Depending on what you are making, you'll probably find pbj's method much easier to deal with (i.e casting.) Injection molding is tricky business, and it's easy to get hurt, especially with make-shift machinery.
The big issue with injection molding is that complicated parts require complicated and expensive molds. Things like a solid fish, or items with holes in the direction perpendicular to the seam, can use a simple two-piece mold. Anything more than that and you need more pieces and more complicated frames to hold them. It's also tricky to properly design vents. The vents not only allow for air to escape, but it also helps to regulate and direct the flow of molten plastic. Demolding can also be a pain. Most commercial molds have ejector pins built in to force the parts out, once they have solidified. Chilled water is run through the mold to expedite this, and to control mold temps.
I actually picked up a tabletop injector, but quickly found that the items I wanted to make were too large or too complex for it to be of practical use. Mold design and cost is also a barrier. 3D printers blow DIY injection molding out of the water for everything other than speed. And if speed is what you need, you can often do better by simply buying multiple 3D printers (up to a point, of course.)