Vacuum forming is a process to let a heated plastic sheet conform to a master part by sucking air between them.
The benefit of using vacuum forming is that you can make thin and hollow part just like a plastic model. Also, once the master part is made, you can make several copies. Example:
http://www.asahi-net.or.jp/~xr2h-fkd...sp/333sp01.htm
Try searching online. There're a few web sites, that show you how to build your own vacuum forming machine. (All you need are wood jewelry box, picture frame, metal cramps, vacuum cleaner and sealing tape.)