Moisture, Dry Air, and Keeping Filament From Going Bad
Last two lessons: PLA, then PETG and TPU. This one's a problem with nothing to do with your printer settings and everything to do with the bag your filament came in.
Filament goes bad. Sounds strange for a spool of plastic. It's true, and it trips up people who've otherwise got their machine dialed in.
Plastic drinks water. Yes, really.
PLA, PETG, TPU, all of it, is hygroscopic. It pulls moisture out of the air and holds onto it. Not a lot. You're not going to wring out a spool like a towel. But enough to change how the plastic behaves when it hits a 400-degree hotend.
Here's the mechanism. The filament holds tiny amounts of water. That water hits the heat and flashes to steam instantly. The steam has to go somewhere, so it forces its way out through the melted plastic as it extrudes. That's your popping and crackling sound at the nozzle. That's stringing that won't go away no matter how you tune retraction. That's a print with a rough, almost fuzzy surface instead of a clean one.
If your PETG suddenly looks like a haunted house prop when it used to look fine, moisture's usually the reason. PETG is worse about this than PLA. TPU's worse than both.
We're up at about 4,600 feet here, and the air along the Wasatch Front runs dry most of the year, which works in your favor some months. But run your printer in a garage in January with a humidifier going in the house, or leave a spool open on a shelf for four months, and you'll get moisture in the plastic regardless of the weather. It doesn't take monsoon season. It takes time and an open bag.
How to tell if it's the filament and not you
Check these before you second-guess your slicer settings:
- Popping or crackling sounds at the nozzle while it prints
- Strings and wisps between parts that weren't there before
- A dull, slightly bumpy surface instead of smooth
- Layer lines that look inconsistent even though nothing else changed
Two or more of those, and you haven't touched your settings? Don't tune retraction distance. Dry the filament first. I've watched people burn a whole evening chasing print quality in the slicer when the real problem was a spool that had sat open since spring.
Keeping it from happening
A few things that matter.
Store it sealed. Filament lives in an airtight container with something to pull moisture out, not on a shelf in the paper bag it came in. A cheap plastic tote with a good seal and some silica gel packets does the job. Mine live in totes from Costco with the big reusable silica bags you can recharge in the oven.
Add desiccant and actually replace it. Silica gel packets go from blue to pink (or orange to green, depending on brand) when they're saturated. Changed color means done working. Pop them in a low oven for 30 minutes or so to dry them back out and reuse them. Don't leave dead packets in the box thinking they still help.
Dry the spool if you think it's already wet. Run filament in a food dehydrator or a dedicated filament dryer at low heat, usually somewhere around 45-55°C depending on material, for 4 to 8 hours. PETG needs longer than PLA. No dryer? Some people use the oven on its lowest setting with the door cracked, but ovens run hotter than they say, especially at our elevation, and PLA gone too warm turns into a sad puddle on your baking sheet. Spend the $40 or so on an actual filament dryer instead of risking your good spool and your oven both.
Buy in amounts you'll use. Print occasionally? Don't buy the giant 5kg spool for the cheaper per-gram price. It sits open longer than it should and you fight moisture problems the whole time. Buy what you'll go through in a couple months.
The overnight thing, again
I've said this before, but it repeats here because moisture problems show up on long prints. Stringy, popping filament is way more likely to snag, tangle, or jam mid-print than dry filament is. And a jam on an unattended overnight print is exactly how you lose the whole thing.
Learned this one the hard way. Set a nine-hour print going one night, went to bed, got up for my run the next morning expecting a finished part. Instead the model had come loose from the bed about two hours in, and the nozzle spent the remaining seven hours dragging filament into empty air. Nine hours of plastic spaghetti, coiled up like a nest, printing nothing. I've got the photo. I show it every session, because plastic doesn't know it's failing. It just keeps doing what it's told.
I don't run long prints unattended anymore. I watch the first layer, check an hour in, and if a print will run past when I go to bed, I don't start it that night. Start it in the morning instead. A moisture problem alone probably won't wreck a print completely, but it raises the odds of something going wrong, and combined with an unattended overnight run, that's how you lose real hours for nothing.
Before next time
Check your filament storage as it actually exists right now, not how you meant it to be. Spools sitting open on a shelf? Get them into something sealed with desiccant before our next class.