Do you need a pressure pot or vacuum chamber? Well ...that depends on how meticulous you are. In a perfect world, we all have vacuum chambers. Those cost money. So before we start shelling out money, lets look at what we want.
We want to cast. If you are looking at this as a way to make money, forget it. Its too labor intensive and time consuming to make a living at it this way. So for the most part, unless you are a perfectionist and even then...for basic home use...skip the vacuum chamber. I would have to say as a self proclaimed perfectionist that for what we are trying to achieve, the results we are after can be easily obtained using a basic pressure pot and compressor.
I bought a 10 liter (2.5 Gal) paint pot on sale for $70.00 Canadian. The compressor I salvaged. Now while I realize not everyone will have my luck, if you know what you want and are willing to hunt around, you can put this setup together for less than $200.00
IF you cant afford a pressure pot system, that doesnt mean you cant cast. It means you need to be efficient in managing your "casting time" and you will need to find alternative methods to disposing the air from your molds.
First, the mold. I would drop my silicon mold on the table right after pouring it. Only an inch or two. You just want to impact it enough to drive the air bubbles up. I would repeatedly hit the frame of the mold with the bottom of my screwdriver handle. I would take my exacto knife and actually 'cut' through the surface of the silicon to slice up any air bubles that were just below the surface. I would use pins to make sure the silicone got into every nook and cranny to help guarantee my mold wouldnt have air pockets in crucial places.
By doing those things, I was able to make some AWESOME casts. I made PEREFECT detailed Magno Chest Hilights and Missiles. Good enough that I said dam! and am using them proudly.
NOW..as for the casting...you really are at the mercy of the gods. You have to hope you made your mold in such a way (sprue or smuch) that resin got into every corner and that there will be no air bubles to mar the surface of the figure. Clear resin casting is out. Seriously. Dont try it. Ok...go ahead...but I warned you.
Clear resin casting WITHOUT a pressure pot wont work. You WILL get air bubbles and you will be upset. Doesnt matter what you try or dont try. They will appear. You are creating a chemical reaction. And they arent air bubbles that will interfere with the cast. Oh no. They arent empty spaces where resin should be. They are little air bubbles everywhere evenly placed throught the cast. That if you didnt see them, the cast would be perfect.
I thought like you. Oh yeah? I said...thats what YOU think. I can do it... And I tried. And then I tried again. Then I got one that was perfect. I was so happy I...oh wait...nope...I couldnt see them at first, then I turned the head in the light...TONS of little tiny bubbles. It was at this point I stopped casting till I had a pressure pot and an air compressor. But I wanted to make production quality style casts. And I was then able to do it with a pressur pot system.
If you just want to make some crafts for the kids, then I wouldnt even bother with the compressor system. If you just want to kill an afternoon with the kids or you want to make a few things for the church social, then forget the compressor.