Layers Splitting or Cracking Apart

Likely causes, most common first

1. Nozzle temperature too lowMost common
   Polymaker calls printing too cold one of the most common reasons for poor layer adhesion. The hotter the new line, the deeper it fuses into the layer below.
2. Too much part cooling, or a cold roomVery common
   Prusa explains that layers cooling at different rates build stress that can exceed the layer bond. For ABS it says to turn the part fan off completely. Polymaker adds that many lower-density materials such as ABS and HIPS usually want the fan off.
3. Large ABS or ASA part printed in the openCommon
   Prusa recommends an enclosed chamber for ABS, heated if possible, and notes that ASA depends heavily on a stable environment. Drafts from a window or air conditioner make it worse.
4. Under-extrusionCommon
   Thin, starved lines have less plastic to bond with. Polymaker lists wrong E-steps after an extruder swap, a slipping extruder gear and a skipping extruder motor. See Under-Extrusion.
5. Printing too fastOccasional
   Polymaker explains that slower printing keeps the hot nozzle in contact with the layer below for longer, which strengthens the bond.
6. Wet or old filamentOccasional
   Polymaker notes that old or wet filament causes problems that are hard to diagnose. See Wet Filament Symptoms.
7. Fiber-filled filamentLess common
   Polymaker notes that carbon fiber reinforced materials have more layer adhesion issues than unfilled filaments.

Fix it, one change at a time

1. Load the right profile. Start from your slicer's preset for this exact material, then compare nozzle temperature and fan settings with the filament maker's printed range. Do not start a test outside that range.
2. Raise the nozzle temperature in small steps. Prusa's fix for cracking PLA is to raise the temperature 5 °C at a time Starting point until layer adhesion improves, staying inside the maker's range. A temperature tower finds the point faster.
3. Cut the part fan for materials that want it off. For ABS, Prusa turns the part fan off completely. For PLA and PETG, leave the fan as the profile sets it and change temperature first; reducing the fan on PLA trades one problem for curling and sagging.
4. Enclose ABS and ASA and keep the printer away from drafts. If a large ABS part keeps cracking in an enclosure, Prusa suggests PETG as a material that suits large objects and cracks less.
5. Slow the print down. Reduce outer wall and overall speed so each line has more time in contact with the hot layer below.
6. Check for under-extrusion. Look for thin walls, gaps between lines and a clicking extruder. Calibrate flow with Flow Calibration and change the multiplier in small steps.
7. Dry the filament if the spool has been open for a while, then repeat the test.
8. Consider a larger nozzle for parts that must be strong. Polymaker notes that a wider nozzle gives a wider contact area between layers.

Confirm it is fixed

Print a tall, single-wall test or a plain box with thin walls using the new settings, let it cool fully, then try to split it by hand along a layer. It should break across layers with effort, or flex and tear rather than peel cleanly. Then print the real part and check the area where the crack used to start, usually on long walls or at the height where the part gets wider.

Why it happens

Each new line has to melt slightly into the layer below to bond with it. Polymaker describes the bond as polymer chains from the two layers tangling together while both are hot. Everything that keeps the interface hot for longer (a hotter nozzle, a slower speed, a wider line, a warm chamber) gives more time for that to happen. Everything that chills it (a strong fan, a draft, a thin starved line) cuts it short.

Cracks that open mid-wall on big ABS parts have a second cause on top of a weak bond. Prusa explains that the upper layers cool and shrink at a different rate from the lower ones, and the stress this builds can exceed the strength of the bond. That is why an enclosure helps so much for ABS and ASA: it keeps the whole part at a similar temperature, so less stress builds up.

Polymaker treats this kind of shrinkage cracking as a warping problem rather than simple poor adhesion: you can have strong layers and still crack a high-shrink part. If the part is also lifting from the bed at the corners, read Warping alongside this page.

Layer lines are always the weakest direction of an FDM print, even when the bond is good. If the part must take load, orient it so the force runs along the layers rather than pulling them apart. See Print Orientation.