Nozzle scraping or scarring the top surface

This guide is for anyone seeing thin grooves, shiny drag marks or raised furrows across flat tops, such as straight lines running from one feature to another. It covers what causes them, how to tell scars from other top-surface problems, and the flow, Z hop, travel and ironing settings that Simplify3D, Prusa, Bambu Lab, OrcaSlicer, UltiMaker Cura and Polymaker document.

What is top-surface scarring?

Simplify3D explains that for each layer "the nozzle can freely move to any portion of your print bed," which is fast, but "you may notice that the nozzle leaves a mark when it travels on top of a previously printed layer." It says the marks are "typically most visible on the top solid layers of your part," and that they "occur when the nozzle tries to move to a new location, but ends up dragging across previously printed plastic."

Scars or something else?

Several top-surface problems look alike at a glance. The pattern of the marks is the best clue.

Step 1: Rule out over-extrusion

Simplify3D says that "one of the first things you should verify is that you are not extruding too much plastic." If you are, "each layer will tend to be slightly thicker than intended," which means "when the nozzle tries to move across each layer, it may drag through some of the excess plastic." It says to check this "before you look at any other settings."

Prusa makes the same link. Its extrusion multiplier article says over-extrusion "causes scarring of bottom layers and unnecessary material buildup on top layers." Its visual calibration method aims for top layers that are "smooth, shiny, and without scarring or filament buildup near the perimeters." If the top isn't smooth, Prusa says to "re-adjust the Extrusion Multiplier accordingly by 1-2%."

Slicers also have a flow setting just for top surfaces:

• Bambu Studio: Top surface flow ratio. Its tooltip in the v02.08.04.57 source says "you can decrease it slightly to have smooth surface finish." In that source, the setting needs Develop mode.
• OrcaSlicer: Surface flow ratio. Its wiki uses similar wording: "you can decrease it slightly to have smooth surface finish." The wiki recommends calibrating the main flow ratio first, "before using a value other than 1."
• Cura: Top/Bottom Flow and Top Surface Skin Flow. Both inherit the main Flow value (100%) in the 5.13.0 definition.

OrcaSlicer also offers Small area flow compensation, which it says helps "the tops of narrow letters or fine features," where "standard extrusion flow may be too much for the available space, leading to over-extrusion."

Our over-extrusion guide walks through flow calibration in each slicer.

Step 2: Turn on Z hop

If flow is right and the nozzle still drags, Simplify3D's next step is vertical lift. It says lifting the nozzle "a set distance above the previously printed layer before moving to a new location" means "this can avoid the nozzle scratch on the top surface of your print."

Where to find it

What each slicer says

• Simplify3D gives the example that entering 0.5 mm means "the nozzle will always raise up 0.5mm before moving to a new location." It warns that "this vertical lift will only occur when the nozzle is performing a retraction." To make every move retract, it says to disable "Only retract when crossing open spaces" and "Minimum travel for retraction" on the Advanced tab.
• Prusa says lift Z "lifts the extruder during movement," and that a lower setting "will improve stringing," but "disabling this feature may cause the nozzle to hit the printed part." PrusaSlicer 2.9.6 also offers Use ramping lift, which lifts along a ramp instead of straight up and "helps reduce stringing," plus Steeper ramp before obstacles (Expert mode), which makes the slope steeper "in case an obstacle might be hit during the initial phase of the travel."
• Bambu Lab says that whenever there's a retraction, "the nozzle is lifted a little to create clearance between the nozzle and the print," which "prevents the nozzle from hitting the print." It says the value "cannot exceed 5mm." Z Hop type offers Slope, Normal, Spiral and Auto. With Auto, Bambu says the printer uses slope lifting unless the travel would cross an overhang that may have warped, where it has "a certain chance of colliding and scraping." In that case it uses spiral lift, which "only passes through the smaller XY area above the part surface, with a lower probability of scratching." Bambu says the printer always uses spiral lift when changing layers. A Z hop upper and lower boundary limits the heights where Z hop takes effect.
• OrcaSlicer describes Z hop as lifting the nozzle "slightly during travel moves to avoid collisions with the printed object." Its On surfaces option offers All Surfaces, Top Only, Bottom Only, and Top and Bottom, and Only lift Z above or below limits it by height. Types are Auto Lift, Normal Lift, Slope Lift and Spiral Lift.
• Cura's Z Hop When Retracted is off by default in the 5.13.0 definition, with a Z Hop Height of 1 mm when enabled. Cura says it "prevents the nozzle from hitting the print during travel moves, reducing the chance to knock the print from the build plate." Z Hop Only Over Printed Parts limits hops to moves over printed parts that can't be avoided by horizontal motion.

Step 3: Keep travel off visible surfaces

Z hop lifts the nozzle. Travel settings decide where it goes. Each slicer can route travel around walls instead of straight across them:

• PrusaSlicer: Avoid crossing perimeters, under Print settings, Layers and perimeters. Prusa says it minimizes "crossings of external perimeters during travel moves," which reduces stringing "and improves overall print quality." It slows G-code generation and adds some print time. In version 2.9.6 it's off by default and shown in Expert mode. A Max detour length skips the detour if it would be too long.
• Bambu Studio: Avoid crossing wall, in the Quality settings. Its tooltip in the v02.08.04.57 source says it will "detour and avoid traveling across wall which may cause blob on surface (when travel length greater than Travel distance threshold)." It's off by default in that source, with its own Max detour length.
• OrcaSlicer: Avoid crossing walls. The wiki says the print head "will detour around it, which can significantly reduce surface defects and stringing," and calls it "highly recommended for detailed or aesthetic prints." It notes the option is "not compatible with Timelapse mode."
• Cura: Combing Mode "keeps the nozzle within already printed areas when traveling," and Cura says "it is also possible to avoid combing over top/bottom skin areas." The options are Off, All, Not on Outer Surface, Not in Skin, and Within Infill. In the 5.13.0 definition, Combing Mode switches to Not on Outer Surface when Monotonic Top/Bottom Order, Monotonic Top Surface Order or ironing is on, and is All otherwise. Avoid Printed Parts When Traveling is on by default.
• Simplify3D: In its blobs guide, Simplify3D describes "Avoid crossing outline for travel movement" in the Movement Behavior section.

A note on infill crossings

Bambu Lab says some infill patterns, "such as Grid, Triangles," cross over themselves within a single layer, "so the nozzle will scratch the infill at the crossing point." It says this "usually does not have much impact on the print." If it does affect a part, Bambu suggests slowing the infill or using a pattern without crossings, "such as Line, Rectilinear, Gyroid, or Concentric." This affects infill inside the part, not the finished top.

Step 4: Look for curling and lifted edges

A nozzle can only scrape what's sticking up. Several makers describe plastic lifting into the nozzle's path:

• Curled layers and overhangs. Polymaker says curling looks like layers "warping upward," and that it "most frequently" happens when printing PLA without an active cooling fan or with too little fan. Simplify3D says curling later in a print "typically points to overheating issues." PrusaSlicer 2.9.6 has an experimental Avoid crossing curled overhangs option that plans travel to avoid "areas where the filament may be curled up," which it says mostly happens on "steeper rounded overhangs and may cause a crash with the nozzle."
• Small, fast layers. Polymaker says that if a small layer finishes too quickly, the previous layer "will still be hot and start to curl upward as the nozzle goes back over it." Its fixes are more cooling, a minimum layer time, or printing more than one object so each layer has time to cool.
• Warped parts. Bambu Lab says that after a part warps, the reduced distance between the warped area and the nozzle can flatten that layer's extrusion and leave a horizontal line on the print.
• Low nozzle temperature at high speed. Bambu says that if the nozzle temperature is too low during high-speed printing, the melt has "higher stickiness and poorer flowability, which easily causes the nozzle to scratch the printing." It suggests raising the nozzle temperature appropriately. Bambu adds that if "the nozzle or the printing is partially stuck with the block," the nozzle may also hit the print, and recommends cleaning the nozzle and printing again with reduced nozzle temperature and speed.

For curling on overhangs, see our overhangs and bridging guide.

Step 5: Consider ironing for flat tops

Ironing makes a second pass over a flat top at the same height with very little flow. It's a finishing step. It won't stop travel moves from scarring the surface.

• Prusa says that as the hot nozzle travels over the top layer, "it flattens any plastic that might have curled up" and extrudes a small amount to fill holes. It has been in PrusaSlicer since version 2.3.0, under Print settings, Infill, Ironing, Enable Ironing. It's not available in Simple mode. The default ironing flow rate in version 2.9.6 is 15%.
• Bambu Lab says that during ironing "the nozzle will rub over the top layer," so the model takes "greater force from the nozzle compared to normal printing" and may come off the plate if it isn't firmly bonded. Ironing is off by default in Bambu Studio's process settings. Bambu says too high an ironing flow "will result in over-extrusion on the surface." Ironing inset, added in Bambu Studio 1.10, keeps ironing away from edges where material can build up.
• OrcaSlicer describes ironing as "printing a second time at the same height, but with a very low flow rate." It offers Top surfaces, Topmost surface, or All solid layers.
• Cura says Enable Ironing goes over the top surface "extruding very little material." It's off by default in the 5.13.0 definition, with Ironing Flow at 10%. Cura warns that too much flow "results in overextrusion and blips on the side of the surface."

Prusa and Bambu both say ironing is for flat tops and does little on curved or organic surfaces. If ironing itself leaves grooves, edge ridges or rough patches, our ironing problems guide covers flow, spacing and speed.

A sensible order

1. Reset to your printer maker's profile for your printer, nozzle and filament.
2. Look at the scars. Straight lines between features point to travel. A rough, ridged top points to flow.
3. Check flow first. Calibrate it, then lower top surface flow slightly if needed.
4. Turn on Z hop, or raise it, for the problem part. In Simplify3D, make sure lift happens on the moves that matter.
5. Turn on avoid crossing walls or perimeters, or check Cura's combing mode.
6. Look for curled edges or overhangs, and fix cooling or layer time if you find them.
7. Add ironing last, if you want a smoother flat top.

Do you need to buy parts?

Every fix the sources above describe is a setting, a calibration or a cleaning step. Before buying anything:

• Check flow on the current spool. Prusa says the right extrusion multiplier "can even differ from spool to spool."
• Clean the nozzle. Bambu Lab links a partially stuck nozzle to the nozzle hitting the print, and recommends cleaning it. Follow your printer maker's cleaning procedure.
• Give the nozzle clearance. Simplify3D, Prusa, Bambu Lab, OrcaSlicer and Cura all describe Z hop as a way to lift the nozzle clear of printed parts during travel.

If the top has holes or dips rather than scratches, see our pillowing guide or gaps in top layers guide.

Print problems: what to check before you buy ↗