Casting part 1
Part 1, creating a mold
I'm going to take you through the steps of creating a mold. For this tutorial I'm using a GI Joe Monoploy USS Flagg game piece, but the idea is the same no matter what. Usually for these types of tutorials we have a "Tools" or "Items Needed" section but as you'll see, there are a lot of ways to go about this and so I'll go a little more in depth into each one and why. I'm also doing this without a pressure pot because I don't recommend anyone spending the money on one until they have a good handle on the basics, myself included.
For the mold itself I'm using Alumilite's High Strength Silicone Rubber. While there are several different companies like Smooth-On who offer similar products, Alumilite is carried by most hobby stores. Which means I don't have to pay for shipping and can get it easily when I run out. In short, personal preference. The "High Strength" means it won't tear easily, but because it is Silicone rubber it is still flexible enough to pop a head out of it without needing a two piece mold.
Before we get into the meat of this, let me preface this with the best advice I've ever received when it comes to casting. Make sure you want a lot of whatever you are creating a mold for. If you only want a few of that item, this is not the most cost effective way of acquiring it.
Building the mold
For this particular piece I used a sheet of styrene plastic as the base. That wasn't the best idea. Usually I use a sheet of corrugated plastic that is similar to those used in mail bins. It is easy to cut and the silicone rubber and hot glue won't stick to it as much as the styrene. I've seen people use Legos, cereal box cardboard, pill boxes, and a slew of other materials to build their mold out of. It doesn't really matter as long as the liquid won't seep into the surface. So anything that would get soggy if wet is out. For this, I used plastic sauce cup like the ones you get for take home condiments. I cut the bottom off and it was good to go.
This isn't something most casting tutorials talk about but you have to think of everything upside down when you're building the mold. You have to think of how the liquid resin will flow and where air will get trapped. You also have to think of how you're going to remove the cast. That means you start off with your opening. For heads with long hair you to put the head upside down so the resin will start with the hair.
I started with a tiny wad of blue casting clay. This is a special kind of clay that the rubber and resin doesn't stick to. This particular clay is made by Alumilite, but again, a lot of companies make clay that does the same thing. You just don't want to use Sculpey or Femo or any kind of arts and crafts clay. It has to be casting clay. This will create a nice even opening for the resin to go into.
Having already created one mold for this Flagg piece, I knew there were some trouble spots. It has overhangs and angular cuts as well as the thin bridge. This was causing all kinds of air bubbles. So, I attached some styrene rods to the base of the mold and connected them to the areas of the Flagg where I wanted the air to be able to escape. If you are doing a head, this would be under the chin and behind the head and possibly the hair if it hangs down. Take a look at any production figure and you can see where the sprue marks are.
Gluing it all in place
It only takes one time when what you thought was a water tight seal to leak all over your workbench before you to take better precaution against it ever happening again. For me that means hot glue and lots of it. I hot glued the plastic condiment cup inverted over the Flagg piece. In retrospect the hot glue warped the styrene. Not enough to ruin the mold or anything, but it did make it a lot harder to get the styrene off of the mold once it dried.
Mixing the Silicone
This step can and will get messy. Make sure you have something put down on your work area or that your work area can be thoroughly wiped down afterwards. You also want to handle these chemicals in a well ventilated area. I like using disposable drinking cups for casting. Not only are they cheap, but you can bend them to form a pouring spout. That comes in handy. This Silicone is a 10:1 mix ratio with an hour working time. That means for every 10 measurements of Base, it gets 1 measurement of Catalyst. For this, that measurement will be a teaspoon. Nice level scoops. I used the coffee stirrer to wipe off any extra to make sure everything was accurate. You do not want to eye ball this mixture.
Stirring is essential. The Alumilite Silicone will go from the Base white to a light pink as the Catalyst gets mixed in. Make sure you stir up form the bottom. At the same time, you want to use nice smooth moves so you aren't creating a lot of bubbles.
Pouring the Silicone
Don't go pouring the silicone right over the top. Go slowly and pour from the outer edges first. The idea is to let the silicone flow into all the nooks and crannies. Take your time. Pop any big bubbles. The hour working time is plenty to stop every few seconds to check how the silicone is covering. I used the coffee stirrer again to make sure the silicone went under the styrene sprues.
Bubbles are the enemy. Bubbles in the resin cast are a minor inconvenience. Bubbles in the mold itself usually means having to start all over again. Once the silicone is poured I like to tap on the bottom repeatedly to try and get any remaining air bubbles to rise to the surface. I then vibrated my hands to shake them to the surface. Then tapped again. After a few minutes, when no more air bubbles came to the surface, I put the mold away so it could dry.
The instructions for this product says 18 hours to demold, but I've never had any mold completely dry in that short of time. So I go with the "does it stick to my finger" technique. If the silicone is still tacky I keep waiting until it doesn't stick to my finger at all. Depending on temperature, precipitation, humidity, and any number of other factors it can take anywhere from 2-7 days.
I used a blade to carefully cut away the hot glue. As I mentioned the styrene caused a bit of a problem so I had to use to snips to cut a little closer to the lip of the cup. Once the styrene was removed I did the touch test again to make sure the bottom (which will now be the top) wasn't tacky. Then I used vice grip pliers to remove all of the styrene rod sprues. Then I spread the opening a little and dug out the blue casting clay. I used the xacto blade again and cut a plus sign (+) into the opening about an 1/18 of an inch deep, just enough to give room to pull casts out. Lastly, I slowly worked the Flagg piece out of the mold by pushing from the bottom and pulling from the top.
Now typically I create several molds at once. This isn't necessary but doing gang molds allows me to make the most out of the silicone and in turn the resin for casting. Since the silicone and resin only has a few months shelf life, the more you can do at once the less you'll end up wasting in the end.
Part 2, Casting
Click on through to Part 2 where we start making cast parts.