Flash back? What causes it?

Hello all,

The photo is the rear of two items cut from the same sheet of wood with the same settings from speed and pew. I had already ran several parts with no issues at all but the last 4 parts all had similar marks on the rear side.

What can I do to stop this or minimise this? Thanks in advance.

If any air gets in underneath the material, like at edges or next to other cutouts in the board, or if the material is warped, you’ll see an increase in flashback in those areas.

To minimize it, leave a decent amount of space from the edges and other cuts when placing your design. Make sure the material is pinned down flat in the cutout areas to not let air underneath. And flashback is one of the reasons that the Proofgrade materials come pre-masked. You won’t destroy the backside of the cuts, just the masking. You can apply your own paper masking, or apply a masking tape to the bed underneath the cuts to keep that from being quite so bad.



Excellent, thanks Jules. I use masking on the front and also magnets, hold down pins on non proof grade material but obviously not well enough on the last few cuts. Thanks again.

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In addition to what Jules said, even slipping a sheet of cardstock underneath will probably stop the bulk of it.


Good information. Thank you. Every days a school day.

This cannot be stressed enough not to let any air under the material, I had my first fire in the unit , my saving grace was I was cutting on top of piece of acrylic . So what happend was the magnet fell off , and just enough air got under and poof.

Thanks Jules

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I’m not sure I understand (and pardon my ignorance) - if material is down on the GF honeycomb bed, isn’t there a lot of air underneath?

There is a bunch of air, but it’s fairly compartmentalized and doesn’t flow well. Introducing extra space gives you significant air that can flow and that’s where you run into trouble.

This is partly why corrugated is so risky to cut compared to other things. There are air channels all through it.

It should be pointed out that some of this is theoretical to me, the actual physical processes that occur when lasing are complicated (a mixture of combustion, ablation and probably sublimation, combined with complicated heat conduction in your material and airflow… plus anything my wee physics brain can’t think of right now… it gets pretty muddy to model), but experimentally speaking, air under your materials is a fire risk. Fire needs a constant supply of oxygen so the theory is definitely fitting the data. Maybe someone who has done more formal research about it can explain it more thoroughly.