Zits are small raised marks on an FDM/FFF print, while blobs are larger deposits of molten material. They most often appear where a perimeter starts or stops, along the Z-seam, during speed changes, or when temperature, flow, filament condition or retraction is unstable.
In short: diagnose the pattern before changing settings. A vertical line points to the Z-seam; a bump exactly at the start or end of a loop points to retraction, extra restart, wipe or coasting; swollen corners point to pressure compensation; random marks with strings or popping point to heat, moisture or inconsistent extrusion. Change one variable at a time and compare the same small test print.
Visual note: the comparisons in this guide are illustrative. Use them to recognize defect patterns, not as dimensional measurement references.
Table of contents
- What are zits, blobs and the Z-seam?
- Diagnose the pattern first
- Checks to make before changing the slicer
- Seam placement, retraction, wipe and coasting
- Pressure Advance and Linear Advance
- A reliable calibration order
- Controlled test table
- PLA, PETG, TPU and wet filament
- Common mistakes
- Sources
- Frequently asked questions
What are zits, blobs and the Z-seam?
Every closed FDM perimeter needs a defined start and end point. Molten filament does not respond instantly: pressure builds and falls inside the hotend with a delay. If extrusion, temperature and motion are not balanced, excess material can remain at that transition.
The Z-seam is the stacked trace of those perimeter start/end points across successive layers. It does not automatically mean the profile is bad; a normal non-vase print still needs a join. The goal is to make the mark small, consistent and intentionally placed. Prusa explains the same relationship between the perimeter endpoint, a vertical seam and small zits in its official seam-position guide.
This guide covers surface defects above the first layer. It does not cover elephant foot, warping, layer shifts or poor build-plate adhesion.
Diagnose the pattern first
The most useful check is to watch the exact moment the bump forms. Simplify3D's official troubleshooting guide recommends distinguishing whether the defect appears at the start or the end of a perimeter, because the two cases require different corrections.
| Defect pattern | Most likely cause | First check | Appropriate action |
|---|---|---|---|
| Marks stacked in one vertical line | Z-seam or seam gap | Inspect seam placement in the slicer preview | Move it to an edge/rear area or test a scarf seam when supported |
| Blob exactly when a loop starts | Too much restart after retraction | Extra prime / extra restart and extrusion stability | Reduce restart compensation in small steps; revert if a gap appears |
| Blob exactly when a loop ends | Residual nozzle pressure | Temperature, wipe, coasting and pressure compensation | Reduce the underlying pressure before adding aggressive coasting |
| Swollen outside corners | Pressure lag during acceleration/deceleration | Flow, speed, acceleration and PA/LA | Calibrate Pressure/Linear Advance using the firmware procedure |
| Random marks with strings or popping | Moisture, excess heat or oozing | Spool condition and a temperature test | Dry the material and test temperature before retraction |
| Chaotic roughness on a previously stable profile | Nozzle, extruder or PTFE-path problem | Contamination, partial clog, leakage or filament grinding | Fix the mechanical fault before changing the slicer profile |
Checks to make before changing the slicer
Save a copy of the current profile and keep the spool, nozzle, test model and speed constant. If temperature, flow and retraction change together, the result cannot tell you which adjustment worked.
- Filament: dry it when moisture symptoms are present and keep it in a sealed container or filament dryer/dry box during testing.
- Hotend: check for residue, a partial clog, a loose nozzle, leakage above the heater block or a worn PTFE tube.
- Extruder: confirm there is no clicking, slipping or filament grinding. If needed, begin with the extruder-calibration guide.
- Baseline profile: return to a validated profile for the exact printer and material instead of inheriting unknown edits.
Seam placement, retraction, wipe and coasting
Seam position
On models with edges, Nearest, Aligned, Rear or Sharpest corner can hide the join more effectively. Random does not remove the seam; it distributes the marks. Prusa notes that this produces a less smooth surface but can be a viable trade-off on cylinders without sharp edges. If the slicer supports a scarf seam, test it on curved walls; it can soften the transition, but it adds print time and is less effective around sharp corners and overhangs.
Retraction distance and speed
Retraction controls oozing during travel moves, but it is not a universal fix for every blob. Direct-drive systems normally use a shorter distance than Bowden systems, yet the correct value still depends on the extruder, hotend, material and profile. Start from the printer manufacturer's profile and change distance and speed separately.
Too little retraction leaves strings and ooze; too much can create an unstable restart, filament grinding or a partial clog. If the defect exists only at the seam and there is no travel stringing, do not automatically increase retraction.
Wipe, coasting and extra restart
Wipe spreads residual material along a short movement instead of leaving it at one point. Coasting stops commanded extrusion before the end of a line and uses remaining nozzle pressure. Extra restart/extra prime changes how much material is restored after a retraction. Each can help, but only when it matches the exact timing of the defect.
Do not copy values between slicers: names, units and implementations differ. If a correction removes the bump but leaves a gap or a weak join, the correction is excessive.
Pressure Advance and Linear Advance
Pressure Advance in Klipper and Linear Advance in Marlin compensate for the delay between a motion-speed change and the pressure response inside the nozzle. They are especially relevant to swollen corners and high-speed profiles.
The official Klipper Pressure Advance documentation states that the value depends on the extruder, nozzle, filament, temperature and extrusion rate. Do not copy a PA value from another printer, profile or spool.
Important: do not stack several pressure corrections without testing. Marlin Linear Advance warns that slicer pressure controls such as coast at end, extra restart and wipe/combing can interfere with firmware compensation; retraction should be checked again after calibration. Klipper may still benefit from a small retraction and wipe, but follow its own procedure rather than applying Marlin rules.
A reliable calibration order
- Record the baseline. Photograph the same cylinder, cube or seam test and save the profile.
- Stabilize material and mechanics. Dry filament, a clean nozzle and consistent feeding are prerequisites.
- Test temperature. Start inside the manufacturer's range and compare 5 °C steps without sacrificing layer bonding.
- Calibrate extrusion and flow. Do not use flow to conceal an uncalibrated extruder.
- Tune retraction. Test distance and speed separately on the same model.
- Choose a seam strategy. Hide the seam on an edge/rear area, or accept the Random/scarf trade-off for the geometry.
- Calibrate PA/LA when supported. Use the firmware's official test for the exact nozzle and spool.
- Validate on a real part. A calibration model proves direction; real geometry shows whether the profile is robust.
Controlled test table
These are safe diagnostic increments, not a universal print profile. Manufacturer limits and a proven profile for the exact printer take priority.
| Parameter | Baseline | How to test | Signal to stop or revert |
|---|---|---|---|
| Nozzle temperature | Manufacturer range or validated profile | One 5 °C change per test | Poor bonding, rough/matte extrusion or gaps |
| Flow / extrusion multiplier | Calibrated extrusion | Small 1–2% corrections | Wrong dimensions, gaps or weak walls |
| Retraction distance | Printer/extruder profile | Small steps, separate from speed | Clicking, grinding, clogging or an empty restart |
| Extra restart / prime | Zero or a proven profile value | Only for a blob exactly at the line start | A gap immediately after the seam |
| Seam position | Based on geometry and the visible side | Compare Aligned/Rear/Nearest/Random or scarf | Scattered zits or a weak/open seam |
| Pressure/Linear Advance | Official firmware test | Calibrate for the exact nozzle and spool | Rounded corners, under-extrusion or extruder skipping |
PLA, PETG, TPU and wet filament
Material changes profile behavior. PLA is often a convenient baseline, but a different color or manufacturer can still require a new temperature and PA/LA calibration. PETG and TPU can show more oozing and stringing, while moisture makes every diagnosis less predictable. For flexible filament, use the dedicated TPU settings guide.
If the hotend pops, the strand contains bubbles or the surface changes randomly, begin by drying the spool. See why a dry box matters in 3D printing. When using Bambu Studio, first confirm that you understand its profile structure with the Bambu Studio beginner's guide.
When the material itself is compromised, a fresh spool from the correct category — PLA, PETG or TPU — is a better diagnostic control than endlessly retuning damaged filament.
Common mistakes
- Increasing retraction by habit. It can destabilize the restart and create a new blob.
- Changing several parameters together. The result has no clear cause and cannot be reproduced reliably.
- Ignoring mechanics. A partially clogged or leaking nozzle cannot be repaired with a profile.
- Copying PA/LA blindly. The value is specific to the printer, extruder, nozzle, temperature and filament.
- Stacking coasting and firmware compensation. The two can fight each other and leave gaps.
- Assuming Random seam is always worse. It is a trade-off: it distributes marks but can be viable on cylinders without edges.
- Testing only on a large final model. A small control print saves material and makes comparisons more reliable.
Sources and further technical documentation
- Simplify3D — Blobs and Zits: start/end diagnosis, extra restart, coasting, wipe and seam placement.
- Prusa Knowledge Base — Seam position: Aligned, Random, Rear and scarf-seam trade-offs.
- Klipper documentation — Pressure Advance: calibration, dependencies and limitations.
- Marlin Firmware — Linear Advance: nozzle pressure, K-factor and conflicting slicer compensation.
Conclusion
There is no single magic value that removes every 3D print blob or zit. The dependable process is to identify the pattern, stabilize filament and the hotend, test temperature and flow, tune retraction, choose seam placement and only then calibrate Pressure or Linear Advance.
If the defect does not respond logically to one controlled change, restore the baseline profile and inspect the mechanics. A good profile should repeat its result on both a calibration model and a real part.
