You hear popping from the nozzle during a print. The surface has random zits and the extrusion looks inconsistent — thin in spots, blobby in others. You check your e-steps, PID tune the hotend, swap the nozzle, and it still looks bad. The problem is water. Wet filament is the single most common print quality issue that gets blamed on something else.
What Moisture Does to Filament
Most 3D printing filaments are hygroscopic — they absorb moisture from the air. PLA absorbs slowly. PETG absorbs moderately. TPU and nylon are sponges. When wet filament enters the hotend at 200-260°C, the trapped water flashes to steam. That steam bubble bursts out of the nozzle, disrupting the extrusion and leaving a void. The pop you hear at the nozzle is literally a steam explosion on a microscopic scale.
The damage manifests differently by material:
| Filament | Moisture Absorption Rate | Signs of Wet Filament | Drying Temp | Minimum Drying Time |
|---|---|---|---|---|
| PLA | Slow (days to weeks) | Stringing, inconsistent surface sheen, weak layer adhesion | 45-50°C | 4-6 hours |
| PLA+ / Tough PLA | Moderate | Same as PLA but worse at lower moisture levels | 50-55°C | 5-6 hours |
| PETG | Moderate (hours to days) | Stringing, zits on surface, popping at nozzle, brittle prints | 60-65°C | 4-8 hours |
| TPU (flexible) | Fast (hours) | Heavy stringing, foaming at nozzle, inconsistent extrusion width | 50-55°C | 6-8 hours |
| Nylon (PA6, PA12) | Very fast (minutes to hours) | Constant popping, foamy extrusion, extremely brittle, yellow discoloration | 70-80°C | 8-12 hours |
| ABS / ASA | Slow | Less affected, but can get surface bubbles | 70-80°C | 4-6 hours |
| PVA (water-soluble support) | Extremely fast | Turns to gel, clogs nozzle, unusable | 45-50°C | 6-8 hours |
The temperatures are conservative. Exceeding them risks softening the filament on the spool, causing it to fuse together in the dryer — a ruined spool.
How to Dry Filament
Method 1: Dedicated Filament Dryer ($40-80)
A purpose-built filament dryer like the Sunlu S2, Eibos Easdry, or Sovol SH01 is the most reliable solution. It holds 1-2 spools at a controlled temperature with a fan for air circulation. The fan is critical — without moving air, the moisture that evaporates from the filament just hangs around the spool and re-condenses when the dryer cools.
The Sunlu S2 runs at up to 70°C and fits two spools. It covers PLA, PETG, and TPU. It does not reach the 80°C needed for nylon — for that you need the Eibos Cyclopes or a modified food dehydrator.
Set the temperature, set a timer for the minimum drying time in the table above, and leave it. If the filament has been sitting out for weeks or arrives from the factory wet (more common than manufacturers admit), double the drying time.
Method 2: Food Dehydrator ($30-50)
A round food dehydrator with a tall lid or a 3D-printed extension collar is the budget workhorse. It circulates heated air and reaches 70-75°C on most models. The Presto 06300 is the classic choice — cheap, reliable, and fits one spool with the tray removed.
The downside: no sealed enclosure means moisture re-enters the filament as soon as you take it out. Print directly from the dehydrator if possible, or transfer the dried spool to a sealed storage container immediately.
Method 3: Heated Printer Bed + Box (Free, Slow)
Place the spool on the printer bed, cover it with a cardboard box with a few small holes at the top for venting, and set the bed to the drying temperature. The holes at the top let the moist air escape by convection. This method works but takes 2-3x longer than an active dryer because there is no forced air circulation. Temperature control is also imprecise — the bed thermistor reads the aluminum plate temperature, not the air temperature inside the box, which is typically 10-15°C lower.
Do not use your kitchen oven. Most ovens cannot hold a stable temperature below 80°C and the thermal cycling swings ±20°C, which will soften and fuse PLA spools.
How to Store Filament After Drying
Dried filament re-absorbs moisture from ambient air. The timeline:
- PLA in 50% humidity: Reaches problematic moisture levels in 2-3 weeks
- PETG in 50% humidity: Noticeable degradation in 3-5 days
- TPU in 50% humidity: Unusable in 24-48 hours
- Nylon in 50% humidity: Unusable in 4-8 hours
Storage solutions in order of effectiveness:
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Vacuum-sealed bag with desiccant: The gold standard. A $25 vacuum sealer kit (Sunlu or eSun brand) with reusable vacuum bags and a 50g silica gel packet keeps filament at <10% relative humidity for months. Squeeze the air out, seal, done.
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Airtight storage box with desiccant: A gasketed plastic tote (IKEA SAMLA 11L with a weatherstrip added) with 200-500g of color-changing silica gel beads. The beads turn from orange to green when saturated — microwave them to recharge. This keeps humidity at 15-20%, good enough for PLA and PETG stored for months.
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Printing from a dry box: A sealed container with a PTFE tube running from the spool to the extruder. The filament never sees ambient air between the dry box and the hotend. For TPU and nylon, this is not optional — it is the only way to print without quality degradation mid-print. As our TPU printing guide covers, flexible filaments absorb moisture so fast that a 3-hour TPU print can start clean and end with steam-popping if the spool is not in a dry box.
The Factory-Wet Problem
Brand-new vacuum-sealed filament is not guaranteed dry. Some manufacturers bag filament hot off the cooling line without a drying step, sealing the moisture inside the bag. If a fresh spool pops and strings immediately, dry it — the factory seal was not a dryness guarantee.
Common Mistakes & What Most Makers Get Wrong
1. Thinking PLA does not absorb moisture
PLA absorbs slowly compared to nylon, but “slowly” means 2-3 weeks in humid conditions, not never. I have pulled a roll of PLA that sat on a shelf for three months through a Florida summer and watched it snap like uncooked spaghetti — classic hydrolytic embrittlement. The moisture breaks the polymer chains, making the filament brittle and the printed part weak. If a spool has been open for more than a month in ambient humidity, dry it.
2. Drying at too high a temperature
Nylon requires 80°C to dry effectively. PLA softens at 60°C. Do not put them in the same dryer at the same temperature thinking “hotter is faster.” You will come back to a spool of PLA fused into a solid plastic brick. Follow the temperature table.
3. Storing desiccant indefinitely without recharging
Silica gel beads saturate in weeks. A container of saturated desiccant sitting next to your filament is doing nothing — it has reached equilibrium with the ambient humidity inside the box. Color-changing beads make this obvious: orange = dry, green/dark = saturated. Recharge in a microwave (2-3 minutes on medium, stir, repeat) or a conventional oven at 120°C for 1-2 hours.
4. Ignoring humidity during long prints
A 12-hour PETG print in a room at 60% humidity starts beautifully and ends with stringing and zits because the filament on the spool absorbed enough moisture during the print to cause problems. A $25 dry box with a PTFE tube feed eliminates this. If you print PETG or TPU for more than 4 hours at a time, a dry box pays for itself in saved failed prints within a month.
5. Not realizing wet filament causes clogs
Steam-pops at the nozzle leave carbonized residue. Over hours of printing, that residue builds up on the inside of the nozzle and eventually restricts flow. The symptoms look exactly like a partial clog — underextrusion, thin walls, clicking extruder — but the root cause is wet filament, not a foreign particle. Before disassembling your hotend to clear a clog, dry your filament and do a cold pull. You might save yourself an hour of unnecessary disassembly.
⚠️ Safety Notice: Filament drying involves sustained heating of plastics. Always dry filament in a well-ventilated area. Some materials (ABS, ASA, nylon) off-gas potentially harmful compounds when heated. Use an enclosed dryer or dehydrator in a ventilated space, not in a closed room. Follow the electrical safety certifications for any heating appliance used for drying. The temperatures recommended in this guide are below the thermal decomposition point of each material.
The Sunlu S2 filament dryer is the most practical drying solution for FPV builders printing PETG components and TPU mounts — it holds two spools at precise temperatures up to 70°C and runs quietly in the background while you build.