Silicone Tin catalyzed cast-a-mold 25T or 30T rubber mold making

Cast-A-Mold 25T is a general purpose rtv silicone elastomer which exhibits excellent tear strength, long library life, and accurate detail reproduction. Cast-A-Mold 25T is a condensation cured silicone (tin catalyzed ) It is used to make rubber molds that can be used to cast polyurethanes, epoxy’s, polyester’s, cement, concrete, soap, wax, plaster, etc.. It is easy to mix and de-air, and will cure at room temperature over virtually any surface. Cast-A-Mold 25T is extremely useful for applications where superior physical properties are required.
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Thanks to IQTAGS.com demonstrating jewelry mold making process.

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Better brush on the silicone first to make sure no bubbles are created and then cast the rest only in one corner, do not cast directly on the piece.
If the workpieces are solid you can also use a pressure pot or degas the silicone first in a vacuum chamber.