LAOS Laser

Or etching, or whatever it's supposed to be called...

(e.g. using a font in inkskape and getting the cutter to engrave it as opposed to cut it out)

I'm making up some keyrings for my wifes business and noticed some "fogging" of acrylic when I'm engraving stuff. If I leave the backing film on, then it's better, but then the film seems to melt itself into the etched surface and looks a bit manky, so I remove the film. I've tried with air assist on and off and neither seems to affect it - also tried peeling the backing off and washing the surface with some dilute soap to remove static (a tip I got from a company who make/etch 1000's of things a day on BIG machines), also tried cutting first, etching 2nd.

It seems to be to just be at the ends of each pass - my guess is that the head is slowing down, then speeding up - so over etching might be an answer (ie. put a dummy character outside my main area to keep the head moving) but that wastes material at each end and slows it down somewhat.

I'm guessing the fogging is vapour that's settling back onto the surface - a bit like the "scorching" I'm seeing at the edges when cutting ply...

If you look at the bottom row of this image: http://unicorn.drogon.net/IMG_20140816_181715.jpg then you see it at each end, but the 2 ones in the middle are perfect...

(that's bi-directional engraving)

So does anyone have any tips to reduce this?

cheers

-Gordon