A vertical line running the height of the print, made of small starts and stops stacked on top of each other. On a cylinder it is unmissable. On a boxy part it hides in a corner and nobody notices.
Accept the premise first
Each perimeter is a closed loop, and a loop printed with a single continuous extrusion has to begin and end at the same place. There is no way to make that junction invisible: the extruder either leaves a fraction too much where it stops or starts a fraction late where it begins, and the two are not going to cancel exactly.
So the useful question is not how to remove the seam. It is where you want it, and what you are willing to trade to put it there.
What each mode actually costs
| Seam setting | What it does | What it costs |
|---|---|---|
| Nearest | Starts wherever the toolhead already is | Fast, and scatters marks unpredictably over visible faces |
| Aligned | Stacks every start on one vertical line, chosen for consistency | One clean line you can orient away from the viewer |
| Rear | Forces the line to the back of the plate | Only useful if the part has a back |
| Random | Moves the start point every layer | Removes the line and replaces it with speckling over the whole surface |
| Sharp corner | Hides the join in an existing edge | Excellent on angular parts, useless on cylinders |
| Scarf or staggered | Ramps flow over an overlap so the join is spread across a distance | Best appearance available; needs slicer support and adds a little time |
The one that catches people is random. It sounds like the answer and it is usually the wrong one for a display piece: instead of a single flaw that can be turned to face the wall, you have spread the same total amount of defect uniformly over every surface. It is a good choice for functional parts where a stacked seam would be a stress raiser, and a poor one for anything you are looking at.
Improve the join itself, not just its position
Placement decides visibility; these decide severity.
- Calibrate pressure advance. A tuned machine leaves far less excess when flow stops, which shrinks every seam on the part. Same calibration as for blobs and zits.
- Add a small wipe distance so the nozzle finishes the loop with the extruder already stopped, dragging the surplus into the path rather than dropping it.
- Turn off retraction at layer change if your slicer offers that as a separate option. Retracting and re-priming at every seam adds a pressure transient exactly where you least want one.
- Enable scarf seams if the slicer has them. This is the one genuine advance in the last few years for this defect: instead of a butt join, the flow ramps down and up over an overlapping distance, so the seam becomes a gradient rather than a step.
Design it out when you can
If you are making the model, put a chamfer, a flat, a rib or a groove somewhere on the profile and let the slicer's sharp-corner mode anchor the seam to it. A vertical detail line on a cylindrical part is a design decision that costs nothing and permanently solves this — a technique borrowed straight from injection moulding, where parting lines are placed on features for the same reason.
Paint it where you want it
Most current slicers let you paint a seam position directly onto the model with a brush, and paint exclusion zones where it must never go. On a figure or a display piece this beats every automatic mode, because you know which side faces the room and the software does not. It takes under a minute and it survives re-slicing.
A seam that is starved rather than fat
If the marks do not line up vertically, they are ordinary restart bumps rather than the seam. If a vertical line appears without raised material — a groove or a colour change rather than a bump — the flow is being starved at each restart instead of overfed, which points at retraction being too long, or at a nozzle running cool enough that flow takes too long to re-establish.
Filaments that flatter a seam, and filaments that expose it
Silk and other high-gloss PLA grades exaggerate seams badly: the surface reflects light in a continuous sheet and the seam breaks the reflection. PETG makes physically larger seams because it holds pressure and strings. Matte grades and fibre-filled grades hide them so completely that a part which looks unacceptable in glossy black is fine in matte grey — sometimes the cheapest fix on this page is a different spool.