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I started in this biz by extending my hobby of rebuilding and restoring old projectors in the 1970's. I used everything under the sun to strip off old chipped and peeling paint, and then there was still all the rust that was under that to deal with.. I finally resorted to taking crates of parts to have them hot tanked to remove paint and rust. But hot tanking left a strange residue that STILL had to be cleaned off. Now this can all be done with high powered lasers designed just for this purpose. These things are really amazing, but expensive, However, no doubt those hot tanking places are now offering this as a service. The only left over trace after laser stripping is some pitting from where any rust was. The metal looks like new. Has anyone out there used or tried one of these devices yet? You can see a quick demo of it in action in this Youtube Video. I sure wouldn't want to get my hand under that thing!
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Have seen those before in some videos, but never in real action. I can imagine those lasers being somewhere into the multiple hundreds of Watts if not even kilowatt ranges. Pretty impressive stuff. Also, interesting sound once the laser hits the surface and starts to cook it...
But how does it actually work? Does it actually break the bond between the metal and the oxygen? It looks like the layer of rust is literally blown off by the intense and sudden influx of energy. Also, the light effect looks more like plasma and/or glowing metal than laser light itself.
Maybe Hollywood should take a slice of this when they come up with new special effects for lasers.
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I don't know exactly how this laser works but I have used a few laser machines at places where I have worked.
In two places, they used laser machines to engrave part numbers and bar codes onto product. Just imagine a laser beam being raster scanned over a surface and, where the beam hits the workpiece, a small amount of material is vaporized away. It's essentially a dot matrix printer except it uses a laser beam instead of a jet of ink.
What I see in the video, above, appears to be a variation on that scheme. The laser is scanned, quickly, back and forth while the user or robotic arm moves the beam over the part, vaporizing minuscule amounts of material as it goes.
In machines that I have used, the power of the laser is adjustable, depending on the kind of material being marked. The epoxy or fiberglass surface of a circuit board would need less power to mark than steel. Yes, the machines I have used are powerful enough to leave a mark on steel.
One thing we had to watch out for is the fact that metal can reflect the beam and damage anything in the way of the reflected beam. In manufacturing, that's a bad thing but it is my guess, in this application, that's the effect we're looking for: Rust, paint, scale or other debris vaporizes more easily and bare metal reflects more of the beam than gets absorbed.
It is my guess that the power and/or frequency of the laser has been adjusted so that it vaporizes unwanted material but reflects off the bare metal once it burns down, deep enough, leaving a clean surface.
What is the plasma plume? What is it doing at the molecular or atomic level? I think that it is just heating the material so quickly that it doesn't have time to burn in the way we think of "burning" and, instead, converts that matter directly to a gaseous or plasma state which then turns to gas or smoke.
The machines I have used for laser engraving are inside a metal enclosure with laser proof glass viewing ports that are colored dark, greenish yellow. (Probably the complementary "anti-color" designed to absorb light at the laser's frequency.) The machines have some pretty robust evacuators to catch and filter out the smoke.
I have been around when the filters stopped working for one reason or another and the whole shop filled up with the smell of burned circuit board. (Usually accompanied by cries of, "Who f•••ed up the laser burner...again!")
Unless they've got some good ventilation in the shop where that laser cleaning machine is being used, I bet the place gets pretty stinky after a while!
Last edited by Randy Stankey; 08-14-2021, 05:56 PM.Comment
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The one in the video above is a 1kw laser. Some how they shape the beam into a wide narrow pattern... Mirrors and lenses inside I suppose. But that laser tube has to be mega expensive., so I see this sort of thing being relegated to places that specialize in metal restoration. They probably charge by factors of SQ inch of metal restored, and how long the laser tube is on during the job. Doing a little searching, that 1kw unit runs 480,000 USD.Comment
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I guess the narrow shape can be achieved by rapidly scanning the laser "up-and-down" via a scanning mirror, which will probably be more effective than a bunch of lenses, which will create more like an oval form. I don't know if laser optics are more susceptible to overheating than "broadband light" optics, but just like those in a high-end projector, those can't be cheap at those power levels.
I'm just armchair guessing here, but it will probably be something like this. Iron turns to gas at a temperature of about 3170K, that's about half the temperature of the surface of the sun... Iron oxide is far less reflective than iron in a solid state, so I guess the iron oxide absorbs most of the energy of the laser and super-heats to a temperature beyond 3170K in a fraction of a second. This will not only destroy the covalent bonds between the iron and oxygen, but the rapid expansion of the material turned from solid into gas will also cause the material to fly off in a cloud of "dust". The material that remains inside the laser beam will be excited even more and turn into a plasma.Originally posted by Randy Stankey View Post
It's probably important to move the beam with sufficient speed, if you keep the beam aimed at the same spot of the now exposed metal too long, I guess it will start to melt rather quickly.Comment
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Must be a giant case to fit all the warning and danger stickers.Comment
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