Compared to other polymer processes, injection molding can be pretty complicated. Deciding where to inject, where to set the ejector pins, runner and gate sizing, shrinkage, warping, and then there is the whole design aspect - making sure that the part can even be molded in the first place, or maybe more correctly, making sure that the molded part can be removed from the mold. Designing an extrusion die is much easier, even in the case of multilayers.
Protomold has a wonderful set of resources on their website regarding injection molding. I know of no equivalent site on the internet. The resources are no substitute for an experienced moldmaker - woe to anyone who reads the stuff and think they know everything, but by becoming a smarter customer, the relationship with the mold builder can't help but be improved.
Full Disclosure: Neither I nor my employer has any financial relationship with Protomold. This post is simply me pointing out a useful website.