[Solved!] Silicone just isn't curing...

[Evil Eye]

UPDATE: I have actually solved the problem, and as such am going to turn this into more of an informational post/tutorial than a question. Original post in the spoiler below.

Spoiler

Against my better judgement, I decided it would be a good idea to try getting back into casting using silicone (of my own designs and use, before anyone asks). Notably I had the idea that I could design models with transparent elements (canopies, crystals, xenomorph-style domes etc) and print the whole lot in opaque resin, then polish and make moulds of the parts I want clear and cast them in clear epoxy.

To that end I bought some Siraya Tech "Defiant" Silicone; a very well-reviewed platinum-cure silicone (mixed in a simple 1:1 ratio) that works with 3D resin. Before I comitted to casting anything printed though I thought I'd practice with a plastic WHFB forest goblin and make some simple cut-open cup moulds to practice. Unfortunately, I've been having difficulties with the moulds; they cure most of the way, but I'm now on my second mould that nearly 24 hours later still has some uncured, runny bits.

 

Given that this stuff (according to reviews) can't even be cure-inhibited by usually difficult to mould materials like UV resin, I suspect the problem is improper mixing. If it IS just this, I might have a solution (mixing a small amount of pigment into one half, or different colours into each, and then mixing until I get a solid colour to confirm the parts are well and truly combined). However, is there anything else that might be to blame?

So as it turns out, my problem was simply down to improper mixing, but I was able to devise a very simple and reliable solution to avoid this in future. As such, I highly recommend anyone making moulds from platinum-cure silicone to use the following method to ensure proper mixing (mostly helpful if your silicone is colourless- if it's already a two-colour silicone then this step is unnecessary).

 

First pour both parts into separate containers, following usual wisdom (slow pour to minimize air bubbles, getting the ratio as close to equal parts as possible). Then, take some silicone-safe colouring agent- I recommend mica powder as it is incredibly cheap, perfectly safe for your silicone (it's often used as a colourant in hobbyist-made silicone products such as colour-coded moulds and prosthetics) and available in many colours, often in large multi-packs. Select two colours that directly oppose each other and have an easily recognizable colour when mixed. For example, I used blue and yellow as these are almost perfect opposites and make green when mixed. Add a small amount of pigment to each part- so blue to part A and yellow to part B in my case. You don't need very much. Make sure the pigment is properly folded through the silicone using a stirrer of some form, making sure to do so slowly and methodically, again to minimize air bubbles. When each part is consistently pigmented, and you're ready to make your mould, combine the two parts (one trick to ensure less waste is to pour part A into part B's vessel, mix and then pour this into part A's original vessel). As always, mix methodically; when the silicone is a consistent colour (in my case green) with no streaks, marbling or irregularities in colour- so no bits that are more blue or yellow than the rest of the mix- your silicone is properly combined and ready to make your mould from! Pour, making sure to do so slowly and carefully to avoid bubbles, and once fully poured leave to set. You can of course use a vacuum chamber or pressure pot to ensure no air bubbles, though depending on what you're moulding and how even-tempered or viscous your silicone is this may or may not be necessary- I don't have either and my moulds came out with some very small, harmless bubbles suspended in the mould itself, not affecting the actual cavity, and that was largely because I wasn't careful with mixing. Wait to cure, and if all has worked properly you should have nicely cured silicone moulds!

 

As a side note, by the way, I can HIGHLY recommend Siraya Defiant 25. It is, as advertised, fully compatible with 3D resin- they recommend the underwater post-cure for parts but I found this wasn't necessary, though obviously it can't hurt. Certainly, the part I moulded from caused zero issues at all. The silicone itself is very well-behaved, not too viscous and when properly mixed sets to a very good consistency- durable and firm but stretchy and yielding enough to easily demould complex parts, and easy to cut with a sharp hobby knife in the case of one-part moulds. Not massively expensive as silicone goes either, not "no-name cheap craft silicone from Amazon" cheap but much cheaper than Smooth-On's products. The worst thing I can say about it is it's colourless out the bottle which can make mixing a bit tricky if you aren't a seasoned mouldmaker and don't use the pigment trick.

 

One note I will add- it's not suitable for casting metal in, so if you're planning on making metal casts from 3D masters then you will need to cast an epoxy resin copy in the mould and then make a new one from heat-resistant rubber.

Edited 11 hours ago by Evil Eye