Optically translucent prototypes are used to check components cooperation, interference respectively, assembly structures and especially interior material working processes. Typically transparent parts are made with resin systems, however resin parts are not durable or adequate for functional testing.
To increase transmissivity of FDM parts:
Make part with ABSi (air gap: 0.004 inchs/0.1 mm).
Infiltrate part in transparent epoxy resin in proper mixing rate of main resin and hardener.
Remove the extra epoxy resin on the surface with tools like putty remover.
Dry the part in the oven (30-35C) for 15-20 minutes and continuously dry it at
normal temperature until it dries perfectly.
For more information, download the full best practices document here.