Here are some shots of the results different techniques yield:
I made a couple of sample molds and some castings for them to use as instructional aids for a casting class I'm planning to do at my local makerspace, Novalabs. The molds are simple one-piece block molds, both of the same part.
First is a poured RTV mold made of RTV that has not been de-aired under vacuum. You can pretty easily see where air bubbles were, leaving voids:
Then, I cast some resin into that mold using a simple pour. On the left outside edge, you can see where an air bubble in the RTV resulted in an unwanted protrusion on the casting. You can also see how air entrapped in the resin has created pits in the casting.
After that, I cast some resin into that mold and let it cure under pressure. The final part shows significant distortion and odd protrusions caused by air bubbles inside the mold collapsing from the pressure. But at least there are no pinholes to fill, right?
Next, I made a second mold. This one was made using vacuum to de-air the RTV. It is an obviously superior mold compared to the other. While I haven't made any molds using pressure instead of vacuum, I think results would probably be similar.
I made a casting not using any pressure. It has obvious pinholes that would need to be filled. Also, note that some smaller details such as the bolt heads failed to fill and would need to be re-worked.
Then I did a pour into that mold, and let the resin cure under pressure. This is the kind of casting I am looking for. No pinholes, no unfilled detail, no distortion.
Finally, just to illustrate the point a bit better, I made two castings using clear resin. The one on the left was done under pressure, the one on the right was done without pressure. It isn't really easy to see in the photo (mostly due to my lack of camera skills), but the pressure-cast part is significantly more clear. The middle piece is there just to give the camera something to focus on.
Lastly, here is a shot of my pressure chamber - a paint pot. I think it is 2 gallons, it has been a long time since I bought it. It is rated for 80PSI.
A closer look at the connections. In the foreground you can see one of the openings has been capped off. Moving up on the left side, you can see the pressure relief valve that came with the pot. I personally would never use nor suggest a pressure pot that didn't have a relief valve. This one opens at about 70 PSI, but I usually only go up to 40 or 50 PSI anyway. Sometimes up as high as 60. Above that is a shut-off valve that I almost never use. I just leave the compressor hooked up in case there is a slow leak. To the right of the valve is the air hose fitting, and above that the pressure gauge. When the resin has had time to set, I just unhook the hose and let the pressure blow out of the air fitting. If you apply pressure and then release it rapidly when the resin is still liquid, the air bubbles trapped in the resin will expand rapidly, splashing resin all over the inside of your pressure pot. Or, uh.. so I have, uh.. been told. By people. People more foolish than me. Who would do that sort of thing. Because I wouldn't.
I hope this is helpful.
Eric