Direct answer: if PETG printed cleanly when it was loaded but becomes stringier after sitting in a Bambu AMS, check the spool's condition and the AMS moisture-control state before changing retraction. The most useful proof is a same-spool, same-file comparison after the filament has been dried according to its maker's guidance. If the wisps and rough travel marks improve without a slicer change, moisture drift was the better lead.
This page covers one exact failure: PETG stringing that appears or gets worse after the spool spends idle time in a Bambu AMS. It does not assume every string is caused by moisture. A hotter-than-needed profile, partial nozzle contamination, feed drag, or a changed print file can create a similar-looking result, so the checks below separate those causes in a useful order.
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Use this symptom pattern before blaming the AMS
The page fits when the PETG spool was acceptably clean at the start of its loaded run, then developed more fine hairs, thicker strings between separate features, or rougher travel scars after it sat in the AMS. The pattern becomes more persuasive when the printer, nozzle, profile, model, and room have not otherwise changed.
| What you observe | Best first lead | What not to change yet |
|---|---|---|
| The same spool printed cleanly, sat loaded, then became stringier | Spool condition and AMS humidity control | Retraction distance, speed, and temperature all at once |
| A newly opened or freshly dried spool strings immediately | Profile, nozzle, or feed path | Buying AMS moisture hardware |
| Several materials changed after printer maintenance | Nozzle and mechanical baseline | Treating PETG loaded time as the only cause |
| Stringing changed only with a different model or profile | Travel path and slicer baseline | Drying every spool without a control test |
First, identify which Bambu AMS you have
The model changes what a valid fix looks like. The original AMS is an airtight, desiccant-assisted storage and feed system; do not treat it as an active recovery dryer. The AMS 2 Pro and AMS HT add active drying, but a drying-capable model still needs the correct power, vent position, cycle, temperature, spool, material, and rotation conditions. A low humidity reading or a closed lid alone does not prove that moisture inside the PETG has been removed.
| AMS model | What it can establish | Correct next move |
|---|---|---|
| Original AMS | Sealed storage, desiccant condition, humidity trend, and normal feeding | Use a controlled external recovery method when the same-file test points to damp PETG |
| AMS 2 Pro | Active drying up to the model's documented 65 C limit, subject to its operating instructions | Verify the complete cycle, supported power, venting, rotation, spool, and material conditions |
| AMS HT | Active drying up to the model's documented 85 C limit, not automatic proof of a recovered spool | Use the exact PETG guidance and confirm the cycle finished under supported conditions |
This page stays narrower than the broad AMS wet-filament symptom guide. Use that route when the material pops, bubbles, pits, or prints rough from the start. Continue here only when PETG printed cleanly, sat loaded, then became stringier.
Use one stop-or-continue gate before the six checks
Continue with this page only if the same PETG spool printed cleanly, sat loaded in a Bambu AMS, and then became stringier while the printer, nozzle, file, and profile stayed the same. Stop this diagnosis if a new or freshly recovered spool strings immediately, several materials changed after maintenance, or the artifact appears only on one new file. Those results point first to the nozzle, feed path, travel moves, temperature, or supported profile, not to loaded-time moisture drift.
This gate matters because drying and retraction can both make a small tower look different without proving the original cause. Preserve one failed file and one last-known-clean reference before changing anything. That gives every later check a pass/fail result instead of another guess.
Likely causes, in the order worth checking
- The spool was not actually dry when it entered the AMS. Passive moisture control is better at slowing change than recovering a spool that already needs active drying.
- The desiccant or internal air has drifted. A sealed-looking AMS is not proof that its moisture-control material is still useful or that frequent opening has had no effect.
- The spool sat loaded longer than your workflow can support. The important evidence is the before-and-after print behavior, not a universal number of safe days.
- The PETG spent time exposed outside the AMS. Bench, dryer, storage-bin, and loading habits all belong to the timeline.
- A non-moisture problem happened at the same time. Profile changes, nozzle residue, feed resistance, or a different geometry can imitate part of the symptom.
Check 1: prove that this is new loaded-time stringing
Start with the last known-clean print. Use the same spool, color, printer, nozzle, build plate, profile, and preferably the same small test model. Confirm that the new sample has more stringing rather than a different seam, support scar, or travel path. Fine hairs between isolated towers are different from a single heavy branch caused by a leaking pause or a nozzle wiping problem.
If you do not have a clean baseline, print a small two-tower or separated- feature test and save the file. The test is not a magic calibration object; it is simply a repeatable comparison. The separate PETG stringing guide is the better route when the spool has always behaved this way and there is no AMS loaded-time pattern.
Check 2: inspect the whole spool timeline
Write down where the spool was from opening to failure. Include time on the bench, in a dryer, in a storage box, during AMS loading, and while the AMS lid was open for service or spool changes. The purpose is not to invent a precise exposure limit. It is to find the uncontrolled segment that changed between the clean and stringy prints.
A humidity display or AMS indicator is context, not a moisture meter inside the filament. A good reading does not prove that a spool loaded in a poor state has recovered. A poor or worsening reading does make the AMS control side a stronger lead. Use the wet-filament diagnosis guide if popping, rough extrusion, or inconsistent surface texture appears alongside the strings.
Check 3: inspect the model-specific moisture-control state
For an original AMS, confirm that the lid is fully closed, nothing is trapped at the seal, the desiccant holders are seated, and the desiccant is being replaced or regenerated under its own instructions. Record how often the unit was opened. Its official role is sealed, desiccant-assisted storage and feeding, so it can help maintain known-good PETG but should not be credited with actively recovering a spool that already prints poorly.
For AMS 2 Pro or AMS HT, add the active-cycle checks: correct supported power, the model's required vent position, a completed drying cycle, supported temperature, spool compatibility, and rotation behavior. Do not assume that a selected temperature means the filament experienced a complete effective cycle. Follow the current model manual and the filament maker's instructions.
A humidity display is context, not a moisture meter inside the filament. Color-changing desiccant is also a maintenance clue rather than laboratory proof. The AMS desiccant box and hygrometer review explains the accessory's limits; the general Bambu AMS dryness guide covers preventive storage rather than this exact PETG-after-idle failure.
Check 4: run one controlled dry-versus-loaded test
Save the failed file and keep the printer, nozzle, PETG spool, model, and profile unchanged. Recover the suspect spool using the filament maker's current guidance and equipment that can hold a controlled condition. Bambu's current filament drying recommendations list material-specific temperatures and times; use the exact filament maker's instruction when it is more specific. Do not copy an aggressive value from a different PETG grade, spool construction, dryer, or AMS model.
With an original AMS, this recovery step happens in compatible external equipment. With AMS 2 Pro or AMS HT, it may happen in the unit only when the documented power, venting, temperature, rotation, spool, and material conditions are satisfied. Let the spool return to a safe handling state if the instructions require it, reload, and print the saved file without changing retraction.
Compare both samples under the same light. Look for fewer fine hairs between separate features, fewer thick strings, cleaner travel marks, and stable extrusion. A clear improvement after correct drying makes loaded-state moisture drift the working cause. No meaningful improvement is an instruction to stop adding drying time and move to the feed, nozzle, and profile checks.
Check 5: rule out feed drag and nozzle contamination
Watch a load and unload cycle. Confirm that the spool turns freely, the filament is not crossing under another winding, and the path does not add unusual resistance. Inspect accessible PTFE routing for tight bends, poor seating, or obvious damage. A dragging path more often produces inconsistent flow or under-extrusion than classic moisture stringing, but it can muddy the same test enough to send diagnosis in the wrong direction.
Then inspect the nozzle exterior and extrusion behavior using the printer maker's safe maintenance procedure. PETG residue on the nozzle can be carried across travel moves and look like stringing. If the extrusion itself is weak or intermittent, move to the under-extrusion guide instead of forcing this AMS moisture diagnosis.
Check 6: tune temperature and retraction only after the material test
If a controlled dry test does not improve the symptom and the feed path is clean, return to the filament and printer maker's supported PETG profile. Print the same small test. Change one variable at a time and keep the result. A temperature that is unnecessarily high can increase ooze; a retraction change can hide one artifact while creating poor restarts or extra wear elsewhere.
Do not copy a dramatic retraction value from a different motion system, hotend, or extruder path. The goal is not to make a tower test cosmetically perfect at any cost. It is to recover a repeatable PETG profile that also starts, feeds, and produces real parts reliably.
Fix the cause that your test actually proved
| Proven result | Fix | Next proof |
|---|---|---|
| The same file improves after correct drying | Recover questionable PETG before loading; refresh AMS moisture control; shorten idle loaded time | Repeat after the next normal loaded interval |
| AMS readings or desiccant condition drift quickly | Correct sealing, opening habits, and desiccant maintenance | Watch the trend with a known-good spool |
| Drying does not help, but nozzle cleaning does | Restore the nozzle baseline; keep the PETG profile unchanged initially | Reprint the saved comparison file |
| Only one model or profile strings | Audit travel, temperature, and supported retraction settings one at a time | Confirm on a real part, not only a tower |
Use limits: know when to stop drying and leave this diagnosis
- Stop after one correctly completed recovery cycle if the unchanged file does not improve. More heat or time is not stronger evidence; move to feed drag, nozzle residue, and the supported PETG profile.
- Stop using this page when the spool never had a clean AMS baseline. Use the broader PETG stringing diagnosis because loaded idle time has not been isolated as the changed condition.
- Escalate popping, bubbles, pits, weak extrusion, or rough texture. Those symptoms need the wet-filament or under-extrusion routes rather than a stringing-only fix.
- Do not exceed the filament, spool, dryer, or AMS model instructions. A supported temperature for one PETG grade, spool construction, or drying unit is not permission to copy it to another.
- Do not approve a production part from a clean tower alone. Reprint a representative part and confirm surface quality, starts, flow, and the required mechanical function before trusting the recovered workflow.
Prevent the same PETG-in-AMS failure
- Load known-good PETG. Do not ask passive desiccant to recover a spool that already prints poorly.
- Keep an exposure timeline. A simple loaded date and last clean print are more useful than arguing about a universal safe window.
- Rotate idle spools out. Return quiet colors and specialty PETG to controlled storage instead of leaving every slot occupied forever.
- Maintain the AMS deliberately. Inspect the seal, holder, desiccant, and humidity trend as one system.
- Keep a saved control file. Use it before changing several slicer settings in response to one bad print.
If a passive AMS workflow cannot hold a proven baseline, compare an AMS heater with an external dryer. That is an escalation decision, not the first diagnostic step.
Common fixes that waste time
- Increasing retraction before drying the control sample. That changes the evidence before moisture has been tested.
- Assuming a closed lid means a dry spool. Enclosure state, desiccant state, and filament state are related but not identical.
- Drying hotter or longer without checking maker guidance. More aggressive is not automatically more controlled.
- Buying an active AMS heater for one unexplained print. First prove that loaded-state moisture drift repeats.
- Using sound alone. Popping can strengthen the moisture case, but the same-file before-and-after result is better evidence.
What to do next
If the same PETG spool improves after correct drying, fix the loaded-state workflow: recover before loading, maintain AMS moisture control, rotate idle spools out, and repeat the saved test after a normal use interval. If drying does not help, stop treating the AMS as the answer and inspect the nozzle, feed path, and supported profile in that order.
If several variables changed and the result is still ambiguous, use the PETG wet-versus-AMS-versus-overcorrection guide. It owns the broader symptom-separation decision; this page stays focused on new stringing after loaded idle time.
Common questions
Can PETG absorb enough moisture to string while it sits in an AMS?
It can become less consistent when the spool, AMS control state, exposure history, and room conditions allow moisture drift. Prove that on your setup with the same-file dry-versus-loaded comparison rather than assuming every string is moisture.
Will fresh desiccant dry a PETG spool that already prints badly?
Do not rely on passive desiccant as a recovery process. Dry questionable filament with an appropriate controlled method under the filament maker's guidance, then use the AMS to help maintain that known-good state.
Should I change retraction first?
No when the symptom appeared after idle AMS time. Preserve the profile, test the spool condition first, and change retraction only if drying and mechanical checks do not explain the result.
Does the original AMS dry PETG?
No active recovery cycle is documented for the original AMS. Its airtight structure, desiccant, and humidity information support maintenance of a spool that entered in suitable condition; use a compatible external recovery process when the same-file test points to damp PETG.
Can AMS 2 Pro or AMS HT dry PETG?
They provide active drying functions, but only within their exact model, power, venting, temperature, rotation, spool, and material limits. Completing a supported cycle and improving the unchanged test file are stronger evidence than a low humidity reading by itself.
Do I need an AMS heater for PETG?
Not from one bad print. Consider active drying only when a repeatable test shows that your normal passive AMS workflow cannot maintain the required PETG baseline after sealing, desiccant, turnover, and pre-drying are under control.