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Sawdust as Facing Ingredient

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vtsteam:
In Ironman's videos (luckygen1001 channel), there's one about using sawdust to replace coal dust in facing sand. He gets very good results in an experiment to compare the two. Both produce an insulating and reducing gas atmosphere adjacent to the sand face preventing adhesion to the sand and scale,, and producing a smooth finish.

Bituminous coal dust is hard to come by locally. Anthracite is the only coal available to me here, and it doesn't have the volatiles necessary to produce much of a gas layer. So I decided to try sawdust.

He recommends using dry wood, and actually saws up some particle board, sifts it to fines, and adds that at 5% to his facing sand. normally he uses 4% coal dust for the same purpose.

So I thought I'd try it. I happened to have a fair amount of fine sawdust from making pine patterns lately on the Gingery lathe. After sieving, I mixed them 5% by weight with my molding sand.

Unfortunately, when it came time to mold for a pour, I found that the new facing sand had almost no bond strength. It would definitely not hold a pattern face without crumbling. So I made a quick decision not to use it (the iron was melted already) and just went with plain molding sand, which left a very rough and abrasive finish on the iron casting.

I've thought about this since - I think the sawdust was extremely dry -- old white pine that had been in the house for a decade, and winter humidity indoors now is abut 30%. That means the sawdust was bone dry. As such I guessed that it might have absorbed moisture out of the greensand. So, tonight, to try to give it some molding strength tonight, I added back a little more water. It improved, but still was quite weak. I didn't want to add too much more water, because of the potential for steam creation at mold time.

I'm thinking 5% may be too much, and actually, less might be needed than coal dust. A few reasons I've come up with .... coal dust is much denser, so the volume of even 4% sawdust would be much greater that coal dust. That greater volume dilutes the clay and sand of the facing material more than coal dust and reduces its bond, by comparison.

Second, my experience in making charcoal (my thread on this is in this forum) burning wood is essentially at least 2/3 gas volatiles, leaving behind about 1/3 carbon as charcoal, while coal dust has a much higher percentage of carbon, which remains as coke after the facing sand has done its job.

So it seems to me that wood dust might be effective at less, not more, than the normal amount of coal dust in facing sand. Maybe even 3%, which, if correct, would improve the facing sand's green strength. And that is the main problem for me when trying to use it right now.

Just some of my thoughts, presented here.... in case anyone is interested in these things.

vtsteam:
I've done some lookups for densities of coal, coke, charcoal and wood, and doing some calculations  and then accounting for 12% moisture in the wood density figure and it looks to me like white pine is closer to 70% volatiles, while bituminous coal is about 22% . That's a ratio of about 3 to 1 in favor of white pine sawdust.

So if that's true and Ironman is getting good results with coal dust at 4%, it seems white pine sawdust should work at 1.3%. Let's say 2%. That would greatly help the green strength of facing sand -- if all the assumptions above were true. Only one way to find out....... :dremel:

awemawson:
If only you could use resinous pine for your saw dust it would act as a binder and produce a lovely aroma while you are casting. But I expect it wouldn't produce the fine dust like sawdust that you are after.

vtsteam:
Actually, Andrew, pine resin is waterproof (it was used as caulking), you may have noticed if you've ever tried to get it off of your hands, or.... a car. As such it wouldn't bind greensand (clay water and sand) well at all -- both resist and interfere with each other.

As for pine scent, the pine I used definitely has that. No mistaking the facing sand for greensand. It also has some pine resin in it, naturally. But bonding strength is poor. I'm about to dilute my present facing mix down to 2%, and we'll check the bond, and then try that in a melt. I've done two melts in two days so far, but the temps are dropping and there is possible snow coming. Might be as much as a week before I can try it out. Fingers crossed.

vtsteam:
Very happy with the bond at 2% fine white pine sawdust. But will it do the job?  :borg:

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