Sometimes, especially with large bones or assemblies, a silicone block mold ends up being too heavy and expensive. A thixotropic mold is a good solution as it combines the best qualities of silicone (detail, non-stickiness) with those of latex (lowered cost, lightness).
To make a thixotropic mold you need the same materials as to make a block mold, but additionally you require
* Thixotropic additive for the silicone, or a standard thixotropic base
* A disposable chip brush
* Burlap and Aquaresin (or Hydrocal) for making a rigid mother mold
* Optional nylon window screen material (Rip stop inclusion in the mold)
The bone is clayed up in the same manner as I described in the Block Mold article, except that the surrounding walls don't need to be as tall. A small batch of regular silicone is then poured over and brushed onto the surface of the bone. This is the "print coat" and ensures an air bubble free surface on any place that detail is required. Brushing every few minutes until thickened is needed to make sure that the silicone does not flow completely away.
Once this coat has partially cured, it is now time for the thixotropic coat. This coat is more silicone, and is intended to form the main thickness of the mold, giving it strength. You may either use a straight thixotropic base 9like with GI series silicones) or add a thixotropic additive to standard silicone (like with Dow HS II or GT brand silicones). It is recommended that you mix the base and standard catalyst in before the thixotropic additive, your arms will thank you for it.
Brush an even coat over the entire mold, clay ropes, pour spout, bone, everything. In order to ensure ripping does not happen while pouring the mold, I like to incorporate some nylon window screen material into the silicone (as strips, covered with silicone) at this time. Allow this to cure fully.
Next you must make a rigid mother mold so that the mold will retain its intended shape. Plaster and burlap works well, but can be heavy. Aquaresin and burlap works better, as you can get decent strength from only a double thickness of burlap. It is imperative that you make this mother mold before either flipping the mold or removing the mold from the item.
If you are making a single-sided plaque mold, you are finished when the mother mold is fully cured. If you wish to make a two sided mold, flip the mold and remove the clay. Put a thick separator on the exposed silicone (and of course release agent on the bone surface) and you are ready to repeat the steps to make another side. |
