Roger D. LaBrant gives us a method of producing solid objects that will render transparently every time.
"In order to create a 3D solid that is 'see-through,' such as glass or acrylic, I came up with a method that would give me a satisfying visual result. First, I draw the shape of the part, whether it's a simple rectangle or a more complex shape. If my part is not already a polyline, I use the Pedit command on my geometry and make my part a closed polyline. Secondly, I offset my polyline 1/32" to the inside of the original polyline. Next, I extrude both polylines to their desired thickness. Finally, I subtract the inner solid from the outer solid to get my finished 'see-through' part."
Notes from Cadalyst Tip Patrol: This tip can help in several ways. Since it essentially creates an opening in the object, the area will be see-through. It also reduces rendering time because there is nothing there to render. However, you won't be able to achieve a reflective look in the rendered image, because there is no surface to reflect onto. It all depends on what your needs are.