Under-extrusion: diagnose missing material

1. Compare the gap with the sliced preview

Inspect the failed region in the layer preview. Does the slicer actually command a line there? Open infill, an intentionally hollow region, or a detail omitted during slicing is different from a commanded line that failed to print.

Prusa’s under-extrusion guide identifies missing material between printed layers as a symptom with multiple possible causes. Record when it appears and what changed just before it began: a new spool, nozzle, profile, high-speed mode, or maintenance task.

2. Restore the matching printer and material profile

Check the printer model, installed nozzle diameter, filament diameter, and material selection. If a project came from another person, verify all of them again. Save a copy before returning to a known manufacturer profile.

Inspect any flow override on the printer as well as the slicer. Prusa explains that firmware flow and slicer extrusion multiplier are separate controls; changing one does not reset the other. An unexplained adjustment in both places makes later diagnosis harder.

Use a small repeatable model and a normal supported speed profile. If that succeeds, compare its saved settings with the failing job before buying replacement parts or changing calibration values.

3. Trace the filament route from the spool

With the job stopped and the machine in the condition specified by its manual, inspect the accessible filament route. Look for filament trapped under a wrap, a spool rubbing its holder, a pinched tube, or a route that pulls tight at part of the travel.

Follow the machine’s load/unload procedure if a feed test is needed. Do not pull against powered feed gears or reach into moving mechanisms. A clicking noise is a reason to investigate resistance; it is not an instruction to tighten the idler until the noise disappears.

If the spool or routing changed immediately before the failure, compare with the previous compatible arrangement. Keep the free end secured when a spool is removed so the next test does not introduce a new tangle.

4. Check whether the requested output is realistic

The hotend must melt material quickly enough for the requested lines. Prusa’s maximum-volumetric-speed documentation describes limiting material volume per second, rather than relying on travel speed alone. Wider lines or taller layers can increase demand at the same movement speed.

Return to the supported profile before experimenting with flow limits. If a slower supported profile improves the same geometry, record that observation; it suggests demand deserves investigation but does not prove the hotend is healthy.

Do not compensate by selecting an arbitrary hotter temperature. Stay within the filament and printer maker’s guidance. Our layer-height guide explains why a change in layer thickness is also a change in the printing task.

5. Investigate a failure that develops over time

Check for a blocked external fan opening, an error message, or an enclosure configuration that differs from the material instructions. A printer that begins normally and then loses extrusion can have a cooling-related problem.

Prusa describes heat creep as filament softening too far upstream, with warm environments and inadequate hotend cooling among the possible contributors. Do not confuse the hotend cooling fan with the part-cooling fan; changing the latter is not a general cure for the former.

Keep guards in place and use the manufacturer’s inspection procedure. If a fan fails or the printer reports a fault, resolve that fault before continuing. A flow multiplier should not be used to push through it.

6. Separate filament condition from mechanical faults

Confirm that the material and any fillers are supported by the nozzle and feeding system. Specialty formulations may have additional requirements. Start diagnosis with a compatible ordinary material if that comparison is available.

Surface defects, popping, or irregular extrusion can make moisture worth investigating, but appearance alone does not establish it. Follow our wet-filament guide and the material maker’s drying procedure before labeling a spool unusable.

If obstruction remains plausible after the accessible checks, use the exact manufacturer troubleshooting guide or support channel. Avoid generic instructions to poke, drill, or dismantle a hot nozzle.

7. Fine-tune flow only after feeding is stable

Once the printer consistently feeds the correct material with the correct profile, a supported flow calibration may be appropriate for a particular filament. Prusa classifies extrusion-multiplier calibration as an advanced process and notes that its published method has specific nozzle and layer-height assumptions.

Change one variable and save the result. Log the failed region, profile, spool, inspection result, and next test in the print log. If the problem persists, those observations plus the saved project give support a clearer starting point than “I increased flow and it still has holes.”