I ve read a story about castration party where a guy's penis is injected with some kind of plastic mass.

Can somebody please tell me the title of the story or author?

Not sure exactly which story you're talking about, but there's one in which the guy's cock is injected with super glue just before it's removed. That one's The Cock Punisher. Then again, maybe check out the Ryn stories.

Thanks but no those are not the one that I am looking for

Here's the story that I read that seems closest to that description. It doesn't fit exactly, but I couldn't help thinking of this story when I read that.

It's The Lottery (http://www.eunuch.org/Alpha/L/ea_13537the_lott.htm) by Otis. It's a fun read, but not really my sort of story. It's also one of the few that I've read, so I'm hardly an expert on the wide variety of stories that we have available, and it's probably not the one you wanted.