Stringing and Blobs When Printing with PETG – Causes & Fixes

As PETG is one of the materials that we can consider to be highly prone to oozing, the appearance of stringing and blobs when printing with PETG filament is a common problem that many users face when switching from a less ooze-prone filament, such as PLA, for the very first time.

In this guide, we will discuss the factors that can cause stringing and blobs when printing with PETG filament, take you through the corresponding solutions that will allow you to resolve these problems in the most convenient way possible, and look at some extra things you can do to reduce stringing and blobs when printing with PETG.

Stringing and Blobs When Printing with PETG – Causes & Fixes

While there’s not a whole lot we can do about PETG being prone to oozing by default, the appearance of stringing and blobs on your 3D prints mostly comes down to the misconfiguration of print settings in some way (or some slight issues related to the nozzle or the filament itself in rare cases), which you can correct to resolve the problem you’re experiencing.

severe stringing example
Source: Robert @ Stack Exchange (CC BY-NC-SA 4.0)

Retraction Distance Too Low

We can consider a low retraction distance value to be the primary factor behind the stringing issue that occurs when printing with PETG, as finding the optimal retraction distance where you don’t experience any problems will usually require some testing when setting your print profile up for the first time.

cura retraction-distance


When the retraction distance is way too low, your 3D printer won’t be able to pull the filament back far enough for it to stay within the hotend during the travel moves it performs, especially in the case of long travel moves where the filament has more time to travel down to the tip of the nozzle, which will lead to the plastic eventually oozing out of the nozzle and creating the stringing problem you’re experiencing as a result.

With this in mind, our recommendation when configuring retraction distance for PETG with a Bowden extruder 3D printer would be to stay within the range of 5 mm to 7 mm as a starting point, which should prevent oozing without creating issues that stem from using a retraction distance that’s too high, whether it’s clogging or filament grinding.

On the other hand, for a Direct Drive extruder, the retraction distance range we recommend starting with would be 1 mm to 2 mm, as the absence of the PTFE tube, which is the component that carries the filament from the extruder to the hotend in the case of a Bowden extruder, makes it possible to prevent oozing without having to pull the filament back as much.

In the case where the stringing problem still continues after adjusting the retraction distance accordingly, our recommendation would be to apply some additional fine-tuning by either increasing the retraction distance by 0.5 mm across a few test prints or printing a retraction tower (you can refer to our guide on configuring retraction settings for PLA if you aren’t familiar with how a retraction tower works), which will allow you to cross retraction distance off your list of possible causes moving forward.

Retraction Speed Too Low

Similar to how a low retraction distance value is the primary factor behind stringing, a retraction speed value that’s too low is the primary factor behind excessive blobbing when printing with PETG, as the retraction speed, just as the retraction distance, will require you to run some tests before you can find a value that works well.

cura retraction-speed


In a case where the retraction speed you’re using is lower than necessary for a clean retraction, the nozzle will end up spending too much time in the same spot as it retracts the filament prior to a travel move, which will give the molten plastic at the tip of the nozzle enough time to ooze out and leave a blob that will become larger and larger the more time it takes for your 3D printer to retract the filament completely.

When adjusting the retraction speed for printing PETG, our recommendation would be to stay within the range of 20 mm/s and 35 mm/s as a starting point, which should ensure your 3D printer is able to retract the filament quickly enough before it can ooze, but not so quick that the extruder gear ends up grinding the filament.

Similarly to the case of configuring the retraction distance, our recommendation when fine-tuning the retraction speed value would be to increase it in increments of 5 mm/s across your tests to ensure that you don’t end up pushing it too high at once or to print a retraction tower configured explicitly for the usage of varying retraction speed values, which will allow you to rule retraction speed out from being a possible culprit behind the appearance of blobs you’re seeing on your prints.

Retraction Minimum Travel Too High / Too Low

Another factor that can contribute to the appearance of blobs and stringing on your prints when printing PETG is using a retraction minimum travel value that’s either too low or too high, with the former causing blobs and the latter causing stringing.

retraction minimum travel


When the retraction minimum travel value you’re using is way too high, your 3D printer will end up retracting the filament across distances where a retraction would actually be necessary to prevent the plastic from oozing as the travel move is taking place, which will contribute to the occurrence of the stringing problem you’re experiencing on your prints.

On the other hand, in cases where the retraction minimum travel value is too low, your 3D printer will end up needlessly retracting the filament over very short distances where stringing wouldn’t take place regardless of whether there is a retraction or not, which will increase the number of retractions that your 3D printer performs, and as a result, increase the number of blobs that appear due to the nozzle stopping to retract the filament so frequently.

With these considered, our recommendation when configuring the retraction minimum travel value would be to use the default Cura value of 1.5 millimeters as a starting point, regardless of whether you’re printing PETG or any other filament, which should prove to be a balanced value that doesn’t create any issues on neither the stringing nor the blobbing fronts.

When it comes to fine-tuning the retraction minimum travel value, our recommendation would be to increase this value by 0.1 mm across test prints if you observe signs such as an increase in the number of retractions and blobs on your prints, to decrease this value by 0.1 mm across test prints if you notice an increase in stringing, particularly across short distances, and to find the highest value where you don’t experience either stringing or blobbing issues, which will optimize the number of retractions your 3D printer performs and save you some time in the long run.

Maximum Retraction Count Too Low (or Minimum Extrusion Distance Window Too High)

Maximum retraction count & minimum extrusion distance are another two parameters to keep an eye on in cases where you’re experiencing stringing and blobs when printing with PETG, as the usage of either a maximum retraction count that’s too low or a minimum extrusion distance window that’s too high can both lead to problems in this department.

maximum retraction count cura minimum extrusion distance window cura


In either scenario, whether it’s a maximum retraction count that’s too low or a minimum extrusion distance window that’s too high, the problem you will come across is stringing as a result of the limit on the number of retractions your 3D printer can perform across each particular length of filament, as this will create a scenario where your 3D printer will be unable to perform retractions even when they would be necessary to prevent the plastic from oozing out of the nozzle.

When configuring the minimum extrusion distance window value, the general rule of thumb we recommend sticking to is to set this value to be equal to the retraction distance you’re using, as this essentially simplifies things and makes it possible to use the maximum retraction count value alone to control the limit of retractions that will be applied.

On the other hand, when configuring the maximum retraction count value, we recommend following the simple formula of multiplying the minimum extrusion distance window value with 5 as a starting point, such as a maximum retraction count value of 30 for a minimum extrusion distance value of 6, which effectively means that your 3D printer will be able to perform a maximum of 30 retractions per 6 millimeters of filament.

To fine-tune the maximum retraction count, you can increase it by 5 at a time if you’re coming across stringing yet again due to a lack of retractions or decrease it by 5 at a time if filament grinding due to excessively retracting the same piece of filament starts becoming a problem, and keep testing until you find the value where there are no issues on either end.

Printing Temperature Too High

Provided that retraction is configured optimally, the next factor that immediately comes to mind when experiencing stringing and blobs on your prints is the usage of a printing temperature that’s too high.

cura printing temperature


When you print PETG with a nozzle temperature that’s above the recommended level, your 3D printer will end up melting the filament at a much faster pace than necessary, which will cause plastic to ooze out of the nozzle uncontrollably (over-extrusion) even when your 3D printer isn’t actively extruding filament and lead to both stringing and blobs based on whether the nozzle is actively moving as the filament is oozing out, or waiting at a particular spot.

Our primary recommendation to ensure that the printing temperature value you’re using isn’t way too high is to refer to the specification sheet for the brand of PETG you’re using by visiting the manufacturer’s website, where you should be able to find the recommended printing temperature range that will point you toward the right direction and to use a temperature value that’s closer to the middle of this range to be on the safe side.

As an example, if the manufacturer recommends the usage of a temperature range of 220 degrees Celsius to 260 degrees Celsius, which we can consider to be pretty standard for printing PETG, regardless of the brand, which can come in handy if the manufacturer of the PETG you’re using hasn’t specified a temperature range, our recommendation would be to start out with a temperature of 240 degrees Celsius to make sure that over-extrusion doesn’t take place.

If the filament oozing is still troubling your prints after the temperature adjustment, our recommendation would be to fine-tune this value further by reducing the printing temperature by 5 degrees Celsius at a time across test prints (or printing a temperature tower with 5°C increments) and to repeat this process until you either find a temperature value where oozing doesn’t occur anymore or until you reach the lower end of the temperature range.

Print Speed Too High

Even though correcting retraction and printing temperature should solve the stringing problem you’re experiencing in many cases, the usage of a print speed value that’s way too high is a non-retraction-related issue that can lead to stringing when printing PETG, which is something to look at if you haven’t been able to solve the problem yet.

cura print speed description


In cases where you’re printing PETG at speeds that are way too high, it becomes possible for the nozzle to drag some of the extruded plastic that’s still in its melted form during movement, as the plastic doesn’t get enough time to cool down completely and solidify by the time the nozzle is ready to start moving to its following location.

To configure the print speed value in a way that will ensure stringing doesn’t take place as a result of printing way too quickly, our recommendation would be to start out with a printing speed that we can consider to be on the lower side, such as 40 mm/s, and to run your test prints with this print speed until you resolve the issue, which will allow you to rule out print speed as a possible culprit.

Once you resolve the stringing problem you’re experiencing, you can then push the print speed value up by 5 mm/s at a time across tests to find the highest point where you don’t experience issues, which will be the optimal print speed value that reduces print times as much as possible without having adverse effects on your prints.

Travel Speed Too Low

While not as commonly encountered as the possible causes we have discussed so far, a travel speed value that’s way too low is another thing capable of causing stringing when printing with PETG, making it a good idea to ensure it’s correctly configured to check it off the list.

cura travel speed


In a scenario where you’re printing with a travel speed that is way too low, the non-extrusion movements (travel moves) where the melted plastic slowly (and unwarrantedly) moves toward the tip of the nozzle will take a whole lot longer than necessary – and even with perfectly configured retraction settings, lead to stringing due to the plastic having plenty of time to eventually ooze out of the nozzle due to the slowness of the movement.

While the travel speed you should be using depends on your 3D printer’s hardware, our recommendation would be to start out with a value of 150 mm/s in a case where you are unsure what your 3D printer can handle, which should be quick enough to prevent any stringing from happening as a result of the nozzle traveling too slowly, but also not so much to the point where issues such as ringing and extruder motor skipping may arise on some hardware.

Similar to print speed optimization, fine-tuning the travel speed is all about testing (10 mm/s increments should do) and finding the highest point where adverse effects don’t occur, and even though optimizing the travel speed won’t have as much of an impact as optimizing the print speed on reducing print times, it’s still one that we recommend doing as the time savings can add up to a considerable amount in the long run.

Printing with a Dirty Nozzle

Considering that everything seems to be adjusted correctly on the slicer side of things, the next thing to look at to solve the stringing issue you’re experiencing is the nozzle itself, as a nozzle that has become dirty over time can also contribute to this problem.

In essence, when printing with a dirty nozzle that has solidified filament residue around (or inside) it, heating the hotend up causes the residue to soften and melt again, which in turn creates a scenario where the melted material starts oozing from the nozzle and leads to the stringing issue you’ve been experiencing.

In this case, the primary step we recommend taking would be to clean any residue off the outside of the nozzle with the help of an isopropyl alcohol wipe (you can warm the hotend up to soften the residue if it doesn’t seem to be coming off, which should help things) until it’s entirely clear with no remnants of solidified plastic around it.

While cleaning the outside of the nozzle should be sufficient for the most part, in a case where the stringing issue continues with everything else seemingly in order, our recommendation would be to follow up with the cold pull process, which will allow you to remove any residue in the nozzle without having to take the nozzle apart and ensure that the nozzle is free of any residue that can lead to stringing.

Alternatively, if you are unsure whether you have been able to clean the nozzle successfully, you can also opt to replace the nozzle with a new one, which will allow you to effectively guarantee that the nozzle isn’t the culprit behind the stringing issue you’re experiencing.

Printing with PETG That Has Absorbed Moisture

Last but not least, if you can’t find the factor causing the problem you’re experiencing anywhere else, it’s often a good idea to look at the filament itself, as a spool of PETG that has absorbed moisture can easily lead to stringing and blobs.

When PETG absorbs moisture, which can especially be the case in scenarios where you have left the spool out in the open in a humid environment for a prolonged amount of time, the moisture inside will end up throwing off both the temperature and the pressure balance during printing, which in turn will lead issues such as inconsistencies in melting, changes in the viscosity of the molten plastic, and steam pressure disrupting the filament flow, with all these issues contributing to the appearance of stringing and blobs.

While there isn’t a guaranteed way to find out whether the PETG you’re printing with has absorbed moisture, some of the signs you will come across in this case, along with the stringing and blobs, are popping, cracking, and hissing sounds, a drop in the visual quality of your prints, inconsistent extrusion, filament sticking to the nozzle, and brittleness, which would practically confirm that moisture is indeed the issue.

Regardless, our primary recommendation to rule this possibility out, especially if you can’t seem to find the problem anywhere else, would be to pick up a fresh spool of PETG to replace the one you’ve been using, as this will practically guarantee that the stringing problem you’re experiencing doesn’t stem from the spool of PETG itself and allow you to focus on other areas without having to worry about this particular possibility.

That being said, once you do solve the stringing issue, we highly recommend referring to another article where we discuss different ways of drying a spool of PETG that has absorbed moisture and storing it in a way that will prevent moisture from getting to it again as it’s entirely possible to restore your spool of PETG and make it suitable for printing once more.

Extra Tips & Tricks to Reduce Stringing and Blobs When Printing with PETG

If you still seem to be having trouble with stringing and blobs, even after ensuring that you have adjusted the print settings we have mentioned earlier to be as optimal as possible, there are some extra slicer features you can utilize to resolve the issue you’re experiencing, which should be worth a try if you’re running out of options.

Enable Combing

We can consider combing to effectively be the best Cura feature to utilize when it comes to reducing stringing and blobs on your prints with how impactful it is, especially considering that it’s a pretty straightforward feature to use with almost no configuration required.

cura combing


When you enable combing, Cura will alter the travel paths in the G-code file it produces in a way that prevents your 3D printer from crossing the walls of your prints during any of the travel moves whenever possible, even if it means that the nozzle will have to travel a longer distance, reduce stringing visible from the outside by hiding the oozed material to stay within the printed parts.

Our recommendation when configuring the combing feature would be to use the Within Infill option, which will practically tell your 3D printer to avoid crossing outer walls, inner walls, and the skin, which will minimize stringing and scarring the most out of all the options at the expense of slightly increasing the print times in some cases.

Enable Coasting

Another Cura feature you can make use of to reduce stringing is coasting, and even though it isn’t as straightforward to use as combing due to some configuration becoming necessary for the feature to work as intended, it’s still a potent tool that will help you considerably in your pursuit to a stringing-free print.

cura coasting


With combing enabled, Cura will tell your 3D printer to stop extruding the material slightly earlier than usual when finishing up an extrusion path, which will effectively create a scenario where the material that’s oozing from the nozzle is used to complete the extrusion path and prevent oozing from occurring during the travel move that follows, which will reduce stringing due to there being no more material to come out of the nozzle as it moves.

Our recommendation when configuring coasting would be to follow the Cura recommendation of setting the Coasting Volume parameter to be close to the cube of the nozzle diameter, such as 0.064 mm^3 for a 0.4 mm nozzle, as a starting point, and to fine-tune it by using increments or decrements of 0.1 mm^3 at most based on whether oozing still seems to be an issue, or under-extrusion is taking place as a result of cutting the extrusion way too short.

Enable Z Hop When Retracted (Z-Hop)

Last but not least, Z-hop (Z Hop When Retracted) is a Cura feature you can make use of to prevent blobs, in particular, from appearing on the surface of your prints, and even though this feature will also require some configuration for optimal results, it’ll create a considerable difference if blobs have been troubling your prints.

cura z hop when retracted


When the Z-hop feature is active, Cura will prompt your 3D printer to lift the nozzle up slightly before each travel move, reducing the likelihood of the nozzle coming into contact with a previously printed area and either scar the surface or leave a blob.

That being said, as incorrectly configuring the Z-hop parameters, such as using a Z-hop height value that’s way too high, can increase oozing and make stringing worse, our recommendation would be to enable this feature as a last resort in cases where the surfaces of your prints get scarred as a result of the nozzle constantly coming into contact with them.

Conclusion

There’s no denying that the appearance of stringing and blobs when printing with PETG filament is both an annoying problem to deal with and also a slightly complex one to solve, as there are quite a few things that can go wrong to create this issue, requiring a lot of testing to find out where the problem is.

That being said, the good news is that when you solve the problem for the very first (and last) time, you will never have to deal with stringing and blobs again, at least when printing with PETG filament in particular, as you will have an optimal slicer profile consisting of perfectly dialed-in print settings that won’t cause you any trouble.