Concrete Abutment – Pt II

Making a latex mold
I used latex for the mold. It was pointed out that it would have been much better to make a mold using RTV. I agree .. but I had the latex rubber already. The RTV mold would have been quite a bit more expensive I think. Hey .. what can I say. I’m cheap. In any case, I only needed two castings so it seemed to me that the latex mold was better from that point of view.

So. Five layers of latex and one layer of latex + cheesecloth.

Negative Casting
Since *unlike the RTV mold which is self-supporting” you have to support a latex mold. That means that I had to form a box around the master and latex mold while it was still on the tile. Plaster poured around it will support the mold when you are making a casting.

Everything worked great – until I tried to get the casting out. I couldn’t. The problem was that everything fit so tight it created a vacuum between the negative and the outside of the latex mold. I ended up having to break the negative. It wasn’t a major problem as I held it back together for the second casting like a giant jig-saw puzzle. The solution would probably be simply to cut the negative in half – or – probably a simply groove cut into the negative would have allowed air to get behind the latex mold. I remembered afterwards that people who cast plaster allow a passage to let them used compressed air between the casting and mold.

You can see in this photo how the casting was set up and how the negative supports the mold. You can also see where I had to break the negative.
A casting
Once I had a casting I could use it to see where I needed to remove scenery so everything would fit.

That’s it for the abutments other then putting some color on them and bridge shoes – but that’s covered in my thread on the On30 Plate Girder bridge .. what I made the abutments for.



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