Well, that sounds like a fun job!
Do you know the answer to this question: in an industrial injection molding machine, are the parts made one at a time, or are many molds "strung" together? I thought maybe the tons and tons of pressure were required if the plastic had to travel a longer distance or for very large molds (ie, if the plastic needed to be injected super fast so it fills the molds before hardening). I just looked at the site that Justin posted and some of those items look to be the same size or larger than a feed. Also, I wonder what type of plastic most feeds are made from...
Next time I talk to my FIL, I'll ask him, but I think feeds are probably polypropylene.
As for the multi tons - most production molds that I've seen him work with have mutiple stringers and cavities for smaller parts (picture a model car... you cut the pieces from the stringers). So something like a feed holder would be N-up mold (where N is some number like 16) so that every cycle of the mold produces multiple parts. This is necessary to keep the run cost down. Each cycle of the mold (close, fill, cool, open, eject) takes a similar amount of time no matter if its 1 part or 20.
Bigger parts means less pieces per cycle, due to the physical limits of the molds (and some of them are HUGE).
But in either case, the pressure helps get the parts filled correctly, and sometimes they heat the mold in the process as well to change the flow rates.