First Lost-Foam Casting

[the artfull-codger]

(03-17-2013, 06:46 PM)Sunset Machine Wrote: <chuckle> We're pouring the metal on top of the foam, correct? Burning it out with a smoking fury, from inside of the mud shell? What's the mud do that the sand doesn't?

It's supposed to make a better finish as it's smoother than the sand & also hold it together better [so I've read] never used it myself but I'll try it out,some use ordinary sand,is this true?? perhaps someone more knowlegable will let us know, I've allways used fairly dry moulding sand,the downside is it makes the sand stink.
Graham.

[Hawkeye]

You guys have sorted out the language barrier as far as drywall mud goes. Lots of different ways to call it.

When you coat the pattern with some kind of slurry, you do get a smoother finish. Some wrap the foam with masking tape. The main reason I use it is to eliminate the chance of the sand collapsing as the metal replaces it. With a hardened slurry coating, you can use dry sand instead of green sand. No problems with too damp a mix causing steam when you pour. Since the coating is thin, it will allow some of the gases to escape into the sand.

My casting bucket is equipped with an air agitator (seen as the copper tube on the back side of the rim). When you run a small air flow up through the sand, it turns into a dry quicksand that allows you to push the pattern down into the sand very easily. When you shut off the air, the sand settles around the pattern. Same method for removing the finished casting. Turn on the air, the sand starts moving and you can pull the hardened metal out. No dumping.

The steel rig on top of the sand gives you someplace to pour the metal. Easier to hit than a small bit of foam. More importantly, lost-foam requires a ready supply of molten metal ready to flow down as the foam burns out. By pouring a few inches of metal into the reservoir, the flow into the pattern is not interrupted. Since it is smaller than my crucible (the bottom half of a fire extinguisher), it will be easy to drop it in for the next melt.

[Sunset Machine]

You've made it all sound very simple, thank you!

[Hopefuldave]

I dunno, took me weeks to work out what drywall mud was, and you blurt it out for all to see.... Mutter mutter...

Hawkeye, nice castings, nothing like bike parts to put a smile on my face
Did you vent the investment? I'm told McDonald's drinking straws are the very thing...

I've seen a few pages / youtubes where the foam's dissolved out with acetone and the investment's part-fired to clear the residue - looks interesting, but the smoke and flames aren't anywhere near as impressive...

Can't see why you shouldn't surround the investment with core mix (sand with sodium silicate / CO2) for support before dissolving and burning the foam residue? It should be a lot firmer than dry sand, tho the fluidised quicksand strikes me as genius!

I must give this a go - 50 gallon drum came my way at the weekend, and another with some gallons of waste engine oil... Shame the refractory costs so much At least I've already got the compressor and an old blower for the oil burner, high pressure propane regulator coming off ebay - just The Details to work out

Dave H. (the other one)

[Hawkeye]

If you take a look back at the first photo, the left end went to the top of the casting bucket. With the funnel shape of the two sections, there was no need for extra venting. I have been thinking of using straws where needed. Thin strips of foam sliced off the parent material would be easy to glue onto the main parts.

I had read that it helps to drill down into the foam after the pattern is supported in the sand bucket. This gives the metal a path to follow to start burning down into the foam. Lots of tips on the internet.

[Hopefuldave]

That makes sense - I have a few parts I'd like to cast with some thin sections linking quite heavy ones, so I think it'll mean a trip out for burgers... Might give the melt-out a try, as at least one (about 8 pounds of aluminium) will need a runner and gates to get a good fill..

As I have a half-bucket of silica gel, I'm going to try some home chemistry and the silicate/sand mix, can pipe some Mig gas through to cure it.

A rude question, I know, but any tips on budget refractories for furnace linings?

Dave H. (the other one)