Three Ways to Make Silicone Molds From Type 1 Silicone Caulk

This instructable is going to recall what i went through trying to create silicone resin molds from scratch. including failures!

Supplies

The caulking silicone im using, but other ones probably work too. This also implies using a caulking gun and nitrile gloves (do not use latex gloves, it reacts weirdly with silicone)

Now onto the main ingredients:

For type 1 we'll use cornstarch

For type 2 we'll use soapy water, so need a bowl with soapy water

For type 3 im going to do some experiments on solvents: spoiler, the one that works is white spirit but stick to the end to know more! (White spirit is Mineral Spirit for Americans)

Contact with solvents is not advised, work in a well ventilated space!

molds made with white spirit are NOT food safe

Step 1: Preparation (optionnal)

You are going to need an object to cast. Luckily i have a 3d printer and i wanted to test if i could cast silicone without it reacting with PLA (it didn't react) So i've used a calibration cube of 2x2x2cm with lego like studs.

I've used cut cardboard goblets and hot glue to secure everything in place, this will be a problem for the first two tries but we'll see later.

Step 2: First Try: Corn Starch

Okay so this recipe is relatively well known, you handmix 1 part cornstarch and 1 part silicone and this can get really messy, hence the gloves. For my try i didn't use enough corntarch it feels like, because it was still a bit sticky and hard to apply. Needless to say it wasn't a perfect mold

Step 3: Second Try: Soapy Water

Directly drop the silicone into a bowl of soapy water and mix it thoroughly with your hand inside the bowl, when you get a nice clump you can mix it in the open air and apply it like the first one.

Here the problem is that my molding guides were too narrow so i couldn't get the silicone to sit flush around the 2x2block, creating some bubbles in the mold.

In hindsight this technique is great but more for imprint molds or jacket molds.

Step 4: Third Try: Failure!

For my third try i've used Acetone to thin down the silicone to make it pourable. Sadly it didn't cure (you can see the two other molds are good after 48h but this one isn't.

Theory time!

Silicone caulking cures because of air moisture. Corn starch brings air moisture into the silicone, and for the second mold kneading it in water does the trick. But 100% acetone doesn't contain watery stuff so it doesn' work!

I have then tried another dillution with added acrylic paint, thinking it would help it get enough water percentage but it was a fail again.

I figured acetone was the wrong solvent.

Step 5: Fourth Try: Success!

The three most common solvent that aren't water are Alcohol,White spirit and Acetone. Since i had no Isopropyl around to test it out i used White Spirit, and using the same logic as before i added a bit of cornstarch instead.

To my surprise, this worked wonders, but it wasn't quite pourable, and i made bubbles.

I decided to retry it with White spirit, cornstarch and black acrylic paint. i mixed it to a thinner consistency but it was still thick, so since i had some leftover mix i added even more white spirit till it was honey like.

Incredibly enough, both cured perfectly in 48 hours!

Step 6: Conclusions:

The mold thinned down to the max was really floppy, not as strong as the other molds for the same thickness. (this means a larger mold would be prone to deformation if doine using this technique)

The details picked up are great, and the resin cured completely on all my molds exept the one with soapy water. (some leftover moisture was in it) i filled it with silica beads, waited another 48 hours and had a perfect resin cast.

Ranking these on different criteria:

toughness: flexibility: reusable:

Cornstarch 9/10 3/10 Kinda

soapy water 7/10 7/10 Yes

WS fudge consistency 5/10 7/10 Yes

WS honey consistency 3/10 10/10 Kinda

My Cornstarch mold was showing signs of wear after two casts, and the final mold may shrink or deform over time so if you're casting geometrically precise pieces be careful not to use too much white spirit!

My personnal Winner is the Third one because it's both tough and flexible and can actually be poured (granted you need to shake it a bit to level it and pop bubbles)