Allowing the material to flame is at best an incorrect use of a laser cutting machine and at worst a very dangerous risk of causing a major fire.

1. Preventing Fires in a Laser Cutting
Machine
Some suppliers include warning devices within their laser cutting
machines designed to indicate the presence of a fire and would
have you believe that material flaming during laser cutting of
potentially flammable materials, such as laser cutting acrylic, laser
cutting wood or laser cutting card is a normal part of the process.
Nothing could be further from the truth.
Allowing the material to flame is at best an incorrect use of a laser
cutting machine and at worst a very dangerous risk of causing a
major fire. Flaming during the laser cutting process will damage
the material as well as deteriorate the system optical components
leading to wasted time, wasted materials and expensive repairs.
Allowing the flaming to develop into igniting the material is
extremely dangerous and nothing short of user negligence.
If the material is flaming it indicates a serious fault in the process
that could relate to a misguided use of the equipment (incorrectly
specified machine and/or inappropriately trained operator),

2. incorrect parameters being used, incorrect focus or optical
configuration (lens) and a variety of other common user errors.
In most cases flaming during laser cutting can be related to two
specific causes:
1) An inadequate rate of laser fume extraction and/or air assist. If
the vapours generated during the laser cutting process are not
quickly and effectively removed from the path of the laser beam
they can combust. In vapour form the material will ignite much
easier than it will as a solid.
The best way to remove these fumes is by way of aggressive
extraction from the underside of the material combined with a
targeted delivery of high pressure through a small bore nosecone.
This keeps the vapours away from the cutting point, preventing
flaming.
2) An inappropriate use of the laser cutting machine. Some
suppliers would have you believe that it is perfectly safe for a 30w
laser to cut 20mm thickness acrylic. Just because the machine can
do such a thing doesn’t mean it should. Using a laser of very low
power to cut thick materials requires a very slow feed rate and
sometimes multiple passes.
Also, using a lens with a short focal length (anything less than
127mm or 5” f/l) to cut thick material (>12mm or ½”) means that
even for waveguide materials, such as acrylic, the majority of the
material is being cut with a beam significantly out of focus. This
maximises the heat absorbed by the edge of the material and in so
doing brings closer the flash-point where the material ignites.
It is therefore more important that when using a lower power laser
to cut thicker material, the user is even more vigilant with their
supervision of the process.
Whenever the machine is laser cutting, no matter what the
materials are in terms of type or thickness, the laser cutting
machine should NEVER be left unattended for more than a few
moments at most. We train operators to have a constant ‘line of
sight’ to the machine at all times that it is laser cutting. In the event
that flaming appears the process must be stopped and the system
configuration and/or parameters adjusted accordingly. In the event
that flaming still occurs our advice is not to continue with the
process. If the user has no option but to continue while the material

3. is flaming then we advise to do so with a great degree of caution
and a real understanding of the risks being taken.
At Lotus Laser Systems we manufacture a wide range laser,
marking and engraving solutions and we pride ourselves on the
quality and depth of knowledge of our support staff. Our experts
would be happy to advise you on this or any other aspect of your
laser system operation.
Source:- http://lotuslaser.blogspot.com/2016/03/preventing-fires-
in-laser-cutting.html