So I am looking for some advice for a friend of mine. He is running his own small start-up business, and would like to get some parts (plastic) made fairly cheaply if possible. Injection molding seems to be the simplest option, but the equipment is quite expensive, and the machining required may be cost prohibitive (this is a small budget). Other options are spin/roto-casting. Since a lot of folks on here have experience with this sort of thing (albeit for vinyl toys and resin) I just thought I might throw it out there and ask for any tips or advice from the experienced hands. Currently we have done some 3D printing, but this is just for the prototypes, and doesn't really translate easily to what he would like to do. Anyhow, if anyone has some suggestions on casting, molding, costs, outsourcing, etc., or just wants to share their knowledge, I would welcome them. Thanks!

Post script - Just to be clear, this is not toy-related, so I hope I am not stepping on anyone's toes so-to-speak. Just asking for general info. Skullbrain is just a vast wealth of information and experience, and I'd like to help out someone who did me a favour.

Im not an expert but I will try to help.
It really depends on how big it is, how many do you need made, and how strong it needs to be? If you don't need a 1000 of them and they are not huge, you could do it yourself for a fairly low cost. You would still need a pressure pot and a compressor. Do you have a pic of the prototype?

Great man, thanks. The part is somewhat small, about 6cm wide by 8cm long (cylindrical more or less). As far as thickness, I think it is about 3mm, but doesnt need to have any high level of strength really. It is not quite assembly line, but I understand he would like to produce about 300 a month maybe to start out (to fill some initial orders).

Oh man, that would be really easy. If they are like cylinders you could make a one part mold. Gang up like 20 in one mold and pour some alumilite (http://www.alumilite.com) or I use Smooth cast 325 so I can make it what ever color I want it to be (http://www.alumilite.com/).