How to Prevent Moisture Blisters When Printing Old PLA in Summer
If your old spool of PLA starts popping, hissing, or leaving tiny bubbles and craters on your prints once summer humidity rolls in, the culprit is almost always moisture that’s worked its way into the filament itself. The fix isn’t complicated: dry the filament properly before use, print it from a sealed dry-box, and store it right going forward so you’re not fighting this battle every time the weather turns muggy.
Why Humidity Causes Moisture Blisters in PLA
PLA is hygroscopic. That means it pulls moisture out of the air over time, and honestly, it happens faster than most people assume. A spool sitting open on a shelf for six months, especially through a humid summer, can soak up enough water to cause genuine print defects. Not just cosmetic ones either.
When that moisture-laden filament hits the hot end (usually somewhere around 190-220°C for PLA), the trapped water flash-boils into steam almost instantly. The filament is already melted and semi-liquid at that point, so the steam has nowhere clean to go. Instead, it:
- Creates tiny internal bubbles that pop right at the nozzle tip, causing that audible crackling or hissing you might have heard
- Leaves pitted, cratered, or rough textures on surfaces that used to come out smooth
- Produces inconsistent extrusion width, sometimes with patches of under-extrusion
- Weakens layer adhesion, because steam pockets get in the way of proper bonding between layers
- Causes a weirder kind of stringing, often wispier and more brittle than what you’d normally see
Old spools take the worst of it because the outer layers have been exposed to air the longest. Got a roll that’s been sitting around for a year, especially one you opened and resealed halfheartedly with a rubber band? That’s a prime candidate for blistering the moment summer humidity kicks up.
How to Tell If Your PLA Is Wet
Before you start blaming temperature, retraction settings, or the slicer, check for these signs first:
- Popping or sizzling sounds from the nozzle while printing
- Steam or faint wisps near the nozzle, visible if the lighting’s good
- A surface finish that’s noticeably rougher than it used to be with the exact same settings
- Filament that feels tacky to the touch, or a spool that just feels heavier than you remember
- A strand that snaps cleanly instead of bending when you flex it
If two or more of these show up, drying is almost certainly your answer. Not a hardware tweak.
How to Dry Old PLA Before Printing
Oven or Filament Dryer Method
- If you own a dedicated filament dryer, use it: 45-55°C for 4-6 hours does the job.
- No dryer? A kitchen oven works fine in a pinch. Set it to 45°C (113°F) for 4-6 hours with the door cracked slightly so moisture can escape. Don’t push past 55-60°C, since PLA starts softening and deforming right around 60°C.
- Rest the spool on a rack rather than directly on a hot surface, so air can reach all sides evenly.
- Let it cool back to room temperature before loading it into the printer. Skipping this step risks warping from the sudden temperature change.
Food Dehydrator Method
A lot of makers in humid climates swear by food dehydrators with adjustable temperature settings. Set it to 45°C and run for 5-6 hours for a standard 1kg spool. Thicker spools, or ones that have been sitting around for a long time, might need 8 hours or more.
Printing From a Dry Environment
Drying once won’t help much if you’re printing in a humid room. PLA reabsorbs moisture surprisingly fast, sometimes in just a few hours if the room sits above 70% humidity.
- Print straight from a dry-box with desiccant packs and a PTFE tube running directly to the extruder.
- Keep your desiccant fresh. Silica gel beads should look orange or blue when dry, and pink or clear when they’re saturated. Recharge them in the oven on low heat when needed.
- During peak summer humidity, think about running a small dehumidifier in your print room, especially for overnight prints you won’t be watching.
Preventing the Problem Long-Term
- Store every spool in a sealed container or vacuum bag with desiccant, even in between quick print jobs.
- Write the date you opened each spool somewhere visible, so you know exactly how long it’s been exposed.
- Don’t leave spools sitting loaded on the printer for days at a stretch during humid months.
- If you live somewhere that’s consistently humid, look into filament sold in resealable, desiccant-lined packaging.
Conclusion
Moisture blisters in old PLA come down to simple physics: water trapped inside the filament turning to steam the instant it hits heat. Dry your filament before you print, keep it dry while printing with a proper dry-box or enclosure, and store it sealed all year round, especially through the sticky summer months. Stick with that routine, and even your oldest, most neglected spools will extrude just as cleanly as they did the day you opened them.