How to Fix PLA Under-Extrusion

PLA is the easiest material to push through a hotend, so when PLA under-extrudes something real is in the way. The three usual suspects: heat creep softening the filament above the melt zone, a partial clog, or a speed profile that outruns even PLA's generous melt rate.

Suspect #1: heat creep (especially in enclosed printers)

PLA softens at about 45 °C — barely warmer than a hot summer room. In an enclosed printer the chamber can drift past that during long prints, and the filament goes soft above the melt zone, buckling and gripping the walls instead of feeding cleanly. The signature: prints start fine and degrade after 20–60 minutes, often ending in a full jam. Open the door, take the lid off, and never use active chamber heating with PLA. The full mechanism is in our PLA heat-creep guide.

Suspect #2: a partial clog

Dust, a charred fleck from an old high-temp print, or debris from cheap filament sits in the nozzle and throttles flow. Signature: under-extrusion that varies moment-to-moment, sometimes with the extruder clicking. A cold pull usually clears it — procedure in the clogging guide. If the machine has printed glow-in-the-dark or carbon-filled spools, suspect a worn bore as well and replace the nozzle.

Suspect #3: outrunning the melt zone

Check the volumetric ceiling. Bambu’s stock PLA allows 21 mm³/s of melt (40 mm³/s for the high-flow variant), and the slicer slows moves down to respect it. Slower machines set far lower limits — Prusa’s and Creality’s generic PLA baselines cap at 12 mm³/s. If someone raised the limit to “make it faster”, they disabled the safety net; put it back to stock.

Filament settings → Setting overrides → Max volumetric speed

Or give the melt more time and heat. Stock PLA prints at 220 °C in a 190–240 °C window; the top of the window exists precisely for fast printing. Gaps that appear only on long fast infill moves and vanish on slow outer walls are the classic melt-rate signature.

Also worth two minutes

Confirming which suspect you have

Time correlates with heat creep (starts fine, degrades); randomness correlates with clogs (varies line to line); speed correlates with melt rate (fast moves starve, slow moves don’t). Print one slow test cube: if it comes out perfect, you have a speed/melt problem; if it is still gappy, work the clog and feed-path list. The material-agnostic method — including extruder clicking diagnosis — is in the main under-extrusion guide; the daily-driver values in the PLA settings guide.

Frequently asked questions

Why does PLA under-extrude in an enclosed printer?

Heat creep is the usual cause: PLA softens at about 45 °C, and in an enclosed printer the chamber can drift past that during long prints. The filament goes soft above the melt zone, buckling and gripping the walls instead of feeding cleanly. The signature is a print that starts fine and degrades after 20–60 minutes, often ending in a full jam. Open the door, take the lid off, and never use active chamber heating with PLA.

What max volumetric speed should PLA use?

Keep it at stock: Bambu’s PLA profile allows 21 mm³/s of melt (40 mm³/s for the high-flow variant), while slower machines set far lower limits — Prusa’s and Creality’s generic PLA baselines cap at 12 mm³/s. The slicer slows moves down to respect this ceiling, so raising it to print faster removes the safety net and causes gaps on long fast infill moves.

How do I find out which cause my PLA under-extrusion has?

Match the pattern: degradation over time points to heat creep, moment-to-moment randomness (sometimes with extruder clicking) points to a partial clog, and gaps only on fast infill that vanish on slow outer walls point to outrunning the melt rate. Print one slow test cube — if it comes out perfect you have a speed/melt problem; if it is still gappy, work the clog and feed-path list.

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