I attended a Shapecrete workshop with MakeICT and cast stuff out of silicone molds. Here is what I did.
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Shapecrete is some wonderful stuff. At MakeICT we did an awesome workshop using Shapecrete. I brought down my geeky Silicon ice cube trays I bought for the occasion. I had done a little casting in college twenty years ago, but the other members down there were knowledge and helpful, so I was quite successful. lemme show you what I did.
Step 2: What You Will Need:
Shapecrete of course. More than you think you will need. Water, and a 5 gallon bucket to hold said water. 5 gallons seems a bit much, just fill your bucket up with enough water to dunk the mould in and then add 10% you should be good (or just do what I did and fill up the bucket with water.) Moulds of course I got mine from various places like Amazon or wish... one ships faster than the other of course. Pigment, this is if you want coloured shapecrete. a small scale. though this isn't critical, I very highly recommend this. Mixing cup, I used a large plastic QT cup. Something sturdy to stir with, I used a wooden paint stirrer, worked rather well. I think that's it.
Step 3: Mixing the 'crete
First you are going to weigh out your shapecrete. if you put any pigment with it that is a good time to mix that in. Above you see someone mixing in the yellow pigment. It is easier to get it mixed well as a powder. Next you add the water. For casting like we are doing you are going to want 1/6th the water (by weight) than you did of shapecrete. There is a big difference between by weight and by volume. When you mix the water in you are going to think that there isn't enough. There is. Get the nooks and crannies while stirring. Also, wear a dust mask. this gets dusty and reacts to water. you likely won't want this in your lungs. So just to recap... dustmask, crete, pigment, stir, water, stir stir stir.
Step 4: Pouring Your Mould
Once it is sufficiently mixed then you are ready to pour. Take a moment and dunk the mould in water. Completely submerged. then take some of the watery stuff and put it in, swish it into the crannies and nooks. Then carefully pour the rest in. The death star mould really needed a funnel, but careful pouring while you tap firmly on the side until the mould if full. In the case of the death star mould, it is going to lie to you. Keep tapping, keep pouring. Inside a cone shape forms periodically you should firmly tap the whole thing on the table to flatten it out. Once your mould is full, scrape off all the excess so that it is just a bit above the top of the mould.
Step 5: Waiting:P
Cover with plastic and put it out of the way. You can demould in 24 hours, I waited a week. This is the hardest part. Build something else like a Wordpress server or a flamethrowing coffee mug... don't think about it.
Step 6: Remove From Mould
if you didn't use a funnel like some people, it may be covered. I used a few taps from a multitool to break it up and get it open. Sillicone is pretty forgiving. the Tardis needed to be completly inverted before it would let go. The Xwing popped right out. Depends on the moulding.
Step 7: Enjoy!
You now have the ability to make concrete objects from silicone moulds! Enjoy!