First, I want to start out by stating that this post is intended for people that are new to 3D printing and could benefit from an objective point of view and one geared specifically towards those real world scenarios that nobody talks about.
Now that we have that out of the way let's dive right in and discuss the technical definitions of both PLA and ABS then highlight some things to consider when and why you should use one over the other.
What Is PLA?
Poly(lactic acid) or polylactic acid or polylactide (PLA) is a biodegradable and bioactive thermoplastic aliphatic polyester derived from renewable biomass, typically from fermented plant starch such as from corn, cassava, sugarcane or sugar beet pulp. In 2010, PLA had the second highest consumption volume of any bioplastic of the world.
The name "polylactic acid" does not comply with IUPAC standard nomenclature, and is potentially ambiguous or confusing, because PLA is not a polyacid (polyelectrolyte), but rather a polyester.
What is ABS?
Acrylonitrile butadiene styrene is a common thermoplastic polymer. Its glass transition temperature is approximately 105 °C. ABS is amorphous and therefore has no true melting point. ABS is a terpolymer made by polymerizing styrene and acrylonitrile in the presence of polybutadiene.
ABS is a terpolymer made by polymerizing styrene and acrylonitrile in the presence of polybutadiene. The proportions can vary from 15 to 35% acrylonitrile, 5 to 30% butadiene and 40 to 60% styrene. The result is a long chain of polybutadiene criss-crossed with shorter chains of poly(styrene-co-acrylonitrile). The nitrile groups from neighboring chains, being polar, attract each other and bind the chains together, making ABS stronger than pure polystyrene. The styrene gives the plastic a shiny, impervious surface. The polybutadiene, a rubbery substance, provides toughness even at low temperatures. For the majority of applications, ABS can be used between −20 and 80 °C (−4 and 176 °F) as its mechanical properties vary with temperature. The properties are created by rubber toughening, where fine particles of elastomer are distributed throughout the rigid matrix.
Now that we have covered what ABS and PLA are from a scientific perspective lets talk about what that means for those of us that don't have a PhD.
Why you should almost never use ABS
The truth is that you rarely need to use ABS and given the fact that it contains material known to be carcinogens even for those of us that don't live in California. The particulates can be hazardous and the odor is strong and unpleasant. Unless you plan to install a filter system you should plan on noxious fumes and possible health risks. Unless you have a genuine need for impact resistant functional parts there is no need to print in ABS. If you do need a model in ABS for one legitimate reason or another you should consider a 3D printing service to avoid exposure to the fumes and airborne particulates.
I know there are some people that don't believe there are any real health risks associated with ABS and they may be right but you should judge that for yourself.
If your concern is a smooth print it can be easy to fall into the trap of thinking that ABS is your only option. This is yet another reason to avoid using ABS since it requires the use of Acetone to get a glass like surface. An alternative that makes PLA an equal performer to ABS is Smooth-On XTC-3D.
XTC-3D® is a protective coating for smoothing and finishing 3D printed parts. Two liquids are mixed together and brushed onto any 3D print. Coating self-levels and wets out uniformly without leaving brush strokes. Working time is 10 minutes and cure time is about 4 hours (depending on mass and temperature). XTC-3D® cures to a hard, impact resistant coating that can be sanded, primed and painted. Adding colors and metal effects is easy. Inexpensive to Use – 1 oz covers 100+ square inches. 90% Time & Labor Savings: XTC-3D® fills in 3D print striations and creates a smooth, high gloss finish. The need to post finish is almost eliminated. Coat any 3D Print Surface - XTC-3D® can be applied to both SLA and SLS prints. It works with PLA, ABS, Laywoo, Powder Printed Parts and other rigid media.
Another reason to use PLA over ABS is the fact that PLA is biodegradable. Regardless of how you plan to use your personal 3D printed models it is likely that these will eventually find their way to the landfill.
Using PLA is the environmentally responsible material to use and can be as effective as ABS in most ways.
Also, given that ABS requires a much higher print temperature and a heated bed it reduces the type of printers that you can use and increases the wear and tear on the hot-end.
If you don’t have a heated bed, forget about it. If you want to print some large objects and don’t have a way of protecting it from breezes or even the ambient air temp, you will fight with splitting and cracking. Avoid ABS if you cannot adequately ventilate the room you’re in, the smell might be irritating.