PDA

View Full Version : Cutting 1/4" Acrylic



Braden Todd
03-13-2015, 8:03 PM
Hoping that someone has some advice for me on this!

I am trying to cut some 1/4" semi transparent red cast acrylic and I keep running into issues with it. My current issue is that the back side of my cut is melted/distorted/scarred around my laser cut lines. Does anyone have any ideas what causes this to happen? Not sure if it's heat, vapor, paper, gremlins or what. I tried cutting without the back paper, but I get a lot of overheat and distortion of the surface without the paper.

Currently I am cutting my shape on top of an already cut area to keep it off the honeycomb and to allow decent vacuum and room for the beam to pass through. I am also using nitrogen for my air assist, started low and have gone up to 15psi roughly. I am using a lot of power ~500 watts at 1.55"/sec, much less power and I lose my edge quality.

Any advice on how to avoid this back melt would be greatly appreciated.

Thank you!

Scott Shepherd
03-13-2015, 8:10 PM
That's way too slow for that much wattage. It's doing that because it's too much heat. You want the power to be just enough to get through it. With that speed, I'd think you could be at about 1/4 the power you are at. I don't know the magic combination for your machine, but that speed and power number are what you'd see on a 100 w machine. High air pressure can also mess the edge quality up. I'd drop that down, drop the power down and speed the speed up if I were looking for a place to start. Either that or crank the speed WAY up with that power and maybe turn the air down. I think Syrad recommends 1 or 2 psi.

Braden Todd
03-13-2015, 8:25 PM
Thank you for the reply!

I tried dropping my power down as low as 35%, but I started to get vertical lines on my edges and the back burn and not a flame polished look. I was afraid if I went to much faster I would again get the vertical lines and move away from the polished edge, I will run some more even faster to see what happens. I do think that some of my sections do cause my laser to limit my speed though :/

Scott Shepherd
03-13-2015, 8:46 PM
The striations can be caused by too high air pressure. You're essentially cooling the material before it can remelt and be smooth.

Keith Colson
03-13-2015, 10:05 PM
Hi Braden

I cut a lot of sign lettering and have found a method that gets me from 95% perfection to 99%. You can read my post here
http://www.sawmillcreek.org/showthread.php?228374-Laser-cutting-10mm-sign-lettering

I have won over a fair amount of business from my competitors from this small improvement.

Cheers
Keith

Joe Pelonio
03-13-2015, 11:12 PM
Keith's two-pass method should solve your problem, but I still wonder why you have it. My machine is 45 watts, and I use 20 lbs of air assist to cut 1/4" acrylic, leaving the paper on both sides. It cuts cleanly in one pass at 100% power and speed 8. With your higher power you should be able to go faster nd cut through cleanly unless something else is going on.

Braden Todd
03-14-2015, 1:46 PM
I know I can run this faster than the angles in my design will allow the machine to maneuver and cut it the whole way through, but I am also trying to achieve a flame polished smooth edge at the same time so that is why I was using so much power.

I could try the 2 pass method, but we are looking at roughly 6000 pieces for this job and I fear the extra time may be to costly in the end.

I think I will re visit the drawing board and see what else I can come up with on my settings.

Thanks

Braden Todd
03-14-2015, 3:15 PM
Ok I tried the two pass method and on the final cut I still did get the distortion on the back edge.

I tried cutting some without paper and on top of another piece of acrylic, I noticed a liquid when done and it rand and as it dried it formed what I am seeing on my pieces. Is this liquid acrylic/vapor, and is there a way to get rid of it?

Thanks

Keith Colson
03-14-2015, 6:06 PM
As Scott said things are getting hot which indicates you may be burning the acrylic. I think a good step here is to measure your kerf which you can do easily with a set of feeler gauges.

I just measured the kerf on my machine with 6mm acrylic, with a cut focused to the surface. I could fit an 8 thou (0.2mm) feeler gauge through easily and a 9 thou (0.23mm) feeler would not pass through the cut. What does yours measure? The wider the slot the more your machine is burning the material. This indicates how tight your hourglass is and how well focused your machine is.

Cheers
Keith

Ray Scott
03-15-2015, 2:20 PM
I don't know of any way to get rid of the acrylic fluid issue. It is just re-solidifying on the surface. It might as well be part of the original acrylic. It sounds like your laser machine needs a better exhaust system. You need to get the fumes out of there before they can solidify on the back surface.

I agree with these other guys.. You need to slow down the speed, less air assist, less laser power. I realize that I can't keep driving all the parameters up. The material properties don't "go up" too. If you want the nicer flame edge, then you need to use less power, move slower .. less everything. .. better exhaust. Keep the paper on.

Just my thoughts.

Braden Todd
03-15-2015, 3:07 PM
Thanks for all the replies.

I have turned my air down to just knock off any top side flames that may pop up, I also have a very wide nozzle so it's helping to give less of a direct shot to my cut line. I found dropping my speed to 1" /sec and power to 55 I have less of the back issue but it's still there.

I tried to measure my cut width but I have a set of calipers that are being troublesome. I am using a 5" lens so my cut is a little wider to begin with.

In regard to vacuum, I am running 2 2hp blowers (and one small roof motor 1/2hp I believe), one for the gantry fume removal and the second for my table. The table is a 52x100, it seems to have really good suction and I was wondering is it possible there is to much? My whole table is covered and the only vent is where the cut occurs. I actually have to turn off the vacuum to remove my piece due to the suction.

I tried to to slow my speed down to .45"/sec and dropped my power to about 55-60 but I had a lot of flame ups and even worse back edges.

Going to try some more tests now.

Thank you all for the input!!

Dan Hintz
03-15-2015, 7:18 PM
I am using a 5" lens so my cut is a little wider to begin with.

I think we may have a winner here...

Scott Shepherd
03-15-2015, 7:21 PM
I think we may have a winner here...

Yeah, I agree with Dan, unless there's something that works quite different than anything else I've seen, which is possible. Do you have a 2" lens, or anything less than 5"?

Braden Todd
03-16-2015, 10:36 AM
I do have a 2.5" I was using, but my cuts had a slight angle so I just used the 5" to avoid this.

Why would the 5" seem to be the issue? I was thinking it would be better since the focal point is longer and the extra beam size didn't affect my design.

Thanks

Dan Hintz
03-16-2015, 11:38 AM
I do have a 2.5" I was using, but my cuts had a slight angle so I just used the 5" to avoid this.

Why would the 5" seem to be the issue? I was thinking it would be better since the focal point is longer and the extra beam size didn't affect my design.

Thanks

With the 5", you are essentially using a flashlight to cut rather than a laser scalpel. To cut through the material at a speed you're comfortable with, you have to really up the power due to the lower power density of the 5" lens. The power density may be lower, but don't let the name fool you... all of that power is still ending up in the substrate (and it's roughly 6 times more power than if you have used a 2" lens for the same cut speed). All of that power is melting the substrate. Sure, you're pouring in enough power to cut, but that extra heat has to go somewhere, and that "somewhere" is "softening the plastic".

Scott Shepherd
03-16-2015, 12:19 PM
Try the 2.5" and focus down into the material .060-.090" and see if that helps your angle issue.

Braden Todd
03-16-2015, 2:04 PM
Ok used the 2.5" and focusing in did help the angle issue, but that watery/charring effect is still there.

Going to try to play with the settings some more and hope that solves it.

Thanks

Scott Shepherd
03-16-2015, 2:09 PM
1/4" thick, you might try leaving the paper on the top too. I can't say I remember going down that troubleshooting path, but with the power you have available, you might try that on a sample if you have time.

Chuck Stone
03-18-2015, 10:32 AM
I have found my best results come from removing the paper on both sides
and replacing it with transfer tape. I forgot that yesterday and had to re-do
a plaque 3 times before the little light bulb turned on over my head.
I also do it in two passes, although I can get through it in one pass. the second
pass means less power, less pooling and smoother edges.