PETG vs ABS: How do they compare?

In this final article comparing popular 3D printing polymer filaments, we will be taking a look at how PETG and ABS stack up against each other in terms of mechanical and thermal properties, printability and other factors such as aesthetics. 

In the previous articles we compared PLA vs ABS, and also PETG vs PLA. 

ABS was shown to be particularly good for printing items that may need higher impact resistance, and PETG was shown to be a rugged plastic good for food safe plastics and for outdoor use.

Let’s examine these thermoplastics in a little more detail and hopefully help would-be users to identify which 3D printing filament of these two would be best suited to their needs.

ABS

ABS (Acrylonitrile Butadiene Styrene) as we saw in the previous article, is prized in industry for its impact resistance, which is why it is used in a wide array of products ranging from automotive bumpers to electronics housings to Lego bricks. 

PETG (Polyethylene Terephthalate Glycol) is a modified form of PET (the most commonly used commodity plastic in the world) [1]. PETG is modified to make it more suitable for 3D printing compared to its industrial cousin. 

PET is valued for its ease of forming, its food safety properties (non-leaking/leaching) and its low cost. It’s also transparent in its raw state. All of these reasons make them attractive for use in plastic drinks bottle manufacturing.

As a filament, PETG comes in a variety of colours and various degrees of transparency/translucency, and like many filaments can be modified with additives to change the properties of the filament.

For specific comparisons in this article we will be referring to this particular variant of ABS [2] and this PETG variant [3].  

Both examples are unfilled varieties and so demonstrate a good baseline to discuss what an average filament comprises in terms of material properties. 

Strength and Toughness

As we saw in the previous comparison articles, PLA was stronger than ABS in terms of tensile strength, while having similar strength to PETG.

PLA had an inferior impact resistance when compared to both ABS and PETG.

So how do ABS and PETG measure up against each other in terms of strength and toughness?

Take a look at the table below, which has been populated with data from our two branded filaments:

From these two specific brands of filament, we can see that the PETG has a much higher tensile strength than the ABS in this case. As we mentioned in the previous articles, there is a lot of variation from filament to filament and from polymer to polymer. 

The graph below plots density against tensile strength for a range of polymers including ABS (red), PETG (purple) and PET (blue).. The green blob shows PLA, just for reference.

Ashby Plot of PET, PETG, ABS

The plot contains polymer data for a range of bulk materials such as injection mold, extrusion, and additive manufacturing. 

Regardless of the exact bulk material, we can say for sure that most PET and PETG are heavier plastics than ABS, and most PETs (including PETG) have a higher tensile strength than the various ABS types. PLA sits above ABS for tensile strength but below PETG, and is a little heavier than ABS in general.

Generally speaking, the majority of filaments will have tensile strength values ranging around 30-50 MPa for ABS, and 60-65 MPa for unfilled PETG.

The ABS also has the higher elastic modulus, meaning it is stiffer than the PETG, and both of these examples have similar impact resistance.

Whether stiffer ABS or more flexible PETG is a good thing or a bad thing will depend entirely on your chosen application!

Thermal Properties

ABS, as we know, has a higher glass transition temperature than most common desktop printer filaments, meaning that the point that the molten polymer freezes solid is higher than the other filaments we have looked at so far. We can think of this in layman's terms as having a higher melting point. 

Plastics with higher melting points/glass transition temperatures tend to have higher Heat Deflection Temperatures also, meaning they can maintain their stiffness (they bend less) at higher temperatures.

The same is true for ABS and PETG. ABS has the higher glass transition temperature than PETG, and so requires heating more in order to induce the same deflection under the 66psi load when compared to PETG.

So on the positive side, ABS stays stronger under heat. On the negative side, it means you may need to modify your printer nozzle and heatbed to reach higher temperatures if you are using an entry-level FDM printer.

Warping

Warping is a problem when dealing with high temperature filaments, and this is true especially for ABS.

When printing a high temperature filament like ABS, differential cooling of the part on the print bed can cause warping, which can result in the part bending up from the printbed and the print failing.

In order to reduce warping on ABS prints, a higher temperature printer bed heater is required in addition to the installation of an enclosure to protect the print from drafts while it is being printed [4].

PETG is much more forgiving, and will print at more or less the same temperatures as PLA and with as much ease [5]. Although as with most filaments, it's best to print in a draft-free environment.

Print Settings

Again, for general print settings we have consulted the Simplify3D website, as they are very well versed in all things related to print settings:

The ABS will need higher temperatures at the nozzle and print bed than PETG so modification may be needed for some printers.

ABS can be chemically reduced to a slurry which can be applied to the print bed to assist with adhesion of ABS, while a glue stick or painters’ tape will suffice for PETG.

Do note that a part cooling fan is recommended for PETG.

Weatherproof 

Plastics are great, long lasting and rugged materials, but they are not all equal when it comes to maintaining those properties when exposed to environments outside of their intended usage (such as in strong sunlight or moisture, or various chemicals).

For this reason, polymers are often assessed for how they can withstand the outdoors by means of various weather testing methods. 

One such test is the accelerated weather test [6] which combines cyclic weather (water/mist) with artificial UV sources. This distresses the plastic at an accelerated rate and shows how a plastic will perform when exposed to the Sun and rain over time.

While PETG and ABS may be very similar mechanically, when you plot UV resistance against Water Absorption for a range of different PET and ABS plastic variants, a different story presents itself:

The graph shows UV resistance on the vertical axis, ranging from “poor” at the bottom to “excellent” at the top. The horizontal axis shows water absorption as a percentage of original mass, rangine from 0.02% up to a little over 1%.  

This graph has been populated with various types of ABS plastic (coloured in red), and various types of PET (including PETG) in blue. You may have noticed that the split between the colours on the graph is pretty clear. All the ABS falls into the bottom right corner of the graph, while the bulk of PETs reside in the top left quadrant.

What does this mean?

It means that most PET (and PETG) scores “good” for UV resistance while most ABS ranks as “poor”.

It is a similar story for water absorption over 24 hours. All of the PET has less than 0.2% water absorption with PETG falling in the range of 0.118 - 0.143%.

All of the ABS has more than 0.2%, and goes as high as 1% for an ABS/PA (unfilled) variety. 

ABS is really not great for outdoor use at all: it degrades in sunlight and it sucks up moisture when it rains.

It’s even worse in the cold vacuum of space, bathed in all manner of particles (including the whole gamut of UV that is normally filtered by our ozone layer).

NASA has performed many tests on the effects of space weather on polymers, having tested several thousand samples by mounting the samples to a plate and mounting them outside the International Space Station for a couple of years [7]. You can see how UV and atomic oxygen has affected the various samples including ABS and PET in the image below.

Image Credit: NASA

To be fair, when exposed to the harshness of space, neither ABS or PET looks great.

The zoomed in image below shows before and after exposure to space for extended periods of times for both polymer samples. Much of the discoloration is caused by exposure to UV radiation, while the physical damage is caused by atomic oxygen ramming into the samples and physically eroding the surface of the polymer.

Image Credit: NASA

But this is an extreme example of how radiation and rare elements affect polymers. You’re likely not going to be printing for such harsh conditions on your desktop printer.

Let’s have a look at what is happening to the plastics anyway when they are exposed to radiation, be it in space for short periods, or on Earth for longer periods.

When exposed to radiations such as UV, gamma, and other high energy particles, polymers can experience two kinds of reaction, being cross linking and chain scission [8]. Both of these can occur in sunlight here on Earth also, not just when tied to the front of a space vehicle, although naturally the effects are quicker in space.

Cross-linking results in formation of chemical bonds between two polymer molecules. This increases the molecular weight of the polymer ultimately binding the material into an insoluble three-dimensional network. 

Conversely, chain scission is the breaking of polymer molecules which results in a reduced molecular weight and higher solubility.

Either of these reactions can lead to unwanted effects such as reduced stiffness (or increased brittleness), lower tensile strength, reduced hardness, release of gases and also aesthetics of the plastic in question.

You can see what happens to a nylon rope when it is left in the sunlight for too long in the image below. The exposed rope on the left has clearly discolored and begun to fray as a result of the prolonged UV exposure.

 Image Credit: Wikipedia

So if you want to build something that is UV-proof for use on Earth, don’t use ABS (or Nylon).

And if you want to print something out of plastic that can withstand the radiation environment of Low Earth Orbit, then none of these particular plastics are especially suitable. 

Most of these desktop filaments outgas a heck of a lot in vacuum too, but that’s a topic for another time.

Post Processing

ABS can be both electroplated and vapor smoothed fairly easily, safely and at low cost.

Certain variants of ABS have higher conductivity and can electroplate, or electroless nickel plate without modification. For some ABS plastics, and some other non-conductive plastics (including PETG), a conductive ink can be applied and plating can continue as normal [9].

It is not recommended to attempt vapour smoothing of PETG in the home environment as the chemicals required are harsh and are not the kind of thing one tends to keep laying around the house. This is best left to research institutes and people with access to industrial vapour smoothing systems, with all their safeguards and controllability.

Like most AM filaments, parts printed in both ABS and PETG can benefit from manual filing, sanding and machining so that is an option if you don’t mind working up a bit of a sweat.

Paint and varnish can be applied to parts printed with either filament to alter the colour and surface finish.

Just remember, most paints are also based on commodity polymers and are also susceptible to UV rays, so be wary that any applied paint may also degrade or discolor in the sun.

Actionable Takeaways 

ABS

1. May need to modify printer to reach ABS temperatures and prevent warping

2. Has higher water absorption than PETG (not good, generally speaking)

3. Not so great for use outdoors

4. Still a good filament for impact resistance

5. Has higher HDT than PETG and is suitable for higher temp environments
   
   

PETG

1. Has higher UV resistance than ABS (and PLA)

2. Has higher tensile strength than ABS

3. Easy to print. Can also be printed on an unmodded, basic 3D printer (check settings though). 

4. Food safe (see previous article on PETG)
   
   
   

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References

[1] "Polyethylene terephthalate | Structure, Properties, & Uses", Encyclopedia Britannica, 2021. [Online]. Available: https://www.britannica.com/science/polyethylene-terephthalate. [Accessed: 23- May- 2021].

[2] "ABS M30 Datasheet", Stratasys.com, 2021. [Online]. Available: https://www.stratasys.com/-/media/files/material-spec-sheets/mds_fdm_abs-m30_a4_0320a.pdf. [Accessed: 23- May- 2021].

[3] CoEx 3D, "PETG Datasheet", Coexllc.com, 2021. [Online]. Available: https://coexllc.com/wp-content/uploads/2019/06/COEX-TDS-CX13-PETG-01-04-19-REV-1.0.pdf. [Accessed: 26- Apr- 2021]..

[4] "Ultimate Materials Guide - Tips for 3D Printing with ABS", Simplify3d.com, 2021. [Online]. Available: https://www.simplify3d.com/support/materials-guide/abs/. [Accessed: 23- May- 2021].

[5] "Ultimate Materials Guide - Tips for 3D Printing with PETG", Simplify3d.com, 2021. [Online]. Available: https://www.simplify3d.com/support/materials-guide/petg/. [Accessed: 23- May- 2021].

[6] "Accelerated Weather Testing", UL, 2021. [Online]. Available: https://www.ul.com/services/accelerated-weather-testing. [Accessed: 23- May- 2021].

[7] "MISSE | Glenn Research Center | NASA", Glenn Research Center | NASA, 2021. [Online]. Available: https://www1.grc.nasa.gov/space/iss-research/misse/. [Accessed: 23- May- 2021].

[8] NASA, "Nuclear and Space Radiation Effects on Materials", NASA, 1970.

[9] R. Bernasconi, G. Natale, M. Levi and L. Magagnin, "Electroless plating of PLA and PETG for 3D printed flexible substrates", The Electrochemical Society, 2015.