A fine, shimmering, fish-scale pattern across flat vertical surfaces. It has no measurable spacing you can put calipers to; it appears as a sheen or a moiré rather than as ridges, and it is far more obvious on some filaments than others.
Very small, very regular
The physical size of this defect is on the order of microns — vastly smaller than any of the other surface faults on this site. It is visible only because it is perfectly periodic, and human vision is exceptionally good at picking regular patterns out of a flat field.
That has a practical consequence: it will not affect a part's dimensions, its strength or its fit. Anyone chasing it is chasing appearance, which is a legitimate goal but worth being explicit about before spending money on it.
What is reasonably well established
Stepper motors do not move continuously. Drivers approximate smooth motion by dividing each full step into microsteps, and those microsteps are not equal in size — the torque available at each intermediate position varies with the motor's magnetic geometry, so the toolhead advances in a slightly uneven rhythm.
That unevenness is tiny and it is precisely regular, repeating once per full step. The nozzle traces it into the wall. Different driver chips and different driving modes produce different amounts of it, which is why the same model of printer can show it before and after a firmware change or a board revision.
What is genuinely argued about
Beyond that, the community does not agree, and the disagreement is real rather than a matter of one side being uninformed.
Driver mode. Quiet stealth-type modes and higher-torque spread-type modes produce different artefacts, and people report opposite results — some machines look better in one, some in the other. It is worth testing rather than assuming.
Current setting. Raising driver current increases the torque available at intermediate microsteps, and some report it helps. Others measure no change and point out that higher current means hotter motors, which is a real cost — see stepper motors running hot.
Power supply ripple. A supply with poor regulation modulates the drivers' output. This is plausible and is occasionally demonstrated, and it is also invoked far more often than it is measured.
The extruder rather than the axes. Some argue the pattern originates in flow variation from the extruder's own stepper, not in the motion axes at all. This would explain why it appears on some machines with excellent motion systems.
The honest summary is that the microstepping explanation covers the core of it and the remaining variance is not settled. Anyone presenting a single confident cause is describing their own machine.
What is worth trying
- Change driver mode and print the same test wall. Free, reversible, and the largest single variable.
- Adjust driver current a small amount in each direction and compare, watching motor temperature.
- Check the machine is on something solid rather than a flexing shelf, which will not fix this but removes a confounding variable.
- Try a different filament. Not a fix, but see below.
When to accept it
If the above produce no improvement, this is inherent to the machine's electronics, and the remaining options are replacing drivers or replacing the printer. On a machine that is otherwise printing well, neither is a sensible response to a cosmetic artefact measured in microns.
The pragmatic answers are a matte or fibre-filled filament, which hides it completely, or a light abrasive pass on the faces that matter. Both cost less than new hardware.
Three look-alikes that do have a measurable period
Ripples that trail sharp corners and fade are ringing, and they have a measurable spacing. Regular bands with a pitch you can put calipers to are Z banding. Blotchy definition confined to shallow curves is uneven layer lines on curves. Salmon skin is the one with no measurable period, on flat faces, visible only as a sheen.
Filaments that reveal or hide it
Glossy and silk grades display it most — a specular surface reflects light coherently and any regular disturbance of that surface becomes a visible interference pattern. Metallic and pearlescent pigments do the same thing. Matte grades, wood-filled and fibre-filled filaments scatter light at the surface and hide it entirely. If a part must look flawless and the machine will not stop doing this, changing spool is the fastest fix available.