When I was producing kits, I was always faced with this same dilemma. I turned to resin casting as well and did a lot of research and testing before I had success. The tricks I'm going to share with you should result in clean molds without the bubbles and clean resin castings. The equipment you need is not as expensive as you might think. I'll give you links where you can buy it. Once you've made that initial investment for the equipment, it will pay for itself over time because you'll use it time and time again to make parts.
I can tell you from experience that first you need a vacuum chamber with pump which you can find on eBay pretty cheap. Here's
one that costs $130.00.
The type of silicone I used was
here from Smooth-on. You want a silicone that is mixed at a 1 to 1 ratio. That way you don't need a scale. I got to the point that I could tell by the color of the silicone when I poured the part 1B into the part 1A when I had added a sufficient amount but small glass measuring cups work great for this part which you can find at any department store like Walmart or Target.
When you mix the silicone for the mold, it needs to be in an oversize bowl. I used to use large 4 quart glass kitchen measuring cups for this. The reason is that when you mix it, you put it in the vacuum chamber to let the pump suck all of the air out. The stuff will start to bubble big time and it will rise. It will rise to a certain point and then seem to explode dropping back down to its normal level. That's when you can shut the pump off and remove the mixing bowl.
It will seem like it still has bubbles in it but that's normal. They'll escape and rise to the surface once you pour the mold. But before you even mix the silicon you have to get the mold setup for pouring the first half.
I took modeling clay and flattened it out making it about 1/2" thick. I built walls all around the clay using Lego's and made the walls about an inch or more taller than the clay. Then I pushed the object into the clay about half of it's total thickness. Here's the tricky part. You need to get some small dowels about 1/4" in diameter and use a pencil sharpener to put a blunt point on them. Then you sink those into the clay at points around the part so that the blunt point is pointing towards the bottom of the mold and the thick part touching the side that will become the top of the mold. These will create air vents.
Next, you use styrene rod that is the same diameter of the blunt end of the pencil dowels. You can just warm the styrene in hot water so that it can be bent into 90 degree angles. What you need to do is to create a tube from the pencil dowels to the actual part to form a complete channel from the part to the dowel for venting air. It's important to get these air vents at points near the bottom of the part - the bottom is that area of the part when the mold is standing up ready for resin pouring.
The final step in making the mold is to create registration points. I would take a brand new pencil and press the eraser into the clay at various points to create dents in the clay. That way when you pour the silicone you'll get small bumps which will fit into small dents created when the second half of the mold is poured.
Once you get your mold set up with these air vents and the part, you're ready to mix the silicon, take the air out of it with the vacuum chamber, and do the pour. Let the mold sit over night to harden. Then you need a special mold release compound that is designed for rubber against rubber. I used one I bought from
Smooth-on.
After the mold has set for 24 hours, remove the mold and the clay from the lego walls by tearing down the walls first, and then peeling the mold from the rubber. Now turn the mold over but leave the part in the mold. Rebuild the walls around the mold again making them an inch or more taller than the top of the mold and part. Then spray the mold with the mold release agent. Spray it good so that after you pour the second half, you'll be able to pry it apart.
Now mix up a second batch of the rubber, vacuum the air out, and pour it over the first mold to create the second half. Let it set overnight. When you remove the second half of the mold, you tear down the lego wall and handle it in your hands. It will be a little difficult to find the junction where the first half of the mold ends and the second half begins. Once you find it, you can use an Xacto knife to cut around this junction to pry the two halves apart. Just work around the entire mold prying a little bit at a time. It will eventually come apart cleanly thanks to the mold release you sprayed before you poured the second half.
Remove your part and the other stuff you used to create the air vents. You will probably have to clean out these air vents so that they create a clean path from the top of the mold to the part. I used tweezers to grab any fragments of rubber that was blocking the tube created, especially where the tubes meet the part.
For the resin casting, you'll need a
pressure pot also on eBay. You probably already have an air compressor but if not, you can get this one on eBay for less than $60.00. The investment in the vacuum chamber with pump and the pressure pot and compressor will be about the same as your quote for $350.00 to make the mold, but now you'll have the equipment to make lots of molds.
Now you are ready to pour the resin. I used this
resin from Alumilite for my resin casting. It's a 1:1 mixed resin that sets rapidly and is very strong. Spray both halves of the mold with the same release agent you used before. Put the two mold halves together and wrap thick rubber bands around the mold so it doesn't come apart. Mix the resin, pour it into your mold, put the mold in the pressure pot and close it up, turn on the air compressor and adjust the pressure for about 50 psi. Let it sit for 10 or 15 minutes. You'll see the resin turn beige which tells you it's almost hard. Wait a couple of minutes at that stage to be sure the resin is hardening. Then turn the pump off and remove the mold. Wait a few more minutes before removing the rubber bands and opening the mold up. You should have a part with no bubbles in it. Clean it up by removing the vent parts.
I've used this procedure hundreds of times in the years that I produced kits and always got good parts. You can get the resin in black even so that the parts don't have to be painted or you can add a special coloring agent which is liquid based. Just a few drops into the resin when you're mixing it works great.
That's the best way I know for casting good resin parts.
Hope that helps,
Bob