Polymer Drying¶
Polymer drying is a complex process influenced by numerous factors.
It's important to understand that filament can absorb moisture through various mechanisms. Moisture can be physically adsorbed on the surface or within the pores and microcracks of the filament-composites based on non-hygroscopic materials can absorb moisture to some extent this way. This also explains the variance in moisture absorption rates of PETG from different manufacturers-PETG is particularly sensitive to production parameters, and deviations can introduce structural defects. More critically, moisture can be chemically absorbed by the polymer itself, as with hygroscopic materials like polyamides (nylons), involving complex mechanisms that we won't delve into here to avoid expanding the article. For those interested, consult polymer science textbooks and resources on polymer processing machinery.
A quick literature review shows that drying efficiency depends on several key factors: the difference in relative humidity, polymer temperature, and air convection.
The role of relative humidity-or rather, the difference in partial pressure of water vapor between the material and the environment-is fairly straightforward. Essentially, the surrounding atmosphere must have lower humidity than the plastic. This concentration gradient causes water molecules to migrate from within the polymer to the surface and then evaporate. This alone can initiate drying. Peltier element-based dryers operate on this principle by dehumidifying the air. Even placing moist filament in a container with silica gel will reduce its moisture content (to an extent), especially when dealing with physically adsorbed moisture.
However, without heating, this process is inefficient-particularly with hygroscopic materials, where water molecules are chemically bound to polymer chains. Heating offers two main advantages. First, it increases the mobility of both water molecules and polymer chains, enabling water molecules to overcome chemical bonding energy more easily and detach. Second, higher temperature reduces relative humidity, increasing the vapor concentration gradient and enhancing moisture migration from the polymer's interior to its surface for evaporation.
Without convection, though, surface evaporation is hindered-especially in the deeper filament layers like the inner coils of a spool. In a sealed environment, evaporating water raises the humidity, gradually slowing and eventually halting drying. Heating alone can only dry filament up to a point and not uniformly. Moreover, heating itself isn’t harmless-raising the temperature accelerates polymer aging and degradation, particularly via hydrolysis (polymer chain breakdown due to water interaction). While outer layers may dry, moisture from the inner layers can remain, leading to increased hydrolysis. After several cycles, this might not be noticeable, but a dozen such drying cycles can leave the inner PLA brittle enough to break by hand.
This is where the convective aspect of drying-hot air circulation-becomes critical. Airflow removes evaporated moisture from around the filament and somewhat ventilates the spool, boosting evaporation and improving drying efficiency, especially deeper inside. Additionally, bringing in fresh air displaces moisture-laden air, maintaining low relative humidity inside the dryer.
Of course, ambient air already contains moisture, which doesn’t disappear upon entering the dryer. Thus, there’s a limit to how low humidity can go, setting a lower bound for drying. In industrial setups (e.g., drying polymer granules before molding), air is dehumidified before being heated and circulated. Such systems may involve complex multi-stage or cyclical drying processes, dew point control algorithms, and more. Still, they all ensure effective air convection around the drying polymer.
A common question in online communities is, “Is this dryer any good?”-usually referring to dryers from AliExpress. One of the most community-approved Chinese dryers is the Sunlu S2 (new revision with a small internal fan), and deservedly so. Based on the above, this dryer can indeed dry filament. It effectively removes most of the moisture from the outer filament coils, which significantly impacts print quality-after all, these are the first to enter the extruder. Even in industrial contexts, a small amount of residual moisture is acceptable and sometimes even necessary.
However, without strong airflow, while the dry outer coils are being printed, the inner coils may be “stewing in their own juices.”
Regarding "ventilation," even a sealed dryer will eventually reach saturation, ceasing to be effective. The dryer in the referenced video is unlikely to be fully airtight, so some moisture does escape, likely due to the humidity gradient with the atmosphere. Thus, additional ventilation may not be crucial. However, upgrading the convection fan could be worthwhile if the original is too weak.
In conclusion, a dryer design with good airflow and hot air circulation dries faster, more effectively, and more safely-preserving the filament’s original properties to the greatest extent.
This article is an adapted version of an answer by Viktor Shapovalov (thanks to him) to a subscriber question about filament drying and the Sunlu S2 dryer in particular. Special thanks to Anton Sovetov (3d-club.ru) and Andrey "Kekht" Korolev for consultation and adaptation.