Filament Drying
Printing while drying
A dried spool starts reabsorbing moisture the moment it leaves the box. For nylon, a long print can finish wetter than it started.
By Michael W. · Published October 5, 2026
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The reabsorption gap, and what it costs
Drying removes water. The instant the spool leaves the dryer, it starts taking water back from the air. How much that matters depends entirely on the material and the print length:
- PLA over a four-hour print: negligible. Ignore it.
- PETG over a twelve-hour print in a humid room: enough to show as stringing late in the print.
- Nylon over a twenty-hour print: potentially enough to undo the entire drying cycle, so the print finishes with wetter filament than it started.
Printing from inside a heated sealed unit removes the gap entirely, because the filament is being dried continuously while it feeds.
Why this is the one feature worth paying for with hygroscopic materials
Because every other dryer specification — temperature, capacity, timer — improves how well you dry a spool, and none of them addresses what happens afterwards. With nylon, what happens afterwards is the problem.
It also explains an experience people find baffling: dry a spool, get a great print, then get a progressively worse one from the same spool a week later. The drying worked. The storage did not.
Your options, ranked by fit
| Approach | Best for | Limitation |
|---|---|---|
| AMS 2 Pro | Enclosed Bambu machines, multi-color, PLA/PETG/TPU | Does not fit the A-series; not hot enough for nylon |
| AMS HT | Nylon, PC, PA-CF on a compatible machine | One spool only |
| AMS add-on dryer | Owners who already have an AMS and only want drying | Compatibility must be verified for your exact unit |
| A dryer you feed from directly | Any printer, including open-frame | Filament path geometry and drag need attention |
| Sealed box with desiccant, fed from | Cheap prevention between prints | Passive — will not dry a wet spool |
Feeding from a dryer on an open-frame machine
If you own an A1 or A1 mini, the AMS options above do not apply, and the practical approach is a dryer positioned so filament feeds from it directly to the extruder.
Two things to get right. The path: keep it short and avoid tight bends, because added drag causes feed problems and is especially bad with TPU. And the height: a spool feeding upward over a long distance adds tension the extruder has to overcome.
Done sensibly this works well, and it is the cheapest route to printing-while-drying on an open machine. Feed problems if it goes wrong.
When the gap does not matter
Be honest about your own situation, because this feature carries a price:
- PLA only, dry room, short prints. The gap is irrelevant. Buy a cheap dryer and sealed tubs.
- Occasional PETG. Dry before printing and accept the gap.
- Long prints in nylon or PC, regularly. Now it matters, and it is worth paying to close.
Where to go next
AMS HT versus a dryer is the same decision in more detail. The dryer roundup for units. Wet filament symptoms if you are not yet sure moisture is the problem.
What to buy
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1. Best for enclosed Bambu machines
Bambu Lab AMS 2 Pro
Four spools, dried actively, feeding the printer from inside the sealed box.
The cleanest answer to this problem if your printer takes it. Four spools, sealed, dried actively, feeding the machine without the filament ever sitting in room air.
The multi-color capability comes along with it, which is why this often replaces two planned purchases with one.
What it gets right
- Active drying means the spool you print from is the spool you dried — no transfer step, no re-absorption window
- Four colors on a machine you already own, for a fraction of a new printer
- RFID logging removes the single most common cause of a ruined multi-color print: the wrong material profile
What it does not
- Four-color printing wastes filament on every tool change; the purge tower is real and it is not small
- If you only print single-color PLA, you are buying a dryer with a filament changer attached
- Needs the space beside or above the printer that your shelf may not have
| Type | Four-spool automatic material system |
|---|---|
| Headline feature | Active filament drying |
| Inherits | Humidity monitoring and RFID logging from the original AMS |
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2. Best for engineering filament
Bambu Lab AMS HT
One spool, high-temperature active drying, printed straight from the unit.
For nylon, polycarbonate and carbon-filled nylon, this is the configuration that matters: high-temperature active drying with no reabsorption window at all. Those are exactly the materials where the gap does the most damage.
Single spool, which is the correct shape for engineering filament. Verify fitment and stated material support for your exact printer.
What it gets right
- Dries at a higher temperature than the AMS 2 Pro, which is what nylon and PC actually need
- Prints directly from the dryer, so there is no window where a dried spool sits absorbing moisture again
- Single spool is the right shape for engineering filament, which you rarely want four of
What it does not
- One spool. If multi-color is the goal, this is the wrong unit entirely
- Costs real money for a capability a standalone high-temp dryer also delivers
- Only worth it if you print materials that genuinely need the extra heat
| Type | Single-spool material system |
|---|---|
| Headline feature | High-temperature active drying |
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3. Best add-on for existing AMS owners
EIBOS Tetras
Active drying designed around the AMS you already own, rather than replacing it.
Cited as best for Bambu Lab AMS owners, with active drying that reuses the AMS you own. If drying was the only reason you were looking at an AMS 2 Pro upgrade, price this first.
More components in the filament path is more places for a feed problem to begin, which is the honest trade. Verify compatibility with your exact unit.
What it gets right
- Built around the fact that you already own an AMS, rather than asking you to replace it
- Active drying while the AMS feeds the printer closes the gap between drying and printing
- Cheaper than an AMS 2 Pro upgrade if drying is the only reason you were considering one
What it does not
- Its whole value proposition depends on you owning a compatible AMS — verify fitment for your exact unit
- If you do not have an AMS, a plain four-spool dryer is simpler and cheaper
- More parts in the filament path is more places for a feed problem to start
| Positioning (cited) | Best for Bambu Lab AMS owners |
|---|---|
| Approach (cited) | Active drying that reuses the AMS you own |
Questions people actually ask
Can you print directly from a filament dryer?
Yes, and with hygroscopic materials it is the single most useful feature available. Some units are designed to feed the printer from inside the sealed heated enclosure; with others you position the dryer so filament feeds out to the extruder directly. Keep the path short and avoid tight bends.
Does filament reabsorb moisture while printing?
Yes, from the moment it leaves the dryer. For PLA over a short print it is negligible. For nylon over a twenty-hour print it can undo the whole drying cycle, so the print finishes with wetter filament than it started.
Why did my print get worse partway through?
If stringing or surface roughness increases as a print progresses rather than being constant from the first layer, moisture is the likely cause - the spool is absorbing water from the air while it feeds. That is the signature of the reabsorption gap rather than a settings problem.
Is printing from a dryer worth it for PLA?
Usually not. PLA is only mildly hygroscopic, so over a normal print length the reabsorption gap barely matters. Dry it if it has been sitting in a humid room and store it sealed; the continuous-drying feature is for PETG, TPU, nylon and polycarbonate.
How do I feed from a dryer on an A1?
Position the dryer so filament feeds from it to the extruder over a short path with no tight bends, and avoid making the filament climb a long way, which adds tension. The AMS-based options do not fit the A-series, so this is the practical route on an open-frame machine.
Sources
Read this next
- AMS HT vs a filament dryerThe reabsorption gap explained, and whether it justifies an AMS HT over a standalone high-temperature dryer for your print mix.
- The best filament dryers in 2026Dryer picks split by temperature range and capacity, because a 70 C unit and a 110 C unit are different products for different materials.
- How to tell if your filament is wetThe three near-conclusive signs of wet filament, the ambiguous ones, and the free test that settles it in one print.
- Bambu Lab AMS 2 ProWhy active drying makes the AMS 2 Pro a different product from the original AMS, and the case for buying it instead of a new printer.
- AMS not loading filamentThe four real causes of an AMS feed failure in diagnostic order, three of which cost nothing and take minutes.