Acrylic Cutting Question; Paper on or off?

[Mark Smith61]

I'm trying to cut acrylic for the first time and the laser is really good at it. I read to leave the paper on, but my concern is a lot of flames shoot out when I do that. The flames are going up into the lense as well as the auto Z axis measuring device. Is it safe to assume all of those things are made to withstand the heat from these flames? I've tried cutting both with the compressor on and with it off and the flames appear to be about the same. The exhaust fan appears to mostly keep them flat to the work piece and aiming toward the back of the machine, but I can see them at times up around the lense and sensor.

[Mike Null]

First, use the minimum power and maximum speed you can use to get through the material. Too much power and too slow will produce flames. Use air assist. Yours may need adjusting. I always leave the paper on to save on clean-up, bounceback from the grid and scratching.

[Martin Boekers]

Not sure how your laser works, but on my Epilogs you have to set the air assist to on or off on the laser control panel.

You may have a compressor on, but it is not getting to the tube. Also make sure that the air is aimed at the burning point.

[Richard Rumancik]

Mark, I if you are worried that the paper will start a fire it probably won't if it is the factory paper. The factory paper won't lift off on its own, and as long as it is secure it won't make things worse.

What you see burning is the PMMA (acrylic) vapor coming off the cut. A properly aimed air assist should direct the vapor down through the kerf and keep any flames underneath the material. You should not let flames get up into the nozzle/lens area.

Is there any way to redirect the exhaust so it pulls a "vacuum" underneath the material? This will help keep the flames down below. I have made my own cutting frame to elevate the honeycomb, with an opening such that the exhaust pulls from below the sheet being cut.

You should not run unattended expecially if your material is prone to flaming. Some grades/brands of acrylic are worst than others.

[Joe Pelonio]

If cutting large pieces, try misting with water or wiping with a damp sponge before you start. If a lot of small pieces, it will evaporate before you get to the end, so it would be better to peel and use transfer tape, which will absorb the water some and last longer. Still, with everything set right you should not get flaming.

[Mark Smith61]

I'm not worried about a fire, it doesn't appear there is much danger of that. My worry is the flame getting to the lense and the Z axis sensor. Seems like the heat wouldn't be good for them. I've got the air assist and I can tell it's blowing air, but I guess I can check to see if it's blowing in the correct way. On the exhaust, the vents are in the back and it appears that it sucks the air from the top of the work piece. I don't know if there are vents in the bottom or not, but I'll look when I get back to the shop.

I used the recommended settings for cutting the acrylic but I'll try adjusting them to see if I can stop the flaming up.

[walter hofmann]

hi mark
if the air asisst is directed right there should be no flames at all. the next after my expierience is to lift the acrylic of the table with some distanc pieces to avoid backlash because this could mess up the piece from below.
greetings
waltfl

[Dan Hintz]

The flames are going up into the lense as well as the auto Z axis measuring device. Is it safe to assume all of those things are made to withstand the heat from these flames?
No...

I'm not worried about a fire, it doesn't appear there is much danger of that.
Ah, my young padawan, you have much to learn about lasers ;) There is great potential for fire, as evidenced by the number of machines here that have been destroyed by fires people "never thought could happen in the 30 seconds I looked away". The biggest contributors are acrylic and wood. Naively proceed at your own risk...

[Martin Boekers]

[QUOTE=Mark Smith61;2057809]I'm not worried about a fire, it doesn't appear there is much danger of that.
QUOTE]

Please post the photo of your machine after your first fire... :) Not trying to be funny, it will happen, search the forum for posts
on fires. Many here have first hand experience with fires that "we won't have a fire" fires.

I am sure your insurance company would consider any laser to be a fire hazard.

Fire can a do happen in an istant, even for the most cautious!

A good air assit and PSI should keep flame ups to a minimal, if not there probably is an issue with it.


Good luck!

[Richard Rumancik]

Perhaps you gentlemen are reading too much into what Mark said. I don't think he said that fires were not possible; I took his comment simply to mean that he did not feel that the material was at risk of igniting in the circumstances presented.

I am not worried about a fire either, as I watch my machine when running and keep flaming under control with air assist and exhaust. I also have a couple extinguishers handy. Mark is doing likewise, trying to reduce flaming so as to not damage his machine. I am quite sure he is aware of the risks of flaming materials and the potential for fire.

[Martin Boekers]

Perhaps you gentlemen are reading too much into what Mark said. I don't think he said that fires were not possible; I took his comment simply to mean that he did not feel that the material was at risk of igniting in the circumstances presented.



The way I read is starter thread was that he was new to cutting acrylic and he had flames shooting up the lense assembly and was asking are they
built to withstand the heat from the flames. The next post says he's not worried about a fire (I didn't see any mention of material damage)

If you have flames coming out of any cutting they should be addressed. I hate to say it but, it seems that the minute one gets distracted or answers a phone while cutting that
will be the exact time something occurs... :) Acrylic can be unpridictable at times and can cause a quick flare up, that I do know from experience.
Lasers can be a dangerous tool if not handled to error on the side of caution.

[Wes Moore]

Mark

Double check that the material is what you think it is.
I have had vendors sub plastics on me a number of times
only to watch in horror and scamble for the CO2 extinguisher.
This issue pops up for me more frequently when dealing with
thin transparent plastics. Some vendors think nothing of
shipping unmarked ABS when Plexiglas is speced.

Check your air assist flow. I do this by cutting a thin sliver
of poly tape - the yellow stuff that is about 1 mil thick -
and attach one end to the air assist tube. It should flap
in the breeze like a sailor's pennant in 30 mph gale with
the right air flow. If it is limp when cutting then you know
you need to address the air flow. I do this because my laser
shuts down the air assist when the carriage is not moving.

As recommended test a small sample for power needed to make
the cut. I usually place a rectangular piece in the laser and then
send multiple jobs to the laser. Each job has a line incremented
a small distance from the preceding job, typically about 1/8 inch.
That way I can increment the laser power two steps at a time
and then review the cuts to determine which power setting does
a good job of cutting but also yields minimum flame up. If you use
use to much power you can also get cuts with optical changes
in the plexiglas along the edges due to spotting heating. Ugly mess.

By the way, if you have a fire extinguisher (and there should be one
next to the laser - do not ask me how I know) make sure it is CO-2.
Yeah it will blow stuff around but dry chemical extinguishers leave
a cockamamey mess and can be corrosive. CO2 is your friend.

[Kevin Groenke]

Enough responses about the flaming... some is normal, too much could be a problem - it takes some experience to know the difference.

As to removing paper/film cover: You can go either way, depending upon the job at hand.

Generally:
When engraving - definitely take the paper off the top for best results, probably leave it on the back to protect from scratches (possible exception to this is if color filling)
Large vector cuts - we usually remove the paper from the top and leave it on the bottom (where it protects from cutting grid blow-back)
Small vector cuts - remove paper top and bottom (so you don't have to do it to a bunch of small pieces) - place a piece of plain old paper between the stock and cutting grid to minimize blow-back
Mixed - whatever from above produces the best results with the least work

-kg

[Mark Smith61]

Okay, yes I realize a laser can cause a fire. I said that in reply to another persons response. I'm aware of the fire danger and take the appropriate precaustions. My question dealt with the flaming up of the material and I got the answers I was looking for here and through some other research.