Issues when cutting thick plexiglass

Hi
I have a 100W chinese laser cutter.
I’m having issues when I want to cut thick plexiglass. Thick means 1cm.
When I cut it, I use the following parameters:

  • speed : 5 mm/s
  • power: 65%

I put the plexiglass on spacers (5mm thick) so it doesn’t lay directly on the cutting bed (which is quite dirty, due to many MDF and plywood cuts). The laser get through the plexi, but I get burning stains on the lower side (I’ll show a picture very soon).

I believe they are caused by some gas created by the laser and pushed down by the blowing system, and which take fire underneath the plexi.
Do you think it is the correct explanation, and how can I avoid this?

20210319_183740

When you cut acrylic you are basically vaporizing the plastic. The resulting vapor is very flammable and it’s common for it to periodically ignite under the acrylic. Does the acrylic have masking on when you cut it?

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Actually no, the example I showed did not have the protective plastic sheet on the face below. For this test, I can try to keep it.

But I originally wanted to engrave on both sides, and I need to remove the protective sheet when engraving. To do 2 sides engraving, I first engrave on side A (so no protective sheet on side A) and cut a surrounding symmetric form which enables to put the form back at the exact same place upside down. Then side A is down and there is no protection. I engrave side B and cut the final shape, which makes stains on side A…

From what you told me, I think that I’ll have those stains whatever the elevation of the acrylic over the cutting bed, right? What if I had a hair blower (?) to create a horizontal air flow underneath the acrylic? (although my wife wouldn’t like that…)

You could add your own masking after engraving and before cutting, I would think:

Thanks, I guess that’s a good solution for you guys who live in modern civilized countries. But I’m not sure I can find this reference here in France…

Can you explain what kind of material it’s made of ?

@NedMan can you tell whether this is laser-compatible?

If so it’s available on amazon.fr as well:
https://www.amazon.fr/transparent-dalignement-silhouette-autocollants-fenêtres/dp/B07KPB38LK

It says it made from vinyl so it that’s the case then it’s not laser compatible. I haven’t run across a clear transfer tape that’s been compatible. The laser compatible ones tend to be paper backed.

The transfer tape I referenced, in the post Michael linked, is a paper backed vinyl lettering transfer tape.

There are some plastic food wraps like Glad Press’n Seal that could work as it should stick fairly well to acrylic.

I also seem to recall someone once talking about using a thin coating of liquid soap on acrylic as a mask and just rinsing it off afterwards.

You can certainly try adding air flow under the acrylic. I suspect that may help.

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Thanks for your help. I’m now trying to cut, but I keep the protections on both sides (no engraving for the moment)

New issues: it seems that the cut path is correctly cut (the laser passes through the whole thickness), but the vapors seem to cool down and stick inside the path so the piece does not get correctly cut. (sorry not easy to explain)
As the cut speed is low and the power is high, and the cut path is quite long, the cutting time is quite long too (7 minutes). Although I put the blower on, the piece is not cut at the end.

Should I try to increase the speed , or lower the power and make several paths?