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:
Answered with a tutorial: https://grabcad.com/tutorials/how-to-increase-transmissivity-of-fdm-parts-part-translucency
Often if you use a 100% infill, the part will turn out more clear. When printing with clear filaments, if you want them as clear as possible use about 10-15 degrees hotter then recommended temp. The slower and hotter you print the clearer the model will be.