When an FDM 3D printer works properly, it extrudes thermoplastic filament and deposits it in a sequence of two-dimensional layers. Sometimes, however, the 3D printer is unable to extrude the right amount of material, leading to noticeable issues like thin layers, missing layers, gaps, holes, and weakness of the printed part.
The problem of a 3D printer failing to supply adequate material is called under extrusion, and it happens to all printer users from time to time. Generally, under-extrusion is not a sign of any major hardware problem with the 3D printer, but the phenomenon can have a range of causes, which makes it tricky to diagnose the underlying issue. Under-extrusion can usually be solved by printer maintenance — cleaning the nozzle, for instance — or by adjustment of print settings.
This article examines what can cause under-extrusion and offers solutions for how to fix under-extrusion problems, both through physical maintenance of the 3D printer and adjustment of print parameters.
There are several potential causes of under-extrusion, and many of these relate to the condition and cleanliness of the 3D printer. Clogs, dirtiness, and incorrect calibration can all lead to extrusion problems, so learning how to identify these basic issues is an easy way to prevent under-extrusion.
FDM printers must feed a spool of thermoplastic filament through various parts of the machine, and there are a few areas where the material can leave a buildup of residue and cause clogging.
If under-extrusion starts happening suddenly (and you haven’t made any changes to the print setup) then a clogged nozzle is a likely culprit. There are few ways to clean and remove blockages from a 3D printer nozzle, including:
Using a brass wire brush (not steel) to remove debris
Using a needle to remove internal debris
Running a special cleaning filament through the printer and performing a cold pull (letting the filament cool while it is in the nozzle, then pulling it out)
Removing the nozzle and immersing it in a chemical such as acetone (best for ABS)
Removing the nozzle and using a heat gun to melt any filament left inside
Another hot end issue that can cause under-extrusion is a worn-out PTFE coupler — a component found on common FDM machines like the Creality Ender 3 and Ultimaker 2. The coupler should be replaced if it becomes deformed by heat.
The hot end isn’t the only place where clogs can occur. Bowden tubes can also get clogged up with filament, and this can equally cause under-extrusion, as not enough filament is able to pass through.
A clogged Bowden tube is often the result of heat creep: when the tube gets hotter than it’s supposed to be and melts the filament before it reaches the hot end. To clean the tube, first remove it, then push a small piece of sponge or cloth through it using a piece of filament.
Another common cause of under-extrusion is dirt on the extruder gears and motor. Any dirt or debris on these critical parts can be removed with a brush.
One reason why debris might accumulate in the extruder gears is incorrect tension on the filament. The tension must be great enough to cause small marks in the filament but not so great that the gears strip chunks out of the material, resulting in thin and weak layers.
Bowden tubes can get clogged when filament melts too early on its journey to the nozzle. This is sometimes caused by issues with the printer’s cooling apparatus: if the fan is not able to regulate temperature, the material can overheat, leading to under-extrusion. The cooling fan may cease to function properly if it is installed improperly or if jamming has occurred. Check it for stray pieces of filament or other debris, and remove these items with a brush.
Problems with the 3D printing material can cause under-extrusion. These problems can include damp filament, knots in the filament, and incorrect material settings (discussed in the print settings section).
Damp filament can indirectly cause under-extrusion. This is because filament expands when it absorbs too much moisture, and a bloated strand of filament can cause clogging within the nozzle. Fortunately, it is easy to keep filament dry by storing it correctly and, if necessary, drying it out thoroughly before use.
Filament can also become tangled or knotted, especially when it is regularly removed from the 3D printer and replaced. Using a filament clip can help to prevent this issue.
Recommended reading: How to dry filament
Under-extrusion and myriad other issues can be caused by minor technical bugs, which tend to pop up when using outdated 3D printer firmware and slicer software. Install updates when they become available.
Cleaning, maintenance, calibration, and firmware updates can all help to prevent under-extrusion and related issues. Sometimes, however, adjustment of print settings (in slicer software like Cura, for example) is all it takes to fix under-extrusion.
A common and easily resolvable cause of under-extrusion is insufficient nozzle temperature. If the hot end does not melt the material enough, it can struggle to push enough of it through the nozzle.
Slicers usually have default printing profiles for certain filaments. But it may be necessary to adjust the printing temperature by 5 or 10 °C to achieve the best results. Also note that non-brass nozzles such as hardened steel nozzles require higher temperatures, as they do not retain heat well.
Flow rate, the amount of plastic extruded per second, can be a cause of under-extrusion. When the set flow rate is higher than the actual capabilities of the printer, parts may come out under-extruded. Flow rate is defined by layer height, nozzle diameter, and printing speed.
In slicers such as Cura and Simplify3D, flow rate can be most easily adjusted via the extrusion multiplier parameter, which is usually set at around 90% (or 0.9) for PLA and 100% (or 1.0) for ABS. When under-extrusion occurs, this value can be increased in small increments until the problem goes away.
Note that the default flow rate will be incorrect if the filament diameter has not been set properly. For example, if you are using 2.85 mm filament but the slicer thinks you are using 1.75 mm, the printer will not feed through enough material at once. For the highest level of accuracy, you can use calipers to measure the exact diameter of your material.
Under-extrusion can be caused by printing the first layer too close to the build surface. Users sometimes calibrate the first layer very close to the build surface in order to improve adhesion, but this can have the negative effect of actually obstructing the nozzle and preventing the natural flow of filament 
First-layer calibration should strike a balance between adhesion and flow. If the print does not stick to the print bed, then a change in build surface material or use of an adhesive should be considered instead.
Recommended reading: How to clean 3D printer beds: Glass, PEI, adhesive
Retraction is when the printer nozzle pulls back a bit of filament before it travels. Retraction can prevent issues like oozing and stringing, but incorrect retraction settings can cause under-extrusion issues.
When using significant retraction, under-extrusion may occur immediately after travel (when the printhead hops from one part of the print to another without depositing material), as the interruption of material deposition causes a loss of pressure. Under-extrusion may be mitigated by increasing the retraction extra prime amount, which increases pressure in the nozzle after retraction.
Although under-extrusion is difficult to stamp out altogether, carrying out regular maintenance and checking slicer settings can help to prevent the issue in most cases. If under-extrusion occurs, attempt the following:
Check nozzle and Bowden tube for clogs
Clean gears, motor, and cooling fan
Store filament in a secure, dry place and check the spool for knots and overlap
Update printer firmware
Try increasing nozzle temperature
Try increasing flow rate
Try printing first layer further from print surface
Try adjusting retraction settings
 How to perform a hot and cold pull [Internet]. Ultimaker. 2020 [cited 2022 Apr 19]. Available from: https://support.ultimaker.com/hc/en-us/articles/360011620800-How-to-perform-a-hot-and-cold-pull
 Under-Extrusion [Internet]. Simplify3D. 2019 [cited 2022 Apr 19]. Available from: https://www.simplify3d.com/support/print-quality-troubleshooting/under-extrusion/
 Under-extrusion [Internet]. Prusa Knowledge Base. 2021 [cited 2022 Apr 19]. Available from: https://help.prusa3d.com/en/article/under-extrusion_2007/