Glass transition temperature: at what temperature a print softens
The glass transition temperature (Tg) is the temperature at which a plastic changes from a hard, strong state to a soft, bendy one. For PLA it is around 60 °C and for PETG around 80 °C.
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Contents
What happens at Tg
The glass transition is not melting. The plastic does not flow, it just softens sharply. Under load the part starts to bend and deform even though it still looks whole. The heat deflection temperature, which is given in data sheets, works in a similar way.
The exact values differ by manufacturer and additives. Take them as a guide: PLA around 55 to 60 °C, PETG around 75 to 80 °C, ABS around 100 °C.
Where it bites
The most common problem is the car. In summer the air above the dashboard of a parked car heats up to 70 °C and more, and PLA cannot cope. Likewise PLA softens by a radiator, in the sun behind a window, by a halogen lamp or in a dishwasher. PETG withstands more, but is also borderline under continuous heat above 70 °C.
A higher temperature means the part softens and does not return to its shape. The residual deformation is permanent.
What to do about it when ordering
If the part will get warm, choose PETG. Unlike PLA it prints at a higher temperature and deforms less easily. Above 80 °C, ABS, ASA, nylon or polycarbonate are more suitable, but we do not print them. If you need heat resistance, write to us at jakub.novak@boostello.com and we will advise, or tell you it will not work.
A stronger infill will not help
A stronger infill stiffens the part, but it does not raise the softening temperature. That is a property of the material itself. There is annealing, in which the crystallinity of PLA is increased and its heat resistance rises, but it changes the dimensions, and we mention it for information only and do not offer it as a service.
Examples from practice
- A phone holder for the car. In the sun and with the heating on, it warms above 60 °C. PLA will sag after the summer, while PETG lasts better.
- A mat under a hot pot. Direct contact with a hot pot is beyond either material.
- An electronics enclosure. Power supplies and motors warm up. If the temperature in the enclosure approaches 60 °C, choose PETG.
- Decorations and models. Tg does not matter here, and PLA is cheap and available.
Common questions
At what temperature does PLA deform?
From roughly 55 to 60 °C, depending on the particular filament. In a hot car that is enough.
Will PETG take the sun behind a window?
Usually yes, better than PLA, but do not design it for long-term heat above 70 °C.
Is Tg the same as the melting point?
No. Tg is the softening of the amorphous part of the plastic, while melting happens at a much higher temperature.
Related terms
PLA
PLA (polylactic acid) is the most common 3D printing material, made from plant starch. It is cheap, accurate and easy to print, but it does not withstand heat above about 60 °C or long exposure to sun.
PETG
PETG is a tough 3D printing plastic, a close relative of the material in PET bottles. Compared with PLA it withstands more heat, impact and moisture, so it suits technical parts and outdoor use.
ABS
ABS (acrylonitrile butadiene styrene) is a strong, tough engineering plastic that LEGO bricks are made from. In 3D printing it warps easily and gives off fumes, so it needs an enclosed printer.
Annealing prints
Annealing a print means reheating the finished part to a temperature above the material's glass transition, where the polymer partly arranges itself into a crystalline structure. It is most often done with PLA so it can take a higher temperature.
PETG vs ABS
PETG and ABS are two common plastics for functional parts. PETG prints more easily and copes better with water, while ABS withstands higher temperatures and can be finished further.
ASA vs ABS
ASA and ABS are related engineering plastics. ASA withstands UV radiation and outdoor weather better, while ABS is usually cheaper and is used mostly indoors.
SourcesSources checked 1 November 2026, 3
- Glass transition, Wikipedia, checked 2026-11-01
- Heat deflection temperature, Wikipedia, checked 2026-11-01
- Thermoplastic, Wikipedia, checked 2026-11-01
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No model? No problem, pick a ready one
These sites have thousands of free models. Download an STL or 3MF and upload it here. If you sell it or use it for business, check the model licence. How to pick a model and licence