Moisture, Dry Air, and Keeping Filament From Going Bad
Last two lessons were PLA and then PETG and TPU. This one's about a problem that has nothing to do with your printer settings and everything to do with the bag your filament came in.
Filament goes bad. I know that sounds strange for a spool of plastic, but it's true, and it's the thing that trips up people who've otherwise got their machine dialed in.
Plastic drinks water. Yes, really.
PLA, PETG, TPU, all of it, is hygroscopic. That means 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 that it changes how the plastic behaves when it hits a 400-degree hotend.
Here's what happens. The filament's got tiny amounts of water in it. That water hits the heat and turns to steam instantly. The steam has to go somewhere, so it pushes its way out through the melted plastic as it extrudes. That's where you get the popping and crackling sound at the nozzle. That's where you get stringing that won't go away no matter how you tune retraction. That's where you get 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 actually works in your favor some months. But go 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 what the weather's doing outside. It doesn't take monsoon season. It just takes time and an open bag.
How to tell if it's the filament and not you
Check for these before you start second-guessing 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
If you're seeing two or more of those, and you haven't touched your settings, don't start tuning retraction distance. Dry the filament first. I've watched people burn a whole evening chasing print quality with slicer settings when the actual problem was a spool that had been sitting open since spring.
Keeping it from happening
A few things that actually matter:
Store it sealed. Filament should live in an airtight container with something to pull moisture out, not just sit 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. I keep mine 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. When they change color, they're done working. Pop them in a low oven for 30 minutes or so to dry them back out and reuse them. Don't just leave dead packets in the box thinking they're still helping.
Dry the spool if you think it's already wet. You can 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. If you don't own a dryer, some people use their oven on its lowest setting with the door cracked, but ovens run hotter than they say they do, especially at our elevation, and if you get PLA too warm it'll turn into a sad puddle on your baking sheet. I'd rather you spend the $40 or so on an actual filament dryer than risk your good spool and your oven both.
Buy in amounts you'll use. If you print occasionally, don't buy the giant 5kg spool because it's cheaper per gram. It'll sit open longer than it should and you'll fight moisture problems the whole time. Buy what you'll go through in a couple months.
The overnight thing, again
This connects to something I've said before, but it's worth repeating here because moisture problems tend to 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.
I 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 I found the model had come loose from the bed about two hours in, and the nozzle had spent the remaining seven hours dragging out 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 it's a good reminder that 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 on it an hour in, and if it's a print that'll 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
Go check your filament storage situation as it actually exists right now, not how you meant it to be. If your spools are sitting open on a shelf, get them into something sealed with desiccant before our next class.