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GE RTV664A silicone

I have been making some rubber molds using GE RTV664A silicone. This
was the first time I have used silicone and found that the viscosity
of the mix was very high. I vacuumed the mix at 29in-g for at least 5
minutes and still do not get out all the air. Are there other grades
that would have less viscosity, but would have a similar density? This
material has a shore hardness of 60.



I don’t know if this will help but I have used a Dow Corning product
for making rubber molds. This product is a two part silicone rubber,
the name is 3110 RTV silicone rubber it has a relatively long working
time and I have had good success in vacuuming out the bubbles. This is
an opaque white material and comes in 1lb and bigger containers with
the catalyst. My use has been for relatively flat items that I have
wanted to mold, I have not made molds that needed to be cut but rather
molds made in two parts.

Douglas Frey

Lee; I have been using the RTV silicone rubbers for molds for about 25
years I use the Dow Corning products myself, I think they are all
pretty thick. In order to remove all the air introduced when mixing I
first vacuum in the mixing container. This helps remove a large
quantity of the air. I usually mix in a 16oz plastic cup so I have a
lot of room for expansion. I only pour one or two molds at a time and
they are usually about 100to 125 gr total wt. I then pour the
vacuumed mixture into the mold frame and vacuum again. As the mold
material rises to the top of the mold frame I release the vacuum until
the volume drops back down and the quickly close the release valve and
increase the vacuum. As you do this repeatedly you will find that the
bubbles burst when you release the vacuum (I only reduce the vacuum,
not completely release it) and restore it, helping to reduce the
amount of trapped air. It usually takes 10 to 15 minutes of this
vacuum and release method to remove all the bubbles. Always works well
for me. Another possible solution is to paint on a contact coat of
mold material with a soft brush, then if you do still have bubbles
they will not be on the model but in the body of the mold. Good luck
and feel free to contact me of line if I can help with any other
questions about RTV molds. Frank Goss