Using Silicone Caulking to Make Molds

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Introduction: 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)
UPDATE: Cornstarch helps molds to try faster.


UPDATE!: Thanks to JDUFFY54, he has provided the following helpful additions to this recipe.

JDUFFY54: I found that using 2 parts cornstarch, 2 parts silicone caulk, and one part mineral spirits works well, more than doubles the ammount of molding stuff you get per caulk tube, and dries compleatley in under an hour. Not to mention, cornstarch is cheaper than caulk, so half the mold is cheaper. I haven't done a lot with it yet, but I have made some test molds which turned out excellent.

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.

Step 2: Measuring

The following ratio is key to successfully molding with silicone caulking

1oz caulk : 3 drops glycerin : 1 drop paint : .5 oz mineral spirits

A standard tube of caulking is 10oz, so you should get that much molding material out of a single tube. The recipe for an entire tube would be:
10oz Silicone, 30 drops glycerin, 10 drops paint, 5 oz mineral spirits.

Measuring without a scale
A scale isn't strictly necessary to pull off this project, but it is helpful.

To measure silicone without a scale, cut the entire tip off your caulk, and prime it to the top. A standard caulking tube has 20cm of caulk in it, so make your lines at 2cm distances. I say centimeters because it's actually something like 8" and who the heck has 1/5 dividers on their ruler?

To measure mineral spirits, use a shot glass. Half a shot glass is 1oz.

Drops can be done with a straw. Dip your straw into the glycerin, then place your finger over the open end. Squeeze out your drops as necessary.


Step 3: Mixing and Setting

NOTE: The vinegar smell here is normal. Silicone caulking is mixed with acetic acid to help it cure. The fumes are harmless, but you will probably mix this outside.

To mix your molding material add the ingredients in the following order:

1. Measure out your Silicone into plastic cup (1oz)
2. Add appropriate amounts of Glycerin (3 drops)
3. Add appropriate amounts of Paint (1 Drop)
4. Add appropriate amounts of Mineral Spirits (.5oz)
5. Mix with plastic silverware until the paint is dispersed through the mixture with no white left. I like to use a knife, some prefer a spoon.
6. Scoop silicone into casting box. I say scoop becausethis material does not pour like molding silicone.  If you're trying to make a two part mold, you're going to have to add pressure to fill in the gaps.
7. Cure times depend on heat and moisture, but is generally 2-4 hours.

That's all there is to it! Follow the directions of some of the other casting instructables, and you should have a mold that you can make your own things with.

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89 Comments

I found that using 2 parts cornstarch, 2 parts silicone caulk, and one part mineral spirits works well, more than doubles the ammount of molding stuff you get per caulk tube, and dries compleatley in under an hour. Not to mention, cornstarch is cheaper than caulk, so half the mold is cheaper. I haven't done a lot with it yet, but I have made some test molds which turned out excellent.

10 replies

Are your ratios based on weight or volume?

How much mineral spirits do you put in a batch?

Is this substituting the liquid glycerine for the mineral spirits?

Sort of, you can use glycerine, instead or in addition to mineral spirits. Spirits has the advantage of being thinner, so less of it will make for a thinner silicone, however, if you add more than about a 2:1 silicone : spirits ratio, the mold will shrink when you're done. Glycerine does not suffer from this as much, if at all.

when i tried this it came out the consistency of floam (airy playdough). im not sure if this is right... i would like to know if this is the consistency that it is supposed to be.

Roughly. I've usually found that a mixture of two parts spirits, one part cornstarch, and one part caulk usually turns out very airy and light, whereas the mixture mentioned above turns out more like a giant wad of rubber bands. Make sure that it dries fully first

Update to this that I forgot to mention: you HAVE to use this type of mold quickly, you only get maybe a day's use before it starts to shrink. It will eventually end up about 2/3 to 1/2 the original size. Works fine if you just want to make something that looks nice, but not usable if it has to be a certain size.

+1 to this mix. I tried the other mix but it took all night to cure. This one takes about 10 minutes to start to solidify and is done in 30. It is a bit thick, so I put some in a plastic bag(like a frosting bag) and apply it to all the detailed areas before filling in the rest of the mold.

Oh, also, despite the fact that silicone cures with water, adding water to the mixture will NOT help it dry faster, in fact, in multiplies the drying time erroneously. I made some a few days ago with one part water, mineral spirits, cornstarch, and caulk. It still isn't even surface-hard after about 4 days.

Yeah, I also found that one part silicone, one part cornstarch, and two parts mineral spirits will pour pretty well (but needs to be mixed with a drill-dremel), but it takes about 6 hours to dry. I just made an axe mould with it. Unfortunatealy, since the mineral spirits evaporate, it is very foamy, and though it still gets detail perfectly, it is much lighter and does not work well to hold up large molds. If you use that method, for a large mold (more than two inches or so) make sure to reenforce with fiberglass (I use house air filters, since cheap ones run as little as 50 cents. Also, MAKE SURE TO USE BREATHING PROTECTION!!

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!

2 replies

Deth, thank you for your comment. I gave up on these molds because mine never cured. Now I know the reason, I used Silicone II. I'll give it another shot.

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.

5 replies

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).

You did the same thing I did; used Silicone II. If I hadn't read Deth becomes you's comment about not using it, I'd have never known why mine never cured. I'm about to try again using Silicone I.

I just bought my 100% silicone and got home to review this instructable. to my Horrors, I bought the Silicone II and didn't even realize it. For anyone reading this, the GE 2.8 Fl oz tubes have a very tiny "II", printed, only at the top right, nowhere else. Glad I didn't open them. My guess is the small tubes are only type II. I went for smaller tube thinking 'test' project, be frugal. Its 35 miles back in to town, but I bet the large tubes (that go in caulking guns) are the Type I. If this is true, I'd recommend updating the instructable to reflect Type 1 and LARGE size tube (only caulk gun size), so others don't make the same mistake.

SiliconeII_oops.jpg

I would use 2 parts cornstarch, 2 parts caulk, 1 part mineral spirits. I literally just finished making a few chess pieces from this (because its hilarous to see people expect marble and get rubber). It cures fully in less than an hour, AND doubles the ammout of material you get per caulk tube.

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....