Annealing PETG involves heating your printed part to a temperature near or above its glass transition point (80–85 °C), holding it there for a set period, then letting it cool slowly. This process encourages the polymer chains to reorganize into a more ordered, semi-crystalline structure, which improves both strength and heat resistance. The tradeoff is some dimensional change and the risk of warping, but both can be managed with the right technique.
Why Annealing Improves PETG Parts
PETG comes off the printer in a mostly amorphous state, meaning its polymer chains are tangled and disordered. When you reheat the part and hold it at temperature, those chains have enough energy to shift into a more crystalline arrangement. This semi-crystalline structure makes the material stiffer, stronger, and more resistant to softening under heat. In practical terms, an annealed PETG part can handle higher temperatures before it starts to deform, which matters for functional parts exposed to heat, sunlight, or mechanical stress.
The strength gains for PETG are real but modest compared to materials like PLA. One study found that annealing at 70 °C for 90 minutes improved tensile strength by about 8%, taking a part from 28.2 MPa to 31.04 MPa. A more aggressive approach, annealing at 110 °C for 2 hours, pushed tensile strength to 61 MPa in a separate study using thin-walled test specimens. The results depend heavily on your specific temperature, time, and part geometry.
Temperature and Time Settings
PETG’s glass transition temperature sits between 80 and 85 °C. You can anneal below, at, or above this range, and each approach has different tradeoffs.
- Low-temperature annealing (60–70 °C): Safer for preserving dimensions and fine details. Requires longer hold times, typically 90 minutes to 4 hours. A good starting point if you’re annealing a part with tight tolerances.
- Mid-range annealing (80–90 °C): Right around the glass transition point. Produces more crystallinity and strength improvement but introduces more risk of warping. Hold times of 1–2 hours are typical.
- High-temperature annealing (100–110 °C): Maximizes strength and heat resistance. Research found 110 °C for 2 hours produced the highest tensile strength in neat PETG. However, parts are much more likely to deform at this temperature without support packing.
If your part contains carbon fiber PETG, the optimal parameters differ. One study found that CF-PETG performed best at just 70 °C for 4 hours, a lower temperature but longer duration than neat PETG. The carbon fibers already provide stiffness, so aggressive heating offers diminishing returns while increasing warping risk.
Step-by-Step Oven Method
A standard kitchen oven works, though a small toaster oven or dedicated annealing oven gives you more precise control. Here’s the basic process:
Place your part in the oven while it’s still cold. Set the target temperature (start with 70 °C if this is your first attempt) and let the oven preheat with the part inside. This avoids thermal shock from placing a room-temperature part into a hot oven, which can cause immediate warping. Once the oven reaches your target temperature, start your timer. For a first run, 90 minutes at 70 °C is a conservative choice that balances strength gains with minimal deformation.
When the hold time is up, turn the oven off but leave the door closed. Let the part cool inside the oven as slowly as possible. Rapid cooling reintroduces internal stresses, which defeats the purpose. Depending on your oven, this cooldown takes 1–3 hours. Don’t open the door until the temperature is close to room temperature.
Using Salt Packing to Prevent Warping
The biggest challenge with annealing PETG is keeping the part’s shape intact. Once the material softens near its glass transition temperature, gravity and internal stresses can pull it out of shape. Salt packing solves this by surrounding the part with a rigid, conforming support.
Fine powdered salt works best. Regular table salt or sand won’t compact tightly enough to hold details. Powdered salt, with its fine, sharp crystals, forms a solid mass when compressed. Start by pouring a thick layer of salt into a metal or oven-safe container. Place your part on top, making sure multiple parts are spaced far enough apart that they won’t fuse together. Add more salt around and over the parts, tapping the container regularly so the powder flows into every crack, hole, and surface detail.
Once the part is fully buried, compact everything by pressing down firmly with a flat object or a second container. This step is critical. Compacting eliminates voids where the softened material could flow, and it locks the part’s geometry in place so warping is minimized. Top off with more salt if needed after compacting, then place the whole container in the oven and follow the normal annealing process.
After cooling, brush or rinse the salt off. Fine details should be preserved if the packing was thorough.
Handling Dimensional Changes
Annealed PETG parts will change dimensions. As the polymer chains reorganize into a denser crystalline structure, the material contracts in some directions. Parts typically shrink slightly in the X and Y axes (the flat plane of each layer) and may grow in the Z axis (the stacking direction) as layers bond more tightly and internal stresses release.
The exact percentages depend on your annealing temperature, infill density, and wall thickness. Higher temperatures cause more dimensional change. If you need precise final dimensions, the best approach is to anneal a test piece first, measure the changes, then scale your model to compensate before printing the final version. For functional parts where a millimeter or two doesn’t matter, you can often skip this step.
Thinner walls and lower infill percentages are more prone to distortion. Parts with 100% infill and thick walls hold their shape best during annealing because there are fewer internal voids where material can shift.
Print Settings That Help
How you print the part in the first place affects how well it responds to annealing. A few adjustments at the slicer level can make a noticeable difference.
Higher infill density gives the part more internal structure to resist deformation during heating. If you plan to anneal, printing at 80–100% infill is worth the extra material and time. Layer height also plays a role. Thinner layers (0.1–0.2 mm) produce more layer interfaces, which can bond more completely during annealing. However, research shows that even 0.3 mm layers respond well, with that layer height actually producing the best percentage improvement in one study.
Print orientation matters too. Orient the part so that the direction requiring the most strength aligns with the layer lines rather than across them. Annealing improves inter-layer bonding, but the weakest point of any FDM print is still the junction between layers. Starting with a favorable orientation amplifies the benefits of the post-processing step.
What to Expect From the Results
After annealing, your PETG part will feel slightly different. The surface may appear more matte or develop a faint haziness, which is a visual sign of increased crystallinity. The part will be stiffer and less flexible. It will resist heat better, meaning it won’t soften as easily in a hot car or under sustained load at elevated temperatures.
The strength improvement is meaningful for functional parts but not transformative. You’re looking at roughly 8% more tensile strength under conservative conditions, with potentially larger gains at higher temperatures and longer durations. Where annealing really shines is in improving the thermal stability and reducing the “creep” behavior where PETG slowly deforms under constant load over time. If your part needs to hold a bolt under tension, support weight in a warm environment, or resist gradual deformation, annealing is worth the effort.