This is a Sheet Set for a Hollow Mini Mendel. Parts were made hollow to cut down on cost of printing. Just fill them with some sort of epoxy resin to give them strength.
Pretty expensive alternative to printing them yourself on a DIY 3D printer, but time is money... right?
These are provided AT COST. No profit here, since they are GNU GPL...
Original single hollow parts done by Marcus Wolschon @ http://www.thingiverse.com/thing:2204
Single sheet hollow parts page on thingiverse @ http://www.thingiverse.com/thing:5074