Under-extrusion happens when a 3D printer pushes less molten plastic through the nozzle than the slicer has instructed, leaving visible gaps between layers, patchy infill, and parts that snap or crumble under light pressure. The fastest way to correct under-extrusion is to clear a clogged nozzle, raise the nozzle temperature by 5°C to 10°C so the filament melts completely, and increase the flow rate, also called the extrusion multiplier, in the slicer by roughly 5 percent. Those three adjustments resolve most under-extrusion cases within a single test print. Persistent under-extrusion, however, usually points to a mechanical or measurement problem elsewhere in the system, from worn extruder gears to filament that measures narrower than the diameter set in the slicer. Working out which of these factors is driving under-extrusion on a specific machine, rather than guessing at settings one at a time, is the difference between a five minute fix and a wasted afternoon of failed prints.
Key Takeaways
- Under-extrusion causes gaps between layers, thin walls, and weak parts, most often triggered by a partially clogged nozzle.
- Raising nozzle temperature by 5°C to 10°C and increasing the slicer's flow rate by around 5 percent fixes the majority of mild under-extrusion cases immediately.
- Slipping extruder gears, caused by loose tension or gears clogged with filament dust, are a common mechanical cause of under-extrusion.
- Filament that measures thinner than the diameter entered in the slicer will under-extrude consistently until that setting matches the actual measured diameter.
- Under-extrusion repeating across several different filament spools usually signals a hardware issue rather than one bad batch of filament.
| Cause of Under-Extrusion | Typical Symptom | Quick Fix |
|---|---|---|
| Clogged nozzle | Thin, inconsistent extrusion or a complete stoppage of flow | Heat to printing temperature, clear with a fine needle or wire |
| Nozzle temperature too low | Under-extrusion worsens as print speed increases | Raise nozzle temperature in 5°C steps |
| Slipping extruder gears | Grinding noise, filament dust, inconsistent feed | Clean the gears, retighten the tension screw |
| Incorrect filament diameter setting | Under-extrusion spread evenly across an entire spool | Measure with callipers, update the slicer's material setting |
How Under-Extrusion Happens During FDM 3D Printing
Every FDM 3D printer calculates how much filament to push based on a formula in the slicer: wall thickness, infill density, and layer height are converted into a target volume of plastic per second, which the extruder motor then delivers by rotating a set number of steps. Under-extrusion occurs whenever the actual volume of plastic reaching the nozzle tip falls short of that calculated target, no matter what value the slicer displays on screen. The shortfall can originate at several points along the filament path, from the spool through the extruder gears, down the bowden tube or direct-drive block, and finally through the heated nozzle itself.
3D printing depends on this precise, repeatable delivery of material layer after layer, which is why even a small shortfall compounds across hundreds of layers into a visibly weak or gap-riddled part. South African hobbyists moving up from a basic starter kit to a faster machine such as those in 3D Printing Store's Creality 3D printer range often meet under-extrusion for the first time simply because higher print speeds demand a higher flow rate than a slower, well-tuned beginner printer ever needed.
Diagnosing Under-Extrusion: Signs to Look For
Under-extrusion rarely announces itself with a single obvious symptom. Most South African makers first notice it on a finished part rather than mid-print, which makes catching the early signs during a job worthwhile.
- Visible gaps between vertical layer lines, most noticeable on outer walls and vase-mode prints
- Infill that looks sparse, stringy, or see-through compared with the slicer's preview render
- Parts that snap, crack, or flex noticeably more than expected for the chosen infill density
- Extrusion that starts strong on the first few layers and tapers off as the print continues
- A thin, inconsistent stream of filament during a manual extrusion test with the nozzle heated and no model loaded
Running a manual extrusion test settles most doubts quickly. Heat the nozzle to the filament's printing temperature, then push filament through the extruder using the printer's manual controls while nothing is printing. A steady, even stream of roughly the expected diameter indicates the hot end and drive system are working correctly, while a thin, curling, or stalling stream points straight at under-extrusion.
Under-extrusion is a distinct failure mode from the interlayer bonding problems covered in 3D Printing Store's guide to 3D print layer adhesion problems, and from the corner-lifting behaviour explained in the guide on 3D print warping, though a neglected printer can easily suffer from more than one of these issues at the same time.
The Four Main Causes of Under-Extrusion
Clogged Nozzle
Tiny bits of burnt plastic, dust, or debris accumulate inside the nozzle tip over time and progressively restrict the flow of hot plastic. A partial clog is often the quiet cause of under-extrusion that shows up gradually rather than all at once, since a print might start clean and only begin under-extruding once the blockage worsens under sustained heat.
Heat the nozzle to the normal printing temperature, then push a fine acupuncture needle or a length of thin cleaning wire up into the tip to break the clog apart. Follow with a cold pull, where the nozzle is heated, a length of filament is inserted, then the temperature is dropped slightly before pulling the filament back out, often bringing carbonised residue with it. A nozzle that continues under-extrusion after cleaning may simply need replacing.
Incorrect Nozzle Temperature
When a printer runs faster than the hot end can melt filament, the plastic behaves like thick syrup forced through a narrow opening rather than a smooth, fully melted stream. This produces under-extrusion that gets worse the faster the print head moves, since there is less time per millimetre of travel for the plastic to reach full temperature.
Increase the print temperature in 5°C increments until under-extrusion clears, or reduce print speed if temperature is already close to the filament manufacturer's upper limit. PLA generally prints well between 190°C and 220°C, while PETG needs 230°C to 250°C to flow properly at typical speeds.
Slipping Extruder Gears
The extruder motor grips filament using a set of small feeder gears. When those gears clog with plastic dust or the tension spring is set too loose, they lose their grip and grind the filament into shavings rather than pushing it forward, causing under-extrusion that often comes with an audible clicking or grinding noise.
Clean the gears with a stiff brush to remove built-up dust, then adjust the tension screw on the extruder so it clamps the filament firmly without crushing it flat. Filament with visible chew marks or flat spots is a clear sign that gear slippage has already been causing under-extrusion for some time.
Incorrect Filament Diameter Setting
Not every metre of a 1.75 mm spool measures exactly 1.75 mm. If the true diameter is closer to 1.70 mm but the slicer still assumes 1.75 mm, the software calculates a larger cross-sectional area than the filament truly has, so it under-commands the extruder motor and under-extrusion follows across the entire print, not just certain layers.
Measure the filament with digital callipers at several points along the spool, average the readings, and enter that exact figure into the slicer's material settings. This single adjustment resolves under-extrusion that no amount of temperature or flow rate tweaking will fix, since the root cause is a mismatch between the software's assumption and the physical filament.
3D Printers and Tools That Help Prevent Under-Extrusion at 3D Printing Store
The equipment below spans FDM 3D printers built with reliable extrusion systems and active flow monitoring, alongside resin printing hardware for makers running a mixed workflow. Resin printers cure liquid resin under UV light rather than pushing melted filament through a nozzle, so they do not experience under-extrusion in the way FDM machines do, though they remain useful additions for a workshop producing both types of parts. Full specifications, stock levels, and local warranty details for every machine below are available through 3D Printing Store.
Creality SparkX i7 3D Printer with CFS Lite 4 Filament Dispenser
Fully enclosed multi-material 3D printer with an AI camera that flags extrusion irregularities early in a print. The pressure-monitored hot end helps catch under-extrusion before it wastes an entire spool of filament across a long multi-colour job.
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Anycubic Wash & Cure 3
Wash and cure station for resin prints, built with a spacious chamber and a gooseneck UV light for hard-to-reach areas. Resin printing does not under-extrude in the way FDM prints can, but this unit rounds out a workshop running both technologies side by side.
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Creality Ender-3 V3 KE 3D Printer
High-speed open-frame 3D printer with a direct-drive extruder that grips filament firmly through fast prints, reducing the gear slippage that causes under-extrusion on older bowden-style machines. The 300°C hot end handles PLA and PETG with consistent flow at speed.
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Bambu Lab P1S 3D Printer
Fully enclosed CoreXY 3D printer with active flow rate compensation that continuously adjusts extrusion in response to acceleration and deceleration, reducing under-extrusion artefacts on fast corners and detailed sections that older printers routinely under-extrude on.
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Bambu Lab A1 Mini 3D Printer Combo with AMS Lite
Compact, self-calibrating 3D printer with a pressure sensor built into the hot end that measures filament flow in real time and adjusts extrusion automatically, correcting the under-extrusion that catches out many entry-level printers switching between filament brands.
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Creality Halot Mage S 14K 3D Printer
High-resolution resin 3D printer with a 14K LCD screen for fine detail work. Since resin curing under UV light replaces the melted-filament extrusion process entirely, it sidesteps under-extrusion altogether, making it a useful complement for makers who also run FDM machines.
View ProductStep-by-Step Guide to Fixing Under-Extrusion
Work through these steps in order rather than changing several settings simultaneously, since altering temperature, flow rate, and mechanical tension all at once makes it difficult to tell which change resolved the under-extrusion.
- Heat the nozzle to printing temperature and run a cold pull to clear any partial clog before adjusting anything else.
- Raise nozzle temperature in 5°C steps, testing a short print between each increase, until under-extrusion stops.
- Check the extruder gears for dust build-up and confirm the tension screw clamps the filament firmly without crushing it.
- Measure the filament diameter with callipers at three or four points along the spool and enter the average into the slicer.
- Increase the slicer's flow rate by roughly 5 percent if under-extrusion persists after the previous steps, then fine-tune from there.
A single-wall test cube printed at 100 mm sides is a useful diagnostic tool for isolating under-extrusion. Measure the printed wall thickness with callipers against the slicer's setting; a wall that prints thinner than commanded confirms under-extrusion and gives an exact percentage to correct in the flow rate setting.
Choosing 3D Printers and Filament That Resist Under-Extrusion
Creality and Bambu Lab both build pressure-sensing and active flow compensation into their newer machines, which catches the early signs of under-extrusion before a print is ruined. Direct-drive extruders generally resist under-extrusion better than bowden-style setups, since the shorter, more direct path from motor to nozzle leaves less room for the filament to compress, buckle, or slip along the way.
Filament quality plays an equally large role. Cheaply produced spools with inconsistent diameter are a frequent, overlooked cause of under-extrusion that no printer setting can fully correct. 3D Printing Store stocks a full range of 3D printer filament manufactured to tighter diameter tolerances, alongside 3D printing filament spare parts such as replacement nozzles and extruder gear sets for makers maintaining an older machine. Buyers comparing new machines can browse the full 3D printers for sale range to weigh direct-drive against bowden-style options before settling on a printer suited to their material list.
Under-Extrusion Prevention for South African Makers
Gauteng's dry Highveld winters bring low ambient humidity that keeps most filaments in reasonable condition, but hygroscopic materials such as nylon and TPU still absorb enough moisture over a few weeks to cause under-extrusion from steam bubbles forming inside the nozzle as the water flashes to vapour. Store opened spools in a sealed container with silica gel between print sessions, particularly during Johannesburg's humid summer thunderstorm season.
Maker spaces and university engineering labs around Hatfield in Pretoria and Randburg in Johannesburg see a predictable spike in under-extrusion complaints whenever students switch filament brands mid-semester without re-measuring diameter or re-calibrating flow rate. Building a habit of testing a new spool with a simple calibration cube before committing it to a long print job avoids the bulk of these avoidable failures.
Under-extrusion that resists every fix above, on a printer already several years old, is sometimes a sign the machine has reached the end of its cost-effective service life rather than a sign of a single broken part. 3D Printing Store's guide on whether 3D printing is worth it covers running costs and realistic expectations for South African buyers weighing a repair against a replacement.
Maintenance Tips to Prevent Recurring Under-Extrusion
Clean the nozzle and check for early signs of a clog every 40 to 50 hours of printing, since prevention takes minutes while a full clog can cost an entire evening of failed prints. Inspect extruder gears for dust build-up on the same schedule, brushing away accumulated filings before they start to affect grip.
Keep a set of digital callipers next to the printer and measure a new filament spool before loading it for the first time. Recording the diameter and the flow rate percentage that worked well for a given filament brand in a simple logbook saves considerable troubleshooting time the next time under-extrusion appears on a similar spool. Replace nozzles showing visible wear at the tip, since a worn orifice widens unpredictably and produces under-extrusion that no amount of temperature adjustment will resolve.
Frequently Asked Questions
What is the fastest way to fix under-extrusion on a 3D printer?
Start by heating the nozzle to printing temperature and clearing any clog with a fine needle or a cold pull. If under-extrusion continues, raise the nozzle temperature in 5°C steps and increase the slicer's flow rate by around 5 percent. These three changes resolve the majority of under-extrusion cases within one or two test prints. If under-extrusion persists after that, check the extruder gears for slipping and measure the filament diameter with callipers, since a mismatch between the true diameter and the slicer's setting causes under-extrusion that temperature and flow adjustments alone cannot fix.
Why does under-extrusion only happen with certain filament brands?
Filament diameter tolerance varies between manufacturers, and a spool that measures consistently narrower than the 1.75 mm the slicer assumes will under-extrude no matter how well the printer is otherwise tuned. Different filament brands also melt at slightly different temperatures even within the same material type, so a nozzle temperature that works perfectly for one PLA brand may run too cool for another, producing under-extrusion that looks like a printer fault but is a material mismatch instead. Measuring each new spool with callipers before a long print avoids this brand-specific under-extrusion entirely.
Can slipping extruder gears cause under-extrusion even with the correct temperature and flow rate?
Yes. Correct temperature and flow rate settings only help if the extruder gears can grip and advance the filament consistently. Gears clogged with plastic dust, or a tension screw set too loose, allow the filament to slip rather than feed forward at the commanded rate, producing under-extrusion regardless of how accurately the software side is configured. Filament with flat spots or visible chew marks along its length is a clear sign that gear slippage has been causing under-extrusion. Cleaning the gears with a brush and retensioning the extruder resolves this mechanical cause directly.