I'm going to review two types of molds using the same materials and principles. The first will be a simple block mold. The other will be the same block mold but it will include multiple castings. This is often called a gang mold.
You're going to need:
- RTV silicone. I will be using Smooth-On OMOO two part silicone molding compound. This is fairly easy and inexpensive to pick up. You can order it online or at some craft and plastics stores. Any RTV (or Room Temperature Vulcanizing) silicone will do. For this process the Polyester casting resin I will be using has some cure inhibition (meaning that it will remain sticky) against tin cure silicone (your standard inexpensive RTV). This means that to get good parts out of the mold, you will have to coat the mold in sealant before casting your parts, bake the parts after they're cured, or apply a clear sealant to the parts after they are molded.
- Latex Gloves
- Stirring sticks
- A digital scale or graduated measuring cups
- Bondo car body filler. You can pick this up at most hardware stores and auto shops.
- Clear Polyester resin. Find it at a plastics store, or order it online. Sometimes craft stores will sell this as a way to make cutesy paperweights with flowers trapped in them, etc.
- MEKP catalyst. This will often come with your resin. You can also find it as fiberglass catalyst in most hardware stores.
- Polyester resin dye
- Respirator, ventilation system, or open well ventilated space to work in. Be mindful of your neighbors, as the polyester resin smells something awful, and will cause some people with sensitivities to feel ill.
- Hot glue
- A mold board. Any piece of smooth wood or plastic will do. Remember to seal the wood before casting on it, or else the mold will be difficult to remove.
- Disposable paint brushes
- Plenty of paper towels
- The touch
- The power
Step 1: Dismantle Your Controller
It's time to suck it up and take the screwdriver to this fancy bit of extruded dinosaurs. You must remove the main body panel before the neck will unfasten. Once you've opened the neck, take a small electronics screwdriver and carefully take out the circuit board (you are saving all the parts, right?) Remove the keys, and take their rubber bumpers off. The rubber bumpers will fit perfectly into the new keys once everything is done.
Step 2: Single Key Mold
This is pretty simple and straightforward. You lay a single key down on your mold board. Then you cut the bottom off of a disposable cup, and hot glue the top portion to the mold board around the key. That's it. You're ready to mix up and pour.
Step 3: A High Pour
Check over your mold and part to make sure there aren't any dust, grit, or fingerprints that will screw up your casting. Remember that whatever you see here will show up in your final parts, and there's no way to correct a screwed up mold. A few extra seconds of inspection will save you the massive headache of recasting the whole part.
Mix your silicone according to the manufacturer's specifications. For the OMOO the instructions called for equal parts of compounds A and B. Stir your mixture for at least a minute, making sure to scrape down the sides and bottom. Try not to whip bubbles into the mix by moving the stick in and out of the mixture.
One of the crucial factors in making a successful mold is making sure there are no voids or bubbles in your casting. This means you pour your mixture from high above the mold in a very thin stream. This stream will pop the large bubbles that would have been included in the casting. Start in one corner of the mold, and let it fill evenly to the other side. When you've poured enough that you can only see the top of your key, go in with the paintbrush, and make sure there's silicone filling up the small divits where the rubber bumpers were. Now you can fill the mold until your silicone is about 1/4" above the level of your key.
Step 4: Gang Mold
This mold is simply a slightly more complex version of your block mold. Start with several rectangles of foamcore. Adhere them to the mold board, and then spray them with a sealant or shellac. Then glue down your parts with a daub of hot glue, this makes things a little simpler to arrange and cast. Fill this mold with silicone exactly as prescribed in the previous step.
Step 5: Casting the Plastic Keys
The Polyester resin I used was clear. For the translucent frosted look I was going for, it had to be modified a little before casting. For a frosted look, you can apply a crystal clear spray into the mold in a thick coat before molding. When the parts come out of the mold you can wipe the spray off of them with acetone. For a super smooth look, you can spray PVA mold sealant into the mold using a Preval Sprayer. It will form a super shiny water soluble layer in the mold. When your parts come out, rinse them in water to reveal a remarkable shine.
To begin, I measured about an ounce and a half of resin on a digital scale. I marked how much resin was in the cup with a marker and measured its height. Then I transcribed that line onto another cup and cut it in half vertically. Now I could put this half cup over each cup I'd use to mix and just draw the line on it, meaning that I didn't have to weigh out my resin each time.
To color the resin, I used red, yellow, and blue dyes made especially for it. I also made the resin slightly more opaque by adding a pea sized daub of Bondo to it before mixing. Make sure you mix your resin thoroughly before adding your catalyst. Each of my 1.5oz servings of resin would need six drops of MEKP. Be careful when using MEKP, as it burns when it contacts the skin, and is very bad for your health. Make sure to work in a well ventilated area or use a NIOSH rated respirator.
After you've mixed your resin and catalyst, you can pour your keys using the same high pour method described before. Make sure to go slowly, and to take out all the bubbles you can with a stirring stick or other small instrument.