PETG: layers that separate or brittle parts that break along the layers

Symptom

PETG parts split between layers when bent or when a screw is tightened, break cleanly along a layer with little force or show horizontal cracks; sometimes the surface is also rough or bubbly.

Likely causes, from most to least likely

1. Too little temperature for the speed: when printing fast, the nozzle has to melt more plastic in less time and Bambu Lab asks you to raise it so layers do not separate. Creality warns that PETG printed too cold separates between layers.
2. Too much part cooling: it cools the PETG before it bonds to the layer below. Prusa explains that a hotter plastic bonds layers better and suggests turning the fan off if you want the part as tough as possible.
3. Damp filament: the steam leaves pores and the part is weaker (Bambu Lab). PETG readily absorbs moisture from the air.
4. Low flow or more speed than the hotend can melt: Bambu Lab puts PETG flow between 0.93 and 0.96 and warns that below 0.93 there is under-extrusion and weaker layer bonding.
5. Orientation: every printed part is weaker between layers. In Elegoo's Rapid PETG data, tensile strength drops from 42 MPa along the layers to 28 MPa across them, and impact strength from 67.6 to 8.9 kJ/m².

Checks to tell which one it is

• Look at where it breaks: if it is always cleanly along a layer, layer bonding is the weak point.
• Compare your profile's temperature, speed and fan with the filament maker's values.
• Purge a little: if it pops or bubbles, the filament is damp.
• Think about how the part works: if the load tries to pull layers apart, orientation matters as much as settings.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Dry the PETG

   Dry the spool before printing: Elegoo asks for 60 ± 5 °C for 8 h for its Rapid PETG, and Bambu Lab for 60-65 °C for 8 h in a convection oven. Store and print it with desiccant below 20% relative humidity (Elegoo).

   Check: No more bubbles when purging and the surface comes out smooth.

   If that does not fix it: Match the temperature to the speed (next step).

2. Temperature matched to speed

   Use the maker's range (Elegoo, Rapid PETG: 240-270 °C; Creality, CR-PETG: 230-250 °C, starting at 240 °C; Prusa, its PETG: 230 °C for the first layer and 240 °C for the rest). If you print fast, raise it within the range so material does not run short and layers do not separate (Bambu Lab). If you slow down, lower it too or you will get strings.

   Check: Layers no longer split when you bend a test part.

   If that does not fix it: Lower the fan (next step).

3. Less part cooling

   Prusa suggests the first layers with no fan and then half power, and turning it off completely if you want maximum toughness. In exchange, overhangs and bridges will come out worse.

   Check: The part takes more load before splitting.

   If that does not fix it: Check the flow (next step).

4. Flow and volumetric speed

   Keep the flow between 0.93 and 0.96 (Bambu Lab): below it, under-extrusion and weak layers; above it, oozing and clogs. If you print very fast, make sure you do not exceed the profile's maximum volumetric speed: going over it also weakens layer bonding (Bambu Lab).

   Check: Lines come out full and gap-free.

   If that does not fix it: Reinforce and orient the part (next step).

5. Reinforce and orient the part

   For more solid parts, Prusa suggests more perimeters rather than more infill, and Bambu Lab stays at or below 6 walls and 50% infill to avoid warping. Orient the part so the main load does not try to pull the layers apart.

   Check: The part withstands its intended use.

   If that does not fix it: If it still breaks along the layers, consider another orientation, another design or another material.

Prevention

• Store PETG sealed with desiccant and dry it if it has been left out.
• If you change the speed, adjust the temperature too.
• Decide the orientation of load-bearing parts before slicing them.

Still not fixed?

Tell usi3d's AI about your case: choose your printer, describe what you see and add photos if you like. It proposes checks step by step, with their sources.

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Material: PETG.

Official sources

This guide summarises the official documentation linked here:

• PETG Usage Guide · Bambu Lab
• Volumetric speed and how it impacts 3D printing · Bambu Lab
• PETG (material guide) · Prusa Research
• Rapid PETG (specifications and mechanical properties) · Elegoo
• CR-PETG · Creality