I never liked candles/soaps/other disposables shaped like actual things, because you can’t use them without ruining them. I dunno, maybe it’s me. The urge to use it as intended causes dissonance with the urge to not destroy this pretty flower/figurine/whatever that someone gave me.
You can buy the silicon mold (some ebay listings say it’s for ice cubes, some for crayons) and make them yourself! I gather all the bits from my classroom and melt them into minifigs a couple times a year. You can also buy the molds for 6x2 Lego blocks, or giant minifigs. 
Could you 3d-print a mold from a printable elastomer? What about an extruder that’d use molten crayons?
You rule. This is going to be a weekend project for my kid and I.
Could you use silicone caulking (with a separator) and make your own moulds from common objects laying around?
I like your idea about 3d Printing a mould, but I wonder if it’ll be too rough…
It will be. The solutions are e.g. (but not limited to):
- mechanical smoothing
- acetone vapor polishing
- nothing, the layers “add character”.
I think buying the silicon moulds will be much simpler. The 3D printed ones don’t seem reusable or particularly easy to peel away from the crayon. Getting them slick enough to remove melted wax? That’d be hard.
It’d be fun to do. Melting the crayons down and pouring/cooling them should be interesting enough for the child and the adult.
If you can live with the lack of customizability and the limited options available (which is a given if the objective is trying out casting, and making a specific shape is a means instead of the end), definitely yes.
Depends on the material.
Silicone caulking does a good job in making a slick surface, and adheres pretty well to many materials. I found that by accident, when I was wiping off fingers and tools onto a piece of paper. The result was a siliconized surface that was smooth, hydrophobic, and surprisingly robust. If you have some on hand, give it 5 minutes and smear some into selected scrap materials (wood, paper, cloth, old 3d-printout) and see the results.
Edit: Another thought: there are off-the-shelf mould-casting rubber compounds. These could be likely used for making the moulds from existing objects. A 3d-printed object then could be multiplied by casting, by first using it for casting a rubber mould around it, then using the mould for further castings from wax or resin or maybe even lower-melting alloys. (Then there’s the option of printing from PLA, and going the lost-“wax” casting from aluminium or even cast iron, but that is a bit higher-end and not even I tried it yet.)
Absolutely no contest here!
Epoxy casting is also fun. Some years ago I was helping with some artefacts at some themed camp, and ended up making, inter alia, a simplified king’s seal. I used a chunk of plasticine, imprinted an end of something round to get a base surface/shape, then imprinted letters and a crown with a toothpick, then cast in about a cubic centimeter of epoxy (with a piece of wire as a handle). It worked pretty well with sealing wax.
Then there comes the Sn-Pb alloy that casts pretty well, or the Wood’s metal (and other fusible alloy families, some of which may be objectionable due to cadmium (and others due to the insane cost of indium), which is volatile so I would advise against open-flame melting and use hot water instead).
But yes, wax is a more than sufficient starting material of low cost, low risk, high usefulness, and instant satisfaction. And there are all those color-mixing options, to add to the fun. (Try pouring two colors simultaneously and then swirl the melt with a toothpick, to yield some nice swirly patterns.)
I’m a little… concerned, about these robot crayons.
Heh! When I was maybe five I got a tiger-shaped “soap on a rope.” Kept it for a month, maybe longer, before I gave in and used it as soap!
Aye, the same goes with sculpted food and cakes. Why should my satiety be prized over a work of art that someone spent hours creating?
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