The topic of mold making and casting is really too much for any one Monthly Column since there are countless types of things to mold, many many materials to choose from, and varying levels of experience required for any combination of the former two. So I'll try to give some basic guidelines that cover a lot of situations and gives a good start to a budding caster.
You may find it helpful to check out my video where I discuss how to understand the properties of mold rubbers. You can see it here: https://www.youtube.com/watch?v=ECE6Db5Vf68
Rather than go into choosing between silicone or urethane rubbers, types of casting resins, and the methods of making a mold box, it occurs to me that the most fundamental aspect of mold making is often not discussed.
*** Understand your master! Before considering materials for molds and casting, you need to consider the object (the 'master') you are molding. First and foremost this will determine the type of mold you need to make. So for this month I thought I would provide a simple guide to get you started. (I've organized them from easiest to more challenging.)
Master - One sided object with relatively shallow detail.
Mold - A single pour, open faced mold
*** Think here of something like a shield emblem you sculpted out of green stuff, but it can also be a paint dropper bottle. Both have a relatively flat profile if you consider the dropper bottle to be a single surface in the shape of a cylinder. This mold only requires you to glue the object down to a board, wall it off and pour a rubber over it. Most rubbers work fine for this since tear strength, durometer, and other properties aren't that important. Though for taller objects or for aggressive textures, rubber properties can be important.
Master - One sided object that is larger than a soda can or similar.
Mold - A single pour mold or a brush on mold with a mother shell.
When objects get large, the amount of rubber needed can get very expensive very quickly. For larger molds, you need a thicker wall so the mold doesn't slump or bow when you are casting. You can choose rubbers with a high durometer, but often times it is a good idea to create a shell around the rubber (the "mother mold") to hold the shape of the mold once you remove the master.
Many times it is easiest to use a brush-on rubber and build up several layers. Then the mother mold is built around this. Brush on rubbers have a much thicker consistency so they don't flow off your master (they are thixotropic). There are rubbers that come pre-formulated for this purpose, but you can also buy additives to thicken normally thin-flowing rubber.
Mother molds can be made from a wide variety of materials including plaster, fiberglass, resin plastics, and more. Cost, ease of application, and strength affect the best choice of material. Typically they are made in two halves (or three parts) to allow you to remove them. If they were a single piece, the mold and master can become locked in the mother mold. Not good... :)
Master - single sided object with undercuts
Mold - single pour mold with high tear resistance, and possibly a clear color
An undercut is any place on the master where it is likely to trap bubbles when the rubber is poured, and/or these areas protrude out away from the main body creating areas that will be difficult to pull from the mold.
Here you should think of an army man (or perhaps a space marine?). The model has a gun that sticks out from the body, a gap in between/through the legs and an arm pointing down along the waist but isn't attached to the body. This presents a much more difficult mold to make.
The basics are simple, create a wall around the model and pour the rubber over it. However, you will not be able to pull the master out due to the gun, arm, and filled gap between the legs. In this instance you will need to create parting lines in the rubber to allow you to remove the master and future casts. This is done by carefully cutting through the rubber for each area so that you can create a split that allows you to pull out each section.
This can be very difficult as you are blindly cutting into the rubber until you get to the part you need to free. Clean cuts are very helpful in re-aligning the mold to get a clean cast later. I've struggled with this almost every time I've done it. However, there are clear rubbers made just for this purpose since they allow you to see the master through the rubber - much easier to cut your way into it cleanly and efficiently.
A mold like this will often need support to keep the cuts together when pouring into it. For simple molds you can often use foam board or hardboard to create plates to push against the sides of the mold. For larger molds or ones that have several cuts, a mother mold may be needed.
Master: A two sided object or sphere
Mold: A two part mold
This type of master would be represented by a something simple like a ball or a coin, or could be complex like molding a Tau drone as a single piece - (tricky!)
For this type of master you need to create a two part mold since none of the model can be left uncovered. To create this type of mold you need to pour the rubber in two stages. The first stage to cover half of the master, and then the second pour to cover the remaining half. This is a more complicated process as it requires you to imbed the master half way in some material, like clay, pour the first half of rubber, flip over the master, remove the clay and pour the second half.
You will need to add bumps or grooves in the rubber to make sure it aligns later (these are called 'keys') and if the shape is complicated (Tau drone) you will need to find the best line possible down the middle to make removal more easy. And if the model has undercuts as well you will need to cut parting lines for those areas...
In general I have stayed away from complicated two sided objects for this reason. Making a two part mold isn't difficult in concept, but the challenge will lie in how difficult your master is. Most of the time, complicated shapes are cut apart and several two part molds are made. Then the cast parts can be assembled later. This explains why most models are cast in sprues, and why you should remove the guns and antenna from the Tau drone before starting. ;)
With two part molds, you will also need to create a pour hole, and at least one vent hole. Since the mold is filled with air, you need a vent (basically a tube in the mold - like gluing a straw to your master when you are pouring to create a channel when you demold it. This allows the air to escape while pouring the resin into it. If the object has several areas that will trap bubbles based on their thickness or orientation, it is often required to create vents specifically to those areas.
Wrap Up -
Also note that none of these guides are hard and fast rules. Each master presents it's own challenges and creating good molds comes with experience. And of course, this is a very brief description of the techniques. All of these techniques are clearly explained in many videos available on YouTube. (That is how I learned.) Companies with great videos include Smooth-On, Brick in the Yard, and Tap Plastics. In the videos, they explain the materials they use so you can use that as a guide for purchasing your rubber and resin. And I'm always here to help you if you get stuck trying to figure out how to do some step, or if you can use some other material for your mold box, release agent, silicone rubber, etc. :)
Mold making is fun, challenging sometimes, and a very different way to enjoy terrain making.