Polylactic Acid (PLA) is the most widely used material in fused deposition modeling. PLA is a hygroscopic material, meaning it readily absorbs moisture from the surrounding air. This characteristic turns an otherwise reliable polymer into a compromised filament, leading to print failures and degraded material properties. Understanding this vulnerability and how to mitigate it is key to achieving high-quality 3D printing results.
The Mechanism of Moisture Absorption in PLA
PLA is a polyester whose molecular structure contains polar groups naturally attracted to water molecules. This attraction causes water vapor to diffuse into the filament, becoming trapped within the polymer matrix. The rate of absorption accelerates significantly in high humidity, but it is a continuous process even in dry conditions.
When this water-saturated filament is fed into the printer’s hotend, it reaches temperatures well above the boiling point of water. The absorbed moisture rapidly converts into steam under pressure. This sudden phase change creates microscopic bubbles that disrupt the smooth flow of the molten plastic as it exits the nozzle.
This process, known as hydrolysis, causes chemical degradation. The steam bubbles break the long polymer chains of the PLA, reducing the material’s molecular weight. The chemical degradation caused by the water weakens the plastic, resulting in parts with reduced mechanical strength and durability.
Identifying Diagnostic Signs of Contamination
The presence of moisture in PLA filament manifests through auditory and visual cues. The most immediate sign is an audible popping, hissing, or crackling sound emanating from the hotend. This noise results from the trapped water turning into steam and escaping the nozzle as the filament is extruded.
Visually, the printed surface will often appear rough, uneven, or pitted with tiny voids. These blemishes are caused by steam explosions disturbing the molten plastic flow as it settles onto the layer below. The inconsistency in flow also leads to increased stringing, where fine strands of plastic are left between separate points of the print.
Prints made with wet filament exhibit poor layer adhesion, causing the finished part to be brittle and easily fractured. The internal steam pockets compromise the bond between layers, weakening the structure. The filament may also snap more easily when bent, indicating the absorbed moisture has weakened the polymer chain before printing even started.
Effective Drying Methods and Preventative Storage
Moisture-contaminated PLA is restored by applying low heat over a sustained period to encourage the water molecules to diffuse out of the filament. A dedicated filament dryer box is the most precise tool, but a food dehydrator or conventional oven can also be used. The temperature must be carefully controlled to remain well below the PLA glass transition temperature, which is around 60°C (140°F).
The recommended drying parameters for PLA are between 40°C and 55°C (104°F to 131°F) for a duration of four to six hours. Using a conventional oven requires constant monitoring and a separate thermometer to verify the temperature, as oven thermostats can be inaccurate at low settings. The goal is to gently warm the filament to release the water without softening the plastic spool or causing the filament to fuse.
Once the filament is dried, preventative storage is necessary to keep it in optimal condition. Spools should be stored in an airtight container or a vacuum-sealed bag immediately after drying. Including a desiccant, such as fresh silica gel packets, helps absorb any residual moisture or vapor that may enter. Monitoring the relative humidity within the storage container using a hygrometer is recommended to ensure the environment stays below a 20% relative humidity level.