When printing with PETG – or any other filament – an essential thing to consider is the glass transition temperature (Tg). This is the temperature at which amorphous solids turn from a solid to a viscous liquid state.
This temperature is different from the melting temperature, and is important to know because at this point the filament will become pliable and easy to work with.
PETG’s glass transition temperature is around 80°C to 85°C. At this temperature, the brittle filament will become viscous, which can then be heated to the melting point and extruded.
When printing with PETG, it is important to avoid heating the bed above the Tg.
Heating the bed above the material’s glass transition temperature can interfere with cooling, preventing the filament from adhering properly to the build plate.
Factors That Affect PETG Glass Transition Temperature
Knowing the glass transition temperature of the material you’re printing with can determine an object’s suitability for a certain application. Knowing this threshold is also essential to prevent printing defects such as elephant foot and warping.
Regardless of the material you’re using, there are a few factors that affect its glass transition temperature.
Chemical Structure of Material
The first thing that affects the glass transition temperature of PETG is the chemical structure of the material.
PETG is a material with a heavier molecular weight than other thermoplastics used in 3D printing. A higher molecular weight results in an increase in Tg, and so does the material’s bulky molecular structure.
These features make PETG a good choice for outdoor applications (the material can maintain its shape at higher temperatures).
Filaments with a higher glass transition temperature than PETG include ASA, polycarbonate, and ABS.
Another factor influencing PETG’s Tg is the amorphous nature of the material.
Amorphous, as opposed to crystalline, is a term that defines materials that don’t have a regular structure or defined shape.
Like most materials, PETG is easy to work with when solid. However, as it starts to soften and become liquid, the molecular bonds weaken – just as it happens with water.
Amorphous materials have high thermal energy, which results in a relatively high glass transition temperature.
While the chemical composition of PETG should be standard, the truth is that each brand tweaks the formula to improve the filament’s characteristics.
When plasticizers are used, they can increase the volume between PETG’s polymer chains, spacing them out.
This allows the chains to slide past one another at lower temperatures, resulting in a lower glass transition temperature.
This is the main reason why PETG filaments from different brands have slightly different glass transition temperatures.
Similar to plasticizers, a high moisture content increases the distance between chain structures and adds free volume between them. As a result, wet PETG has a lower glass transition temperature than dry filament.
Since PETG is a highly hygroscopic material, not drying it properly before use could result in a variety of printing defects. Some of these are related to the glass transition temperature, such as the bed temperature.
When using wet PETG, you risk not calibrating the temperature of the build plate properly. As the material becomes viscous faster, it might develop the elephant’s foot. A too low bed temperature could lead to adhesion problems.
Ideal Glass Transition Temperature Of PETG
The ideal glass transition temperature of PETG is 82°C to 83°C. However, most filament manufacturers and 3D experts recommend a wider range, usually between 80°C and 85°C.
This wide temperature range allows you to make adjustments based on the actual conditions of the material. For instance, the same filament can have a Tg of 83°C when dry and only 80°C when using it wet.
You may also observe slight differences between brands. Glass transition temperatures printed on PETG labels differ from brand to brand due to the additives used.
This is why most experts printing with PETG recommend a bed temperature between 65°C and 70°C – despite manufacturers claiming that their product can support bed temperatures up to 80°C.
However, you should consider recommendations from your hardware brand and adjust the bed temperature in small increments if necessary.
Prusa, for instance, recommends a bed temperature of 85°C for PETG. Yet, Creality recommends heating the bed between 50°C and 60°C. These differences are explained by the different bed types that equip these printers.
The original build plates from Prusa are usually PEI with a smooth, powdered, or satin surface. The brand also sells magnetic heat beds, but these normally don’t come as standard with the printer.
Meanwhile, Creality equips its Ender 3 printers with textured glass beds – which can be used to print PETG, as long as you turn them upside down and use an enhancer, such as glue stick or hairspray.
PETG vs. PLA Glass Transition Temperature
Besides PETG, PLA is one of the most popular filament types.
Standing for polylactic acid, PLA is a common choice among environmentally conscious users. This material is not plastic, meaning that it doesn’t pollute the environment.
However, because of the different molecular structure and diametrically opposed chemical composition, PLA has a glass transition temperature much lower than PETG. This material becomes viscous at about 50°C to 80°C.
This extremely wide interval is explained by the formula variations from brand to brand. Additives can lower or increase the Tg point, just as it happens with PETG.
PLA’s lower glass transition temperature makes it less suitable for outdoor applications.
A 3D printed object exposed to the elements for a long time – especially during summer – can lose its structural stability and deform or even collapse.
Likewise, this material isn’t suitable to use for any other application that requires the use of heat. PETG is a much better choice for these purposes.
It goes without saying that a lower glass transition temperature is usually synonymous with a lower melting point and lower heat retention.
When it comes to printing quality, this means that it’s easier to obtain quality prints with PLA rather than PETG – and perhaps this is the reason why beginners tend to favor it.
PLA has a melting point between 170°C and 180°C, which means that heating the nozzle between 180°C and 220°C is enough for proper extrusion.
The material also cools down faster, minimizing the risk of stringing, oozing, and other printing problems.
PETG has a melting point of about 260°C and requires a nozzle temperature of at least 230°C to 250°C.
As you can expect, the material isn’t completely melted at this point, but it’s oozy enough to flow out of the nozzle. Heating the nozzle to the filament’s melting point would result in excessive oozing and stringing.
Frequently Asked Questions
How hot can PETG get before warping?
Due to its high glass transition temperature and melting point, PETG is challenging to work with. However, warping isn’t usually the issue.
Warping happens when the printed layers don’t adhere properly to the build plate or the already printed rows. The edges and corners of the layer curl up, compromising the part’s structural integrity and affecting its visual appeal.
PETG has a high melting point, but it transitions from a solid to a viscous liquid much faster. Hence, the printed layers stick easily to the build plate or the already laid rows.
Sometimes, it sticks so well that the problem is removing the print from the plate. It isn’t generally recommended, however, to heat the nozzle over 260°C.
A too high temperature can affect the material’s chemical formula and in this case, warping could occur.
More common problems with PETG are elephant’s foot, zits, blobs, and stringing. When PETG warping does occur, it is more likely the result of a too cold bed, too fast cooling, or printing with poor-quality/wet material.
Can you reduce glass transition temperature?
The glass transition temperature can be manipulated by manufacturers, by adding plasticizers (polymers with a low molecular weight) to the PETG. This is why filaments from different brands can have a different Tg, even though they are all PETG.
Once the filament has been manufactured, you can’t manipulate the glass transition temperature. Yet, you can reduce it (at least practically), by using wet PETG.
Due to PETG’s hygroscopic nature, the material absorbs moisture from the air like a sponge. Moisture increases the free volume between the material’s chemical chains, lowering the glass transition point.
Just keep in mind that it alters the filament’s melting point, too. This is why stringing and oozing are more common when printing with wet PETG.
PETG has a glass transition temperature between 80°C and 85°C. Heating the build plate over 80°C when printing with this material can result in a variety of printing defects. However, knowing the glass transition temperature can also help you decide whether PETG is the right filament for your part, based on application.
We hope this guide can help you set the printer correctly and use PETG with minimal issues.