Nozzle Hitting or Dragging Across the Print

Likely causes, most common first

1. Curling edges and overhangsMost common
   Polymaker describes layers curling upward, mostly from too little part fan on PLA, hot enclosures and fast small layers. OrcaSlicer notes curl compounds over several layers, which is how a small lift grows into something the nozzle hits.
2. Part lifting off the bed (warping)Common
   Prusa describes corners of large parts lifting, from temperature differences, poor adhesion, wrong cooling or too little first-layer squish. A lifted corner rises into the nozzle path. See Warping.
3. Travel moves crossing the part with no liftCommon
   OrcaSlicer suggests Z hop for tall, thin features and for plates with several objects, where the nozzle could otherwise strike the part during travel.
4. Too much plastic or nozzle too closeOccasional
   If the nozzle drags across whole surfaces rather than catching one raised edge, check flow and first-layer height before travel settings. See Over-Extrusion.

Fix it, one change at a time

1. Fix curling first. For PLA and PETG, make sure the part fan runs and the profile sets it on after the first layers; open the enclosure for PLA; set a minimum layer time so small layers can cool. Keep slow-down-for-curled-perimeters enabled if your slicer has it. See Overhangs Drooping, Sagging or Curling Up.
2. Stop the part lifting. Clean the plate, check the first-layer height, add a brim, and keep drafts off the printer, as in Prusa's warping steps.
3. Turn on Z hop (called lift Z or travel lift in some slicers). OrcaSlicer offers normal, slope, spiral and auto lift types, and options to apply it only on certain surfaces or above or below a set height. Start small and raise it only as far as needed to clear the part.
4. Turn on avoid crossing walls (Cura calls it avoid printed parts when traveling), so the nozzle goes around the part rather than over it.
5. Check flow and first-layer height if the nozzle drags across whole surfaces. Calibrate flow and re-check the Z offset; see First Layer Fundamentals.
6. Support tall, thin features with a brim, a sacrificial support beside them, or a different orientation, so a light touch does not snap them.
7. If Z hop causes new bands on an older single lead screw printer, Polymaker found that removing it stopped layer bulges; use avoid crossing walls instead. See Z Banding and Z Wobble.

Confirm it is fixed

Reprint the same part and listen at the layers where it used to scrape. There should be no ticking or scraping, top surfaces should have no furrows, and curled edges should sit flat enough to pass a fingernail over without catching. Check the sides for new bands if you added Z hop.

Why it happens

The slicer plans every move on the assumption that the layer below is exactly where it should be: flat, and exactly one layer height down. The nozzle is never more than a fraction of a millimeter above it. Anything that rises above that height, even slightly, is in the path of the next pass.

Curl is the usual culprit because it grows. A lifted overhang edge or corner is reprinted on the next layer a little higher, and the next layer higher again, until the nozzle catches it. Fixing cooling stops the growth at the source; Z hop only lifts the nozzle over it during travel and does nothing while the nozzle is printing that edge.

Z hop has a cost too. On a Z axis with backlash, Polymaker found the nozzle may not come back down to exactly the same height, leaving bulges. That is why it is a targeted fix: use it where travel actually crosses the part, and use OrcaSlicer's surface and height limits to keep it off elsewhere. For how retraction and travel settings interact, see Retraction.