Your PETG print is stringing like cotton candy, the PLA snaps mid-spool, and the nylon part smells burnt. You’ve tuned retraction, temperature, and speed — but the filament absorbed moisture from ambient air and those slicer settings are irrelevant. Drying filament fixes problems that no slicer adjustment can touch. Here’s what moisture does to each material, how to dry it, and how to keep it dry.
What Moisture Does to Each Filament
Moisture damage isn’t one problem — it’s different failure modes per material. And it happens faster than most people think. PLA left out in 50% RH for 48 hours is compromised. Nylon in 60% RH is compromised in 4-6 hours.
PLA
Absorbs moisture slowly but becomes brittle. Wet PLA snaps during retractions inside the Bowden tube, not at the spool — the filament fractures inside the extruder path and you spend 20 minutes disassembling. The print surface shows small pits (steam bubbles) and the extruder clicks from the inconsistent melt.
Drying: 45-50°C for 4-6 hours. Do not exceed 55°C — PLA’s glass transition temperature is 60°C and you’ll fuse the outer loops of the spool together.
PETG
PETG is hygroscopic and aggressive about it. Wet PETG strings ferociously because the water turns to steam in the nozzle, the steam pressurizes the melt zone, and plastic oozes during travel moves. Surface finish turns matte and rough. Layer adhesion drops because steam pockets create micro-delaminations between layers.
Drying: 60-65°C for 4-6 hours. PETG can handle this temperature without deforming. Longer drying (8+ hours) for filament that’s been exposed for weeks.
TPU
TPU absorbs moisture faster than PETG. Wet TPU prints with a rough, foamy texture because steam expands the flexible filament as it exits the nozzle — the extruded line looks puffy. Layer adhesion tanks. The filament also sticks to itself on the spool, causing feed jams.
Drying: 50-55°C for 4-5 hours. TPU softens at lower temperatures than PETG — stay under 55°C to avoid deforming the filament.
Nylon / PA
Nylon is the worst. It absorbs 2-3% of its weight in water within 12 hours at 50% RH. Printing wet nylon sounds like popcorn — audible pops at the nozzle as steam explodes out of the melt. The part looks foamy, has zero strength, and the nozzle clogs from charred filament residue.
Drying: 70-80°C for 6-12 hours. Nylon requires the highest drying temperature. Most budget filament dryers max out at 55-60°C — insufficient for nylon. Use a food dehydrator, a dedicated high-temp dryer, or an oven with accurate temperature control.
ABS / ASA
Less hygroscopic than PETG or nylon, but not immune. Wet ABS shows surface defects (small blisters) and reduced interlayer adhesion. The bigger issue: printing wet ABS in an enclosure concentrates styrene fumes, which is a health concern.
Drying: 65-70°C for 4-6 hours.
| Material | Absorption Rate | Wet Symptom | Drying Temp | Drying Time | Max Safe Temp |
|---|---|---|---|---|---|
| PLA | Slow (days) | Brittle, extruder click, pits | 45-50°C | 4-6h | 55°C |
| PETG | Fast (hours) | Heavy stringing, rough surface | 60-65°C | 4-6h | 70°C |
| TPU | Very fast (hours) | Foamy extrusion, feed jams | 50-55°C | 4-5h | 55°C |
| Nylon/PA | Extremely fast | Popping, zero strength, clog | 70-80°C | 6-12h | 85°C |
| ABS/ASA | Moderate | Surface blisters, weak layers | 65-70°C | 4-6h | 80°C |
Commercial Dryers vs DIY: What Actually Works
Commercial Filament Dryers ($40-80)
The Sunlu S2/S4, Eibos Polyphemus, and Sovol filament dryers are purpose-built enclosures with PTC heaters and fans. They maintain a set temperature and circulate air.
What they do well: Set and forget. Consistent temperature control. The Sunlu S4 fits two spools and runs for 12+ hours. The Eibos Polyphemus rotates the spool for even heating.
What they don’t: Most budget dryers max at 55°C — fine for PLA, PETG, TPU. Inadequate for nylon. Check the max temperature before buying. The Sunlu S2 claims 70°C but the actual internal temperature at the spool center is 5-10°C lower because the heating element is at the bottom and hot air rises past the spool, not through it.
Food Dehydrator ($30-60)
A round food dehydrator with adjustable temperature shelves is the dark-horse winner. Remove the internal trays, stack two spools vertically, set temperature, and run for the required time. Most dehydrators hit 70-75°C — nylon territory.
What it does well: Higher max temperature than budget dryers. Better airflow if the fan is at the bottom pushing up through the spool stack. Costs less.
What it doesn’t: No enclosure — heat escapes and the room heats up. Inefficient for small batches. Takes up counter space.
DIY Oven Method (Use With Caution)
Home ovens cycle ±10-15°C around the setpoint. A 50°C setpoint means the heating element kicks on at 40°C and off at 60°C. The 60°C spike softens PLA on the spool edges. The temperature swing on a gas oven is even worse.
Only attempt with an electric oven and a standalone thermometer inside to verify temperature. Preheat, place spool, monitor for 5 minutes to confirm temperature hasn’t spiked. If in doubt, don’t.
| Drying Method | Cost | Max Temp | Best For | Worst For |
|---|---|---|---|---|
| Sunlu S2 | $45 | ~55°C actual | PLA, PETG, TPU | Nylon (temp too low) |
| Sunlu S4 | $70 | ~60°C actual | PLA, PETG, TPU, ASA | Nylon (borderline) |
| Eibos Polyphemus | $75 | 70°C | PETG, TPU, Nylon | PLA (heats evenly) |
| Food Dehydrator | $40 | 70-75°C | All materials | Small batches, space |
| Home Oven | Free | Varies | None (last resort) | PLA (temp swing risk) |
Storage: Keep It Dry After Drying
Drying is pointless without dry storage. Filament re-absorbs moisture at the same rate it originally absorbed.
Desiccant Dry Box
A sealed plastic container with 500g of color-indicating silica gel beads. The beads turn from orange to dark green as they absorb moisture. When they’re dark green, microwave them to recharge.
This is the cheapest effective solution. A 20L gasket-seal container holds 4-5 spools and maintains <15% RH for 2-3 months before the desiccant needs recharging if you only open it briefly.
Active Dry Box (Print-From)
A sealed box with a PTFE tube feed to the printer. The filament stays in the dry environment during printing. Commercial options like the Sunlu S4 support printing directly from the dryer. DIY options use a gasket box with a bowden fitting and desiccant.
This is mandatory for nylon and recommended for PETG in humid environments. Printing from a dry box means the filament never sees ambient air between the spool and the hotend.
Common Mistakes & How to Avoid Them
Mistake 1: Drying at too high a temperature
PLA at 60°C fuses the outer windings of the spool together. When the extruder pulls, it can’t overcome the fused loops and the print fails mid-spool. Fix: Know your material’s glass transition temperature. Stay 5-10°C below it.
Mistake 2: Assuming new filament is dry
Vacuum-sealed filament with a desiccant packet means it was dry when sealed — but not necessarily dry now. Desiccant packets can be saturated by the time you open them, and pinhole leaks in the vacuum bag let moisture in during shipping. Fix: Dry every spool before its first use. Consider the factory seal a courtesy, not a guarantee.
Mistake 3: Leaving filament on the printer between sessions
In 60% RH, PETG absorbs enough moisture in 8 hours to affect print quality. Nylon is compromised in 4 hours. If you print weekly, the spool sitting on the holder has re-absorbed significant moisture between prints. Fix: Return filament to sealed dry storage after every session. It takes 30 seconds and prevents 4 hours of re-drying.
Mistake 4: Using a dryer without a fan
A heated enclosure without airflow heats the outer layer of filament on the spool. The inner 80% stays wet. Fix: Verify your dryer has a fan (most commercial units do). For DIY solutions, include a small 40mm fan to circulate air through the spool.
⚠️ Safety Notice: Filament drying involves sustained heating of plastic materials and electrical appliances. Always follow the latest 2026 fire safety and electrical standards for your region. Never leave a filament dryer or dehydrator running unattended. Ensure smoke detectors are functional in the room where drying occurs. Use appliances with thermal fuses and automatic shutoff features. Proper ventilation is required when drying materials that may off-gas (ABS, ASA, nylon).
For optimizing print quality after your filament is dry, see our stringing solutions guide — dry filament plus tuned retraction eliminates the problem at both ends. Material choice matters for structural parts — our PLA vs PETG guide covers moisture sensitivity and when each material justifies the drying time.
For FPV pilots printing TPU mounts and PETG brackets, dry filament is the difference between a mount that snaps on the first crash and one that survives a season. The Sunlu S4 filament dryer handles two spools simultaneously, hits 60°C consistently, and prints directly from the enclosure — the workhorse we use for every spool of PETG and TPU before it touches a printer.