I am trying to develop a machine called Manufacturetron, or RUGDMMAC. It is supposed to be an open source project. It makes highly accurate molds without limitations on the geometries, through a 3d printing like process combined with machining. But I would like to get rolling on identifying and obtaining some samples of some alloys which have good properties. I thought I remembered reading that there are some alloys which expand slightly upon freezing, counteracting the thermal contraction (pewter?). The mold is a block of plaster with a void inside it, which is the mold cavity. The plaster will be disintegrated by an ultrasonic cleaner bath after casting, to remove it even from delicate castings, deep cracks etc. I would like to focus on non toxic stuff, if possible. Any good ideas? The stronger and more accurate the result, the better.
There is a family of low-melting-point metals some of which contain cadmium (toxic if you consume it, but not if you use reasonable precautions) with various CTE's, expansion properties on freezing etc. Belmont metals has a data sheet detailing them. https://www.belmontmetals.com/wp-content/uploads/2017/09/LM5.pdf Every couple weeks I melt and cast Cerrobend that contains cadmium (I don't eat it or breathe its fumes) and am reasonbly careful with it. My exposure to the molten metal is of a few minutes duration in a well ventillated space. Not a concern. Great stuff when used properly. The folks that historically got into trouble with cadmium had long term repeated exposures either industrially or through ingestion of contaminated foods daily over long periods of time. Denis
I believe the expansion comes primarily from bismuth, cadmium really isn't needed unless you need melting points below 95°C.
hm, lol I don't think I want to use Cadmium, but maybe it will lead to the discovery of some other alloy . That stuff is extremely carcinogenic. There must be some modern alloys that are good...
Denis' pdf lists many alloys with no cadmium. Both 60/40 Tin/Bismuth and 58/42 Bismuth/Tin looks nice.
OK, I’ll give a hint. Cadmium is not conclusively known to cause cancer (as opposed to alcohol, for example). It is a suspected carcinogen. If someone wants some science-based risk assessment here is a pretty decent reference: https://www.epa.gov/sites/production/files/2016-09/documents/cadmium-compounds.pdf Our biggest source of cadmium exposure is likely in food. Burning of fossil fuels is likely next. Melting a metal alloy does not release significant levels of cadmium though smelting of some metals does. Zinc is an example. Denis
Rose's metal is pretty good, cadmium free. Roughly half bismuth, quarter lead, quarter tin; melts at the boiling point of water.
Thanks re the scientific input on toxicity. I had thought it was extremely carcinogenic in microgram quantitites. Probably you would have to hit up google scholar to get a good, accurate idea of toxicity. However we are talking about using quite substantial quantities...Lead is another thing I would have to read up on, I have heard it has low acute toxicity for adults, but the amounts that cause lead poisoning in children over a chronic basis are remarkably small, so I don't want to be messing with it. It would be easy to get into a complicated discussion about toxicity... it's a worthwhile undertaking for sure, but could we leave it to another thread perhaps, if someone else would like to undertake that initiative, it's not on my agenda for today... Anyone tried those zinc aluminum alloys ? I was hoping there was some awesome technology where the recipe had really been worked out to deliver something surprisingly strong, precise etc. Metallic glasses can copy detail very precisely, and some have very reasonable critical cooling rates, but they still shrink due to thermal expansion/contraction, and I expect they are hard to obtain/impossible, and expensive. I once read a paper on making scalpels out of metallic glasses that were already sharp right out of the mold. The cutting edge of a scalpel! If that's not good detail reproduction, I don't know what is .
I’m unsure what you mean when you say precise alloy. It sounded like you meant a metal that would be dimensionally similar to your pattern/mold but for replication of detail, that is more a matter of casting process and mold media. Best, Kelly
Yes, touche. Technically precision is not the right term, that would imply essentially the repeatability. What I need is very low or ideally zero expansion/contraction. Obviously everything has a thermal expansion coefficient so that's one of the problems, but something that is better rather than worse, at least. Steel for instance is terrible, with all kinds of phase transitions along the cooling path. Repeatability would be better than nothing. Steel isn't even very repeatable, the contraction/expansion varies with the exact path through temperature vs time taken during cooling. I was under the impression this would be a common problem in casting, to get good detail replication and low contraction.
You can get very repeatable casts with very high detail with ceramic shell or investment casting but asking for zero expansion isn't going to happen as you already mentioned everything has an expansion coefficient. A lot of us oversize parts by the percent that the metal is known to shrink, for bronze/brass its around 2-3% shrinkage, so if you make your pattern 2-3 bigger the part comes out very close to the right size. Sand casting something lets the walls get pushed out by the pressure of the metal as it enters the mold cavity so it tends to be less accurate. If you want to see what I mean google canon casting and see the youtube videos of the casting difficulties from the pressure of the metal. Ceramic shell doesn't usually have this issue since the mold is completely bonded. The other option is to cast your part slightly oversized and then machine to a precise size. That's the approach the industry has gone. Ex: all precision car parts, engine, gears, etc
Yeah, I am trying to tackle the problem across as many fronts as I can. Lower temperatures help. A mold with as close a CTE as the material as you can get helps. The right alloy helps, as I mentioned steel has issues well beyond CTE. The thing about compensating the size is that, suppose you are making a fork. The tines of the fork are determined by the mold, so if the metal shrinks relative to the mold, you can't really compensate for the stresses that will result. Where the ends of the tines of the fork will end up will become a bit messy. Compensation is a big one, though, especially because with this process I am trying to develop we can make the mold very precisely of any shape, so compensation even for complicated distortions could be possible. Re machining, yes, but actually the place I am coming from is that I am a CNC machinist! . So I know that machining is very expensive, time consuming, not something you want to do if you don't have to. That's why I want to try to push the limit of casting by exploring these other ways to improve. I don't think I am likely to have much use for tin bismuth alloys, on reflection, because carbon reinforced plastics can be stronger anyway, copy the mold perfectly, and ate actually sligthtly more temperature resistant. I was hoping there was something I had heard pewter under goes phase transitions that compensate for the shrinkage to some degree. By mixing things just right things could be adjusted for very low contraction. Also, there are some metal alloys that have very low coeifficients of thermal expansion, https://www.advancedsciencenews.com/a-material-for-all-weathers-with-zero-thermal-expansion/
And always makes things difficult. You want cheap and safe and low CTE and low melt temperature and geometry insensitive. Sounds like what you really want is casting without the complications of casting. If you think plastics copy their molds perfectly you're living in a different world than I am.
There are the polyurethanes used in so called resin casting, with the right one, slow setting, as much filler as you can get, you can copy a mold to within microns, yes. Yes, what I want is casting that is better than usual casting, premium casting. I will have premium molds, so that's a start. What I want is advances in technology, which gives fast cheap, and good, to a higher degree than previously...
Unfortunately the accuracy that powder bed or any other 3d metal printing process gives is very poor, like a hundred microns or less, depending on dimension size. Even 300 microns is typical. The world needs more like 15 microns over 5 cm. Also such machines require very expensive lasers, making them very expensive, like $100k, not $10k. They are otherwise not that expensive, but the laser really is. It has to be a very powerful fiber laser, basically. Even co2 lasers don't cut it, the wavelength doesn't give good enough absorption.
