Flamepolishing after laser cutting, does it help?

[Kalle Pihlajasaari]

Hi All,

Mostly curious to hear from you old hands. There have been various threads about the small flame polishing machines but I have not seen specific mention if this is primarily for use after routing or if it is also recommended after laser cutting.

I am thinking I need an excuse to make a small flame polishing machine one day. I am confident that I can do it safely after reading all I have researched and am not worried about that. I have no active use for one except to play so I am not planning to purchase a commercial unit. I might go the safe route and have separate H2 and O2 lines and use a standard jewellers torch, having explosive mixture in the gas line in a HHO + solvent system is still a small gamble in my view even with a flash back arrestor or two. I know someone who fitted a crude HHO generator under his bonnet and it detonated one day and he says it gave him quite a fright. The inside of the engine compartment was covered with bits of plastic and caustic soda solution when the small amount of mixed gas in the generator ignited.

All the laser cut samples I have seen and the few little bits I have tested on my recently prepared brain transplant laser look like a pretty decent finish as it is.

My questions are, does flame polishing add any structural value to a laser cut Perspex edge by relieving stress?

Secondly, will the faint ripples smooth out?

Kalle
--
Johannesburg, South Africa

[Dan Hintz]

It definitely does not add any structural value... for that, you need to anneal the piece for several hours in an oven.

Not all laser-cut edges will be perfect, especially thicker pieces, which will show some striations. In those cases, another pass with a flame polisher can help the edge quality greatly. for thinner pieces, if you still need it, you should spend some more time tinkering with your laser settings.

[Steve McKee]

It definitely does not add any structural value... for that, you need to anneal the piece for several hours in an oven.


Dan, can you expand on that comment? What does annealing do to acrylic?

Thanks,
Steve

[Dan Hintz]

Annealing helps release the stress points in the plastic. Those stress points are created when you first cut it by quickly heating and then quickly cool it... and that's where it will crack when certain chemicals hit it (like alcohol or ammonia when people try to clean it).

[Steve McKee]

Annealing helps release the stress points in the plastic. Those stress points are created when you first cut it by quickly heating and then quickly cool it... and that's where it will crack when certain chemicals hit it (like alcohol or ammonia when people try to clean it).

I see. So does it only help with chemicals or stress in general? I had heard the term before just never really knew what it meant.

Thanks,
Steve

[Rodne Gold]

Heat stresses acrylic big time , especially short polymer chain extruded. You need an hour per mm thickness at either 70 or 80c (extruded/cast) for every mm of thickness of material , and if is machined , stress relieving often then distorts it...
You still need a good edge to get good results from flame polishing. wont smooth out striations much if they bad.
Best used on saw cut/routered material , especially thick stuff.
Just as a matter of interest why did your friend stick a HHO generator in his car?

[Chuck Stone]

when the acrylic has been quickly heated and cooled, it becomes brittle.. much like steel that
has been quickly heated and cooled can get brittle and lose its temper.
At the point of heating/cooling, it is subject to cracking, crazing and fracturing. Annealing will
soften the matrix of the material and allow the stressed areas to relax. Temp must be
controlled both while heating and while cooling.

Place in an oven at 175°F (80°C) for at least two hours. For thicker material, use 1 hr. per
3mm of thickness. Cool slowly .. no faster than 1°F per minute till it is below 140°F. Quick
cooling causes more stress, so make sure your oven can hold heat after being shut off.