Layer separation and cracking

Layers are not welding to each other. FDM parts are only as strong as the bond between layers, and that bond is made by heat — a layer must still be hot enough to melt into the one below when it lands. Nearly every cause on this list is a heat problem.

Looks like: Layers split apart, usually part-way up a tall print, and the part snaps along a layer line.

Fixes, in the order worth trying

1 · Try this first

Raise the nozzle temperature

The most direct lever on layer adhesion. Hotter plastic melts further into the previous layer. Layer strength climbs steadily with temperature right up to the point where stringing and heat creep become the limiting factor.

Printing Temperature: Cura → Material → +5 to +15 °C, toward the top of the spool range
Nozzle temperature: OrcaSlicer → Filament → Filament → +5 °C per test

2 · Try this first

Reduce part cooling

The fan is actively working against layer bonding. PLA tolerates full cooling because it bonds easily; ABS, ASA, PC and nylon do not — running them at 100 % fan will produce parts that fall apart in your hand.

Fan Speed: Cura → Cooling → ABS/ASA 0–20 % · PC 0–20 % · nylon 0–30 % · PETG 30–50 %
Fan speed: OrcaSlicer → Filament → Cooling → Match the material, not the printer default

3 · Common cause

Dry the filament

Moisture ruins layer adhesion as badly as it causes stringing. Steam voids inside the extrudate leave the bond porous, and the part breaks along a layer under light load. Nylon and PC are the worst offenders — they can absorb enough moisture in a single humid day to matter.

Dry at 60–65 °C for 6–8 hours for PETG, ABS and nylon; 45–55 °C for 4–6 hours for PLA. For nylon and PC, print straight from a heated dry box rather than drying once and leaving the spool out.

4 · Common cause

Enclose the printer

For high-temperature materials, ambient temperature is part of the process. In a cold room an ABS part cools too fast between layers to bond properly, and the internal stress that builds cracks it apart part-way up.

Any enclosure helps. For ABS and ASA aim for a chamber around 40–50 °C. Keep the electronics and stepper drivers outside that heated volume.

5 · Less common

Lower the layer height or raise line width

A layer taller than about 75 % of the nozzle diameter has too little contact area with the layer below. On a 0.4 mm nozzle, 0.32 mm layers bond noticeably worse than 0.2 mm ones.

Layer Height: Cura → Quality → ≤ 0.75 × nozzle diameter (0.3 mm max on a 0.4 mm nozzle)
Line width: OrcaSlicer → Process → Quality → 105–120 % of nozzle diameter

Change one thing at a time. If you adjust temperature, retraction and speed together and the problem goes away, you have learned nothing and you will be back here next month. Re-print the same test object after each single change.

Frequently asked questions

Why did my print crack only after it finished?

Internal stress. Each layer shrinks slightly as it cools, and in a tall part those forces accumulate. If the layer bond is weaker than the accumulated stress, the part splits — often hours later, sometimes with an audible crack. It points at layer adhesion and cooling rate, not at a mechanical fault.

Will printing slower improve layer adhesion?

Usually yes, but less than temperature does. Slower printing gives the layer below more time to cool, which cuts both ways — the real benefit is that the hotend has more time to fully melt the plastic, so it lands hotter and more consistently.

Is layer adhesion always weaker than the plastic itself?

Yes, for FDM. A printed part is typically 30–60 % as strong across layers as it is along them, depending on material and settings. Design around it: orient the part so the load runs along the layers rather than across them.