The Home Machinist!

Hello, I have a question regarding green sand. I ordered 30lbs of premixed sand off ebay. I wanted to do it myself but I couldn't source a fine enough sand or bentonite clay. Plus it was convenient and saved me time. I got the sand and they also included extra bentonite to "refresh" the sand. I've finished building everything. Furnace, tools, flask, and my first pattern. So I started to work with the sand hoping to soon cast something. I slowly added water and mixed. I finally got the sand to the point it would hold the shape of my palm somewhat, only leaving a thin coat of sand on my palm. When touched it would break fairly cleanly. The problem is its not really rigid at all in any form. When smashed in my hand it will hold its shape but I can just touch it and it collapses. To me it acts like plain old beach sand. I'm lost.

I watched a video of a guy prepare his commercial green sand on youtube. When he smashed it together it appeared that it snapped apart between his fingers like when compressed it was more of a solid. Mine just falls apart.

Is my problem a water issue?
Is it possible my green sand is just sand?

If it would help I could upload a video of how the sand acts.

Thanks for any help!

If you do not have the experience with green sand, it is dificult to explain, so that it is clearly understood.. Example, how high is up, how big is a rock, how hard is steel. Without some help on the concepts, that are standard, you will be lost.

Green sand usually benefits from more water, for more strength. then the mistake is made to add too much, to the point where the hand squeeze actually allows drips from your hand. At that point, you will NEVER cast ANY metal into that sand and get anything good. The amount of steam created will blow and bubble and ruin anything poured. The green strength is determined by the amount of the 'sticky' binder, which can be quite high , until you close the sand off. That is, filling the pores between the grains with clay, until it cannot pass gas or vent. At that point, all your castings get 'gassy' and will either bubble holes in it, or, like aluminum, absorb some of the steam, and get spongy, where the metal is actually porous, and weak. Bronzes suffer from steam badly too.

ok, now that I confused you, let me try to square this away. Bentonite is a fine clay, and is 'sticky' enough to provide the bond you need. When fisted and released, it should form a near perfect image of the void inside your fist. When 'tickled', individual grains will fall away, and when gently folded, should break cleanly, and not crumble with many pieces. If correct, it can be left in air to dry, and it will skin, and hold shape, getting dry on the outside, and perhaps a few grains being lost to your feel. If there is no green strength, just drying it, or touching it, will destroy its form. You would need more binder. The binder only works if damp. Too dry, it wont hold shape, too wet, you get mold blows.

Need more?

To do some troubleshooting, weigh out 10 ounces of sand mix and mix with water in a container until there is standing water over the sand. Mix it up really well and let it settle for two or three minutes. The sand will settle out quickly and the clay binder will stay in the water. Pour off the liquid into a separate clear container and let it sit overnight. The clay should be settled out on the bottom. You can then estimate the amount of clay in the sand mix. The most accurate way is to evaporate off the water and weigh the clay and the sand to get the percentages.

The mix I use has about 12% clay, 3% water and the balance olivine sand by weight. The clay can range as low as 5% or as high as 15% and still work reasonably.

Occasionally old used sand will have sand fines in it from the shattering of sand grains that are in contact with the melted metal poured into the mold. The solution to sand fines is to wash out the sand in water and after a minute or two of settleing, drain the water off which will have clay and fines in it and remix with fresh bentonite so that you have large sand grains coated with bentonite.

My guess is that the clay content is low and you need to add bentonite, but measuring the weights of the constituents is the best way to be sure.

Jim,
Can you talk a little about olivine sand? Where you buy, and the cost relative to other types of sand? I get the idea it's the sand of choice if you intend to cast metals that melt at higher temperatures.

Harold

Hi;
Thank you guys that's the problem I have been having. Thank you ,Thank you, now if I can just get enough enregy to get back on the problem I will try again.
chaz

I did get to the bottom of the green sand problem. I didn't get to try the water method but I will next time I have any doubt. I put some wet sand on a metal plate and let it dry. Then I wiped away the sand hoping I'd see the clay still stuck to the metal. It had a very faint haze on it. I figured that it did contain clay but very little. I separated about 5 lbs of sand and started adding the "Refresh" clay they gave me. It took a few ounces to get the clay to what I thought was close to what I needed. The clay actually started clumping and became workable. So I'm way closer to what I needed. Now I can slowly work the rest of the sand more precisely and see what happens. Thanks everyone! I'm sure I'll have more questions!

Harold,

Olivine is a magnesium iron silicate that is stands higher temperatures than silica sand. I bought mine at LaGrand industrial supply in Portland, OR. There are fewer concerns about getting silicosis from olivine sand than ftom silica sand as the silica sand grains tend to shatter and make more dust than olivine.

Jim

Thanks, Jim.

While it's premature (not ready for it yet), I fully intend to explore the source you provided. Considering my plans to cast ductile, it sounds like olivine would be the best choice.
Haven't seen you around much of late. Hope you and Cindy are well. Tell her hello for us.

Harold