Plastic expands when it gets hot/melts, then retracts when it cools/hardens/solidifies. Because of this, You do have to fill up the Sprue/Gate, BUT, I've found that after filling the mold cavities, if You continue to hold Injection Pressure going into the mold, You can feel when the Plastic retracts/cools and the cavities "need" a little more plastic...the Injector Plunger will fall a little as it feeds more plastic into the mold.
The DrawBack to this is the plastic on the Tip of the Injector cools on/in the Sprue, kind of "adhering" the Tip to the Sprue. In any case, always fill the Sprue after Injection. This is an on-going thing, every time You melt plastic, until someone comes out with a Plastisol that doesn't expand when liquified...
