Designing and Making a Silicone Mould

The first principle of a good mould is to begin at ‘the end’ and work your way back to the beginning. ‘The end’ being the piece you want to cast and the material you want to use, plaster, polyurethane resin, epoxy, silicone etc; Knowing this you can select the right silicone for your mould. If you aren’t 101% sure, confirm with your supplier that your preferred casting material will set, in the mould.

Not all resins or epoxies will set in all silicones. If you pour wet plaster into a plaster mould, without using a release agent, the whole thing will become one large block of plaster

Silicone applications

Most Silicones are suitable for multiple castings, but a few are only good for ‘one shot’, some are skin safe, and a few are food safe, others can be brushed onto vertical surfaces. All set at different rates, with varying degrees of flex, hardness, tensile strength ratings, etc.

Silicone mould design

After material compatibility, Design is the most important part of a mould. A poorly designed mould may result in awkward join lines through the middle of the face of your Venus De Milo, or Jedi, where it may be difficult if not impossible to clean. Poorly designed moulds may distort the casting, undercuts may cause air bubbles under chins, on the tips of delicately up turned noses, or at the end of your lightsaber. If you are planning a cold cast metal finish, bad mould design can prevent you from laying down good metal skins. Brushing gel coats do not run and important details can be lost, moulds for this application need to be as open as possible.

Original model

Original model

The mould making process

To keep this mould simple I don’t intend to talk about sprues or chimneys, while not inherently difficult they do make a simple process more complicated.

Join line

Join line

Defining the join line; each mould needs to be made in a way that will assist with the easy removal of the piece once it is set. Our horses head is basically symmetrical, a line drawn from the base along the center of his mane, between his ears, down the snout, back up under the snout, down his neck back to the base. The two halves are clearly defined and it’s apparent that it will be easy to remove the piece from the mould. The texture of the horse offers considerable leeway as to exactly where the join line runs. Following the high points will make cleaning up easier and less obvious.

Beginning of rough clay blocking

Beginning of rough clay blocking

Locate the horse horizontally on your baseboard using a ball of clay and begin roughly blocking in an outline under the head. Use small logs of clay and build up towards the join line. Using small logs of clay will make life easier later when you have to pull down the clay. When the wall is built up to the join line, smooth out any sharp edges The ledge needs to be approximately 15mm wide all the way round the head and more or less flat.

Completed rough clay blocking

Completed rough clay blocking

Now the slow delicate work of pushing home the clay begins.

Good dry edge against the horse

Good dry edge against the horse

You need to create a ‘dry connection’ between the head and the clay. Silicone is very viscous and will leak thru the smallest gap, it’s also expensive and wasting it is a ‘no no’!

As well as being dry, the connection needs to be flat, a rolled edge on the mould may cause difficulties locking the two halves of the mould together. This flat connection needs to extend out from the piece for 2 to 3 mm.

Poor rolled edge

Poor rolled edge

Don’t rush this, it’s slow, careful, detailed work, the better it’s done the better the mould, the better the end result.

Poured via original base, black shows probable air bubble

Black shows probable air bubble

With the dry connection completed, it’s time to plan the extremities of the mould and the construction of the dam walls. Where will the base of the mould be? It needs to be flat so it will stand easily for pouring, the base line and the top of the pour hole will be parallel, so draw a line across the pour hole the base line becomes obvious.

Correct pour hole

Correct pour hole

Chose the right place for your pour hole, it’s not always in the obvious place! The obvious location for a pour hole is at the base of the horses neck. But if it was there the head would end up with a large air bubble where the snout should be. So the head is rotated until the snout is from the perspective of pouring the resin ‘down hill’. The base line is established and the pour hole will be above. Again the end process needs to be considered early.

The sooner you get used to the idea of working upside down, back to front, inside out, half way upside down, neither straight nor level, top to the bottom, the easier mould design will become

Two dam walls with dry connection

Two dam walls with dry connection

Building a silicone mould dam wall

A dam wall is built along the base line, the base of the mould needs enough integrity to hold its own weight plus the weight of the resin, without distorting, in this case the 15 to 20mm ridge is enough. The wall needs to be at least 25mm higher than the highest point of the horse.

Last clay wall with pour point

Last clay wall with pour point

With the first wall built, the next job is to make a dry connection between the ridge and the wall. Again slowly and carefully being sure not to put pressure on the work already done. The best option here is to add extra clay, not drag the existing material, which risks breaking the carefully completed, dry connection. Then the second wall can be built, it connects to the base wall and the flat ridge, next the opposite wall. The top wall, the one with the pour hole is the last to be built and unlike the others it is pushed hard up against the base of the horse.

Pour hole sealed to wall

Pour hole sealed to wall

Where it touches the horse will become your pour hole, the rest of the ridge is then completed and we are almost done. The joins between the ends of the dam wall need to be sealed, this can be done on the outside, they need not be pretty, but should be strong and dry.

Some mould makers build the walls first and then make the dry connections. For me one wall at a time allows greater access to the work area while limiting the risk of damaging the work already done.

Location holes

Location holes

Small location holes are created in the clay, (these will locate the two pieces of the mould relevant to each other) the round end of a pencil is as good a tool as any. Don’t push down! Drill, 2 or 3 mm deep is plenty, with your finger tip, flatten any sharp edge you may have created in the process.

Half silicon mold

Half silicone mould

Mixing and pouring the silcon mould

You are ready to pour the first half of the mould. Mix the silicone, always a ‘two part’ material, according to the directions and mix it well. Bubbles in the silicone should not be an issue, but poorly mixed components may not set. Make a habit of choosing a low point in the mould box and pour all the silicone, slowly in the same place. Don’t pour directly onto the piece and fight the temptation to pour all over the place. Silicone is very clever stuff, it’s self-leveling and will find its way into all the little nooks n crannies, without your able assistance.

TIP: All silicones are two part mixes, make sure you understand and follow the manufacturers directions. If the directions say 50/50 or 100/5, clarify is this by weight or volume.

Check that your dam wall is dry, no leaks please, leaks are expensive and tricky to stop once the silicone has made good its escape. The most effective way to stop a leak is to hold a small piece of absorbent paper against it for a minute or so. Be warned, squashing down on a leaking clay wall, usually makes matters worse. If all’s well, nothing for it but a cup of coffee and a well earned break. Drying times for silicones vary from 15 minutes to 12 hours, so again check first.

With the first pour dry, carefully pull down the dam walls and lift the mould + horse and clay all in one piece, roll it over and lay it on its back. Now you can deconstruct the clay being careful not to lift the horse even one poofteenth up out of the mould. A good plan is to push down on the horse with one hand and deconstruct with the other. Now the advantage of building with small logs becomes obvious.

“Why” I hear you cry into the wilderness “isn’t the horse allowed to rise up”? Because if he does he wont lay down again in exactly the same place and when you pour the second half of the mould, the clever silicone will get in and ruin all your good work! Remember you can lead a horse to silicone but you can’t make him lay down in it!

Clean up any small bits of clay around the edge of the join line and rebuild the dam walls. Again they need to be at least 20mm higher than the highest part of the horse. Mark the inside wall at a height that will give you approx 15mm coverage over the highest part of the horse.

Walls built, corners and edges sealed, OK we are ready to pour the second half of the mould. Well not quite! Liquid silicone poured onto dry silicone will make a perfect bond and Neddie will be trapped inside never to roam the plains again. A release agent is applied to the surface of the mould, over spray onto the piece shouldn’t be a problem. This done you can pour the second half of your mould.

ANOTHER TIP: as a matter of habit I like to apply release agent to all sides of the first mould before building the walls. It can be difficult creating good dry connections between the walls and the silicone so leaks are a good chance. This way any leaked silicone will peel off when its dry. Don’t forget, make sure the release agent is suitable for the silicone being used

When the second half is set, tear down the walls with gay abandon or anyone else who might help.

Now the Grande Opening, starting at the pour hole, peel apart the two halves of the mould. If it feels like its not going to seperate, panic! Then when you have calmed down, apply a little more pressure and slowly but surely like a giant clam the two halves will open. Remove the horse and stare in wonder at the exacting detail of your two piece mould and congratulate yourself on a job well done

Should we continue on and make the first replica? Why not!

Pouring the resin casting

Because we were clever and decided at the beginning what we wanted to do at the end, here in the middle we can rush headlong on without a care in the world, knowing our resin will set in the comforting confines of our silicone mould.

Strap the two halves of the mould together with some rubber bands, fill it with water, then pour the water into a measuring jug. Now you know the total volume of resin needed.

Rubber bands are OK for a quick volume measure, to pour a piece I suggest a tad more sophistication. A couple of pieces of board, cut to the size of the sides of the mould and a clamp (or rubber bands if you must). The boards ensure even pressure is applied to the mould, so as not to distort it. Mix the resin as per the instructions and pour it slowly into the mould.

Go away and leave the resin to do its own thing, maybe you could do some cleaning up! Like silicones, different resins have varying setting or demould times. This is a chemical process during which a great deal of heat is generated. When you think its set tap the top of the resin with a pencil to check, make sure its cold and demould your first replica.

If you have done everything according to the rules and I haven’t forgotten something critical to the process, you should have an exact replica of your original piece.

If you have a perfect result feel free to email me with your glad tidings. If something went wrong I am traveling overseas and will get back to you.


One last tip or two

Read material labels carefully before use, especially safety information. I know its un-Australian but do it anyway, for your own good

Work in A VERY WELL VENTILATED area.

A very small number of people are allergic to polyurethanes.

Polyester fumes can be dangerous, to everyone, even in small amounts. These fumes can effect your nervous system. Polyesters are banned in many schools and colleges.

Paper masks? Don’t make me laugh!