Think that making a mold and casting your own parts is too hard? Think again. The hardest part is what you already do - make a model. No, really, that's the hardest part! After that, you're just measuruing two liquids, stirring them together and pouring the mixture in place. Once you get to casting the parts, you don't even have to worry about insane calculations because it's a one-to-one mixture! So, let's get started...
The first thing that I need to tell you about casting your own parts is that you should not use this method to duplicate commercially available parts. The model manufacturers paid good money to develop a kit for you to use, and making a mold of a commercial part is not only unethical, the commercial part is likely copyrighted and/or trademarked so making a mold of a commercial part may be illegal (but don't take my word as legal advice, I'm not a lawyer). If you make your own master, not only will you gain a few extra points on NMRA judging, but you won't have to worry as much about the legality of copying your own work.
So, now we know we're going to make our own masters, what considerations are there in preparing the master? For making flat molds, your master part needs at least one flat surface. It is through this flat surface that you will be pouring the resin once you get to that step. The part should generally either be flat and shallow or tall and narrow; this will help you reduce the amount of resin that is needed to make the casting because your part won't need as much resin to cast it. I've found that a resin part can be quite strong at as little as 1/16th of an inch thick; yes, it will break if you stress it hard enough, but you don't need a lot of resin to make a strong part.
Your master part cannot have anywhere that would form a loop. Making a master that can be used like a link in a chain will create a mold where it's impossible to remove the master without either tearing the rubber or breaking the master. Even if you do get your master out of the mold, you're going to have the exact same problem with any part that you cast in it. You will run into the same problem if your part has a large ledge; while you might be able to get the part out without breaking it, your chances of tearing the rubber increase as the size of the part's undercuts increase. This is one place where the KISS principle (Keep It Simple, Stupid!) really works. Parts that come out as boxes, cones, cylinders, pyramids or similar shapes work very well as castings. Think of it this way: if you put the undetailed side of your master down on the tabletop, you want to be able to just lift the mold straight up to release the cast part.
Glue the flat, undetailed side of your master parts to a flat sheet of plain styrene. I like to use Elmer's Stix-All to glue my masters to the base because I can remove the master parts when I'm done. Almost anything thicker than .010 inches will work as a base material as long as you have a flat and level surface where the mold box can sit while the rubber cures. If your parts are small and your mold will encompass more than one of them, set your masters apart by at least ½ inch on all sides. This will give the rubber enough mass between the parts to stand up as you pour the resin.
Around your master parts, build a wall of a similar non-porous material (styrene sheet works well for this too) to form a box around your master parts The box does not need to be rectangular, but that configuration is the simplest to build. The walls of the mold box should be about ½ inch away from the master parts and the walls should extend at least ½ inch above the highest point of your master parts. Use a gap-filling cement to glue the walls to the base and to each other; again, Stix-All works well here. After the glue dries on your mold box, use modeling clay to seal all of the corners around the outside of the mold box. The one edge that you don't seal is the edge where the rubber will seep out before it's cured.
The next step depends on the manufacture of the rubber that you've purchased. I'm a fan of Alumilite products, and their rubber is very easy to work with. If you've got a two-part rubber and catalyst mixture, combine them per the manufacturer's instructions using a paper or plastic bowl that you're willing to throw away (although be sure to use one that does not have any holes in it; if the bowl will hold water, it will work for rubber). Mix up less ruber than you think you'll need to make the mold. You can always add more, but it's not so easy to take it away, especially if you overflow your mold box.
When the rubber is mixed, slowly pour it into the mold box. Pouring it slowly minimizes the risk of forming bubbles against your master parts and it maximizes the control that you have in the pour. The rubber that Alumilite sells allows about 10-15 minutes of pouring time, so you don't need to hurry. If you find that what you've mixed isn't enough to cover your master, don't panic. Rubber will bond to rubber, so just mix some more and pour it on top of what you've already poured. You don't need to have it mixed and poured immediately; take your time, measure carefully and pour it as if it was the first pour. The top of the rubber should be somewhere between ¼ and ½ inch (leaning more toward ½ inch) above the highest point on your master part.
When you're done pouring the rubber, you'll have something that looks like this:
Set the newly poured mold aside for a few hours while the rubber cures. I like to leave mine to cure at least overnight. Different rubber mixtures will have different curing times, so check the manufacturer's instructions. Alumilite's rubber sets up in about 4 hours, so you could theoretically pour a mold and then use it the same day. However, letting the mold cure longer will make a more stable mold when you use it.
Once the rubber is cured, you can remove your new mold and reclaim your master parts. If you used Stix-All to assemble the mold box, you can pull the walls away from your new mold and peel the mold box base away from the mold and embedded master part. You can peel the glue off the mold box parts and use the base and walls again for another mold if you're ready to do so. Stix-All, however, is a rubber-based cement, so it will stick to the mold.
After you peel away your mold box, your mold will look something like this:
Gently pull the rubber from the sides of your master parts and pull your master parts out of the rubber. You will probably find some rubber flash has seeped under your master part; a sharp hobby knife will easily cut this flash away and help to remove the parts without tearing the rubber. You will probably notice at this point that the rubber has crept up the sides of the mold box as it was curing. Use a sharp hobby knife to cut this lip off what is now the bottom of the mold so that your mold can sit flat on the table; this step will help ensure that your castings are not warped before they are cured.
Once you've removed your master parts from the mold, you're ready to start casting copies. Like I said earlier, I like to use Alumilite products for my casting needs. The resin that Alumilite manufactures is a two-part epoxy that is mixed in a one-to-one ratio. Follow the manufacturer's instructions on mix ratios for other products.
The next thing you'll need to know is how much resin to mix. One trick that I've seen is to fill your mold with water, then pour the water from the mold into a measuring container to get the complete volume of your mold. If you do this, however, towel dry the mold and let it air dry unil whatever droplets are left have evaporated completely. Mix a little less resin than you think you'll need to fill the mold, and pour it into the mold. Drizzle the resin into the mold from one corner of the mold cavity. Let it flow around and through the cavity on its own; this will help minimize the number of bubbles that form in the resin against the mold walls.
The Alumilite resin that I use changes color as it cures, so you have a visible indicator of when your new castings are ready to remove from the mold. Removing the part from the mold early, you may be able to invoke a warp or bend into the part before the resin is fully cured. This can be helpful if the wall you're casting needs to be curved to fit the scenery around it or if the prototype wall is curved, you have a few minutes that you can work with the curing resin to get it into the shape you need.
At this point, the steps for creating another copy of your master part are reminiscent of the instructions on a shampoo bottle. To cast another part, mix, pour, demold, repeat.