Making Better 100% Silicone Caulk Molds

Silicone Caulk Molds for Plaster Casting

I’ve been having a lot of fun painting up the flat cat head magnets, but I’ve finally run out of plaster blanks… so it was time to cast some more!

But when I went to make them I realised that I only had one mold and it had some minor issues. The plaster and wood glue mixture that I use takes an overnight to cure, so that means I can make one cat head a day. Add to that the cure and prep time and I’m looking at a lot of waiting for not a lot of cat…

The weather has been great in Virginia recently, so it was time to try my hand at making a new batch of 100% silicone caulk molds! ๐Ÿ™‚

(For my previous attempts, check out the Materials: 100% Silicone Caulk tag.)

TL;DR: Add the caulk starting on one edge of the box and moving to the other to reduce air pockets. Then use another box to firmly press the caulk flat and force it against the piece.

Gathering Materials

The materials I used this time are the same as the last few attempts. Right now I’m trying to find the best method to use with this setup. I’ve had a lot of good comments on the other posts, so I’m planning on trying out new mixtures later this year!

DO NOT DO THIS INSIDE! It’s incredibly stinky and you get water and soap everywhere. They aren’t kidding when they say to use caulk in a ‘well ventilated area’, this is an outdoor sort of craft! (In fact you will need to leave the new molds outside for several days to finish off-gassing.)

Silicone Caulk Mold Making Trying New Methods
A Sunny Day and a Bit of Art!

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  • Item to cast – This needs to be an object with a flat back.
  • Glue – to temporarily glue the object to the plastic box. (I used Loctite Gel.)
  • Two stackable plastic boxes that are larger at the top than the bottom – I used Pink Square Divider Trays By Recollections.
  • Bucket/Plastic container – needs to be deep enough to knead the caulk underwater.
  • Water
  • Cheap Dish Soap – grab the dollar store stuff, we’re going to use a lot!
  • 100% Silicone Caulk – I normally use GE Silicone I 10.1-oz Clear Silicone Caulk.
  • Scissors/Box Cutter – to open the tube.
  • Long Nail – to puncture the tube.
  • Caulk Gun – to squish out the caulk! Like this one.
  • Disposable Gloves – these are sort-of optional and I don’t always use them, but you really should for safety!
Use GE Silicone I not GE Silicone II
Red = Bad, Blue = Good… just like the Force!

Note: Do not use GE Silicone II because it will not set properly! I’ve always used the clear version of Silicone I, but Lowe’s was out and the white turned out to work perfectly as a substitute.

Note II: Test out the superglue on your box with an object of similar material that you can afford to lose. What you want is something that the superglue will stick too, but that will pop free after making the mold.

Prepping the Mold Box

Once of the problems I had with silicone caulk molds was that I’d end up with gaps around the edges. The resulting plaster casts are fixable, but it’s annoying to have to clean up. I want to end up with mold where there are no gaps around the edges and where the surface isn’t too rough.

100% Silicone Caulk Mold Gaps Around Edges
Not pretty, but fixable!

I’ve had luck before in squishing the silicone into a confined area so that it’s forced up against the sides of the object. I tried a few things that didn’t work (more on that later), but the best result was using a second box to compress down the silicone.

Preparing the Mold Box for 100% Silicone Caulk Mold Making
Time to make some cats!

I am using a Super Sculpey item and super gluing it to the bottom of the plastic box. Any plastic container will do, it just needs to be sturdy and slightly larger that your object. You can always trim down the mold later and the caulk is cheap, so it’s better to work big!

I have had no problems with the sculpture not popping free after the bath in soapy water. The super glue will build up a bit on the bottom, so you will need to scrape or sand it off after a few uses.

Kneading the 100% Silicone Caulk

Fill your bucket/tub with water and then add a large amount of dish soap– until the water is a slightly tinted and feels a bit slimy. The soap is what will keep it from sticking to your hands/gloves, so make sure you rub some on them before you start working.

Dish Soap and Water for Silicone Caulk Mold Making
Green and goopy!

Cut off the tip of the caulk tube further up than normal so that you can squish out a bunch at once. I don’t clip mine all the way back because I was planning on using it more than once and I needed to be able to cut further up after it solidified at the tip. You then need to puncture the tube with a nail (or the tool on the caulk gun, if you have a fancy one.)

How to Open a Silicone Caulk Tube
(Not Rocket Science, I Promise!)

…I mention this because I saw a negative review online from someone who didn’t realise you have to poke it open. *sighs*

Then put the tip of the tube under the water and squish away!

For my project I ended up using roughly half of the tube for each mold, with a little left over. You’ll need to experiment a bit to find out how much you need.

Since it’s exposure to the water and soap that trigger the caulk to set, you want to make sure as much of the material comes in contact with it as possible. So thin and fold and thin and fold.

Or knead it like bread or… other things that require kneading. (I’m not much of a cook, sorry!)

You want to work with the caulk until it starts feeling like soft clay and less like toothpaste. Unlike push molds you don’t want to get it firmer like playdough, since we want it soft enough to squish into the nooks and crannies.

Creating the Mold

Now that you have the caulk ready to go, smooth the surface by running your hands over it under the water. This will help brush off any undissolved soap and smooth out any cracks.

Sidenote: Undissolved soap will make some interesting patterns in the mold, so if you are going for something abstract, give it a try! ๐Ÿ™‚

Soapy Texture versus Smooth Texture
Soapy Texture versus Smooth Texture

Starting from one edge of the box, lay the caulk down like you would unroll a pie crust, pushing down slightly as you go. Moving from one side to the other will help get rid of the larger air pockets.

Silicone Caulk Mold Made in a Plastic Box

Then use the other box to firmly press down and even things out. You want to have enough caulk that doing this will force some of it up the edges a bit. If the caulk won’t seep up the edges, it was kneaded too long and you may need to try again.

Let things sit before you attempt to demold — for about 30 minutes or so, if you’re in a rush, or an hour if you aren’t. The easy way to tell if it is ‘done’ is to poke it with a toothpick. If it springs back instantly it’s done, if the toothpick gets stuck, then it’s not.

100% Silicone Caulk Mold Making Experiments End Results

You’ll need to trim the edges of the back of the mold so that it sits flat. You will also need to trim any areas where the caulk seeped under the mold. For my molds, I ended up with some overhang on the ears I had to fix.

I also ‘fixed’ the center bottom mold by adding caulk to the back of the bad mold you saw earlier. I had to trim a lot of material off in order to make it plaster friendly, but I could have used it as-is in the picture with resin.

Leave the finished molds somewhere outside to dry for at least a three days before you try and use them. While they are outgassing they will stink of vinegar and will cause issues with the plaster casts.

Silicone Mold Fails

For your amusement value, I figured I’d include some of the failures! ๐Ÿ™‚

All of these molds turned out to be usable, the plaster casts just needed a little more cleanup than the best ones.

Mold Making Fail #1

I tried was using a smaller piece of caulk and trying to start in the middle and push it down around the object the way I would normally make the molds. Then I went back and added a second layer immediately to try and take advantage of the confined area idea (aka: use the walls to force it to squish into the sides).

Silicone Caulk Mold made in a Box - Take 1
Take 1… FAIL

Didn’t work so well.

As you can see the caulk stuck to itself quite nicely, and it popped out of the mold box with no issues… but I still had some gaps around the object. I was also using my fingers and not the second box to squish things flat, so the back was very rough.

Mold Making Fail #2

Then I went in with a big handful of caulk. I started on one end of the box and slowly unrolled it, hopefully pushing out the air bubbles and filling in the gaps as I went. I also flipped the box over (since this was a different box and the plastic was translucent) and tried to make sure that I squished it in next to the sculpture as much as possible.

Silicone Caulk Mold made in a Box - Take 2
Take 2… Slightly Better FAIL

Unfortunately this was also a failure.

As you can see I did get up to the edges a bit better in places (even got some caulk in the small gap between the sculpture and the base), but I still didn’t get it perfect.

Mold Making Fail #3

This one was just for fun. I has some leftover silicone and wanted to use it up, so I figured I’d try making an old-style mold… which resulted in the fail you saw earlier.

100% Silicone Caulk Mold old style attempt
Round and round we squish…

I started with a ball of caulk that I pressed down over the cat. Then I went back around the edges, trying to carefully press it in on all of the edges. It looked like it worked… but no.

Even though the caulk is see-through, it’s not enough to tell you if you’d gotten all the edges or not. (Alas)

Martha Bechtel

My name is Martha Bechtel and I write fantasy and science fiction stories, paint small model horses silly colors, cast resin and plaster magnets, code random code (and Wordpress plugins)... Come on in and join in the fun!