10 easy steps to dirt cheap mold making

or

how I've learned to (ab)use caulking silicone through trial and error

This is a method I use for molding and casting small items that I make. It is very similar to the typical DIY 1-part / 2-part mold making process, but uses cheap caulking silicone for the mold instead of more expensive RTV silicone.

It is very similar to using brush-on silicone of various viscosities to form a kind of jacket and outer mold. It is fairly quick; the example mold I made for this howto took about two hours total. The mold needs to rest for a bit after, but should be usable the next day.

Materials required for the silicone:

• caulking silicone: the 100% Type 1 stuff
• white gas
• dishwashing soap

You'll need some other materials, these are common to most mold making. Details below.

The ten steps:

1. prepare the mold container: a box of lego and sulphur-free plasticine
2. lay out the pieces, the top has ALL the channel entry+exits. Minimise overhangs
3. cover it all with hairspray, the cheaper the better. Let it dry.
4. mix up thinned silicone, optionally tinted with a drop of white paint
5. cover all the mold parts with it. Use a stippling brush. Let it cure.
6. fill up the mold box with soap-and-water treated silicone. Let it cure.
7. flip the mold over in the box. Remove ALL plasticine.
8. repeat steps (3) through (6), inclusive.
9. remove the mold from the lego, pry apart, remove the master.
10. wash the mold in fresh warm soapy water. Let it dry!!!!

In slightly more detail:

1) prepare the mold container: a box of lego & plasticine

This is similar to using RTV silicone, and the internet has a lot of good info on this.
This is the 8-part series of videos that got me started. It's old, but gold.
UPDATE: Here is a post on dakkadakka that uses the same techniques, but uses Alumilite mold putty instead of silicone. And here is another blog post which does something very similar using Smoothcast silicone.

The box will contain our mold, and should be deep and wide enough to hold the item we want to mold, plus any channels through which the resin should enter the mold, with space around the outside for the mold walls to not get too thin.

I use sulphur-free plasticine. Not sure that it is necessary for this kind of silicone (it is required for RTV platinum silicone), but I haven't experimented with anything else.

Fill the box about halfway with plasticine.

2) lay out the pieces, the top has ALL the channel entry+exits. Minimise overhangs to prevent air pockets.

For this example I am using a plastic acorn that my kids were playing with. First I embed the object in the plasticine, then I form channels using bits of plastic from lollipops and cotton ear swabs that I cut to size.

There should be (at least) two channel entries at the top of the mold: one for the resin to enter the mold (where I have the white piece of plastic), and one (or more) for the resin to exit the mold. You want the resin to flow into each shape from the bottom. You do not want the resin to fill into a hole from which the air cannot escape.

The parting line is important: pay attention to getting the plasticine to form a 90-degree angle with the incident surface of the piece. The cleaner this is, the better your mold and castings will be.

Also add some key / registration marks: I use the end of a brush to push little dots in the empty space between the container walls, channels and the master. Don't make these too deep, we need to get the silicone in there.

3) cover it all with hairspray, the cheaper the better. Let it dry.

The hairspray acts as the silicone de-molding agent. Without it the silicone will stick to your master, and you may end up destroying it (or the mold!) when taking it out. You also don't want bits of it (paint layers, etc) sticking around in the mold.

Give it a good 2 or 3 sprays, and let it dry. We don't want the silicone to rub it off, instead the silicone should stay on top of the hairspray layer.

I'm using the cheapest & nastiest can of hairspray I could find at Ralph's.

4) mix up thinned silicone, optionally tinted with a drop of acrylic paint

The way I do this is by putting a squirt of silicone into a yoghurt container or something similar, then adding a squirt of Coleman white gas (anything containing naphtha should work) from an eye-dropper, then stirring with a mixing stick. This thins the silicone a bit. Don't overdo the naptha though. We don't want runny silicone, just nicely spreadable.

The optional paint (I use tungsten white) speeds up the curing process, due to the moisture it contains. With the paint, my thin layer cures in about 20 minutes. Without it the cure takes a bit longer, up to an hour. It also makes the mold surface easier to inspect, since the pure silicone caulk is transparent.

5) cover all the mold parts with it. Use a stippling brush. Let it cure.

Basically, brush the thinned silicone all over the master and the channels. The thinned silicone gets into all the details of the model, which is what we want. (You'll note I built the box up to its full height now).

You'll know the silicone has cured when it is touch dry, and when you can peel the remaining silicone from the mixing container easily.

6) fill up the mold box with soap-and-water treated silicone. Let it cure.

Take another yoghurt container, fill it halfway with water and add a good squirt or three of dishwashing liquid to it. Now squirt enough silicone into it to fill half the mold. Work it with your hands for a couple of minutes. Try to get the entire silicone surface exposed to the soapy water.

The soap will stop the silicone sticking to your hands. The silicone will cure throughout, and fairly fast, around 20 minutes, from contact with the water, which is why you should massage it in the water properly. In other words, work well it in the water, up to a minute or two. The longer you work it, the stiffer the silicone will become and the shorter the cure time will be.

Now take the silicone and work it into the mold box. I use a lego plate to smooth the surface of the silicone, since I want the surface to b flat, and the silicone won't run by itself.

7) flip the mold over in the box. Remove ALL plasticine.

Once cured, take the mold out, and remove all the plasticine. Try to not disturb the master and channels that should now be embedded in the cured silicone. If anything pops out, try to place it back in the same way it was.

8) repeat steps (3) through (6), inclusive.

What It Says. Remember the hairspray. If you don't, the next layer of fresh silicone will stick to the cured layer of silicone, and you will end up with a nice single block of silicone and your master embedded inside it. You don't want that.

9) remove the mold from the lego, pry apart, remove the master.

The silicone will fight you, even with the hairspray. Be gentle, try to find a corner that gives you an opening, and gently try to get it open. Your master may stick to the silicone. Don't rip it out, again try to find a forgiving corner and work from there, or your master may take some damage.

PS: The blue tint in the corner is leftover pen ink from the white piece of plastic that I used for a channel. Ignore it :)

10) wash the mold in fresh warm soapy water. Let it dry!!!!

We need to get the hairspray off. And anything else. The naphtha has to evaporate as much as possible too. Any resin we pour into the mold will react with any contaminants left on the silicone and most likely not cure properly. As in, become a sticky mess. You don't want that. You can also wash the master, if you want to use it somewhere else.

I leave the mold immersed in water overnight, to make sure it is completely cured, and to leech out any of the above-mentioned chemicals. Just make sure it is dry before you cast resin into it!

What you have now is a typical 2-part silicone mold. To cast resin into it, you can use the same technique as with RTV silicone molds. Again, the internet has a lot of good info on this. Refer to that video series I linked to in step (1).

An example cast

This is an example cast I made using the mold above. I used a 2-part 45-minute pot-life / 12 hour initial cure / 3 day full cure epoxy resin that I tinted with some sepia ink. This resin is not ideal, it made lots of bubbles, and has a horrible cure time, and has ultra-low viscosity (so it leaks everywhere). So not ideal.

First up you can see the upright closed mold, with supporting structure all taped up. I injected the resin using a 10ml syringe into the entry hole, unitl the resin came out of the exit hole.

The next image is the finished piece still inside the mold. On the left you can see the original.

You can see that the resin level sank while hardening, but luckily the entire piece was still cast. All the detail is there, it looks pretty good. There is very little flash. Also, since this is the first cast, it will clean up the mold by taking any leftover clay with it.