Glycol-modified Polyethylene Terephthalate (PETG) is a clear, extremely durable 3D printing filament with higher temperature resistance than PLA and easier printing than ABS. PETG is rapidly becoming more popular so here you can find everything you need to know to choose and print with the right PETG filament.
PETG is a copolymer of the synthetic polymer Polyethylene Terephthalate (PET) which was first patented 10 by E. I. du Pont de Nemours and Company in 1949. The production of PETG from PET was first demonstrated by Kibler of Eastman Kodak Co. in 1959 11 however it wasn't until 1977 that PETG was commercially produced by the Tennessee Eastman Chemical company under the product name Kodar PETG 6763. PETG was initially intended for use in commercial blow moulding due to its clarity and toughness however it was not particularly well received, many thought that production would be better focused on PET.
Despite its clarity, one of the biggest problems with commercial PETG was its colourisation. Significant processing and catalysis was required to reach an acceptable colour, often using cobalt. It was not until 1993 that a low colour PETG was marketed as Spectar Copolyester as a result of some significant breakthroughs in PETG colouring 13 and toners 12.
PETG is an oil based polymer and is made from 3 components: terephthalic acid (TPA), ethylene glycol (EG) and cyclohexane dimethanol (CHDM).
PETG is made using a two-step, melt-phase polycondensation process. This is a relatively simple process that joins together two different monomers accompanied by the release of a small molecule, like water. The first step in PETG synthesis is esterification of TPA, EG and CHDM at around 230 in a nitrogen atmosphere, this is when the first polycondensation reaction occurs in the presence of a metal catalyst, usually cobalt. The second polycondensation step is carried out at high temperature (usually around 280) under high vacuum conditions. This animation 8 shows how an industrial melt-phase polycondensation reaction works and you can read more about PETG production in the Handbook of Thermoplastics.
As previously mentioned, PETG is a copolymer of PET. PET is a polymer synthesized from TPA and EG. PET is extremely widely produced due to being extremely tough, lightweight, thermally stable and hydrophobic. These properties are a result of PET polymers being able to pack very tightly against each other giving a crystalline structure. This is possible as EG is a relatively small, linear monomer. PET's high thermal stability makes it very difficult to 3D print with, requiring a high printing temperature and often having adhesion issues.
The key difference between PET and PETG is the presence of cyclohexane dimethanol (CHDM). CHDM is incorporated into the polymer chain, replacing EG at variable ratios. For the polymer to be classed as PET then it must have 50% or less CHDM modification, anything above 50% is classed as PCTG.
CHDM monomers are much larger and bulkier than EG, this means that once it is incorporated into PETG the polymer chain cannot fit tightly against its neighbouring chain. This greatly reduces the crystallinity of PETG making it an amorphous polymer. This also results in PETG having a significantly lower glass transition temperature and printing temperature than PET. It is for this reason that PETG is used for 3D printing: It retains many of the desirable mechanical and chemical properties of PET whilst being much easier to print with!
|Chemical Formula||C10H8O4 . C16H18O4|
|CAS Number||25640-14-6 5|
|SPI Resin Identification Code 15||1 or 7|
|Melting Temperature||No strict melting temperature as PETG is amorphous|
|Tensile Elongation At Break*||120-190%|
|Other Solvents||Benzene, Glacial Acetic Acid. Full list of PETG Solvent resistance 17|
|Example Technical Date Sheet||3DXSTAT TDS 2|
|Example Safety Data Sheet||3DXSTAT SDS 1|
* ASTM D638 Test Method 3
Note This table contains generalised properties and should not be considered as material specifications. PETG products from different manufacturers may differ considerably from the data given here.
One of the key advantages of PETG is its physical properties. Being a copolymer of PET means that PETG retains many of the extremely desirable strengths of PET. Not only is it a very lightweight material, PETG is impact resistant, strong and flexible. It is slightly softer that PET, ABS or PLA making it more difficult to break but also more susceptible to wear. PETG is resistant to water as well as having great chemical resistance.
Getting the settings right for PETG can be a little tricky but once everything is set up correctly PETG will print very well with great Z-axis adhesion. PETG adheres very well to the base layer although in some cases it adheres too well and can be difficult to remove! For this reason, it is also not suitable as a support structure. PETG demonstrates very little shrinking so is suitable for printing large parts. PETG is odourless when printing but you should still make sure you're printing in a well-ventilated area.
PETG filament is generally considered food safe, especially as it can be sterilized. PETG is often used in food and drink containers. Low CHDM PETG is often used for soft drinks bottles as it is more flexible than PET. PETG resin has been FDA approved for food contact however this may vary from different manufacturers and filaments depending on additives that are used. Always check the Safety Data Sheet. When 3D printing products that will be in contact with food and drink you must always remember that bacteria can easily build up in the small ridges characteristic for 3D printed products. Using a food safe sealant or epoxy can help to prevent this build-up. You should also use a food safe hot end/extruder (usually made from stainless steel). This is because other extruder materials can contain contaminants that may enter your product, for example brass nozzles may contain lead.
Whilst PETG is a very strong material it tends to struggle printing bridges between parts as the printed PETG will often sag. Active cooling and slightly lower print speed and temperature can help to mitigate this. PETG is also susceptible to producing thin hairs on the surface of the print due to stringing and oozing. An important consideration when printing with PETG is its clarity. Whilst this is usually seen as an advantage, it will also make any imperfections within the internal layers of your print visible.
Many find that 3D printing with PETG can be difficult to begin with but once the correct settings have been found it is easy to print with and produces high quality products. To help find the correct settings for 3D printing with PETG you can use our Go-To print settings tool. This tool collates the settings from all the successful prints uploaded by the Filaments.Directory community to find the right settings for you!
PETG is traditionally classed with a Resin Identifier Code of 1, this is the same as PET and other PET copolymers. In recent years however, concern has grown over the recycling of PETG with PET. Despite being chemically and physically similar, PET and PETG require different processing temperatures during mechanical recycling. This has led to some recyclers claiming that PET and PETG act as contaminants for each other when recycled in the same stream! This is particularly important as PET and PETG are not easily distinguished from each other using the common sink-float separation process 14 used by plastic recyclers. This has resulted in some areas excluding PETG from RIC 1 18. Some local recycling programs will accept PETG for recycling whilst others will not so it is always best to check
Whilst PETG has a relatively low flammability HMIS 7 flammability rating of 1) it will burn after prolonged exposure to high temperatures. It will release carbon oxides, including carbon monoxide, on combustion. Powdered PETG may form an explosive dust-air mixture. Dry chemical or chemical foam sprays are appropriate extinguishers. Direct water streams should be avoided, especially if the plastic is molten. Always check the Safety Data Sheet before use.
PETG is hygroscopic, meaning that it will absorb water from the air. This results in increased brittleness, filament breaking, bubbling when printing or filament degradation. In some cases, this can lead to filament jamming in your printer's hot end, it may even require replacing the hot end if the jam is severe. This can be avoided by storing your filaments in vacuum bags with silicagel "dry packs". You should also avoid storing filament for a long period of time. To read more about protecting your 3D printing filaments from humidity you can read our blog post here.
Unlike ABS or other 3D printing filaments, PETG does not release foul odours during printing. Despite this you should still print in a well-ventilated area as printing with plastics of any type may be a source of ultrafine particles 19 which should not be inhaled.
Using a good base layer material is imperative to ensure that the first print layer adheres properly and does not unstick during printing. To find out more about the importance of adhesion and the base layer you can read our blog post.
The first print layer is essential to get right in order to prevent unsticking during the printing. There are multiple factors to consider here:
PETG is particularly prone to stringing and oozing when printing. PETG can also form droplets, particularly when the nozzle is stationary. In some cases, droplets of PETG have been known to stick to the nozzle and burn or drip onto other parts of the prints. At best this will reduce the quality of your print and at worst it may be a hazard next time the nozzle moves through that area. It is therefore important to get the printing temperature for PETG just right! The Print temperature for PETG is generally between 220°C and 250°C however this can vary greatly depending on the filament. Always check the Technical Data Sheet. You can also consult our Go-To print settings tool which has been created by analysing all the successful print settings that the Filaments.directory community have uploaded. The optimal printing temperature will also be affected by your printing environment so it is a good idea to do some test prints and vary the temperature by 5°C increments until it runs smoothly.
PETG has a print speed slightly slower than that of ABS or PLA. Print speed is often in the range of 30mm/second to 90mm/second however this may vary depending on the filament and the printer, it is often not likely to go above 60mm/second. Higher quality prints will generally benefit from speeds at the lower end of this spectrum. Much like printing temperature you may find that it is best to start with a speed and increase or decrease it incrementally until you find an optimum. Faster printing will also tend to require a higher printing temperature as it requires a lower viscosity which can be achieved at higher printing temperatures. Always consult the Technical Data Sheet for the manufacturers recommended print speed.
Most of the problems associated with printing with PETG arise from incorrect print settings. The best way to address this is to experiment with some tests first to find the optimum settings for your printer and the PETG filament you're using. A good way to start this is with the recommended settings from the manufacturers Technical Data Sheet. Another great way to help find the right settings is to use our Go-To print settings tool which has been created by analysing all the successful print settings that the Filaments.directory community have uploaded.
Some specific problems you might encounter are:
If you have a dual nozzle printer you can print your supporting structures in a low-density water or chemical soluble 3D printing filament which can simply be soaked in the relevant solvent to remove. HIPS and PVA are both commonly used as support materials. As previously mentioned, PETG adheres very well to itself. This can makes removing PETG supports very difficult without damaging the rest of the print.
You may find that your print has little burrs or lips that need to be removed as they can often feel quite sharp and prevent pieces from fitting together properly. A Dremel 6 is commonly used to grind down 3D printing products, alternatively you can file down any excess by hand. PETG is a good medium between PLA (which does not sand well) and ABS (which sands extremely well).
Cold Welding is the process of using a small amount of solvent to fuse together multiple printed parts of the same material. This technique is often used for ABS products. As previously mentioned, PETG has good chemical resistance. Whilst cold welding PETG is possible, the solvents required to do this are generally very strong and can be very hazardous. Cold welding is not recommended for PETG 3D prints. Instead of welding, consider using a cyanoacrylate glue (superglue) or an epoxy (see below). Gluing may be significantly improved by sanding, cleaning and de-greasing the area.
Epoxy can be used to fill gaps in the print and give the product a clear, smooth outer shell. It is worth noting that the 3D printing layer lines will still be visible under the epoxy coat.
PETG is not an easy material to paint (if you've ever tried to paint a plastic water bottle you will know this) however with sanding, cleaning and de-greasing it should be able to hold a priming layer that you can sand again and paint over.
PETG is an extremely widely used plastic. Its chemical and functional differences from PET make it an ideal material for multiple uses. Due to being very light, impact resistant and clear, PETG is often used for packaging purposes. PETG's ability to undergo rigorous EtO Sterilization 20 makes it ideal for medical instrument storage/packaging. PETG resin is FDA approved food safe and often used in food packaging. PETG is becoming an increasingly popular 3D printing filament due to its excellent mechanical qualities and a printing difficulty similar to that of PLA and ABS.
PETG is becoming an increasingly popular filament for use during 3D printing, in fact our [2018 State of the Filament] LINK HERE market survey showed that it has become the second most popular filament used by our community, in place of ABS. As such, we can expect to see many more PETG filaments being introduced with a wider variety of superpowers and modification such as UV resistance. Much like for PLA 9, we may even begin to see PETG resins that have been specifically designed for 3D printing.
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