Introduction: Silcone Matrix Mould / Cavity Mould

About: One day I shall own a Delorean.

In this instructable I will show how to make a simple silicone matrix/cavity mould, single piece, the simplest form of matrix mould, it is possible to do multipart matrix moulds but I wont be covering that in this instructable.

I needed to make a silicone mould of a part and I didn't want to use a load of silicone by doing a block mould for such a large part so I thought I would give matrix moulding a go.

there are a few methods for creating a thin silicone mould with a fibreglass jacket, one method is to add a thickener to the silicone paint it on until you think its thick enough then let it set, and build a fibreglass jacket on it. problem with this method is that you can end up with thin spots on silicone mould which would be prone to tearing.

A matrix mould allows a uniform thickness of mould over the whole part you want to cast and it uses significantly less silicone than a block mould so it works out slightly cheaper. the down side is that it requires a little more time for the setup.

This kind of mould is also referred to as a cavity mould as you are creating a cavity between the master (the part you want to mould) and the jacket.

Step 1: Equipment and Materials

you will need the following materials and equipment, some are optional and are noted as such. and a suitable master to make a mould of, I used a MDF elbow gimble master from my life size ED-209 project

Like the facebook page to keep track of the progress of that project.


Condensation Cure Silicone - You could use and mold making silicone but I am using condensation cure Shore 25

Polyester laminating resin - for fibre glassing

Fibreglass Matt - I used 450 gsm its cheaper but you use more resin, you could use 300 gsm matt

Foil Tape - optional

M6 Bolts - 6mm bolts

M6 Penny Washers

M6 Wing Nuts

A thick disposable board

Equipment and consumables

Latex Gloves

Tongue depressors - for mixing resin

2" GRP Brush - this is a brush that can be used with the fibre glassing resin, normally its a natural fibre brush as they wont melt with the resin.

Paint mixing cups 600ml

Counter top food mixer - Optional, I used it to mix the silicone but you can mix it by hand.

Vacuum Chamber with Pump - Optional, I used this to degas the silicone to help prevent surface bubbles.

Saran wrap / Cling Film / Pallet wrap

Clear Straws

WED Clay or Oil Based Modeling clay

Blue PVA Mould Release.

Vaseline / Petroleum Jelly

Silicone mould release

Old clothes / overalls - you don't want to get resin on good clothes

Retractable Knife

A drill

A drill bit the same size as the bolts.

A drill bit the same size as the straws, this happened to be the same size as my bolts so I only needed one.

Step 2: Preparing the Master

what we don't want is our Master, the part you want to make a mould of, moving around so we need to secure it to the board with some wood screws.

I found it easier to turn the master upside down, put it on some paper to keep it clean and put the board on top and screw the board to it that way, it really depends on the shape of the part. I put enough screws in to hold it firmly on the board.

I am using an MDF board and I didn't want the polyester resin soaking in to the board and fusing the whole lot together. by using foil tape to cover the board I can avoid that and it creates a nice smooth surface.

I found it easy enough to apply with no bubbles if I used a plastic card like a store loyalty card to smooth out the bubbles. you don't have to go 100% up to the part as the silicone should come away from the MDF without too much trouble.

Step 3: Adding the Clay and Preparing for Fibre Glassing

so the first thing we need to do is protect the master from the clay so I wrapped it in blue pallet wrap before I applied the clay.

Next thing is to roll out the clay to a uniform thickness and apply it to the master. if you are using blocks of WED clay you can cut it with a wire, I am using old oil based modelling clay so I need to roll it out.

to get the same thickness I use a board with 2 bits of timber glued to it, this gives me exactly 12mm thickness all over. this will be the final thickness of the silicone when its poured. Make sure you blend any seams in together and try and eliminate dips as this will be places where air bubbles can form. Around the base of the part I come out about 2 inches (50mm) and I carved a dip in to that so it site perfectly on the jacket later.

when you have a full covering we need to add in some registration keys, this will allow the silicone mould so line up perfectly with the fibre glass jacket later. to make the keys I lay 6mm thick layers of clay in a cross over the part making sure not to leave a gap between the 2 pieces of clay.

Once all that is done I used a piece of 100mm plastic pipe I had laying around and pushed it in the top, which will be the pouring spout for the silicone later on.

Then i put on 3 layers of blue pva mould release letting each one dry before applying the next, this should make it easier to pull the clay out of the jacket later, if you are using a water based clay you could skip this as you can just wash out the jacket later.

Put in some bits of clay around the outside as pry points so you can remove the the fibre glass shell when its set.

Step 4: Fibre Glassing

Next step is to give it 2 layers of fibre glass, I like to put a bit of colouring in my resin so I can see if its soaked in properly. Initially I tried the first layer without it as i have seen many people do before and I have now ended up with bubbles which I have had to fix. I wish I had done what I normally do in the first place.

The fibre glass will act as a jacket keeping the thin later of silicone ridged and in shape when using the mould.

Be sure that your matt is fully soaked in resin and try and push any air bubbles out. TIP: you can help with limiting the air bubbles if you tear your matt and not cur it.

Step 5: Removing the Clay

Once the fibre glass has fully cured, I normally leave it over night to be sure, we can remove it, but before that we have to drill holes for bolts. using a sharpie mark out at regular intervals where you want to drill holes, about 4 - 6" inches apart should be fine.

Drill the holes and go right through the board, you may want to put the board on blocks before you do this so you don't drill in to a table of workbench.

Now you can start to carefully pry the fibre glass off, if you are lucky it should come right off the clay, if like me you are not lucky then the clay will remain in the jacket.

You should be able to peel the clay away from most of the jacket, if you are using oil based clay you may have to scrap everything you can out of it. if you used water based clay you can wash the clay out with a soft nail brush or toothbrush then dry it with paper towels.

Step 6: Preparing to Silicone.

if you have some air bubbles in the fibre glass don't worry too much just use some epoxy putty to fill them in.

using a little clay make sure the bottom of the master is sealed to the board so silicone can't leak under it.

I wanted to be sure I had a solid seal on the edge of the jacket when i attached it so I coated it with Vaseline and sprayed it with silicone release as well as the master, then bolted it to the board.

Put the bolts on finger tight then use a spanner to tighten them up but not too tight you don't want to damage the jacket.

Next we drill holes at the high points where air will pool, these holes will allow the air to escape when you pour the silicone.

I use the clear straws to add sprues to the holes this allows me to pour the silicone without having to rush to cover the holes, I can pour the batch I am working on and plug the straws. when I put the straws on I attach them using a bit of oil based clay.

Step 7: Pouring the Silicone

I use a kitchen mixer to mix my silicone which makes things easier, it does mix a little air in to it but not much more than doing it by hand. I mix it up in small batches of 1kg to 1.5kg at a time and then put the whole bowl in the vacuum chamber which is why I bought a large one so I don't have to mess about putting it in another container.

The vacuum chamber expands the air inside the silicone which allows it to come to the surface, so in the vacuum it looks like the silicone is boiling as it expands and eventually it expands so much it collapses in on it self. at this point we hold the vacuum for about 90 seconds then let the air back in the chamber.

By degassing the silicone I can reduce the chance of a bubble forming on the surface of the part I am moulding, this doesn't mean I can just dump the silicone in to the mould though. I still pour it at a steady rate but I can afford to do it a little quicker than hand mixed silicone.

You don't have to use a mixer you can do it by hand and you don't have to use a vacuum chamber to degas the silicone what you can do it pour much slower and form a very thin stream so the bubbles break on the way down.

as you pour each batch give it a minute for the silicone to settle in the mould and you should start to see it come up in the straws when you do block them off using a little clay on the top of the straw, this will stop them filling up any more.

Keep mixing up batches of silicone and pouring them in, you should be able to tell how full the mould is by how far up the silicone is on the straws. When you start to fill up the sprue just fill it about 1 - 1.5cm high, then leave it to cure for 24 hours. the time to cure depends on the brand of silicone you have bought so always read the instructions.

Step 8: Demoulding and Using the Mould

Now 24 hours have passed and the silicone is cured its time to demould it, undo the bolts and then carefully pry the jacket off, the silicone may stick slightly to the jacket and come off with it that's ok.

I can then just peel out the silicone from the jacket and trim off the sprues from the straws and any flashing from the pouring sprue.

The silicone should now fit back in to the jacket perfectly and light up with the registration keys.

you can now use the mould, lightly spray it with silicone release and prop up the jacket just on some blocks to keep it level and steady. you can now coat the mould in what ever medium you want I will be casting this part out of fibre glass so I will coat it with gelcoat and lay it up like any other fibre glass part I would make.