High Temperature Metal Casting
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Basically it is plaster and sand. http://www.chicagoartistsresource.org/node/9297 I do not know if the mixture can be air-cured, or if is only safe and functional after being heat-cured. The heat-curing probably helps evaporate the mechanical water.
You need Investment, a special mix of plaster and non-plaster materials in order to do aluminum or bronze casting.
Ceramic Shell Method
Direct Printing Moulds
You can print a casting mould with a reprap directly using a motor driven paste-dispenser instead of the FFF-head.
Pastes made from dust or milled particles tend to stick in thin dispenser needles, as the sharp-edged microparticles sometimes nests and clogg the flow.
Better fluidity is achieved by using spherical particles instead of milled material and dust.
Some tests were performed with glass-spheres from sanding, comercial cenospheres (hollow aliminium-silicate-spheres with some microns diameter), hollow glass-spheres and rock-spheres - all mixed with sodium silicate (waterglass) and water.
This pastes dries at air and can be heat-cured for faster drying or sintering to a more solid ceramic material.
Hollow glass-spheres are good heat isolators but only stable until 600 centigrades.
On December 2, 2009 06:23:05 pm Vik Olliver wrote: > Sebastien Bailard wrote: > > Vik, > > > > Do you want to type up some notes on the wiki under > > casting/metal? > > Yeah - when I get a chance to come up for air! Spent the last day > resurrecting my work laptop. Still not quite done. > How long has it been since I backed up my machine? ...
> > Also, you may want to buy some jeweler's investment if you want to go to silver or bronze. > > Less chance of steam explosions and all that. Or look up some investment recipes that use plaster. > > S'okay, I bake my moulds out at 250C. I used to do a lot of low-temp > casting. Mind you, the new furnace just about melts aluminium :) Just > needs a blowtorch to keep it at a pourable temperature. > Are you using pure plaster? Or plaster and a refractory aggregate?
This is a bit of a ramble, sorry:
Baking out your investments at 250C is going to dry out all the water that didn't find dry plaster to turn back into gypsum crystal. ('Mechanical Water')
But if you pour in bronze or aluminum, you're looking at having the remaining H20 ('Chemical Water') in the gypsum crystal disassociating from the gypsum and escaping. Possibly. Possibly violently?
Bronze and aluminum are different animals from pewter, and deserving of healthy respect.
"Plaster of Paris is a type of building material based on calcium sulphate hemihydrate, nominally CaSO4·1/2H2O. It is created by heating gypsum to about 150 °C.
2 CaSO4·2H2O → 2 CaSO4·0.5H2O + 3 H2O (released as steam).
'You get it _hot_, water comes out.'
http://users.lmi.net/drewid/plaster_faq.html#Anchor-the-24925 says you need refractory aggregate.
My copy of From Clay to Bronze says the same. He just uses plaster and sand. And that for bronze, solid investment, the burn out is 540C for 2 days.
This looks good and straightforward, via: http://www.chicagoartistsresource.org/node/9297 "Though there are many variations of the ratio between the sand and the plaster, the formula that I have successfully used combines one part plaster with one part sand by weight. Since plaster and sand are commonly packaged in 100 pound bags, it is easiest to premix one bag of each in a large tin washtub and then add this mixture to your water."
This is nice and technical: http://www.artmetal.com/brambush/forum/bramyak1/messages/109.html
"If there is ANY chemical water left or if there is any wax residue left in the plaster there is likely to be pitting, boiling, gas released from the mold and even a violent BURP or explosion like a geyser that will blow the metal right back out the top at worst or just bubble away at less worse.
Both are real bad.
Both make for casts less desirable (being real kind here... personally I call them poorly shaped remelt ingots)."
As I (partially) understand it from "From Clay to Bronze", we bake out the mechanical water, and then the chemical water, and trust to the wiremesh wrap to keep the investment together mechanically, because the gypsum crystals are toast, so it's just a big weak matrix of calcium sulphate hemihydrate and sand, as opposed to good strong plaster (gypsum).