Glass transition temperatures: not what they seem
 

Glass transition temperatures: not what they seem  

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metacollin
(@metacollin)
Eminent Member

Glass transition temperatures, or simply Tg, is something anyone doing 3D prints is probably fairly familiar with. I understood the Tg of a given material to be the temperature where it sees a sharp loss in rigidity, and could generally be described as 'melty' (though, it isn't really molten yet, that occurs at a much higher temperature). I understood Tg to be a property of a material. You know, like density or the melting point of something. Tg is surely a property specific to a given plastic, or blend of plastics, and is something that we measured empirically and now it's just a number in a table of material properties.

This is definitely how Tg seems to be universally treated within the 3D printing community, including by the manufacturers.

The problem is that everything I just described as what I understood the glass transition temperature to be was wrong.

And, for a lot of you, it doesn't actually matter. But anyone who prints in stuff beyond PLA, or especially anything more exotic than ABS like PETG, Nylon, ASA, etc., you may find the rest of this post fairly useful. It's a bit of a read, and it isn't going to do anything life-changing for you, but I think some will find it interesting. It may help solve a few small mysteries at the very least. 🙂

Anyway, enjoy (or if this is boring as bat shit to you, there is a TLDR at the bottom 😉 )!

The down and dirty of glass transition temperatures:

  • They're made up. They are not a property a material has, but rather something completely artificial that we, well, made up. I think wikipedia puts this the best: "Different operational definitions of the glass transition temperature Tg are in use, and several of them are endorsed as accepted scientific standards. Nevertheless, all definitions are arbitrary, and all yield different numeric results". So, like the cake, Tg is a lie. More importantly, it means that any mention of a material's Tg that doesn't specify which definition for Tg is being used should be taken as a fairly vague and approximate thing, as the Tg you're interested in might be 5-15 degrees different from this number. This is why the same plastic from two different manufacturers can often differ on just about everything related to temperature by ~5 to 15 degrees or so.

  • Plastics are incredibly hysteretic materials. That is to say, they exhibit significant hysteresis, which means that the history of the material determines its current properties. Or, another way to look at it is that plastics/polymers 'remember' everything that has happened to them. Mechanical strain, the temperatures it has been at and what sort of rate of temperature change it saw, how quickly it was melted, how fast it cooled, all of these things will effect its properties. Including...no, especially the somewhat arbitrary concept of the glass transition temperature. This is readily apparent: the plastic on just about any spool will 'remember' how it was originally extruded and the mechanical strain it was put under. We see this as the 'curl'. The filament want's to be curved, not straight, and that is because of hysteresis.

    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.

  • Glass transition temperature behaves wildly differently for some materials. We can again thank the arbitrary nature of Tg for this. Case in point: both PLA and ABS exhibit very sharp changes that truly deserve to be called 'transitions'. The exact temperature it occurs might have a bit of variation, but they both go from rigid to doughy goop with a mere 5 degrees difference between the two. The change is sudden and quick. However, Nylon is a completely different beast. Taulman will list a Tg of 45°C for Bridge, and recommend this temperature for your heated bed. This is, frankly, incorrect. Speaking from personal experience, you want your bed as hot as your i3 is willing to get. I print bridge with a bed temperature of 110 for the first layer, then 120 after that (though it never actually hits 120. And I have increased my PSU to 13 volts and change). I get *much* more reliable prints this way, both in adhesion and warping problems (I still have warping issues, but only on the largest parts and it is usually not enough to ruin the print).

    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.

  • This isn't going to really effect or change what most of you have been doing for the most part - but I think a few people may find this helpful. Regardless, I was glad to learn all this myself, and it is nice to have some conceptual understanding of some of the weirdness that sometimes happens with 3D printing.

    TLDR; Always take glass transition temperatures with a grain of salt, and place only the most cautious significance on them. It works pretty well for PLA and ABS, but for things like Nylon (and I believe Polycarbonate, though I am not sure), you can essentially just completely ignore their Tg - it simply isn't useful in the context of 3D printing.

    Posted : 18/11/2017 12:30 pm
    jim.c5
    (@jim-c5)
    Trusted Member

    Interesting. Nice to know, but what do I do about it? What do we do to reduce the effect of the "plastic memory"?
    Does heating the plastic up and drying it out, similar to tempering steel, before printing reduce all the "history" each reel has seen? Would this reduce the differences we see from reel to reel?

    Posted : 18/11/2017 1:59 pm
    3D Print
    (@3d-print)
    New Member

    Thanks great information presented very clearly.

    Posted : 06/08/2018 4:26 am
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