Is this normal when cutting acrylic?

[Dan Simone]

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Just wondering if there is anything I can do to reduce the lines I am seeing on the side of any acrylic I cut? The pictures are of 1/4" cast clear acrylic cuts. I ran at about 60 percent power and 10mm/s. I have since done another alignment which made the lines less apparent but, if I turn the pieces just right in the light I can still see them. Is a completely smooth fire polished edge not achievable? If so any tips on what I might be doing wrong?

[Dan Hintz]

What frequency are you running at? (and on that note, what machine are you running? Put machine and power in your sig)

A high frequency will give you a more polished edge, but be aware that as thickness increases you'll have a more difficult time getting a clean cut.

[Larry Bratton]

Dan,
It appears that you may be cutting with your frequency set too low. I have an Epilog machine and I always cut acrylic at 5000f and I get nice smooth edges, almost as good as flame polished. Since you don't say what kind of machine you have that's my best guess.

[Frank Corker]

Dan you have what they call 'bottom drag' you can actually see it sometimes on thick acrylic as the heat sears through the acrylic, as the beam continues forward the cutting edge drags behind almost like light refraction. If you slow the cutting speed down, the drag is less. If the frequency is high or the ppi (pulses per inch) are closer, this helps to illimiinate it. When it comes to acrylic and dependent on the thickness of your material and the wattage of your machine you might not be able to achieve a perfect smooth finish. That looks a bit like 10mm thick and it's not an unusual finish. I have a 45 watt but even I can see bottom drag. That is why there are a few of us who use flame polishers. Flame polishing is when that rough surface is rubbed down and heat is applied which melts the surface to a shiny finish.

[Frank Corker]

Here are a couple of little samples. 1st pic should be the bottom drag. 2nd pic rubbed down. 3rd pic treated with heat. I just did these quickly to show you how it can be cleared up. I deliberately ran my machine faster than the correct speed setting to exaggerate the bottom drag.

[Chris Terrell]

I've noticed this too, though it's not been enough to cause problems in our school context (I've had enough other issues with the laser to deal with!). Thank you everybody for the knowledge shared.
Chris

[Dan Simone]

I updated my signature but, I don't see it updated on this thread. I'm using a Shenhui SH-G1410 with a 100W RECI tube. I haven't seen anywhere in the RDCAM software that I can change frequency or PPI. I'm sure it is in there I just don't know what the Chinese call it? Also, I'm using the default lens that came with the machine which I believe is a 40mm focal length lens. It uses a 9mm focusing stick if that helps.

[Rodne Gold]

Check the focus , that it's spot on and then when cutting thick acrylic , focus about 1/3rd of the way into the acrylic , use potent air assist as well. The coaxial with the beam air assist doesnt work as well as if you direct a tube outside the nozzle as air assist
You can also try "dotting" mode , varying the settings till you get a better edge.

[Rich Harman]

I haven't tried the "Dot" mode with the new main board but the old one was awful. In dot mode it would stop moving for each and every dot.

[john banks]

It keeps moving on v5, but I haven't yet found a setting that I prefer to cut mode for cutting efficiency, but haven't put the time in yet to play with acrylic edge finish. I tried some ridiculously low numbers to request a very high frequency and it did obviously nothing like with on-offs of say 5mm length. Some of the intermediate settings made different noises and might achieve the flame polished effect a little better.

Rodney's comments about air assist ring true, I see a few more striations since I improved the coaxial air assist I think. Perhaps the air is cooling it down too much rather than letting the acrylic melt?

This is not talking about the "bottom drag" effect which is easy to solve with the right combination of speed/power/lens/focus, but the striations.

[Rodne Gold]

The conventional wisdom re air assist and acrylic is to drop it till it flames and then up it a notch or 2. I dont like anything thats even remotely near "flames" so tend to up it a few more notches...
The stock lens of the shenhuis is 40mm , which is quite a short lens , I think this shows off finer engraving than a 50 so their laser looks "better" , but its not that great for cutting thicker stuff , if you got a spare lens try a 50 or a 63mm...

[Dan Simone]

So there is no such thing as a frequency or ppi setting for a Shenhui laser?

[john banks]

There is, but how well it works is not well documented in the instructions or forums. There are three settings in dot mode you'll see if you select it, and a check box to "center". There is a distance for on, distance for off (and these make sense), but also a time and I'm not clear how this interacts with the other setting. Try playing with it and share you findings?

In brief testing the other day I found that the ammeter showed 50% of usual maximum current when I had the distance for on and off the same, so it is doing something. Perhaps using larger values to show the pattern much larger would show how it works.

[Joe Pelonio]

Great educational post Frank. I can get a smooth cut up to 1/4" with my 45 watt but anything thicker I get that too but it's slight, and not all that noticeable, but not acceptable if making templates.

[Frank Corker]

There are a few odd things that happen with laser cutting acrylic, generally they are suprising for me, sometimes cut something at say 6 sp and 100 power, cuts through no problem. Then 6 sp 70 power..... cuts through exactly the same! Don't understand it, but it does happen on occasions and it all seems to stem with the speed setting, not the power setting. Against all the theory of freq, I have cut acrylic and had very similar results with 5000, 2500 and 1000. So for me, I look at it as 'whatever does the job' use it.

[john banks]

I fiddled more with dot settings and can't improve the edge on acrylic over using cut. Where I thought it was helping I got similar results by dropping the power and speed, as again I found that dot mode with equal on-off distances was dropping the average power and I needed to reduce the speed to compensate. No free lunch on this setting on the Shenhui I think.

[Thomas Baranowski]

There are a few odd things that happen with laser cutting acrylic, generally they are suprising for me, sometimes cut something at say 6 sp and 100 power, cuts through no problem. Then 6 sp 70 power..... cuts through exactly the same! Don't understand it, but it does happen on occasions and it all seems to stem with the speed setting, not the power setting. Against all the theory of freq, I have cut acrylic and had very similar results with 5000, 2500 and 1000. So for me, I look at it as 'whatever does the job' use it.
Frank often I have seen issues when color mapping is on during a cut and having issues, I now always make sure color mapping is off when using vector since I always make my cut lines red.