Sodium Silicate Shakeout Additives

Not sure if this has been discussed, interesting reading in this paper from the 1960s on SS additives to promote better shakeout. I'd be interested in actual experience comparison to these points.

https://scholarsmine.mst.edu/masters_theses/2676

Things that made me go hmm...

Sodium silicate with a alkaline:silica ratio of 1 : 2.40 and 52 degrees Baume is the most universally used silicate according to the various suppliers of sodium silicate. A sand bond with this silicate has good reactivity with the carbon dioxide, high gassed strength, some green strength, and fairly good flowability.​

You can make SS RU from SS N (1:3.2) grade by adding NAOH to SS N.

My CO2 cores use Sodium silicate that has an SG of 1.37 and is 8.55% Na20 27.45%Si02 i.e. the ratio is 3.2 now it is said that the best ratio for core work is 2.4 so I add 3% each of NaOH and water in the form of a 50/50 solution (be careful if you make this solution up as it can boil and spit) this addition should bring the ratio to 2.4. - OlFoundryMan​

Optimal SS percentage increases with AFS grain fineness


SS doesn't fuse at aluminum temps so shakeout should not be a large concern. (This seems suspect given practical experience) Iron temps it matters.

Baking after gassing adds green strength and drives out water.

1% fireclay can work as an additive with excellent surface finish up to 1500 F but adversely affects moldability. Compressive strengths increase at 1500F and above.

 

here is what the US Navy said about SS cores. I did not see any Change in the 1993 edition which is the last one made. the only addition you may need is to add colapsability ( that is very important in Aluminum,) very bad to have a rock hard core rip apart your casting ... seen that very disheartening . 400Lb Brass Valve Manifold for the USS New Jersey feed water system, extremely complicated, one of the flanges tore off

https://books.google.com/books?id=c98mAQAAIAAJ&pg=PA175&source=gbs_toc_r&cad=4#v=onepage&q&f=false


V/r HT1

 

FWIW, that's been my experience, I've never had any trouble removing SS bound cores from my aluminum castings and that's without any additives to assist the shakeout. I've never cast bronze or iron so I can't speak to how similar cores might behave in those metals.

 

SS cores in cast iron made without a weakening agent (table sugar for me) results an absolutely rock hard core that was impossible to chip out even though access was fairly good. My mentor at a large foundry had to scrap 800-pound iron castings when his supplier failed to add the specified table sugar as a weakening agent to the SS he bought. There is not a question in my mind that, for iron, weakening is essential. Tom Cobett has a nice paper on SS cores. I have yet to find what he said in that paper to be at variance with actual experience. http://forums.thehomefoundry.org/index.php?threads/sodium-silicate-ru-for-core-making-in-cast-iron-molding.702/

I reported my own experiments with baking SS cores here http://forums.thehomefoundry.org/in...-ru-for-core-making-in-cast-iron-molding.702/

Denis

 

Thanks for the link, I've got a copy of the 57 revision but nothing on the training course. I'm intrigued by the references to the training videos. The sugar ratio for SS recipe is helpful.

I believe SS does not reach full strength until around 1500F. A little sugar insurance can't hurt shakout though. It does make me want to make an aluminum pouring basin out of fused SS.

For cast iron everything I've seen concurs with your and Tom's experience. It was your thread and the windmill gear thread that started me down SS rabbit hole. I made a batch of SS last weekend - concentration is a bit suspect but should be good for a mold test.

 

Sodium silicate also expands when it reaches high temperatures to form a foam and gives off steam, so it's used for fire barrier caulking mixes around pipes going through walls.

https://en.wikipedia.org/wiki/Intumescent

 

Hmmmm, I have not seen the expansion or breakdown of the sodium silicate when used as cores. Maybe that relates to their relatively low percentage as a fraction of the total sand/silicate mass. In fact, wiki more or less says so regarding intumescence: "The intumescence disappears in the presence of finely divided mineral dust, whereby the waterglass becomes a mere matrix." Likewise foaming is not evident in cores---perhaps the very thin layer of silicate on each sand grain actually foams , but there is so much space between grains that the foam just occupies part of the space? I suppose there could be some breakdown as the molten iron hits the sand/silicate core and the surface of the core exceeds 2000 degrees briefly, but I must say the detail rendered by a silicate-bound mold is extremely fine. So, the quoted 1950F ish melting point of NaSi stands in contrast to its withstanding the onslaught of molten iron rather nicely.

And it is good to remember there are quite a few forms of liquid sodium silicate out there. RU grade is the standard form usually discussed in core making. Other forms may have varying (read that frustratingly varying) properties.

Denis

 

They do mention using other silicates (calcium) as well for firewall seals, my only experience with foaming was a homemade refractory mix made from cement, fireclay, perlite, and sodium silicate which formed a foamy green glass on hot areas before it even got hot enough to be usable in a furnace. It only had a small amount of silicate so the the other ingredients heat performance would predominate. I did drizzle some sodium silicate into the refractory cracks at the bottom of my furnace to try and seal any oil escape points with no visible foaming or even sealing for that matter.

 

I have no first hand experience. Based on the paper I linked, I thought it fused or sintered at 1000-1500 and broke down at 2300. Thats why shakeout post iron temps is difficult. I'm admittedly keyboard casting at this point so I'll defer to those who have actually put metal in bonded sand.

 

I got my silicate from a pottery supply place, it's N-grade, it works quite well although I found that I need to use more than is typically recommended for the RU-grade. For the N-grade, 6% by weight seems to about right, I tried 5% and my cores were a little too weak.

 

Mark,
perlite makes a nice fluxy green glass at relativity low furnace temps. I glassed my first furnace with a similar mixture as refractory. But successfully patched the furnace with ss and sand (heavy on the SS). I didn't get that furnace over brass melting temps so I don't know what it does at Iron temps.

 

From post #9 of that thread:

 

Brunhildas Bra held up to high molten iron just fine. BUT, to be fair, the iron was in contact with it for only 20 seconds or so. (As an aside, were I to make a pouring funnel today, it would not be round or conical in shape, but rather a tapered but flat square-sided to prevent the poured metal orbiting as it descended.)

Denis

 

As an experiment I heated up some home made liquid sodium silicate: the stuff foams up like popcorn, whether the CO2 cured silicate does the same is another matter.

 

You can see on the sample that it's already melting round the edges into a glassy substance that I'm guessing is dehydrated solid sodium silicate and is probably water soluble to a certain degree. I imagine it'd function like mineral glass wool fibre but messier and harder to handle as it needs heating to puff it up and it would easily crumble.

At any rate it's not an issue for sand moulds as Denis mentioned, the Wikipedia article states that fine dust mixed in with it prevent the expansion from happening.