I have a closed poly-surface that has a Material property of Plastic - with a semi-transparency. (The actual object will be made out of smoke-grey plexiglass). On the “Render” view, it appears perfectly. However, when I go to Render out a nice image using Rhino Render, any transparency is lost and the object becomes totally opaque.
The Rendered mode and Rhino Render handle transparency different. I wrote about it back in 2017 - then for the Raytraced mode, but the same is pretty much true for Rhino Render.
Create a material with 100% transparency, then assign a grayscale color to the transparent color channel. The (diffuse) color channel should be set to black - easiest is to just use a Glass (possibly Plastic) material, then convert that back to a Rhino Custom material and play with the transparent color channel.