Hello Forum,

I hope I am posting to the right place. I am new to mold making, and need some advice on making 2-part molds with liquid latex.
For a test, I made a box out of Lego, lined to bottom with oven-bake clay. Using an old plastic figure, I embedded in halfway in the clay, made some location lugs and poured on some liquid latex. However, after 24 hours it is not set (the surface has a skin, but below is still liquid).

I am starting to think that I have bought the wrong casting material. I have seen posts that suggest adding talc or fine plaster to thicken the latex. However, other posts have suggest that this retards the setting process and would still not set under the top skin. I have seen lost of places using RTV rubber rather then liquid latex. However, RTV is more expensive and I have bought 5 liters of liquid latex!

Any help / suggestions on this would be gratefully receive.

Kind Regards,
Harold Clements

The liquid latex you are using is air drying. Its used for making moulds by painting on the object and gradually building it up in layers exposed to the air. The skin is then peeled off and you use it to cast the new part. Its used commonly to cast plaster of paris parts and garden ornaments.

As its air drying, this is why it doesn't go hard in the bottle its supplied in and will not cure in a mould.

You should use something that either cures by chemical reaction (thermo setting) or by cooling down (thermo plastic) There are hundreds of types available.

Try here Tiranti for mouldmaking . Vinamold is reusable and the silicon is not that bad in price. Just find what you want, then google the brand name if you are not in the UK to find a local supplier.

You will struggle to use what you have for complex shapes, but you could use the build up technique and add cotton wool in the later layers, then back it up by casting plaster of paris behind it. This gives you a hard mould but with a softer latex skin near the part which helps when demoulding.

Thank you very much for your reply.

I will give your idea of building up the mould, adding cotton wool to the layers. Out of curiosity, I have read that its a very good idea to use neat latex to get the initial shape of your object. However, would thickening the latex help after the initial build up, or should I just add cotton wool and neat latex? Also, I was going to get the plaster of paris backing a go. I take it this will adhere to the latex with on problems?

Thank again for your response.

Kind Regards,
Harold Clements