Weak layer bonds and delamination: what heat, cooling, enclosures and moisture change

Prusa defines layer separation as delamination of printed layers: as layers cool at different rates, warping forces build up, and when they exceed the bond strength between layers the part cracks ^[1]. Bambu Lab describes the same failure as interlayer cracking, seen when bonding is insufficient and the material shrinks a lot, and names ABS, ASA, PC, PET-CF and PA-CF as the usual materials ^[2].

Crack, gap or weak bond

Three different defects get called layer separation, and they have different fixes.

• A crack that opened during the print. Prusa’s tell is a clean cut with edges bent slightly upward, which separates cracked layers from missing ones ^[1].
• A gap where a layer was barely extruded. Bambu Lab notes that a partial nozzle clog can stop extrusion on one layer and look like cracking; the fix is a cold pull, not a temperature change ^[2]. See under-extrusion.
• A part that looks fine but snaps along a layer line under load. That is a weak bond, and the data-sheet section below shows how much Z strength varies with material and print speed.

Symptom, likely cause, what to check

Temperature, speed and flow

Prusa says PLA can crack and separate if printed too cold, gives 195–220 °C as the typical PLA range depending on brand, and suggests raising the temperature in 5 °C steps until bonding improves ^[1]. A temperature tower does the same search in one print.

Bambu Lab’s advice for interlayer cracking is to raise the nozzle temperature somewhat or slow down, so the new line has more time to fuse with the one below; for ABS it gives 150 mm/s inner walls and 100 mm/s outer walls as an example ^[2]. Polymaker’s PolySonic PLA data sheet shows the speed effect in numbers. At 46.7 mm/s and 210 °C the printed specimens reach 46.0 MPa in XY and 37.3 MPa in Z; at 300 mm/s and 230 °C they reach 43.9 MPa in XY and 32.4 MPa in Z ^[13]. XY strength drops by about 5% at the higher speed, Z strength by about 13%, even with a hotter nozzle.

Prusa also suggests raising the extrusion multiplier in 5% steps, on the basis that more material makes a stronger bond ^[1]. Calibrate flow rate first so the change starts from a known value.

Cooling, drafts and enclosures

Part cooling and room air both remove heat from the last layer before the next one lands. Prusa calls ABS highly susceptible to separation when any cold air reaches the print before it has cooled fully, suggests fans off for ABS, and recommends an enclosed chamber ^[1]. Bambu Lab notes that drafts cool a model faster and increase the stress between layers, and that high-temperature materials are usually printed with little or no part cooling in an enclosed printer ^[3]. For interlayer cracking it also suggests raising the bed 5–10 °C and, on printers with active chamber heating, raising the chamber by 5 °C ^[2].

The data sheets agree on direction but not on a single number:

• Bambu ABS: chamber 45–60 °C, cooling fan 0–80% ^[11].
• Polymaker PolyLite ABS: enclosure needed, cooling fan off; its test specimens were printed in a 90 °C environment with the fan off ^[14].
• Prusament PC Blend Carbon Fiber: fan at 0% (up to 20% for bridges), with a note that cooling significantly decreases interlayer adhesion with little effect on print quality ^[19].

For the trade-offs between the two styrene-based materials, see ABS and ABS vs ASA.

Safety: Prusa’s ABS guide says ABS releases fumes that could pose a health risk, including styrene, and recommends printing in a well-ventilated room while keeping drafts away from the print itself ^[4].

Layer height and moisture

Prusa says layer height should stay below 80% of the nozzle diameter, about 0.32 mm for a 0.4 mm nozzle ^[5]. OrcaSlicer gives 20–80% of the nozzle diameter as the usual range and warns that above 80% the risk of layer adhesion problems rises; its table lists 0.08–0.32 mm for a 0.4 mm nozzle and 0.12–0.48 mm for a 0.6 mm nozzle ^[6].

Moisture works against the bond from inside the filament. Prusa’s drying guide says a high water content can cause low layer adhesion, blobs and bubbling ^[7], and Bambu Lab says moisture vaporizing in the nozzle forms bubbles that lead to holes, rough surfaces and reduced strength ^[8]. Symptoms and drying steps are on the wet filament page.

Nylon adds a second moisture effect after printing. Polymaker’s PolyMide CoPA sheet lists 45.8 MPa Z tensile strength for dry specimens and 13.7 MPa after the specimens were soaked to 6.16% moisture ^[15].

What data sheets publish about Z strength

Several makers test tensile bars printed in the XY plane and along Z. The Z bar breaks across layer bonds, so the gap between the two numbers shows how much weaker the bond is than the extruded strand.

All values are ISO 527 results on each maker’s own printed specimens. Bambu Lab states its values are for design reference and comparison only ^[9], so compare within a brand rather than across brands.

Prusament reports the same idea as a tensile strength anisotropy coefficient, the ratio of Z to XY strength with 1 as the maximum ^[17]. Its product page lists 0.60 for PLA High Speed, 0.33 for PLA and PC Blend, and 0.38 for PETG ^[16]. Prusa Polymers also publishes an in-house interlayer adhesion figure: 17 ± 3 MPa for Prusament PLA and 41 ± 3 MPa for PLA High Speed ^{[18, 17]}. The filament table collects the published values.

Order of checks

1. Decide whether it is a crack, a missing layer or a weak bond, using the tells above.
2. Rule out a partial clog if a single layer is thin.
3. Check nozzle temperature against the maker’s range and run a temperature tower.
4. Lower part cooling for the walls and slow the walls down on shrink-prone materials.
5. Close the enclosure or shield the printer from drafts; for ABS, ASA and PC, use the chamber temperature the data sheet lists.
6. Keep layer height under 80% of the nozzle diameter.
7. Dry the filament if the extrusion pops or the surface looks rough.