But first, a little note on tools. One tool that I accidentally made a number of years ago, but has served me very well, is this little gem:
It's left over silicone rubber from a mold I once made. I obviously mixed up too much, and this was left in the mixing bucket. I use this on almost EVERY project I do, as it is the perfect surface for mixing putty. Any time I'm using some two part putty, like the stuff I used earlier in this build, I mix it on this. I will also squirt the air dry putty on here, and then scoop it off with an exacto or pallet knife to apply it to my master. Why?? Because nothing sticks to this! It makes for super easy clean up once the putty cures, as it just flakes off. Some people like to use post it notes or paper for mixing, but I find this solution to be much cleaner and easier. Anyhow, just wanted to pass that along.
Like you care.
But enough of this episode of "Tool Time", let's get down to business.
Tonight I'm pouring rubber on the belt master, that currently resides in a foam core box, sitting on top of a very flat famed poster. Nice!
I'll be using a Sil-Pak product tonight. The deal with this stuff is that you mix it 10 to 1 by weight. But who has time for that? I'm going to do it by volume. The good thing about RTV is that the mixing ratio is very forgiving. So if you pour in too much or too little catalyst, it will still kick. It will just change the time it takes to cure, and the resiliency of the final product. So if you OVER catalyze the rubber, you'll end up with a mold that cures faster. It will be more rigid, and it will provide fewer pulls. It will be a bit more frail. If you under catalyze it, you get the opposite. It cures slower, but will be more flexible and softer.
Ideally, you would mix it at the recommended ratio. For the hobbyist, this is fine, though in professional situations, where time is of the essence, and you don't expect to need more than a few pulls, you may end up having to "hot rod" your mold.
Here's the stuff. My expectation is to do two, ten ounce mixes. I've got my mixing cups and stiring rods at the ready.
No real trick to mixing the stuff. I pour in the rubber first, and the catalyst goes on top of it. I use the markings on the side of the cups to indicate how much I need to pour. I use a different stirring stick for each mixing. That's a good habit in general, though it may not be necessary in this case.
Also important is NOT to pour and catalyze both containers at once. Pour one, mix it, dump it, THEN move onto the next one.
Next up is probably the single most important piece of the mold making process, and is the step that most people forego. Folks, if you want good molds, you've GOT to vac the rubber. The idea here is that when you mix up your rubber, you stir in air bubbles. When you pour the mold, some of those air bubbles will escape, but many will remain. Why are air bubbles in your mold bad?? Because they screw up your positives! For the most part, when you pour a resin into a mold, it heats up. That heat causes the air trapped in the rubber to expand, pushing INWARD on your resin, resulting in tons of little pock marks. That sucks, and usually means TONS of time spent filling those in, sanding them, etc. It's just a giant mess. I'd ALMOST go so far as to say that it's just not worth making a mold unless you can vac the rubber. But, since most people don't have access to one, I guess I'll have to back down off that.
If you don't have a vac chamber, the next thing to do is pour your rubber from high up. I mean really high up. Like standing on your toes, with your arm stretched above your head. By pouring high, you stretch the liquid rubber out into a thin stream. Because it's all stretched out, the bubbles tend to burst on their way down. It's not HALF as good as vacuumed rubber, but it's better than just dumping it onto the mold.
I prepped my molding surface by making sure it was level. My garage floor, er, I mean my WORKSHOP floor is graded, so I like to level the surface off so that the mold pours flat. I guess it's not manditory, but it saves time when it comes time to pour up the master. Here you can see the level sitting on top of the frame, with popsicle sticks stacked up underneath it to set it level.
Then I just dumped it into the mold. That's why this type of mold is often referred to as a "dump mold". It's also called a "one part mold", because, well, it's one part. Open faced mold? The list goes on.
Despite all the prep work that had to be done, and all the effort that went into mixing, vac'ing, and pouring the rubber, this is still the easiest type of mold that can be made. It only gets more complex from here. If you're new to molding, I'd recommend starting with a simple "dump mold" like this, and then moving on to try something more complex like a two part mold.
The catalyst I used for this rubber makes it kick in about six hours, but I always like to let it sit overnight just in case. So by this time tomorrow, I should be all set to pour up a positive. yay.
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