Why Does My PETG Become Brittle After Printing? (Solved!)

It’s no secret that printing with PETG is more challenging than printing with PLA, causing many enthusiasts to have issues with their 3D printed models after switching to PETG for the first time.

While some issues are less difficult to solve than others, some, due to the fact that they can be a result of many different factors, can take a substantial amount of time, effort, and experience to solve.

PETG becoming brittle after printing is one such problem, which can be extremely frustrating considering that the reason behind most enthusiasts switching to PETG from PLA is the added flexibility that comes with it.

So, why does PETG become brittle after printing?

PETG can become brittle after printing due to numerous reasons, which we have listed below:

  • The nozzle temperature is too low.
  • The cooling fan speed is too high.
  • The printing speed is too high.
  • The printer nozzle is clogged.
  • The filament has absorbed moisture.
  • The filament is low quality.

As there are so many reasons that can cause PETG to become brittle, finding out the root cause will require you to look for other symptoms that come with these possible causes.

In the next section, we will analyze each of the possible causes in greater detail, look at what other symptoms they would cause, and what you can do to resolve the issue.

Why Does My PETG Become Brittle After Printing?

As PETG can become brittle after printing due to numerous reasons, we will look at each of these reasons individually, which will be helpful to pinpoint the root cause.

Nozzle Temperature Too Low

A low nozzle temperature is often the primary reason for PETG becoming brittle after printing.

When the nozzle temperature is too low, your PETG filament won’t be able to get to the desired temperatures for it to form strong bonds between layers.

The failure of the formation of these bonds causes the model to have an issue known as poor layer adhesion, which can present itself in the form of the model becoming brittle and weak.

If a too low nozzle temperature is the culprit, here are some of the other symptoms that may come with it:

  • Under-extrusion
  • Filament grinding
  • Nozzle jam
  • Poor bed adhesion

Fortunately, fixing a low nozzle temperature is not the most challenging task, as the manufacturer usually mentions the temperatures you should be using to print with the filament.

To fix the low nozzle temperature issue, tune your printer’s nozzle temperature to the values the manufacturer of the filament you’re using recommends.

If the temperature information isn’t available, we recommend using a nozzle temperature between 240 and 260 degrees Celsius and experiment until you get optimal results.

Cooling Fan Speed Too High

As the cooling fan also directly contributes to the filament’s temperature, a cooling fan speed that is too high can also cause PETG to become brittle.

When the cooling fan speed is too high, the filament ends up cooling down way too quickly, preventing the layers from forming strong bonds with each other.

The absence of these strong bonds causes the layers of the model to adhere to each other very poorly, causing the model to become brittle and susceptible to cracking and breaking.

If you suspect the cooling fan speed is the issue, here are some of the signs you should also observe:

  • Poor bed adhesion
  • Shrinkage
  • Warping

Resolving the issue of a low cooling fan speed usually comes down to experimentation to find the optimal values.

We recommend running a series of test prints while incrementally reducing the cooling fan speed by 10 percent until you get the optimal results.

While it’s possible to print PETG with 0 percent cooling fan speed, a high cooling fan speed helps improve your model aesthetically, making it a good idea to keep it at the highest point possible where you don’t face issues.

Printing Speed Too High

While a high printing speed sounds fantastic in theory, as it allows you to get your prints done as quickly as possible, it can, unfortunately, be a contributing factor in PETG becoming brittle.

When the printer prints at a very high speed, the model’s layers don’t get enough time to adhere to each other, as more and more layers keep coming, causing pressure on the lower layers.

As a result, the strength of adhesion between the layers of the models becomes very weak, which can even cause them to separate in some scenarios.

Here are some other signs that a high printing speed can cause:

  • Under-extrusion
  • Ringing (ghosting, echoing, rippling)
  • Warping
  • Blobbing
  • Stringing

While finding the optimal printing speed can require some experimentation, starting off with a base value that is accurate enough should solve your problems.

We recommend using a print speed between 40 and 60 mm/s with PETG filament, as values over 60 mm/s often cause problems with layer adhesion.

To optimize the print speed, you can adjust the value in increments of 1 to 5 mm/s until you find the results that work best for you.

Printer Nozzle Clogged

While it’s not something that we usually notice, the nozzle of the printer becoming clogged can cause a wide variety of issues, including brittleness.

A partially clogged printer nozzle will prevent the printer from extruding as much plastic as necessary, causing the model to become weaker than intended.

As a clogged nozzle will usually produce more noticeable problems than the model becoming brittle, it’s most likely not the culprit if brittleness is the only problem you’re facing.

Here are some of the symptoms a clogged printer nozzle can produce:

As there are plenty of ways to unclog a printer nozzle, the procedure you should choose mostly comes down to the severity of the clogging.

Unclogging the nozzle with the help of a needle or a piece of old and unused filament often does the job if the clogging isn’t too severe, which should get you up and running in no time.

On the other hand, using a technique such as cold pull or even entirely removing the nozzle from the printer to clean it may be necessary in more severe cases.

As the nozzle runs very hot, ensure that you wait a sufficient amount for it to cool down before you apply any of the unclogging procedures for safety purposes.

Filament Has Absorbed Moisture

Filament absorbing moisture can be a complete nightmare as it’s almost impossible to notice until the symptoms start presenting themselves during the printing process.

A moist spool of PETG filament causes a wide variety of issues, which can also include your 3D printed objects ending up brittle.

That being said, the symptoms you will observe while printing with a moist spool of filament are usually very apparent during the printing process, which will most likely prompt you to stop the process before the object is ready.

Here are some of the signs that moist filament will produce during printing:

  • Cracking and popping sounds during print
  • White smoke during print
  • Weak bed adhesion
  • Weak layer adhesion
  • Uneven extrusion
  • Blobbing
  • Stringing

There are a few different methods you can use to dry PETG, with the oven method being the most popular due to the availability of ovens.

Here are the steps for drying PETG with the oven method:

  1. Preheat your oven to 65 degrees Celsius and wait until it’s ready.
  2. (Optional) Check the temperature of the oven with an oven thermometer to ensure that it’s accurate.
  3. Place the filament on one of the oven racks and leave it in the oven for at least three hours.

As the process for the filament to dry can take longer than three hours in some cases, the best course of action is to run a test print after the drying process and repeat another three-hour cycle if the results aren’t satisfactory.

Filament Is Low Quality

If all else seems to be in order, the only remaining option is the PETG filament itself being low quality.

If your prints are becoming brittle even after ensuring that everything is in order, the best course of action is to try a fresh spool of PETG filament, preferably from a reputable brand.

Since lower quality filaments often contain lesser amounts of actual PETG combined with cheaper materials, issues during print can occur as a result.

Wrapping Up

While finding out why your PETG is becoming brittle after printing can be a very tedious process, you can think of it as a learning experience that will come in quite handy down the road.

To recap, a nozzle temperature that is too low, a cooling fan speed, or a printing speed that is too high, or a clogged nozzle can all easily contribute to PETG becoming brittle after print.

If you still experience issues after configuring and maintaining your printer, the problem is most likely a result of the filament absorbing moisture or being low-quality.

We hope that this guide has been helpful for you to resolve your issues, and we will see you next time.

Happy printing!