CO2 assist?

[Keith Upton]

Has anyone ran CO2 as their air assist? Some of you will recall my issue cutting foam where it was melting back together as I cut more of the sheet. One of the suggestions was to freeze the foam and another was to increase my air flow in order to cool the foam before they could fuse back together. I was thinking I could use CO2 instead of air. As it expands from a liquid to a gas it drops in temp a lot. It is also non flammable which is always a bonus.

Just wondering if anyone has used it and what issues I might run into?

[Dan Hintz]

You would have to have a really fast decompression to cool the charge any significant amount (i.e., you'd fill the room with CO2 pretty quickly, unless you're exhausting outside), and it's typically the cylinder itself, not the charge, that cools down due to drop in pressure.

[Matt Turner (physics)]

If you go this route, make sure you have good ventilation in the room you're in. It doesn't take much CO2 to reach dangerous levels.

I think that any cooling effect you might obtain with the CO2 could be accomplished with higher air flow. As Dan says, it's the remaining liquid, not the blowoff, that cools. If you were able to extract the CO2 at a significantly low temperature, I'd be worried about cracking the air lines if you got them too cold and brittle.

[Kev Williams]

Nitrous Oxide?

[David Somers]

Ooooh! Kev!

Keith could change his business name from Aerodrome Accessories to the Giggling Pilot! The jobs might look terrible, but he wouldn't mind!! <grin>

Dave

[Dan Hintz]

Nitrous Oxide?

Putting any form of oxidizer within close proximity of a heat source (i.e., laser), particularly one in gaseous form, is a scarily bad idea.

[Dave Sheldrake]

Nitrous!

It would be interesting, .......for about 2 seconds. Foam and NO2 is used by the home brew rocket boys as Fuel.

Air is your best bet, lots of it at good pressure (30+ PSI) the lines on most basic lasers aren't rated for gasses other than air.

cheers

Dave

[Mike Null]

Here's a thread which might help. http://www.sawmillcreek.org/showthread.php?189588-Inert-Gas-Air-assist

[Keith Upton]

Thanks everyone. I use CO2 to air up my Jeep tires after being out on the tails. I know the gas exchange during that is pretty fast, but the tank, hose and valve stem all get very cold (tank and hose will freeze over if I'm airing up a lot). The Epilog Mini manual says the air assist is rated to 30psi max. I'm already pushing 25psi though it (what my airbrush compressor is rated at) and it's not giving enough cooling power to keep the foam from melting back together. My though was that CO2, even if only dropping the temp down 10° or so might be just enough. I could get another compressor (or run my large shop compressor down to the workshop) to pick up the 5psi I'm leaving on the table, but I don't think that is going to make a difference.

What psi are you guys pushing through your systems and what are they rated for?

[Dave Sheldrake]

40PSI normally at high flow rates (on a system rated at 150psi)

An airbrush compressor isn't going to hold 25PSI Keith, that may be it's rating but it refers to the peak pressure and doesn't allow for volume.

I run individual compressors on each laser at 16CFM / 125psi working pressure with a 70psi cut in pressure and just about stay inside a 50% duty cycle on the compressor to get 25psi flow.

cheers

Dave

[Ray Scott]

I think we might want to look at how to actually do the air assist option for the laser machine to control the air flow... and when to turn the gas on and off.

Rabbit Laser USA has made a nice tutorial for how to do the modification of EXTRA air assist. This is a rather nice DIY mod for those import laser systems.

If you want to add air from a high pressure tank, then you would need to add another regulator capable of that pressure.

Ray Scott

[Keith Upton]

40PSI normally at high flow rates (on a system rated at 150psi)

An airbrush compressor isn't going to hold 25PSI Keith, that may be it's rating but it refers to the peak pressure and doesn't allow for volume.

I run individual compressors on each laser at 16CFM / 125psi working pressure with a 70psi cut in pressure and just about stay inside a 50% duty cycle on the compressor to get 25psi flow.

cheers

Dave

Thanks for the info Dave. The airbrush compressor I use does not list a duty cycle. Instead, it has a thermal cutoff switch that will shut the unit down if it overheats. As long as it is running, it compresses and blows air. But it sounds like CFM might be more important than PSI for air assist??? The compressor I'm using is only rated at .25CFM @ 30psi.

Off to do more research...

[Kev Williams]

Kinda thought you guys would red-flag nitrous! :D

[Dave Sheldrake]

CFM and PSI are intrinsically linked, to get a given pressure of xx PSI through an orifice you need xx CFM to sustain it at that given pressure.

CFM on compressors is usually the swept volume of the cylinder(s) but FAD (Free Air Delivery) at pressure is the important figure.

cheers

Dave

[Richard Rumancik]

Keith, if you want to experiment you might try a vortex tube to cool the foam. This uses regular compressed air. You will need a decent sized compressor. They can be a bit on the noisy side. You might talk to some local precision machine shops for advice - sometimes they use them to keep parts cool while cutting if the tolerances are really high. They are a bit pricey new but it avoids the regulator and tank, and there are no expendables. Perhaps there are a few eBay deals.

I have no idea what it will do - you need to heat the material to cut it. I guess you are trying to reduce the heat affected zone. But since the whole volume is being cooled in the cutting zone, it might negatively affect cutting speed, so there is a tradeoff. Only way to know if it is acceptable is to try.

[Keith Upton]

Thinking about this more, I think I really need to cool the foam after it has been cut, not while it's being cut. So if I was able to direct the air flow behind the point of focus, I'd still have the heat needed to cut the foam, but the air hitting it just after cutting would cool it down fast enough as to not let it melt back together. Another way to do this might be to chill the entire cabinet. Teh use of a vortex tube for spot cooling or as part of the cabinet chiller is very interesting.

But all of that sci-fi stuff will have to come after testing to see if just a better compressor will do the trick. I've got a shop compressor up in the garage that I think is a 3 or 6 hp unit with a 30 gallon tank. I guess I'll see what I need to do to hook that thing up and give it a try.

[Richard Rumancik]

Thinking about this more, I think I really need to cool the foam after it has been cut, not while it's being cut. . . . .

That is what I was thinking would be optimum . . . but it might be hard to do that. You would need to cool it fast enough to have an effect on the heat affected zone, and it might be difficult to get repeatibility as you would need to hit it within a very short time frame. I have no idea of what kind of mechanics it would take to follow the beam some distance behind with cold air. Sounds complicated . . .

On the other hand, it might still work okay to cool the cut area as you originally proposed. Although you would have to use the laser to raise the material temperature back to melting/vaporization/degradation point, the extra few degrees of temperature rise might not be a big issue overall. And with the adjacent material already cold, you would reduce the speed of heat transfer to the maximum. So even if not optimum from a physics point of view it might end up more practicable to do it that way.

Try the simpler things first but if you do try some cutting tests with cooling, make sure you report back what you find.

[Matt Turner (physics)]

I wasn't sure whether to post this on the original thread or here, Keith, but have you tried dropping the frequency on your cut?