MadModder
The Shop => Metal Stuff => Topic started by: vtsteam on February 12, 2022, 11:37:39 PM
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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.
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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:
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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.
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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.
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Very happy with the bond at 2% fine white pine sawdust. But will it do the job? :borg:
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By weight or by volume?
-russ
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In casting, ingredients always by weight, at least for me.
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As a follow up, yes the 2% pine sawdust facing sand was successful in giving a much better finish to my castings. Spruce sawdust did not do quite as well, though it did have some effect. I'm thinking that hardwood sawdust might be better than either, and could be used at greater percentage (by weight) since it is denser, and takes up less room, affecting bond strength less therefore (I hope.)
Another thought is that adding dry sawdust to facing sand too soon will cause it to absorb moisture out of the sand. There may be a difference in performance between sawdust facing sand mixed a week ahead of time and sand mixed during molding. This would logically seem to reduce the bond strength of the sand, and increases the ignition time of the sawdust. Something more to test.....
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There used to be a spirit based spray on that was applied just before closing the cope and drag. While the mould was still open it was flashed off with a gas torch, burning and leaving a carbon like surface deposit. This was to improve surface finish and didn’t interfere with the facing sand bond.
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Yep, Andrew, washes have traditionally been used for improving casting finishes. but the topic here is experimenting with sawdust in facing sand.
I'm interested in knowing how much and what type and how mixed and when should it be added to optimize. If you are planning to do any sand casting in future with your induction furnace and want to give sawdust a try in traditional greensand, I'd greatly like to hear your results. :dremel:
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I mainly use dry silver sand and sodium silicate Steve - far less messing about and moulds are robust to handle. It also gives a good open mould that breathes with little venting required.
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How is the finish on cast iron?
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It's always been pretty good - certainly OK for my stuff and better than green sand by far. I suspect that the hard skin is less as well as there is less chilling effect as no moisture to speak of.
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Post some photos of the out of the mold finish, some time when you're less busy Andrew, I'd like to see that. Maybe you could do a thread on how you cast with pics of the whole process. :dremel:
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Cylinder casting, poured 3 hours ago, as broken out:
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No immediate plans for casting at the moment Steve, life’s got in the way - or rather sorting stuff after storm Eunice. Working on an external isolator and socketry for a site wide generator- surprisingly complicated to do it safely AND get the earthing right for a PME supply. Extra motivation as we had yet another half day power cut on Tuesday
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Here are all of the cylinder and piston castings for the Rider engine. All using sawdust facing sand.
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Beautiful!
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The surface finish looks great to me so the sawdust does work.
When I made that video about sawdust I was going to use a Aussie hard wood called iron bark which has less than zero chance of getting it else where in the world. So I used particle board which is available every where and I suspect that it is mostly pine sawdust. The video was made because so many people could not get coal dust. I got the idea from when I spilled iron on a iron bark plank and the spilled iron was very shiny instead of the rough surface when exposed to air.
I will be still using coal dust as I have a lifetime supply.
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Hi Ironman, thanks greatly for visiting here again!
I'm thinking particle board might be good also because of the binder in it. Maybe also denser and less quickly water absorptive -- I don't know, just guesses.
My first try with white pine sawdust at 5% didn't work because it just wouldn't hold together, I might have had better luck if I'd added more water, but I didn't want to go overboard with that.
I did get pretty good results at 2% white pine sawdust. It held together okay for molding. I'm only mentioning the exact type of pine to say exactly what I did, not because it is necessarily better. Just helps to identify what was tried so far.
I will probably try more experiments here with different types and amounts to understand sawdust in facing sand a little better.
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Beautiful!
Thanks Tom! :beer:
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Ironman! Nice to see your around and making vids still.
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Ironman! Nice to see your around and making vids still.
Yes I am still making videos. Have a look at this video where I have go at casting crankshafts and machine them then destroy them to see how much punishment they can take.
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I saw that when it came out lots of good information thanks for doing them.
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Yup, Ironman, I've probably seen every one of your videos at least twice, and several many more times than that! :beer: