FDM Filament Troubles: Keeping Hygroscopic Materials From Degrading
Regardless of the status of polymers used with FDM 3D printing like nylon, ABS, and PLA as being typically indestructible, they do include an entire vary of moisture-related points that may have an effect on each the printing course of in addition to the ultimate outcome. Whereas the idea of ‘baking’ such 3D printing filaments previous to printing to take away absorbed moisture is well-established and with many industrial options out there, the precise extent to which these completely different polymers are affected, and what these adjustments appear to be on a molecular stage are typically much less well-known.
One other query with such hygroscopic supplies is whether or not the identical problems with embrittlement, swelling, and long-term injury inflicted by moisture publicity that impacts filaments previous to printing impacts these supplies post-printing, and the way this impacts the lifespan of FDM-printed objects. In a 2022 paper by Adedotun D. Banjo and colleagues a lot of what we all know as we speak is summarized along with an examination of the molecular results of moisture publicity on polylactic acid (PLA) and nylon 6.
The scientific literature on FDM filaments makes clear that past the shiny advertising and marketing there’s a great world of supplies science to discover, one which might train us rather a lot about find out how to get good FDM prints and the way sturdy they are going to be long-term.
Why Water Wrecks Polymers
Though the results of moisture publicity on FDM filaments are inclined to get tossed collectively right into a simplified mannequin of ‘moisture absorption’, there are literally fairly completely different mechanisms at play for these completely different polymers. A superb instance of this from the Banjo et al paper is the distinction between nylon 6 and polylactic acid (PLA). Whereas nylon 6 could be very hygroscopic, PLA is generally hydrophobic, but this doesn’t save PLA from getting degraded as nicely from moisture publicity.
Within the case of nylon 6 (C6H11NO), the extremely polar purposeful teams equivalent to amides (−C(=O)−(N) ), amines (−NH2) and carbonyls (C=O) make this polymer hydrophilic. As these purposeful teams are uncovered to moisture, the ensuing hydrolysis of the amide bonds step by step impacts the fabric properties of the polymer like its tensile energy.
Just a few % moisture within the polymer filament previous to passing via the new extruder of an FDM printer will correspondingly trigger points as this moisture quickly evaporates. And after printing a nylon object, moisture will as soon as once more hydrolyze the amide bonds, weakening the fabric over time. That is one thing that may be prevented considerably by sealing the thing in opposition to moisture intrusion, however that is not often sensible for purposeful elements. This degradation of polyamides might be noticed by the cracking of nylon gears in toys gearboxes, servo motors, and related high-stress purposes.
Within the case of PLA ((C3H4O2)n), far fewer polar purposeful teams are current, making PLA successfully hydrophobic, though it’s soluble in numerous natural solvents like ethyl acetate. PLA’s weak spot lies in its ester bonds, that are topic to hydrolysis and might thus be damaged like amides. The sort of hydrolysis in PLA could be very sluggish, nonetheless, with research discovering that it barely degrades even submerged in water. The usually cited ‘composting’ of PLA thus requires chemical hydrolysis, making choices like incineration the sooner and simpler route for disposal. Consequently, for long-term stability PLA does price extremely, no matter its different materials properties.
Naturally, within the case of all hygroscopic polymers the speed of degradation is determined by each the moisture content material of the air, and the temperature. Within the earlier referenced research by D. Banjo et al., the FDM printed samples had been totally submerged into water to speed up the method, with three forms of polymers examined at 21 °C and 70 °C.
Freshly Baked Polymer
Drawing the moisture out of the polymer once more might be completed in quite a lot of methods, with making use of warmth over an prolonged time frame being the commonest. The appliance of thermal power motivates the water molecules to make their manner out of the polymer once more, however you will need to perceive right here that hydrolysis is a everlasting, non-reversible course of. Which means that the main focus is totally on eradicating any absorbed water that may be problematic throughout extrusion, and to stop additional degradation of the polymer over time.
A paper offered by Xuejun Fan on the IEEE EuroSimE convention in 2008 titled “Mechanics of Moisture for Polymers: Elementary Ideas and Mannequin Research” covers the basic ideas associated to moisture intrusion which in the end allow the degradation. Specifically it’s of be aware that the results of submersion (water sorption) versus publicity to the air (moisture sorption) result in very completely different transport mechanisms, and that there’s a distinction between certain and unbound water contained in the polymer. This unbound water is contained inside microscopic pores that exist throughout the materials, and would thus be a very good goal for compelled eviction utilizing thermal means.
Precisely how a lot warmth must be utilized and for which period differs wildly, primarily based totally on the kind of materials, with industrial filament dryers typically having presets programmed into them. Filament drying charts can be found from all kinds of sources, such from Bambu Lab. They suggest drying PLA filament at 50 °C – 60 °C for eight hours, whereas Prusa recommends drying PLA for six hours at 45 °C (and PA11CF bolstered nylon at 90 °C). This highlights simply how onerous it’s to make particular statements right here apart from not heating up a spool of filament to the purpose the place it softens and sticks collectively. The query of ‘how lengthy’ could be ideally answered with ‘till all of the moisture is gone’, however since that is onerous to quantify with out specialised tools, longer might be mentioned to be higher.
Maybe the largest take-away right here is that stopping moisture from getting even close to the polymer is by far the most suitable choice, which means that conserving spools of filament in vacuum baggage with desiccant gel between printing periods is very really helpful.
Endurance
If water molecules trigger bodily injury to the polymer construction, how extreme is the impression? Clearly having unbound moisture within the filament is a horrible factor when attempting to soften it for printing, however how lengthy can an FDM printed half be anticipated to final as soon as it’s completed and put into use in some form of moist surroundings?
For PLA and nylon we will see the results illustrated within the D. Banjo et al. research, with parameters like moisture absorption, crystallinity adjustments, and mechanical efficiency examined.
Maybe most fascinating about these outcomes is the efficiency of PLA, which at first seems to outperform nylon, because the latter initially reveals a pointy lower in mechanical properties early on. Nevertheless, nylon stabilizes whereas PLA’s properties at both temperature water utterly fall off a cliff after a couple of week of being submerged. This brittleness of PLA is already its principal weak spot when it’s not subjected to hydrolysis, and clearly accelerated getting old on this vogue reveals simply how rapidly it turns into a legal responsibility.
The large asterisk right here is after all that this research used an absolute worst-case state of affairs for FDM-printed polymers, with water sorption in comparatively heat to very heat water. Even so, it’s illustrative of simply how a lot completely different polymers can differ, and why selecting the optimum polymer for an FDM print utterly is determined by the surroundings. Clearly PLA is completely advantageous for a lot of conditions the place its disadvantages aren’t a problem, whereas for extra demanding conditions nylon, ABS/ASA, or PC would be the more sensible choice.
Preserving filament dry, vacuum-packed and much away from moisture will considerably enhance printing with it in addition to its longevity. Printed elements can have their floor handled to seal them in opposition to moisture, which might make them final for much longer with out mechanical degradation. Finally FDM printing is simply part of the messy world of supplies science and manufacturing, each of that are concurrently marvels of recent science whereas additionally giving engineers horrible nightmares.