Diachi - I'm currently building a Nylon based printer as such, building a basis before moving onto DMLS. I've chosen Al because of it's low melting point and general practicality.
Encap - Just to clarify here, my question was in regard to how one determines the opacity of a material - as the title states. To answer this requires no elaboration. The EOS M280 you mentioned is also capable of printing Titanium; a metal with a melting point near 3 times that of Al, an absorption coefficient half that of Al, and all within a build chamber held at 35 Celsius. As for the 680k price tag, that isn't a pure reflection of what it costs to build the machine but rather the cost to support a business that does.
The software / electronics side of things is within my background, I've pretty much covered that. I think creating the inert environment will be the most challenging of tasks but I have some ideas around it.
After doing some digging around I've found that Al has an absorption coefficient of around 0.13 @ 808 nm. Some information I found particularly helpful:
http://www2.ensc.sfu.ca/~glennc/e894/e894l15g.pdf,
Refractive index of Al (Aluminium) - Rakic.
I see Aliexpress sells 5W diode lasers for around $100 US a pop, not sure if that is average or peak power though. These should be adequate as I will be externally heating the build chamber to near the melting point of Al. The next step is to model / determine how the heat will be dispersed within the medium and thereby determine how much power is required. I've been working through some simulations in Solidworks to get a feel for this but they're crude as it doesn't have the capacity to model powder. However I presume solid Al will be more conductive so the required power derived from this will be ample.
Good luck with whatever you try to do building the type and quality level 3D metal printing machine you attempt.
It's not only wattage of the laser it is the area of the laser spot on the surface of the powder and method of focusing it so energy density is high enough to do the job. Normally a high power fiber optic laser output is collimated then focused to as small a spot as can be done to generate the energy density needed. It is the critical energy density of beam/spot needed and not just power/surface area of a metal powder because as the process proceeds the melt zone will be conducting heat out in three dimensions + scan speed and a whole host of other problems and considerations.
I might be wrong but if a 3D metal printer could be easily made using a $100 5W 808nm laser diode on AliExpress it would have been done a
1000 times over already.
There are probably a lot of good reasons expensive commercial DMSL printers use a 200W Yb fiber optic laser other than cost of running a business. There are dozens of low cost hobbist finished product 3D printers and DIY kits that can print nylon available but no 3D metal powder printers for real reasons cost, technical, and hazard/safety -- it looks like 3D printing of Nylon is child's play relatively speaking.
Here is a link to < $2000, 3D printer developed by Michigan Technological University’s low cost open source 3D metal printer project. It is not laser based but MIG welder based and it actually works, althought the quality of thing produced is not very good.
http://www.appropedia.org/Open-source_metal_3-D_printer. Lot of good links there as well.
A lot of techincal challanges would need to be overcome. If you're using lasers and powders, you generally have to get the material up to white heat to get the materials to fuse together and in a atmosphere that is oxygen free. Getting a result that is close to 100% metal density is not easy. Warping, surface roughness, and numerous other aspects are not so easy to control/deal with, not to mention the hazards that need to be mitigated.
Here is what one guy has done. He has created a production capable 3D metal powder bed fusion 3D printer is which metal powder is lain on the print bed, after which a high power laser melts it in precise locations according to a CAD file (computer-aided design). Once the layer is complete, the computer adds a new metal powder layer and repeats the process.
Estimated cost is much cheaper than currently available machines but still in the $80,000 to $100,000 range.
Good Video with guy who created it:
https://www.youtube.com/watch?v=wRXymDoYoWQ
Article:
MatterFab Metal 3D Printer Is Extremely Cheap, at 10% of Current Prices - Softpedia
Web site:
MatterFab | MatterFab
If you are interested in the technical challenges associated with making a powder bed fusion 3D metal printer, here is a paper produced by National Institute of Standards and Technology April 2015 titled
Measurement Science Needs for Real-time Control of Additive Manufacturing Powder Bed Fusion Processes-- where
"manufacturing researchers at NIST have created a set of guidelines for powder bed, metal 3D printing fusion processes they say identifies key unknowns in the additive manufacturing process in the hope they can help make those methods capable of being fine-tuned automatically. " NIST says powder bed fusion of metal parts is “beset by system performance and reliability issues that can undermine part quality, problems shared by other additive manufacturing methods.” They say problems like dimensional and form errors, voids in fused layers, high residual stress in finalized parts, and poorly understood material properties including hardness and strength are holding back the process"
"The NIST research team broke the method down into a dozen “process parameters,” 15 types of “process signatures” and the half a dozen categories of “product qualities” they charted to identify the “cause-and-effect relationships among variables” in each of the three categories." from article below
Article about the NIST work:
NIST Releases Additive Manufacturing Metal Powder Report to Hopefully Improve Manufacturing - 3DPrint.com
NIST paper:
http://nvlpubs.nist.gov/nistpubs/ir/2015/NIST.IR.8036.pdf
In the NIST paper they say: "today, variability in part quality due to inadequate dimensional tolerances, surface roughness, and defects, limits its broader acceptance for high-value or mission-critical applications. While process control in general can limit this variability, it is impeded by a lack of adequate process measurement methods. Process control today is based on heuristics and experimental data, yielding limited improvement in part quality. The overall goal is to develop the measurement science1 necessary to make in-process measurement and real-time control possible in additive manufacturing."
