Blobs and zits in 3D printing — 6 causes of lumps on the walls

Symptom: you can feel small lumps on the outer wall of the model, most often lined up in a single vertical stripe running the full height of the print. Sometimes instead of a lump there is a dimple or a short scar. Dimensions and geometry are correct, but the surface feels like sandpaper along that one band.

Blobs and zits form at the point where a layer starts or ends. In that instant the hotend has to change the pressure in the nozzle from zero to working pressure or the other way round. Molten polymer, however, is compressible and viscous, so it responds with a delay: at the start of the line there is briefly too little material, and at its end an excess flows out and stays on the wall as a dot. On top of that come all the situations where the nozzle touches the print during a travel move. Fixing it therefore means doing two things at once — getting the pressure under control and hiding the place where the layer closes.

Most common causes and fixes

1. The seam is placed somewhere visible

Lumps arranged in a perfectly straight vertical line are no coincidence: that is the seam, the point where the perimeter closes. By default the slicer puts it wherever the previous layer happened to end, and that is how you get a band visible over the whole height.

How to check: open the G-code preview and turn on the display of perimeter start points. If they coincide with the band of lumps, you have your answer.

What to do: set the seam position to Aligned or hide it in a corner (Rear, Sharpest corner). The Random option does not remove the lumps, it merely scatters them over the whole surface, which can make the model uglier than one tidy seam. In OrcaSlicer and PrusaSlicer you can additionally paint on where the seam should run.

2. Retraction and coasting are not dialled in

Retraction takes the pressure off at the end of a line, and coasting stops feeding material just before the line closes so that the excess already in the nozzle fills the rest. Settings that are too aggressive give a hole instead of a dot; settings that are too weak leave a lump.

How to check: look at the defect through a magnifier. A bump means too little retraction, a dimple — too much.

What to do: change the retraction distance in 0.2 mm steps and print the same small test pattern after every change. Only once retraction is right should you add a little coasting. Watch out with flexible materials: TPU Flex 98A tolerates large retractions poorly and works best on a direct drive at low speeds.

3. Pressure advance / linear advance is not calibrated

This is the most effective tool for this particular problem. Feed-forward pressure compensation reduces feeding before the hotend decelerates and increases it before it accelerates, so the flow keeps up with the motion. Without it corners swell and line ends leave dots, no matter how well retraction is set.

What to do: print the calibration pattern available in Klipper, Marlin and OrcaSlicer, then read off the value at the height where the corners are sharpest. Run the calibration separately for each material and repeat it after changing temperature or nozzle diameter — the same settings for PLA and for PETG will not work, because the materials have completely different viscosity at working temperature.

4. The filament has absorbed moisture

Wet filament produces lumps of a different kind: irregular, scattered over the whole surface, often with an air bubble inside. Water flashes to steam in the nozzle and tears the strand apart, and the material thrown out this way settles on the wall.

How to check: listen to the nozzle. Crackling and hissing are an unambiguous sign. The second symptom is a matte, rough surface where the material should be smooth.

What to do: dry the spool according to the table below and print straight from the dryer if you can. PA absorbs moisture fastest and drying it is mandatory. No amount of retraction tuning substitutes for dry filament.

5. The hotend drags across the printed surface

During a travel move the nozzle stays hot and coated with a thin film of polymer. Every graze against a freshly laid wall leaves a mark, and the material collected this way sooner or later breaks off and sticks to the model as a lump.

What to do: enable lifting of the Z axis during travel (Z-hop, Lift Z) by roughly one layer, and at the same time Avoid crossing perimeters, to cut down the number of travels over the model. Use Z-hop sparingly, because every lift lengthens print time and with some settings it aggravates stringing.

6. Deposits build up on the nozzle

A drop of polymer growing on the outer edge of the nozzle will sooner or later fall onto the model. This is encouraged by too high a temperature, a worn nozzle coating and printing abrasive materials without a suitable bore.

What to do: clean the nozzle hot with a brass brush before longer prints and lower the temperature by 5 °C within the material's range. With PLA/CF a hardened nozzle is essential, because brass quickly loses the shape of the bore, and a worn edge does not cut the strand off cleanly.

Material	Drying temperature	Time
PLA and PLA variants	30–40 °C	2–4 h
PETG	30–40 °C	2–4 h
PETG Transparent	30–40 °C	2–4 h
HT PETG	50–60 °C	6–12 h
ABS / ASA	40–60 °C	6–12 h
TPU Flex 98A	40–60 °C	2–4 h
PLA/CF	55 °C	4 h
PA (nylon)	65–80 °C	8–12 h — mandatory
HIPS	Not required — a non-hygroscopic material

Source: the official FDM technical data table from the manufacturer, Porima. Full guide: filament drying — temperature and time for every material.

How to tell lumps apart from other surface defects

It is worth spending a minute on identification before changing anything, because three different problems feel similar to the touch. Seam lumps line up vertically and occur exactly once per layer. Ringing from vibration is a wave that always repeats after a sharp edge and runs horizontally, in the direction of hotend travel — that is covered by the page on ghosting. Roughness scattered irregularly with no relation to the geometry is almost always moisture or over-extrusion.

There is also a case where the settings are not to blame. Filament with a fluctuating diameter causes pressure swings in the nozzle that neither retraction nor pressure advance can compensate for, because the compensation assumes a constant volume fed per millimetre. Porima filaments are produced to a ±0.03 mm tolerance, which lets you keep the settings you once found for the whole spool. If lumps appear suddenly on a material that previously printed cleanly, suspect moisture first, and a change of filament batch straight after.

Related problems

• Stringing — the same nozzle-pressure mechanism, only visible over a void.
• Wet filament — bubbles, crackling and a rough surface.
• Ghosting / ringing — echoes after edges mistaken for lumps.
• Over-extrusion — excess material building up on the nozzle edge.
• Clogged nozzle — uneven flow gives irregular lumps.
• Filament grinding — the result of overly aggressive retraction while tuning the seam.
• Burn marks and discolouration — deposits on the nozzle return to the model as a dark dot.