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Filament Drying

Table of Contents

Essentials
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  • New filament is not normally dry filament.
  • If your new filament came in a metallic bag, it might be dry.
  • Clear vacuum-sealed bags are not moisture-proof1. Add desiccant to them.
  • Desiccant doesn’t dry filament. Drying filament well requires heat.
  • You can use your printer’s heatbed to dry filament.
  • Some filaments can be dried with a temp less than their target drying temp; you have to dry longer though.

  1. Water Vapor Permeation in Plastics by Pacific Northwest National Laboratory ↩︎

How filament drying works
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Heating “pulls” moisture out of the filament. You use desiccant in the drying chamber to absorb that moisture (so it doesn’t go back in the filament) and to keep new moisture from entering the filament. This dries the filament.1

This video from Lost in Tech explains some of the theory and application of filament drying really well.

Drying and Desiccation

How dry is dry enough?
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Dryer is always better. 2

Put your filament in a sealed container with a hygrometer. Let the hygrometer settle for a few hours. 20% RH or less is great for most filaments.3

PLA is pretty tolerant of moisture, and is often fine in “room air” that’s generally <50%RH.

TODO: PETG, ASA/ABS, PET

TPU and Nylons (PAs) are hygroscopic and can absorb enough moisture to cause problems in hours outside of sealed, low-humidity boxes.

Filament Dryers
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What to look for
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  • Accurate heating at the temperatures your filaments need
    • <= 70C is fine for PLA, PETG, and ASA/ABS
    • >= 70C for other filaments
  • Air circulation around the filament spool
  • Ability to print from the dryer (preferably while it’s drying)
  • A way for moisture to leave the chamber
  • A hygrometer or way to add one

Notes: Bambu AMS 2 Pro and AMS HT
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The AMS 2 Pro and HT are combination AMS units with drying capabilities.

Some caveats:

  • Can’t print from an AMS that is drying
  • Can’t use the AMS functionality for regular TPU
  • They suggest unloading the filament from the inlet from the AMS before drying

The ability to dry 4kg of filament overnight, with automated rotation and humidity venting, without needing space for a dedicated dryer might be worth the caveats.

Notes: AMS mods/add-ons
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There are commercially available products that add drying capabilities to the AMS. They’re new, and their quality and effectiveness isn’t well-known yet.

Recommendations
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Check out the dedicated Filament Dryer Buying Guide for recommendations on which dryer to get!

Desiccants
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There are three types of desiccant we normally see in the 3d printing world:

Summary
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Material Less Effective Below Typical RH Floor Achievable Approx. Capacity (g H₂O / 100 g desiccant) Notes
Silica Beads <30% RH ~20–40% RH ~25–35 g Strong at moderate humidity; weak at very low RH. Good all-round choice.
Activated Alumina <10–15% RH ~10–15% RH ~15–25 g Consistent performance; faster than silica; harder to recharge.
3A Molecular Sieve >70% RH (inefficient) ~1–5% RH ~20–22 g Excellent at low humidity; good for nylon/PVA/TPU, etc. Single-use for most.

Silica
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Silica is the most common type of desiccant; it’s easy to find, inexpensive, and easy to recharge.

If you’re using indicating (color-changing) silica beads, you should avoid the blue beads; they contain heavy metals.

Silica’s extra capacity (vs alumina) helps keep already dry filament dry over time.

Can be recharged in your filament dryer.

Recommended: Dry & Dry Mixed Orange Silica ↗ 4

Activated Alumina
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Can’t store as much moisture as silica, but it’s more effective at low humidity. It’s also more expensive.

Absorbs moisture faster than silica; useful if you’re opening an AMS, cereal box, or Polydryer often.

Activated Alumina dust isn’t good for you; wear an N95 mask when handling it.

You can recharge Activated Alumina in an oven at 240C/450F for 1-2 hours; it won’t be as good as new, but it will be better than nothing.

Recommended: Wisesorb Activated Alumina ↗

3Å Molecular Sieve
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Unless you know you need this, you probably don’t.

Extremely effective at low humidity. Not as common as silica or Activated Alumina.

Requires 550C (>1000F) to recharge. This is single-use if you don’t have industrial equipment.

Best choice for expensive, extremely moisture-sensitive filaments like Nylon, PVA, TPU, and some support filaments.

Recommended: Wisesorb 3Å Molecular Sieve ↗

Keep Exploring

Filament Dryer Buying Guide
AMS2 PLA Temperatures

  1. This is simplified. It’s good enough unless you want to learn about things like “Vapor Pressure”, “Surface Energy”, “Adsorption”, “Hysteresis”, or “Moisture Sorption Isotherms”. ↩︎

  2. This is true for most filaments. PA/Nylons can be overdried! ↩︎

  3. This method depends on Moisture Sorption Isotherms. ↩︎

  4. Mixed rather than just Orange, because Dry & Dry’s datasheets say Orange absorbs less than non-indicating: Dry & Dry Silica Beads. ↩︎