I have been getting to try my various 3D printed PLA patterns with some different sands: 1. Petrobond 130 from Teton 2. Homemade green sand sieved through a 'screen door' (15 mesh?) and mixed with 10% sodium bentonite clay and water. This was mixed by hand and sieves a week or two later to remove 'tar balls' of clay. 3. Homemade green sand sieved through 40 mesh sifter and mixed with 7% clay, half sodium bentonite and half calcium bentonite. Mixed by misting in water, rest overnight, then stomped, hoed, raked outside and put back into its container. The Petrobond results have been consistently good, the green sand went the other way: the sand I thought should do better via screening and clay has a more pronounced texture than the 'take what I got' sand before I got the finer screen and both clays. I attribute my success in drawing the patterns from the sand from the layer of graphite followed by calcium carbonate used as parting powder Denis mentioned elsewhere. Is there anything at first blush that I could be doing better/different to get a better texture from my green sand? Seriously considering making a muller... PS: LH is Petrobond, RH/rough is 'fine sand mixed clay', Bottom/better is 'coarse sand simple clay' with an open faced mold. Thanks!
I did a homebrew section to better see porosity. Bandsawed the previous rough goods into melting fodder at work and potted in epoxy at home and orbital sander 40, 80, 120, 150 grit then hand sanded 320 and 800(?). We have a lab grade machine for this at work... Old sand, open mold, metal from end of pour: lots of pores New sand, closed mold: less pores but still some there To be clear, patterns for the left and right piece are the same, pattern for the bottom piece is different, smaller in size and relief. Looking to get my green sand castings looking more refined like the Petrobond, and to better understand the mechanism so if I 'want' to make a rougher casting I could do that too.
Tops, If you haven't seen Martins videos, I highly recommend subscribing and watching every single one of them. Each one has some info crucial to good casting practices.
You’re probably having more than one issue going on here. Maybe several. An open faced mold will kill the deal right off the bat. Referring to your original post your greensand looks to be just plain too wet, maybe insufficiently mulled, especially the one on the upper right. Again, if that was an open mold, all bets are off. Lots of things affect surface finish including gating, moisture content, etc. Give that sand another chance. The porosity can be attributed to several factors as well: furnace atmosphere too rich, dirty or painted metal, held in a molten state and exposed to the atmosphere for too long, or just plain too hot. There are other factors I’m sure, but pouring just as soon as the metal is hot enough is good practice. It might take 15 minutes for the metal to get to 1250F, but once it’s molten, it’ll increase to 1500 (too hot) in a couple of minutes so I make sure all my molds, tools, etc at ready well before the melt is. I might have mentioned it before but the Navy Foundry Manual is available online in PDF and has much to say about sand. Don’t be put off by the vintage of the book.(1940’s). Permeability is an important factor and is affected by clay and moisture content as well as grain size in greensand. Also if you watch Luckygen or olfoundryman on YouTube you’ll see them use both facing sand as well as backing sand in their molds. It’s a topic well worth exploring. Pete
Thanks Pete. I don't think I am getting soot, been checking, but for sure I am not aware of the exact onset of the liquid state of the aluminum. I need to be ready and waiting instead of walking around getting ready at that point. I still do not have a pyrometer either. Normally I 'geek out' over stuff like that but have not pulled the triger on one of those yet. With the onset of warmer weather that will help too as I won't be tempted to go into the house to warm up while things are working. I appreciate the titles and channel names, I have seen a couple videos from each and leafed through the manual but have not sat down to it. Looking forward to using the new mixer/muller and trying some pre-graded foundry sand.
Looks like a moisture issue to me. Gas inside the casting is created by the excess moisture in the sand. The sand should just be wet enough to activate the clay which bonds it together and then mulled until you get a good consistent texture...should be able to squeeze the sand in your hand and feel if it sticks together or crumbles apart...should just be sticking together good...you can usually feel it if it is too wet.
Thanks Billy. I was expecting the second batch with mixed clay and less water to work better than the first but it was the other way around. That was before I got a cement mixer. Maybe I should mull them both again and try a little pour with each. I also have graded sand 50-130 and 120-270 that could be made up as well.
I went and mulled batch 2. It was sticking to the side of the drum and did feel 'wetter' than batch one. My hands also seemed almost 'soapy' afterwards. Could that mean too much moisture and too much clay? I was going to leave it air out for the night or a day in the mixer.
Sounds like it is still too wet but really hard to say when cant feel it for myself. Im pretty sure I saw a thread about testing the moisture in the sand on here but I may be mistaken.
I had 50 lbs in the mixer with way to much water, I blasted it with a weed burner as it was tumbling but be careful...there's a rubber gasket between the halves of the drum and you don't want to melt it. In your case with just a little sand, lay it on cardboard out in the sun. It will dry in no time this time of year.
Billy and Fish, thanks for the tips. I left it out in the mixer overnight and tried it again the next day. Better, drier.
Since the last post I made up the 50-130 sand and got one casting with a really nice finish and tried to reuse that sand after cooling and quick re-mulling and the second one was not as good. The second time also saw from fraying at the pattern edges, the sand could have been too dry plus not enough ramming. Went to mull again last night and added some water...too much water! I think I need to get ahold of this piece of equipment as mentioned in the thread below [I about spat out my coffee reading this for the first time ]: Good reading/pictures/videos in that other thread: https://forums.thehomefoundry.org/index.php?threads/rough-surface-finish-from-green-sand.1095/
Not the same Emmet cup, sorry. But maybe it's just right for your sand... Just a matter of doing a little trial and error, you'll figure it out. Jeff
Thanks Jeff. I take it that it would be foolish to compare the volumes of the various tapered cylinders and sand make up and local weather conditions in an attempt to 'normalize' my (virtual) Emmet cup... Things are looking better. A couple hours out in the warm dry wind yesterday and two cycles through the tumbler, one last night and one this morning:
