PLA, also known as polylactic acid or polylactide, has a rather pleasant aroma when heated, sometimes described as "waffles" or "honey." Along with its lack of warping, the aroma makes for a very satisfactory 3D printing experience.
ABS, also known as acrylonitrile butadiene styrene, has a very different printing experience, particularly when your 3D printer is poorly ventilated. Its smell is very sharp and sometimes can burn your throat if you breathe too much of it.
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