I"ve Worked in injection molding for 29 years, Setup, and shooting molds, and taking them apart to repair them. First, some of these molds are shot in a injecton machines between 350-500 ton machines depends on the size of the mold & how many cavity. So it's between 2000-2500 psi. Injected at a high rate of speed and presure. The reason you will not find air bubbles is because the molds all have air vents in them. The bigger the cavity the more air vents. The presure a speed in witch the plastic go's in the mold forces any air out of the vents, witch are about 0.002-0.004 deep. In turn if the vents become dirty the parts will have burn marks on the because the hot air can't get out. I've used scotch tape to add venting to a mold. If you do see a bubble in a model kit its because some of the plastic did not melt right. As far as a kit with a lot of flash on it, three things come to mind. The mold is warn, maybe because of a lot of parts being smashed in the mold from the operator not watching the mold. I get this a lot at work. yes plastic does damage steel 2- The mold is getting over packed with plastic, 3- The mold was in a smaller machine, tonage for mold to small. Not to be a know it all, I'am not. hope this helps Thank you Joseph barbato