Laser Fire Incident

[Chris DeGerolamo]

While cutting some 3/8" clear acrylic this morning, I heard some interesting noises from the back of the office where our machine is located. When I got back there, I noticed flames licking the top glass and a melted boiling pile of acrylic. After changing my pants, I grabbed the fire extinguisher (and with a bit of effort) was able to deliver a quick blast to put it out. I removed all of the now junked material and attempted to salvage the job...oh with all of that, the machine remained fully functional; I will not have to replace a single part.

I essentially made a marble base (see photo) on which to rest the acrylic, figuring this way there would be no possibility of fire....I was wrong. I sat and watched as seemingly from no where puffs of flames (both orange and blue, not at the same time of course) and caught the material on fire. I want to specify: the blue flames ignited almost the same as if you were to drop a match on gasoline - quickly and from seemingly no where. Does cutting acrylic release flammable gases??? I have never heard of this or witnessed this before. FWIW, the acrylic was cast not extruded.

I ended up having to tell the client that I cannot cut the material for them any more due to the fact that it's a fire hazard and I cannot risk it. What do you all think?

[Ross Moshinsky]

Acrylic is flammable and it's the source of most laser fires. We've never had a laser fire. For me, every time I hear someone have a laser fire it makes me think their exhaust system is not strong enough. If you have a really good exhaust system, it should make it pretty darn difficult to catch fire.

[Joe Hillmann]

When you say there are blue and orange flames when you are cutting (not talking about the burning melted acrylic) do you mean a small flame, about the size of a lighter flame, that follows the beam around? If so that is common when using the laser to cut if you don't have air assist. And putting the acrylic on a marble base probably wasn't a good idea. Any gasses that get between the acrylic and the marble have no where to go until they explode which gives you a little pop every few seconds. As long as you keep an eye on the machine it really isn't dangerous, but if you walk away from it it can get dangerous very quick.

[David Fairfield]

Were you running air assist when it happened? No flame damper is a bad idea. Leaving the machine unattended is really bad idea. Been there, done that, took it as a warning!

You can only get away with that if you are engraving a nonflammable like glass or metal.

[Gary Hair]

I ended up having to tell the client that I cannot cut the material for them any more due to the fact that it's a fire hazard and I cannot risk it. What do you all think?

Because of the way a laser works, anything you cut is a technically a fire hazard, it's how you mitigate the hazard that is important. First, never leave the laser alone - several here have shown pictures of what can happen when you walk away for "just a minute". Second, always use air assist and exhaust extraction when cutting acrylic. The air assist helps cut and will also blow out the flame. Third, raise the acrylic off the table and make sure your exhaust extraction draws from above and below, that will keep the gasses emitted when cutting from building up and igniting. Before I raised my honeycomb off the table and allowed airflow under it, I had a couple of times where there was a small "poof" and the acrylic actually raised off the table. I don't know the physics of what happened, but it scared the bejesus out of me. Later that day I made a frame to raise the honeycomb and it has never happened again.

Gary

[Joe Pelonio]

I would attribute it to trying to cut too thick a material for your wattage. Running that slow is what does it. With 35 watts I'd stick to 1/4" or less.

[Kasey Maxwell]

I use a 5000 freq with acrylic and air assist is an absolute must !

[Mike Null]

You should be able to cut this material with no problem but follow Gary's suggestions---airflow above and below the work.

Experiment with your settings on the scraps you have left to see if you can make them more efficient.

[Chris DeGerolamo]

Yes, I was running the air assist and as far as the exhaust is concerned, the system should be adequate (6" round to a blower above drop ceiling vented externally).

As far as the size of the blue flame, I would estimate it to be the size of my palm...much larger than a lighter. I am familiar with the ins and outs of the machine but this is the first I've ever seen of this nature.

My settings were 100% power at 1% speed...the settings *were* adequate to cut the material in a single pass, albeit dang slow.

Unfortunately, I do not have the time to dedicate to watching the machine in operation 100% of the time. I check it periodically, especially when doing a production run. Again, this is the first time I've ever seen anything like this.

Gotta agree with you Joe, the marble base surely trapped the gases in place.

Live and learn I suppose...

[andrew zen]

As a man who knows a lot about laser fires, I know about your pants. I keep a little Zout on hand for that occasion. A little trick we learned is to have a bicycle tire inflator handy (the one with the co2 little modules) when the flame acts up we blow on it a blast. An adapter and a paintball canister filled with co2 at a paintball place makes the co2 last longer. I think you could use a grid under the acrylic as heat and acrylic vapor needs to escape the violent event known as lasering. If the laster blows thourgh the acrylic, it still heats the things under the acrylic as well as the molten acrylic as it is vaporized. When heat builds up, FIRE FIRE HEH HEH HEH.

The laser does not cut, it is a vaporizer.

Air assist blows the heat away. We have an elbow on our assist tube to direct the air towards or away from the point where the laser hits the medium.

Oh yeah. I put a network camera on the cover of my laser to watch the laser process as I am in a nearby room. There will always be flame ups. Especially at curves, well especially anytime unpredictable usually following Murphy's law.

Flame ups lead to fires if left unattended as you noticed.

I also noticed that the covering for acrylic contributes to the fire factor. The dang glue. Transfer paper is the solution there.

I know, basic knowledge for the lurkers present.



.

[Glen Monaghan]

Well, the marble base would ensure that the base didn't catch fire, but then so would an aluminum or steel honeycomb, pin table, etc. I suspect that having the acrylic sit directly on marble, along with running 100% power at 1% speed, caused a significant heat buildup that promoted the fire. Seems like the marble would act as a heat sink, absorbing and holding the heat, letting it build up as the beam slowly moved along. Couple that with vapors and... I'd suggest raising the acrylic well off whatever base you use, ensure there is exhaust airflow above and below the acrylic, and have adequate air assist running.

-Glen

[Ross Moshinsky]

I still think exhaust might be your issue. If your exhaust is really sucking air it should make catching fire very difficult. I bet if you went up to a higher CFM exhaust you'd never have this issue again.

If your exhaust isn't strong enough to lift a small piece of plastic off the table, then it's not moving enough air.

[andrew zen]

Yeah exhaust helps. I have a 2HP dust collector and it almost sucks the flames through the exhaust pipes. But then sometimes it doesn't and I get charring on the acrylic which I have to clean. Enough cleaning, and this job is now losing money and I am not able to catch up on Burn Notice, Psych, or Dexter episodes.

[Bill Cunningham]

While cutting some 3/8" clear acrylic this morning, I heard some interesting noises from the back of the office where our machine is located. When I got back there, I noticed flames licking the top glass and a melted boiling pile of acrylic. After changing my pants, I grabbed the fire extinguisher (and with a bit of effort) was able to deliver a quick blast to put it out.

Ahhh...There's another problem... Never Ever, wait until you change your pants, before grabbing the fire extinguisher.. When seconds count and all that stuff!

Glad any damage was kept to a minimum, it certainly 'could' have been a lot worse.

[paul mott]

Unfortunately, I do not have the time to dedicate to watching the machine in operation 100% of the time. I check it periodically, especially when doing a production run. Again, this is the first time I've ever seen anything like this.

I really think you need to review your thinking here when you are working with flammable materials.

Paul.