We have been making rigid, re-usable face shields on a glowforge from 1/16" acrylic. We are donating them to high exposure folks around the country. We have made a few thousand to-date. Time lost on focusing and scanning every time is worth minimizing. Speed matters so, despite advice, we have been trying to figure out if we can get decent results, faster by cutting multiple stacked sheets of acrylic at a time without welding them together. It appears as if parchment paper between the layers was the magic we needed. It’s working well. It seems that multiple passes is a good idea. If others have advice, we would be thrilled to figure this out faster with your input. Right now we are cutting a stack of 4, each 1/16" thick at full power, 145 speed, 3 passes. Thanks. - Luke and the DIY Face Shield Team
Noooooooooo! Don’t want to rain on the parade, but most of the completely totalled machines happened from acrylic fires.
Stacking materials will void your warranty if you set fire to your machine. You would have to pay for any repairs.
(And if you do continue doing it, never take your eyes off of it.)
Hmm if you’re trying 3 pieces in three passes, it seems that the bulk of your time savings is in setup?
Clever trick. I’d say just watch it carefully and you’re fine. Acrylic is nasty if it catches fire, but sounds like you heard that already.
How? If I want to cut something that’s 1/2" (advertised capability) I can’t put it on the crumb tray. I’m told by GF to put it on several pieces of wood or draftboard or the like so that it comes up above 1.5" from the floor of the GF.
So if I do that and I cut thru (say balsa wood or some other easy cut wood) and it hits the PG Draftboard I’m stacked on, does that void the warranty?
This is similar to the defying-physics advice around magnets - they tout the steel crumb tray (most lasers use aluminum honeycomb) which is magnetic & initially said to use magnets (Dan) but then they backtrack and say that you should flatten the material using pieces of scrap PG. Scraps of PG don’t weigh much and are physically inca[able of weighing down a piece of warped PG.
In this case it’s stack it to get it in range of the lens focus area but btw, your warranty is now voided. I don’t know that any FNL or even UnFNL would be able to make that argument with a straight face or prevail.
An acrylic fire cutting stacks of 1/16" acrylic would not have the GF materials warranty because it’s not PG and GF doesn’t sell 1/16" acrylic. It’d also not be warrantied because it was a fire and they tell you to watch it every minute so catching on fire is on you. There’s nothing special about the stacking that voids the warranty - it’s the acrylic fire which may result which based on the OP has not over thousands of cuts. Might it? Sure. But it might also blow up due to a power surge but we don’t warn people not to use full power.
I don’t stack because when I have done it (on the Redsail, no GF warranty violation involved) it was hard to keep the cut pieces from welding themselves to each layer. I never tried parchment paper before though. Looks like they’ve found a possible solution, although to @evansd2’s point, it only saves setup time since they’re still doing 3 passes. Since I do 1/16" acrylic at 195/100 what with the parchment paper layering and the slower speeds on each pass, I expect my throughput is as good as theirs so I won’t bother but it is an interesting technique.
It’s proscribed in either the User Manual or the Terms of Service. (Or it used to be…I’ll admit I haven’t had time to read the latest versions.) 
It’s always been a bad idea to stack materials. Any air in between them can feed the fire. (Like corrugated cardboard…the air in the chambers causes more of a fire hazard, so you have to be more careful cutting it.)
I’m not saying not to do it if you want to, just be aware that it likely voids the warranty if you set fire to the machine. The risk needs to be understood by the people doing it. They can decide what risk level they are willing to assume.
I just wanted to follow up on this with folks who might be interested. Thank you everyone for the caution. I hear you loud and clear.
We have been keeping a really close eye on these and continuing to have solid results. To be clear, the results are not quite as good as one-at-a-time cutting. Also, we had a significantly lower failure rate when we went to 3 stacked instead of 4. Yes the primary savings is in setup. Also from a workflow perspective it just works better to have 3 shields take 7 minutes instead of 2:20 per shield. It saves on the rescanning, focusing time. It also makes it so you can have 7+ minutes to do something else with your hands while your eyes watch the cutting. That means less time wasted on task switching.
One more caution note: it seems that if you want to do this thing --which you should not do – make sure that things are well secured. Air gaps between the stacks are the places where burn marks show up like folks have flagged above.
Thanks all.