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Latex Moulds

Latex MouldsAndrew 'Kishkumen' Nelson cast the tyres shown in this piece using latext moulds that he made himself.

These moulds were Andrew's first attempts at mould making so he very sensibly chose a relatively small and simple item for his first efforts:

I took tires from toy cars and stuck them together with tacky glue and stuck them to a flat surface with more glue. Some of you might recogize the smaller single tire castings as 1970s-era Lego tires (the bald ones before the chunky treaded ones were made). Several other types of types were also used.

I then brushed on a thin coat of latex, making sure to get everywhere. Once dry, I dabbed on subsequent coats of latex until it was thick enough. It took two or three days to make a mold, so I made a few at a time. Use a cheap brush. You will never wash the latex completely out of the bristles and the brush will be ruined after three coats.

Latex is stronger than it seems at first. It seems to be awfully fragile as it begins to cure, but after a day or two it's like balloon rubber, very strong and flexible even when very thin. With hindsight I realised that my first moulds were too thick. Not only did they take several days to make, but they are difficult to turn inside-out when extracting a finished casting.

Another problem that I encountered was that when I removed the new mould from the original I found that some of the latex had not fully cured, especially in places where it was thick. It seems that some of the latex next to the original, had been cut off from air and didn't set. Applying the latex in thin coats is the key too success but watch out for indentations and other places on the original where the latex may collect into a thinkness that will not set properly.

Latex MouldsThe photo to the left shows some of Andrew's first castings along with his moulds (on the right of the soldier), plus a few other experiments that were on his workbench at the time.

It's is worth observing that Andrew chose to make moulds of individual tyres, or groups of two or three, even though the eventual stacks would be much higher. This was done so that the castings could be glued together in different arrangements for maximum versatility.

Probably the biggest problem with latex moulds is that they can be stretched by the weight of casting material being put in them. Imagine trying to fill a rubber glove with plaster in order to cast a hand shape: the glove would stretch. This problem can be worked around by making a 'support jacket' but this is not an issue with items as small as Andrew's tyres and he simply used a few Lego bricks to prop his moulds in the correct position while pouring the casting plaster.

Latex MouldsMy first castings had a lot of air bubbles in them. By making a thinner plaster mix I was able to reduce the number of bubbles a lot. A spray of soapy water reduced water tension in the plaster mix and helped prevent bubbles. I also tried spraying some soapy water inside the moulds and rubbing it around with my finger. I don't know if this did any good, but my later molds had a lot fewer and smaller bubbles.

There are various techniques that can be used to reduce the number of bubbles:

  1. Take care when mixing the plaster such that you are more likely to 'stir' air out of it than 'whip' air into it.

  2. Pour the plaster in a thin stream such that bubbles are realeased as it goes over the lip of the pouring vessel.

  3. Pour the plaster such that it runs down the side of the mould and fills it up, rather than pouring it into the middle so that it 'splashes' around.

  4. Keep stopping as you fill deep moulds to massage the mould and thereby dislodge any air that has been trapped. Then continue filling.

  5. Part fill a shallow mould and use a brush to 'paint' the plaster around the inside to dislodge bubbles before contunuing to fill the mould.

The final picture below shows Andrew's tyres being using on another of his terrain piece and this perfectly illustrates the benefits of having made a mould: once you've done it you can use it over and over again.

Latex Moulds

Note that while the methods described above work well for small items, larger casts will tend to distort the latex mould and the use of a support jacket becomes necessary.

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