Hello,
I am having issues with the transparency of a lid that is placed face down on a wooden worktop. The material is Clear Shiny Plastic White. I have tried raising it above the surface and increasing the ray bounces with no luck. The lid shows up black as can be seen from the image. I need it to look naturally transparent with the worktop visible through the material.
You will probably also have to enable "Ground Illumination" if your parts are sitting on the ground plane.
If that wooden part is a piece of geometry, move your model up a very small amount (.1mm is fine) to add an air gap between the parts.
I've had issues with transparent plastics. Try "Glass" and "Solid Glass". Test between "2-sided" and not. You can tweak the color and refraction to get to the white. And yes, a very slight air gap between the cap and table is essential.
Bill G