Welcome! This forum is a great place to get information and feedback on ideas you may have.
TL;DR My suggestion is to stick with PLA unless necessary. PETG-CF was easier/better than HTPET. PA6-GF is hard to print, but may be a rigid option if your printer can print at 300℃.
I’m in Arizona, so my garage does get to about 115℉ (46.1℃), and when I first built my MPCNC I worried quite a bit about sag over time. So I printed that machine from HTPET (Fusion Filaments.) It prints at a high temp (280-300℃ ish on my home-built printer.) I thought is worked fine, but the rigidity of the machine was always a question on my mind. There were a lot of factors, so I’d say the HTPET was “fine.”
A few months later I was facing a table rebuild, and I decided that the LR3 was a better form factor, so this time I chose PETG-CF. It also appears to be “fine.” I wish I had the rigidity of PLA, but I worry about the stepper mounts during long cutting sessions in such hot weather (the stepper may have trouble dissipating heat and may run hotter.)
While I was working alone in my shop (on my LR3 table) some knucklehead snagged the loose gantry on his pants and it fell about 3 feet onto the concrete. The core cracked across the layers near the top bearings. I felt like the the layer adhesion was good, and I had used 3 x 0.6mm perimeters, which was insufficient to withstand the drop.
So, I’d recommend using PLA first, even if it’s to get an idea of how the machine performs according to spec. But, in my extreme climate, I felt like PETG-CF was a better option, since it printed easier, and seemed a little more rigid than the HTPET. The main area than I struggle is with the nut traps. I struggle with the nuts spinning, so I’ve started epoxying them in place. This works “fine.”
I have started experimenting with Polymaker PA6-GF (I was too cheap to get CF.) The nut traps work pretty consistently! I am focused first on the stepper motor mounts, since I feel like those are the main concern in the heat. However, (of course) I tried printing the core. It appears that once the heat from the bed is sufficiently far from the newly printed layer, slight curling begins to occur, which leads to print failure. So I’ll hold off on the core until I have the other parts printed and have used my LR3 to build an enclosure. It “feels” more rigid to me (and my fingernails), and I think the stats support it, so I did use it for Doug’s “hidden Y mod” where the idlers are extended. Might be placebo?
Note - I once got in my Truck and the dashboard said the exterior temp was 137℉ (58.3℃). I don’t believe it was that hot outside, but I did want to have the option to move my table out onto the driveway for big projects that may be awkward in my garage. At least I use that scenario to explain why I’ve burnt through so much filament in the name of heat tolerance… 