There are a number of methods of making hollow thermoplastic mouldings. Some have been used since before WW11.
As I don't feel inclined to do the basic homework for you, try a google search. If that produces no results, repost here with a bit of an explanation as to what you are trying to do exactly.
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.