Why are there small fires in my GF?

When I cut acrylic sometimes, theres a fire where the laser shoots that comes and goes, but it never lingers. Any idea what could be causing this?

Video: https://youtube.com/shorts/PvwXN–4_To?feature=share

1 Like

Just a thought; Make sure the air assist fan is super clean before you start cutting. Also, you could try increasing the cut speed somewhat, so the head doesn’t linger in the same area to get the plastic hot enough to ignite.


Everything @swmisher said is correct. Normally I wouldn’t chime in since they answered your question but I thought you might like a little confirmation since flares can look alarming.


And more:


The bottom line is that you’re applying heat to flammable materials and flares happen. They’re usually not bad by themselves but you need to ensure that it doesn’t turn into an actual fire.

This sort of thing is why Glowforge is so adamant that you should watch your cuts all the time. While I personally think that’s the lawyers talking I do take care especially with acrylic and other plastics. Plastic fires are particularly bad because there’s so much energy in plastics — they’re essentially forms of oil. No fire is good in your Glowforge but acrylic fires seem to cause more damage than other materials. Keep your fans clean and use correct settings with clean SVGs and you should be fine.


Oh and to answer your question as to why: armchair physicist here so take it with a grain of salt but I believe that the laser is ablating/sublimating the acrylic and the resulting gases are particularly flammable. The laser applies just enough heat to ignite the gases before they disperse and so they flare up. The air assist fan should prevent the bulk of this, the incoming air (usually) disperses the flammable gases so they aren’t concentrated enough to ignite.

While woods and other materials can flare up acrylic is particularly prone to it. Compared to other materials I think the gases that are liberated are either more flammable at lower concentrations, have a lower ignition point, or both so we see more flare ups.

Kinda like this:


You can sort of tell by the smell. Acrylic fumes just smell more combustible than things like wood smoke, yeah? It has that petrochemical vibe like gasoline or mineral spirits etc.


I’ve done that trick before but this is the first time I’ve been able to watch the flame traverse down the smoke stream. I think the little blue dot against the black background stands out better.


There is another hidden issue, not covered above, that is that the laser beam does not penetrate as far through one if those flame things, so where there was a flame the cut may not be all the way through. There is a tendency to want to increase the power when it does not cut through, but enough power is not the issue and in that case could make the situation worse.

I have seen the even cutting near the top edge will cause turbulence in the air flow and make the difference in that flame showing up. In those instances some scrap just laid along side the top edge solves that problem. As noted above almost every glowforge I have seen destroyed by fire has been cutting plastic while the owners back was turned. If wood catches fire it gets a little red coal that will travel and eat the wood but takes more to actually sustain a flame. Acrylic has no such issues and goes right to a fairly large flame.


That’s not a small fire… THIS is a small fire!

(No glowforges were harmed in the cutting of this piece…)


What species was that? Maybe oak?


Rough-cut pine picket, 5/8" thick. One pass.


Another unfortunate example.

I keep a small aerosol fire-extinguisher next to my machine, with shrink-wrap removed and I know how to use it. Never had to but I am prepared.

I also keep a damp rag in a zip-lock, also never had to, but it is out of bag and ready when cutting materials that I know can flare up. Again, never had to use it either.

You’re cutting with fire, it just so happens it’s rare for it to be a problem, but is it worth the risk?


I was doing that but found that the rag was not very nice when I opened the zip lock again. So now I keep a spray bottle of water on hand and the rag nearby with an emptied hand sanitizer bottle filled with water and both marked so one does not grab actual hand sanitizer by mistake. A fine spray of water before cutting sometimes helps with flammable stuff like zebra wood.


This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.