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Determining material opacity

Junkers

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Hi there

I figured this may be the best place to start, hoping to tap into some knowledgeable people here as I'm a newbie to the physics behind lasers. How do you determine a materials opacity relative to a particular wavelength of light?

The reason I ask is I'm attempting to build a 3D printer and need a laser which is will pass through glass but not aluminium. Can anyone help?
 



diachi

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Hi there

I figured this may be the best place to start, hoping to tap into some knowledgeable people here as I'm a newbie to the physics behind lasers. How do you determine a materials opacity relative to a particular wavelength of light?

The reason I ask is I'm attempting to build a 3D printer and need a laser which is will pass through glass but not aluminium. Can anyone help?
Pass through glass but not Aluminum? Any visible laser will do the job just fine then. Regular window glass reflects about 5-10% - you can get anti-reflection coated glass that will reflect less for whichever wavelength you choose to go for.
 

bostjan

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I don't think you need to worry about your laser passing through aluminium.

If you want to determine the absorbance of the glass, there are a few different ways to do so, depending on what you have at hand, how much you want to spend, and how much precise you need to know the absorbance of the glass.

It might be as simple as getting a piece of the glass you want to use, getting the laser you want and shining it through the glass to see how much light goes through and how much is reflected. If you want some numbers, but don't care about precision too much, you can use a CAD cell or PV module, depending on the power of the laser, to see if there is a measureable change in the beam intensity.
 

Benm

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Question is what happens with the aluminium: does it get reflected or absorbed?

This would be fairly important if you want to attempt to heat aluminium through a glass plate or something like that: the light will pass through the glass, reflect off the aluminium and pass trough the glass again, not resulting in significant warming up of the aluminium nor the glass.
 

Encap

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Question is what happens with the aluminium: does it get reflected or absorbed?

This would be fairly important if you want to attempt to heat aluminium through a glass plate or something like that: the light will pass through the glass, reflect off the aluminium and pass trough the glass again, not resulting in significant warming up of the aluminium nor the glass.
Exactly -- the degree to which a lasers wavelengths are reflected and/or absorbed is important if you are attempting to heat aluminium.

A laser beam has no temperature - there is no inherent "temperature" to a laser beam. Heat is the random motion of matter particles (atomic or molecular particles). A laser beam itself is not made of matter but of photons, which have no mass, thus a laser beam can have no temperature.
"Heat" is caused by a laser beams energy being absorbed by a material surface and turning light energy into heat energy.
 
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Junkers

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Question is what happens with the aluminium: does it get reflected or absorbed?
Spot on. This is what I need to clarify. Ideally I need a wavelength which won't be absorbed by glass and will be fully absorbed by aluminium. I just need to know where to start when attempting to solve this problem.
 

Encap

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Spot on. This is what I need to clarify. Ideally I need a wavelength which won't be absorbed by glass and will be fully absorbed by aluminium. I just need to know where to start when attempting to solve this problem.
Have a look at https://en.wikipedia.org/wiki/Opacity_(optics)

A CO2 laser tube operating at 10,600 nm is absorbed by and will heat aluminium fairly effectively.

Why don't you have a look at what others are offering in 3D kits and machines rather than reinvent the wheel.

see: https://www.3dprintersonlinestore.com/electron-3d-prusa-i3-kit and google 3D laser printers
 
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diachi

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Encap

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CO2 is going to be heavily adsorbed by regular glass unfortunately.
Yes, I know. I mean't a CO2 laser would be pretty well absorbed by and thus heat Aluminium of the top of my hat. I corrected the post.

Without knowing a lot more about what he wants to do and why, it is impossible for anyone to help him. His budget for making a 3D printer is a key consideration not mentioned, also -- $500 or $5000 or $50,000. All we know is he has a daydream about doing something with glass, aluminium, and using a laser with both.

If you look at his original question " I'm attempting to build a 3D printer and need a laser which is will pass through glass but not aluminium" there are a lot of laser wavelengths can pass through glass but not aluminium . What wavelengths pass through aluminium?
Glass transmits pretty well between wavelengths of about 350nm to 2500nm --- so all he can use is a laser in that range---none of those wavelengths will pass through aluminium so it is exactly as you said:
Pass through glass but not Aluminum? Any visible laser will do the job just fine then. Regular window glass reflects about 5-10% - you can get anti-reflection coated glass that will reflect less for whichever wavelength you choose to go for.
Aluminium is highly reflective over the entire visible spectrum. If he wants/needs a wavelength that glass passes but that aluminium absorbs and causes the aluminium to become hot then he is SOL pretty much. Can't get there from here not with CO2 anyway.
If Junkers just wants a wavelength that will not pass through aluminium then all of the wavelengths that ordinary glass passes are good to go--none of them will pass through aluminium.

The ability of glass to transmit IR falls to zero at about 2500nm(ability of eyes to see IR falls to zero at about 800nm) A good paper explaining what wavelengths glass can and can't transmit is here: http://fp.optics.arizona.edu/optomech/references/glass/Schott/tie-35_transmittance_us.pdf

What is the actual need --- why does Junkers need a wavelength that glass will not absorb it but aluminium does absorb--and to what extent is either case?? If he actually needs same in a real world situation would a CO2 laser and a ZnSe window or lens do? Why is it necessary and at what expense?

What does Junkers actually want to do and with what materials in the real world other than create problems and technical challanges that need not exist or for which there is no solution and can not be solved?

Thus my suggestion to look at what exists in the real world of 3D printers using technology available today rather than reinventing the wheel, if actually what he wants to do is make a 3D printer.
 
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Junkers

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The objective is to build a DMLS printer. The glass will act as a canopy to the powder bed, sustaining high temperature and an inert environment. The aluminium is the material to be sintered. I'd like to keep the galvanometer / laser external so not to have to expose it to extreme temperature, as doing so would further complicate matters. My total budget is around $4000 - 6000.

Ideally the glass will transmit 100% of the light whilst the aluminium will absorb 100% of the light, reality will obviously prevent this. I'm just looking for some material / information that will help me get as close as possible to the ideal circumstances.
 
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diachi

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Encap, thanks for updating your post. You are entirely correct - and now we have some more information too! :)

The objective is to build a DMLS printer. The glass will act as a canopy to the powder bed, sustaining high temperature and an inert environment. The aluminium is the material to be sintered. I'd like to keep the galvanometer / laser external so not to have to expose it to extreme temperature, as doing so would further complicate matters. My total budget is around $4000 - 6000.

Ideally the glass will transmit 100% of the light whilst the aluminium will absorb 100% of the light, reality will obviously prevent this. I'm just looking for some material / information that will help me get as close as possible to the ideal circumstances.
Well CO2 lasers are out the window. Your options now are high power fibre coupled bar diodes in the NIR region of the spectrum - 810nm, that sort of area, or, a Q-switched YAG. I'm not sure if you'd quite enough power from such a system (everything I've seen so far says 200W-400W fibre coupled in commercial systems which is $$$$ just for the laser) - although maybe I'm wrong, in which case this is probably the way to go. Collimate the output from the fibre, feed it into the galvos and off you go (Complexity greatly simplified). Much easier to power, much easier to modulate/blank, no ridiculous peak powers, no high voltage. This is the way I'd go if it works.

The other option (and the one I imagine most commercial units use, although I may be wrong) is to go with a Q-Switched Nd:YAG (Or some other Q-switched SS laser) system. However, you're not going to get a new diode pumped Q-switched YAG laser at with those kind of budgets. At least not if you want to have any money left over for anything else. You could pick up a used Laser Scope for cheap ($1000-2000) - But it'd need to be converted for your use - if you can find anyone who'll give you that information these days. They'll run at nearly 100W average (Few kW to a few 10s of kWs peak) when in IR mode. Not to mention the 6kW@230VAC power requirement on those + however much power the rest of your system needs. That said, the high power would probably mean you can use a lower temperature for the Aluminum.

Not a laser you want to be messing around with if you don't have experience with lasers - even as an experienced user I'd avoid these, they are just about the most dangerous laser I can think of that you can actually get your hands on. With maybe the exception of a big CuBr. Plus blanking is a bit of a PITA with a Laserscope.

Would you be able to go with plastic SLS using nylon or some such polymer instead? Or does it need to be Aluminum? That would greatly simplify the build. You'd need significantly less optical power for a start. From what I'm seeing you'd be talking 0.1W to 1W as opposed to 200W.


Edit: I was wrong - looks like the commercial units I've been looking at use a 200-400W Yb:Fiber.
 
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Encap

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Well CO2 lasers are out the window. Your options now are high power fiber coupled bar diodes in the NIR region of the spectrum - 810nm, that sort of area, or, a Q-switched YAG. I'm not sure if you'd quite enough power from such a system (everything I've seen so far says 200W-400W fibre coupled in commercial systems which is $$$$ just for the laser).
Yes, CO2 is a dead duck is what I was thinking also if Junkers has to have glass involved. A YAG or a Yb Fiber Optic laser would be optimal but the biggest hurdle might be the software...and or hardware to recognize files to be printed -not to mention all the other technologies needed and hazards associated with same--dangerous laser, toxic fumes, toxic metal powder in the air inhaled, inert gasses that can suffocate -- a DMLS unit needs an inert atmosphere a argon or nitrogen chamber to work with Aluminium and needs to purge all oxygen out of the system first.

I would think $4000 to $6000 is a drop in the bucket for all the technologies you would need.

I found one guy who made a DMLS machine to make things with steel powder however the YAG laser cost was $100.000 and the machine manually operated--not computer run automatically.
He says "I'm working on just such a rig. It is a box about 3 x 6 x 6" in stanless steel and would be airtight to vacuum out and purge with argon. The unit would have glass windows and sit beneath an 80 watt YAG laser with galvo. Everything including the powder elevators and spreader are metal, so nothing should burn. I am working with steel powder until things are working well." " The laser alone here is over $100K, so that will be the limiting factor. That can probably come down to about $15K to get a raw laser in the minimum power and then a few thousand bucks worth of mechanical stuff." see: http://www.3dprintforums.com/showthread.php?t=90


The objective is to build a DMLS printer. The glass will act as a canopy to the powder bed, sustaining high temperature and an inert environment. The aluminium is the material to be sintered. I'd like to keep the galvanometer / laser external so not to have to expose it to extreme temperature, as doing so would further complicate matters. My total budget is around $4000 - 6000.

Ideally the glass will transmit 100% of the light whilst the aluminium will absorb 100% of the light, reality will obviously prevent this. I'm just looking for some material / information that will help me get as close as possible to the ideal circumstances.
Do a search of what exist in the real world of DMLS 3D printing machines.

I would guess $4000-$6000 is not enough money to even begin to purchase or create any of the technology needed for a functional DMLS machine.

You could look into a high powered Ytterbium fiber optic laser. Yb has center wavelengths ranging from about 1030 to 1080 nm which would transmit through glass.

the DMLS was developed by the EOS firm of Munich, Germany.
Interesting information on the EOS system, and in particular the EOSint M 280, their flagship metal printer. see: http://ip-saas-eos-cms.s3.amazonaws.com/public/e1dc925774b24d9f/55e7f647441dc9e8fdaf944d18416bdb/systemdatasheet_M280_n.pdf
Some fun facts...
- Costs $680k per unit
- Has a build area of 9.85" X 9.85" X 12"
- Uses a 200W Yb fiber optic laser (for an extra $50k you can have 400w)
- Has an integrated nitrogen generator
- Can scan up to 7M/S (23FT/SEC)
 
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Junkers

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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.
 

Encap

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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."
 
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