Clear Acrylic Water Marks When Laser Cut

[Cailline Lea]

Hi everyone :) and Merry Christmas

I am new to the forum so I am sorry if there are threads already covering this subject, but I have searched for an age and can't find any info.

I am cutting 3mm clear acrylic cast sheet on an 80w machine using power 58, speed 15 and I keep getting water mark type stains on the edge of the material. I am leaving the protective sheeting on the side that it touching the bed but when peeled back there is this residue like it has seeped out of the plastic. I have also cut without the protective layer and am still getting the marks in some instances.

Any info would be greatly appreciated.

C

[Steve Kelsey]

I never considered what causes this and at this late hour it is too much for my simple brain to ponder but I've found that denatured alcohol generally cleans it all up with no problems.

[Dan Hintz]

Are you talking about the mottled appearance on the edge itself? If so, that means you need to adjust your settings and/or air. I start with air high until the acrylic frosts, then I adjust down until the frost disappears. Make sure your pulse frequency is set high, too.

I suggest against using alcohol, as suggested by Steve... the acrylic can craze along the HAZ (heat affected zone) almost instantly or even weeks later due to the chemical reaction.

[Mike Null]

I think you need to experiment with settings. I usually leave the protective paper on both sides. If it's plastic I remove it and use transfer paper to protect it.

Are you using a grid for cutting? If not this may be your problem.

I use denatured alcohol on cast acrylic as my go to cleaner but not on extruded acrylic.

Welcome to SMC.

[David Fairfield]

I think the OP refers to a sort of ripple that appears on the horizontal surfaces near the cuts. You have to look closely to see it and for most, its not an issue. But for miniature work as I do, it can be a problem. Its not actually a residue or surface deposit, its more like a shock wave that passes through the material as its lased. So it can't be cleaned off with a cleaning solution. I haven't dealt with this recently as acrylic is not one of my primary materials, but as I recall, leaving the paper on doesn't seem to work. I haven't actually come up with a solution but I would think working with minimal power to make the cut might help. Or experiment with speed vs power and see what happens. It may be related to the the acrylic manufacturing process, or it just be something you have to live with when working with acrylic.

HTH
Dave

[Scott Shepherd]

David, I don't think that's what they are talking about. When cutting thicker acrylics, it's common to see what looks like water coming out of the bottom of the cut. When it comes out, it leaves a "stain" around the cut that looks just like a hard water spot. It's something I haven't solved yet either. It's one of those unpredictable things. I ran a job recently that was 1/2" thick acrylic, large pieces, and every so often one would do it. You can see it happening, but you can't stop it. I don't know if air pressure is the answer or not. I don't have the ability to change air pressure mid cut on that machine, but I will soon. When I do, I'll do some extensive testing to figure out how to get through it.

It's like a micro stain. Novis #2 won't take it off, so it's got a little depth to it. If anyone else knows the solution, I'd appreciate it too!

[Mike Null]

I think I may have removed that with some auto rubbing compound--it's not the aggressive compound but a mildly abrasive polish and Novis.

Whatever I'm doing I haven't had it happen for some time.

[Richard Rumancik]

I am thinking the same as Scott - the "water" mark is probably PMMA (acrylic) vapor that has boiled off and re-deposited back onto the surface.

Cailline - what kind of mask does your material have? If it is poly mask then you need to replace it with paper mask. Poly is terrible for laser cutting. But it would be a lot better if you can get your material with factory paper mask. I have never has a problem with vapor when using factory paper mask. Unfortunately it is not always available as an option. If you are using your own paper mask, then you have to make sure it is adhered well. Maybe you need to use a roller. You could also look for a higher tack material. Some paper masks will work better than others.

Are you using air assist? If you can blow the vapor completely through the cut there is less chance of residue. But to me the key is using a better mask.

[Joe Hillmann]

Have you tried cutting with the acrylic not touching the bed of the laser?

[Scott Shepherd]

Have you tried cutting wit the acrylic not touching the bed of the laser?

I can't speak for the original poster, but we never cut it on the table. I have a box of 1/2" acrylic circles I use to space it off the table.

[dave cox]

I haven't had this problem for awhile but certainly factory paper mask is a must and i found wet brown paper underneath on the vector bed seemed to solve the problem, my theory was it either absorbed what was coming out or cooled it quickly. I seem to use wet brown paper a lot it seems to solve quite a few problems, cleaning isn't necessary.

[Scott Shepherd]

I used to use the paper all the time until I had a long run job that I just couldn't stop the paper from catching fire. Once it caught fire, it caught the acrylic on fire. I tried the old wetting the paper trick, but with air flowing as exhaust, it would dry the paper out before the job was done. That was with 45W cutting 1/2" thick. With more power now, maybe it's time to revisit that since cutting it is much faster and maybe the paper won't dry before the job is finished.

[Doug Griffith]

I don't have a solution either except that quicker with more power produces less of this problem than slow and low. But then the edge quality suffers. Also, to clarify, this appears on the top side.

[Dan Hintz]

Time to install that automatic sprayer, Steve ;)

[Mike Null]

We haven't determined whether the OP has a grid or not but I am betting that cutting on the table is the problem.

[Richard Rumancik]

Cailline - you need to clarify the problem and let us know if anyone has shed any light on the issue. Otherwise we are all guessing. Lots of people are happy to help but we need you to tell us more.

[Richard Rumancik]

I don't have a solution either except that quicker with more power produces less of this problem than slow and low. But then the edge quality suffers. Also, to clarify, this appears on the top side.

I'm confused, Doug . . . what appears on the top side, and for whom? Seems Cailline is seeing the staining on the side of the material touching the bed.

(And if the material is actually in full contact with the bed, then I agree with Mike - you have to have a place for the vapor to go; the material needs to be raised off the table if a grid is not used.)

[Doug Griffith]

I see it on the top surface. It looks like a water stain and polishing does not get rid of it. David Fairfield did a pretty good job of explaining what it might be. The bottom-side problem the OP is referring to is possibly from off-gassing that creeps beneath the masking. I see that sometimes too. It usually has a slight honeycomb pattern to it.

[Mike Null]

The only time I can recall getting this mark on the surface was when I didn't remove the protective plastic on extruded acrylic.

[Scott Shepherd]

I've gotten it on the top and on the bottom. My acrylic doesn't have the mask on it (I removed it) and it's suspended above the table 1/2", so it's not touching anything. I suspect 2 things from my watching it close. 1- it's air flow sensitive. When you plop down a 1/2' acrylic sheet that's the same size as the table, it doesn't get much air flow under the part, and 2- it might be air pressure related on the air assist. I just picked up my air line and couplings yesterday to put a regulator in place, so we'll see if that helps.

It's just as described, it's like molten plastic in it's liquid form, seeps out of the cut. It can travel 1/4"-3/8" away from the cut on the surface.

[Ronald Erickson]

I had started masking my acrylic because of the "watermarks" I was seeing on the top side. I have the acrylic above the table (LEGOs :o). Without the mask I get a lot of "watermarks" on the top side that are very difficult to remove. Magic eraser kind of works, but invariably I'll end up scratching the acrylic if I'm too aggressive with it. I'm interested to find out if regulating the air source helps.

I noticed this video on Youtube of a 60 watt Speedy 300 cutting 15mm acrylic : http://www.youtube.com/watch?v=AIstywWkcoU

At around the 20 second mark, when the author goes in for a closeup, you can see fluid getting pushed around by the air assist. I'm not sure if this is fluid from the air source, or liquid plastic from the cutting. As the fluid exists during all the cuts, I would assume it's liquid acrylic I'm seeing. (?)

[Paul Phillips]

Scott, I've seen what you are describing happen when cutting 1/2" Acrylic with no mask at all, but with standard paper mask it seems to turn out fine, when cutting with no mask you can occasionally see a weird "plasma" looking effect that takes place but it's totally random in both location and duration. FWIW, I'm using 120w with 30-40 psi air assist.
Paul

[Mike Null]

Ron

I'm of the opinion that's condensation. I'm also of the opinion they have inadequate exhaust.