Flame on trailing edge of cut with 1/8” acrylic

Anyone else seeing a flame on the trailing edge of the laser when cutting acrylic? I’m cutting basic 1/8" Inventables opaque blue acrylic (using PG settings) and when the gantry moves from front to back, i’m seeing a faint elongated flame on top of the acrylic.

Anyone else see this at all? It’s not leaving a mark on the masking, so (to me) it doesn’t seem intense enough to affect my cuts. But figured it’s worth an ask.


Pretty sure that’s the depth sensing laser

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It’s flame-yellow and flickers - definitely not anything constant or consistent.

I’ve seen it occasionally coming up from underneath the material. I think it’s normal if there’s an air feed to the flame. (Like cutting lines close together, or close to an edge.)


I saw it tonight when cutting some Proofgrade clear acrylic.

I don’t remember seeing it before, but the last time I cut acrylic was around a month ago, so perhaps I’m just not remembering. But it’s also possible that they’ve changed the Proofgrade settings since then.

I do see it occasionally when cutting acrylic.

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An MSDS on Acrylic shows potential for flare ups…

Hazardous Decomposition Products: In case of thermal decomposition, combustible vapors are formed, which are irritating to eyes and respiratory system, mainly consisting of: methyl methacrylate.

I had no idea what methyl methacrylate was so another MSDS search shows it as a liquid used for making of plastics and it has an amazingly low flash point.
(Not sure these numbers are valid when it is a vapor, but I would think that, as a rule, a vapor flashes easier than a liquid form).

Flash Points: CLOSED CUP: 13°C (55.4°F). OPEN CUP: 19°C (66.2°F)

Easy to connect the dots. As the plastic melts//burns it is gassing a potentially flame producing byproduct.

Interesting, but overall this looks//appears harmless since no danger of it becoming concentrated with the fans running.
An added plus. Since the methyl methacrylate vapor is irritating, then having it burn off is probably a good thing.

Happy Trails…


Yes I have seen (and recorded) the flame.

Picked up some 3/16 and 1/4 clear cast acrylic locally. Between the thickness and the paper covering, it is not uncommon to get a flame. I have seen it suggested to play with the speed and power settings in order to minimize the ignition of the vaporized acrylic.

Here are some examples (best flames at the last 15 seconds of video 3)

Flaming Acrylic - 01

Flaming Acrylic - 02

Flaming Acrylic - 03

I’ve seen this with all acrylic other than ProofGrade. I had a hunch and ran a test piece without the paper masking and that’s what it was.


It’s always nice to know when you are in good company. Thanks all!


I’m going to giggle and tell you that’s normal. It happens on all lasers, not just the Glowforge.

The lasers are combusting the material - burning it away in the reductive process. This is why lasers are industrial machines and it is highly suggested that you keep a fire extinguisher nearby!

(This is also why I always, always leave the masking on the bottom of materials. Flashback is a thing.)


Ive tried that too with my flaming forge and the heat on the metal of the crumb tray distorts the acrylic on the bottom.