I have been making two part RTV molds for quite some time now. Lately
I have started using the aluminium mould frames designed for
vulcanised molds to contain the silicon rubber. I place the frame on a
piece of glass. A little vacuum grease between the two surfaces
prevents any of the liquid RTV from leaking out. I then pour in a thin
base layer and cure this quickly by placing the mold in the oven at
about 60 degree celcius. Once this is hard I attach the sprued
original to the mold frame. Most of these frames have holes drilled
through them for this purpose. I then stick two tefflon pieces to the
base layer, using a little RTV as adhesive.These will form holes in
the next layer, which will act as lock lugs for the two parts of the
mold. You can use almost any plastic or metal shape for this purpose.
The only requirements are that the RTV does not stick to it and that
the shape is not wider at the top than at the base (I know - kind of
Once the tefflon pieces have adhered to the base layer, I pour in the
next layer. At this point you have to be sure where you want your
dividing line to be. With simple pieces this would just be a
horizontal line. With complicated pieces you may have to incline the
mold at various angles, allowing successive layers to set before
continuing. You can also block off areas that you do not want filled
at this stage with plasticine.If your original is made out of wax,
this part is quite time consuming. Otherwise curing can again be sped
up in the oven. This can be messy if you did use plasticine, as it
releases oil when heated.
Once the second layer has set, you can remove the spacers (lock
lugs). I then spray the surface of the second layer with a tefflon
spray to act as a barrier coat. You can also buy special fluid, from
the RTV suppliers, that can be painted on. (Check with your supplier
what can be used as some RTV’s will not set if in contact with the
wrong materials.) Once this has dried, I pour on the final layer. Once
this has set you can remove the mold from the frame, and your 2 part
RTV mold is ready.
If you have originals with a lot of surface detail, it is best to
paint a bit of RTV onto the bottom surface before pouring the lower
layers. Otherwise you are likely to trap some small air bubbles.
Hope this helps.
Now, does anyone have some detailed instructions for making 2 part
vulcanised molds? I have not had much success with them.
All of the best from a chilly South Africa.
Johannesburg, South Africa