Fire needs oxygen, fuel, and heat to get going. You’ve got the fuel, sure… but the layers are smooshed together, so there’s not a great deal of oxygen that can get in there. That just leaves heat. If you are cutting quickly at high power it’s surprisingly generally less fire risk than lower and slower, because of [insert nerdy thermodynamics stuff here]. I’ve done this cardstock trick many times with no issue, but like anything you put in the laser, especially acrylic and other synthetic materials, watch it and be careful.
Nerdy thermodynamics stuff if you like
OK so not too nerdy, I’ll try to make it relatable.
So the laser dumps a lot of energy into a small space, and can very quickly heat up a small area, but if you are moving it quickly it doesn’t have time to heat up the surrounding materials to the point where they can burn*. All materials absorb heat at a specific rate, and how much heat it takes to warm a given amount of something is a property called “thermal mass”. Paper doesnt absorb heat too quickly, but it has a low thermal mass, so you can get it on fire pretty readily if you give it a second with a lighter. Think of a candle, almost everyone has run a finger through the flame. Go fast, no problem. Slow down? OUCH. You can do the same with paper, you just have to be a bit faster.
So, like the candle, when the laser is running hot and zipping along, the surrounding material just doesn’t have time to heat up and burn.
* There are exceptions, like living hinges, where the cuts are very close to each other. Even if the laser is moving very quickly, you have to be careful that it doesn’t make too many cuts too close to each other, as they will gradually heat up the material and that can cause warping or even fire. The same goes for very “noisy” or jaggy lines, if there are all sorts of tiny details the laser basically hangs out in one area too long, and risks heat damage or fire.
If you take this logic to its end, the laser cuts thinner materials a lot more cleanly and with less fire risk than thicker stuff. Cutting cardstock at 350 speed and 100 power has almost no char or risk of fire compared to 1/4" hardwoods, where you have to go a lot slower. Ask @rbtdanforth about his adventures with thick zebrawood sometime, it’s a prime example of this phenomenon.
For even further nerdiness, look into the “coefficient of thermal expansion”. Basically as you heat things up, they get larger. This has been shown to be a noticeable problem with acrylic if you’re making lots of small parts. Great thread started by @patsucebox here: