Here I am using work photos I took of a simple 2-piece block mould I made some years ago to illustrate the basics of the mouldmaking process. There’s more than one way to make a mould for any given object and more than one choice of material to use. Actually the options, both for the material used and the type of mould, are not so many though, they can be counted on one or both hands .. but this is enough to make the decision a considered one. The choice depends on the level of accuracy and detail-reproduction you want; the material and shape of the prototype; the type of casts you want to make; the amount of money you can afford to spend and the amount of time you’re prepared to put into it .. see ‘quick view’ comparisons of mouldmaking materials in the Materials section.
The prototype here is a carved, wooden puppet head measuring altogether 10cm long. It has simple features and the surface detail is not especially fine or intricate. It was painted and given a satin varnish as a seal. The parts which will need more attention to during the process are the protruding chin, nose and the ears. A 3D head like this can be ‘divided up’, so to speak, for mouldmaking in a number of ways, illustrated below.
If a mould in two halves is planned, a line around the object needs to be found which would then allow the two pieces of mould, once completed, to be pulled away relatively easily in opposite directions from the object. The two parts of a head mould are most often divided like the form on the left above .. a line bisecting the neck which travels up, usually following the outer edge of the ear, to meet at the top of the head.I say ‘relatively’ easily because using a flexible mouldmaking material such as silicone rubber does allow a certain amount being in the way .. in other words undercutting .. because the mould will flex and stretch over obstructions, at least to a certain degree. The extent to which the silicone is expected to do this will determine whether you use a silicone that is particularly soft and flexible i.e. with a low Shore A value, or one which is tougher with a higher Shore A value. At least, that’s the idea. In actual fact I think that most mouldmakers, like myself, settle upon using a tried-and-trusted general-purpose silicone rubber with a medium Shore A to manage most things.
Setting up is probably the most important, time-consuming and exacting part of the mouldmaking process. By ‘setting up’ I mean preparing the prototype to receive the mouldmaking material .. something which often involves separate stages, each beginning with their own set-up.
Above is the completion of the major part of the set-up for the first mould half which in this case involves embedding the prototype up to a half-way mark in soft modelling material. I’ve already cut the plasticine bed to a shape, the reasons for which I’ll come to later. At the time when I was making this mould I used standard plasticine for this bed (in the UK the ‘Newplast’ type in long cellophaned packets). This was cheap, re-usable and easy to work with. Nowadays I prefer to use soft modelling wax, which is all of the above but easier to clean up (if needed) from the surfaces of prototypes and moulds. See the ‘worklog’ in Modelling wax in the Materials for an account of using it for mouldmaking set-ups.
But for this one, plasticine was packed fairly loosely around the puppet head first just to secure it in place on the baseboard and then built up to reach the half-way line. Although the plasticine must form a clean, smooth and ‘watertight’ seal where it meets the prototype form, the top surface around this doesn’t need to be clinically smooth and usually won’t be flat since it has to undulate around the form. In fact the less flat the better to some extent, so that when the two silicone halves take this form they will lock together better. That’s the reason for the spaced depressions I’ve made in the surface with the smooth end of a paintbrush. The silicone poured on top will fill these marks and the second silicone half will reproduce them, and that will help these halves to lock together in exactly the right place. It’s important though that the initial marks made are just enough to be seen i.e. not too deep, because otherwise the natches (as they’re called) created in the silicone may catch when trying to put the mould halves together prior to casting.
Since I’d decided to divide it straight down the middle the half-way point was fairly clear so I didn’t need to mark it on the prototype first. But one has to mark it somehow for more complicated or less symmetrical forms, and after scratching my head about the best way to do this without defacing the prototype I came up with what I think is the best solution. Below I’ve rolled a thin string of plasticine and am using it, since it can be repositioned, to rehearse possible dividing lines. Once the right one is found this can usually be kept in place when embedding.
The next stage, below, is making a sturdy containment wall into which the first silicone half can be poured. I could have just cut a strip of cardboard to the right height, bent it round and pressed it firmly into the plasticine, for example. But I chose to slice the plasticine bed straight down in a shape first, because I wanted to re-use the cardboard wall for both halves of the mould. I made the cardboard strip to fit around the shape, scoring on the outside to help it bend cleanly in the right places. I’ve sliced the plasticine away completely at the neck base so that when the mould halves are made it is left as a pouring hole. I’ve made the cardboard strip high enough to extend approx. 1.5cm above the highest point of the prototype, to make sure that the silicone block would be thick enough here (in total it should be high enough for both halves of the mould if it is to be used again later, see below) . At the time I didn’t take photos illustrating the final details before mixing and pouring the silicone .. taping the wall securely in place; making sure there were no gaps, including smoothing the plasticine edge to meet the cardboard, and Vaselining the inside of the cardboard wall to help it release cleanly from the silicone .. but you can get the idea.
The next photo I took jumps ahead to dismantling the first mould half once the silicone had cured i.e. once it had hardened properly. It’s not exactly necessary to illustrate how to mix silicone rubber .. but there are a few things to say about choosing, mixing up and pouring it!
In the first place, there are different hardnesses available indicated, as I’ve said, by the Shore A number. Because I was making this 2-piece mould just in silicone and unsupported I chose one which was a little tougher than I would usually for a small form .. T30 from Tiranti in London, which has a Shore A of 30. I judged that if the block as a whole was thick enough it would have the strength to be cast into on its own, but still be flexible enough for some slight undercutting i.e. in this case, the ears.
T30 has to be mixed with 2.5% of its catalyst to cure properly, and it’s best to be as accurate as one can. The catalyst supplied is designed to be mixed by weight rather than volume (this is the case with almost all silicone rubbers, and many resins). This means that digital weighing scales are needed, but nothing special .. inexpensive kitchen scales are fine (see the equipment list in ‘Beginner’s Basics’ – mouldmaking and casting explained). But before one can portion out by weight, one needs to know roughly the volume .. I needed to at least estimate the amount of silicone I would need to fill the containment for the first half, partly because silicone is too expensive to waste. The best way to measure the volume needed is to fill the containment with something which can be easily tipped out and measured .. I’ve done it many times now with rice, for example. This needs to be done before the mould setup is Vaselined, for obvious reasons! I found that I needed 260 cubic centimetres (or ml) of fluid silicone to fill the first half. One could then just measure out 260ml of silicone rubber in a calibrated mixing pot (another item that’s often essential for this work) and whatever the weight comes to needs 2.5% added in weight of catalyst .. or one could multiply 260 by the SG (the ‘specific gravity’ which is the weight of 1 cubic centimetre) which is included on the packaging info, to get the exact weight needed and therefore not need a calibrated pot.
Once catalyst is added to the silicone the chemical reaction .. the curing .. will start but with most silicone rubbers there is plenty of time to mix properly and pour. This should not be hurried .. it needs to be thorough! I use strong, straight-sided, translucent polyethylene beakers and disposable chopsticks for stirring. The bamboo chopsticks are very resilient .. they need to be because silicone rubbers are quite viscous. It is important not to forget either the bottom or the sides of the beaker while mixing. As a ‘rule of thumb’ I make sure that I spend at least 5mins mixing. Nowadays I prefer to use a silicone with a pigmented catalyst .. when there are no more streaks visible you know when to stop .. but T30 has a colourless catalyst. According to Tiranti T30 has a 20mins pot life i.e. the amount of time for mixing, pouring or applying it before it starts to change. This is short compared to many silicones but it also has a shorter curing time of 8hours, compared to the more usual 24hours.
If the prototype form has a lot of fine surface detail, particularly recesses, it’s usually worth brushing the surface with a coat of the catalysed silicone first to make sure that all detail is filled. Because the puppet head didn’t have much surface detail I just started pouring the silicone slowly into one corner of the containment, letting it flow round the form and gradually rise at its own pace. This is quite a slow pace, but patience is necessary because otherwise air can become trapped.
I swear that many years ago .. I mean about 20 .. when I first started using silicone rubber I read something about it needing a certain amount of moisture in the atmosphere to assist curing, and I always used to get better results putting moulds in the bathroom while curing, perhaps even with a wetted flannel or two placed near them. I don’t think this is necessary anymore because silicones have developed, but I still do this in the summer if the air is particularly dry. What’s important though is that you put the moulds in an undisturbed place with a level surface for curing.
Below, after about 8hours (recommended for T30) I dismantled the mould containment carefully, making sure that I could use the wall again.
To make the second mould half the whole setup needs to be turned over and, firstly, all of the plasticine (or whatever is used for a temporary bed) must be removed. This is usually straightforward and can be quick, as long as one’s been mindful not to pack the modelling material too tightly all over the prototype in the first place.
I had some cleaning up to do where the plasticine had stuck. Most can be removed with a wooden modelling tool or a cocktail stick, but if need be plasticine can be dissolved either with Vaseline or white spirit. I think one has to resist the temptation at this stage to take the prototype out of the mould. It would make cleaning much easier, but there have been rare occasions when I’ve tried to put the prototype back after cleaning and it hasn’t sat properly .. i.e. slight gaps visible that weren’t there before .. because some detail on the prototype was catching.
I had made sure that the cardboard wall was cut to twice the height I had needed for the first half, so that when re-assembled it was already the right height for the second mould half. Now it’s really just a repeat of everything done for the first half from this point .. with one very important extra! Do not forget that now you are pouring silicone onto silicone, which will fuse completely unless the cured silicone surface is greased with Vaseline as a barrier! As I say, just greasing the surface, no more than that. Since Vaseline is transparent it is difficult to check whether you’ve been thorough. I usually go over it with the brush 2-3 times to make sure (not adding more Vaseline, just moving it around) or .. I’ve done this occasionally .. put a little pigment in the Vaseline to make it visible.
Silicone rubber moulds can be used as soon as the recommended curing time has passed. A tough rubber such as T30 should last many years and could deliver hundreds of casts if using non-aggressive materials such as plaster, Sculptamold or Jesmonite. Far fewer perfect casts are achievable with resins, because over time resins such as polyurethane or polyester will alter the silicone, making the surface more brittle .. small bits of surface detail can break off eventually.
Being able to make successful castings is a separate subject in itself .. there are many different materials and as many ways of casting with them! But in order not to just leave it at that, here are photos of a lightweight, hollow casting I’ve recently tried using filled resin (polyurethane resin with Fillite).
If you do a lot of casting you will know that there is always a seam where a little of the casting material has seeped into the join between the two silicone halves, but if all has gone well this can be the easiest thing to clean up.
For current suppliers and prices of silicone rubbers see ‘quick view’ comparisons of mouldmaking materials under mouldmaking in the Materials section. This will also give an overview of the other properties or advantages of silicone rubber, as well as the drawbacks.