Why does PLA filament get brittle? Let’s solve it!

Not so long ago, I tried to print something using a filament color that I had not used in a while. Much to my surprise, the filament broke multiple times, and the overall quality of the resulting print was underwhelming. It goes without saying that I had not put much thought into properly preserving the spool while stored. If it happened to you, you may be also wondering why your PLA filament got brittle.

PLA can become brittle if it’s exposed to moisture since it’s a hygroscopic filament, meaning that it absorbs moisture, but being exposed to UV radiation (sun light) will damage it even more, making it lose its color and become brittle, weak, and easy to snap. To avoid this, proper storage is recommended.

Why does PLA become brittle and snap?

The mechanical structure of PLA or Poly-Lactic Acid is known to be brittle, but environmental conditions will, and do, contribute to its already brittle state. PLA is hygroscopic and absorbs moisture from the atmosphere, which changes its mechanical structure making it more brittle.

In addition to this, the sun’s UV rays break down both PLA filament and prints. Finally, not all PLA filament is manufactured and packaged to the same standard, so some brand types will be more fragile than others as a result.

It may have absorbed moisture

As mentioned, PLA is hygroscopic, meaning it absorbs moisture from the air causing it to swell, and alter its properties that can result in printing complications. Although it should be mentioned that PLA is less hygroscopic than Nylon or TPU, is still able to absorb enough moisture over short periods to negatively affect print quality.

Moisture-ridden filament swells and may jam in the Bowden feed tube or snap due to normal tension on a now fragile filament. Additionally, the water content in the filament boils at the extruder, causing steam that bubbles and pops and will affect the consistency of the extruded filament. These printing complications will ultimately affect the quality of the print. Just take a look at the following photo to get an idea of the impact that humidity can have on overall printing quality.

Left: PLA that absorbed humidity.
Right: PLA filament, properly preserved. Source

It may have been exposed to UV light

Like I previously mentioned, the biggest culprit when it comes to PLA becoming brittle and snapping easily, is the sun.

Leaving the filament roll near a window in direct sunlight won’t affect it if it’s only for a day or two, but leave it there for a month or two, and you’ll start to notice some degradation, mostly in the form of color loss.

However, that’s not all that happens to PLA when it’s in contact with the sun since it also becomes weaker and more brittle.

PLA should always be stored in a sealed container with something that absorbs moisture, such as silica beads, and it should never be in direct sunlight.

More on how to make a storing container for your filament at the end of this post.

Low-quality filaments

Not all manufacturers make 3D filaments to the same standards. In many cases, the ingredients and process are kept secret in order to capture a bigger market share. This is all fine and well from a business perspective, but it is the uninformed consumer who pays the price for inferior filament being sold at “affordable “prices.

Some color dye types cause the filament to become brittle, while in other cases, the packaging and shipping of filament are at fault. Being a biodegradable product, once opened, especially in humid conditions, the life span is greatly reduced unless strict storage measures are implemented.

To ensure the quality of the filament you are using, it is best to find a brand that produces quality filament and stick with that brand. You can experiment with other brands in order to create your own experience register of the best brands to use. Just be sure to have your 3D printing processes in order and always go the extra mile to correctly store and preserve your filament and prints.

How to print with brittle PLA filament

Printing with brittle filament is like setting yourself up as a crash test dummy; you will crash, and your print will fail. But on the other hand, if you are not aware of the fact that the filament has become brittle beyond normal use and only discover this when printing, then you can try and increase the nozzle temperature in the hope of getting rid of the moisture problem. However, this little trick does not always work reliably, so I tend to recommend not to use that filament until it is dry and in proper conditions again.

The printing community holds the idea, somewhat based on logic, that affirms that removing the humidity from the filament will revert the material to its original state. Although this is both plausible and possible, there is a certain element of luck involved.

Degradation occurs naturally with biodegradable products made from natural resources, and moisture and UV rays simply speed up the process. In cases where there is minimal moisture absorption and the filament is still within its shelf life, then attempts to salvage the filament are very feasible, and trying to dry the filament is definitely recommended.

The shelf life of PLA is very dependant on the manufacturer and the overall quality of the filament. Nonetheless, if properly stored after opening the package, you can expect your filament to remain usable for up to two years.

Under these conditions, it is possible to restore the filament to its original, so it is definitely worth the effort to not only dry the filament but to store and use it correctly as well.

How to dry PLA filament

Sunlu Filament Dryer: one of the most popular devices for drying filament. Source

PLA filament has a shelf life, and there are other thresholds that limit the use of the filament, one being moisture absorption. If said absorption has taken place over a long period of time and degradation has become permanent, then drying the filament will not help. But if the moisture absorption is minimal and has occurred over a short period of time, the filament can be dried and returned to its normal state.

Here are a few ways that have been used to successfully dry the filament to the point where it is still fine to print with.

  • Filament dryer: A filament dryer is a handy piece of equipment to have, especially if you stay in an area with high humidity levels like coastal regions. A filament dryer heats up the filament before use and, in doing so, allows for any moisture to evaporate. The temperature is always below the glass transition temperature of the filament (approximately 65C° for PLA), so you do not have to worry about melting your filament in the dryer.
  • Food dehydrator: Using a food dehydrator is actually ideal because the temperatures are conducive to drying without breaching the glass transition temperature of the filament. The process will be slow, but the results are worth the time.
  • Convection oven: A normal convection oven can be used to bake dry the filament. The process is very slow and should take from four up to six hours on average to dry the filament. If you have no other alternative, you should set the oven temperature between 40°C – 50°C if it can be set that low. I should also state that it is probably not optimal to use the same oven to cook food, but I certainly did it multiple times.

How to keep PLA from absorbing moisture

Regardless of the average humidity levels in your area, it is advisable to store your filament correctly. An easy solution is using a Ziploc bag with silica gel to absorb the little moisture captured in the bag. Airtight or sealed containers with silica gel are also a good option and might be even better for storage.

If you have a designated room for your 3D material, then using a dehumidifier will help control the humidity.  This will obviously depend on how much filament you have in storage and how often you print, as moving in and out of the room will increase the humidity.

Protecting your filament from external elements should be standard practice, and dispensing the filament into the printer should not unnecessarily expose the filament to open air. A storage box with silica gel to control the humidity can be used, and the filament would be fed straight from the dry box into the extruder. This will literally eliminate any moisture problems you might have had.

The bonus of using a dry box is that the end of the filament is fed through a sized opening directly into the extruder, which in addition to preventing the filament from moisture, also will prevent the filament from tangling up.

How to make Filament Dry box

A homemade filament dryer box. Source

Filament dry boxes are an important component of 3D printing as they ensure that your filament is perfectly preserved and ready for use. Although a dry box is in essence just an airtight box with silica gel to extract the moisture in the box, improved designs include a heater to facilitate the drying process.

There are several companies (brands) that make heated dry boxes, but you can easily make your own heated dry box for a fraction of the price, and it will work just as well as the branded models.

The main idea is to keep your filament dry and print-ready. The beauty of making your own heated dry box is that you can design it to suit your 3D printing activity. For example, if you don’t print too much and seldom change filament types or colors, then a smaller unit will serve your purpose. Conversely, if you work with two or three different types of filament, a bigger heated dry box will work better.

What you will basically need to build your own heated dry box are the following inexpensive items:

  • A plastic container big enough to take the number of spools you normally work with. The container should preferably have a sealed lid to limit airflow; however, it is not necessary for the container to be 100% airtight as heated air will need a small outlet.
  • A system that allows the filament spool to roll freely in the box. Rollers are often used, but this limits the spool sizes you can use in the box, so consideration is required based on the reel sizes you use.
  • A temperature and humidity gauge so you can monitor both these values.
  • A flat electrical reptile heater that is placed below the spool or spools which is normally set at about 32°C.
  • Silica beads or packets to absorb moisture. Silica beads are orange and turn green when saturated with moisture. They can be dried in the oven and, when they turn orange again, can be reused.
  • You will need to drill holes into the container for the filament to pass through, and it’s a good idea to attach a standard plastic pipe to each outlet that can feed the filament straight into the feeding mechanism of your printer. This will keep the filament warm as it enters the printer.

You can check out this awesome video of Becky Stern showing how to do you own filament dryier box:

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