I'm trying to cast a small robotics part for a local college. Its small in size but needs to be heavy. So far I've created one with epoxy resin and it is okay. However, for the same size, I wanted something denser to make it heavier. I was hoping to know if a mix of white cement + water only is strong enough to withstand a few light knocks and if so what ratio would they need to be mixed in? Since its small, using large gravel would not be possible.
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I can't imagine concrete holding up for a small part. What about mixing some buckshot in with the epoxy? – JACK Sep 27 '20 at 15:23
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Talk to the metal work instructor at the college. – Jasen Sep 28 '20 at 08:20
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How small, how dense ? Maybe a low temperature metal casting should be considered ( lead, tin, zinc, etc.). Otherwise cement + white silica sand will make a very strong part ( after a couple weeks cure). Mixing ratio is very flexible , commonly 20 % cement makes a strong concrete . Plaster of Paris ( calcium sulfate) will cure very quickly but is not very strong.
blacksmith37
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Once it has set (become initially hard), submerge the part in water for the rest of the curing time for the best cure. There are some interesting temperature / strength relationships (elevated temperature gets stronger faster, but moderately low temperatures eventually become stronger than the parts cured at elevated temperatures.) – Ecnerwal Sep 27 '20 at 16:40
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Thanks. The part is about 270mm x 250mm x 50mm. It has to be about 1-1.5kg in weight. We don’t really have a couple of weeks for cure time. The epoxy one I made is about 770gms as of now. I added some large bicycle ball bearings in the casting and that upped the weight a bit. – electrophile Sep 27 '20 at 17:16
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As noted, elevated temperatures (on up to autoclave in some processes, but current advice suggests 140F maximum) accelerates curing, but the ultimate strength is reduced .vs. cool but slow curing. Much information if you search Accelerated Concrete Curing. Depending what you need, it might work at that lower strength achieved faster. Treating the part dimensions you give as a rectangular solid (which it probably is not) the density seems rather low. Like "aggressively floating in water" low - what's the actual volume of the part? – Ecnerwal Sep 27 '20 at 18:40
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Try stone dust or sand in epoxy, unless it really needs to be (expensively) dense, in which case tungsten powder, or the most economical approach to "partly tungsten" which is scrap tungsten carbide (from machine tooling teeth/cutters, particularly the replaceable ones) crushed and sifted if need be.
If less dense will do, less dense (and less expensive) metal powders, or perhaps the waste from a metal grinding operation (a combination of metal dust, oxidized metal, and grinding wheel particles.)
You can also get epoxy already mixed with steel powder as a product. Interestingly, the published density of that product and concrete are (within margin of error) the same - 2.2 - 2.4 g/cm**3 or 2.3 times denser than water.
Ecnerwal
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Some exercise weights use sand size steel particles in a plastic bag, cheap and available. – blacksmith37 Sep 27 '20 at 16:37