-- [ From: Al Hastbacka * EMC.Ver #2.10P ] --
I have received many requests for how to make tools and parts from
silicone RTV when using the Model Maker to make the master pattern.
Here are a couple of ways that we have tried with good luck. Since
many of the subscribers to this list may have similar questions, I
decided to distribute this info via the RP mailing list.
1. We have used the investment casting process to make RTV models by
substituting RTV for metal. The way this works is as follows: Make
the master model on the Model Maker. Make a ceramic (or plaster of
Paris) mold by pouring the slurry around the master model. After the
ceramic has dried, put the ceramic mold into an oven to burn off the
Model Maker build material. At this point, you will have a mold with
air where the build material had been. Use a vacuum on one of the
holes in the mold to draw out any loose particles. While applying the
vacuum, squirt in an oil based mold release to coat the inside of the
mold. DO NOT USE A SILICONE BASED MOLD RELEASE. A Teflon release
agent is OK.
Mix up the two part RTV and pour into the ceramic mold. This is best
done in a spin casting machine, so that the RTV will flow into all the
crevices via centrifugal force. Let the RTV cure and then break away
the ceramic mold by hitting with a mallet or dropping on a tabletop.
If done as described, the resulting RTV part will look like it came
from a production tool. In our work, we use a medical grade of RTV.
2. In the more conventional way of making RTV tools, the pattern
produced by the Model Maker is immersed in a slow curing RTV (a ten
minute cure RTV produces exothermic heat that could melt the pattern).
If the pattern is very intricate, use of a mold release is advised, or
you can simply melt out the master pattern by heating to 300 degrees
Fahrenheit and pouring out the green liquid. After the pattern has
been removed from the RTV, subsequent models can be cast from any of
the polymers that are compatible with RTV (we have not had good luck
with polyester resins, but most of the other materials such as the
epoxies and urethanes work fine).
Regards,
Al Hastbacka
precision@sanders-prototype.com
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