Hey All, I am new to the forum and new to green sand casting all together. I have made a couple cool items already and am learning fast but had an interesting question I could not find an answer to. When you are done casting and cut the sprue, riser, feeders, etc. off the product being produced, these scrap pieces seem to have some sand imbedded in the surface. I have been tossing them aside in a bucket not sure what to do with them. I figure they aught to be useful for another casting but have some hesitancy about the sand and other impurities now stuck in the surface. If I melt these down for another casting, will the sand float up in the dross and come out when i skim it? or will the sand sink to the bottom so I can just pour off the top and discard what is remaining in the bottom? How do you guys normally deal with this? Thank you, Ken
I clean the scrap with a wire brush and it falls off easily. Not a good idea to allow sand into the crucible. Is your sand coarse? Might be the reason why the aluminium is sticking to it, Cheers Charlie
I put the runners and sprues on top of the furnace lid and the heat makes most of it pop off and disappear. I remember reading that it was a problem for Ford in World War 2 when they were making radial aero engines as the silicon dioxide reacted with aluminum and was reduced to silicon which then increased the silicon content of the aluminium. Also sand and aluminium have densities of 2.3 and 2.7 respectively so the sand is not that buoyant when it comes time to skim unlike sand in molten brass and iron.
I am using "Feldspar #90 Silver Sand" from P.W. Gillibrand It is rather fine sand that has been giving me excellent results with amazing detail. I had a bunch of this stuff on hand from a past project and mixed it with Bentonite clay to make my green sand. I cant be certain there is much sand there, under 10x magnification I cant really tell. However there are smooth presumably clean spots on the sprue and some pitted spots i have to think there must be sand in... why else would it look so different? (photo: https://ibb.co/yN3FRgb ) Would you expect there to be contamination in that rough patch? So if i hit the surfaces with a wire brush and it "seems" to be clean (e.g. sand not obviously shedding off) you think it is ok to re-melt and use? Does the rough spot in my photo cause any concern? Thanks, Ken
The data sheet you referenced, mentions it's silica sand or quartz or silicon dioxide so all the problems with sand in the melt applies. It's fine to use as normal aluminium foundry sand, it has a density of 1700 kilograms to the cubic metre, the fine stuff I use is still silica but is 2200 Kg per cubic metre but is very fine so it would pack denser I guess. If your crucible is decently large you can let it sit in the fuel burning furnace while it's shut down to avoid swirling the hot metal and let the sand float to the top where you can be generous when skimming and waste a bit of the metal to get as much sand off the top as possible. The photo below is of a casting made from recycled runners where no effort was made to remove sand off the surface and you can see particles of sand embedded in the metal. Clean metal going into the crucible is the best measure, even if you have to sand blast the surface or sand it with a belt grinder.
Thank you everyone for the responses! Seems like it isn't a big deal for ornamental parts where structural integrity is not a consideration.
You could make a test ingot with your "most-suspect" sprues and such and take some cross sections. I doubt you'll see anything alarming. But your right about the ornamental stuff. It won't make much difference. It could be potentially harder on your tooling, but again, probably not going to be a concern. Pete
That is what I do and the rough spot looks normal. Stick to the procedures; Clean the scrap as much as possible. i.e. sand, oil, grease, dirt. Remove steel components from the scrap as much as possible. Use a ceramic crucible. Cast iron pots are ok but they still release gas into the melt. Remove steel parts from the melt with a hook, or small skimmer e.g. piston reinforcers. Steel produces Hydrogen into the melt I believe. Skim the melt of dross. De-gas the melt by plunging the de-gasser to the bottom of the crucible. Skim again – it is surprising how much dross is brought to the surface the second time. and you shouldn't have much trouble, Cheers Charlie
