Power lowering during long cut jobs

[Rodne Gold]

Well,if a mirror overheats it can distort and the laser loses alignment and I have heard of lenses that have a hairline crack that reveals itself and interrupts the beam as it strikes the compromised lens and heats it more

[Giacomo Cheslaghi]

thanks for the infos Rodne!
when you talk about distortion of the mirror are you talking about a temporary status or a real deformation?
Cause when I engrave in raster mode (where the temperature is lower, I know) my laser works perfectly..

[Rodne Gold]

Temporary - when it cools it relaxes. The mirrors will have to be damaged or dirty to allow them to overheat tho.
To avoid the flashbacks don't cut on the honeycomb , put a piece of anodised aluminium on the bed/table (it absorbs laser) and elevate the perspex on blocks or pieces or perspex so its off the table.
If you can expose or open the tube chamber the tube on your machine , allow a very strong fan to blow on it after cleaning all the heat fins /fans of dust (with a dry paintbrush or canned dry air) , if it still loses power when being "supercooled" , then its most likely nothing to do with tube temperature