Good news / Bad news

Anything to do with air assist, vacuum tables, fume extraction and related...

Good news / Bad news

Postby Mark K » Fri Dec 23, 2011 4:33 am

First the good news. A 100 cfm bathroom fan, internal baffles in the laser cutter and a second hose providing makeup air from the outside will completely exhaust fumes and odors when cutting wood and acrylic.

The bad news. Sitting in the same room you do not notice a fire until the flames start melting the polycarb in the lid. I only turned around because the long job finished running and I was going to load another piece of acrylic to make the next Christmas gift. I could see the flames licking the lid, grabbed the extinguisher and blasted it into the box. I only had a dry powder extinguisher in my office, the halon ones are in the shop, so that made the biggest mess. I am not sure what started the fire. Most of the 6"x12" acrylic sheet was melted and had dripped down into the bottom of the unit. The grid was distorted from the heat, the air assist nozzle was destroyed and the lens was cracked. The compressor for the air assist sounds horrible after ingesting the dry powder.

2011-12-22_18-46-37_109.jpg
fire damage

Even though I do not run jobs unattended I am seriously thinking about adding a flame detector like this one. http://www.acroname.com/robotics/parts/ ... N-PKG.html. They are sensitive enough to find a candle on fire fighting robots. It could at least shut off the fan to slow the supply of oxygen and sound an alarm when there is a flare up.

Oh well, everyone is safe, and it was time to work on Laser Rev 3 anyway.

Happy Holidays
Mark
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Re: Good news / Bad news

Postby canadianavenger » Fri Dec 23, 2011 4:20 pm

That's terrible news... glad to see you caught it fairly early though, the damage appears to be limited. The fires do build up very quickly due to constant fresh air being drawn in by the fan, one only has to see my blog, and the laser I am rebuilding to see the effects of a fire that went for only a few minutes.

A flame detector is a great idea, and is on my list of potential upgrades for the laser once I get it going. Thanks for posting the link, it'll help get me started on researching possible solutions. I'm going to have mine kill the fan, laser, and possibly turn on a fire suppression system mounted inside the chassis.
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Re: Good news / Bad news

Postby lasersafe1 » Fri Dec 23, 2011 5:00 pm

Yes, terrible news. Won't the intense far infrared light from the laser falsely trigger a flame sensor? It says "high sensitivity UV", but I would think they mean IR.
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Re: Good news / Bad news

Postby canadianavenger » Fri Dec 23, 2011 5:35 pm

Optical flame sensors can be UV or IR based. Yes an IR based one would be a problem in this application. This one, however, is indeed UV based, which should eliminate/reduce false detections.
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Re: Good news / Bad news

Postby dirktheeng » Fri Dec 23, 2011 6:10 pm

wow!! I am so sorry that that happened and so thankful that nobody was hurt and the damage was isolated to just your machine.

I want to suggest that using a good PPI solution may help prevent this as well. I'm going to do some testing to be sure. I have noticed when I was cutting acrylic before (that is with a continuous on system) that I would get a sustained flame jet out of the bottom of the material as I was cutting. To sustain a flame, you have to continuously give it fuel. I have a feeling that using a PPI control will reduce/eliminate the continuous supply of fuel and keep the flame from forming. I don't know, but that is what my intuition is telling me. I haven't tested cutting acrylic since I built the PPI system on my laser. I will try some this afternoon and see if I get a sustained flame as before.

That really scares me! I will not be cutting acrylic without actually watching it the whole time until I know that I can either eliminate the flame or have a system in place to cut air.

Another thing we can do to reduce the risk is put in an inert gas air assist like nitrogen or co2 to lower oxygen content.
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Re: Good news / Bad news

Postby Mark K » Sat Dec 24, 2011 1:43 am

Thanks for the kind words and input, but I was really hoping that KyleAdam would let me know how to solve the problem. :D
The heating system flame detectors are UV because there would be many false IR sources in the furnace.
The whole family pitched in today to get the laser back online. Even though I only gave it a quick burst, there was extinguisher powder everywhere. Any bare metal parts had a layer of oxidation on them. I am not sure if it is from the fire or the extinguisher but it cleaned up ok. I turned a new nozzle from aluminium, the old one was plastic with an aluminium insert. I had another lens and all of the mirrors cleaned up well. The mirrors needed a major alignment. The table is beyond repair, I will have to replace it later.
I am still spooked by the fire and will probably hover while it is cutting. One of the halon extinguishers from my lab is next to it now.
Cheers
Mark
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Re: Good news / Bad news

Postby lasersafe1 » Sat Dec 24, 2011 2:04 am

I still have to wonder about that UV. It will be interesting to hear if it works. Considering the focused beam is much hotter than the surface of the sun*, it wouldn't surprise me if there are mechanisms of creating UV at the target.

*Correction: The photon energy of 10.6um light is only 0.1eV, which makes the "temperature" of the beam only about 5 times room temperature. The power density of the beam though can be kilowatts/cm2. Reference: http://www.laserk.com/newsletters/whiteCO.html
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Re: Good news / Bad news

Postby mikegrundvig » Mon Dec 26, 2011 12:52 am

Sadly, I think the fire sensor you are looking at is far too sensitive for this application. They can detect a candle flame from >10 feet away easily. Even the slightest flare up will trigger it. I'm not at all suggesting that this is a bad idea, I think it's great. I'm just not sure that any of the fire sensors used by the Trinity robot guys are the right way to go. Is a heat sensor set off a touch from the beam a possible option?

-Mike
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Re: Good news / Bad news

Postby mikegrundvig » Mon Dec 26, 2011 12:57 am

Ah, here we go. Found the link I was looking for right after I submitted my post:
http://www.sparkfun.com/products/9570

I'd imagine something like this detecting the temperature of the area around the beam might work well.

-Mike
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PPI Makes Acrylic cutting Much Safer

Postby dirktheeng » Mon Dec 26, 2011 6:25 pm

All,

I did a bunch of testing with acrylic cutting today with the new PPI system. I am 100% convinced that it is much safer. I could not sustain a flame on the underside of the cut as long as my duty cycle was low.

I did several tests. The first tests I did was to take the table out and secure the acrylic to the table frame. I tried to get a spontaneous flame by setting the PPI pulse width very high and the power to maximum and changing feed rates. I could not do it. I got a bunch of smoke, but nothing ignited on its own. (this does not mean that it could not happen, just that it is not likely).

That really confused me because I would almost always get a flame with the continuous on control and my old table. Setting the pulse width to something like 1s is the same as full on with a PPI control, so that should have done it. So I thought, well maybe the danger comes in when the laser is cutting and hits the table or something stuck to it. I put my new table back in (which is one of the aluminum light vents) and tried again. On a clean area of the table, I could not get a flame. Then I set up the cut to go over a piece of plywood that was stuck to the table from all the cutting I did for Christmas presents this year (I still hadn't cleaned up everything yet). Sure enough, I got ignition and the flame was sustained as long as the laser was on.

Next I set the PPI settings back to what I would normally use to make a cut, 3ms pulse width and some reasonable PPI and feed rate (like 400 PPI, and 200 mm/min). I passed over the same area and when it hit the wood, the flame ignited, but did not sustain after the laser moved off of it. Further the flame did not seem to be very hot as it did not produce any bubbling of the acrylic on the bottom like it did before (it seemed to be pulsating and not like a candle flame like before). I tried with both 1/8" and 1/4" acrylic, same results.

Moral of the story: when cutting acrylic use a non-flamable table material (like aluminum) and always clean any gunk off the table from cutting things like wood. Always clean up cut pieces that fall through the table as the laser can burn those too. Set up a PPI system either through EMC2 like Ben or by hardware like I did. Run the material at a slow feed rate with PPI to reduce duty cycle to keep flammable gas concentrations low and reduce heat input to the material.

PS: I ran 1/4" acrylic at 50mm/min, 1200 PPI, and 3ms pulse width and got some stunning cut quality results. Sides were nice and smooth and there were no bubbles from overheating the material, not even in sharp corners. The only distortion on either surface was very small marks from where the laser crossed a rib in the table. The piece came out with no effort and was so tight and accurately cut that it squeaked as I took it out. It was done in a single pass and there was no flame problems. I am continually amazed at the difference PPI makes in cutting. I spent an entire day trying to get the settings right with just "on/off" control and I couldn't even come close to these results, especially with one pass. Granted the feed rate is slow so it takes time. That said, I couldn't even think about running the laser that slowly without PPI as it would just melt/bubble the plastic. To get good results with PPI, raise the PPI, lower the pulse width, and lower the feed rate.
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