by 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.