At the time my laser decided to misbehave, having completed my tests on plywood and MDF, I was part way through testing acrylic sheet. I had noted that there is a difference between cast and extruded acrylic and that cast is the best for lasering but I was not sure what I had bought from my local hardware store although it was likely to be extruded. I believe if branded Perspex then it is cast. Anyway, I had seen videos of cutting through acrylic like a knife through butter and although I could cut 4mm in 3 passes by raising the material above the bed, it had a rough edge and produced a candy floss type debris. Engraving simply melted the surface so I looked around for a hack that might achieve a better result but could find very little. In the past I have found all manner of techniques including coatings for engraving many materials with my diode laser. Do you have any suggestions?
My K40 last time I tried cut through 4mm acrylic in one pass( maybe 2 ) and the edge was clean and smooth. Having an adjustment on your air assist can help because more air gets you better cutting but it looks like a saw blade cut the acrylic while just a little or no air will keep the edges hot and they will flow back to a smooth glass-like edge.
Cast acrylic is better for sure but I forget the reason. It’s gotta be listed all over the internet since it’s pretty much Laser Cutting 102, ie basic materials info and up there with don’t cut vinyl.
Cast is better for laser engraving since it looks frosted. Extruded tends to engrave clear. Cutting is basically the same.
Acrylic should be easy to cut on a K40 on my experience.
What I am trying to do is to create a panel with the text on the rear so although I would prefer it to be frosted, if the text is well defined maybe I can infill with paint. I think I need to experiment a little.
Is this a visible light laser? You mention diode and your profile has no information to help me figure out what kind of machine you are using…
I cut lots of acrylic with my co2… I find it performs poorly with higher air pressure…
If it’s a visible light diode, it can’t really cut acrylic. Acrylic passes visible light.
On my co2, I order extruded when I have to pay for it… as my cast usually has some issues when I cut it… Since I have a batch of it for free from one of the custom engraving business… It’s all scrap from them, but is very usable and the price is right…
I have three types of lasers only the co2 cuts acrylic well.
Good luck
Sorry for any ambiguity. I had a number of blue diode lasers for about 3 years but I got my K40 a couple of months ago and then discovered this group. So, yes this is using a K40 laser. I was aware that diode lasers will not cut acrylic and that a CO2 laser can but I had not appreciated that there were 2 types of acrylic. As I say, it is extruded I would like to use as I can get it from my hardware store so I simply want to get the best I can from it. I note that you say you need lower air pressure which presumably is to get a more liquid edge. My air assist is very strong so so is great for wood but I do need to do need to restrict the flow for acrylic. However, it is engraving that I’m particularly interested in on the reverse side so making panels and coasters where I used glass with my diode lasers. I have noted on videos that they have a frosted effect (cast?) but mine come out melted so it seems the only way is to maybe paint into the groove. I will get back to experimenting but if you have any good tips do let me know.
I asked Russ Sadler about this a couple years ago… He advised lowest pressure to keep the lens clean…
If you have the optics to look at the damage the laser does to the material, it will help you diagnose the proper speed/power to do best work.
I put a small dish of water under the nozzle and increase it until I see the water just barely ripple… Seems like the lower the pressure the better the result… for both engraving and cutting.
I purchase extruded, it the most common and, I think, cuts substantially better… I have had some (donated) cast and wasn’t real happy with it’s machining ability on the laser.
If you are vaporizing the acrylic it should be distorted by clear… If you’re melting it, I’m not sure how it’s working… I think air would just exacerbate the problem…
Try it and see… 
Good luck
Many thanks for the response. So I need to tame my air assist first. It’s interesting that you find extrudes better so yes I will give it a try.
I have only been a laser fan for a couple of years now and I have learned that Jack has great knowledge
and wisdom. He has taught me a lot and I respect him for that. Now, my question is, (Somewhere, I thought that I read some info that stated that when burning extruded acrylic it gives off a vapor gas that you really don’t want to be breathing, ( choke, choke, die, die) and that cast does not. Is that correct or did I miss read something somewhere?
Randy
Thanks for the additional information. I am not sure but I keep the lid down when I am cutting acrylic and my extraction does work well but I will be careful. My air assist is too strong so I am in the process of incorporating a speed controller (used the same one on my CNC spindle) to help tame it but when I switch it off it does provide a smooth edge but the cut part tends to re-stick in the hole. Once air assist is tamed, my next project is to recreate a new panel for my K40 to accommodate the changes I have made and I hope to use some of the principles in this excellent video: Making Flight Sim Panels with my eBay Laser - YouTube. I have done a couple of prototypes so far but need to sort the ait assist first.
Every day, in every way, I grow a little wiser!
Brilliant tip, thank you.
I have now successfully incorporated a PWM motor controller to tame my air assist so I am now in a position to test cutting and engraving acrylic.