OrcaSlicer Detect Thin Walls: Why It Disappears With Arachne

Why Detect thin walls disappears with Arachne

By default, walls are printed as closed loops. When a wall is too thin for two line widths, Detect thin walls prints it as a single extrusion line instead of leaving it empty. Thin walls printed this way may have reduced surface quality and strength because they are not closed loops (Walls settings).

Arachne uses a different variable-width method, adjusting extrusion width along the model so thin features and wall-count transitions are handled inside the path. With Arachne active, Detect thin walls is disabled because the fixed-width workaround it controls is not in use. That is not a reason to choose Arachne, nor a promise every thin detail survives; the setting no longer applies. Depending on the build, the checkbox may be disabled, hidden, or labeled differently; that is a UI variation, not a universal physical change.

Confirm the setting and record the build

Detect thin wall is documented under Strength > Walls (detect_thin_wall), the generator under Quality (wall_generator) (Wall generator settings). Turn on Advanced visibility, then record the version and profile; labels vary between builds.

Classic or Arachne

Choose Arachne when variable width suits the part; stay on Classic when fixed widths are the point. A nozzle swap does not replace that decision, and disabling validation only hides the warning.

Three outcomes that look alike

A model geometry problem means the intended feature is absent, is not a valid solid, or does not intersect the inspected layer; no generator prints geometry that is not there. A skipped toolpath means the geometry is present but slicing rules omit it, for example because the cross-section falls below the threshold or Detect thin walls is off under Classic. A printed failure means the path exists in Preview but adhesion, cooling, or flow failed. A missing path in Preview alone cannot distinguish the first two: verify the feature in the model or cross-section, then inspect the generator and thresholds.

Minimum feature size is not minimum wall width

In Arachne, both fields are percentages of nozzle diameter, not fixed millimeters. Minimum feature size is the threshold for printing a feature at all: narrower features are skipped, while one above the threshold but narrower than Minimum wall width is widened to match that width. Minimum wall width sets the width used for those features; it is not a guaranteed physical minimum or an accuracy specification.

Upstream examples such as --min-feature-size=20% and --min-bead-width=20% are CLI examples, not presets. Do not copy 20% across materials, layer heights, or nozzle sizes, and do not assume a blanket 20%, an arbitrary nozzle swap, or disabled validation will fix a missing wall. If a validation error appears, identify the field that triggered it; the warning in OrcaSlicer line width too small is a separate check from a skipped thin wall. Measured hole fit is separate, handled by hole compensation.

A controlled same-layer check

1. Save a copy of the profile and model so the comparison stays reversible.
2. Keep the same affected object and layer selected for every slice.
3. Verify the feature exists in the model geometry or cross-section.
4. Note whether Classic or Arachne is active and record the version.
5. Make one suitable change, such as the generator or one width threshold.
6. Re-slice, then inspect continuity and widening through the affected feature and its adjacent and full relevant layers, not only the sampled layer.
7. Print only after the preview is acceptable; a toolpath shows intent, not adhesion, strength, or printability.

A single sampled layer isolates a symptom but does not prove the whole path is continuous. Repeat physical checks, one change per trial.

Symptom and next check

Record what you changed