I have a project where I have to burn off black paint from a plexiglass. (a transparent scale for a tank with liquid) I got a sheet of transparent, fully clear, not matte, plexiglass (acrylic glass) I spray painted it with black paint (have two choices, from a spray can, of can use an airbrush and paint it with water based acrylic paint, same results) Then I remove just the paint with my 60W MOPA Fiber laser (15% power 100ns 50kHz, 0.005mm) leave the black as some marking, a scale, numbers, etc. The result looks nice, the black paint is removed fully where needed, the plexiglass is unharmed.
BUT the plexiglass turned into a matte surface where the paint was burned off, but I need it to be clear transparent as it was before the painting. First I thought I applied too much power that matted the plexiglass, but the results were worse with less power. Then I had a closer look at the plexiglass and saw it was some residue of the paint the laser left, it could be scraped off with a blade. Then I tried with more power, at some point the residue was gone but also the plexiglass melted, even bubbles appeared in it.
Does anybody have experience with a similar thing? The task is similar to lasering off just the paint from a powder painted tumbler and leaving the metal intact. But the plexiglass is not that forgiving as stainless steel, Should I keep on trying, is there some fine setting to completely burns off just the paint, or is it impossible, and should I make peace with not using my laser (do it with screen printing, whatever)
Thank you for any clues.
…when you engrave the surface the acrylic reacts to the heat, this is as it is. Can’t you invert your text so everything else gets engraved but not the actual font?, I use this approach sometimes (I have a co2 machine).
I suspect that @bernd.dk is correct on why it’s fogging up.
This is also probably the wrong tool to use with acrylic. It’s more likely that you could engrave with the co2 and fill it with some color, more easily.
There is also dual color acrylic… which might be another angle if you only have a fiber machine.
For the fiber, I would think you would have to try and figure out pulses per mm to see if you can dial it into the perfect speed, power, frequency and q-pulse to do what you want… Probably a pretty fine adjustment from acceptable to not.
The paint thickness needs to be considered… as it will heat differently and require changes in one or more of machines parameters to be modified to compensate for variations in thickness.
The issue will probably resolve to heat, enough to vaporize the paint but keep the acrylic surface below softening point. Acrylic starts to soften at 210F (99C), below the boiling point of water, you could probably see that effect. If the paint must exceed that to vaporize, then it might be a near impossible battle.
I have an MP-60 also. I have a spread sheet to detemine pules per mm and I’ve been working with similar issues regarding power applied and pulses, at least applying it to coating removal…
Maybe it can assist and guide you in your adjustments…
This is in the jpt manual, but shows the q-pulse output waveforms. It should help you decide on the heating duration you wish to apply. Note the peak kW applied with a pulse.
Thank you very much for both comments, I am also a new fiber laser user, have my gear since about a month or two, Did not yet had enough time for many experiments…
I made the signage already negative, It is a vertical gauge to show a liquid level in a tank with a thicker transparent stripe that should allow to see the liquid itself thru it. Imagine the scale like a classical thermometer marking.
I managed to fine tune my settings when the paint is evaporated fully, and the acrylic is melted just a little, became somewhat frosty, but has not that yellowish hue it had at my first tries. And most importantly no bubbles now in the acrylic. Also tried to trick with the focus, made it deliberately a 1-2mm higher, so the laser hits harder the upside than the inner material. Don’t know…
The result is quite usable. I guess a totally clear acrylic is not possible, should use glass for that maybe…
The best result were by making a first pass with 1000mm/s 40% 100kHz 50ns and a second, meant to clear up the surface and not melt too much the acrylic. Cross hatch 500mm/s 20% 200kHz 25ns. (with a 60W JPT MOPA)
PS. Thank you for the chart also, I did not realize the longer pulses have a declining power level, so the heating effect is not linear with the pulse length.
Do I assume right that having a MOPA with those extremely short pulses can help me evaporate the surface without heating up the deeper parts of the material?
I’m guessing, that if you figure out what the paint takes to remove it, these pulse changes could help you zero into the perfect settings… you have to determine if you need more or less energy depending on the results of the acrylic.
I might be full of bs, too…
It’s back to the material… What temperature does the paint need to allow it to be removed and what effect dose that have on the acrylic…
I think you’ll see the same thing with glass… the fiber will go through it and any pain you want to remove will probably leave a frosted glass.
Have you tried masking with tape all the plexyglass, then engrave in negative (as @bernd.dk mentioned), later paint with spray can and finally remove the tape on the numbers, so they are clear?
Thank you, the masking tape is also a good idea to try, btw. I have a vinyl cutter, do that masking and painting thing sometimes, this thing is however too small signage for that, that’s why I am trying with the laser.
I can try the masking tape with two methods, trying to evaporate the tape with an area fill (maybe the very small parts), and also a second try when only the outlines are cut and I remove the tape using tweezers
Btw. small font. Just for fun, I tried to laser a 0.4mm high text into the paint layer, it was perfectly readable of course not for the naked eye but with a magnifying glass, very cool ! we have more advanced technology at our home workshops now than the NASA had 15 years ago for millions!
