Injection Molding is done for both thermoplastics as well as thermo setting plastics. But it is dominant in the case of thermoplastics nowadays. The big reason is the one ( i.e. processing parameters like zone temperature, homogeneous mixing, etc) while thermo setting plastic couldn’t easily meet the desired processing parameters.
Apart from the processing parameters, the blended material grade absolutely meet the needs in engineering plastics as well as commodity plastics. The blending could not be easily done in the case of thermo setting plastics. In the case of exact end use property, tailoring of plastic materials need to be done & thermoplastics are good kind enough.
Another important factor is the nature of reuse. Thermoplastics are easily get grind & the grinded material is used for remolding. But in the case of thermo setting plastics, it couldn’t be reused once it is subjected to molding because of the permanent stabilization of molecular bonds.
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.