in Liquid Rubber & Plastic for Moldmaking & Casting

Sculpture & Foundries

Tips from the Mold Room

By Angus Macaulay

How do you repair a silicone mold?

Very few substances will stick to cured silicone mold rubber. This is a tremendous advantage when casting but a problem when trying to patch a damaged mold.
When using the TinSil® 70 Series silicones, the answer is clear. Since you presumably have more product on hand, use more of the same TinSil® product for patching. The TinSil® products stick to themselves very well. In the Polytek® Mold Room, we like to use 4 parts FastCatTM in addition to the standard 10 parts A to 100 parts B mixture for patching and gluing TinSil® molds. The addition of TinSil® FastCatTM to your A and B mixture will hasten and improve the cure in the thin sections encountered in repair work.
When using PlatSil® 71 or 73 Series products, the answer is not so obvious. Platinum-cured silicones generally do not bond well to themselves when fully cured. For patching or filling holes with a lot of surface area open to the air, some single-component silicones work well. We use silicone aquarium sealant in the Polytek® Mold Room. For gluing two or more pieces of PlatSil® together, you will need an adhesive that does not require exposure to air for curing. In this case use a TinSil® product with FastCatTM, as described earlier. TinSil® 70-25 works well.

How can I make a silicone mold over a sulfur-containing modeling clay?
Sulfur clays can cause cure inhibition in many mold materials, particularly silicones. Proper selection of materials and surface preparation will improve your results.
To start off in the right direction, rule out any platinum-cured silicones. Platinum-cured silicones are drastically affected by sulfur clay; it is best to avoid the combination. Choose a TinSil® product.
The TinSil® 70 Series products are very tolerant of sulfur clay. TinSil® 70-10, 70-20, 70-25, and 70-30 are the best choices for use over sulfur clay. The addition of FastCatTM to the standard A and B mixture will improve the surface cure. The TinSil® products tend to stick slightly to sulfur clay. For the cleanest results, try a coat of Krylon over the clay.
If you want to eliminate these special procedures, consider sculpting in sulfur-free modeling clay. Polytek®'s Poly Plasticene is sulfur-free. Most other clay suppliers also offer sulfur-free formulas. Sometimes evading the problem is easier than solving it.

Can I use shellac to seal a sulfur clay model?
Yes, shellac is an adequate sealer for making a TinSil® mold, but you must use a release agent over the shellac. Silicones will often stick to shellac. A liberal coating of Pol-Ease® 2350 will prevent TinSil® products from sticking to shellac. To eliminate the release agent, try sealing with PVA, lacquer, or enamel. As always, a test cure is recommended to determine the suitability of any sealer for your specific application.

By Angus Macaulay

Polytek® has developed a silicone rubber with an unequalled combination of performance and ease of use. Revolutionary new 1:1-mix- ratio PlatSil® 71-20 offers the option of brushing or pouring, a fast 4-hour cure time, and high tear strength. It can be mixed by weight or by volume. PlatThixTM liquid thickener can be added by weight or "by eye" to adjust the flow. The fast cure allows for the construction of the flexible mold and the mother mold in the same day. Molds can be kept thin due to the toughness of PlatSil® 71-20.

PlatSil® 71-20 Is Remarkable
The feature that most people find remarkable about PlatSil® 71-20 is the easy 1:1 mix ratio. An accurate scale is a tool no moldmaker should be without, but this material can be mixed 1:1 by volume. The best procedure for mixing by volume is to fill two identical containers to exactly the same level. Keep them side by side on a level surface so you can accurately judge the volume. Pour and scrape the contents of each container into a third container for mixing. Part A and Part B are two distinctly different colors to act as a mix indicator. If you are making a poured mold, just mix until the color is uniform, then pour.

Brush on a Mold in the Morning
For brush-on molds, the first coat is generally applied without any thickener. This will facilitate bubble release through a paper-thin layer of PlatSil®. Subsequent coats should be added every 20 to 40 minutes, depending on the ambient temperature. These applications will be thickened with PlatThixTM, a liquid additive that creates a thixotropic, brushable consistency. Typical buildup coats would be mixed with 0.5% to 1% PlatThixTM. If you are adding the PlatThixTM by weight, make sure your scale is capable of accurately measuring such small amounts. If you are adding the PlatThixTM by eye, it must be added gradually and carefully. Too much PlatThixTM in the mix will slow the cure and possibly affect the physical properties of the cured rubber. After one or two buildup coats, apply a coat with little or no PlatThixTM added. This procedure will smooth the mold for a better fit in the mold shell and a neater appearance.

Make a Shell in the Afternoon
Once the final coat has lost its tackiness, the mother mold can be constructed. At room temperature this will take 2 to 3 hours. If a low heat source is available, this can be accelerated. The warm air from a hair dryer can reduce the waiting time to less than an hour after the final coat. The mold can be removed from the model 4 hours after the application of the last coat.

Great for Complex Parts
PlatSil® 71-20 has excellent physical properties. The combination of flexibility and high tear strength allows you to demold very complex parts that would be difficult or impossible to produce in most rubbers. The finished mold is suitable for casting wax, resins, plaster, Forton MG, and even concrete.
Silicone moldmaking has never been easier. With a 1:1 mix ratio, you don't even need a scale to use this silicone. In many cases, vacuum degassing is unnecessary. The versatility of being able to brush or pour with ease makes PlatSil® 71-20 a rubber for all occasions. The 4-hour cure time saves time and money. Physical durability and chemical resistance round out the features of this rubber. Moldmakers across the country are expanding their capabilities with this fantastic new product.

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