Instructables
Picture of 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.
 
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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!

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.

You're scary welcome artdollist.

jduffy542 years ago
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.

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
+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!!
flogy40313 years ago
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 :-)

A thin brush works, too. If you wipe off the excess goop straight after you've finished painting your part you can use it quite a few times. After the mixture sets you can also get the goop out by running a metal trowel over it a few times.

tvane12253 years ago
How well does this process actually work. will the silicone dry all the way through even if it is thicker than 1/4 inches?

Cornflour/starch will make the mixture set all the way through. The more you use the faster the silicone will set. About half to one part of the starch/flour to one part silicone is about right. Any more and the stuff will set while you are mixing it.

Lonecoon (author)  tvane12253 years ago
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.
danzo3212 years ago
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.

Chalk and plaster of paris and gypsum will fast set your silicone. Don't use them. If you use colorants try minerals that do not interfere with setting time.

sdomph danzo3212 years ago
Can you use side walk chalk that is for kids? Also to make homemade chalk it's craft paint and grout powder. Can these be used?
jdunne5253 years ago
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.

You are missing the essential cornstarch. This sets the mixture all the way through, not just where it is in contact with the air, or close to it. You need half to one part cornstarch to each part of silicone, optional drops of paint, optional drops of glycerin and one to two parts of mineral spirits or paint thinner. You just used the wrong formula. Try again.

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 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....
__lane__1 month ago

Hi,

I live outside the US, and I was wondering if I could use something else instead of the Mineral Spirits, since you can't bring it aborad and there's no exact equivalent here in my country (I've searched the internet). So, any suggestions for the thinning the silicone part? Thanks in advance

Aragh. So many variations. Equal parts silicone and corn starch and 1 part glycerine works?

arick2 months ago

Is the mixed between silicone and spirit dangerous ?

Lonecoon (author)  arick2 months ago

Not at all. It smells bad, but it's not dangerous.

arick2 months ago

Can I use soap for mould release ? thanks

crashaire3 years ago
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?

Usually with things with a lot of detail you want to mix up a couple of thin batches first and brush it on with a chip brush to make sure to get into all the details. The more layers the more detail you will grab out of the finished item. Just thin the mix out with a little more spirits. Then go in with a thicker top coat. And let it dry some between layers so you don't mess up the layers before hand. And if molding larger objects be sure to add in a layer of cheesecloth or burlap to give the mold some added strength. Just lay down a layer of silicone and put the fabric pieces down and use a brush to press them into the silicone.

Lonecoon (author)  crashaire3 years ago
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.
jsrubianoch10 months ago

Hi I have a few questions, is this food safe? What are mineral spirits? Can this be made without the spirits?

No! The silicone itself can bleed off some nasty stuff to food. you will need to look into food safe silicone for that.
mineral spirits are used most as brush cleaner. you can get away without it, but it will result in a thicker casting material
cherbert19 months ago
so how well does this work for molding something like an ear? can i go so far as to use it like body double and do a face cast? will it burn the skin? how safe is it in that respect? thanks guys..
I would go with algenate for life casting. Its a little more expensive, buy not much. it's 100% safe to use on the body. its actually the same stuff dentists use for dental molds.
padbravo1 year ago
Sorry for asking... but, what are "MINERAL SPIRITS" ???

I does not live on USA, nor in english-spoken country (I live on latin america)... so... what is this? what is for? its a paint thiner?

Mineral spirits is a petroleum solvent product that is a mid quality thinner for oil based paints. Cheaper is "paint thinner." Tops is turpentine.

Tks for the data... I learned a new phrase...

Inresca10 months ago

CAn i use this to make chocolate and that kind of stuff?

Personally, i don't use mineral spirits or turpentine or paint thinners like some suggest with silicone and cornflour/cornstach mixes. I use a little vinegar to thin the mix. Works great, and isn't bad to breathe.
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