There are many reasons why FDM users choose PETG (Polyethylene terephthalate) as a 3D printing material. Stronger and more impact-resistant than PLA, this inexpensive thermoplastic offers a good level of flexibility and is suitable for mechanical parts.
Other advantages of PETG include its good chemical and moisture resistance, as well as its natural transparency.
But while PETG offers several benefits, it can also cause headaches for FDM users. Without the right preparation and calibration, PETG prints can exhibit large amounts of oozing and stringing — an undesirable phenomenon where a printhead nozzle deposits hair-width strands of plastic as it travels from one part of the build to another.
This article takes an in-depth look at PETG stringing, discussing why stringing occurs, how to diagnose it, and how to prevent it.
What is PETG stringing?
FDM 3D printing is an excellent technology for making accurate, low-cost parts from thermoplastics. The FDM process involves heating up a filament to its melting point, extruding it from a nozzle, and depositing it in 2D layers.
Unfortunately, any technology that deals with hot liquid plastic is liable to run into occasional problems. One such problem is stringing: when the printer nozzle leaks excess molten material as it travels from one area to another. This leakage results in fine strands of plastic being deposited over the build — sometimes making the part unusable.
Stringing can occur with any kind of filament, but it is particularly common with PETG as the material has a high melting point and requires a high nozzle temperature in order to flow and print properly. High-viscosity materials, on the other hand, are less prone to stringing.
How to identify stringing
In most cases, stringing is easy to spot. Fine strands of plastic resembling hairs or cobwebs will be visible on the build, particularly when features are spaced far apart.
It is possible to gauge the extent of a stringing problem by printing a stringing test print or stringing tower, a simple STL model (like this) that is susceptible to stringing by design. Users can print the model to see whether changes to certain print parameters help mitigate the problem.
How to reduce PETG stringing
There are several troubleshooting options for reducing stringing on PETG prints, from proper filament storage to adjustment of print settings.
Keep PETG filament dry
Moisture can cause stringing, regardless of the material. This is because moisture creates small air bubbles in the filament that burst when the material is heated. This bursting can cause leakage of the molten filament, which leads to stringing if the nozzle is in motion.
Because PETG is especially prone to stringing, it is imperative to keep spools of PETG filament dry: by keeping them in a sealed container with a desiccant, putting them through a filament dryer, or gently heating them in an oven before printing.
Adjust retraction settings
Retraction is a feature of FDM 3D printers in which the nozzle pulls back a small amount of filament before it travels to a new location. It is a feature designed specifically to prevent oozing and stringing, so setting the right retraction parameters is key to successful PETG printing (particularly when using a Bowden extruder over a direct-drive extruder).
There are four main parameters to focus on here, each of which can be adjusted to reduce stringing:
Retraction distance: One way to reduce PETG stringing is to increase retraction distance, which means the nozzle will retract a longer section of filament before travel. Try increasing the distance in increments of 1 mm, using the stringing tower to see the effect.
Retraction speed: Another means of reducing PETG stringing is increasing retraction speed, i.e. making the nozzle retract the small portion of filament more quickly. Try increasing the retraction speed in increments of 5 mm/s.
Minimum travel distance: Stringing is most likely to occur over large travel distances (when the printhead makes a large jump along the X or Y axes), so your default settings may not enable retraction over smaller distances. Try reducing the minimum travel distance so that retraction occurs even for small jumps.
Vertical lift: Z-Hop is a feature of the Cura slicer application that instructs the printhead to move vertically (along the Z axis) during travel to avoid dragging along the surface of the part. Disabling Z-Hop can reduce PETG stringing.
Adjust travel speed
If adjusting the retraction settings alone does not fix stringing, adjusting the printhead travel speed might.
Travel speed is the speed at which the hot end moves between two points without (intentionally) depositing material. If the printhead travels more quickly, there is less time for blobs to form or for excess material to drip out of the nozzle.
Combining a faster travel speed with a lower print speed may also help to reduce stringing.
Adjust nozzle temperature
Extrusion of PETG requires a high-temperature nozzle to ensure good material flow and layer adhesion. MatterHackers recommends an extrusion temperature of around 245 °C. However, higher temperatures and freely flowing material can lead to stringing.
When using PETG, try reducing the printing temperature in increments of 5 °C to see if stringing can be reduced without compromising print quality and durability, causing clogs, or negatively affecting adhesion (first layer or otherwise). You can also try increasing the cooling fan speed to cool the PETG after it leaves the nozzle.
Printing PETG can be a challenge without due consideration of filament storage and printing parameters. However, while PETG may not be as user-friendly as PLA, the advantages of using the material — including impact resistance, chemical resistance, and flexibility — make it worth the effort.
 Everything you need to know about PETG 3D printing [Internet]. MakerBot. 2020. [cited 2022 Feb 15]. Available from: https://www.makerbot.com/stories/engineering/everything-you-need-to-know-about-petg-3d-printing/
 How to fix stringing [Internet]. Ultimaker Support. [cited 2022 Feb 15]. Available from: https://support.ultimaker.com/hc/en-us/articles/360012016280-How-to-fix-stringing
 MH Build Series PETG [Internet]. MatterHackers. [cited 2022 Feb 15]. Available from: https://www.matterhackers.com/store/c/mh-build-series-petg