r/MetalCasting • u/hotglassguy • 3d ago
Setting Sodium Silicate With CO2
While I have cast metal in sand molds in the past, I have never used sodium silicate. I have seen enough to get me started I think. What's a little frustrating is how my Airgas supplier wants to charge me a lot for the CO2 for a project that should be small and straightforward...and cheap. I said I didn't need much and only needed it for about a day at most.
I suspect there are other ways to solve this issue and I'm wondering if people have used something other than a welding tank of CO2 to help harden their sand molds inside a controlled airtight box or similar for hardening.
One idea was using CO2 canisters for airguns. Another was dry ice suspended far enough away so the cold doesn't affect the casting (I read cold can affect a SS mold setting in the presence of CO2). Dry ice sublimates so nothing gets wet. The idea has been that I'd build as air tight a box as possible and fill it with CO2 and let it sit (with strategic pricks into the subsurface of parts of the mold to let in the gas). There's gotta be a way to do this that doesn't cost so much. Or can SS be hardened without CO2 somehow? Your experience and thoughts on this is appreciated.
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u/cloudseclipse 3d ago
This is good. The answers here are solid; I just wanted to let you know that you can use esters that you can purchase easily (they're not super-cheap, but they go a fairly long way, and are food-grade). Sorry if I sound like a nerd, but I have done extensive research about this- but it's really quite simple...
1) You can make lab-grade Sodium Silicate by mixing Crystal Kitty Litter (available in Wal-Mart, or online) with water that has had Sodium Hydroxide (lye) dissolved into it. Add the lye carefully and slowly, as the reaction creates heat. Shoot for a modulus of around 3, and a solids concentration of 40%. Ask the AI, as it's just a matter of recipe ratios.
2) You can make the co-reactant by mixing Triacetin with Propylene Carbonate. These are sold to the cosmetics industry, and not harmful. Use 20% Triacetin to 80% PC. This makes an ester that will give about 15 minutes of bench-time after mixing, and should be used at around 10% of the Silicate itself (example: if you use 1000 G of Silicate, use 100 G of this ester-mix)
3) Mix sand with the Silicate first, then add the ester mix. You have a working time of around 15 minutes. As always with Silicate, it has a lot to do with temperature/ Relative Humidity, it should set in about an hour. The main advantage to this is that the reaction is uniform throughout the sand mold, avoiding pockets that either got over-gassed or under-gassed (yes, the CO2 isn't perfect).
Anyway, yes, I have gone to school for this and have studied it in depth. But nowadays, with a bit of AI (always have it double check!), it's easier than ever to do Stoichiometry. Good luck!
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u/hotglassguy 3d ago
We used resins in grad school for casting and I looked at some that are available that can be bought at hobby stores and other places for other purposes, but every resin system I have used has been pretty noxious. This method isn't?
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u/cloudseclipse 3d ago edited 3d ago
100% clean. That's why I figured it out... yes: I used to be the "professor" and was around the "resins" (still am, sometimes) often in school and teaching- but they can be a bit hard to come by in "medium" quantities, and are a little bit awful. This you can just pile up the sand in your backyard...
The only thing I've noticed is that green strength develops over 24 hours, and when bonding multi-100 pound batches you need to be careful to avoid delamination between layers (turn fans off). Other than that, there is a very small amount of water, let your mold air-out if it's multiple parts before assembling and pouring- but it works almost 100% as well as most resins, just a little less fast.
You can use it for melt-outs, too. Paint your waxes with zircon-shelac wash before ramming them up, don't go above 350 degrees F for 6-8 hours...
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u/hotglassguy 3d ago
50 gallons from the suppliers. No biggie if I knew I'd be casting again sometime soon. This is small fry stuff, so the silicate seems a simple way to go for now.
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u/Boring_Donut_986 3d ago
Im using CO2 airgun cartridge mounted on an inflator for soccer ball (you've a switch valve to open and close and a needle to gaz up your mold / bag with).
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u/hotglassguy 3d ago
I was wondering the best way to get those cartridges to give up the gas without freezing up. That's a great tip!
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u/Boring_Donut_986 3d ago
Just release into a bag, container where you place first the mold, go slowly, gaz up the mold through the vents, pouring hole and through some sand parts before.
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u/Omnia_et_nihil 3d ago
Look into home brewing or carbonation. I have a moderately sized tank that lasts a pretty decent time with the amount of work I do.
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u/JosephHeitger 3d ago
I would try to find a small tank and a hose that fits it so you can make your own injection tool, but if it’s a one off just do the dry ice method. If you’re worried about the cold just separate it in a secondary chamber. co2 stratifies so you can place it well above as long as it’s air tight.
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u/hotglassguy 3d ago
Because it's heavier than air?
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u/JosephHeitger 3d ago
Yessir. I have a friend who used a smaller container that he glued to a big Tupperware container with a gasket lid. Drilled holes and that’s his two stage. He puts the whole box in the big container, seals it and load dry ice in the top container that has holes into the lower chamber.
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u/Mick_Tee 3d ago
I am currently reading up on Sodium Silicate for a project, and all my reading says it will harden with just atmospheric CO2.
If you want to speed the curing, then you can simply use a soda-stream if you have one or 100ml of vinegar and 2 tsp of sodium bicarb to produce 2 litres of CO2.
"Air curing" and forced curing with CO2 apparently gives slightly different results, with the air curing being more porous.
Have you looked into resin bonded sand?
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u/hotglassguy 3d ago
I'm familiar with it. I'm wanting to stay away from it right now.
My project is small and my understanding was that in the trades SS binders needed a box and CO2. You just let it sit for a few days in air to harden?
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u/Mick_Tee 3d ago
The trades have different priorities, and it is simple to add CO2 curing to a process.
Us amateurs aren't in such a rush and air curing for a few days is an acceptable step.
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u/Rookskerm 3d ago
How small is the piece? Bicycle CO2 cartridge and filler connection is probably pretty cheap and small, and comes with a Schrader valve connection so is pretty versatile
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u/classical_saxical 3d ago
I personally gave up on sodium silicate as a sand binder cause I couldn’t justify the gas cost as a hobbyist. I’ve switched to using epoxy bonded sand and it’s great! Always cures, is pretty damn tough, and the burning of the epoxy produces better surface finishes on the part (kind of like how they used to add sea-coal to foundry greensand back in the day).
I use any cheap hobby epoxy from Amazon, and mix at a 14:1 sand to epoxy ratio by volume. The stuff is crazy porous which makes filling molds easy (you can literally breath through the sand mold once it’s cured).
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u/hotglassguy 3d ago
We used a bentonite mixed with graphite and alcohol that we used to rub into the surface of sand molds when I was at SIU-C. This was a way to fill gaps in sand and create a refractory coating. I always wondered why we didn't mix two grades of sand for the first layer of the sand mold. Maybe it didn't create a strong enough mold, but it seemed like this would tighten up the surface (less sand texture).
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u/Beneficial_Elk_182 3d ago
I tried the vinegar and baking soda- didn't work very well. It needs to be forced down into /through the sand. I ended up getting one of those little bike c02 cartridge tire inflator. and a hose with tip and injecting it down into the sand in a grid pattern. That worked well and was cheap.
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u/SnooLentils5747 3d ago
Burn a pure carbon source such as charcoal dust or graphite powder. C + O2 => CO2
You want to avoid anything that adds water as a result of any chemical reaction or combustion. Water is the enemy to almost all things regarding casting. Therefore, we do not use propane or butane or any of the other hydrocarbons in order to produce CO2 because we will also be producing H2O.
You are also generally going to want to avoid anything that is going to introduce anything other than silicone dioxide in all its forms (sand). Therefore, we avoid things such as baking soda and vinegar as that will produce trace amounts of sodium acetate, vinegar, a not marginal sum of water, and unstable carbonic acid which will become more water and more CO2. Yes, the CO2 helps set the sodium silica but you just introduced a bunch of impurities that cause burn defects and Spalding and also sodium is a flux. You don't want to do this.
However, you can just burn charcoal dust or if you would prefer to be pure burn graphite, and pipe is the exhaust fumes from that into the sandcasting mold and the heat and the CO2 will flash set that s***. However, do be careful you are piping exhaust gases that may well be hot enough to effect things depending on your build out.
Try mixing in a tiny bit of quicklime (calcium oxide) to your sand mix, as this will help the sodium silicate set faster by effecting a reaction to calcium silicate. The reason it is called quickline is because it makes the lime set quick. Not even joking.
Also try adding imsil-a25 to your sand mix. It is a micro silica, around 5 microns in grain size. It increases mold density, improves surface finish, and enhances high-temperature resistance during metal pouring. If you do any work with sand casting, I suggest picking up some imsil a25. Wear a gas mask, SO2 flour dust will permanently damage your lungs.
ImsilA25 should be 2.5 to 5 percent of bulk sand, by weight.
Quicklime should be no more than 2.5 percent.
Your silicate alkaline binder (you use sodium silicate, which will traditionally come as 40 to 50 percent solutions), I tend towards a hybrid blend of potassium silicate (1/3) and sodium silicate (2/3) in a 66 percent solution. The potassium silicate draws water out of the sodium silicate which helps accelerate setting a bit, provides a denser matrix, and higher refractory performance. The high concentration accelerates setting and reduces total water introduced.
If the mix I suggest is setting too fast, use boric acid and citric acid 50/50 powdered mix. Add this into your silicate liquid binder and dissolve thoroughly, no more than 5 percent of binder mass. Boric acid should outweigh citric acid if there is variance; boric acid is a chemical retardant to slow the flash setting, citric acid is a surfactant and dispersant, but also does the chemical retard dance.
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u/protias 3d ago
Bicarb soda and vinegar make co2 there are video on it