Keeping Moisture Out of your Stored Filament

An occasional topic around 3D Printing is that the material filaments, especially PLA, are hygroscopic, meaning that they will absorb moisture from humidity in the air. Many reports indicate that the absorbed  moisture can cause various printing problems, so it’s probably a good idea to keep moisture out of your PLA as best as you can. It seems that ABS can also be affected by moisture, but to a lesser degree. Nylon and Polycarbonate are even more seriously affected by moisture than is PLA.

A roll of blue 3mm PLA filament mounted on the TAZ 3
The challenge is to minimize the amount of moisture absorbed by your filament

With summer and higher humidity levels coming here in the USA Midwest, I wanted to take some preemptive steps to keep my filament supply dry. I did some web searching, and I was inspired by this web page on using desiccant to keep a geocache box dry to prevent rust and mold. The author makes his own desiccant packs that include visual indication of when the desiccant is saturated with moisture.

About the desiccant

The desiccant that I am referring to is called silica gel, which is a man-made product that readily absorbs humidity from surrounding air. Silica gel is usually available as tiny beads, and is included in small packets inside the sealed bag whenever you buy filament. The purpose is to remove any small amount of residual moisture from the bag during shipping and storage, so you get fresh dry filament when you open the bag.

Two small desiccant packets with ruler showing that they are less than 4" (100mm) in length.
Commercial desiccant packets included in sealed filament packages

However, these small silica gel packs have 2 problems:

  1. They are always very small, so they can absorb only a limited amount of moisture before they become saturated;
  2. You have no way to know if they are saturated or if they are still able to absorb moisture.

After you’ve had the filament bag opened a few times, there’s a very good chance that the silica gel pack is fully saturated and can’t absorb any more moisture. Then it can’t do it’s job. No matter how tightly you seal the filament bag, whatever humidity is in the air will stay in the bag, to be absorbed by the filament. You need a supply of fresh desiccant in  your filament bags, so the desiccant can absorb the moisture from the air before the filament does.

Here I will describe my method of making and  using my own reusable desiccant packs. These home-made desiccant packs overcome both of the problems mentioned above:

  1. They are larger than the small packs that come with the filament, so they have much more capacity for absorbing moisture. You can make them as large as you want to.
  2. They have visual indicators so that  you know when they are saturated and need to be rejuvenated (dried out in an oven) and/or replaced with a fresh pack.

Supplies needed

  • Bulk silica gel desiccant with colored indicators
  • Package of children’s socks
  • Package of 2-gallon (7.5 liter) zip-sealing freezer bags for food storage
  • Wire twist-ties or rubber bands
A bag of silica gel, sold in craft/hobby stores, for use in flower drying
A bag of silica gel, sold in craft/hobby stores, for use in flower drying

I went to a local outlet of a hobby & crafts store to buy a bag of silica gel desiccant. This desiccant is packaged and sold for the purpose of flower drying. The flower crafters pour the desiccant beads into a container, add some fresh flowers, and then seal the container. The desiccant will eventually draw the moisture out of the flowers, leaving dried flowers for use in various crafts projects.

An open desiccant bag showing that the indicator beads are pink, telling us that the desiccant is saturated with moisture
Pink beads indicate that this silica gel is saturated and needs to be rejuvenated

This desiccant also has a visual indicator. The silica gel beads are normally clear or white, similar to glass beads. The particular brand that I bought has a small number of blue beads. As the desiccant absorbs moisture, the blue beads turn to pink. When there are no more blue beads, it’s time to change to fresh desiccant. The saturated desiccant can be baked in an oven to rejuvenate the beads, turning the pink  beads back to blue. Note that different brands of silica gel may use different coloring schemes, so check the instructions of the brand that you get.

I paid about US$12.00 for 5 pounds (2.3 kg) of silica gel desiccant, and noted that they were out of stock on a smaller bag that sold for about $5.00.

A low-cost package of children's socks used as the bags for my desiccant packs.
A low-cost package of children’s socks used as the bags for my desiccant packs.

Then I stopped at a local discount department store and bought a pack of the cheapest children’s socks that I could find. I use these small socks to form my reusable desiccant packs.

A box of ten 2-gallon Ziploc freezer bags.
These 2-gallon (7.5 liter) freezer bags are the ideal size for holding a spool of filament.

At my local supermarket I bought a package of zip-sealing food storage bags for freezing. The freezer bags tend to use thicker plastic, and so are more durable than lower-cost bags. My bags are 2-gallon (7.5 liters) capacity which is a good size match for storing my 1 kg filament rolls. These bags have double zipping closures for a better, more reliable seal. (I’m  not suggesting to freeze the filament; just that the freezer bags are very suitable for this application.)  

Making the desiccant packs

Using a roll of electrical tape as a form to hold a sock open while I scoop new silica gel desiccant into it
My method for filling the desiccant packs

I use the children’s socks as the outer bag for the desiccant packs. I fill the socks about half way, which provides a generously sized desiccant pack that fits into the hub of a filament roll for compact storage. As shown in the photo above, I use a roll of electrical tape to hold the sock open while I scoop silica gel into it. Then I twist the upper end of the sock a bit and secure it with either a wire twist-tie or a rubber band. (If you use wire twist-ties, be careful that the wires don’t poke holes in your freezer bags.)

Placing the desiccant in the freezer bag with the filament
Placing the desiccant in the freezer bag with the filament

Then I place the new desiccant pack into a zip-sealing freezer bag with a roll of filament and carefully seal it. It helps if you kind of hug the bag while sealing it, so that you can force out all of the air possible. This makes things easier for your desiccant packs because any air that’s left in the bag will contain moisture that needs to be removed by the desiccant.

Remove as much air as possible from the freezer bag as you seal it
Remove as much air as possible from the freezer bag as you seal it

Once the freezer bag is sealed, you can safely store it. Make sure everyone who has access to the filament knows the basic rule — the freezer bag should always be sealed, with a minimal amount of air inside.

A humidity gauge placed in a sealed PLA filament bag with large desiccant pack shows a reading of 16% relative humidity.
A humidity gauge placed in a filament bag to verify low humidity level

As a test, I sometimes insert a humidity gauge into a filament bag. This is a low-cost temperature & humidity indicator that I can buy locally for $15 – $20. For that price it’s not highly accurate, but it seems to be reliable as a relative indication. The low cost sensors in these gauges are designed for household use, and are not accurate at very low and very high humidity levels. For example, this one seems to have a minimum reading of 16%, as that’s the number that I get every time I do this test. It does take some time to stabilize after sealing the bag, and this is a good indication that the desiccant is doing its job.

Checking your desiccant packs

I suggest checking your desiccant packs every time you put a filament roll back into its freezer bag. In the web page I referenced earlier, the author describes  how he fabricated special desiccant packs using one clear panel so the desiccant can be viewed easily. If you’re ambitious, that’s a good idea. However, I  chose an easier route by using the socks as simple “bean bags.” So when I put a spool of filament away, I just open the sock and check the bead color. If it’s pink, I put in a fresh pack, and set aside the old pack for rejuvenation.

Here are some important considerations on managing your desiccant. This applies to the new desiccant in the bag, as well as your filament bags as you remove filament for use and then put it back away. It’s very important to make sure your bags are always closed and sealed. As soon as you remove some desiccant from its bag, seal the bag again so that it doesn’t become saturated by trying to remove moisture from the air in the room.

Same with the filament bags. When you remove a roll for printing, put the desiccant pack back in and seal the bag, removing as much air as possible. Being careful with these steps will make your life easier because you won’t have to bake your desiccant so often to rejuvenate it.

Rejuvenating your desiccant packs

The spent desiccant packs in their glass dish ready for baking. Then they can be sealed and stored in the same dish.
The spent desiccant packs in their glass dish ready for baking. Then they can be sealed and stored in the same dish.

The vendor of my silica gel recommends baking the gel at 250 degrees F (125 degrees C) for 45 minutes to drive out the moisture and prepare them for reuse.  Here are two ways to bake your desiccant:

  1. You can pour the silica gel into a pan or baking dish and bake for the recommended 45 minutes. You can verify completion when you see that the indicator crystals are back to their original color. Then you can refill the socks or set aside the dried desiccant for later use.
  2. You can leave the silica gel in the socks and place the packs into a pan or baking dish and into the oven. Since the silica gel is held in the packs, this method requires additional baking time. I found that 1.5 hours is about right.

In either case, put the silica gel or desiccant packs into sealed storage soon after removing from the oven so they will be available for future use.

Three sock desiccant packs in the glass dish in which they were baked. The sealed lid has been installed.
The desiccant packs were baked in this dish and covered with a sealed lid shortly after removing from the oven



I’ve introduced the reasoning and methods for keeping filament dry. This method should help you to store filaments for longer periods of time, so it can be more feasible to have many  colors on hand to enable some choice and variety in your printed objects.

If you have thoughts, experiences, or ideas to share, please leave a comment below.


Article text and photographs copyright 2014 by John Burt. All rights reserved.

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