Layer Separation and Cracking: Why Layers Don't Bond
Contents
- Symptoms
- Why does layer bonding get weak?
- 1. Raise the nozzle temperature
- 2. Reduce the part cooling fan
- 3. Control the ambient temperature
- 4. Fix under-extrusion
- 5. Layer height and line width
- 6. Lower the speed
- 7. Cooling time on small layers
- Summary by material
- Think about part orientation
- Frequently asked questions
If you flex a part slightly by hand and it splits in two along a layer line, the layers were only stuck on top of each other and not fused. On tall parts this separation can also appear as a horizontal crack during the print. The problem is common with ABS and ASA and rarer with PLA. The cause is most often low temperature, too much cooling or a cold room.
Symptoms
- Horizontal cracks on the side of the part.
- The part breaks with light force along the layer direction.
- On tall parts the top section separates and leans during the print.
- The crack usually starts at the corners or the thinnest section of the part.
Why does layer bonding get weak?
When each new layer is printed, the hot plastic briefly reheats the surface of the layer below, and the two layers fuse. If the layer below is too cold, or the new layer is not hot enough, the fusing stays at the surface. The result is layers that are only stuck to each other, not fused.
1. Raise the nozzle temperature
This is the setting that affects layer bonding most.
- Raise the temperature in 5 °C steps.
- Print in the upper half of the filament's recommended range.
- To see which temperature gives the best bond, print a temperature tower and try breaking each section by hand.
2. Reduce the part cooling fan
The fan cools the newly printed layer quickly, which gives nice results on overhangs and bridges, but weakens layer bonding.
- PLA: 100% fan is usually no problem. If you get cracking, drop it to 50–70%.
- PETG: 30–50% is enough.
- ABS/ASA: 0–20%. Raise it only for bridges and overhangs.
On functional parts that need strength, lowering the fan gives up a little appearance for a serious gain in strength.
3. Control the ambient temperature
A cold room or a draft on the printer makes the layers cool quickly.
- Keep the printer away from windows, doors and air conditioning.
- Use an enclosure for ABS and ASA. The warm air inside both strengthens layer bonding and reduces the warping risk.
- An enclosure temperature of about 40–50 °C is ideal for ABS. When printing PLA, open the enclosure door, because PLA softens at high ambient temperature and the risk of a clog in the hotend rises.
4. Fix under-extrusion
If not enough plastic is printed, the contact area between layers shrinks. If you see gaps in the walls or thin lines, solve the under-extrusion problem first.
5. Layer height and line width
- Layer height should not exceed 75% of the nozzle diameter. On a 0.4 mm nozzle use at most 0.3 mm. Thicker layers stop the nozzle from pressing enough on the layer below. See Choosing layer height.
- Keeping line width slightly above the nozzle diameter (100–120%) increases the contact area between layers.
6. Lower the speed
Layers printed too fast neither melt enough nor are laid down with enough pressure. Especially on tall, thin parts, lowering print speed by 20–30% improves layer bonding.
7. Cooling time on small layers
On very small layers the opposite problem occurs: the new layer is printed while the layer below is still very hot and the part melts. The minimum layer time setting in the slicer balances this. This setting is a safeguard against melting, not against cracking, but if you set the value too high it can weaken layer bonding on small parts.
Summary by material
| Material | Tendency to crack | Main fix |
|---|---|---|
| PLA | Low | Raise the temperature a little, lower the fan if needed |
| PETG | Low | Fine in most cases; avoid too much fan and low temperature |
| ABS / ASA | High | Enclosure, low fan, a place away from drafts |
| Nylon | Medium to high | Dried filament, enclosure, high temperature |
Think about part orientation
The bond between layers is always weaker than the layer itself. Orient a part that will carry load so that the force acts parallel to the layer lines. For example, if you print a hanging hook standing upright it breaks between layers, and if you lay it flat it is much stronger.
Frequently asked questions
The part cracked a few hours after the print finished. Why?
Internal stress, especially with ABS, can turn into a crack as the part cools. Using an enclosure and letting the part cool slowly inside it after the print reduces this risk.
Does raising infill prevent layer separation?
Infill raises the overall strength of the part but does not directly strengthen the bond between layers. Increasing the wall count and setting the temperature right is more effective. See Infill settings.