Have you ever licked a cactus? Probably not. But it might be interesting to try it out, in another form. Time to create some special sensations for our tongues! In this instructable I'll make a cactus-inspired popsicle and demonstrate the two-part silicone casting process.
Here's an instructable that introduces you to mold making basics. Making a two-part mold has versatile applications and is beginner friendly. Here I used a combination of one-part and two-part mold-making techniques.
Step 1: Model & 3D Print the Original
I modeled the cactus shape in Rhino, exported as .stl file, and got it printed on an Objet 3d printer. If you don't have access to a 3d printer, you can always use online 3d printing services like Shapeways and Materialise.
As you can see the model is very symmetrical, with a clear front and back (where the spikes are). So for our two-part mold the parting line will be the center line between the front and the back. If you're working with a more complex geometry, try to mark your parting line so that it avoids delicate details. Note that the parting line doesn't need to be on a flat plane.
Step 2: Silicone Casting Materials & Tools
Smooth-on offers a wide range of silicone products, but for food safe applications there're three kinds: Smooth-Sil® 940 , The Sorta Clear® Series and the Equinox® Series. Out of the three only the Sorta Clear® Series is translucent (as its name suggests, it's sort of clear), which is very helpful when model visibility is important. Here I'm using Sorta Clear 40, the hardest in the Sorta Clear® Series. The mix of A (yellow bucket) and B (blue bottle) is 100:10 by weight. You'll need a digital scale that has at least one digit after zero.
b. Mixing tools
Prepare a lot of mixing tools including paper/plastic cups and mixing sticks (i used disposable spoons). Even if you know that you'll mix a lot of material, it's better to do it in small batches. This is to ensure thorough mixing and easy handling.
c. Mold release
I prepared both food grade silicon mold release and corn starch. Both of them failed unfortunately (well it's not the end of the world), but I would recommend vaseline and wax-based mold release.
d. Molding Clay
Plasticine is most often used in making silicone mold. Here I used white sculpey. It's more expensive than plasticine but it's food safe. Note that food safe is not crucial here since the clay will only be in short contact with your silicone mold and you can always clean your silicone mold afterwards.
Whatever clay you use, it need to be oil-based non-sulfur clay (sulfur reacts with silicone).
e. Vacuum chamber
The vacuum chamber is used to remove the air from high viscosity materials like our Sorta Clear 40 silicone, so that we can end up with bubble free castings.
Working with silicone can be messy. Wear gloves and protect your working surface with butcher paper.
Step 3: Prepare for Casting
First brush a little bit of corn starch to the surface of the original so that it won't stick to the clay. (Corn starch and spray-on silicone mold release both works for this purpose)
Plan your pour spout (where you'll pour your actual casting material into the mold) beforehand. Here it's on the bottom of the original. You can use a small piece of sculpey (either soft or hardened) and stick to the bottom of the original.
Prepare a mold box (doesn't need to be fancy, here it's laser-cut acrylic sheets). Start laying out clay on the bottom of the box, put the original in, secure the clay for pour spout, and just keep putting clay down until you have a smooth surface along the parting line. Lastly, apply mold release.
Step 4: Embed Registration Keys
To make sure the two parts silicone mold align perfectly, we'll make a few registration keys. There're many different ways to do this. You can use a stick to make some holes on the clay, or draw a dented line around the original, or embed some metal dowels in the clay.
Step 5: Silicone Mixing & Degassing
Always mix in small batches to make sure you stir thoroughly. When not mixed properly the silicone will not cure, which wastes your material and precious time. Here I did 4 small batches at the same time.
One very important thing here, make sure your silicone mixture doesn't go higher than 1/3 the height of your mixing cup. This is because during degassing the silicone mixture will rise up very quickly, and if you don't leave enough room for it, the silicone will come out of the mixing cups.
Put all mixing cups with silicone (mixed already) into a vacuum chamber. Start the pump and you'll see the surface of silicone mixture rise up with a ton of bubbles. Keep observing the bubbles: they will rise to a very high point, then start to go down very slowly. When the bubbles become bigger and the height no longer changes, it means the degassing is successful. Note that at this point you can still see a lot of bubbles and slow breathing motions from the silicone mixture.
Slowly open the air vent on the vacuum chamber. You'll see the bubbles gradually disappear and in the end the silicone mixture is clear with no visible bubbles. Otherwise put them back and continue degassing.
Step 6: Pour the First Half of the Mold
Now we're ready to pour silicone!
Tilt your mixing cup and let the silicone flow down. Don't rush this process or stir it violently, because you'll get air bubbles in the silicone again. Pour at one corner of the casting box and let the silicone flow to other parts of the mold box naturally. This is also to avoid adding more air bubbles to the silicone.
For the last bit of silicone left in the mixing cup, you can use a stick or a spoon to get it out, and you'll find more air bubbles getting in because of your scooping motion. Put your mold box back in the vacuum chamber to get rid of the new air bubbles. In the end you should see a very consistent, clear and bubble-free silicone mixture in your mold box.
Note that it's ok if you still have some small bubbles left in the end. During the curing process, those 10% small bubbles will disappear magically. The Sorta Clear 40 silicone cures in 16 hours at room temperature. However if you have a kiln/oven, you can cure the silicone much much faster at a low heat (150 degress F).
Step 7: Pour the Second Half of the Mold
Once the first half of the mold is cured, disassemble the mold box and clean the clay off the silicone. Please make sure that you don't move your original during this process. Put the first half of the mold with the original back into your mold box, and apply mold release.
Mix, degas, and pour in the second half of the silicone into the mold box. Repeat the process in the last step.
When the second half cured, disassemble the mold box and you should be able to get out a two-part silicone mold that aligns perfectly through the registration keys. However if your mold release fails (which happened to me), you will end up with a big block of silicone because the two parts stick to each other.
It's not ideal but it's not the end of the world. You can still cut open your "one-part" mold open carefully with a scalpel (you'll need another person to help hold your mold still and pull the mold apart along the cutting line). Make sure the cutting line follows where you want to part the mold, and also try not to cut straight all the way. A curvy parting surface will help the two mold parts align better.
Step 8: Popsicle Casting
Now we finally have our two-part silicone mold ready. Time to cast some popsicles!
Clean the mold thoroughly and wipe it dry with paper towel. Align the two parts and use some pressure on your hands to stick them together. Secure the mold with rubber bands. Mix whatever recipe you want for your popsicle, however low sugar high water percentage works better in holding the details (spikes) in this case. Tap the mold during and after pouring, and make sure no air bubbles are trapped in the spikes. Here you'll see the benefit of having a translucent mold because you'll clearly see where the air bubbles are.
I lasercutted a small acrylic piece to hold the popsicle stick in place. Put the whole thing in a freezer, and after 24 hours, the spiky cactus popsicle is ready!
Step 9: Dangerous Popsicles Party
Now we have our delicious dangerous spiky popsicle! Lick the spikes!
Surely you can get creative with the shapes and flavors. I actually made two families of dangerous popsicles, the cacti family and the viruses family. Would you like an HIV popsicle?