Is PETG Filament Moisture Resistant?

PETG, or polyethylene terephthalate glycol, is one of the most durable, food-safe filaments out there. Prints made from PETG are much more stable than those made with PLA or ABS, but is it moisture resistant?

PETG filament is moisture-resistant, but it isn’t completely waterproof. That said, though PETG filament absorbs water, it does so at a much slower rate than other types of filament. So any water it does absorb won’t damage its structural integrity.

This article will cover how PETG filament reacts with moisture and water. Then, I’ll tell you about how PETG reacts to submersion and air humidity and teach you about the best applications for this robust and water-resistant filament. 

PETG Filament Is Hygroscopic and Moisture Resistant

Hygroscopy is a term used to describe how much moisture any given material absorbs. Anything from soil to wood can be hygroscopic as they take in moisture from the air and any water from surrounding sources. 

Hygroscopy is primarily based on the chemical makeup of an object. In the case of filaments, some absorb moisture more than others since each one contains different chemical structures and elements. 

For example, TPU (Thermoplastic polyurethane) is hydrophobic, which means it repels water and moisture.

In contrast, PETG is hygroscopic, which means it absorbs moisture and humidity from the air, even after you use it to print an object. So, you’ll have to keep it dry and seal it away from moisture when you store it. 

That said, the rate at which PETG absorbs moisture is much slower than other filaments and better still, that moisture doesn’t affect its structural integrity, meaning the piece won’t warp or damage.

So, provided the model has sufficient thickness in the walls, it should remain moisture resistant. Still, that doesn’t mean it’s also waterproof.  

Moisture Resistance vs. Waterproofing In 3D Filament

When working with filaments, understanding which ones will absorb the least and most moisture can help you decide how to store your filament and how to use it. However, hygroscopy doesn’t play a huge role in whether something is waterproof. 

If you’re anything like me, you probably thought that water resistance means that a material repels water a little bit less successfully than a waterproof material. However, this isn’t the case, and water resistance means something different altogether. 

Moisture Resistant Materials Can Absorb Water

PETG is one of the most popular plastics used to make water bottles and food containers. That might make it seem like PETG is waterproof – but it isn’t. 

Instead, PETG filament is only water-resistant. That means it absorbs moisture, but the integrity of the 3D print will not degrade, even after days of soaking in water. 

Water-resistance only refers to a material’s ability to stay strong and intact when exposed to humidity and water. 

For example, a sponge is water-resistant, even though it’s absorbent. That’s because sponges are solid and flexible, even when you soak them in potent detergents and acidic dishwater. Likewise, PETG is water-resistant, and it stays tough even though it draws in moisture from water and air humidity. 

So, if you plan on keeping water inside or outside your PETG print, you can expect the material to stay rigid and intact. Because of this property, PETG is often an excellent material for printing vases, cups, aquarium decor and equipment, and pipes that’ll channel water through them. 

Waterproof Materials Repel Water

On the other hand, waterproof materials will completely repel water, not allowing any moisture to absorb inside. 

So, without a waterproofing treatment, your PETG print may seep a bit of water, which will look a bit like condensation over time. This expression of water is typical for this filament, which is why most people coat PETG prints in epoxy resin, varnish, or wax before storing water in them.

Alternatively, ensuring the walls are made thicker with no gaps should help keep any item from expelling water, even over time.  

Should You Dry Out PETG Filament Before Printing?

Although PETG won’t suffer any damage from containing or being exposed to moisture, 3D printers don’t perform their best when there’s water in your filament. 

Understanding which filaments are hygroscopic and which are hydrophobic (waterproof) is the key to understanding when to desiccate and when to use the filament straight from the spool. 

Printing with a moist filament is never a good idea. That’s because humid filaments swell, which may cause a nozzle blockage. In addition, wet filaments generally have weaker layer adhesion, resulting in a less durable, stringy, rough, and potentially failed print. 

It’s advised to dry out PETG filament before printing to ensure strong layer adhesion and that your printer nozzle doesn’t get plugged up or damaged by steam. Use heat from an oven or filament dehydrator to dry the filament entirely for the best results. 

How To Dry PETG Filament Before Printing

When using any hygroscopic filament such as PETG, ABS, Nylon, PVA, or PLA, you must use heat to remove the moisture. 

Using a dry, unheated desiccant such as silica gel won’t do the trick here. That’s because silica gel is just as hygroscopic as the PETG. So when you bury PETG filament in silica, the desiccant will absorb moisture from the air, but it can’t remove the water from the PETG. As a result, silica and PETG come to a moisture stalemate, where both of them are just as moist as the other. 

When dehydrating PETG filament in an oven:

1. Preheat the oven to 104ºF (40ºC).
2. Unspool the filament enough that it’s loose enough for the heat to reach the inside loops.
3. Place the filament on a tray and into the oven.
4. Heat for 4-5 hours.
5. Remove from the oven and allow to cool before use.

If you plan on storing the filament for use later, keep it in a desiccant such as silica and seal it in an airtight container or bag to protect the PETG from air humidity. 

Final Thoughts

PETG filament isn’t moisture resistant, but it’s water-resistant, meaning that it can stay durable no matter how much water you expose it to. 

This hygroscopic filament absorbs moisture from the air, which means it’s necessary to dry out the filament before use and keep it stored in a dry place in an airtight container to ensure that your prints succeed.