TARDIS Chocolate Mold

In case fish fingers and custard aren't your thing, here's some instructions on how you can make a mold for your own chocolate TARDIS!

I made this one as a gift to the students of E-Entry in MacGregor Dorm at MIT, hence the "E" and the "MACGREGOR" but you can put whatever words you want on there.

Let's get started -- allons-y!

Step 1: Tools & Materials

Tools:

Desktop CNC Mill (although you can also carve wax by hand)

A vacuum de-gassing chamber (you can probably jerry-rig something with a normal vacuum)

Sonic Screwdriver (optional)

Materials:

Machinable wax

Smooth-Sil 940

Disposable cups and mixer (you can use a popsicle sticks)

Disposable gloves

Chocolate

Step 2: A Bit About Molding & Casting

There's loads of different ways to do molding & casting depending on what you're trying to make, if you want the mold to be reusable, and so forth. The important bit is to understand the difference between a positive and a negative mold. A positive mold is the same shape as what you want to make at the end. A negative mold is the inverse of that shape.

Because we're making chocolate, we want to have a negative mold that will be flexible so we can pop the chocolate right out. If we were to use a hard mold, then we're going to have to break either the mold or the chocolate. We can't machine the soft mold directly, since it comes in liquid form, so we have to make a positive mold first. Using that positive mold, then we can make the flexible negative mold we'll need to cast the chocolate.

Step 3: Making the Positive Wax Mold

I made the positive mold out of a machinable wax, although you really could make it out of any hard material depending on what you have access to. The nice thing about machinable wax is, well, it's machinable. It's not super hard, so it doesn't take long to machine, and it's reusable (you can always melt it back down). There's also some instructions on how to make your own machinable wax.

I machined my mold on a Modela desktop CNC mill. (The files are attached). Depending on what CNC software you use, you may need to add the recessed pocket into the file directly. Either way, when you make the pocket, make sure it's deep enough so that the back of the mold isn't too thin.

I used a 3 step approach:

1. Mill out a pocket with a 1/8" end mill.
2. Mill out the main details of the TARDIS a 1/16" end mill. It won't get the details of the text.
3. Mill out just the small features (the text, the small windows because I wanted them to have sharper corners) with the 1/64" end mill.

After words, you can clean up the excess wax shavings with a Q-tip or an x-acto knife.

One more thing - if you don't have access to a CNC mill, don't fret! You can carve wax by hand with basic tools (I've carved stuff out of jeweler's wax and it's come out just fine). You don't even have to use wax for that matter - anything you can pour the Smooth-Sil into will do.

You can also try using your sonic screwdriver, but remember if you make a wooden mold, it won't work.

Step 4: Casting the Rubber Mold

I used Smooth-Sil 940 since it's food safe. There's a few things you need to know about working with this material.

1. It comes in two separate parts. The mixing ratio is given on the instructions (unless it's changed, it should a 1:10 ratio by weight)
2. The pot life is 30 minutes. That means that once you mix the two parts together, it takes 30 minutes to solidify, so you need to work fairly quickly. Have everything you need (disposable cups, mixers, etc.) on hand so you aren't running around looking for stuff while it's solidifying.
3. Smooth-Sil can be quite messy, so make sure your workspace is covered in newspaper or something, that you have disposable gloves.
4. Smooth-Sil is really, really viscous, so don't be too surprised when you start mixing it.
5. When you mix the Smooth-Sil, it traps air bubbles which can make your final mold look, well, bubbly. That's why we need some kind of vacuum chamber so we can EXTERMINATE those bubbles.

That means you have 30 minutes from the time you mix the Smooth-Sil to pour it into your wax mold and remove air bubbles. The whole process from when I mixed the two parts together took me 20-25 minutes.

Smooth-Sil 940 comes with instructions on how to use it, and if you use a different material you should definitely check the instructions before using it! The most important things to know are the mixing ratio and the potlife. This is what I did:

1. I measured out the two parts into two separate cups (used a scale since it's by weight), then mixed them together. Because A is super goopy, I poured B into the cup with A, rather than risk losing a lot of A from it sticking to the cup if I'd tried to pour it all into a third mixing cup.
2. Stir. There's supposed to be some fancy way of stirring the two together so you avoid trapping air bubbles, but because this stuff was so goopy, I decided not to worry about it and to just stir it as well as I could. If you don't have a way to pull a vaccuum on the mold, you might want to look more into it.
3. After mixing it, I put the cup in the vacuum degasser and held a vaccum for 3 minutes at 29mm Hg. Be careful here, because the stuff expands to around 3 times its volume - so keep an eye on it AND DON'T BLINK! I put a piece of seran wrap on the bottom so if it spilled over, there wouldn't be a mess. It won't be totally un-bubbled by the end, but you'll put it back in the chamber after you pour it anyway.
4. After that, I poured it into the mold. This Smooth Sil is really goopy you actually wind up pouring out more of a thin ribbon that folds on itself. This is fine, it can trap more air in when it folds on itself. Because of the air bubbles, I put it into the vacuum degasser again. By this point it was thick enough that some of bubbles got pulled to the top, but didn't pop. That's fine though, as long as they're out of the way of the actual molding surface.

Step 5: Curing & Baking the Mold

The Smooth-Sil 940 requires 24 hours curing at room temperature, and then baking at a higher temperature. The website says it should be cured at around 100C for 4 hours. The instructions that came with the box suggested heating it at 80C for 2 hours, and 100C for 1 hour. I wound up doing 200F (93C) for 2 hours and then raised the temperature to 225F (107C) and that worked fine. Anyway, time is all relative, it's just kinda this wibbley wobbley timey-wimey... stuff.

Getting the rubber mold out of the wax block can be a bit difficult. It wound up taking two of us, 4 popsicle sticks, and a scraper to pry it out. I was actually pretty surprised by the level of fidelity. The little wax shavings I hadn't been able to clean out about didn't pick up, and the text came out very crisp.

Step 6: Chocolate!

The best part of all, of course, is making the chocolate! I got some help with this from someone who actually knows how to cook and bake.

We used some regular melting chocolate from the grocery store. You'll want to temper the chocolate as well - something I'm not sure we did the best job of so take my instructions with a grain of salt. Tempering gives chocolate that nice shiny texture and keeps it from being all crumbly at the end. The way we did it was by melting most of the chocolate we had, and then, once it was melted, lowering the heat and adding in the last bit of unmelted chocolate and stirring the whole thing together until that melts as well. In theory, what happens is that the last bit of chocolate you added in is already tempered, and by adding it in to the now melted (and therefore untempered) chocolate, it contributes the crystalline structure of the tempered chocolate.

Unfortunately, we definitely needed more practice casting chocolate, because it came out a bit messy! As you can see in the first one light at the top broke off because we were impatient and tried to take it out of the mold while the chocolate was still soft. (It didn't happen with the other ones that we let cool in the fridge for a longer time, so try to be patient).

Lastly, remember not to eat too many of them at once - they're bigger on the inside!