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I've heard that PETG, what we print with, is slightly different than PET or PETE, which is commercially available. Yet, PET bottles seem easy enough to recycle into prints according to these videos:

#1 Multicolor printing-Details of free filament production from recycled bottle (PET)for 3D printer

Precious Plastic channel

https://www.youtube.com/channel/UCqA-SppbeUi5ou0isB27mgg

What properties change between PET and PETG? Obviously, the additives will change, but what are the properties and parameters that change that make PETG more suited to 3D printing? Especially given the fact that PETG is fairly hygroscopic, which I believe took a while for the community to uncover.

While more science is better as far as an answer, please keep it relevant to practical 3D printing as well.

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I use PETG when I want the object to be somewhat rigid, but flexible enough not to break, especially object having thin parts. PETG is strong enough to print two layer 0.5 mm thick and be very flexible in the z direction, such as printing straps.

Basically PETG is amorphous while PET/PETE is crystalline. Amorphous gives a slightly lower melting point and less out-gassing, characteristics better for 3D printing. PETs crystal structure makes it more rigid than PETG, but leaves PETG more impact resistant and easier to print. The more rigid PET translates to more dimensional accuracy as with PLA.

From https://www.oberk.com/packaging-crash-course/differences-in-pet

PETE (or PET)

PETE is the most common variation of the thermoplastic polymer resin. The clarity of PET is considered to be the best of all plastics with very good chemical resistance. PETE is formed into containers via 1-Step or 2-Step Injection Blow Molding.

PETG

Polyethylene Terephthalate Glycol (PETG) is an amorphous variation of PET. PETG cannot be oriented and therefore you will not find pre-forms of this type of resin. Extrusion, Injection Blow Moulding and 1-Step Injection Stretch Blow Moulding are all possible for PETG, it is also used frequently for 3D printing. Clarity and chemical resistance are considered good when compared to the previous three variations, however PETG has poor impact resistance.

Why is PETG used for 3D printing?

Quote from https://www.3dnatives.com/en/petg-3d-printing-guide-181220194/#!

The characteristics of PETG

PETG is therefore a copolymer, combining the properties of PET and glycol. The addition of the latter reduces the overheating issues of PET and therefore its brittle appearance. Among the main characteristics of PETG are its hardness, impact and chemical resistance, transparency and ductility. It is an easily extruded material with good thermal stability. It is particularly appreciated for its food compatibility. On the downside, note that it requires a heating plate to avoid the warping effects found in ABS 3D printing – even if the warping rate is low, it is better to use a BuildTak sheet to make sure the material grips. It is also more prone to scratches than PLA. Finally, it can quickly take moisture and will keep better in a cool and dry environment.

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To add to Perry's answer, which is already very informative:

PETG is subject to creep (permanent deformation under continuous mechanical load), especially at temperatures above room temperature. As a result, many 3D printer designs utilizing printed parts caution against using PETG. PET is generally regarded as not subject to creep below around 80°C.

Unlike PETG, which is amorphous and does not crystallize, PET can be annealed (made to crystallize intentionally after printing is complete) at temperatures between 80 and 100 °C, after which it can handle loads at higher temperatures, reportedly up to 100 °C. However, annealing produces some dimensional changes, which may make it difficult to utilize this property.

PETG has considerably lower glass transition temperature, melting point, and heat deflection temperature than PET.

While this varies by PETG formulation, generally PET can (and often must, due to crystallization concerns when printing slow) be printed at much higher speeds than PETG.

PETG gunks up and sticks badly to most nozzle materials when printing. Certain coated nozzles, and tungsten carbide nozzles, can significantly reduce but not eliminate this effect. PET does not gunk up or stick to the nozzle.

PET is less sensitive to buildplate temperature, and can even be printed on an unheated bed, although this may lead to corners lifting without sufficient adhesion. PETG is very hard to print well without high bed temperature; layer delamination and failure of first or second layer to adhere are common issues with insufficient heat.

PET has extreme layer bonding, giving near or fully isotropic part strength. PETG generally has issues with delamination/separation at layer lines. In my experience, it is far worse than PLA in this regard.

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