I have been casting Smooth-Cast 300 resin into my OOMOO 30 silicone mold for my BJD's head and have learned a lot from the mistakes I have made. So let me just list them.
1. Be patient. Prepare, cast, and carefully remove your resin from the mold. These things don't go well when you rush. If you don't have the time or patience, don't bother because you will just waste your time and materials. Worst yet your impatience can cause you to tear or damage your silicone mold.
Ex. I did not thoroughly mix components A and B for the resin well enough. I rushed because I thought I didn't have enough time to pour into 2 molds before the resin would start curing. The end result was a casting I could not use. Because I rushed the mixing, the resin did not cure properly in various areas. Some spots were mushy and sticky. The cast was a waste. Not only of material, but the time. I should have mixed well and settled for pouring into just one mold. I may have wasted some resin because I mixed enough for 2 molds, but that would have been better than wasting all the resin and time.
2. Measure out components A and B in 2 separate cups instead of using just 1 cup. You might be thinking you are wasting less or will be cleaning less since you are only using 1 cup, but the reality is that it can be a huge waste. When you mix them in the same cup, you waste valuable pot life time pouring and closing the containers. Time that could have been better spent mixing well. You are also likely to feel a little more rushed. The resin I am using has a pot life of 3 minutes. The instructions say to mix for 90 seconds then pour. So when you pour each component into separate cups you buy yourself time to prepare yourself for the process. The other benefit is that you can pour the mixture into the other cup, stir, and pour it back to the other cup. This seems to ensure a better mixing for me. If the resin is not mixed together well you will have resin that does not cure properly.
Ex. I thought I would cut down on my cleaning if I used 1 cup to measure out both components. (Cured resin comes out easily from the plastic cups I use. Then I soak in soapy water and wash it later.) It took an extra 5-10 seconds just to get from pouring to finally being able to mix. I watched my timer and poured into my molds. I thought I had done a good job mixing and saving some clean up. I was wrong on many accounts. The resin did not cure properly and created a cast that had some gooey spots. Even when I tried to wash the resin with soapy water it continued to ooze. The cast was a waste. And to top it all off, the cup I used to do the mixing was also a mess. Because the resin was not mixed well enough the resin did not cure in the cup well. So now I had a gooey mess to clean up as well. This took more time to clean then if I had mixed the resin well and had cured properly.
Of course your mileage may vary. Maybe you have better luck then I do. But I am learning quickly that when I pour in one cup, the mixing is not as good as when I measure them out in separate cups and then pour back and forth a few times while mixing. Plus, with the resin curing properly in both cups it is much easier to clean.
3. You need to have a good mold to get a good cast. So this has more to do with the silicone mold than resin casting, but it is important still. Because of how I planned the silicone mold, I had difficulty removing the resin casting. I had to be even more patient when removing the cured resin casting or I would break parts or worse yet break the mold. My biggest problem is from my pour holes. They created such an odd angle that I had to try to clip the vents off first before attempting to pull the cast out with the pour hole. But my vents were not without some issues either. It is really hard to explain it in words what is wrong. In a nutshell, vents placed on the top part of the sculpt would have made removal easier than vents placed on the sides of the sculpt. I hope the next silicone mold I make does much better than this one. Also, the locations of your vents is important. Trapped air on your resin cast might mean that the air inside is not escaping properly.
4. Make sure a 2 part (or more) mold is held together. It doesn't have to be very tight, but this helps keep resin from seeping through where you don't want it to go. I used rubber bands. I would have used board, but I had the pour hole and vents at such an odd location that using boards didn't make much sense.
Ex. One time I thought it wasn't a big deal to use rubber bands and forgone the use. When I separated the molds I realized my mistake. The pressure the resin creates inside the mold can cause it to seep where you don't want it. So even if you think it isn't much of a big deal, it still might be.
5. Have a face mask. OMG, this stuff smells awful. I don't care if your room is ventilated well enough with a fan pulling it out the window. My mask of choice is a 3M 7502 Half Face-piece Silicone (Medium) with 3M 2097 P100 filters. You need to make sure you properly fit it every time you use it. Basically, if you can smell this stuff through your face mask something isn't right. Protect your lungs. It only took me one time to realize to USE A MASK. Wearing gloves is a must too. Eye protection is another one. Do read the Material Safety Data Sheet (MSDS).
Disclaimer: Research before you pick a product to protect your health. I do not offer medical advice. I am just mentioning what I am using at this moment, which may change in the future.
6. Protect your work space. I placed a trash bag on my work space then put newspaper on top. The newspaper helps soak up any spills, but it will soak through to whatever is beneath it. The plastic trash bag underneath stops the soaking right there so that it doesn't reach the table surface or any other surface. I reuse the trash bag but replace the newspaper at the end of the day.
7. Shake the bottles (A and B) well 10 minutes before you measure out what you need. Why? I notice B tends to get a lot of micro bubbles when you shake it. The containers tell you to shake both bottles before using. If you allow the bottles to sit a few minutes it reduces the bubbles when you measure out the components. If you must use immediately, you can tap the cup you poured B into to help get rid of a few bubbles prior to mixing with A. But I find it better to plan in advance and shake both bottles real well. Then I go prepare and double check everything prior to measuring out and mixing the resin. It doesn't have to be 10 minutes. To be honest I am not sure how much time I do wait. But I do know using mixing them right out of the bottle seconds after you shake them is not going to be pretty. There will be lots of tiny bubbles. Figure out how much time you need to wait and use that for preparation and double checking your process.
Well, that is all for now. My resin casts from my silicone molds are getting better with the more practice I get. Once I am happy with a few good casts I will start working on finishing one sculpt to become my prototype. That would involve sanding, filling holes, refining detail, etc.