I just got my K40 last week. I just tried engraving and cutting 3/32" Lexan tonight and where I was cutting a simple circle the plastic turned black and re-melted. Can anyone suggest some initial settings to try for engraving and cutting 3/32" Lexan?
@Steve_Prior Lexan / polycarbonate is a no go on a laser cutter. At least that’s my experience.
According to this article (http://atxhackerspace.org/wiki/Laser_Cutter_Materials) cutting Lexan produces lethal chlorine gas. Also corrodes the hell out of the machine (meaning your machine will end up damaged). Possibly best to avoid it.
Noooooooooo lexan = bad!
You want Acrylic. They are easily confused. Polycarbonate is gummy and soft if you cut the edge with a pocketknife,
Acrylic is brittle and hard. That’s what works on a laser cutter, cuts like butter. You still want good ventilation, but it’s much less nasty than Polycarbonate.
The down side to acrylic is the brittleness. Drill it with a very dull drill, and be careful breaking through, Or you can finish the hole running backwards. (you can make a nice acrylic drill bit by drilling concrete for a few seconds with an old, cheap bit.)
If you remember that hard and brittle are the way to tell, you can a find acrylic in the pile of scraps at a glass cutting center in the home center. (sometimes you can buy the whole can of drops for 10 bucks) (material hint, hope you don’t raid my home center…)
@Scott_Marshall I drill acrylic with a laser 
Tap Plastics is pretty common across the US and you can find acrylic there. You can also get it in various sizes and colors from Amazon. I use cast acrylic for my projects as it’s more uniform and consistent than extruded acrylic but it’s probably over engineering on my part - it’s not like extruded acrylic is hugely inconsistent 
BTW, it’s almost never the size advertised if you’re looking for 1/4, 1/8", etc. It’ll be something smaller. That’s a good thing to know if you are making things where you use a thickness setting for sizing like fingers on box joints.
The other thing is that if you’re making boxes and things you might want to use “dog bones” - small circular holes - on cut corners. They relieve cracking stress.