I had seen @dan mention the problem with knots in plywood. And today got a lovely example of it (pretty sure at least).
I tried to replicate a turtle from online with my laser cutter. This worked fairly well (Copy image to Photoshop, Magnet Lasso tool, solid fill, copy that layer to Illustrator, trace to vector, stroke, done). But when I sent it to my cutter, I had the problems arise.
Where you can see the extra smoke damage on the left near the mouth I had observed some significant flaring as the cut happened. At the time, no idea what was happening, I assumed I was over one of my holes under the honeycomb and the mixed air drafts were making things look bad, but not changing the cut.
But this was the other side of my poor turtle. Those spots where the trace fails (laser didn’t penetrate) are precisely where the flaring happened. My assumption is: a knot in the mid-layers.
Now… my laser doesn’t have a lovely alignment system. In lifting the wood I am pretty much guaranteed unable to return it to where I had it. Otherwise I would just modify my initial file to only cut those particular areas, and run it again. Instead I will have to use an Exacto Knife or something to finish the cuts.
So… you certainly want plywood free of knots. Even if you do put your settings significantly overkill to penetrate knots (tossing away a fair chunk of extra laser life in all other areas), there is a lot of extra scorching where knots arise, which changes your end appearance. This of course assuming I am correct in declaring this problem to be mid-layer knots. Could just be that my laser has one more calibration issue to solve.
I did go back and adjust the location to cut a new one, just so I would have a trinket to take home to the kids.