Optimal air assist configuration... to prevent fire!

[Bob Crimmins]

Until now, I have been using the aquarium pump that came with my 80W CO2 for my air assist. It's been ok-ish but the air flow has always seemed week and I've never really been happy with the smoke/residue trail left along the kerf. So I setup a direct connection to my shop compressor, including a lever ball valve to control flow. Now I get plenty of air. So... on my very first cut session following setting up this new improved air stream, the 3/8 acrylic I was cutting caught fire and melted my drive belt. I've been cutting the same 3/8 acrylic for over a year and never had a flair up. So naturally I'm feeling like all that extra air was a contributing factor. Needless to say, I only want to replace that dive belt one time so I'm asking for help. Is it possible/likely that extra air flow "fanned the flames"? If so, would more or less air correct the problem? Is there some pre-treatment of the I need to do? I have read that I should install an air dryer in-line, which I will do, but can moist air cause this sort of thing? I thought that the air dryer was just meant to protect the lens from accumulating moisture.

That for the help!

[Dave Sheldrake]

The small oiless compressors that come with the lasers have a tendency to "pulse" in-line with the diaphragm, this in part is what prevents flame outs or fires, a shop compressor providing a steady stream will usually have to operate at a higher pressure to achieve the same results. The nozzle on the Chinese machines doesn't lend itself very well to providing *air assist* in reality unless you extend the nozzle and reduce the hole exit on the cone.

Easiest method is a baby bottle / feeding bottle teat, cut it off and place on the nozzle to get both a smaller hole and closer to the job without the usual problems that aligning a metal nozzle will have.

cheers

Dave

[Bob Crimmins]

Thanks, Dave. That's a very clever idea. I'll give it a try. If you have a pic of that in action I'd love to see it. I suppose I should just attach the nipple and let the laser cut it's own hole, right? Will that minimal hole be big enough for sufficient air flow or should I make the hold a big bigger than the exit hole that the laser cuts on its own?

[Scott Shepherd]

I don't how good you get it, cutting acrylic is dangerous. Not "it's going to explode" dangerous, but it's 100% "Don't go to the bathroom while it's cutting acrylic" dangerous. We have co-axial air assists on both our lasers, so it's blowing air right down the cut. I believe the air flow under the piece has much more impact on flames than the air going through the cut. If you have little vortices (did I spell that right?) pooling in various places, and the conditions happen to be right, then it's going to catch on fire. It's the combination of the air through the cut, the air across the top, and the air across the bottom that have to be managed.

I watched thick acrylic cut for 100's and 100's of hours, just starting at the cut, watching it, trying to figure out what causes flare ups. I've tried high pressure, low pressure, medium pressure, and I have yet to find the one thing that fixes any of it. I can minimize the risk by controlling various conditions, but acrylic is just one of those materials we watch with a sharp eye.

[Dave Sheldrake]

302526

just like that :)

I use a hole punch to make the hole about 1.5mm

cheers

dave

[John Noell]

Dave, how many psi or kps can you push thru that without either blowing the nipple off or the surgical tubing connecting to the nose cone? I plan to use a Gast wth my Shenhui like I used to use on my Epilog, with a regulator set to 20-25 psi / 138-172kp. Does that sound right?

[Dave Sheldrake]

You won't need the pressure John, the smaller exit hole reduces and directs the air to where it needs to be, you won't get laminar flow but you will get much less staining . I run them at 8 - 10 psi through a 1.5mm exit hole and it works well.

302531

pretty much zero staining at 180 watts

cheers

Dave

[Rich Harman]

I run my air assist through a 1/8" tube adjacent to the nozzle a little over 1/2" from the surface - so that it can pass over the magnets that I use for hold downs. I have never had a top surface flare up with the air assist running. I do occasionally get flames on the bottom when the beam hits a nail (I use a nail bed half of the time), it is the accumulated gunk on the nail that sparks a flame.

I use the tube type air assist so that there is no possibility of water on the lens, plus the lens rarely needs cleaning now that the air isn't going through the nozzle. If I didn't need the magnets so often I would try Dave's nipple idea.

[Bob Crimmins]

Nice. How far off the surface is the nipple at run time?

[John Noell]

Thanks Dave! Is that enough pressure to blow stuff out of the way when cutting acrylic? (MDF is just about impossible to source over here. Acrylic is stocked locally.)

[David Somers]

John,

I can't imagine MDF in Fiji! I would think it would behave like those highly compressed dehydrated sponges you can buy. Thin as a sliver till you drop em in water and then they turn into full size sponges. I picture mdf doing that in the tropical humidity! By a 3/8 inch sheet of mdf, then come check it in a few days and it would be about 4 feet thick! Yeeks!! <grin>

Dave

[Dave Sheldrake]

Yup works fine on acrylic, it's around 5mm off of the job in that picture but even if it hits a tipped part it just pulls the silicone nozzle tip off rather than damaging the machines :)

cheers

Dave

[Matt Turner (physics)]

How much air do people use when cutting acrylic? The recommended settings from Kern have us run at <10psi, or <5psi for their "flame-polished" settings.

I've never had a real fire, but our laser does let us run at much higher speeds than most. I did have a little flareup once, when I'd spaced some acrylic off of the honeycomb with some cardboard pieces and accidentally put one under a cut. The airflow from the vacuum hold-down through the cardboard corrugations must have been just right to fan the flames. The back side of the acrylic got a little toasted in that spot . . .

[Dave Sheldrake]

As little as I can get away with Matt, it tends to cause rapid cooling and striations on DC lasers although the PPI ability of RF's seem to reduce this pretty much to zero. Diameter of nozzle outlet makes a huge difference (to velocity I believe, but fluid dynamics isn't my strong point)

On the Chinese machines with a 5mm nozzle hole 20+ PSI, on the Mitsu with a 0.75mm exit...5 psi max or in the case of metal cutting 150 psi max for nobels and 80 psi for Oxy.
The bottle teat mod needs about 5-8psi if the exit hole stays under 2mm to get decent results but most of that is due to the nozzle being effectively closer to the work.

BOC have a decent technical reference for gas pressures on big industrials but the principles are the same (Laminar flows etc)

https://www.boconline.co.uk/internet.lg.lg.gbr/en/images/laser-cutting410_39553.pdf

page 10 is useful

cheers

Dave

[David Somers]

Dave,

Are you using the laser to cut the hole in the nipple end after it is in place? Or are you cutting it with something else before mounting?

My wife is going to express great consternation when I show up at the house at age 57, carrying a bag of baby bottle nipples. <grin>

Dave

[Dave Sheldrake]

I normally use a hole punch but you can do it just as easily with the laser, just be careful if you do it with the laser the existing hole may be too small and the nozzle blows off before the laser opens up the hole.

cheers

Dave

[David Somers]

Ooops! Just saw where earlier in the post you talked about using a punch hole to do it. Sorry! Should have read more carefully!

Sorry!

Would burning the hole with the laser work if the air pump were off long enough to punch the hole? Or is it the pressure of the protons pushing against the inside of the nipple forcing it off? <grin>



Dave

[Bob Crimmins]

I haven't done this yet, but my strategy is going to be:

1) Affix latex nipple, secure with zip tie.
2) Fire laser through the end of the nipple to mark the exit path of the laser.
3) Heat up a 4 penny nail to red hot and pierce/melt a hole in the nipple at the exit hole.
4) If more air flow is needed then I'll heat up the next sized nail until I get it right.