Glass transition temperatures: not what they seem  

If anyone has used a filament that is hydroscopic without baking it first (relying on it coming pre-dried, or at least hoping that it is), then baking it after that initial usage and using it again, you may have noted that the temperature settings you dialed in initially now seem to be slightly off, and you find yourself tweaking things to get results as good as you got originally. Maybe you bump up the bed temperature 5-10 degrees to get the same level of adhesion? Well, this is because even baking the filament at a seemingly harmless 60°C for a few hours or whatever had a very real impact.

Now imagine what happened to a filament during its journey on a shipping truck to you....

This is why 3D printing is, and will probably always be, so fiddly. It is almost impossible to ensure true consistency between two spools that lived different lives. So, it is actually a good idea to buy multiple spools from the same location at the same time (at least if it is the same material on each spool) because anything that happened to one of them probably happened to all of them, and so you'll not have to fiddle with anything when changing these spools.

Now, most of the time, the differences are not significant enough anyway, but I think anyone who has been printing for long enough hasl experienced at least one 'weird spool'. Well, this is why.

But Nylon's Tg is below even PLA's! Shouldn't it be a goopy mess at such temperatures?

Nylon 'transitions' about as fast as a sloth on ketamine. That's dead. Very very slowly. So slowly, in fact, that you might be a bit surprised to learn that Nylon (depending on the exact application and alloy/blend) has a service temperature that, at the lowest, is just above PETG, but for the most part, exceeds that of ABS. By service temperature, I mean the temperature you can use a nylon part, like a gear. Meaning, this is a temperature range where you can expect Nylon to retain sufficient mechanical rigidity and strength to be in active service. Not only is Nylon a very fine alternative to ABS' temperature resistance, many blends of it are superior in this respect to ABS.

If you don't believe me, try it yourself. If you have a blow dryer or heat gun, find some nylon thing you printed, and heat that sucker up real hot. Way way above 45°C. It will feel as rigid as ever.

Tg is almost useless as it is defined for materials like Nylon. Sure, above 45°C, it begins to undergo some vague mechanical changes that below that temperature, it wasn't. But for our purposes, it doesn't matter, and won't matter until a much much higher temperature. Beyond that, it has no 'transition'. It doesn't have any sort of sharp, 5 degree gradient where it goes from rock solid to wet noodle.

Just to illustrate how significant this is, let's take Nylon's slightly higher Tg form, Nylon6/6. It's the nylon with a Tg of about 70°C. Despite this, Nylon6/6 has a service temperature of about 199°C. Yes, it will still be very much rigid and mechanically sound (albeit with some loss of strength - but come on, it's nearly 200°C, cut it some slack!) to nearly twice the temperature of ABS. But you'd never even guess this if you took Tg numbers at face value.