Optimizing Cooling / Fan Speed for PETG – Detailed Guide

We can consider cooling fan speed to be one of the trickier slicer settings to get right, especially when printing with PETG filament, as both too much and too little of it can considerably affect the quality of your prints in a negative way.

In this guide, we will discuss the optimal cooling / fan speed values for printing with PETG filament, take you through the process of optimizing the fan speed with a fan tower, and explain the effects you might observe when printing PETG with way too much or way too little cooling.

What Is the Optimal Cooling / Fan Speed Value for Printing PETG?

There isn’t one correct answer that will work flawlessly in every case when configuring the fan speed value optimally for printing PETG, as the amount of cooling you should use for the best results possible purely depends on the 3D model you’re printing and its purpose.

The first reason behind the optimal cooling fan speed varying based on the purpose of the part you’re printing is the fact that the rate at which the plastic is cooled has a noticeable impact on both the visual quality and the strength of the printed part, with these two attributes on the opposite sides of the spectrum.

When the cooling fan speed you’re using is on the higher side, the layers that your 3D printer puts down will solidify quickly, which will increase visual quality at the expense of decreasing the strength of the part, as the layers quickly solidifying will stop the material from sagging and drooping as a result of it staying in its molten state for too long, but also prevent them from forming bonds that are strong enough with each other.

On the other hand, with a lower cooling fan speed value, the layers of your prints will take a longer time to solidify, which will increase the strength at the expense of visual quality instead, as the layers will have more time to form bonds that are strong enough with each other before solidifying, but the longer amount of time that the material spends in its molten form usually ends up producing a rough surface with flaws.

Another factor that goes into deciding the cooling fan speed you should be using when printing PETG is the size of the layers, as how large a layer is has a direct effect on the amount of time it will take for your 3D printer to finish printing it, and move on to the next layer that it will place on top of it.

For 3D models with smaller layers, with the top parts of a miniature Eiffel Tower being a fantastic example of this, where it doesn’t take longer than a few seconds (~1-5 seconds) to finish up a layer, using higher fan cooling fan speeds will practically be a necessity, as not being able to get the printed layer to solidify before your 3D printer starts building on top of it will severely reduce quality, and in more severe cases, practically cause these layers to become a complete mess.

On the other hand, for 3D models where the layers aren’t way too small to the point where there’s no time for one layer to cool down correctly before the next one comes on top, you will have more freedom when choosing the fan speed value you’ll be using, at which point you can make adjustments based on the purpose of the part you’re printing once again.

Finally, the last deciding factor when optimizing the cooling fan speed for printing PETG is whether you’re printing overhangs and bridges, which require a whole lot more cooling than usual, as the lack of support beneath them causes them to be much more prone to sagging and drooping, meaning that it’s critical to get these parts to solidify as quickly as possible to stabilize them.

Putting everything we have talked about so far into consideration, our fan speed recommendations for printing PETG, which you can use as starting points, would be as follows for the various different cases you can come across:

  • General Purpose (Balanced): 40-60%
  • Functional Prints (High Strength): 0-20%
  • Figurines & Miniatures (High Visual Quality): 80-100%
  • Models with Small Layers: 90-100%
  • Overhangs & Bridges: 90-100%

Additionally, it can also be a good idea to make use of the Regular/Maximum Fan Speed Threshold, Maximum Fan Speed, and Minimum Layer Time parameters in Cura for a more in-depth adjustment of the cooling fan speed, which will especially come in handy when printing models that have a mix of both large and small layers that can benefit from different levels of cooling.

cura regular/max fan speed threshold description cura max fan speed description cura minimum layer time description


For instance, by setting the Regular/Maximum Fan Speed Threshold value to 15, Regular Fan Speed value to 60%, and Maximum Fan Speed Value to 100%, you can create a scenario where layers that take longer than 15 seconds use the Regular Fan Speed value of 60%, and the layers that take shorter than 15 seconds use a higher fan speed value that gets closer to the Maximum Fan Speed instead.

Due to the way that the Regular/Maximum Fan Speed Threshold parameter works, you will also notice that Cura automatically brings the speed of the fan closer to the maximum the shorter it takes for the layer to print, such as a layer that takes 14 seconds seeing a slight increase to the fan speed over the Regular Fan Speed, whereas a layer that takes only 7 seconds will be printed with a fan speed that’s much closer to the Maximum Fan Speed value.

Furthermore, in cases where no amount of cooling will allow the layer to cool down on time, even with the fan running at max speed, you can also add Minimum Layer Time (and Lift Head) into the mix with a value that’s lower than the Regular/Maximum Fan Speed Threshold, such as 5 for the purposes of our example, as this will give the layer extra seconds to cool down by slowing the print, or by lifting the head after it’s done and waiting for the time to go by in cases where the minimum speed limit is reached.

So, with these configurations in place, any layer that takes longer than 15 seconds will use the standard Regular Fan Speed value of 60%, any layer that takes less than 15 seconds will have a gradually increasing fan speed value that gets closer to the Maximum Fan Speed value of 100% as it gets shorter, and any layer that by default would have taken less than 5 seconds will take 5 seconds instead, whether with the 3D printer slowing down or idling for the extra seconds, printed with the Maximum Fan Speed value of 100% from start to finish.

Last but not least, when it comes to configuring the initial layer cooling fan speed, where things differ due to the dynamics of the adhesion between the bed and the first layer being distinct from the dynamics of interlayer adhesion, our recommendation would be to start off with a value of 0%, which we can consider to be the standard that should work without issues in most cases and to go up to 20% (test with 5-10% increments) at most if you’re experiencing the elephant’s foot issue, where the bottom layers end up getting squished due to solidifying on time.

That being said, since the cooling fan speed values we have mentioned in this section are merely starting points (except for the initial layer fan speed, where the rules are clear cut for the most part), which will help you to print successfully, but not in the most optimal way by any means, further fine-tuning your cooling fan speed value will become necessary for the best results possible, which we will cover in the upcoming section.

Optimizing Your PETG Cooling / Fan Speed Value with a Fan Tower

The best way to optimize your fan speed value for printing PETG is to print a fan tower, which we can describe as a 3D model that consists of different sections, with each printed at a distinct fan speed value to make it possible to find out the level of cooling that works the best, without requiring more than one test print.

cura petg temp fan tower example


For the purposes of this guide, we will explain how you can get a fan tower going with the Calibration Shapes plugin in Cura, which we have found to be the most convenient way of configuring calibration towers, whether it’s for retraction, temperature, fan speed, or practically anything else that comes to mind.

So, if you don’t already have the plugin installed, start off by clicking the Marketplace button on the top-right corner of the Cura window, which will bring up Cura’s Plugin Manager.

cura marketplace


Next, type “Calibration Shapes” (with the quotes) into the input box on the top of the window, click the Install button next to the Calibration Shapes plugin on the list, and follow the on-screen instructions to complete the plugin installation, and finally, restart Cura for the plugin to load.

installing cura calibration shapes plugin


Once that’s done, hover over the Extensions option on the menu bar located at the top of the window, followed by part for calibration, and click the Add a PETG TempTower option from the list, which will insert a temperature tower 3D model that we can also use as a fan tower into the workspace.


Now, you will need to hover over the Extensions option again, but this time, follow it up by hovering over the Post Processing option and clicking the Modify G-Code option, which will bring up the Post Processing scripts window where you will be configuring the parameters of the temperature tower.

cura post processing menus


For this process, start by clicking the Add a script button on the top-left corner of the window, and choose the TempFanTower script, which is the Calibration Shapes script that makes it possible to print the temperature tower with dynamic temperature and fan speed values.

cura tempfantower script in post processing


Next, check the Activate Fan Tower checkbox, and enter the percentage fan values you would like to test (seven in total for the seven sections of the fan tower), with semicolons between each value, into the Fan Values in % input box.

So, as an example, if you would like to test fan speed values for printing bridges with PETG, in specific, you can go for a range of 70% to 100% with 5% increments, which would translate to inputting 70;75;80;85;90;95;100 as your Fan Values in % value to test fan speed values of 70%, 75%, 80%, 85%, 90%, 95%, and 100% respectively, starting from the bottom and moving upward.

cura fan tower speed values


Once you configure the fan speed values, input the printing temperature value you usually utilize for printing PETG into the Starting Temperature input box and input 0 for the Temperature Increment input box, as we will be using this tower for testing fan values alone, meaning that the temperature should stay stable throughout the print.

cura fan tower temperature values


Next up, to ensure that each segment of the tower correctly corresponds to the fan speed you have chosen, you will need to configure the Change Layer and Change Layer Offset values, as this is the only way for the script to be able to to tell the layer numbers at which new sections begin, and change the fan speed value accordingly.

The quickest way to do this is to use the print settings noted in the PETG Temperature Tower information page of the Calibration Shapes plugin, which we will also share with you below, as using these settings will ensure that things fall into place with a Change Layer Offset Value of 5 and a Change Layer value of 52, and save you from further calibration.

  • Nozzle Size: 0.4 mm
  • Layer Height: 0.6 mm
  • Initial Layer Height: 0.2 mm
  • Line Width: 0.4 mm
  • Wall Line Count: 3
  • Top/Bottom Thickness: 0.8 mm

On the other hand, if you cannot use the specified parameters for any reason, such as using a bigger nozzle, which would naturally require you to use larger layer height and line width values, you will need to find the correct values manually instead to be able to print the fan tower correctly.

For this process, start by slicing the model and navigating to the Preview tab by pressing the button on the top-middle part of the Cura window to see a layer-by-layer preview of the fan tower you will be printing.

cura preview fan tower example


Next, bring the layer slider on the right side of the window all the way to 1, and slowly increase it until you find the last layer of the base, which is the one that’s right before the layer where the two squares appear on the left and right sides, and note it as the Change Layer Offset Value.

cura fan tower layer offset


Once that’s done, bring the layer slider all the way up to see the total layer count of the fan tower, and note this value down as well, as it will be necessary for our calculations.

cura fan tower layer count


Now, subtract the Change Layer Offset Value you noted earlier from the total layer count value you noted just now, and divide the resulting by seven (the number of sections), which will produce the Change Layer value.

So, for our example, the total layer count is 370, and the Change Layer Offset Value is 3, meaning that our Change Layer value should be:

(370 – 3) / 7 = 53 (rounded up from 52.42)

Finally, the last steps you will need to take are to uncheck the Use Adaptive Layers checkbox and to check the Enable Bridge Settings checkbox in Cura’s print settings menu, as recommended by the developer of the plugin, which will mean that your fan tower is ready to be sliced and printed.

cura adjusting print settings for fan tower


Once you have the fan tower at hand, you can quickly observe each of the sections (in our example, it would be the bridges in the middle in particular), pick out the one that has the best quality, find the fan speed you have used for that section (you can refer to the post-processing scripts area again if necessary), and use that value as your new fan speed value, or if we go off our own example, as the new bridge fan speed value.

Effects of Overcooling PETG During Printing

If you cool your PETG too much during printing, you will most likely observe the majority of the signs we have listed below, which should act as a guide that will allow you to find out whether you should reduce the fan speed.

Poor Layer Adhesion

Poor layer adhesion is the most common sign of overcooling PETG during the printing process, as cooling the layers down way too quickly will cause them to solidify without they can bond with each other, effectively preventing each of the layers from having a robust connection with the layers above and below.

Source: Valmond @ Stack Exchange (CC BY-NC-SA 4.0)

In a case where the part you have printed has been affected by poor layer adhesion, it’s likely that you will notice gaps between the layers, practically as if the part you have printed is cracking or separating horizontally between layer lines, which will also get worse once you exert force onto the part.

Additionally, it’s worth mentioning that severe cases of poor layer adhesion can also lead to layer delamination, which is a phenomenon where the layers effectively end up completely detaching from each other due to the interlayer adhesion being way too weak, in which case you will most likely be able to easily split the printed part into multiple pieces without too much force.

Brittleness on the Printed Part

Another widely experienced sign of overcooling PETG is the part you have printed turning out to be very brittle with almost no flexibility, which can be a dealbreaker in cases where you’re going for a functional print that needs to be able to bend a little at times.

When this is the case, you will notice that it’s practically impossible to deform the plastic in any way without snapping a piece of it off completely, and while this can make it seem like the part is more robust due to how rigid it is when you’re applying force to it, the brittleness will actually cause the printed part to become a whole lot more susceptible to shattering completely in most scenarios.

Poor Bed Adhesion & Warping (Initial Layer Cooling)

Last but not least, in a scenario where you end up overcooling PETG as the first few initial layers are being printed, by using an initial layer cooling value that’s way too high, in specific, the primary sign you will notice is poor bed adhesion that will almost always lead to warping.

warping example
Source: Zeiss Ikon @ Stack Exchange (CC BY-NC-SA 4.0)

In such a case, you will notice the printed part is peeling off the build plate from one side due to shrinking way too rapidly as a result of the overcooling, as this causes the force of the contraction to be greater than the force of adhesion between the part and the build plate.

Since the part peeling off the build plate reduces its stability and also changes its positioning, such a situation can sometimes lead to the nozzle bumping into the print, which, in more severe cases, can even cause the part to separate from the build plate completely and topple, effectively causing the print to fail.

Effects of Undercooling PETG During Printing

While rarer than overcooling, undercooling can also be an issue when printing with PETG, and if your fan speed value is low enough to create such a situation, you are likely to stumble upon some of the commonly observed signs we have listed below.

Poor Visual Quality

When you undercool PETG using a low cooling fan speed value, the first issue you will notice is a compromise in the visual quality of the part you have printed, as the plastic will either droop or become squished and eventually solidify elsewhere instead of solidifying in its correct place.

poor visual quality on 3d printed part example
Source: Daniel Cantarin @ Stack Exchange (CC BY-NC-SA 4.0)

In such cases, you will notice that the surface of your 3D printed part looks very rough with a lot of bumps that create a “smudgy” or “melted” look, as if you randomly rubbed molten plastic across the surface of your print, as parts of the plastic will spread around and solidify unevenly across the surface instead of solidifying at their correct place due to the plastic spending too much time in its molten state.

Dimensional Inaccuracies

While this sign practically goes hand in hand with poor visual quality, the appearance of dimensional inaccuracies is still a sign that we think is worth mentioning specifically, as it can be a considerable problem for functional prints where the dimensions of the model need to be close to perfect to fit in its place correctly.

dimensional inaccuracy example
Source: Adrian Maire @ Stack Exchange (CC BY-NC-SA 4.0)

Dimensional inaccuracies, in this case, will usually present themselves in the form of plastic that’s either curling upward from the edges or bulging from the sides due to not solidifying in time, which will prevent the part you have printed from having consistent dimensions across the board, with some layers becoming are wider than others.

Edges & Corners Curling

Even though this one can be harder to notice, a sign that you can also observe as a result of undercooling PETG is the edges and corners (if any) of the part you’re printing curling up, as failing to solidify these areas quickly in their place causes the layer that comes afterward to squish them, which ends up with these areas of the print solidifying in the air instead.

corner curling example
Source: R.. @ Stack Exchange (CC BY-NC-SA 4.0)

When this is the case, it will look like parts of the edges and corners are bulging outward & upward and forming a rough surface in the areas affected instead of a smooth and straight finish with no visible imperfections, which can also create issues for the dimensional accuracy of the part in more severe cases.

Overhangs & Bridges Sagging

If the 3D model you’re printing has bridges and overhangs, and you end up applying too little cooling when printing these areas, you’ll notice that they slowly start sagging, as molten material with support beneath it will end up getting pulled down by gravity before it can solidify.

Source: Kenny Wyland @ Stack Exchange (CC BY-NC-SA 4.0)

While this issue will primarily cause the undersides of the bridges and overhangs to become low quality due to the sagging material forming a rough and uneven surface once it solidifies, it can also lead to the bridge or the overhang curling downward or even falling off completely.

Elephant Foot (Initial Layer Cooling)

Finally, even though the widespread recommendation when it comes to tuning the initial layer cooling fan speed is to shut it off for the first layers, which works for the most part, it’s possible to come across a scenario where you experience the elephant foot issue due to the lack of cooling when printing PETG.

elephant foot example
Source: X Builder @ Stack Exchange (CC BY-NC-SA 4.0)

In this case, the layers close to the build plate end up being squished by the weight of the layers that come afterward due to them not being cooled down quickly enough to solidify in time, which causes the molten plastic to pop out from the sides near the base of the printed part.

Conclusion

Using the optimal fan speed value is one of the most effective improvements you can make to your slicer settings when printing with PETG, as the amount of cooling plays a critical role in determining both the visual quality and the strength of the part you’re printing.

While some testing will definitely be required to dial the fan speed in, especially considering that there will be situations where you will need to re-adjust it to make it compatible with the shape and the purpose of the part, the benefits that your prints get out of it will definitely be worth the effort.