The following pictures show the same as the top view but are looking at the bottom of the cut. The order and direction are the same.
- Bottom 1
- Bottom 2
The following shows the cut depth as a function of feed rate:
- Cut Depth Chart
The following shows a table of the quantified results:
- Results Table
Discussion:
In general, PPI did a much better job at controlling the heat distortion effects in the materials cut for the same cut depth in about every metric possible. It turns out that one of the best indicators of heat distortion was looking at the bottom of the cut pieces after letting them sit a little bit. The acrylic scaps I had laying around to do this testing were extruded, not cast, so they stress cracked much easier. When a lot of heat is transferred to the material, it melts and when it solidifies it shrinks, just like metal in a weld. That shrinkage causes stress int he material which is relieved by cracks. When looking at bottom, the white blob-like things (great technical term there
) are those stress cracks. It was kind of neat to watch them form. The stress cracking was the worst on the left side of the cuts when the heat was the highest. This was because the heat would cause the sides of the materials to melt and it would begin to flow. The laser traveled in the direction from left to right so as the cut progressed, the expanding gasses and air assist pushed the melted material down the cut face, across the bottom and up to the left side where it re-solidified. You could watch it flow during the cut... also kind of neat to see.
In terms of bubble formation, the PPI was cleaner as well, especially as you approached depths deeper than 10mm (which is really the practical limit for acrylic anyhow). At 6mm, things were clean for both methods. The distortion of the manufacturers edges was worse at all depths without PPI. The kerf widths for 2.5 and 3ms PPI seem to be narrower than for on off control (I will cut some parts out to measure their dimensions with my micrometer and that will give a much better indication of kerf width), but the 5ms seemed comparable. Melting the material and the flow of it during the cut is (I think) what leads to the "long order" waviness of the cut. That is most evident by looking at the bottom of the cuts with the non-PPI cuts that were deep (last 2 in particular). You can see the cut that went all the way through the 25mm material is not strait at all at the bottom.
As mentioned, the real practical limits of the system is about 10mm. The cuts that made it to between 9 and 11 mm were No-PPI F100, 2.5ms F25, 3ms F50, and 5ms F100. Of these, the best result seemed to be the 3ms pulse width PPI. It had no bubbles and no distortion of the sides. The amount of stress cracking from melting was comparable for all of them. The sides of the 5ms seemed to be a bit smoother than the 3ms. However, the kerf width was the narrowest with the 2.5 and 3ms. The non-PPI cut started to exhibit definite long order waviness near the bottom, whereas the others didn't. The kerf didn't bend as much with PPI either.
In terms of 6mm depth, any of the PPI solutions seemed to beat the Non-PPI. None of the PPI solutions showed any surface distortion, and the 5ms showed only 1 small bubble I could see with my eye. It seems that you could set the surface finish you wanted with PPI as well... more PPI = smoother. The non PPI 6mm cut had more heat in it and showed slight distortion of the surface, had some stress crack a the bottom. That said, the heat did polish the edges nicely. The 2.5ms kerf width was the narrowest. None of them had much stress cracking at the bottom.
Just a note here: the stress cracking wouldn't really be expected to happen if you were to cut through because the melted material would not collect in the cut. However, this probably correlates to how much you would expect to melt corners in cuts and possibly re-bridge the kerf. The actual effect would probably be less in a real cut because you are sweeping the surfaces with more flow of cold gas from the nozzle, but it is comparative anyhow.
Finally, the cut depth seemed more consistent in terms of max/min with PPI than without. Not that this is really an important thing for a lot of applications, but it does show that you have tighter control of the laser with it.
All in all, the PPI solution allows a user to have a greater amount of control over the cutting process and gives quantitative means for a user to control the quality and kind of cut you want to get. I would expect that this would be more uniform from user to user as well given that we don't have to fiddle with a knob setting (that said you could use PWM as a digital means to set power as well). PPI gives users more quantitative "knobs" to play with to get a desired result.
Finally, it occurred to me that the real power of PPI may be the ability to dial down the laser for control of cuts with thin materials without having to go to really excessive feed rates. On these systems, one really doesn't want to have to vector cut at speeds above 400-600 mm/min because mechanical problems like belt stretch and backlash can begin to show up in your cuts. The minimum cut depth of the laser without PPI was 3.69mm at a speed of F400. I couldn't turn down the laser any farther than that, so I would have to increase my feed rate to cut with less energy. The quality of cutting things like veneers, paper or thin plastics is likely to see larger gains with PPI than thicker materials. A user could decrease the pulse width below 2.5ms or simply turn down the power knob to get less power for those materials. Looks like more tests are in order.
Does anybody have any tests that they would like to see?