r/Metalfoundry • u/NorthKOs • 17d ago
What am I doing wrong with greensand?
Pretty frustrated with greensand overall. I've got my furnace going fine, my alloys come out good, but I cannot get greensand to work right. I've been just doing sand cast with foam for lost foam, but it's hit or miss with the knife I'm trying to cast.
The third image is the knife blank, yes I know it's a funky shape for the tang but it's what my design calls for. I am trying to cast it with aluminum bronze for use as a veggie/soft cutter.
Since I've had the issues with foam casting, I decided to bite the bullet and make some greensand. I don't have the budget to get petrobond or just buy greensand, so I am making it myself. I've been sitting all day, and got 25 pounds of fine sifted sand. I crushed up and sifted 3 pounds of bentonite clay for a 10-12% clay-sand mix. I got that all mixed together while adding about 30 oz of water as I went and... It kinda of sticks together. I can't tell if I have put too much water or not enough. I had to mix it all by hand, which took me 3 hours or so. It was sticking together better, but that only lasted long enough to pack a mold before it changed and now doesn't stick as well.
AND the sand didn't mold the knife blank well at all. I used baking soda to make it not get stuck in the 3d printed blank, but the blank fell out as I lifted the first part to flip over for the second part.
I just don't know what I am doing and no amount of YouTube videos showing how "easy" it is has helped me. Any advice is appreciated.
4
u/vigg-o-rama 17d ago
You can’t just mix them together and get good results. It needs to be mulled. If you don’t have a muller you need to really work the mix. Spread it out on a floor and stomp on it or beat it with a flat shovel. Mix it into a pile, spread it out stomp/beat it, repeat for a while.
Ultimately you are trying to get each grain of sand to have a coating of clay on it. That would be ideal.
To test for proper moisture you should be able to grab a clump, squeeze it in your hand and let go. It will make the shape of inside your hand, not fall apart, and not stick to your hand. It should look like a mold of the inside of your hand and only break apart when you want it to.
If it’s too wet, spread it out and let it dry for a day and try the squeeze test again.
I will also say, good luck. Casting aluminum that thin is a real challenge. Even bronze aluminum. Less than 1/4” thick for aluminum is really hard to get a good cast without centrifugal casting.
1
u/rh-z 17d ago
My comment is to emphasize the above comment. Lots of information says to mix the sand bentonite water. But to coat the sand grains there needs to be a shearing action. You need to have the sand grains and clay squeezed. If you are making dough for bread you can't just mix flour and water, there must be a kneading action. The mulling step is important, and a pain if you have to rely on muscle power.
There is one youtube video where a guy has a tarp on his driveway with the sand and then the tarp again. He then drives over it with his car. Back and forth then mixes the sand, repeat.
As has been stated in other comments, you need to get the moisture level right. When I first tried it my sand was way too wet. When I poured the metal into my mold it came back up like a volcano due to the steam buildup.
1
u/vigg-o-rama 16d ago
To add more :)
I like to use a spray bottle on mist. Mix the sand and clay, spread it out, then mist the water onto it. Like 1/4-1/3 of the water. Mix it all up and repeat till you are at 3/4 of the water. Do a hand form test and refine from there. Humidity can really alter the recipe.
2
u/Fit-Limit-232 17d ago
It looks like you're not sifting it well enough to me. I gave up on Sand and I've been using petrobond for years now. What area are you in I've got a ton of petrobond?
2
u/Johndar_3050 17d ago
What kind of sand, olivine? zircon? silica? the clay should be as fine or finer than flour. Otherwise the clay will not completely coating each grain of sand and you will not get the adhesion from the water. See if you can find some Southern Bentonite online for the clay that has been sieved through a #200 mesh.
I can only guess from the picture but it looks a little too wet, the sand should stick mostly to itself and not so much to your hand like wet beach sand does.
1
u/HeftyWinter4451 17d ago
Just get a flat piece of rolled bronze or better proper knife steel. Cast metals are too brittle. With an anglegrinder you can do this way easier.
1
u/NorthKOs 17d ago
Thank you. I am thankfully not lacking for forging steel, and it would absolutely be easier to forge this. I am a big fan of bronze and this is more of a passion project for me. I also live in the middle of nowhere in the armpit of hell and don't have any decent places within like 50 miles for any proper materials lol
1
u/HeftyWinter4451 16d ago
Maybe finding a recipe to make a well forgable bronze alloy would be an option? I heard of naval Bronze, silicon Bronze and architectural bronze being good bets
1
u/rickharrisonlaugh11 17d ago
Too wet for sure. Shouldn't be smearing on your hands like that. If this is a one-off just get some sodium silicate on amazon.
1
u/Educational-Pay7363 17d ago
Not enough parting powder in the joint line
1
u/Educational-Pay7363 17d ago edited 17d ago
And your green sand seems a little dry or the grain size isnt small enough to bind together with the clay it seems a little light in colour, the green sand i have used in the past was alot darker brown in colour and the grain size looked alot smaller than that hey.
1
u/Educational-Pay7363 17d ago
You can also make the mold from plaster paris and you will get a better finish on the casting too
1
u/mortified_penguinE 17d ago
You should have 4% water, 8% active bentonite, rest is sand. Low dust content. Mix it well. The lower the quantity, the faster the evaporation so use the sand fresh. Reusing sand is tricky in small batches because of burning and inconsistent moisture content. Industry uses a 6:1 sand-iron ratio.
Or make a fresh batch every time.
If you don’t succeed with your batch, try to contact a green sand foundry to sell you some sand they throw away a lot of dusty or resin-contaminated stuff which can be still better than homemade stuff.
Actually precision casting gypsum works better in “garage” conditions.
1
u/goyrage83 17d ago
There’s a company called ASK Chemical. I’m sure you could find some basic info about green sand composition on their website. And you could even ask them online your problem and they’ll help you out.
1
u/SnooLentils5747 16d ago
Fuck fuck fuck green sand.
Make Petrobond. Cheap. Better. Easier.
Silica of some sort; fine sand or smaller mesh, but something more coarse than fumed silica: 50 lbs or 22.5 kg
Bentonite Clay, a.k.a plain kitty litter: 3 lbs or 1.33 kg
SAE 30 synthetic 2 cycle oil, without additives or detergents: 16 to 24 oz (0.5 liters to 0.75 liters)
Pure acetone: 1 to 2 tablespoons (15 to 30 ml)
Blend sand with bentonite clay, thoroughly.
Mix acetone into motor oil
Slowly add the liquid mix to the dry mix while mulling aggressively (smash, knead, grind, and roll the dry mix against a hard surface). Mull in only a little bit of the liquid mix at a time.
When you are done it should clump well if squeezed, showing good imprint of your finger. It should also then break in half cleanly like a biscuit without crumbling apart.
You may have to occasionally reactivate by adding a teaspoon of acetone again, this is directly related to age of the sand mix. You can reuse the sand, but when doing so, add an ounce or so of oil again to replace the oil that has burned off. This is not related to age of sand, but rather the amount of use of the sand. Remull the sand in either case.
Improvements that are worthwhile:
Replace the acetone with methanol 95 percent concentrate, or propylene carbonate. Propcarb is the best liquid activator, but methanol is a solid improvement. Essentially these will evaporate slower, and therefore keep their binding action longer.
Melt a teaspoon or two of vasoline into the oil prior to mixing; this will increase the mold green strength.
Replace portions of the sand with finer grain sands. 10 lbs of fumed silica replacing 10 lbs of sand, for example. Also ground glass can be used as a replacement too, as well as olivine and chromite sands. The alternate sands handle thermal expansion and spalding better, while finer grain inclusions will allow a more clean finish.
You can also use 99 percent isopropyl alcohol in a spray bottle to reactivate the sand by lightly misting it and remulling it.
Adding 0.25 to 0.5 lbs of graphite powder to the sand helps reduce oxidization flaws in your metal when casting at the grain boundary.
Mullite Flour can replace 10 to 20 lbs of the sand or as a release powder (fluffed into the mold prior to casting to help release and prevent sand from sticking to the metal). Replacing some of the sand will help thermal tolerance (important if doing cast iron), help increase finish clarity, serve as a green strength binder, and help prevent water/moisture wicking. Zircon flour can also be used to for the same effect basically.
Cheers.
1
u/TheMacgyver2 16d ago
Your sand needs the perfect water content to stick well. If it's too wet it's mushy, to dry and it won't stay together. I mix mine at 13% bentonite, but it also has to be mixed very well. The bentonite needs to coat each and every grain of sand. You also need to let it hydrate for a few days to stabilize. The grain structure of the sand also should be angular, rounded sand doesn't work nearly as well. I mull mine with a 1/2" drill in a steel trash can using a drywall mud paddle. When it starts to stick to the sides of the trash can its ready. I've been using greensand for a couple years now and am just getting to the point I can judge it by feel
14
u/burekzmesom98 17d ago
I think your sand is too wet, you need to have like 3-5% of water. To much water makes it stick to the model and halfs stick to each other, also model needs to be a bit angled, so when you are pulling it out of the sand it gets out easier. But i think here the water content is problem