I have broken the tutorial into 7 stages that reflect the order of the processes. Pictures can be found here. They are:
1. Preparation of the donor piece to be cast
2. Setting up of the donor piece to be cast
3. Making the mold
4. Separation of mold and donor piece
5. Preparation and pouring of the Resin
6. Release of the New Part
7. Preparation of the New part
Step 1: Preparation of the donor piece to be cast
The first step is to determine what you want to replicate or copy, and to assess its suitability for casting. Some parts will cast well using these techniques and others won't:a lot will depend on the shape of the piece you are casting. For the purposes of this tutorial I am going to be making a full figure casting of a vintage C-3PO action figure:the non-removable limbs variety was the kind I had lying around, so that will do. When you make a latex mold from a donor piece, it will usually pick out the details very well, so prior to making a mold the donor piece needs to be cleaned up if applicable. In this instance, the C-3PO figure had a few chunks of plating missing so I gave certain parts a light sand with 1200 grade wet'n'dry sand paper.
Being a full figure casting job, I next separated the various parts. Split figure! With most figures, the "boil and pop" method can be used to do this, however older figures like the gold plated original C-3PO are made from more brittle plastic. I just went at him with a knife. You need to be VERY careful when doing this. Many a time have I cut open my thumbs when the knife has gone through the plastic just that little bit faster than expected. Cut carefully.
Once the parts have been cut, certain bits will need to be trimmed:usually the "pegs" that were previously inserted in the torso to give articulation. I cast in resin, which is extremely hard and brittle. Full figure casting in resin generally means no articulation:you can be smart and tricky and use pins etc to give articulation, but I've never tried it. My full figure customs are always simply glued back together.
Step 2: Setting up of the donor piece to be cast
In order to make a mold, you next need to set up your donor pieces. I just glue the bits to a piece of cardboard and go from there. Like this The section of part that is glued to the cardboard will eventually become the opening to the mold through which you will pour resin.
When you are setting up the pieces to be cast, you need to be thinking firstly about how you are going to get your donor piece out of the mold to make it useable, and following from that obviously how you will get your cast resin piece out once it has hardened. What this means is that you need to consider where the opening in the mold should be, bearing in mind the shape of the piece you are hoping to squeeze out of it.
Generally, the places you will glue the parts down from will be areas that you do not see. For legs it will usually be the feet, for arms, the flat place where the arms meet the torso, heads will be the base of the neck. With modern figures generally the pelvis is separated from the upper body and this join becomes your glue mounting spot. With vintage figures it is not possible to separate the upper and lower body because they are not separate parts:instead the torso is made by a front side and a back side stuck together. Take a look at the pictures. You can see with the 3PO that the legs, arms and head are easily done by gluing down the areas described. With eth torso, I'm taking a punt here that I will be able to stretch the neck hole wide enough to be able to squeeze the chest out. Latex is pretty stretching, so we'll see how it goes.
Some other prep work may be needed depending on the shape of the part. For instance, if a leg had a large thigh, it may be too big to squeeze out of the foot hole of a mold made by standing the leg up. Also, sometimes especially with modern figures, the area where the shoulder joins the torso may not be flat. In these cases you may need to build up an area:with sculpey or plastic sheet, to form the mounting point. I don't have a picture, but say you wanted to cast a leg that had a small foot and a large thigh. You may need to have the mold opening at the largest end, the thigh:where the leg would have met the pelvis. With the way modern figures are made, the top of the leg normally has an L cut out shape when looked at front on:you need to build up this area to give you a smooth mold opening point so that the leg can be cast lying down, similar to the way standard arms are molded.
It is also difficult to cast excessive angles. Say an arm original comes with a bend at the elbow that goes past 90 degrees. You may consider cutting the piece and taking two molds rather than trying to make one possibly difficult to use mold.
Once you have stuck down your parts, you are ready to start making the mold.
Step 3: Making the mold
Once you are confident the position of the parts will yield a good mold, you are ready for the next step. First task is to dust the parts in talc or baby powder. I give them a light brushing with the stuff, then a gentle blow to get off any loose dust:you don't want to be casting powder, so blow off any loose dust before you start molding.
Once the part, and the cardboard it is glued too, is dusted you can start with the latex.
The first coat of latex is the most important and the most time consuming. I pencil on the cardboard the coat number I am applying, so that later I know how many coats have been done:just use a tally system.
It is extremely important to make sure there are no air bubbles on the first coat of latex. Think about it in reverse. An air bubble in the mold, will be an extra area that will fill with resin when you pour it, which will translate to lumps and bumps on the final resin piece. Using an old brush make sure you work the resin into all of the detail of the donor piece and keep working until there are no air bubbles. The first coat should always be thin, and as free from bubbles as possible. This can take some time, and remember that even if you see a bubble, it may not be a bubble on the part, but on top of latex already applied. Rub the brush around thoroughly just to make sure there is a thin, complete layer of latex on al parts. Also, you need to make a base to the mold. See the pictures. I make a square around the base of the piece. Like this! Thinking forward, when the mold is ready for use, it will sit upside down so that you can pour resin into the hole:the square base around it enable you to rest it on little model paint tins or whatever you find handy to use. This part can be thick and messy. The stronger the base, the better it will hold a load of resin.
Once the first coat is done, the rest is easy. Make sure the first coat is fully dry and then apply the next coat, which can be thicker and a lot more slap dash, just be careful not to rip or tear your first coat. You can apply it more thickly to build it up faster, but you need to be careful not to make the end result too thick or you will really struggle to squeeze out the parts. It needs to be flexible.
I find that 1 thin coat followed by 4 or 5 thicker coats is enough. Finished molds
Step 4: Separation of mold and donor piece;
Once the mold has dried and how are happy you have done enough coats, dust the outer mold with talc or baby powder. The next thing you need to know is that the rubber mold will stick to itself if you are not careful:it needs the powder to keep itself from sticking together. Using a knife, lift an edge of your rubber mold base off the cardboard and get some powder under it. Keep doing this until the base is completely separated from the cardboard, and now dusted. Next, snap the glued piece off the cardboard. You should now be left holding your mold, with the original piece still in it. Pull at the opening a bit and get a little nit more powder in there. This will help in separating the mold from the donor piece. You can now start to unroll the mold and squeeze out the donor part:do this slowly, dusting the rubber along the way so it doesn't stick together. Heads are easiest:you can pop them out and dust the inside of the mold once it is inside out. Generally most parts come out easily:just dust with powder as you go - some may take some squeezing but that depends on the thickness of the piece and the size hole opening in the mold you are trying to squeeze it through. Take your time:try not to rush it - ripping a few molds is all part of the learning process though.
Step 5: Preparation and pouring of the Resin
Now you have your rubber molds ready to use, it's time to mix up some resin. Once again, this is a process that you become more proficient at with practice. Before you start pouring and mixing resin, make sure you have all the bits and pieces you need ready:once you add the hardener to the resin you need to work relatively quickly:no need to rush, but you don't want to be looking for or creating tools after you've added the hardener. Make sure you have your molds arranged and supported. I like to rest them on small model paint tins:you can stack them if you need height, e.g. for legs and longer pieces. Also have ready a couple of paper clips that you have bent out so that they are straight lines of wire. These will become your little tools for making sure there are no air bubbles in your resin.
The key is to only mix up a small amount of resin at a time so you have time to work with it. I use the little containers that come in sports drink powder tins (Gatorade, etc) as my little mixing pots. First, using a syringe draw some resin out of your tin and put into your mixing pot. Using either another syringe or a dropper, draw out some hardener, which is normally quite thin compared to the resin, and careful add drops of hardener to the resin in the mixing pot. This is where trial and error comes into play. You need to add enough hardener so that the piece will harden properly. The hardening process is a chemical reaction that creates heat. Add too much hardener and the mixture will bubble and burn and ruin you molds. Add too little hardener and the resin will not set and you'll be left with sticky goop in your molds that will be more trouble than it's worth to clean. You need to find the right ratios yourself for the particular product you are using. The ambient temperature can also affect slightly the amount of hardener you need to use. On a cold day for instance you may need to use more, on a hot day slightly less:it's all practice. Try practicing the mixing ratios before actually using your molds if you are unsure. It's not too hard to get right:usually the instructions on the packaging of the resin you are using with give you a rough guide.
Once you have added what you think is the right amount of hardener, using your large straightened out paper clip, mix and mix and mix. Make sure the hardener is thoroughly stirred in. You now have the choice of tinting or not tinting the mix. 95% of the casting I have done previously was untinted, and it's only recently I have experimented with adding colour. What I have learned is that if you add thinners based paint, like out of the little Humbrol tins, you need to add a fair amount to thoroughly colour the mix, and it also has the effect of speeding up the hardening process, making the working time shorter. In making my recent vintage Dagobah playset rehash diorama, I used an Apple Barrel colour to the resin swamp I made. This seemed to need much less paint to tint the mix, and it didn't speed up hardening. Playing with colours is something you will just have to experiment with. For the purposes of this C3PO casting I will add a yellow tint. Add the amount of colour needed and then stir in as well so the mix is of uniform colour.
Now that your mixture is stirred fully it is ready to pour. Holding your mold in one hand, carefully pour the resin into the opening of the mold. Fill it about three quarters of the way and then use a smaller straightened out paperclip to "jiggle" around inside the mold. You need to careful scrape along the inside of the mold to get rid of any air bubbles. Particular areas to focus on include details, fingers and noses which are common for holding a bubble. Once you are happy there are no air bubbles add the mixture to the top, complete the scraping process on the rest of the mold, and then rest back on the supports so that the mold is level. I always add a bit of extra resin to the opening of the mold, let it overflow a bit. When resin hardens it shrinks a little, so if you only pour the resin flush to the top of the mold the end result will be that the hardened piece will not be completely formed:there will be a hollow or missing section where the resin meets air on the mold. Better to use more than necessary:you can always trim up the piece later.
Repeat the process until you have filled all of your molds. I would not recommend mixing up too much resin at once:only enough to do one or two parts at a time, otherwise you may find yourself rushing to beat the resin hardening. It's best if you take your time and do it carefully. Resin is really sticky too, so Acetone or nail polish remover is also handy for cleaning up your hands afterwards. Resin also smells too so make sure you do it in your shed or somewhere with lots of ventilation. Definitely not something for doing in your bedroom LOL.
Step 6: Release of the New Part
After the resin has set, usually a few hours at least, you can squeeze the part out of the mold. I like to dust the piece in talc powder as I am doing it because after a few hours the piece can still be tacky. I find it is best to remove the piece before it has gone rock hard and solid, otherwise the continuous heat on the mold can cause it to stick. It's easier to squeeze the part out when it is a bit "slippery" but use the talc to avoid ruining the piece with your fingerprints. Make sure the mold is dusted inside, then it will be ready to use again.