I was planning on making my WALL-E head/eyes mostly out of MDF with the critical parts being made out of aluminum. It looks like I was underestimating the total weight I would be dealing with. As it stands, each eye is estimated to weight about 9.5 pounds! Wow.. Even though each eye is about 8 inches wide and 16 inches long, 9.5 pounds is way too much.
The density of MDF varies but is 30-60 lb/ft^3 with ultralight MDF being close to 30 lb/ft^3. Aluminum on the other hand is 168 lb/ft^3. Some alternative materials are balsa wood which is about 8 lb/ft^3 and then there is precision board which starts at 4 lb/ft^3 and goes up past 40 lb/ft^3.
What I will probably have to do is start by first removing as much weight from the design as possible. Most of the pieces will end up being made from balsa with some of the larger pieces being made from precision board if I can't find large enough pieces of balsa. The more critical pieces will be made from MDF and I'll have very few pieces made from aluminum if need be.
The weight will be very important, especially for the head and arm pieces because they are extended out so far from the main body... =\