Pins, nails, wire, toothpicks, you name it. Basically you're just injecting a bait with a light colored and somewhat transparent plastic after having found a piece of wire or a small nail that will run from the runner to the end of the bait or at least as far back as you want the core color to run. Inject the mold and when slightly cooled just cut the baits free of the gate and pull the core rod or whatever out. Trim the baits so that a hole near the back is left so the next injection color can run without binding, put the baits back in their corresponding mold cavities and inject the other color at a slightly warmer temp [360] than the 340-350 one normally would inject..
Doing the cores is a balancing act between several things. The core rod size becomes an issue based on the physical size of the bait. The core has to be large enough to allow the second color to flow freely.
To get started at this I suggest you use some junk plastic that's still transparent enough for you to see what the core plastic is doing as it goes in. Play with the second color temps until you get full fills. A lot of how quick you pick this up depends on how much experience you have with working with the plastic, but using discared plastic will let you do your discovery before you start using the new plastic.
Look to finishing nails, coat hangar wire, copper electrical wire....the sky's the limit on this stuff. Once you find something that fits your needs give it a quick rub with some plain worm oil before setting in the mold and injecting....this will make removal much easier for you.
