A nice summary of material properties

By Ryan Carlyle on the Google+ FlashForge forum:

https://groups.google.com/forum/#!msg/f … nW_c-cBQAJ

Gotta remember that there's a difference between STIFFNESS (elastic modulus) and STRENGTH (yield stress) and TOUGHNESS (elongation at break). You also gotta remember that print settings play an enormous role in the as-printed polymer properties, so take anyone's test data (even Taulman's) with a big grain of salt.

PLA is the stiffest non-composite filament. It's also brittle and creeps at room temp under heavy stresses, and has a low Tg. Do not use PLA for 3D printer parts unless you're desperate or the component is significantly removed from heat sources such as motors, build plates, and extruders. Some advanced  blends like PLA/PHA overcome some of these issues, but not really enough that I would recommend it for printer parts.

ABS is the highest-Tg "standard" filament, and is quite stiff. These two factors combine together to make it warp. Yes, that's right, the same properties that make it difficult to print also make it ideal for printer parts. It's an excellent material for that, basically the best STRUCTURAL option except for polycarbonate and PC blends like ePC or PC-ABS. Only real performance downside is that it's flammable.

PETG has an intermediate Tg and is considerably more flexible than ABS. This is why it doesn't warp much. The great low-warp properties we all love MUST come at the cost of lower stiffness and/or Tg, that's just the physics of FDM. One really big advantage to PETx variants is that they are naturally somewhat flame retardant -- they will combust if directly heated, but will not sustain a flame when the heat source is removed.

Nylon has a surprisingly low Tg and most grades are very flexible. The Tg is somewhat misleading: a nylon part under minimal load will maintain its shape and integrity significantly above its Tg. It works fine for stuff like fan shrouds next to extruders. But it will creep if loaded much while above Tg. Between the flex and low temp resistance, nylon is a pretty terrible structural part for printers. But it's a great material when toughness is required. Something to keep in mind about nylon is that its properties vary ENORMOUSLY with the amount of strain/drawing that the polymer has experienced since last melted. Printing it fast with low layer heights produces a completely different set of performance parameters than printing slowly with large layer heights. Loading a nylon part past yield can re-align the molecules and make it stronger and stiffer. The crystalline structure does all sorts of weird things that you don't really see much in other filaments.

PC is just ABS on steroids. Higher Tg, higher strength, similar stiffness, much better toughness. The high Tg and stiffness make it warp like crazy, it's almost unprintable (except spiral vases) without a heated chamber. PC blends like ePC sacrifice some of that exceptional stiffness and depress Tg in order to reduce warping to an acceptable level more similar to ABS. So ePC is mechanically somewhat better than ABS, but not as much better as regular PC. (The fact that it's printable without jumping through hoops makes it worth using, I think.) Polycarbonate also experiences some strain-drawing type effects similar to nylon, just to a lesser degree. Printing fast and with low layer heights will make it stiffer and more brittle. It gets more flexible and tougher if annealed after printing. Like PETG, PC and ePC are moderately flame retardant. This makes PC the best possible printed material we have access to for parts like extruder carriages and delta effectors.

You can take any of these and add particles or fibers to make them stiffer. Most composite filler materials (glass, carbon fiber, metal, etc) will also significantly reduce warping because the filler shrinks less than the plastic when cooled. I don't know why people waste time with PLA composites like CFR-PLA (which is stiff but not actually very strong) when you could be printing really bad-ass engineering materials like glass-filled nylon or glass-filled polypropylene or carbon-fiber ABS. The filler makes the filament lower-warp, so normally-difficult materials actually get easier to print. (Just need a hardened nozzle, of course.)