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- PLA as a material is at least food safe (though food safe and body safe aren't the same). But! The material passes through nozzles which probably have lead in them etc. even if that's of negligible consequence though the main issue with making something body safe is the layer lines, which retain moisture and breed bacteria.

- yes, concave surfaces are dang near impossible to smooth.

- plenty of household release agents don't work in a silicone to silicone mould, or impede the cure of platinum cure silicone (the body safe kind), and if you bugger it up you're just wasted the mould and the dildo's worth of silicone which is $$$.

- I just used PLA. the beeswax doesn't touch it for long, you literally pour it in and immediately back out. Because the print is hollow on the inside and because beeswax has such a low specific heat capacity it cools instantly, and this all works, my pics are proof.

- silicone doesn't bond to PLA, no, but you need to smooth the layer lines, which brings us back to point 1....

Thank you for replying in such useful detail!

I've been orbiting this array of technologies lately - 3d printing + silicone, and also epoxy resin - so this is all really useful info for me. Your comment about having to figure out working processes by trial and error rings true. I'll be sure to bear beeswax in mind - maybe it'll work some magic on epoxy as well.

In the spirit of reciprocal knowledge sharing, I've been using filler primer spray or smoothing prints. You're probably aware of it already, but it seems very underrated in the 3d printing community. Sands to a mirror finish amazingly quickly and easily, especially under running water. Obviously that's not a body safe surface, and I don't know what effect it would have on silicone curing, but still - one worth keeping in the toolbox.

Which comment about trial and error sorry? I honestly rarely do the same thing twice... ADHD and all. lol

Yes, filler primer is great and I recommend it often. I think I actually mention it in the article? It's just a LOT more work than pouring some wax in and out of a mould.

>And there's so little info out there about the topic! What materials are safe/work? So many processes to master to get a good result, such expensive supplies to be dabbling with! Urgh! Not worth it, right?

Ah, it seems I projected the "trial and error" part. I certainly don't often get it right first try! I would never have guessed about dish soap preventing curing, for instance. So easy for a project to end prematurely in a sticky mess...

I wonder whether warming the mold up, e.g. immersed in hot water, would make it easier to get apart.

> Why would you not just print a negative directly - as indeed she later does? Is it because she considers the concave surface difficult to sand smooth?

I think the problem with sanding a mold is that it would be very hard to sand both halves equally, so you'd end up with a step where the two halves meet.

Using the wax to smooth the mold when the two halves are already stuck together is quite ingenious, as the wax will also smooth the part where the halves meet.

I wouldn't fear a PLA thing first use; but cleaning it thoroughly enough to re-use is going to be a challenge without special effort applied to finishing it after printing.

Beeswax temp: Thermal mass of the print is probably high enough that it sucks the heat out of the wax before melting much surface feature. in this case, smoothing bumps is beneficial.

Beeswax melts at about 65 degrees centigrade. Going higher than that discolours it and if you go very hot, when it sets it cracks. These things would seem unlikely to be an issue when using it more like paint.

> PLA (the most common 3d printing material) is nontoxic and used for utensils

Utensils are (generally) not inserted. however more crucially in this respect, depending on the mix, silicon is more compliant. You want to have a bit of give. (I know I know, glass dildos exist, but thats a specific, advanced usage)

> […] presumably because silicone bonds to itself […]

Silicone absolutely loves silicone. If you pour silicone into a silicone mould you end up with one piece of silicone — which now includes the mould. So a barrier is absolutely essential.

Yes, that's why I typed that.

The question is, why make a silicone mould in the first place?

I guess silicone is a good material to make a seamless mould of a smooth object. Since it's flexible you can remove it from the original object without cutting it.

this is the reason.

Even after it's been set or cured, two silicone things will kind of melt each other. It can be disturbing.