PLA and ABS are the two most common filament materials for desktop 3D printing. Both materials are thermoplastics, which means they can be heated and extruded through a nozzle, so as to build (3D print) a part layer by layer.

PLA (Polylactic Acid) is a thermoplastic that is biodegradable and bioactive, derived from renewable resources such as corn starch and sugar cane. Outside of 3D printing, it used in applications as diverse as disposable cutlery and medical implants. When it comes to 3D printing, the main benefit of PLA is that it's easy to print.

ABS (Acrylonitrile Butadiene Styrene) is a very common oil-based thermoplastic, commonly found in LEGO, kitchen implements and protective headgear. Objects 3D printed in ABS tend to have higher impact strength, flexibility and durability, compared to PLA, at the cost of a more complicated 3D printing process.

Typical properties of ABS and PLA that are relevant to 3D printing are as follows:

Thermal Properties

Two characteristic temperatures of a thermoplastic that are relevant to 3D printing are glass transition temperature and melting point, which for ABS and PLA are as follows:

The glass transition temperature is the temperature at which the thermoplastic transitions from a hard, glassy material into a soft, malleable material. The melting point is the temperature at which the material subsequently liquifies, i.e. turns from a solid into a liquid. ABS is amorphous and so does not have a true melting point. However, at temperatures above 190 °C it can be considered to be well liquified.

In the nozzle of the 3D printer, we would like the temperature to be above the melting point of the filament, such that the material is well liquified and can be pushed though the small diameter opening without excessive viscous friction. Hence, PLA is often printed at 180 - 220 °C, whereas ABS is often printed at 210 - 260 °C.

Additionally, we need the temperature to be low enough upstream, in the cold end of the extruder, such that the material there is still stiff enough to be gripped by the extruder and driven through, pushing the more malleable downstream filament ahead of it. This is why many extruder hot ends feature fins and cooling fans. In a well designed extruder, the filament temperature in the cold end should be well below the glass transition temperature, whereas the temperature in the nozzle should be above the melting point.

Once the filament has been deposited onto the print bed, it can be advantageous to keep it at an elevated temperature by means of a heated bed. Often the heated bed will be set at a temperature near to the glass transition temperature of the filament being printed, such that the filament remains somewhat malleable and 'sticky' so that it sticks well to the bed. Another advantage of this is that the printed part will cool more gradually after printing, alleviating warping issues, especially with ABS. Typically, PLA is printed on a heated bed of 20 - 60 °C (which is optional), whereas ABS is printed on a heated bed of 80 - 110 °C (which really is a must for successful prints).

The higher glass transition temperature of ABS (104 °C), compared to PLA (57 °C) means ABS is more heat resistant and thus more suitable for high temperature applications after printing. For example, the knobs on saucepan lids are often made from ABS and are dishwasher safe. Objects made from PLA would start to droop if put in a dishwasher and certainly are not suitable for cookware applications.

Accuracy

PLA is semi-crystalline and has a sharp transition from solid to liquid, whereas ABS is amorphous and has a far more gradual transition, whereby the connectivity between it's individual polymer chains continuously degrades over a temperature range. Consequently, it is often found in 3D printing that PLA flows better from the nozzle and is able to be deposited more accurately. This is especially apparent when 3D printing sharp corners, which tend to be more rounded in the case of ABS, compared to PLA.

PLA and ABS both shrink when cooling after being extruded. Typically, the amount of shrinkage is as follows:

These figures may seem low, but if the accuracy and dimensional tolerance of a 3D printed part is important, then shrinkage needs to be accounted for. The amount of shrinkage of ABS is twice as high as PLA and can cause issues with warping, for example where the part partially lifts off the print bed due to the induced thermal stresses.  

Mechanical Properties

When it comes to assessing the mechanical properties of PLA and ABS, several characteristics can be looked at:

• Tensile, Compressive & Flexural Strength: The stress (i.e. force per unit area) at which the material fails when being pulled apart, squashed together, or bent, respectively.
• Elongation at Break: The amount the material has stretched prior to breaking, i.e. a measure of the failure mode (brittle or ductile).
• Young's Elastic Modulus: A measure of the material's ability to withstand elastic deformation, i.e. non-permanent deformation whereby the original shape is returned to.
• Impact Strength: A measure of the material's ability to withstand a suddenly applied force (impact).
• Hardness: A measure of the material's ability to withstand localised plastic deformation, e.g. through scratching or indentation.

The precise values of these characteristics for PLA and ABS depend on many factors, including:

• The material grade: not all ABS and PLA is manufactured equally
• The direction of loading: materials behave differently when subjected to tension, compression or shearing. This is especially important for 3D printed materials, which are effectively laminates, consisting of individual layers that are fused together. Often, the limiting factor on the strength of a 3D printed part will be its inter-layer strength, which tends to be significantly lower than the inherent filament strength, since individual layers are never perfectly fused together.
• The rate of loading: materials behave differently under a sudden application of force (impact testing) than a gradual application.
• The testing method: there are different standardised testing methods to measure the above characteristics, which give vastly different results.

Typical values for these material characteristics are as follows:

Generally, with regard to the above, the following distinctions can be made between PLA and ABS:

PLA has a higher tensile, compressive and flexural strength than ABS. However, PLA fails in a brittle manner (fracture), whereas ABS fails in a ductile manner. ABS is also more elastic, harder and has a higher impact strength. Hence, ABS is more suitable for end use applications where it is subjected to wear and tear, whereas PLA remains popular for rapid prototyping where form is more critical than function. 

Surface Finish

Generally speaking, parts that are 3D printed in ABS have a matte finish, whereas parts in PLA are more glossy. However, ABS parts can be smoothed and made very glossy with Acetone smoothing, whereby some of the outside layer of the part is dissolved using Acetone vapour. This is not possible for PLA, as it does not dissolve in Acetone. Both materials can be sanded and painted, although this is somewhat easier for ABS.

Heat Resistance

The higher glass transition temperature of ABS (104°C) makes it better suited to high temperature applications than PLA (57°C). PLA can drastically lose its structural integrity and deform as the temperature approaches its glass transition temperature, particularly when also subjected to loading. ABS is commonly found in kitchenware such as frying pan handles.

Biodegradability

PLA is stable in general atmospheric conditions, but will biodegrade slowly in water (over ~50 months) and a lot faster in industrial composters (~50 days). ABS is not biodegradable, but it is recyclable.