In general its quite a simple process , build a box to hold your item , keep it close to the item as your going to cover it in casting compound and you need to know the volume of the box, the box can be made out of almost anything , but it must be firm enough and have close fitting edges as you going to pour liquid casting compound and it'll leak out if you have the smallest tiny gap . I use plywood and duct tape as I re-use my boxes all the time. fix your item to the bottom of the box - either glue it or use blu-tack or an adhesive pad or whatever - now get your self some Room Temperature Vulcanising Silicon Rubber compound . RTV as it is known as, and a can of spray mold release , both of these can be bought from specialist art shops or specialist suppliers or bought over the internet from a variety of sources - as a general rule buy the most expensive you can afford , as that will give you very good detail reproduction and a good mold life - as each time you use your mold it will wear out - most RTV's come as a two part mix, remember earlier I asked you to calculate the volume , this is the amount of compound you will mix and pour into the mold until it covers ,by about half an inch, your item , follow the instruction to measure and mix the compound , spray the release over item and box ,let dry for a minute , then very carefully pour it all over the item until you covered it . Now let it set , overnight is best. Next day de-mold, the box remove the item carefully by easing the edge away from the item, be careful as you don't want to tear the rubber. Ease it away until you get to a certain point and it should pop out, now go wash both the item and the mold in warm soapy water , let dry in a warm place. Now you are ready to cast , since you say small plastic items are what your making , I would suggest a 'plastic' resin 'fast cast compound' this also comes in two parts , now you need some protection as resin is 'nasty stuff' a respirator mask, latex gloves and old clothes and a work apron are recommended . Plastic resin sets by 'thermotropic' reaction, it uses self-generating heat to set itself , get this on you skin and it gets hot, extremely hot!, and will burn you -you'll only do it once !!
So to start fill the mold with water and measure it , this the volume of resin needed , dry your mold and set aside I spray it with release at this point.
Reassemble the box around your mold -this hold the rubber and prevents it from deforming when you pour the resin in , now measure half and half of the resin to the desired fluid volume , I use plastic disposable cups for this marked A and B . a third cup and a lolly stick are used for mixing pour both part into the mixing cup and using the lolly stick mix together. try not to be too vigourous you don't want air to get in to the mix, this causes bubbles, bubbles that can get cast in and that would ruin your cast Work quickly and pour the resin in until the mold is full almost overflowing , in fact try to get close to that. The reaction starts the second you pour one into the other , which is why I said work quickly - stop for any reason- and the reaction will set it in your cup as a solid mass, it will take depending on what sort of resin you bought 5 to 10 minutes to set . Nnce set pop it out of the mold, if it feels a bit warm let it cool, finally file off any loose bits of set resin from round the edges, don't forget the base and well . if you've done everything properly, you now have a perfect double of the item . Wash it with warm soapy water to remove any release agent. f you have any pits, bubbles and holes these can be filled using car body filler, plastic padding, or plumber's putty or similar two part epoxy filler -sand smooth with coarse medium and fine sandpaper, in that order and your ready to prime, paint to finish. You can re-use the mold many times, ensure it's clean and free from any bits of resin - box it , spray release , mix and pour, leave to set, de-mold , wash dry, and repeat as many times as you want , but remember the mold will breakdown over time . You should expect to get 40 to 50 casts for a good mold , before you notice your casts aren't as good as the first. I usually keep a first cast or 'pull' as it's known with the mold so I can make a new ' master' at some point in the future. To store your mold wash it in warm soapy water - let it dry and store it in a plastic bag or box until next time .
