Baking Soda Cores for Aluminum Casting

[dang]

Recently I was experimenting with coremaking and found that baking soda hardens to a rock like state when it's baked in a conventional oven.
My question is, has anyone else used %100 baking soda to build "sand cores"? I'll soon be testing a soda core against some molten aluminum. If you're interested keep your eyes peeled.

[ratrace2]

WOW, that sounds great, could you send me a picture. Actually, I think you should be careful when you do that pour. Do you think it might explode?

[dang]

Well, there's only one way to find out. I'll test it over the weekend and snap some photos.

[jrrdw]

Do you ever preheat your sand molds?

[dang]

I don't use sand molds anymore. I gave up after numerous & constant failures. Now I machine my molds out of steel, which I pre-heat in my stove before using, at least for the 1st pouring. The steel molds cool so quickly that I can unload the finished castings and reload them within minutes. Die Casting is great for production!
The problem I have is that steel cores aren't really effective at all due to thermal contraction of the aluminum during solidification. Any kind of core basically gets stuck inside of the casting. It's a disaster, especially after all that machining. So I'm gonna experiment with the sacrificial baking soda cores.
If this doesn't work then I'm gonna bite the bullet and order some oil bonded sand for the cores and all the one off castings that I wanna do.

[jrrdw]

i allways thought you just cracked out the cores once the metal was solid. My step father makes his own black powder bullets, i got him 50 caliber mold and oak handles to match for Chrismas. The 1st one allways turns out junk, the rest are nice. He says it's because the mold temp isn't hot enough.

If you don't mind me asking, what are you molding?

[dang]

Yeah, you just knock the cores out once the casting is finished. Hence the term sacrificial. I was trying to use steel cores, that didn't work.
50 Caliber bullets????????? That firearm must be gigantic!

I build and sell printing machinery. I used to weld the machines together, but found that molding them would be much faster with nearly the same durability. I'll soon be branching out into photography equipment as well, using the same Die Casting process.

I have basically the same results as your father with my 1st casting. Pre heating is really important. Once the mold is warmed up though, it's one casting right after the next.

[Robin Hewitt]

Sodium bicarbonate? That's like sodium carbonate with an extra hydrogen atom.

It breaks down when you pass 50 degrees centigrade to caustic soda and carbon dioxide

NaHC03 -> Na0H + CO2

Suggest you try very hard not to get the dust it in your eyes when you clean down your parts

PS: Also suggest you don't delay casting after baking, it's deliquescent, sucks moisture out of the air.

[307startup]

I know that a lot of die-cast and permanent mold castings use salt cores. The salt is heated til molten and poured into core molds, and when cooled are stationed in the permanent mold using core pins or registers. I'm pretty sure they use sodium chloride or potassium chloride. No nasty chemical byproducts and because they have a higher melting point than aluminum, they don't sag or shift in the mold.

http://www.diecasting.org/library/cg...list_page&and=

that should be the link to an old paper written about the technology

Conversely, but along the same line of thinking is AquaCore, which I have used and is BEAUTIFUL to use...

http://www.acrtucson.com/products/Aquacore/index.htm

[dang]

Wow. Thanks for all the advice, especially about Sodium Bicarbonate chemistry. Looks like I'll be using good ol' jupiter blend instead.
Later on of course, I'll try out that salt and alpha alumina core mixture as well.
Thanks all.

[Robin Hewitt]

Originally Posted by dang chemistry.

One very useful feature of caustic soda to keep in the back of your mind is that a strong solution will dissolve aluminium but not iron.

Handy if you ever need to remove a broken/seized aluminium part from a steel fixture

[dang]

Originally Posted by Robin Hewitt a strong solution will dissolve aluminium but not iron.

Is that to say that a %100 Baking Soda core would in fact dissolve whatever aluminum was poured over it immediately? Wow, what an intensely bad idea I had.