Something I know absolutely nothing about; hydraulics. But now's the time to learn. I got a nice discount on this old kit with a pump and a piston, which I will use for the prototype injection molding machine. At the moment I put in around 30hours to print each kit on one of my FDM 3D-printers, which makes it hard to scale up. With injection molding it could take only a few minutes.

A metal pipe filled with ABS pellets, heated by three 280w band heaters, and a piston that goes down and pushes the melted plastic through a nozzle into an aluminium (or perhaps SLA-printed) mold. Simple! 

Why pay $10k when you can make on at home? My customers will thank me when they can buy quality parts without getting a second mortgage.