Using Silicone Caulking to Make Molds

This Instructable should be used in conjunction with other silicone molding instructables.

Let's face it: molding is an expensive hobby. Materials aren't cheap, tools are expensive, and it takes time to make good molds. It's worth it though to see your army of mass produced thing-a-ma-jigs.

But let's say you only need to make one casting. Do you just want to try this hobby?, Do you really want to track down specialty silicone from obscure vendors in your area? Is it even available?

Here's how to make your own molding silicone from what you can find at your local big box store.

You will need the following materials:

100% Silicone caulking
Liquid Glycerine
Acrylic paint
Mineral Spirits
Plastic Cups and Silverware
Mold Release (Also Optional, but highly recommended)

Step 1 Gather your materials

Let's take a trip to our local box store, shall we? Grab a shopping basket and pick up the following items:

100% Silicone caulking
Cost: $3-$5
Caulking is available in all types and brands, but what you need is simply 100% Silicone caulking. Any brand will do, so buy whatever on sale. I prefer the white kind rather than clear, as it's easier to tell when it's thoroughly mixed. This forms the bulk of your mold. Get a caulking gun too, if you don't have one already.

Liquid Glycerin
Cost: Usually $5-$6
Liquid Glycerin is available in the pharmacy as a either a skin protectant, or as a laxative. You may even be able to find it in prefilled syringes. Either way, may sure it's liquid glycerin you're getting, as the suppositories are completely useless to you. This is to add moisture to the mold and allow it to dry throughout. Without this, the molding doesn't work.

Acrylic paint
Cost: $2-$3
Any acrylic paint of any color will do. I use hobby paint because that's that I have lying around. You don't need much of it, so buy as little as possible. This is used to give a consistent color during mixing.

Plastic Cups, Silverware, and Straws
Cost: $3-$5
I always use disposable plastic cups to mix my silicone. The silicone won't stick to the cup after it's cured, and you can throw it away when you're done. The straws are helpful if you don't have any pipettes or eyedroppers.

Mineral Spirits
Cost: $5-$6
 Mineral spirits thin out the silicone,, making ti easier to work with. While not strictly necessary,  I find that I get slightly better impressions with it, and it seems to cure faster. The downside is that mineral spirits combined with the acetic acid in the silicone cause some pretty noxious fumes. So do this outside.

Mold Release (also optional, but highly recommended)
Cost: $5-$6
This item may be harder to find in your area, and is not strictly necessary unless you're making two part molds.. It does make demolding a lot easier though. You can find it at nearly any craft store like Michael's or Hobby Lobby. This isn't used for making the silicone, it's for getting your molds to release once you're done.

Now that we've got our materials, let's head home and get cracking.

Comments

[Deth Becomes You]

Do not use Silicone II. If it has gotten too cold or even frozen it will never cure. Use only 100% Silicone I. Only use acrylic paint. Because it is water based it can actually help to get water into all of the silicone. The over the counter silicone cures because of the moisture in the air. The point of glycerin is to get moisture inside the areas the air is not touching because it is harder for the silicone to cure once the outside has cured. I suggest using a little corn starch. It really speeds up the cure and makes the silicone harder when it cures depending on how much you add to your mixture. I have gotten very firm 2 piece molds doing that. Good luck!

[danzo321]

Other good sources of pigment are mineral color, which masons put in mortar.. OK, you don't want a 20-lb sack of color? Try the chalk powder meant to go in snap-string line which is sold in hardware stores and comes out of a ketchup type squeeze bottle. Art stores may also have powdered pigment so your casting can be an exact hue.

[jdunne525]

I tried this recipe and my mold is still gooey 2 days later.. What am I missing? Here's what I did:
3 oz of silicone,
9 drops glycerin,
3 drops paint,
1.5oz mineral spirits.

I poured (scooped really) it into a wood frame about 1 inch tall, 2 inches wide, and 4 inches across. I also made a small glob (maybe half an ounce) off to the side which also hasn't cured yet.

I had it in my garage for the first day and I thought maybe it was too cold or humid, so I brought it inside and it still hasn't cured after another day.

[jdunne525]

5 days after making this mold, I tried pulling it out of of the frame to see how it did. It was a horrible mess of what seemed like the consistency of sour cream. I threw it out and mixed up another batch with a different recipe that I got from this site:
http://www.therpf.com/f9/can-you-use-silicone-caulk-mold-4634/

Here is what I mixed up:
2 oz of silicone (I had extra last time so I figured I only needed 2 ounces)
12 drops of glycerin
NO paint and NO mineral spirits.

I then stirred it for about 5 minutes (similar to the last time) and glopped it onto my parts (Again about 1 inch deep). After 24 hours I removed the mold from my wooden frame and found that it did much better than the first batch, but still hadn't cured all the way through. I gave it another 15 hours (just a couple minutes ago) and found that it was closer to cured, but STILL wasn't fully cured.
I think pulling it out of the wooden frame helped to get some air in there, but even if I had given it a couple more days I'm not sure if it would have fully cured.
I've now ruined the mold as I pulled it apart when it was still gooey and wrecked the surface where the part was.

I'm not sure what I did wrong. Lonecoon said he's made molds an inch thick with no problem. Maybe its the silicone I'm using?
Here is what I'm using:
GE Silicone II Kitchen and Bath White 100% Silicone (home depot)
Barr Oderless Mineral spirits (home depot)
Humco Glycerin Skin protectant (Meijer)
Generic Acrylic paint (Meijer)

I've been reading on these sites:
http://www.therpf.com/f9/can-you-use-silicone-caulk-mold-4634/
http://www.myheap.com/chapter-8-silicone-caulk-molds.html

They say NOT to use colored silicones and instead opt for clear, though they say the colored silicones DO work. Also they specifically call out GE Silicone II saying that it WILL work. I have seen some information saying that I need to get 100% Silicone RUBBER, not 100% Silicone. I guess I need to go back to the store and see if I can find that. I'm at a loss at this point. The only thing I can say is that most likely the mixture I made of Silicone and Glycerin WILL work as long as you do it in layers about 1/4" thick (boo).

[spikenwee23]

I believe your problem is related to the ingredients...specifically your use of Silicone II....you want to use 100% silicone I only. I have used this method numerous times...all without fail....

[Happy2Think]

Thanks, this is a really nice instructable, I will try it
to cast some items, and I have a question
Can this mold be used to cast poliester resins on it?

[crashaire]

Does anyone know if you could use these molds with plaster? for example if I was to make a mold of a sea shell and fill it with plaster for a plaster sea shell would that come out okay, or detailed enough, or is a shell too porous and/or the mold would not get all the little lines on the shell?

[Lonecoon]

I've gotten enough detail out of this to recast 25mm figures that were out of print, so a seashell should be no problem. The plaster will snap out easily, as it's just silicone.

[tvane1225]

How well does this process actually work. will the silicone dry all the way through even if it is thicker than 1/4 inches?

[Lonecoon]

I've made molds that are an inch thick with this recipe, with no problem. The glycerine is the key to making sure it dries all the way through.

[bac512]

please help me understand this.... You say hobby paint, make sure its acrylic, not latex.... well, your hobby paint is enamel, not acrylic.....???? Soooo....?????

[Lonecoon]

Actually the only thing that matters is that it's not latex. Further experimentation proved this.

[bac512]

ok, cool, cause I went to the hobby store, picked up a .5 oz (?) little bottle of Testor's paint for $1.49, then walking to the register, noticed a bin full of 2 oz bottles that said 'Acrylic paint', so I looked at the Testor's... 'Enamel'... 3 for $1.00 So, I immediately put the $1.49 stuff down, and grabbed the $0.33 stuff :) then came home and figured I'd better clarify before using it :)
(so, just FYI people, Acrylic paint is much cheaper :) )

[lperkins]

Acrylic vs latex is what it's made of. Enamel vs lacquer is the kind of coating it makes. You don't want latex because it won't play nicely with the silicone. The kind of coating is most likely irrelevant for what's being done with it.

[bac512]

what is the paint for?

[Lonecoon]

The paint is to ensure that you get a proper mixture of your ingredients. Without the paint, It's much harder to tell if everything is properly mixed.

[bac512]

so basically, it's just for coloring... when it's all a uniform color, you know it's mixed well?

[flogy4031]

nice technique ,but what about trapped air bubbles?... if you try to replicate a fine texture it would be better to spread by hand a thin layer of transparent (to be able to see eventual air bubbles) silicone caulking, also keep your hands wet (dip in soapy water) so the silicone doesn't stick to your precious fingers :-)

[rimar2000]

Definitely I will try this! Thanks.