Under-Extrusion Leaving Gaps and Thin Lines? Flow, Temperature and Clog Fixes

Thin, patchy walls with visible gaps between extrusion lines mean the printer is laying down less plastic than the model calls for. Under-extrusion has three usual culprits: a flow rate set too low, a hotend running too cool to melt filament fast enough, or a partial clog choking the nozzle. Work through them in that order and consistent, solid walls come back.

Quick Answer

Start by raising flow rate in 5 percent steps until gaps close, then lift hotend temperature by 5 to 10 degrees if extrusion still lags at speed. If both are correct and the problem persists, you have a partial clog: do a cold pull or push fresh filament through a heated nozzle to clear it. One of those three almost always fixes thin, gappy lines.

Reading The Symptoms

Under-extrusion shows up as gaps between perimeter lines, weak or missing infill, and walls you can sometimes see light through. It is the opposite of over-extrusion, where blobs and rough surfaces appear. The tell is consistency: if early layers print fine but walls thin out as the print speeds up, the hotend cannot melt plastic fast enough. If gaps appear randomly and the extruder clicks, you are likely fighting a clog or a feeding problem.

Fix One: Flow Rate

Flow rate (also called extrusion multiplier) controls how much filament the extruder pushes per move. If it drifts low, every line comes out underfed. Nudge it up in 5 percent increments and reprint a small test cube, checking whether the top surface fills in solidly. Most well-tuned setups land between 95 and 105 percent, so if you need far more than that, the real fault is elsewhere, usually temperature or a clog.

Fix Two: Hotend Temperature

Plastic that is too cool is too stiff to flow at print speed, so the extruder skips and the line comes out thin. Raise the nozzle temperature 5 to 10 degrees within the filament's recommended range and watch whether gaps close. PLA usually runs 200 to 215 degrees, PETG higher. If you print fast, you need the hotter end of the range simply because the filament spends less time in the melt zone.

Fix Three: Clearing A Partial Clog

A partial clog restricts flow and makes extrusion unpredictable. Heat the nozzle, push filament through by hand and feel for resistance, then do a cold pull (atomic pull) to drag out the debris. A worn or low-quality nozzle clogs more often, so keeping spares on hand saves a session. The most popular printing accessories include nozzle and cleaning kits that make this a five-minute job rather than a teardown.

When It Is The Machine, Not The Settings

Sometimes the extruder itself is the limit: a worn drive gear, a weak spring, or an underpowered hotend that cannot keep up with faster profiles. If you have chased flow, temperature and clogs and still fight thin walls, a more capable printer with a higher-flow hotend removes the ceiling entirely.

Frequently Asked Questions

How do I know if it is flow or temperature?

If walls thin out only at higher speeds, temperature is usually too low. If gaps appear evenly at all speeds, raise flow rate first. Test one variable at a time so you know which change worked.

What flow rate should I start with?

Default is 100 percent. Adjust in 5 percent steps and stop when top surfaces fill in cleanly. Needing far above 105 percent points to a deeper issue rather than a flow setting.

Can wet filament cause under-extrusion?

Yes. Moisture makes filament hiss, pop and extrude inconsistently. Dry the spool before assuming your flow or temperature settings are wrong.

Why does the extruder click during under-extrusion?

Clicking is the extruder skipping because it cannot push filament through. That points to a clog, too low a temperature, or a feed jam rather than a flow rate number.

Is a partial clog different from a full clog?

Yes. A full clog stops extrusion entirely, while a partial clog lets some plastic through but restricts and destabilises the flow, which is exactly what produces inconsistent thin lines.