The answer is that you don’t. And you can’t using your method.
Silicone rubber does not have the same specific gravity (or density)
as water, so the volume of water you measured has no relation to the
weight of the same volume of rubber. Think of a pound of feathers vs.
a pound of carrots. Different volume entirely.
What you need is a scale. If the ratio called for is 9 to 1, then
you need 9 grams of rubber to 1 gram of catalyst. Or 18 to 2. Or 27 to
3 , etc.
How many grams do you need for that mold? There are formulas, but far
easier to is take an already finished mold a weigh it. Each mold won’t
be exactly the same, but it’ll be close. Yes, you may have mixed a
little too much on occasion, but what is the cost of a total
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