Not long after getting into 3D printing, you start hearing about all the different ways people store PLA (Polylactic acid). The Internet is scattered with systems ranging from the very simple to elaborate PLA humidors. Often accompanied by scant details, but ominous warnings ‘PLA absorbs moisture from the air’.

A photo of an elaborate PLA dryer and storage container.

It was enough to spook me, and change the way I stored my stock of PLA filament. But I was still skeptical, I had more questions then answers. The first came up often:

Absorb water? That doesn’t make sense. Isn’t PLA a plastic and waterproof?

PLA is a bioplastic and derived from plants like corn, potato and rice. Despite being food safe, PLA does breakdown (some even like to compost it). Left in the environment, it only takes several months to two years for pure PLA to degrade into carbon dioxide, water and methane. Older forms of plastic (like disposable drink packaging) probably helped form the impression of a impermeable waterproof material. These older plastics are derived from petroleum and can last in the environment for 500 to 1000 years. If PLA can breakdown so fast, it makes sense that it also easily absorbs water to help the decomposition process along.

How long does it take for PLA to absorb water?

Some scientists performed a great experiment that explored how moisture and different amounts of starch weakened PLA plastic. In their experiment, they submerged PLA in stainless steel tubs filled with water for 30 days. Comparing the weight before and after 30 days in water, they found that the PLA had absorbed a considerable amount of water and increased its weight by 1-6%. So starting with 1kg of plastic, it could have absorbed as much as 60 mls of water in only 30 days? I don’t store my PLA in water, but I live in the tropics and it is the wet season. Relative humidity has barely been below 80% for three months. At a wild guess, had I left my PLA exposed it would have taken on a similar amount of water.

Conversely if you live somewhere dry, there isn’t as much moisture in the air for the PLA to absorb. So it will be a much slower process.

What problems arise from using moisture damaged PLA in 3D printing?

Moisture damaged PLA can cause two main problems with 3D printing:

  • 3D Printer damage: Absorbing moisture makes the PLA filament swell. Feeding swollen PLA into a 3D printer can cause your hot end to jam. In some cases, severe jams require hot end replacement.

  • Poor 3D Print quality: The moisture trapped inside the PLA filament can turn to steam when rapidly heated by the 3D printer. These tiny pockets and bubbles of steam interfere with the flow of plastic out of the hot end.

  • Weaker 3D prints: In the same experiment above, the scientists also tested the tensile strength of PLA in three conditions:

    1. Dry - Spent no time in water.
    2. Wet - Spent 30 days in water.
    3. Re-dried - Spent 30 days in water, then completely dried.

    As you would imagine, the more water absorbed by the PLA, the weaker it becomes. Taking pure PLA, storing it in water for 30 days reduces its tensile strength by 6%. Doesn’t sound too bad right? Well. In the Re-dried condition, the tensile strength of PLA drops by a massive 33%. Enough to be noticeable. If you are heating up PLA enough for absorbed water to turn into steam, you are going to be drying the PLA as you go. So 3D printed ‘wet’ PLA surely falls into the re-dried category. That is, it will be up to 33% weaker than ‘dry’ PLA.

What are the best ways to prevent PLA from absorbing water?

So if you want to avoid any problems with PLA (including weakened prints), it has to avoid contact with moisture. If you live in the desert or somewhere else dry, you are going to be alright. Living in the tropics or somewhere else that is humid, use the following tips:

  • Avoid storing unused PLA filament for extended periods (Greater than 12 months). Buy and use just enough to keep you going for the year.
  • Store your PLA filament in an airtight container with some sort of desiccant. Airtight pet food containers are relatively cheap and large enough to hold a decent supply of PLA. As for desiccants? You could buy some silicon packets. But I just keep all those ‘do not eat’ packets that I find in the bottom of packaged food. Throwing them in with the PLA in my storage tub.
  • Photo by Eric Young.
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