How to reduce stringing
Today we want to talk and give some tips to reduce the stringing in our 3D prints.
A common issue in FDM 3D printing is the so-called stringing effect, which leads to stringy or hairy parts. The name comes from the plastic strings that remain on the printed part after the printing process.
This artifact happens because of the sticky nature of the hot plastic. When the plastic is hot enough it becomes fluid and that enables it to be extruded through the nozzle. However, in this state, the plastic is prone to stick to the model, and when the print-head moves to another region of the model it can leave a trace in the form of a plastic string. Just the same as would happen with glue, honey, or any other viscous and sticky fluid.
PICTURE 2, PICTURE 3
It's important to note that the degree of stringing is not the same for all the printing materials available. When it comes to the most common 3D printing filaments, PETG and TPE/TPU filaments are more affected by this issue than PLA or ABS. This is due to the chemical and physical nature of those materials themselves. PETG is more viscous and sticky than PLA. The extra viscosity and stickiness make the parts stronger in terms of impact resistance but have the disadvantage of producing a worse stringing effect.
When our 3D printers are printing a model, sometimes the print-head is extruding material while moving, but other times the print-head is traveling from one region of the model to another without extruding. In these situations is when the stringing happens. The extruder stops pushing, but the melted plastic in the tip of the nozzle is in contact with some edge of the part, and when the print-head moves, a trail of plastic is left even despite the extruder is not pushing filament at all.
There are several things that we can do to improve or get rid of the stringing problem. However, is important to understand that in some situations is not possible to completely avoid the stringing. For example, in worse scenario cases like using PETG in a Bowden-extrusion printer and printing a model with a lot of spiky features and lots of traveling.
I will group the actions we can do to improve stringing in two categories: 1- Slicer-level actions and 2- Printer-level actions. I will start talking about the things we can do within the slicer because sometimes a little, fast tunning makes the problem go away. However, if the problem persists I recommend following the printer-level actions. These are good not only to fix the stringing but general tips to get the most out of our printers.
Lower the temperature
If we were to give one single tip that would be to decrease printing temperature. In fact, too high a temperature is the most common cause behind printing quality problems. This is also true when it comes to stringing. When the temperature is too high the melted plastic is more likely to ooze or leak down.
If you are having stringing issues, the first tip is to use a temperature in the lower range recommended by the supplier. Our suggested temperatures to print to avoid stringing would are:
PLA - 185 degrees.
PETG -220 degrees.
ABS - 235 degrees.
Retraction plays an important role in FDM 3D printing. When printing, normally the print-head moves from one point to another while the extruder pushes down material through the nozzle. This way the print-head deposits a track of plastic that allows to "print" the parts.
However, there are situations when the print-head needs to move but we don't want filament to come out of the nozzle. For example, if we print several parts at the same time, the print-head needs to travel from part A to part B but we don't want to extrude plastic during this travel movement.
Unfortunatelly, just stopping the extruder is not enough to prevent some melted plastic to come out of the nozzle, because of gravity and momentum.
For that reason, FDM 3D printers use the retraction mechanic when we want to suddenly stop depositing material. When retracting, the extruder pulls up the filament releasing some pressure inside the hot-end and finally preventing the unintended deposition of material.
The retraction movement is defined by 2 parameters, the retraction speed, and the retraction distance or size. The optimal retraction distance and speed depends on several factors such as the material used, the printing temperature, the hot-end, and the extruder. Because of this, is difficult to give general settings that work well for every user. The best way to go is to perform a retraction iterative test to find the optimal settings in each case.
In any case, we can suggest the following retraction parameters.
ABS/PLA in Bowden printer: 4 mm & 40 mm/s
ABS/PLA in Direct extruder printer: 1.5 mm & 30 mm/s
PETG in Bowden printer: 3.5 mm & 30 mm/s
PETG in Direct extruder printer: 1.5 mm & 30 mm/s
Note how we suggest a different setting depending on the type of extruder used. This is because the extruder setup makes a huge difference when it comes to retraction performance, being the direct extruders more accurate and less prone to produce stringing problems.
Increase travel speed
We explained how the string happens during print-head travelings without extrusion. Therefore, one way to reduce stringing is to increase travel movement speed so the time the filament has to leak is shorter.
Normally, travel speed is limited by the firmware, so we can only choose a speed lower than that. Too high travel speed could lead to step loss and compromise quality due to vibrations.
The maximum travel speed can be found using a retraction test.
In our profiles for Ender 3, we use a speed of 150 mm/s
Reduce the distance between parts
The stringing problem can happen between regions of the same model or between two models printed at the same time. If we are having stringing issues when printing several models at once, there is an easy fix to improve stringing performance. You may just arrange the parts closer in the slicer, reducing the space between them and the distance the print-head needs to travel.
Another great way to get rid of the stringing when printing several objects at the same time. Instead of printing all the parts at once, print them sequentially which is printing them one after another. You can do this by simply repeating the print job several times. This method is easy to buy time-consuming, because we need to detach the part and prepare the machine as many times as parts we want to print. However, modern slicers allow to prepare a printing job in such a way that the printer prints several parts sequentially. For your convenience, here you have some links that explain how to perform sequential 3D printing in different slicers.
Try different slicer
This may sound like an unconventional tip, but if we have tried all the other options and still struggle with stringing it is worth a try a completely different approach and use another slicing software. Despite slicing software are supposed to produce similar printable gcodes, sometimes there are big differences in the computed printing paths and there are subtle differences in how
some mechanics are performed. So in case of desperation, we would suggest trying another slicing software before giving it away.
Check hot-end and extruder
3D printing is a precision process and is mandatory that all the parts of the printer are in perfect condition to achieve good results. Within the 3D printing process, the mechanics that can lead to stringing problems are particularly delicate. To perform a clean retraction the extruder and the hot-end need to be in optimal condition. Sometimes, a 3D printer can be apparently working well because we are getting acceptable results with some material in some printing jobs. However, when we change to other materials or try printing parts with different geometry and requirements problems such as stringing arise. Is not uncommon that a printer has a latent problem that only shows under certain circumstances.
For this reason, our recommendation is not to give for granted that our printer hardware is not causing the problem. Regarding the stringing, the printer elements that we may want to double-check are the following:
Hot-ends PTFE tubing - The PTFE tubing inside the hot-end deteriorates over time especially when printing at high temperatures. If we keep having stringing problems, we may want to check the state of our PTFE and replace it in case it's worn or burnt.
Nozzle - Nozzles wore overtime and the rest of the crystalized plastics built up inside. Nowadays nozzles are fairly cheap, so if can't find a solution to our stringing problems replacing or cleaning the nozzle is worth a try.PICTURE8
Cooling fan and layer fan - Most hot-ends include 2 fans, one to cool down the hot-end, the other one to cool down the extruded plastic. It is very important to ensure that both fans are working well. The things to check are: 1- No blades are broken, 2- No dirt is blocking the airflow, 3- The funnel is well oriented and aligned. A humming sound normally indicates a problem. Take into count that fans deteriorate over time and need to be replaced.
Drive gear- This is the gear attached to the extruder motor. It may be dirty and not transmit the motion accurately to the filament. Check also that is firmly attached to the motor's shaft.PICTURE9
Extruder clamp- The mechanism that applies pressure on the filament against the drive gear. These can disadjust and get loose over time. PICTURE10
Bowden fittings and tube- Ensure that the fitting grips firmly the Bowden tube. It's an Ender printer's known problem that worn or defective Bowden fittings can lead to clogs and other problems such a stringing.
Is not uncommon that consumer-grade 3D printers are sold with not-so-well calibrated extruder steps. This means that the extruder might not be pushing or pulling the amount of filament it should. As mentioned, 3D printing is a precision process and retractions are subtle mechanics within this process that will not work correctly if something is out of place. When the extruder steps (e-steps) are not well calibrated the amount of filament pushed or pulled is bigger or smaller than it should and can lead to stringing issues. Follow this guide to check if your extruder is well calibrated and calibrate it correctly in the case is not:
Consider using a direct extruder
No matter how well tunned our slicing settings are or how well adjusted our 3D printer, when it comes to extrusion accuracy direct-extruders are always going to outshine Bowden ones. Nowadays most consumer-grade 3D printers like the Ender series use Bowden extruder. The Bowden has it's own advantages, removing weight from the X-axis, allowing faster speed, and being easier to assemble/disassemble. But despite these advantages, the extrusion and retraction of Bowden systems are less accurate making them more prone to produce stringy prints. Is perfectly possible to get non-stringy parts with a Bowden setup, but if we are close to the worst-case scenario, using a stringy material like PETG or TPU and printing parts with a geometry that needs a lot of travel movements may be the only way to completely avoid stringing is switching to a direct extruder.