Top pour aluminum molds have always been a pain to pour into because of the hollow cavity in the front part of the bait especially if the mold is cold. Bear solved the problem by making an expanded well above each cavity and no bubble has ever formed in any of his molds. I have stick and worm molds from other companies and most form bubbles when the mold is cold. even after a few pours. The bubble can take up 1/3 to 1/2 of the lure making that part hollow, making the lure unusable.
To remedy this, take a socket from a socket set that can sit over the pour hole and place it on top of the mold before filling the cavity completely. The hot plastic gets sucked down into the socket just like in Bears molds and prevents any hollow spaces. I've tried this with four different cold, stick molds and a two-part plaster creature mold I made a few years ago and no bubbles.
If you don't want to buy injection molds for the same designs you already own, this works. Plus if you look at most injection mold openings, they also have a well above before the smaller pour hole and bubbles rarely form inside the bait.