[Tom Kay]

Hi Gents;

I know what lost wax casting is, and I've had some parts done for me (never done the whole thing myself). I know that once the ceramic shell is made, the wax is melted out, or LOST in the process.

But is lost foam different? Is the foam left in a sand mold to simply be burned away once you pour molten aluminum into the mold? Or have I just not understood the lost foam process?

My concern is that if there's a big mass of foam blocking the entry of the melted aluminum, wouldn't this slow down the entry of the metal into the mold, and lead to premature freezing of the metal before it reaches all the intricate corners of the mold? Or, wouldn't it lead to so much off-gassing that you'd have tons of extra gas to vent, plus maybe porosity problems?

I don't get it. Anyone explain how this works? And can you do a very detailed casting, such as an engine block complete with internal webs, etc? Is lost foam usually done with sand (not ceramic shell)?

Thanks, Tom.

[paprjam]

The simplest way is to bury a piece in loose sand and pour molten Aluminum in or I've used green-sand packed lightly around the part. Here's a couple shots of an experiment I did last week with different foams. The green piece was a failure and the rest were about equal.





There is a lot of smoke and fire but you just have to hang in there and keep pouring. I have the metal hotter than usual and try to pour fast and steady or the sand will try to collapse.

[Tom Kay]

Hi Paprjam;

Thanks for the quick reply. The blue and pink foams just look like house insulation. Correct? They're easy to get in bulk, if they are from Home Depot.

Second, if I'm doing a pretty intricate part, would it work to use lost foam and some kind of sand binder, everything from green sand to furan, petrobond, mollasses, etc? I'd hate for the foam to burn away too quickly and have a mold collapse.

Cheers, Tom.

[paprjam]

Some have used a plaster of paris mix for a coating. I haven't tried it, it's supposed to help with surface quality too. I think they mix POP with some fine sand and maybe some silica flour. I believe it would require more venting, or it may crack from the pressure. I'll try to check and get back to you.
FrankG has done some pulleys in lost foam, I can't find a thread about it, but here is his website. www.theworkshop.ca

[Geof]

I have not done this but did a lot of reading about it one time. I remember it was stated to use only the white expanded styrofoam with no additives of any type. The hot metal does burn out the foam and pure styrofoam when it gets hot enough only produces water vapor and CO2 when it burns. Sometimes if the burning is not hot enoguh you do get quite a lot of soot which is just unburnt carbon.

Some foams have fire retardants in them so of course these will not work and the colored foams may have metal compounds for the color so these will not burn away in the same manner.

[Tom Kay]

Thanks Geof;

That makes sense. It still surprises me that a large volume of foam can be burned away fully so that aluminum has time to fill the mold before freezing. But, I guess it's just a case of try some experiments and see. I suppose the foam could also be made hollow in thicker areas, as long as ramming sand around it doesn't cause it to collapse.

Tom.

[Geof]

Thanks Geof;

That makes sense. It still surprises me that a large volume of foam can be burned away fully.....

Tom.

You may have a large volume of foam, but you do not have a large volume of polystyrene. The density of solid polystyrene is about 0.9 and the density of the foam is far less than this; the foam is mostly gas which fills the bubbles.

[paprjam]

I think the white does burn out the best. Sometimes the beads show up in the casting where the blue and pink will show any details or flaws in the pattern or sandy surface if your using a coarse sand. I'm not sure about the metal content, I just try not to breathe anything when I'm casting. I found Franks post on foam and as he and others mention, Don't use the yellow construction foams or the Foam-in-a-Can. They can have some nasty stuff in them.
http://www.cnczone.com/forums/showthread.php?t=7349

Here's a rammer I did with some white foam. It was horizontal in green-sand, which a fellow caster told me shouldn't work. I had already done so I didn't know, neither did the foam.

I did my initials in hot glue in the last picture.

[paprjam]

This is from the MSDS on Dow blue foam board, looks like mostly C, CO2 and CO. I was curious about this myself. Still not good to breathe.

Dow MSDS

5. FIRE FIGHTING MEASURES
FLAMMABLE PROPERTIES
FLASH POINT: 670F, 354C Flash ignition temperature.
METHOD USED: ASTM D1929 Proc. B.
FLAMMABILITY LIMITS
LFL: Not applicable.
UFL: Not applicable.
HAZARDOUS COMBUSTION PRODUCTS: In smoldering or flaming
conditions, carbon monoxide, carbon dioxide and carbon are
generated. Evolution of small amounts of hydrogen halides
occur when burned or heated above 250C (480F). Under fire
conditions polymers decompose. The smoke may contain polymer
fragments of varying compositions in addition to unidentified
and/or irritating compounds. Studies have shown that the
products of combustion of this foam are not more acutely toxic
than the products of combustion of common building materials
such as wood.

[Hodgie]

a good surface finish and resistance to clapsing is to dip in Molochite like the bronze foundries then support in ether green sand or sodium silicate remember this is how gm do there engines it is cnc pattern making for my induction furnace and iron casting that brought me to this site i now make patterns from virtually any drawing or photograph

Ian

[appicnc]

Hello group

I did some lost foam casting when building a CNC router, I basically used Dave Kush's ideads from buildyouridea.com but with one slight change.

Dave dips his faom patterns into plaster, I was doing that but was getting " pimples " of the cast surface, I tried dipping the part into water with a small amout of detergent in it. The detergent " wets " the foam so you get a better adhesion of the plaster to the foam. I think the pimples were small air voids, the attached photo is a casting done that way, no pimples and an exact copy of the foam master.

The plaster provides a good way of keeping sand out of the surface of the casting, I used gyprock base coat, usually two coats and allowed to dry between coats and pouring.


Ed

[mcphill]

The foam is probably 95%+ air. It will leave carbon deposits, but small parts will melt "instantaneously". You can also shell it and burn it out, or you can use epoxy sand and either remove it from the mold, or burn it out prior to pouring.