The 3D system I saw used a highpowered CO2 laser, with
green laser to see wher the beam was, writing in a liquid
plasitic solution. As the CO2 beam heated the misx it
"firmed up" and became solid. I have a little model of a
wildcat that the made for me. The plastic is not all that tuff,
and is used as a master to cast other, harder plastic.
Most often they make what might be thought of as negative,
hollow half images. A guy lost half of his mandible in a bike
accicent. They took a CAT file of the remaing half,
mirror/reversed the image, made a hollow master and cast
a replacement from a phospher rich plastic that his body
acepted. After a year or so this body had bone growing through
out the plastic you couldn't tell he had been a bad accident.
I worked in the University of Kentucky's TV department
and that was one of the neater things I saw.
Terry