Hello! New owner here! We have the Glowforge Pro and we tried different settings but none seems to work. We have checked other comments and information in the community. We definitely can cut the mylar but the problem is that it creates a little bump at the edges of the are that was cut. We need to have a clean cut to be able to use them. Our mylar thickness is 6 mil. We really need help on this one… we have been trying for a couple of days and nothing seems to work. 
We feel we spent to much money for a machine that wont work for our needs.
With every material, it is necessary to test to get the result you desire. There are several test templates in the Free Files section. I suggest using one of those and keep testing in addition to using the set focus tool. It sounds like you are close to getting what you want - try changing the speed or power just slightly.
for example.
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Before I get started: This is a thin synthetic material, ues caution when cutting it more than most other materials. Thin stuff is more prone to fires than thicker stuff, just keep an eye on it at all times more than you might with hardwoods or other thicker materials.
OK, now that that’s said:
Under normal circumstances I would walk away after sharing the testing post and tell you you are on your own, because for most materials it’s a simple process.
However, you’re doing mylar, which is both synthetic and also very thin, and that’s where settings can get really tricky.
Conventional wisdom says cut hot and fast, you get the best results for most materials if you are at full power and as fast as you can go and still get a good cut… but in this case, you are also seeing the effects of overheating and possibly overburn on corners more than some other materials might show.
To that end, I might try to find the power that matches the speed you want to hit – it’ll probably be in the middle. Too high and you’ll have more overburn effects (e.g. melted corners or start/stop points), too slow and you’ll get more thermal effects (e.g. melting or warping) as the laser has more time to heat up the surrounding material.
So.
If it were me, I’d shoot for 2 passes at about 250 speed max. I’d make my test template cuts all have 2 passes at 250 speed, and then vary the power between 20 and full in 10 point increments, see what you get. What you’re looking for is the cut that succeeds consistently and gives you the best edge quality. Why two cuts? Well because this is a fragile material, and you’re concerned about edge quality and melting, settings that barely cut through with 2 passes will be much gentler on the material than doing it hotter in one pass.
The low end of 20 is a wild guess really, for all I know even 20 might be overpowered, but it’s worth trying and seeing.
If all else fails, you might be looking at needing to engrave your stencils, like some people do with paper. Take a look at “engraving paper” to find out what I’m talking about.
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Thank you… we are trying those metrics right now 
Be sure your optics are clean before you start your test. You don’t want to skew the results with dirty glass
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Thank you… None of the options works… I have seen so many companies using laser cutters for stencils but this machine does not seem to work. We have tried literally everything… We are very frustrated because of the cost of the machine…
I cut 10 mil mylar at 500 speed/80% power. (Smooth edges for that.) For 6 mil, I’d start with about 60% power and work up from that. But keep the high speed - Dave is right. A few small tests are always a good idea on new material before you commit to the final product.
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Many people use the machine successfully for stencil cutting and have shared their results here. It’s either your settings, or choice of material.
I’ve been using my Glowforge for almost three years. I still have to invest a little time to determine settings when I start working with a new material.
Perhaps you could try a different material for the stencils. I have had good luck with Lucent from Johnson Plastics Plus. https://www.jpplus.com/lucentcollection They also have several other products for making stencils. I know you have invested a large sum of money in the Glowforge with the hope that it would make your production more efficient. I hope you don’t give up yet. There are many here in the forum that will try to help you dial in successful settings.
It might be helpful if you would share a photo of the failure (the bump) and give a little more information regarding the size and intricacy of your stencils.
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