PETG vs PLA: How do they compare?

PLA (Polylactic Acid) is a good all-round biodegradable thermoplastic for 3D printing components for use in ordinary environments. It is inexpensive, easy to print, and strong. 

This article is part of our ongoing series about desktop printer filaments. In previous articles, we compared the two most common 3D printing materials PLA vs ABS. We also looked at how ABS measures against PETG. These articles are suitable for both beginners, new to 3D printer filament and experienced printers who want to learn more about the physical properties of common materials. 

This article compares popular filaments, PETG vs PLA (Polyethylene Terephthalate Glycol), a thermoplastic material chosen for its printability and mechanical properties.

PETG

PETG is a modified form of PET (Polyethylene terephthalate) that is more optimal for additive manufacturing than its industrial counterpart. PET is the world's most popular thermoplastic [1], and is classed as a commodity polymer. Commodity polymers are plastics produced in high volumes for applications where performance material properties are not needed.

PET is clear in its raw resin state, making it suitable for an array of applications including usage in fibers and plastic drinks bottles. When used in producing synthetic textiles, it is more commonly known as polyester.

Printed PETG (Image Credit: Taulman3D)

Like all thermoplastics, PET and its variants such as PETG can be melted and remelted over and over and can be shaped into a variety of forms such as sheets and filaments.

PLA

PLA is a bio-plastic derived from starchy raw materials such as cassava, corn starch, or sugar beet. It has mostly been used for the manufacture of biodegradable food and beverage containers in an effort to reduce wastage of single-use plastics. PLA is also food safe in its raw, unmodified state, however, the use of additives in the manufacturing process can void that status.

Recently PLA has found a new home in the 3D printing community and is known as an easy-to-print polymer that can be printed on any entry-level desktop printer without modification. It is the most common plastic used in FDM printing, popular for its ease of printing. [2]. It can be used for final parts as well as prototypes.

It is available in a wide range of colors, and even in a clear filament (just like PETG). In its raw state, PLA appears clear, but with a yellow tint. It does have a tendency to discolor when thermoformed, so additional processes must be performed on the base material in order to modify it so it remains clear when 3D printing.

PLA is soluble in a variety of organic solvents which makes it useful as a soluble support material when using other non-soluble modeling materials such as ABS or PEI, in a dual-filament system.

Strength and Toughness

The strength properties of these two materials vary depending on the manufacturer, the quality control, the polymer additives, and the printing process. To provide one example, in this article we compare a PLA [2] and a PETG [9] from the same company.

In these examples, the PLA filament is based on the NatureWorks Ingeo Biopolymer 4043D resin (a common resin in filament manufacture) and the PETG filament is based on the Eastman Eastar 6763 PETG Copolyester.

Let’s look at the mechanical properties of these two specific variants of PLA and PETG. You might remember that PLA held up pretty well against ABS (a polymer prized for its flexibility and impact resistance).

How does this particular PLA perform against PETG from the same company? In the table below you will find mechanical information taken from manufacturer datasheets.

As can be seen from the table, based on these specific PLA/PETG datasheets we cannot draw any clear conclusions regarding tensile strength. For this one, PLA wins by a megapascal. When only looking at data from an individual manufacturer, the bigger picture may be obscured. 

The Granta polymer database shows that the yield strength for PLA generally falls in the range of 48-60 MPa, while PETG falls in the range of 47.9- 52.9 MPa. There is clearly some overlap here, and our own examples in this article sit close to that overlap.

So what does this mean?

If you choose PETG for its mechanical properties over PLA, make sure you check the technical data sheet of the specific filament you're considering. This applies to all filaments but specifically to types of filaments that share some similar values in their properties. Its important to know which properties are more impactful for your project. Do you need to consider shrinkage? Or is heat resistance more critical?

PETG Spool

In terms of flexibility, PETG is typically a better performer, but as you can see in our own sample, PLA can be equally flexible as PETG. For the most part, PLA and PETG are similar mechanically and can be compared, though PETG is more impact resistant in general. 

If your part needs to be outside, it is important to consider the material's UV resistance. When left outdoors for extended periods of time, polymers such as ABS and PLA can degrade in the Sun's UV rays. This can result in discoloration, other aesthetic defects, and also a loss of mechanical strength. PETG is far more resilient and is, therefore, more suitable for outdoor applications.

Thermal Properties

In terms of thermal properties, we're most interested in the Heat Deflection Temperature (HDT) and the Glass Transition Temperature (Tg). As we mentioned in the previous article, the HDT is the temperature at which a load placed on a specimen will deform the specimen by 0.25mm. The glass transition temperature (in layman’s terms) is the temperature at which the molten plastic freezes (hardens) while cooling.

Based on our two sample datasheets, we know the HDTs of PLA and PETG are higher than most plastics. Happily, there is no ambiguity here, as the sample datasheets show that each filament conforms to what has been reported in the wider literature. If you randomly purchased a roll of PLA and a roll of PETG from Amazon, you could be fairly confident that the PETG is going to require a hotter temperature at the nozzle to extrude the plastic, and that the PLA will generally retain its strength at slightly higher temperatures.

It’s only a few degrees difference in HDT (according to the datasheets), but for some applications, those few degrees may swing a designer in favour of using PETG.

Warping

Another point worth mentioning here is that PETG experiences lower warping while printing, due to the lower extrusion temperature compared to ABS, which can experience warping if not printed in a controlled environment. This makes PETG a fairly easy filament to print even with entry-level printers, just like PLA.

For the most part, you can pretty much print PETG with the same hardware you print PLA with, as long as you can eke a few extra degrees out of your hotend and print bed.

In addition, it means you can print significantly larger prints with both PLA and PETG than with ABS without fear of the print warping off the bed.

Print Settings

Again we have referred to Simplify3D website to get the print settings for PLA [6] and PETG [7], as they cover a range of setting values that will likely cover your filaments, regardless of where the filaments have been sourced from.

Post Processing

One of the main draws when dealing with PETG is the range of transparent/translucent materials available. The layer height can alter the clarity of the final print, with greater clarity being achieved with larger layer heights.

But to get the most out of your clear PETG prints, you may wish to do some processing. Both PLA and PETG can be sanded with fine-grit paper to get a smooth finish, but manual sanding on complex geometry can be difficult. Vibratory smoothers are an excellent processing tool for this.  

Like PLA, there are chemical vapour smoothing processes available, but (also like PLA) these particular chemicals fall into a health and safety grey area, so we won’t provide details here. Suffice to say, if you want vapour smoothed prints, ABS will be your go-to polymer feedstock, as it can be smoothed with commonly available acetone.

The other alternative is to simply paint/varnish your printed parts.

Food-Safe Plastics

Food-safe plastics such as PET/PETG clearly offer the appeal of using them for certain applications. So what exactly is a food-safe plastic? 

According to the 3A-Sanitary Standards, three main areas of concern are examined when determining how safe a plastic is to use for storing and handling food and drink.[3]

These are:

• Design
• Materials 
• Manufacturing process

For an item to be food safe, it must not cause injury (no sharp edges), must not leak food, or allow other objects to enter from outside. The material should also not release chemicals into the food. As far as the printing process goes, when 3D printing, the printer must not impart any contamination to the plastic, so certain nozzles should be avoided, and one must be cautious of other contaminants, such as lubricants that come from the printer. 

Simply put, if your printer is made of food-safe materials, you can use food-safe filaments.

Takeaways

In summary, if you want a filament that is strength compatible with PLA, but with the flexibility of ABS then perhaps you may wish to consider PETG for your task. However, the natural fluid impermeability and UV resistance of PETG give it the advantage for if your printed object is destined for outdoor use. For everything else, PLA is a solid choice.

PLA

1. Good all-round filament
2. No need for a modified printer
3. Cost-effective

PETG

1. Has superior impact resistance to PLA.
2. Strength can vary significantly compared to PLA. Check the specific datasheet before purchase to check.
3. Easy to print. Can also be printed on an unmodded, basic 3D printer (check settings though). 
4. PETG has high resistance to UV rays, unlike PLA and ABS. It can be used outdoors in direct sunlight for extended durations compared to the other plastics mentioned.

References 

[1] Britannica, “Polyethylene Terephthalate,” Britannica.com, 2009. [Online]. Available: https://www.britannica.com/science/polyethylene-terephthalate. [Accessed: 03-Apr-2021].

[2] Grand View Research, 2020. “3D Printing Filament Market Size, Share & Trends Analysis Report By Type (Plastics, Metal, Ceramics), By Plastic Type (Polylactic Acid, ABS), By Application (Industrial, Aerospace & Defense), By Region, And Segment Forecasts, 2020 - 2027”. Grand View Research

[3] P. Keane, "Guide to Printing Food-Safe Plastic Products - 3D Printing", 3DPrinting.com, 2021. [Online]. Available: https://3dprinting.com/food/guide-to-printing-food-safe-plastic-products/. [Accessed: 26- Apr- 2021].

[4] CoEx3D, "PLA Date Sheet", Coexllc.com, 2021. [Online]. Available: https://coexllc.com/wp-content/uploads/2019/06/COEX-TDS-CX02-PLA-01-04-19-REV-1.0.pdf. [Accessed: 26- Apr- 2021].

[5] 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]..

[6] Simplify3D, 2021. “Ultimate Materials Guide - Tips for 3D Printing with PLA”. [online] Simplify3d.com. Available at: <https://www.simplify3d.com/support/materials-guide/pla/> [Accessed 21 April 2021].

[7] Simplify3D., “Materials Guide: PETG,” Simplify3D.com. [Online]. Available: https://www.simplify3d.com/support/materials-guide/petg/. [Accessed: 03-May-2021].