hollow meshes?

aren't meshes hollow by default in blender
i mean whenever we delete a face we see what's in it
if they aren't, how do you hollow them out
and is it possible to make a print with more than one material?

I'm not familiar with blender, but I assume it generates surfaces with zero thickness. The 3D printer will not handle this as hollow but as solid. To hollow an item you should offset the surface to the inside and thus build a hollow product with walls that have thickness. Add a hole so unused polymer can be removed.

That is, I'm planning to create a sculpture that way and I assume it should be done that way.

thanks
and my mesh has a transparent window (its a car) so what should i do to keep it transparent?

I suppose you'll need to print the window in transparent polymer in a separate batch. Note that no printed polymer that I know of is completely transparent when it leaves the printer. Maybe some treatment afterward is possible, but that I don't know.

3D printing doesn't really allow for different materials, as in transparent windows for cars. Printed materials are a translucent at best, they tend to be dusty or even milky.

thanks for the replies
it really helped a lot

They do not cast here, but some prototyping shops do. Then you can print a model and they will use the print to make a mold out of silicone. With casting you can get almost any property you need. Of couse the extra work involved means another pricetag.

You can also do it yourself, but it is a lot of work and expensive because of the materials. http://www.wilsor.nl/produkten/polyesterharsen.htm
Vacuum forming using the print as a master may also be a solution.

Then again, I still wonder if the translucent material can be improved by a coating of some sort. But it will always stay somewhat yellow I think.

As a standard in 3d programs, Objects such as a sphere are hollow on the inside as it would be very hardware intensive on the computer to try and calculate the "filler". I have not used blender in a long time, but in 3DS max there is a modifier called "Shell" which can give the effect of an object being hollowed out by adding another mesh inside or outside the object that represents the other side.
As for the windows, you can get plexiglass or some sort of acrylic and cut to shape then fit it in the windows. If you want them to be curved, all you have to do is heat the material and bend it to the proper shape then let it cool.