Why does bubble appear on injection molded plastic parts?

While there may be a hundred reason to why injection molded parts may have bubbles in them, the most common one is water dissolved in your plastic pellets. The melting operation releases the absorbed water, as steam bubbles.

It is no accident that all good injection molding machines have a combination hopper/dryer for getting water out of pellets before extruding, melting.

Sometimes .. dissolved air can be hard to diagnose. So can gases in coatings and other unexpected sources. This is a specialty that some of us here make a living at after looking at your setup.