Quick diagnosis
Blobs and zits are small, localized bumps that show up at specific points on an otherwise clean print, most often along the vertical seam line or wherever the nozzle stops and restarts extrusion. They are not the same as over-extrusion, where the whole part comes out oversized and walls bulge everywhere, and they are not stringing, where thin hair-like strands stretch between separate features instead of leaving a solid bump.
1. Check retraction settings first
A blob often forms right where the nozzle retracts and then pushes filament back out, because pressure that built up in the melt chamber releases all at once the moment extrusion resumes. Confirm retraction distance and speed match the extruder type: a bowden setup typically needs a longer retraction distance than a direct-drive extruder to clear the same amount of oozing filament, while a direct-drive setup usually needs only a short, fast retraction. If the slicer offers a “retract only when crossing open spaces” or “combing” option, enable it so the printer skips retraction on short internal moves where it is not needed, since every unnecessary retract-and-restart cycle is another chance for a blob to form.
2. Lower temperature and check for moisture
Printing hotter than the material needs lets filament ooze more freely at every pause, and that extra ooze is exactly what a blob is made of. Lower the nozzle temperature in small steps within the range recommended for the filament and see whether the bumps shrink or disappear. Wet filament makes the problem worse in a different way: trapped moisture turns to steam in the hot end and can push out an extra pop of material right as the nozzle restarts, so dry the spool and store it with desiccant if blobs cluster on a filament that has been open for a while.
3. Add or tune coasting
Coasting stops active extrusion a short distance before the end of a line and lets the remaining pressure in the nozzle finish the line on its own, which reduces the burst of material that would otherwise land at the stopping point. Start with a small coasting distance, on the order of a few tenths of a millimeter, and increase it gradually if blobs persist. Too much coasting starves the end of the line instead, leaving a gap or a thin spot, so treat this as a per-filament value to dial in rather than a fixed setting to copy between materials.
4. Check where the seam lands
Because the seam is where each perimeter loop stops and a new one starts, it collects more retraction and restart events than anywhere else on the model, which is why blobs so often cluster there. Confirm the seam is not set to land on a highly visible face, and pair it with a wipe-on-retract move if the slicer offers one, since a short wipe drags any built-up ooze along the wall instead of leaving it as a raised dot. The seam position and coasting guide covers the full set of options for hiding this line if the seam itself, not just the occasional blob on it, is the visible problem.
5. Rule out a systemic flow issue
If bumps are appearing everywhere rather than at specific retraction or seam points, and the part also measures oversized across flat walls, the cause is more likely systemic over-extrusion than isolated blobs and zits. Check e-steps, filament diameter and flow rate as covered in the over-extrusion guide before spending more time on retraction and coasting tweaks that will not fix a calibration problem.
Blobs and zits checklist
- Retraction distance and speed match the extruder type (bowden vs. direct drive).
- Combing or “retract only when crossing open spaces” is enabled where available.
- Temperature is at the low end of the material’s range and filament is dry.
- Coasting is enabled and tuned in small increments.
- Seam position and wipe-on-retract are set to minimize the visible bump.
- Bumps are localized, not paired with oversized walls across the whole part.