What layer separation is and how to confirm it
FDM parts are built from layers that must partly remelt each other to bond. When the new layer is too cool, too thin or lands on plastic that has already cooled too far, the bond is weak. The result is delamination: cracks between layers, or a part that breaks cleanly along a layer line with little force.
Confirm it by looking for horizontal cracks, especially near corners and on taller parts, and by a simple bend test: print a thin vertical bar and snap it. A clean, flat break along one layer line suggests poor fusion. A jagged break with stretched strands indicates decent bonding. In ABS and ASA, cracks that open while printing are classic separation from shrinkage stress.
Note where the cracks appear. Separation low on the part usually comes with warping, while cracks higher up on tall parts point to the air cooling the top layers too fast.
Causes, most likely first
- Nozzle temperature too low for the material or speed.
- Too much part cooling, especially for PETG, ABS and ASA.
- Under-extrusion, leaving thin layers with little contact area.
- Cold or drafty environment for shrink-prone materials.
- Layer height too large, beyond about 75 to 80 percent of the nozzle diameter.
- Wet filament, which bubbles and weakens bonds.
Fixes, cheapest and fastest first
1. Raise the nozzle temperature
Increase in 5 °C steps toward the upper part of the filament's range. Typical ranges: PLA 200 to 220 °C, PETG 230 to 250 °C, ABS 240 to 260 °C, TPU 220 to 235 °C. Print a temperature tower and check strength by bending each section.
2. Reduce part cooling
For PLA, 100 percent fan is usually fine but try 60 to 80 percent for strength-critical parts. For PETG, 30 to 50 percent. For ABS and ASA, 0 to 20 percent, with more only on bridges and overhangs.
3. Check extrusion
Measure a single-wall print. If walls are thin, address under-extrusion first: clogs, slipping extruder, speed too high for the hotend.
4. Use a sensible layer height
With a 0.4 mm nozzle, keep layers at 0.28 mm or below. Thinner layers bond better. Wider line widths of 0.45 to 0.5 mm also increase the contact area.
5. Slow down
Lower speed gives the plastic more time in the melt zone and more heat at the interface. A 20 to 30 percent reduction often helps on fast printers.
6. Enclose the printer and block drafts
For ABS and ASA, an enclosure keeping the chamber around 40 to 50 °C greatly reduces cracking. Keep the printer away from air conditioning and windows.
7. Dry the filament
If you see bubbles, popping or a rough surface, dry the spool before further tuning.
Material notes
- PLA: bonds well; separation usually means low temperature or very fast printing.
- PETG: strong layer bonding when fan is moderate; heavy cooling weakens it.
- ABS/ASA: most prone to separation from shrinkage; enclosure and low fan are key.
- TPU: rarely separates; if it does, raise temperature and slow down.
Prevention checklist
- Print within the upper half of the filament's temperature range for strong parts.
- Match fan speed to material.
- Keep layer height below 75 percent of nozzle diameter.
- Calibrate flow so walls measure correctly.
- Enclose shrink-prone materials.
- Store filament dry.
- Orient parts so loads do not pull layers apart where possible.