Drying filament is important but drying filament properly is even more important. Learn the science behind what moisture and drying does to polymer filament.

3D printing hobbyists and pros have gotten the message about wet filament and the stringing and print failures it causes. So much so, that a new issue is popping up: over-dried filament.

Too much drying — whether it’s at the wrong temperature, for too long, or too often — can damage the molecular structure of some polymers. This negatively affects strength, layer adhesion, and results in brittleness. It’s also very difficult to print with over-dried filament because the filament itself can break, extrude unevenly, and any resulting prints can show surface cracking.

Whether you have hundreds or tens of thousands of dollars invested in filament, it pays to have a plan to keep it dry and dry it correctly. Here, we’ll explore how to tell if your filament is too wet or too dry, the best practices for drying properly, and how to keep it dry (both budget and pro solutions).

Too Wet or Too Dry?

You may already be familiar with the problems that wet filament produces, such as popping or cracking sounds when extruding; severe stringing, blobbing, or oozing; uncharacteristically textured or “fuzzy” surfaces on prints; and uneven extrusion lines. Less noticeable is the reduced part strength and layer adhesion.

Wet filament may feel pliable to the touch, it could even have a moldy smell, or faded color.

On the other end of the spectrum, when filament is too dry, there are equally negative results and similar characteristics. You may notice poor layer adhesion or rough surfaces. Brittle filament can snap or break easily during handling, and there could also be stringing, blobbing, or other extrusion issues due to changes in the flow characteristics of the filament.

Dry filament may feel brittle, be easily cracked or broken, and may stick to itself on the spool.

Because too-wet and too-dry filament share some characteristics, how can you tell them apart?

One way to tell is the weight of the filament. When the material has soaked up water, it will weigh more. Measuring this, however, isn’t quite as easy as it sounds. That 1kg spool doesn’t weigh exactly 1,000 grams; there’s the weight of the spool and there’s also slight variations in the precise amount of material wound onto each spool. Plus the amount of moisture that could affect printing can be in the tiny milligram range.

Readers have told All3DP that they often weigh a new spool right out of the pouch on a precise scale and record that number for reference. Then dry the spool and weigh it again. Although this is a way to prove or disprove that your material was moist and now isn’t, it doesn’t provide information before drying. Plus, cardboard spools can hold more moisture than plastic ones, so you can lose milligrams of water just from the spool.

Another method is to cut two lengths of filament, maybe 200 mm each. Dry one of them then compare their weight. You could also put the filament in a sealed pouch with a moisture indicator (widely available for under $30) for a few hours. If the moisture indicator says the relative humidity in the pouch in above 40%, you likely have moist filament.

Because it’s not easy or quick to determine how much moisture your filament already contains, it may seem like the best route is to just dry it before use. In fact, this is recommended by many material producers regarding many types of polymer filament, such as nylon.

However, drying already dry filament can have consequences depending on the material and your drying process. But to be clear, most polymer types can be dried repeatedly with no damage as long as you follow the manufacturer’s recommendations for drying closely. Where drying typically goes wrong is using the wrong temperature (too high) and repeatedly allowing your filament to soak up water then drying it.

Let’s take a closer look.

Do Dry or Not to Dry?

If you’re someone who reads 3D printing forums or watches 3D printing YouTube videos, then you’ve come across two types of people: those who have never dried their filament and haven’t had any problem and don’t understand what all the drying fuss is about, and those who religiously dry their filament before each use. Everyone else is somewhere in the middle.

The point is, if your printed parts turn out with a layer adhesion, strength, and surface quality that’s acceptable to you, keep doing what you’re doing. But if you are not getting the quality you expect, the problem (very often) is the condition of your filament.

If you 3D print for or as your business, you need company-wide processes to follow to ensure your materials are always at the optimal condition.

When Should You Dry?

In a perfect world, the best way to keep filament in top condition to avoid ever drying it, especially with heat. There “should” be no reason to dry brand new filament that comes in an air-tight pouch. Presumably, this material was manufactured and packaged in a suitable environment. But this isn’t always be the case. In fact, some manufacturers, like Bambu Lab and Prusa, recommend that you dry specific materials, such as TPU filament and PA (nylon), before each use. (All3DP has been to the Prusa filament factory and it is state of the art, so the dry recommendation is likely just a precaution.)

This brings up the fact that all polymers are not the same when it comes to their reaction to moisture. Some polymer filaments, like PLA, may never need to be dried, while others, like nylon, quickly absorb moisture from the environment.

It’s Hygroscopic, Not Hydroscopic

You may have heard the term “hygroscopic” when it comes to filaments, which relates to how the polymer’s molecular structure absorbs moisture, e.g. highly hygroscopic. Fortunately, the characteristic is measurable and it’s a useful data point to know about your filament.

There are various methods for measuring this feature of a polymer, but when it comes to 3D printing filament you may see a mention of “moisture absorption rate” (MAR) by the manufacturer — although it’s not a standard measure on a material data sheet.

MAR is typically expressed as the percentage increase in the material’s weight after exposure to a humid environment for a specific period, often 24 to 48 hours, under controlled conditions of temperature and relative humidity, usually 50%. For example, a MAR of 0.1% isn’t the amount of moisture absorbed, but rather how much more the filament will weigh when exposed to typical moisture for a day or two.

Don’t put too much importance on the 24 to 48-hour measurement used in official MAR testing. This doesn’t mean that leaving your filament out for six hours or even one hour is fine. The rate of moisture absorption is directly influenced by the relative humidity of your environment. Higher humidity levels and temperatures increase the amount of water vapor available, leading to greater absorption by hygroscopic materials faster.

Likewise, a filament with a low MAR of 0.1% doesn’t mean that this is the maximum amount of moisture the material can absorb. The longer a material is exposed to moisture, the more it can absorb up to its saturation point, which also called moisture equilibrium or equilibrium moisture content (EMC).

Every material has a unique moisture equilibrium, which is the point where no more moisture can be absorbed into the filament and its fully ‘wet’. For materials like PLA, this equilibrium is under 1% (by weight) and “generally anything under 1% is negligible,” says Luke Taylor at filament maker PolyMaker. “This is why PLA is so friendly and you can leave a spool on top of your printer for months and it will still be fine.”

How to Dry Filament Properly

“When the moisture is absorbed into filament, it’s not like water in a sponge where it can be squeezed out easily,” notes Taylor at PolyMaker. “The water molecule forms a polar bond onto the polymer chain and acts as a plasticizer, reducing tensile strength and stiffness, whilst simultaneously increasing toughness and elongation. For this reason, it takes quite some hours to dry the filament as the heat needs time to break the polar bond and evaporate the moisture out of the filament.”

This is the goal of all filament drying, but how to do it takes many forms.

There are several methods for drying filament — filament dryers, ovens, dehydrators — each with its own set of perimeters that can get confusing. In fact, there is a lack of clear guidelines on drying filament in general, which can be blamed for the prevalence of over drying.

For example, Bambu Lab suggest drying its PC filament in a forced air oven for eight hours at 75 – 85 ºC but recommends drying the same PC in a heated build chamber of a Bambu Lab printer at 90 – 100 ºC for 12 hours. Sounds straightforward enough.

But what if you’re using a filament dryer, like the PolyDry from PolyMaker? That machine doesn’t let you set temperatures and instead has “power levels”. PolyMaker recommends drying any PC filament at power level three, which it says is less than 70°C, for six hours. But this is a lower temperature and less time than Bambu Lab recommends.

How can you be sure of drying when your filament maker says one thing and your filament dryer says another? Let’s take a closer look  filament dryers.

You dont have to buy a filament dryer to dry your filament but it is the best option in our opinion because most come with failsafes that help prevent over drying, such as automatic shut-offs. Many industrial FDMs, such as the Stratasys F3300, the Apium P400, and the 3ntr Spectral 30, have embedded material dryers. Yet these are typically only for four or six spools at a time.

Many drying solutions, from the consumer-focused Sunlu Filadryer S4 to the BigRep Drycon, have pre-configured temperature and duration settings of commonly used filaments, such as PLA, ABS, PETG, as well as TPU, PA and PC. Yet, as weve already covered, polymers can vary widely by brand. Ensure that your filament dryer enables you to manually override the settings to what each filament maker recommends.

Most filament dryers use heat and forced air, yet some only provide heat and youre required to open them every so often to let the moisture escape. Heat alone is not an effective dehydrator.

To ensure that your filament doesnt soak up moisture during printing, some dryer models feature an automatic filament feeding system via teflon tube.

Dry Storage Without Over Drying

Dry storage boxes or cabinets are the best way to keep your filament away from the harmful effects of moisture, but don’t make the mistake of thinking that these can’t ever lead to over drying. Even desiccant pouches (if you use enough of them) can lower the moisture content in a box to the point of damaging filament left there for extended periods, although we admit, this would be rare.

This is why moisture level indicators are critical in any filament storage solution. Only by monitoring humidity levels can you know if your storage is too moist or too dry, or perhaps you didn’t reseal the box properly last time your opened it. Hygrometers, at less than $20 each, or color indicating desiccants packets, are easy ways to ensure the right moisture level in your storage.

In the 3D printing industry, there are several solutions for single-spool storage, but these can get pricy if you have hundreds of rolls.

Other industries have a need to keep products and materials dry, so dry cabinets have been around for a long time. Professional photographers have long stored their cameras in dry cabinets while units in a wide range of sizes are used for printed circuit boards, laboratory samples, and electronics. It turns out that many of these are also ideal for 3D printing filament.

Of course, they lack the bells and whistles of dedicated filament storage units and there’s no way to feed filament directly from these cabinets to your printer, but for general or longer-term storage, these can be a good choice as long as they can maintain a relative humidity (>30%) that your filament needs. Note that typical camera dry cabinets have a minimum RH of only 35% so be sure to find one that can achieve a lower RH.