Warning: everything I am about to say is just my observation and some experience with thermodynamics, as with all advice from random internet people, use your judgement… but this hasn’t steered me wrong. Just be careful and don’t bet your house/life on what I’m about to say.
You’re not wrong necessarily, but it all depends on the material type and the settings in question. lase and fire risk is a sometimes counterintuitive relationship.
To get fire you need a few things, fuel, heat and oxygen. You definitely have fuel and oxygen, but heat is the wildcard. The laser isn’t hot exactly, but it makes whatever you hit heat up a great deal. So, as it passes over the material, if the laser is sufficiently powerful it’ll cause whatever it hits to just get absolutely vaporized, a process called ablation. It’s not burning precisely, it’s almost being blown apart.
Some of the heat generated during ablation will bleed into the surrounding material. How quickly that happens depends on the material, but most fabric isn’t very heat conductive, so it takes a little bit of time. If you’re moving the laser quickly, the fire risk is reduced, as there is just less time for that heat to spread to the surrounding material and thus ignite it.
So, when you worry about fire is when you are lasering something flammable at lower speeds. Speed alone can be deceptive though, because the design of the cut matters here. Let’s say you cut a straight line at some speed (arbitrarily let’s say 500), the laser whips right by at 500 speed, and the fabric just can’t get hot enough to ignite. Now, if you have a really complicated line with many small turns, the laser “hangs out” in an area for longer as it zigs and zags, even at higher speeds. That’s a fire risk, because the laser heats up a specific area long enough to create enough heat in that area to catch fire.
This is the basic gist for all particularly flammable materials, cardboard, paper, even acrylic. Big simple shapes at relatively fast speeds aren’t much risk. Slow cuts or small/complicated shapes? Watch it like a hawk.
PS, not all fabrics are equally flammable. A heavy canvas is going to be really different from a thin polyester. Wool felt versus artificial felt, etc etc. Test and be careful.
One pierce of fabric on sacrificial plywood held in place with low stick spray is safest. You do not even have to cut through but only make it fragile at the cut.
Cutting a top piece of plywood both bigger and smaller leaving a thin channel might keep the oxygen out of the equation as would having it damp. Do all that and keep the total under a half inch thick and you should be able to be problem free, less is more dangerous depending on your toleration for risk,
Stapling cardboard with cloth underneath, to that sacrificial plywood could keep air and flame from getting between layers but only if stiff enough to do so, with the fabric compressed and oxygen cut off and perhaps damp that would be the goal.
I suspect that this will work because the water acts as a heat sink more than anything else. The heat density of water is huge, both in liquid form and the heat of vaporization (the amount of extra energy it takes to transition from water to steam).
I can only add that for my wife’s music theater costumes and props I routinely cut pattern pieces of fabrics from polys to felt to poly fleece. Had to test to find the settings that work best for us (faster speeds, lowered power and 2-pass) and still have to tweak from time to time, but no worries with any fire issues.
I say this nicely, if you bought the GF with just 15 minutes of research, please take the time now to learn more about the machine, how it works, and how to design with it. Otherwise, you are going to be more than frustrated when it arrives.
Although you may be able to do a few things when it arrives, to really understand how it works, most users really need to put in the time, especially when it comes to designing and troubleshooting. This forum is a wonderful place of knowledge.