Zits and Blobs on Bambu Lab Prints: Causes and Fixes

Pattern 1

Small dots scattered over the walls

Why it happens

Bambu calls this the most common cause: the seam position is set to Random. Every layer starts and ends somewhere else, and each start leaves a small mark. Bambu's seam settings page says it outright: Random “will cause some ‘zits’ effects”. The second cause Bambu names is damp filament: the moisture boils off as the plastic leaves the nozzle and leaves small gaps or imperfections in the lines.

What to do

1. Switch the seam to Aligned or BackIn Bambu Studio: Quality → Seam → Seam position. That stacks the starts in one line instead of spreading them over the part.
2. Or paint where it should goThe Seam Painting tool lets you draw the seam onto the model, ideally where nobody looks.
3. Dry the filament if the dots stayBambu recommends drying every filament before printing, new spools included. Temperatures and times: our drying guide.

Fixed? Good. Still there? Next: it may be too much flow or pick another pattern.

Source: Dots on the print: Causes and Solutions; Seam settings

Pattern 2

One vertical line of bumps or a notch

Why it happens

That line is the seam: the point where each layer's wall starts and ends. Bambu calls it unavoidable in FDM printing, except in spiral vase mode. On a part with corners the seam can hide in one; on a round part like a cylinder it can't, so it stays visible. The goal is to put it where it doesn't matter or make it smaller, not to make it vanish.

What to do

1. Move it to a corner or the backNearest prefers concave or convex corners; on a part with sharp corners Bambu says this effectively makes the seam invisible. Back puts it behind the model, for example on a mask that should look clean from the front. Or paint it.
2. Try a scarf seamAvailable since Bambu Studio 1.9: in Quality, turn on Override filament scarf seam setting and pick Contour (outer walls) or Contour and Hole. The seam then ramps up like a scarf joint, which Bambu says reduces its visibility in most cases. Since 1.10 you can also switch it on per filament.
3. Use Bambu's general seam tipsThree wall loops, a slightly higher nozzle temperature with a slower outer wall, and fewer models per plate so the toolhead travels less.
4. Skip the seam on simple shapesSpiral vase mode prints one continuous spiral with no wall seam at all. It only works for simple shapes: a single outer wall, no infill and no top.

Good to know: Bambu Studio stops each wall a little early so the excess has room. That gap is the Seam gap setting, 15% of the nozzle diameter by default. Some seam options only show in Develop mode (Ctrl+P).

Fixed? Good. Still there? Next: the bumps may be oozing or pick another pattern.

Source: Seam settings; Seam (print quality)

Pattern 3

Blobs where a part starts, often with strings

Why it happens

Molten plastic leaks from the nozzle while it travels. Bambu lists six reasons: damp filament, long travel moves with too little retraction, many separate parts or wide spacing, a nozzle that is too hot, low-density filament such as PLA Aero, and a nozzle that is too large or worn.

What to do

1. Dry the filamentWet filament is first on Bambu's list.
2. Shorten the travelPut the models closer together and turn on Avoid crossing wall.
3. Lower the nozzle temperature a littleA nozzle that is too hot makes the plastic runny, says Bambu.
4. Check retraction and wipeRaise retraction length or speed slightly, but keep the length at 2 mm or less: Bambu warns that more can cause clogs. Wipe while retracting moves the nozzle along the last line as it retracts; Bambu says this can minimize blobs when a new part starts after a travel.
5. Match the nozzleThe nozzle size in the slicer must match the one installed, and a worn nozzle leaks more. For PLA Aero and other low-density filament, Bambu suggests a flow ratio of 0.5 to 0.7 and a lower temperature.

Strings are the bigger problem? The stringing and oozing guide goes step by step.

Fixed? Good. Still there? Next: it may be too much flow or pick another pattern.

Source: Stringing and oozing; Retraction

Pattern 4

A lumpy surface, lines that overlap too much

Why it happens

Too much flow. Bambu's flow rate guide describes over-extrusion exactly like this: excess material that forms blobs or zits, and layers that seem too thick.

What to do

1. Start with a clean, dry setupBambu says to calibrate only with dry filament, a clean hotend and clean extruder gears. A partial clog skews the result: do a few cold pulls first (how).
2. Run Flow Dynamics CalibrationBambu suggests the flow rate calibration only after Flow Dynamics Calibration, if the problem stays.
3. Then Flow Rate CalibrationIn Bambu Studio's Calibration tab. Pick the smoothest test sample; slight bulges at the start and end of the lines are normal, says Bambu.

Fixed? Good. Still there? Next: check the nozzle for a lump or pick another pattern.

Source: Flow Rate Calibration

Pattern 5

A big lump on the nozzle, chunks on the print

Why it happens

This is hotend clumping. If a part sticks too weakly to the build plate, or touches it over too small an area, it can stick to the hotend instead. The nozzle keeps extruding, the plastic piles up, and Bambu warns it can damage the hotend. H2 series printers can flag it with the HMS error 0C00-0300-0002-001C, Potential Hotend Clumping Detected.

What to do

1. Stop the printMore extrusion only makes the lump bigger.
2. Clean it with Bambu's guide for your printerBambu softens the plastic by heating the hotend slightly above the print temperature (240 °C for PLA in its X and P series guide), then removes it with tweezers. Wear heat-resistant gloves. Guides: X1 and P1, A1, P2S, X2D, H2D, H2S, H2C.
3. Fix the adhesion so it doesn't happen againBambu's advice: wash the plate with warm water and dish soap, hold it by the sides afterwards, and use glue if needed. More in Warping and bed adhesion.

Fixed? Good. Still there? See what to do next.

Source: HMS_0C00-0300-0002-001C: Potential Hotend Clumping Detected; Hotend Clumping/Blob Cleaning Instructions for X and P series