If im not the 1st person to do this, feel free to tell me.. But if i am, your welcome!
I learned how to make 2 piece plaster molds on YouTube, and all the ones i saw were injected in one side and vented out the other... After a little trial and alot of error i learned if vent holes are too big, too much plastic escapes and baits are left with hollow spots.. If holes are too small, too much pressure builds up and plastic blows out in the mold leaving a ton of flashing to tear off...... So... Tried an experiment and it worked great!
Drill a 1/4 inch hole through the top of the mold (starting from bait side) near the end of the bait and connect hole to specified point on bait via dremel (i use an engraving bit).. Flip the mold over and enlarge your hole (i use a 5/8" spade bit) only drilling about 1/2 way through to create a reservoir... Plastic should be injected from same side of the mold as the vent (i also saved $60 on an injector by watching a guy on YouTube use a copper pipe, works great but this method will work with an injector too)
Since liquid always levels itself, as long as your injection point and vent are higher than the cavity, it will always be full....