are expanded with steam inside of steel dies to form the foam patterns. In hobby use, a pattern is made out of extruded polystyrene insulation panels, specialist foams, or good old white beaded packaging material, like what a VCR comes in. The foam is cut to shape with a hot wire cutter, turned on a foam pattern lathe, or cut and sanded with woodworking tools. The foam pattern is optionally coated with a refractory mixture to improve finish texture, and gas vents are added.
The foam pattern is completely embedded in dry sand, and molten metal is poured into the foam pattern via a sprue. Heat from the molten metal vaporizes the foam and the metal takes its place. The dry sand cavity does not collapse because either the hydrostatic pressure of the metal or the gas pressure from the vaporized foam is pushing against the walls of the cavity. The liquid metal then cools and solidifies into a replica of the foam pattern.
pro's:

easier, quicker than working wood patterns
no need to worry about draft on patterns, or undercuts
finished casting is closer to the original size of the pattern than a greensand casting
no need to mix greensand or petrobond. dry sand is great.
more intricate patterns can be made
no dragging, coping, parting, or lugging fragile sand molds around. just stick the pattern in a (metal) bucket and pour sand around it
comes in 1/2", 1", 1.5", 2", 3", 4" sizes
stock is relatively cheap compared to wood, or free from construction site dumpsters
- blue board: $5 for 4'x8'x0.5", $20 for 4'x8'x2" (Indiana 2005)

cons:

not quite as good of a finish as petrobond (depending on what kind of foam and coating)
smelly/toxic when burning, stinks up your sand and clothes
must build a hot wire cutter (actually I use woodworking tools and sandpaper on foam and it's much better than hotwire)
can not do as thin sections as with blank mold (how thin?)
one foam pattern, one cast... its gone. Screw up? Gotta make another
- not so good for large quantities of one pattern

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troubleshooting

My sand cavity keeps collapsing! Help!

lack of hydrostatic pressure - pour continuously and as fast as sprue will accept metal, increase sprue and riser height
excess gas pressure - increase sprue size or add soda straw vents
make sure sand is evenly distributed around pattern and compacted. Try vibrating the container by hitting it several times with a mallet. (be careful though too much compaction can distort delicate patterns.)
for patterns with extreme undercuts such as an upside-down cup shape, try lightly packing the cup with greensand or dry sand before turning it upside down. Or, leave a hole in the bottom of the flask into which you can pack more dry sand from underneath.
sand is lifting up from lack of sand pressure. increase flask height or fill higher with sand, use a vacuum clamp or clamping board on top of flask. More of a problem with heavier metals.

I get crappy finish, help!: if using a refractory coating:

you _are_ using pink or blue insulation, right???
try breading technique to cut down on bubbles and cracking
spray the foam pattern with latex house paint or wax in solvent (test solvent first to make sure it won't dissolve styrofoam)
decrease pour temp for large items

I get cold shuts, incomplete parts, embedded bits of foam,

increase pour temp
increase hydrostatic pressure by raising sprue/riser height
make sure sprue is larger than the section thickness of the part
add runners, risers, increase section thickness, put large chunks nearby to keep the entire area warm during casting

I get sand erosion, embedded bits of foam, cold shuts

metal velocity is too high, add flared gate to slow down the metal stream
see http://www.flow3d.com/pdfs/Boxcast.pdf on page 7

My finished casting has tiny bubbles in it!: see aluminum#porosity

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Tips and tricks:

Q: I've heard about people dipping their lost foam patterns in wax. Is this a good thing?
A: I would suggest _not_ dipping lost foam patterns in candle wax for the following reaons: the heat will distort your pattern; built up wax will make the pattern inaccurate; you get drip marks sometimes; there is a _lot_ of gas formed when the metal hits the wax - read: lots of collapsed sand cavities and voids in the metal; you have to have enough wax to dip the pattern in; it's a fire hazard; and i'd rather not mess around with a scalding hot pot of wax if i didn't have to.
The gain in surface finish isn't really that much in the end if you are using blue or pink foam, and is totally negligible if you aren't also applying some kind of refractory shell. I would suggest spraying on a couple coats of latex house paint instead. Some oil base paints like spraypaint will dissolve the foam.

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casting with solid cores

Baked sand cores: Pack the foam pattern with greensand, pop out the core and microwave (or bake) it until it is hard, then put it back in once it's cool.
Steel cores: coat a non-galvanized piece of steel of the finished bore diameter with soot from a candle flame or a reducing acetylene flame, let it cool if it got hot, and push it into a hole you drilled slightly smaller. Knock the core out with a hammer and punch to avoid mushrooming the ends.

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casting with dry sand cores

Say you have a pattern like a coffee cup and you want to pour with the cup pattern upside down for some reason. If you just pour the sand in, the cup will trap a sand-free cavity underneath it. Instead, try this:

Set the pattern over a hole in the bottom of the flask, fill the flask completely with sand, then turn it upside down and fill up the hole with sand. The sand compresses a little bit when you turn it right side up again, so you may wish to pack the sand in and rap the sides of the container while filling up the hole.

Some links with _very_ good ideas

www.buildyouridea.com's lost foam casting primer

David Reid's vacuum clamp page and the technical paper on the same page.

Association of backyard metal casters - lots of lost foam innovation here, I especially liked the foam pattern lathe (why didn't I think of that one??) http://www.abymc.com/