Hello, new here, I just made my first attempt at mixing up green sand, the sand is playground sand from Lowes the bentonite clay is what I had leftover from putting a well down have about a hundred pounds, I mixed it fifty pounds of sand to six pounds of clay in my home built muller, for about ten minutes before adding any water and noticed it coated everything in the muller when I did add water. I added enough water so it broke clean when squeezed by hand without a lot of sand sticking to the hand, when I rammed up a mold it acted like it was on the dry side and I noticed the clay was clumping and balling up, sooo what nubie mistakes am I making, appreciate any help.
What sort of muller did you build? Was the bentonite in powder form? How long did you run the muller after adding water? Pictures might help clarify what you're describing. Jeff
was the sand DRY before you started, Playground sand here in FL is soaking wet in the plastic bags, wet sand wont coat well with binder you really need both southern and western Bentonite (One of the college educated fellows will pop in and tell you the scientist names, rather then the foundry names ) for now remull the sand to break up the lumps and try again, it can only get better v/r HT1
Thanks for the reply, the muller had the wheel turning on the inside the drum doesn't turn, yeah the bentonite is in powder form I probably ran it for twenty to thirty minutes after adding water.
Thanks for replying, yeah the sand was dry I made sure to check it before adding the bentonite I don't know if its Southern or Western bentonite and I don't know what the difference is between them. I sifted the sand through a screen and threw out the big balls, I was a little short on sand for the mold yesterday so I mixed up another batch this morning I ran the muller a lot longer before adding any water and added water just a very little at a time as a spray over a lot longer period I think it worked somewhat better and instead of just dumping it into the bench I sifted it as I added. I rammed up a mold this morning had to add quite a bit of water, the mold is sitting in the shack now its been raining off and on all day so didn't fire up the furnace hopefully tomorrow, I'm on a steep learning curve here I haven't rammed up a mold since my high school days and that was a looong time ago, (in the 70's) thanks for help guys!
Hey! I resemble that remark... From the Carpenter Brother's website: https://www.carpenterbrothersinc.com/foundry_products/molding-and-core-making/bentonite/ "The best bonding clays for metal casting are sodium bentonite and calcium bentonite. Sodium bentonite occurs naturally in the Western United States and thus is referred to as Western bentonite. Green sand produced with it will have medium green strengths and high dry strength, which increases resistance to erosion of metal but can cause issues at shakeout. Western bentonites are commonly used for steel casting. Calcium bentonite is mined in Alabama and Mississippi and is thus referred to as Southern bentonite. Calcium is more widely distributed. Green sand produced with Southern bentonite will have high green strengths but low dry strength, causing low erosion resistance and prone to scabbing and other expansion defects. Most of the available products used for green sand molding are blends of Western and Southern and include specific additives to achieve the desired mold characteristics. In addition, clays absorb water from the atmosphere, so they should be stored in dry conditions." I went down the road of buying cheap sand from home improvements stores, screening out the rocks and trying to let the dust blow to the wind. The I sourced the different bentonites locally and online. I was able to get it to mold and cast but not without a couple additional mixing, drying, wetting cycles. I usually recommend pre-made Petrobond as a way to eliminate the learning curve of greed sand, since foundry practice has many learning curves in of itself.
In addition to the above, Western/Na bentonite holds 15x more water than Southern/Ca Bentonite, and it expands a lot when wetted. It's also why it can take several cycles to get it saturated/fully activated and why allowing new sand to "rest" in a sealed container and remixing improves properties. This gives it better green and hot strength at the cost of higher water content which can yield poorer sand flow and casting quality. Most hobby blends would recommend somewhere between 4:1 to 2:1 Na/Ca for the best performance. Na is most common because it is widely used in agriculture, well service, and pond liners. In addition to casting finish, sand grain size can have a pronounced effect on strength and amount of binder required as well. Smaller particles have more surface area/volume than large. This means it requires more binder per pound of sand and also is typically stronger because it has more contact area for binding, but it is less permeable and fouls faster. Sand grain shape can also play a role. If you search threads here, you may also find other additives such as small amounts of wood flour. Though the fine sands (often sold for mortar mix) at the big box stores appear much finer than the play sands, they are still quite coarse by foundry standards. I measured it with sieves and >80% was in the 50-55 mesh (Tyler scale) range. I must say, this is one area that makes me appreciate lost foam casting. Other than keeping it dry, I treat my sand with total disregard and reuse it many, many, times. Best, Kelly
The way you describe it it looks like I have the Western bentonite and I had a feeling the more I worked with it that it would come around and you confirmed that thanks for all the good information obviously I have much research to do.
And yet more good information, I thought the play sand looked pretty fine, then two days ago I got a mold rammed up made a shaft flange for a 20" disc sander I'm building I got a pleasant surprise it come out very well though the surface texture was quite coarse. Being the tight ass that I am went the economical rout already had the bentonite and play sand is cheap sooo I guess sometimes you get what you pay for? Funny you mention the lost foam casting at the same time I poured the flange I also poured a drive roller for a belt grinder I'm building this was my third attempt at the lost foam attempt for this, I think I know where I went wrong this time I poured the flange first when I poured the roller I was getting short on metal and near the end of the pour I hesitated and I believe that was my downfall this time, I'm building a wood pattern for the roller so we'll see how that works AND I'll make sure I have enough metal! Thanks for all the help guys!
