Why dry filament is critical for quality FDM prints

Filament is hygroscopic — it absorbs moisture from the air even when sealed in a bag without fresh desiccant. For FDM this is not a cosmetic detail: moisture changes melt rheology, weakens interlayer bonding, and visibly degrades the surface. The cover image shows the same material, same machine and same slicer profile — left: a freshly opened spool, right: the same spool after a few days in a humid environment. ## What physically happens in the nozzle When filament carrying absorbed water enters the melt zone (200–290 °C depending on the material), water flashes to steam. Steam expands ~1700× vs the liquid phase and: - causes micro-explosions and audible popping/crackling in the hotend, - leaves bubbles and craters on the layer surface, - interrupts continuous flow (under-extrusion, stringing), - hydrolyses the polymer — for PA, PC, PETG and TPU this permanently shortens the polymer chain and lowers strength in a way drying cannot fully recover. ## What moisture actually affects **Mechanical properties.** Tensile strength and interlayer adhesion drop 20–60 % for PA-CF, PC and PETG-CF. PLA is milder but measurable (10–20 %). Impact toughness drops even more, because voids act as crack initiators. **Dimensional accuracy.** Unstable flow means varying line width, so outer dimensions wander ±0.1–0.3 mm. Holes become oval, threads fail to seat, snap-fits stop clicking. **Surface and appearance.** Matte, rough texture, whitish micro-foam marks, stringing between walls, visible flow lines. Colour looks washed out. **Heat resistance and ageing.** A hydrolysed polymer cracks earlier under load and tolerates UV/heat worse. ## Which materials absorb moisture fastest From worst to most forgiving: 1. **PA (Nylon), PA-CF, PA-GF** — unusable within hours, drying is mandatory. 2. **TPU** — very hygroscopic, bubbly extrusion. 3. **PC, PC-CF, PC-ABS** — hydrolysis at high temperatures, dramatic strength loss. 4. **PETG, PETG-CF** — visible popping and stringing after just a few days. 5. **ASA, ABS** — moderate but measurable surface impact. 6. **PLA** — most tolerant, but long-term moisture makes it brittle and dull. ## How to dry filament correctly Use an active filament dryer or an oven with precise temperature control. Reference values: - PLA: 45–50 °C / 4–6 h - PETG, PETG-CF: 65 °C / 6–8 h - ASA, ABS: 70 °C / 4–6 h - PC, PC-CF: 80–90 °C / 6–10 h - PA, PA-CF: 80–90 °C / 8–12 h (very wet spools up to 24 h) - TPU: 50–55 °C / 6–8 h For demanding materials (PA-CF, PC) we recommend printing directly from the dryer — the spool stays at 55–70 °C during the whole print. ## Storage between prints - Vacuum bags with **fresh** indicating silica gel (blue/orange type). - Dry cabinets with active dehumidification for technical materials. - Never leave spools out overnight, especially PA and TPU. ## When the print is already going wrong If you hear popping, see bubbles or stringing — stop the print, dry the spool, then continue. Pushing through with a "slightly wet" material means delivering a part that looks printed but is mechanically far below the datasheet. For functional parts this is not an option — that is why at 3D4U every technical spool is checked for moisture before printing and, when needed, actively dried. The difference is visible on the part, and more importantly — measurable in testing.