Which 3D printable materials (preferably FDM compatible ones) have the lowest opacity (equivalent to that of glass) ?
Which 3D printable materials (preferably FDM compatible ones) have the lowest opacity (equivalent to
Probably Polycarbonate. Next would be either natural PLA or MABS, depending on where you get it (clarity varies wildly from distributer to distributer and even batch to batch). Nylon 645 is pretty clear too. Due to the layering and crosshatching, you’ll never get prints that are optically clear (like for a lens), though you can get closer to that by putting an MABS part in an acetone vapor bath.
You can also solvent bath Polycarbonate to get a very smooth glassy finish. I used dichloromethane to do mine, worked very well.
There’s going to be another option very soon that should be even better. Wish I could spill the beans but they aren’t mine to spill.
@Ben_Malouf
Well let me know what it is when it’s available.
I will try to remember! Just watch the 3D printing blogs next week.
@Ben_Malouf , talking about Taulman, right?
@Whosa_whatsis
After surface smoothing would the inner layers still be more opaque ? if so which technology would make the most transparent object ?
Yes, the inner layers are more opaque. With a very long sloothing and/or multiple runs, you can get thin objects more clear, but you lose some dimensional accuracy.
@ThantiK Maybe. 
I once waaaay-overextruded natural PLA on a small print – because the nozzle was in constant contact with the small print, the resulting object was almost as transparent as the feedstock – perhaps slightly golden from being cooked by the heat – despite the over-extrusion, the finished object was still a good looking print. That was more of an accident, though, and I’m not sure that it’s a usable technique for anything substantially large. But it might be enough for some applications - if I ever have to make custom light pipes, for example, I’ll try to use this technique again.
If optical clear parts are desired, one might consider 3D printing a mold, and then casting clear resin into the mold.
Finally, I know that Objet has some offering for optical-grade 3D prints. That’s a different category than our printers, though.