Rich,
I have purchased several injection molders starting with a very small one with a had clamping mechanism. When I started selling my first product there was no way to keep up with the demand. I had people in my garage hand cranking parts for 18 hours a day and they left with sore arms and fingers. What a joke! I bought a real molder with pneumatic toggle and it paid its self off in a few days.
I would like to point out that you may be able to make your own machine but you will waste a lot of time that you could have been making parts. There is no reason to re-invent the wheel.
Consider buying the one I just posted on ebay or buy a broken one and fix it.
SP3 Injection molding machine & mold like Morgan Press | eBay
The cycle time on this machine can be in the seconds where one with a hand clamped mold is in the minutes.
This video might be helpful to you
SP3 injection molder for sale - YouTube