Cast then machine printer system concept
Okay, if you have a look at my edit history Greenatolsecondtry 07:42, 20 October 2011 (UTC) you can see I have been doing a lot of library research into methods by which to produce full strength objects, especially metal objects, with all the sort of desirable properties that can be obtained with normal production processes, but with lower capital cost, running cost, material cost, low training requirements and labor input per unit productivity, and set up time for a given part. Ultimately it doesn't really matter whether the process is additive or not of course.
Unfortunately there are some real, although probably surmountable problems with making highly accurate (like IT5) parts with normal material properties, with additive processes. Tempering hardening and other control over the crystal structure of the material can be done to some degree by modifying the process characteristics, so I don't want to put that off the table.
But if the goal here is more to make the sort of equipment that can make the sort of products that are already economically important rather than invent yet another production process like electron beam manufacturing, the conventional casting followed by machining may be the route that is most reliable and requires the least product development.
So this concept marries 2 different concepts, and of course there are other ways to do it but just to make this a concept rather than just a few even more vague thoughts. It should be self replicating to whatever extent possible of course, which will help a lot to keep the price of the second generation down:
Automatic casting production machine. This machine prints casts using some process, probably similar to FDM. The material the cast is made from has to be highly temperature resistant, and recyclable, as well as non-porous, easy to remove from the cast and preferably have a similar thermal expansion coefficient to the metal being cast. Preferably the cast is fairly accurate too, and has good surface finish, like 20 microns per centimeter. It doesn't have to be very strong though, not does it have to have low residual stresses, and the process only needs to work with one type of material.
Removing the cast from around certain complex objects can only be done by essentially liquefying or gassifying it so the cast in this concept is not reusable.
Finding a combination of materials and processes that meets all these requirements will no doubt take some searching but seems altogether doable. Potential options:
- Use a volatiles only wax as the support material. Boil some wax and capture the condensate and you have the volatiles only fraction, which can be used in an FDM like printing process.
Then have a material that is temperature resistant etc. dissolved in a small amount of solvent so it is gel-like, a thick paste. Extrude this, allow the solvent to evaporate to produce a deposit of the build material. Machine using a diamond bit for higher accuracy.
The resulting printed object can have the support material removed by placing it in an oven, which can double as the pouring chamber, where the metal is poured in to the mold.
The build material can then be removed by dissolving in the solvent again. CTE can be adjusted by adding ceramic powders to it as desired.
- Another option could be to use a very high temperature FDM process or other additive process, basically, with a material like a ceramic that melts at a temperature even higher than the metal being cast. Plasma spraying followed by milling with a diamond bit could work, and then the material can be removed by dissolution.
- 3d printing using a solvent with a salt dissolved in it instead of an organic binder.
- Although it reduces the accuracy of the casting, it is possible to 3d print casts with organic binder resins, as is done by some commercial companies like I think the name of one is "direct cast" which sells a 3d printing system to produce reusable casts for foundries. The resin pyrolizes releasing some gas due to the high temperature, but it is manageable and can still produce good castings.
Casting process: Casting is not entirely trivial to get good casts every time, there is substantial design involved, but the software to simulate and do such design is widespread. Voids are the main problem, and shrinkage which reduces dimensional accuracy is another one, but since we will be machining it anyway it is sure to be manageable. It would be nice to produce highly accurate castings though, as with "premium" investment casting accuracy on the order of 80 microns over 2 centimeters is being advertised, which is enough to produce many useful products while skipping the machining stage. Also, higher accuracy reduces the amount of machining needed and increases productivity thereby.
There are many standard and well known techniques to produce void free castings too, and we can use them as well as techniques taht may be to low productivity in a normal production context, like briefly exposing the molten metal to a vacuum before pouring into the mold, to reduce teh partial pressure of hydrogen and other gasses present in the metal, which will reduce gas bubbles caused by dissolution of gasses. We can also preheat or pre-cool the cast using the oven functionality as desired, do casting under argon or a vacuum, whatever.
Machining: The idea of a so called "intelligent machine tool" fits here. Basically you hand the tool a casting or piece of stock, and a cad file, it machines it while recovering from any errors, then scans the object or uses a CMM device to ensure the dimensions of the object are correct, and outputs the finished part plus a quality control file that describes the part and compares it to what you wanted.
Unfortunately the development of such a tool has not been pursued. Probably due to the mass manufacturing nature of modern production and the cost of labor being too low - it's cheaper just to pay someone peanuts to do it. It is also however mentioned by some authors that the lack of availability of such a machine, and perhaps the technical hurdles involved with developing such a machine is part of the reason for the interest in additive manufacturing methods, which are easier to automate.
So this would be a substantial project in itself, however a more ordinary less intelligent machine tool would still be okay and produce a very useful unit, especially if low cost, such as self replication which could keep the cost far lower while keeping the quality high compared to buying the parts. termpaper.biz
The machine tool has to be capable of machining a wide range of parts, more than ust a lathe or just a mill, like the Multimachine, and preferably more. More axis can be added to increase flexibility. See the open manufacturing google group thread "A self-replicating open source machine tool that can make practically any part (that you can make in a normal cnc shop) " and the metal print head pages on the wiki for ideas. There are commercial machines that combine the functionalities of both mills and lathes on the market already, and I don't mean those crappy 3 in one hobby tools that can't do anything right, but machines like the mill-turn machines that supposedly can do both well, and without refixturing too.
So that's it, produce casting, heat treat it to anneal or harden or whatever you want, and then machine, heat treat again if you wish. This could produce very good parts although it would require substantial training and intervention to produce unique parts, a lot of the work is apparently writing the program for the machine tool, so the programs can be shared in an open source way to alleviate the training and set up time substantially for such parts.