Thanks TicToc,
I have done quite a bit of trial and error, and have been lucky on most occasions. Aside from pouring a bubble-free silicone, (which decides in the end if the time invested was worth it or if it is time to go back to the drawing board), I think the most important part is having a decent plan to make your mold.
As far as materials go I use Lego building blocks, they come in a variety of sizes, making it easy to make a damn to fit any object and do not let the silicone leak out (even using the single row blocks).
In the begining I used wax paper to provide a bottom barrier, legos on top (back then I was using Duplo blocks... works but definatly not as good as normal legos), and a thin layer of Plastilina molding clay on top of the wax paper to make an air tight connection to the piece I am trying to copy. The configuration worked pretty good, but the Plastilina that I have tends to retain bits and pieces of silicone in it and is at times smeary. VERY important, when choosing a molding clay make sure it is sulfur-free, most of your "normal" clays have sulfer in them and this does not react good with silicone.
Since then I have replaced wax paper with "TetraPak" cardboard... not sure if it is called that in the US, but here in Germany most liquids i.e. milk, juice, right down to your tomato sauce is in a thin cardboard packages that have a plastic coating on the inside. I know it is not as common in the US as it is here, but I do remember seeing some items in the local grocery store being sold in these when I was in DC last year. Anyway, this works excellent... silicone does not stick to it, Plastilina does not stick to it... really a dream. I just cut up enough TetraPaks until I have a large enough work area for a given project, but I believe you can buy sheets of this if you want something without the fold marks.
For demolding I use Talcum Powder,,, to be honest I bought a spray can of release agent just in case, and have never used it. Talcum powder is awsome, it works great and is very cheap... doesnt get any better than that. If you do decide to use a spray, make sure to apply it very lightly, if applied to heavily you will loose some of your fine detail and you may get what is called "orange-skin" which will make the surface of your casts rough and bumpy like an orange. Anyway back to talcum, when applying, use a normal dark-haired painting brush, the kind you get in the watercolor sets... dip it into the Talcum bottle and apply very lightly to the higher areas first (as the powder falls from the higher points it will automatically coat the lower points and you won't end up with a ton of talcum powder at the base). I do not expicitly coat my legos but I do coat any molding clay that might come into contact with silicon. Before doing this, make absolutly certain that your mold, legos, and what have you not is completly dry. Talcum is a powder and will suck up any moisture and create big ugly clumps... my first mold the plastic was out in the cold stairwell and when I brought it in to mold... well you get the picture, beware of moisture. Lastly you have to get rid of the access talcum powder, if you applied it sparingly there should not be too much, nevertheless here is where I advise caution, I do not think that talcum is dangerous if you get it in your lungs, eyes, or any other openings, but play it safe...
- if at all possible, use an air compresser to "lightly" blow away any access
- Do this in a well ventelated room, or even better outside
- Do not do this in a room with other people, childeren, animals.
- Leave the room after your work is done to let any dust settle.
unfortunatly, just tipping it upside-down doesnt work.
all that being said and done, you are ready to start pouring silicone. Any experts out there, please feel free to correct me... as mentioned I am far from an expert on this topic, this is only what I have learned through trial and error.
Hope that helps,
Jason